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chem geek
12-22-2006, 03:12 PM
OK, I'm starting a thread to examine the downsides to a salt pool. Yes, there are many benefits and these have been talked about on other threads, but my search through various forums led me to one guy who posted some information that was correct and some that was not. But it was the correct information that got me thinking about presenting a more balanced discussion to salt pools. I am starting this thread in The China Shop because the issues are tentative and may get technical.

So first, take a look at this link (http://thepoolbiz.blogspot.com/) and realize that this is just a pool cleanup guy who doesn't know science [EDIT] (no offense intended and I'm sorry I didn't write that better -- the word "just" was particularly inappropriate and "maintenance" would be better than "cleanup" and "who isn't a scientist" would be better than "doesn't know science" -- I'm a skinny weakling who is also not a scientist if it helps balance that out) [END-EDIT] and he sometimes takes sources that have accurate information and misinterprets them. Nevertheless, there are a few points worthy of discussion:

1) Does the splash-out (and dripping from bathing suits, etc.) of salt onto coping and other concrete and grout surfaces cause them to wear, pit or have other problems?

2) Does the splash-out and dripping of salt onto metal surfaces (such as diving board mountings) cause them to rust more quickly?

3) Do SWG pools increase corrosion rates of metal surfaces in the pool water such as copper heat exchangers? If so, why?

[EDIT]
4) Does the increased electrical conductivity from higher salt levels lead to greater shock risk with improper bonding?
[END-EDIT]

I'll just leave this as questions to mull over and think about for now. The responses may lead to ways of alleviating any of these issues, if these issues are real. I look forward to your comments and will add my own. No rush in this; I just wanted to put a placeholder here so we don't get too swayed by how perfect SWG pools are. Like most things in life, nothing is perfect and it's all about understanding the tradeoffs and compromises and making intelligent choices.

[EDIT]
Though this list can be debated, here's a starting point for discussion. I will expand and update this as we get more comments and information.

PROS:
• Less maintenance. Chlorine added automaticaly. No need to buy chlorine, physically carry it home, and add it regularly (possibly daily).
• Lower possibility of developing algae, though this may be more related to the continual addition of concentrated chlorine rather than the SWG, per se.
• Less burning or dry feeling in eyes. Salt in water is closer to salinity of human tears.
• Water feels silkier.
CONS:
• Up-front cost is higher than other chlorine sources. On-going electricity cost may be lower. Salt cell replacement cost is high (every 3-5 years?).
• pH often has a strong tendency to rise (so lots of acid needs to be added) unless partially mitigated by lower TA and use of Borates.
• Increased corrosion rate of unsealed coping and hardscape surfaces through repeated splash-out and evaporation.
• Increased corrosion rate of steel and cheap stainless steel surfaces that are not designed for salt water exposure, especially if no CYA is used.
• Increased corrosion rate of copper heat exchangers and copper piping (if any) if no CYA is used and chlorine levels get high.
• Splash out water may evaporate to leave more solid salt on surfaces than with non-salt pools.

Look at this recent thread (http://www.poolforum.com/pf2/showthread.php?t=6364) and now this thread (http://www.poolforum.com/pf2/showthread.php?t=6383) for examples of corrosion posted to this forum. Also, this post (http://www.poolforum.com/pf2/showpost.php?p=41770&postcount=14) describes corrosion of exterior lighting from the splashing of the pool cleaner.
[END-EDIT]

Richard


This could be a good one so I stuck it to keep it at the top for now.
Al

Davenj
12-22-2006, 08:39 PM
Had the pool vinyl IG for about 18 months, SWCG. Kool Deck, haven't noticed anything with that. The SS railing had a white gritty paste/slurry in the aluminum mounts in the deck. Noticed it when I removed them for the winter. Any suggestions for next season.

Dave

chem geek
12-28-2006, 08:55 PM
Dave,

My guess is that the white paste is salt. If these mounts were in the deck, then splashed water could go into the mounts and evaporation would get rid of the water leaving the salts that are in the pool. Multiple cycles of water entry and evaporation would leave more and more salt. For an SWG pool, this would mostly be sodium chloride, but there would also be some calcium chloride as well (I assume that the bicarbonate would outgas with evaporation [EDIT] though you could have some solid sodium bicarbonate as well [END-EDIT]) and some sodium cyanurate. The next time this happens, or if the paste is still there, you can test it by putting a little in distilled water (or tap water, if you test the tap water separately) and then 1) see if the paste dissolves in water and 2) measure the salt (chloride) level. If it's high, then the paste is likely to be salt. You could also measure the CYA level to see if I'm right about the sodium cyanurate.

As for what to do, there doesn't seem like much except putting some sort of waterproof caulking around the top of the mount against the stainless steel railing. You could probably very easily remove the caulking when you want to remove the railing at the end of the swim season. I'm not much of a mechnical guy so others would likely have better suggestions.

Richard

Phillbo
12-28-2006, 10:52 PM
I've yet to have my system a full season so my 'down side' is limited. So far, its been all up side similar to:

• Less maintenance. Chlorine added automaticaly. No need to buy chlorine, physically carry it home, and add it regularly (possibly daily).
• Lower possibility of developing algae, though this may be more related to the continual addition of concentrated chlorine rather than the SWG, per se.
• Less burning of eyes. Salt in water is closer to salinity of human tears.
• Water feels silkier.


My cons are limited as well:

-The up front cost was well within my comfort zone.
-I add a little more acid than I use to but thats a minor negative.
-I have no metal ladders, rails or heat exchangers in my pool.
-I hose off my pool deck and patio weekly during the swim season (did so before installing the SWCG) so I see no build up.


basically, So far, so good.

Davenj
12-29-2006, 08:35 AM
Richard,
I'll test the for salt in the paste next season. My CYA ran 50ppm. Good suggestion about the chaulk. I will try some DucSeal, it stays plyable and will be easy to remove.
Thanks
Dave

chem geek
01-08-2007, 03:46 PM
Well so far, the two reports of corrosion in salt (SWG) pools on this forum have had possible alternative explanations other than the increased salinity. In this thread (http://www.poolforum.com/pf2/showthread.php?t=6364), it appears that the corrosion of stainless steel could be from the high effective chlorine levels due to not using CYA, though the very rapid speed of this corrosion may be due to multiple factors (including salinity) combined together. In this thread (http://www.poolforum.com/pf2/showthread.php?t=6383) it appears that the quality of limestone is the critical factor and that salinity may play a far lesser role in this case.

My gut feel on this is that different factors affect corrosion and can combine together as well. Therefore, blaming a single factor for corrosion is sometimes just used as an excuse. That does not mean that there are not situations where salinity causes greater corrosion, but that it may be getting blamed as the sole or primary factor in some situations where this may not be the case.

Please continue to give us your stories and facts about corrosion you experience, as only a large number of cases will allow us to sort all of this out.

Richard

nater
01-10-2007, 01:38 PM
My experience:

Experience with pool prior to SWC: 18 months
Time with SWC: 12 months
Salt Level: 3200 ppm
FC levels (Ave): 4-6
pH levels: 7.2-7.6
TA (Ave) 120 through late this summer, now at 80 to reduce pH rise
CH: 300
CYA: 120 to current level of 70
Temp range: Winter 40-50 deg, summer 78-86 deg
Pool type: Vinyl Inground
Metal Elements: Stainless Ladder in deep end, light ring in deep end, handle on skimmer lid, plates is SWC cell. (No Heater, no copper, etc).

Observations:
After reading the threads about possible corrosion due to SWC operation, I took a very good look at my pool and equipment.
-I see absolutely no staining or degredation of the concrete at the pool exit points.
-I see some corrosion of my light ring. This was a pre-existing condition and most likely due to low pH (<6.8) resulting from puck usage during the off season (and before finding this forum :)). It has not increased during the year of SWC operation.
-I rigged a kitchen cabinet pull handle to my skimmer lid for easier removal. The screws inderneath the lid have corroded badly. They are NOT stainless. This is not unexpected as I usually add salt to the skimmer when needed. No surprises here.

I believe that prior to purchasing such a system, the buyer owes it to themselves to do a lot of homework. Ignorance is no excuse, and unfourtunately the majority of people in this industry seem to have an aboundance of it. (Anybody that's been "Pool Stored" knows of what I speak)

A SWC system is not a magic bullet. As with any other technology in any other industry, education and awareness of the system's pros and cons are required to make a good descision. For me, it is an absolute perfect fit. For others it may not be the best solution. This is where the "Industry" fails and salt chlorinators in general get blamed for problems resulting in poor execution.

I hope Waste chimes in on this subject. He's got loads of experience with various setups and I'm interested to know his thoughts.

Waterworks
01-10-2007, 05:54 PM
We've been installing SWC's in residential (40+), and commercial (around 10) applications for a few years now. Most of our residential jobs have concrete decking. I have heard no complaints of pitting concrete and not one case of ladder or copper-nickel heat pump corroding. I have one customer in particular with cantilever limestone coping that has noticed no pitting at all, at either steps or the rest of the pool. Another customer has flagstone coping and also noticed no damage after one season. Our commercial jobs are all full-tile and we haven't had any problem with pitted grout on the tile or deck.
I'm going to go over these problems with my service men before the upcoming season, and get them to keep a closer eye on ladders and decking materials. Once I hear from them, I will update in this topic.

Update, I just remembered one issue in a commercial setting. After a few weeks of the pool being open there was staining on the light ring, which transferred a rust stain onto the returns and also the stainless steel heat exchanger corroded to the point that it wouldnt heat at all. The problem was initially blamed on the "corrosive salt". I took a water sample back to my shop and tested the chlorine to be 42 ppm. I think that a lot of the pitting and corrosion issues could be related to high chlorine levels. Since many ppl use dpd test kits, they don't notice that their chlorine level rises way above the norm. Also, since the person doesn't have to add their own chlorine they have no idea how high it can get.

Brad
Waterworks Pools

chem geek
01-10-2007, 07:19 PM
Brad,

Thanks for the info. PoolSean also mentioned (in an E-mail to me) how high chlorine levels can be corrosive and that those with SWG systems may sometimes forget to check their chlorine levels and find them to be too high. The important thing to remember is whether CYA is being used or not. If CYA is not used, then the chlorine level is much, much higher even when it "seems" not to be. An FC of 4 ppm without CYA is over 30 times more powerful as a disinfectant and oxidizer as the same 4 ppm FC in a pool with 30 ppm CYA. I presume that the corrosion ability of chlorine is related to its oxidation capability so a pool without CYA would be particularly at risk.

So my question is whether your pools, the commercial ones in particular, use CYA. Though one might think that they would if they are outdoors and exposed to sunlight, I have heard that some commercial pools with continuous chlorine sources (chlorine gas, hypochlorite liquid, or SWG) do not use CYA. That would be interesting to know and could explain a lot.

Richard

Waterworks
01-11-2007, 12:50 AM
Most of the commercial pools, including the one with the 42 ppm chlorine reading are indoors and have 0 CYA. Most of the residential pools are outdoors and keep their CYA readings between 30-60 ppm.

Do you have an article showing the correlation of disinfecting/oxidation power of chlorine when related to CYA? I've never found a decent source, except when only comparing ORP levels at different CYA levels.

Brad
www.waterworkspools.com

chem geek
01-11-2007, 02:08 AM
Brad,

I answer your question about chlorine/CYA relationships at this post (http://www.poolforum.com/pf2/showthread.php?p=41531#post41531) so as not to get off-topic from corrosion here. Please respond over in that thread.

Richard

chem geek
01-11-2007, 01:31 PM
I received this PDF file (http://richardfalk.home.comcast.net/pool/Corrosion_and_Swimming_Pools.pdf) from PoolSean that refers to careful studies that showed that high chlorine levels are the primary metal (mostly Type 304 stainless steel which is the most common and is not marine-grade) corrosion factor. High chlorine (20 ppm) with no CYA is very corrosive. Chlorine levels of 3.0 ppm and below (without CYA) did not initiate corrosion in one year. CYA levels inhibited corrosion and seemed to confirm that it is the hypochlorous acid concentration that determines corrosion. So typical outdoor pools that use CYA and have normal chlorine levels (even "shock" levels) will not see metal corrosion (assuming pH and other factors are normal). It was noted that owners of SWG systems often do not test their chlorine levels or they use a test method that is inaccurate at high levels (i.e not using the FAS-DPD drop test method) so chlorine levels can sometimes get very high (especially over the winter when chlorine levels are typically not checked [EDIT] in cool, but not cold, climates since the SWG shuts down below around 50F [END-EDIT]) and in indoor pools without CYA this can be a problem (and very high chlorine levels may degrade CYA so that even outdoor pools would be at risk).

The study also looked at salt levels and found that levels of 3000 ppm and below were not a problem and that corrosion was primiarly initiated at 6000 ppm or above. [EDIT] This was in testing of an electrolytic unit, so presumably a chlorine generator, but did not indicate the level of chlorine, the length of the test, nor what metals were being tested. [END-EDIT]

What was not looked at, in the laboratory, was the combination of salt and chlorine, but field studies indicated no issues with properly maintained pools. This is consistent with the reports we are getting from Waterworks (Brad) and I suspect will hear from others. Keep those observations coming in! waste, if you're reading this, we'd love to hear from you.

[EDIT] Note that the study was initiated (and probably paid for) by ELTECH Systems Corp. and they provide technology for the production of chlorine by electrolysis (mostly coatings for anodes and cathodes) and are therefore biased. That doesn't mean the data is bogus, but should be looked at keeping this in mind. I would prefer the original study data, rather than a summary. [END-EDIT]

Richard

Waterworks
01-11-2007, 02:18 PM
Another story just occured to me.
I have a customer with a 12' X 28' X 4' Deep indoor lap pool made of one peice of 316L stainless steel at his cottage. After a few months with the pool he noticed rusting and degradation of the steel. He called in a metalurgist and they both blamed the salt. I couldn't convince either person that salt at low levels was not corrosive to stainless steel. Every time he noticed the rusting he would drain the pool quickly to rid it of the 'evil' salt, and I was never able to test for chlorine. The pool was originally started up in the summer, and the customer also has an outdoor pool at the cottage, as well as a pool at his house, and the cottage is located about 250 feet from the Atlantic Ocean. I doubt that the lap pool was ever used. It was kept in the mid 70's so the chlorine demand would have been very low. He had an aquarite system on the pool. My guess is that the Aquarite produced waaayyyy too much chlorine for such a tiny pool and caused the rusting. He also had 0 ppm CYA which would make the probably high FC readings much much worse/ He ended up getting some type of sealer to go over the stainless to protect it for now. I think I will give him a call and get him to put in some CYA and see how it works.

Brad

chem geek
01-11-2007, 02:28 PM
And 316L is marine-grade salt-resistant stainless steel! So it would indeed be very good to get to the bottom of that customer's particular situation. In addition to checking the chlorine level, be sure to check the pH and also see if the stainless steel has been bonded to other metal exposed to the water. If the chlorine level and pH seem OK (ah heck, check the CH, TA, temperature and salt level as well, while you are at it) and the steel is electrically bonded, see if you can put a current meter between the bonding wire and the steel (assuming it can be easily removed -- or put a voltage meter between the steel and a grounding wire or ground post). I'm just thinking about different possibilities -- stray currents (and voltage) could be an issue, though I'll bet you are right that it's just high chlorine levels.

Richard

Waterworks
01-11-2007, 03:15 PM
I think that we could definitely learn a lot from this particular pool. The customer is a great guy, and is also very interested in getting to the bottom of the problem. As soon as the problem started to occur I asked him if it had been grounded and he immediately said yes, in four places and to every other peice of metal around the pool. I didn't actually think to check it out, but it seemed like he knew for sure that it had been done. Then I told him that an experienced salt guy (Sean) had told me that Stainless does not rust below 6000 ppm. He told me that his metalurgist and chemist said the salt was the problem, and I basically took his word for it. If I knew then what I know now I would have tried harder to convince him that it wasn't the salt's fault. I will try to convince him to let us add more salt and try to figure out exactly what happened.


Brad

CarlD
01-11-2007, 03:16 PM
I see no reason for this thread to be stickied. I have un-stuck it.

chem geek
01-11-2007, 09:02 PM
Then I told him that an experienced salt guy (Sean) had told me that Stainless does not rust below 6000 ppm. He told me that his metalurgist and chemist said the salt was the problem, and I basically took his word for it. If I knew then what I know now I would have tried harder to convince him that it wasn't the salt's fault. I will try to convince him to let us add more salt and try to figure out exactly what happened.
I wouldn't be so quick to dismiss a chemist and a metalurgist. If they were qualified and knew about studies on metal corrosion from salt, then maybe they do know something. This shouldn't be about convincing anyone about salt being the cause or not being the cause. It should be about finding out the truth and it may be that salt is a problem in some circumstances, but not in others. [EDIT] Nevertheless, your plan of having him use CYA and also monitoring his chlorine level is a good one -- definitely let us know the result! [END-EDIT]

The stainless steel rusting example in this thread (http://www.poolforum.com/pf2/showthread.php?p=41424#post41424) was at FC levels of 3-5 ppm though there was no CYA. This isn't at the level of 20 ppm where corrosion is extremely rapid, so maybe on the high side of FC this could be the cause. The study said no sign of corrosion in a pool or in a tank with 1-3 ppm FC after 1 year, but maybe that is still "on the edge". Using CYA would cut the effective FC down considerably.

The question still remains as to why some outdoor pools show sign of metal corrosion when they are using CYA. Are the FC levels really high relative to CYA levels and people don't know it? That is the assertion in the study I posted earlier. And metal corrosion is one thing, but corrosion of stone is quite another. Though corrosion of metal may have a non-linear effect where some critical amount of oxidation must occur at a rate faster than the stainless steel can "heal" itself through creation of a passivating film, the corrosive effect on stone is a different process that is more mechanical. So even lower amounts of salt, repeatedly splashed, could build up and through evaporation/wetting cycles this could be more of a linear effect. So it might take longer for lower salt levels to corrode stone, but it will still corrode (so even non-salt pools might corrode unsealed stone, but could take 3-10 times longer depending on salt level).

I think I'm going to see if I can find a true corrosion expert (or more than one) at some universities and see if I can't get this sorted out. The experiences of different people are different -- some see more corrosion in salt pools while others see equivalent corrosion in both -- and the high chlorine level hasn't been seen in all these cases.

Richard

chem geek
01-12-2007, 01:16 PM
I found the following graph or similar variations of it in several documents talking about corrosion.
http://richardfalk.home.comcast.net/pool/CorrosionSalinity.gif
Though it is hard to read, first note that this is about corrosion of iron (raw steel) and NOT stainless steel. Also, the scale on the X-axis says "Sodium Chloride, g/l" but that is wrong as the scale is actually Sodium Chloride g/100ml (or possibly %). The saturation level of Sodium Chloride is about 36g/100ml (which is 26.5% by weight or 265,000 ppm). The interesting thing to note is that the relative corrosion rate of iron increases to a peak around 1g/100ml (9,900 ppm) and then declines so that sea water at 3.5% (35,000 ppm) is actually less corrosive even though it has more salt. This is because the amount of dissolved oxygen in high salinity waters starts to decrease and dissolved oxygen is a larger factor in corrosion than salinity.

In fact, for iron, it would be inaccurate to say that sea water is much more corrosive than a salt pool just because it has over 10 times the salinity. The rate of corrosion for iron would actually be about the same. But for iron, almost any amount of salt is corrosive and iron still corrodes in even near-salt-free (though still wet with dissolved oxygen) environments, so you can only take this so far, but it is still interesting. [EDIT] By the way, cast iron corrosion isn't always fatal as some large commercial pool pumps use cast iron and though they do show corrosion, the rust (iron oxide) does seem to partially protect the pump from disintegrating -- it probably still corrodes, but is relatively slow given the thickness of the cast iron. Also, the relative corrosion rate only increases by 50% (factor of 1.5) going from a salt level of around 100 ppm to the peak at around 1g/100ml (9900 ppm) so this does not appear to be a huge difference, at least for iron. [END-EDIT]

The corrosion of stainless steel operates with a different mechanism since stainless steel normally has a passivity layer that protects it. This layer is thought to be (a possibly hydrated) very thin layer of chromium oxide (which is why stainless steel has chromium in it) which forms from the combination of the chromium in the stainless steel with oxygen. So there appears to be two competing reactions that occur -- oxidizers (such as hypochlorous acid) break down the passivity layer while oxygen combines with the chromium to form a new layer, but is inhibited in doing so by the presence of chloride ions (which apparently compete to form chromium chlorides). With two competing mechanisms, corrosion would appear to have something of a "threshold" where these rates were equal. Nevertheless, it would be inaccurate to say that corrosion does not occur below the threshold -- it still does, but at a slower rate (and would be less visible due to the "healing" of the stainless steel forming a new passivity layer).

So the study referred to in this post (http://www.poolforum.com/pf2/showpost.php?p=41537&postcount=12) (above) suggested that corrosion from salinity had a threshold around 6000 ppm. There was no indication (with these electrolytic cell tests) as to the chlorine levels nor to how long the test lasted nor what materials were being tested for corrosion. It may be that the threshold is somewhere near 6000 ppm, but it could also be that at 3000 ppm the stainless steel corrodes or at least gets thinner over 2-3 years compared to a pool with 500 ppm salt that may have the stainless steel last 10-20 years.

Corrosion of grout, stone, cement and other coping and hardscape surfaces is a totally different matter. This appears to occur from repeated wetting and evaporation to concentrate salt. Some processes appear to dissolve or pit the stone or to crack it while others are more mechanical with forces having to do with crystallization. It seems that these processes may be more linear and not have the "threshold" effect as shown by stainless steel (or other metals with passivity layers). As has been reported by some users seeing corrosion of limestone in non-salt pools, the quality of the stone and its maintenance (sealing, rinsing) is a big factor in its ability to resist corrosion. So even non-SWG pools that have a minimum of 300 ppm salt and usually more may still be corrosive to grout/stone/cement, but that with one-fifth to one-tenth the amount of salt, it may take 5-10 times longer for the equivalent corrosion to occur. I don't have definitive data about this so this is clearly speculative.

So for me, at least, I am not convinced that salt pools are not more corrosive than non-salt pools to the degree that perhaps recommendations should be made regarding higher quality materials to be used (Type 316L stainless steel; cupro-nickel heat exchangers; sealed stone or concrete that is more resistant to salt). Also, for all pools, the chlorine levels need to be watched and even indoor pools may need to have some CYA (10-20 ppm). That is not to say that there aren't pools with SWG systems out there (using CYA) that show no signs of (common Type 304) stainless steel corrosion in the first year or two -- it's the longer term (when using less salt-resistant materials) that I'm still not sure about. The jury is still out, I need more data, this does not compute, does not compute, compute...

Richard

[EDIT]
P.S.

I communicated with a pool installer from Australia who indicated that they see very little corrosion in their pools (SWG or non-SWG), but nearly all of their installations use the marine-grade 316 stainless steel, the paving is limestone or clay that is fired and that the supplier warrants for use in salt pools or it is coated to make it salt/sulfate resistant. They normally install heat pumps with titanium coils, rather than gas heaters (and such copper and cupro-nickel heaters do show corrosion). They also typically see higher TDS due to higher salt and sulfates in their tap water so even non-SWG pools have high salt levels, especially after seasons of chlorinating liquid usage.
[END-EDIT]

CarlD
01-12-2007, 01:34 PM
While Richard knows more chemistry than I do there is at least one gaping hole in this argument, which is "stainless steel". What is "stainless steel"?

While I cannot answer that too expertly I can tell you this:
Stainless Steel is not a single alloy, but rather a whole CLASS of alloys all of which have different properties. S/S can be customized for the application needed.

For example, many knife blades are made of stainless steel. Much work has been done to create an sub-class of alloys that is nearly as fine-edged as carbon steel, but it still stainless. These tend to be harder for their edge-holding abilities, and their strength when abused. However, they are fairly easily damaged by chemicals, and do NOT do well when left wet--if dried they won't rust, but wet they can, easily. I have several that have water or chemical damage. Some ARE tough enough for both a good edge and chemical impervience. They are expensive--surgical steel in scalpels is a good example.

Other stainless steels are softer, better looking, and stand up to the elements better than knife grade--but they aren't for knive blades. These S/S will generally be impervious but if totally immersed will, over time, rust.

Then there are the chemical-grade S/S. I don't know a lot about them but I do know they are designed to remained immersed without rusting or even pitting. Some are even used for stents in the body.

So when stainless steel fittings are going to be used for a pool, you'll need to know what kind of S/S it is, and whether or not it's rated for total immersion. If it's not, it WILL rust--and too quickly to!

chem geek
01-12-2007, 02:06 PM
Of course you are right, Carl, there were over 180 different types of alloys in the stainless steel group in 2001 and some are described in this EPA PDF file (http://www.epa.gov/tri/lawsandregs/alloys/alloysreport.pdf). You have to use the right stainless steel for the application. My point was only that pools with more salt in them may require stronger materials and that this is something that is not currently being said by the SWG folks. Instead, the claim appears to be that there is insignificant corrosion with the implication that the same materials can be used and one can expect the same lifetime of such materials. This just doesn't seem to fit all the facts and I want to know the real truth.

So this thread will hopefully sort all of this out with some hard data, from studies, good analysis, and from users experiences. That's how we got to those other gems of insight such as the chlorine/CYA relationship, first discovered through experience by Ben and then more finely honed through analysis of the chemistry by me, the explanation (after chlorine usage) of chlorinating liquid and bleach being fairly pH neutral while Dichlor lowered pH, and the techniques of lowering TA to reduce the rise in pH from carbon dioxide outgassing and (from Evan) the use of Borates to further reduce the pH rise in SWG pools, plus all the other important information about metal stains, test kit intereferences, sheet method for dilution, and the like from all the great contributors to this forum including yourself.

Richard

nater
01-12-2007, 02:08 PM
Good point Carl.

Richard, here's an interesting article on CSCC (Chloride Stress Corrosion Cracking) that led to the failure of a stainless steel roofing structure over a community pool due to corrosion:
http://www.imoa.info/FileLib/swimming_pools.pdf

Here's a good link for general info on the different types of stainless:
http://en.wikipedia.org/wiki/Stainless_steal

It's hard to find Stainless Steel types listed on vendor sites for pool ladders, but most are advertised as 304 with a mirror polish.

chem geek
01-12-2007, 03:03 PM
nater,

Thanks for the info. I did check the Wiki link a while ago, but I always try to find independent information since Wiki can sometimes be wrong (as it is modified by anyone), though usually it isn't wrong for too long. At any rate, you probably were still writing your post when I responded to Carl and gave this EPA PDF (http://www.epa.gov/tri/lawsandregs/alloys/alloysreport.pdf) link which, out of the many many sources I've looked at, seems to distill the essence of stainless steel corrosion. It's focus is more on steel in dirt or atmospheric exposure near the sea (at least for some of its studies), but it also contains a wealth of information on the types of stainless steel and their relative corrosion resistance. Specifically, refer to the following sections:

II. Definitions of Alloys and Corrosion - talks about metal corrosion generally, not specific to stainless steel.

V. Seawater of Marine Environments - though this has higher salinity and other chemical and organic components compared to pool water, it still talks about various factors affecting corrosion rates.

VI. Types of Stainless Steels - the most useful section for understanding corrosion resistance of different types of stainless steel.

VII. General Corrosion of Stainless Steels - charts of specific corrosion measurements in multiple studies. This validates the general Class groupings described in section VI. It is this section that contains the following interesting paragraph:

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

I am not so concerned with stress corrosion cracking since that doesn't seem to be as applicable to the pool environment. It would be critical for a mountain climber, however (and see this link (http://www.corrosion-doctors.org/Forms/swimming.htm) similar to the one nater gave above)! Note that the statement I put in bold above talks about 0.3% chloride which is 3000 ppm if the % chloride is measured as % salt (sodium chloride), but more likely this is literally % chloride which would be 3000 ppm chloride which is about 5000 ppm salt. Either way, it says that the chloride level close to what is found in salt pools can produce severe corrosion when sulfates are present (perhaps the 5000 ppm salt level is close to the 6000 ppm level reported in the SWG study, but I would be surprised if corrosion were truly "insignificant" in a 3000 ppm salt pool over more than one year, especially if there are sulfates in the water or if CYA is not used so that the chlorine level is too high). Unfortunately, it doesn't say what level of sulfates start to cause this problem, but be aware that dry acid (sodium bisulfate) and non-chlorine shock (potassium monopersulfate) both introduce sulfates into a pool so should probably be avoided in salt pools. It also means that fill water high in sulfates may make corrosion worse in salt pools.

IX. Copper and Copper Alloys - useful for understanding what might be found in a heat exchanger (in a gas-fired heater, for example).

XI. Specific Properties of Cast Copper Alloys - mentions how Copper combined with Nickel improves strength and corrosion resistance.

Richard

P.S.

I also found this study (http://cat.inist.fr/?aModele=afficheN&cpsidt=13525544) on the corrosion of Portland cement by salt (though at much higher levels of 5% which is 50,000 ppm -- higher than the sea, but with regular wetting and evaporation, could be achieved) and this study (http://journalsip.astm.org/JOURNALS/CEMENT/PAGES/1.htm) on Portland cement and blended concretes (at sea salt levels, probably around 35,000 ppm) in the presence of sulfate and though the sulfate did not make the initiation of corrosion start any faster, it did make the progression of corrosion (once initiated) faster. Also, magnesium sulfate was worse than sodium sulfate. Fortunately, dry acid has sodium while non-chlorine shock has potassium (which is chemically more similar to sodium than magnesium). However, fill water "hardness" typically has magnesium at about one-third to one-fourth the amount of calcium on a molar basis, but the bottom line is that the pool is mostly sodium and calcium, not magneisum (for posistive charged ions, aka cations). If the salt levels in these studies were closer to salt pool levels, then I'd probably fork over the money to get the full study to find out the sulfate levels, but it's not worth it when the salt level is so much higher. This link (http://www.concretethinker.com/Papers.aspx?DocId=30) gives a decent overview of corrosion issues with concrete. I'm sure there's lots more, but what I am looking for is a valid scientific study that relates corrosion rates for specific materials to chloride and sulfate levels (if there's a study with actual pool water, that would be even better, of course, since calcium carbonate saturation *may* reduce corrosion rates for certain materials). With that kind of information, we can set some guidelines for the kinds of materials to be used, estimate their expected life, and make recommendations with regard to chlorine level (including CYA) and identify other risk factors (e.g. sulfates) and their impact. It would be nice if manufacturers would take up the slack in this area, but as we have seen from the "lack of full information" on the chlorine/CYA relationship (independent of salt pools), this is something we may have to do ourselves first.

Though Taylor does not appear to offer a sulfate test kit (they have a sulfite test, but that's not the same thing), there do appear to be test kits from Hach (http://www.hach.com/hc/search.product.details.invoker/PackagingCode=225100/NewLinkLabel=Sulfate+Test+Kit,+Model+SF-1,+5-200+mg&frasl%3BL,+100+Tests,+Extinction+Method), Hanna Instruments (http://www.hannainst.com/downloads/instr/ISTR38000.pdf), and LaMotte (http://www.lamotte.com/pages/domwat/testkits.html) and probably others as well. Any data gathering that is done on pools to try and figure out causes of corrosion should probably test for sulfates in addition to all of the other standard water chemistry parameters (pH, TA, FC, CC, CYA, Salt, Borates, Temp).

[EDIT]
P.P.S.

I want to remind everyone that most people (and servicers/installers) on this forum are NOT reporting corrosion problems with salt (SWG) pools. I do not want people scared off of SWG just because I'm trying to investigate what is going on with a few reports of corrosion and some servicers/installers who believe they see more. I am trying to be as unbiased as possible and just want facts that can be disseminated as information so that people can make intelligent choices. That's all.

waste, Ben, and others who have experience servicing multiple pools (some with SWG, some without), please, please give us your feedback.
[END-EDIT]

waste
01-17-2007, 01:59 PM
Hi all! Nater and Richard have requested my 'take' on this, here it comes:

Davenj (post #2) is wondering about the 'white sludge' in his anchor cups - DavidD and I talked about it in 6237 (#5+)

I have also been in 2 threads discussing SS rusting (5114 / 3310)

The 'bolded' section in the last post may well have something to do with PatL34's admonition against using dry acid (sodium bisulfate) in SWCG pools.

Aside from this, I don't really have much to say, if there's been any degridation of deck or equipment - I haven't noticed it, but we've only been using these units for a few years (the co. I worked for in Va used the Lectronator and did mostly shotcrete pools w/ precast coping- but that was 12 years ago and I wasn't looking for premature failing due to salt and my memory isn't so great that I can remember if any of those pools had coping or deck problems at the ladders or stairs)

The only other thing I've noticed with salt pools is the accumulation of salt 'crust' at the exit areas, and rust on NON STAINLESS bolts on deck chairs, etc where people sit with salinated dripping wet bathing suits.

One thing is for sure; next season I'm going to take a very close look at the deck areas which are routinely exposed to the water from a SWCG pool! (ok, so this is the 'long term approach', but good studies are done over long periods - BTW, who's funding this study :D )


{I've been 'subscribed' to this since it went to page 2 and will follow it through - it's an excellent discussion on a possible pitfall to having a salt pool} - Waste:)

chem geek
01-17-2007, 02:50 PM
Thanks waste. What I think is important is to look out for corrosion in both salt and non-salt pools and to take careful measurements of water chemistry parameters when such corrosion is found. We don't want to bias ourselves by only "looking" for corrosion in salt pools. I knew about your response to one of the two corrosion links I had, but didn't know about the other. Thanks for those references -- every piece of information helps to put the puzzle together, including the reference to avoiding sulfates (dry acid) in some situations.

I reread this statement from the EPA document that I quoted earlier, and this time am including a different section in bold.

Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.

The term "oxidizing chlorides" may not be a specific issue with having only compounds of chloride. It may also occur with the combination of a strong oxidizer in the presence of chloride, but this is just my speculation. Interestingly, the "ferric" form of iron is indeed an oxidizer with a rather high (standard) reduction potential of +0.771V while the "cupric" form of copper is a much weaker oxidizer with a low reduction potential of +0.3419V. By comparison, oxygen has a very high reduction potential of +1.229V while hypochlorous acid has an even higher reduction potential of +1.482V. One can only compare these potentials from a molar equivalent basis and in actual pool water the actual potentials (based on actual concentrations or activities) are completely dominated by hypochlorous acid (hypochlorite ion is tied to this as well, with a lower +0.81V molar reduction potential) followed by dissolved oxygen. In other words, there may be a very strong effect between chlorides and both sulfates and oxidizers (e.g. chlorine) in terms of severe corrosion (pitting) of stainless steel. The reference to "halide salts" would include sodium chloride (and other metals with chloride, including iron and copper already mentioned), but this paragraph from the EPA is wholly qualitative and not quantitative and therefore unsatisfactory. It can guide us for what questions to ask, but does not give us specific answers.

Richard

waste
01-17-2007, 03:27 PM
Richard, (as often happens when I try to keep my 'word count' down), I misrepresented what I was trying to say (go figure;) ). Though I specificly said I'd look at the SWCG pool areas, it was my own 'shorthand' for 'looking' at them compared to pools of the same age and ~ usage which diddn't have the SWCGs. This also raises the issue of my not being 'blind' (ie 'double-blind experiment') - undoubtably, I'll report some more or less corrosion to the pools in question. However, since I work in a relatively small area, many other factors will be removed from my observations. (in my 'miss-spent' youth, I studied psychology, and as it's a 'soft science', they made sure to 'pound-in' the necessity of using the 'hard science' methodology) :D

Waterworks
01-23-2007, 02:02 PM
I just found another potential problem, or at least a very good source for testing. A residential customer of ours with an indoor salt pool called and said his ladders and light were corroding, so I went to check it out with my service man. The ladders were so corroded that they were falling apart. His pool is full-tile and his original white grout lines are now black, his original blue tiles are now greenish. The light ring is black also. We went and checked his cartridge filters and they were black, caked with rust. The pump basket was also black. The ladder rails that are above water are rusting, as you'll see in the pictures, but nowhere near as bad as the underwater parts.
Outside the pool there is a slide, metal deck drains, metal exhaust vents, tile decking, tile safety grip coping and a flagstone deck section with grout. The slide posts showed no corrosion, but I think they are aluminum. One of the metal deck drains is showing some rust, one is just fine. the metal exhaust vents show no corrosion. The flagstone and tile deck sections show no corrosion.

We installed the SWG in May 2005. The pool was installed around 2000. Before the SWG came along there was no corrosion. We went on a service call in March of last year to clear up the pool, as it was cloudy. At that point, the generator had been turned all the way down. We had not had any complaint of corrosion, and did not actively look for it at this point. My service man turned it up to 30% and told the customer to keep checking the chlorine levels and to keep them high until the pool was clear again, then turn the SWG down. The customer has not brought in a water sample since, and while I was speaking with him today he did not know where his test kit was. When I looked at the SWG today, it was still set on 30%. The pool room had absolutely no chlorine smell and was not hard on my eyes. His kids use the pool almost every day and have never complained about anything.

Here are the results of my water test:
Chlorine - I used my FAS-DPD test kit. I filled the sample to 10ml, then added two scoops of powder. As the powder hit the water it turned bright pink, but then went back to clear. As I shook the sample, it would periodically turn to pink, but then back to clear. I'd seen this before so I added my R0871 and after 25 drops the sample had turned pink and stayed that way. I kept adding drops until I had done 110. Then I mixed 1 part pool water with 3 parts distilled water and it still took 100 drops in a 10 mL test, meaning a chlorine reading of somewhere between 55-100 ppm. I did an OTO test and the sample turned bright red.
pH - phenol red test was off the charts so I used my Hannah meter and got 8.4
TDS (Our city water is about 650ppm) - 2850
Salt - 3200 on the Aquarite machine and 3000 on my Taylor Kit
Alkalinity - 80
Calcium - 140
CYA - 0

It seems to me that this info coincides with what I already thought. Most SWG's create WAY too much chlorine for indoor pools. This excess chlorine causes corrosion. Since this corrosion ties in with the installation of the SWG, the SWG takes the blame.

I brought one of his ladders back to my shop so I could try to do some kind of test on it. I was thinking of setting up 2 buckets, one with high chlorine and one with high salt and setting each leg of the ladder (because they are evenly corroded at this point) in it's own bucket. I would take pictures before and after and compare the effects. I would also test the water every day or two and keep it balanced. I wish I had a way to test a third part so I could do high chlorine with CYA. Does anyone think this test would be worthwhile, and does anyone have any tips to make the test better?

If anyone can tell me how to post a picture, I will post a couple pics of the ladder.

Brad

chem geek
01-23-2007, 02:21 PM
I brought one of his ladders back to my shop so I could try to do some kind of test on it. I was thinking of setting up 2 buckets, one with high chlorine and one with high salt and setting each leg of the ladder (because they are evenly corroded at this point) in it's own bucket. I would take pictures before and after and compare the effects. I would also test the water every day or two and keep it balanced. I wish I had a way to test a third part so I could do high chlorine with CYA. Does anyone think this test would be worthwhile, and does anyone have any tips to make the test better?

If anyone can tell me how to post a picture, I will post a couple pics of the ladder.

Brad, I think that this forum is now limiting uploading of pictures, but if you send them to me (via E-mail -- click on my name on any post and there will be a link to E-mail to me) then I can host them on the website I use for the other pool stuff I have and then your post can link to that.

Sorry to hear about this customer, but man, that much chlorine with no CYA is horribly corrosive. As for what test to do vis-a-vis salt, part of what you want to compare is moderate chlorine levels (say, 5 ppm FC with no CYA) with salt at 3000 ppm vs. the same chlorine levels with salt at around 300 ppm. We already know and understand the effects of CYA on chlorine so that isn't as useful. What we really want to know is whether the salt in the presence of chlorine accelerates the corrosion so you want the same chlorine level in both buckets with the only difference between the two being the salt level. To make this a reasonably fair test, go ahead and add baking soda to raise the TA and maybe even some calcium chloride for CH. That would make it very much like pool water (so try and keep the pH around 7.5 as well). Whenever you test or add chemicals, be sure and do the same sort of mixing (circulation) in the two buckets as that also affects corrosion (if there is no circulation, then the buildup of "rust" products probably slows down additional corrosion).

This story really emphasizes the need for customer education about SWG and especially about testing your water regularly. I can hear Carl's words now...

Stay ahead of your water!
Take 2 to 5 minutes every day for pool maintenance!

Richard

chem geek
01-24-2007, 07:02 PM
I thought of another possible reason why SWG manufacturers recommend high levels of CYA (typically 70-80 with 80 "best"). In this thread (http://www.poolforum.com/pf2/showthread.php?t=4495) (starting at around post #30) I said that the reason for high CYA in SWG pools is that this is more efficient at producing chlorine. Though this appears to be true (based on pool owner's experiences with runtime), I didn't say what happens when there is a buildup of chlorine at the plate. In addition to production of chlorine, there is also a competing reaction of producing oxygen. If the reaction producing chlorine slows down, then the reaction rate of producing oxygen can speed up in its place, though the production of oxygen is normally less favored than that of chlorine (technically, the production of oxygen is more likely to occur in an equilibrium sense, but it has a much higher activation energy than the production of chlorine so results in an overvoltage such that chlorine is more favored than oxygen).

It is interesting to note that different SWG systems use a different voltage to drive their cells. Some operate in the range of 6-9 volts, but others are in the range of 22-30 volts. Though the voltage level required to only produce chlorine and not oxygen is quite low (perhaps between 1.16V and 1.45V at normal pool concentrations of pH and chlorine), higher voltages will probably produce both chlorine and oxygen with chlorine being produced more. I do not know if higher voltages change this relationship, but suspect that it might, with the oxygen proportion increasing somewhat at higher voltages (though still produced in lower quantity than chlorine). At any rate, having lower CYA levels may very well increase the oxygen proportion and this can be dangerous since the other plate is producing hydrogen. Though hydrogen by itself is not explosive and in water does essentially nothing, the combination of oxygen and hydrogen can be dangerous if this combination of gasses were to accumulate (if the pump shuts off but the SWG cell keeps running, for example).

The blog link at the first post in this thread has been reporting some explosions of SWG systems and I wonder if those pools were using lower CYA levels than recommended and produced more oxygen that could have been at explosive levels with hydrogen. It is also possible that the explosions weren't actual ignitions of hydrogen and oxygen, but high pressure bursts from the buildup of gas pressure. It should be understood that under normal operating conditions when the SWG only operates when the pump is running, that the risk of explosion is virtually nil. I'm just trying to sort out some unusual circumstances (where the salt cell didn't shut off when the water flow stopped) to better understand what is going on.

[EDIT] A further note: Even the most dangerous levels of hydrogen gas and oxygen gas in a 2:1 ratio will not spontaneously explode unless the pressures are very high (much higher than what pipes would handle, so bursting would occur before explosion) or the temperatures are very high (at least 400C) so generally some sort of spark or flame is required to trigger a literal thermal explosion. In other words, the risk of oxygen production and explosion is low; the risk of a pressure burst is high if the salt cell operates in a closed valve environment (which would only occur if some component failed to operate properly). [END-EDIT]

Richard

giroup01
01-29-2007, 08:05 AM
At any rate, having lower CYA levels may very well increase the oxygen proportion and this can be dangerous since the other plate is producing hydrogen.

Richard,

Can you expand a bit on the chemistry behind low CYA levels favoring O2 production ?

Thanks,

chem geek
01-29-2007, 01:00 PM
Can you expand a bit on the chemistry behind low CYA levels favoring O2 production ?
Sure.

First let's take a look at the only reaction that appears to be reasonably possible at the cathode (the negatively charged plate). The other reactions that can occur at this plate include the plating of metals (iron, copper) that are in the water. Also, the [EDIT] high (sorry about saying "low" at first -- I was "thinking" the right thing but "wrote" the wrong word) [END-EDIT] pH in the region of this plate make scaling of calcium carbonate much more likely.

2H+ + 2e- --> H2(g) ...................... Eo = 0V

In other words, this is the generation of hydrogen gas. It can also be written as coming from water to produce hydroxyl ion:

2H2O + 2e- --> H2(g) + 2OH- .......... Eo = -0.8277V

The different Eo readings reflect the fact that Eo is defined to be at standard conditions which means that each of the species dissolved in water have a concentration of 1 mole/liter and each gaseous component has a partial pressure of 1 atmosphere.

Now let's look at the two different competing reactions that appear to be reasonably possible at the anode (the positively charged plate):

2H2O --> O2(g) + 4H+ + 4e- ........... Eo = -1.229V

2Cl- --> Cl2(g) + 2e- ....................... Eo = -1.35827V
Cl2(g) + H2O --> HOCl + H+ + Cl-
----------------------------------------
Cl- + H2O --> HOCl + H+ + 2e- ......... Eo = -1.482V

The first reaction is where oxygen is produced from water. The other reactions are the production of chlorine where I show the additional reaction of chlorine dissolving in water to produce hypochlorous acid. You can see that the Eo value is more negative after chlorine gas dissolves in water to make hypochlorous acid and that would seem to imply that the reaction is not favored, but again this is only because the Eo is defined for standard conditions (which are theoretical and may not always be actually achievable) of 1 atmosphere pressure of chlorine gas with 1 mole/liter concentration of hypochlorous acid AND 1 mole/liter concentration of hydrogen ion (i.e. a pH of 0), AND 1 mole/liter concentration of chloride ion.

For the cathode, I listed the reduction potentials as oxidation potentials (i.e. with negative numbers) to indicate that a voltage needs to be applied to make the reaction occur. Note that it takes a lower (absolute) voltage to produce oxygen than chlorine. This represents a difference of about 25 kilojoule per mole in favor of oxygen formation over chlorine formation. HOWEVER, chemical reactions have what is known as an activation energy which is an energy barrier that must be overcome before the reaction can happen. In electrochemistry, this barrier is called overvoltage which represents the additional electrical potential needed to get an electrochemical reaction started. Oxygen has a very high overvoltage of 700 mV while chlorine has an overvoltage of only 50 mV.

So if we add the overvoltages to the standard potentials, we see that to produce oxygen it takes 1.229+0.7 = 1.929V while to produce chlorine it takes 1.35827+0.05 = 1.40827V. Or we can look at producing hypochlorous acid directly as 1.482+0.05 = 1.532V. Now these voltages do not account for the actual concentrations of the various species in pool water as I noted above.

If we want to really know about the true voltages required and which reactions are favored, we need to convert from standard conditions to actual concentrations. There is a formula for doing that called the Nernst equation as follows:

E = Eo - RT/nF * ln(K)

where "K" is the ratio of product concentrations (raised to powers corresponding to their stoichiometric quantity in the formula) to reactant concentrations. "R" is the gas constant, "T" is the temperature in Kelvin, "n" is the number of electrons in the equation and "F" is the Faraday constant. Sometimes this Nernst equation is written with a "+" and the "K" value is inverted to have reactants divided by products.

Anyway, if I assume reasonable initial conditions of a pH of 7.5 to get [H+] = 3.7x10^(-8), normal dissolved oxygen levels (based on 21% oxygen in the air and using Henry's Law) to get [O2] = 2.6x10^(-4) moles/liter and converting to 0.0065 atmospheres (partial pressure), using normal chlorine levels of 3 ppm with 30 ppm CYA to have [HOCl] = 5.8x10^(-7) moles/liter and chloride concentration of 3000 ppm to have [Cl-] = 0.052 then this adjusts the oxygen equation by (0.02585/4)*ln(0.0065*(3.7x10^(-8))^4) = -0.47V so E = -0.75V for oxygen. The chlorine equation is adjusted by (0.02585/2)*ln(5.8x10^(-7) * 3.7x10^(-8) / 0.052) = -0.37V so E = -1.11V for chlorine. I'm not certain on the handling of the oxygen quantity in terms of molarity vs. partial pressure, (I believe I did it correctly) but it turns out it doesn't matter much in this case so let's keep going.

Applying the overvoltages again gives is E = -1.45V for oxygen while we get E = -1.16V for chlorine. So at least initially, the production of chlorine is favored over the production of oxygen and if the voltage applied to the salt cell is set to be BETWEEN 1.16V and 1.45V, then only chlorine can be generated and oxygen cannot. [EDIT] I neglected to do the concentration adjustment calculation for the cathode for the production of hydrogen, but that would modify the overall voltage by the same amount regardless of whether oxygen or chlorine were produced at the anode so it's not that relevant to this discussion. [END-EDIT]

If higher voltages are used, then both chlorine and oxygen will be produced with chlorine favored over oxygen. If you have ever done the "home" experiment of taking two carbon cores from D-cell batteries and connecting them to a 6V transformer (as I have), then putting this into salt water produces both chlorine and oxygen (you can smell the chlorine and you can "pop" the gasses with a flame where the oxygen and hydrogen combine) and can somewhat vary the ratio of the two depending on the salt concentration. If you use sodium bicarbonate as the electrolyte, then you get only hydrogen and oxygen and not chlorine.

The salt cells appear to use voltages that are much higher than these minimum amounts that would ensure that only chlorine gets generated. I'm sure that this is partly due to the rather low chloride concentration in pool water (especially today, since early salt generators wanted 6000 ppm instead of 3000 ppm). Otherwise, you would need very large generating plates to get decent generation rates. So with voltages of 6-9 volts for some manufacturers and 22-30 volts for others, it would seem that both chlorine and oxygen could get produced. My guess is that the higher voltage units have a greater tendency to produce more oxygen proportionately than the lower voltage units, but that's just a guess.

Now what I do not know is whether the materials (coatings) used for the generating plates in a salt cell are designed to somehow inhibit the oxygen generation, but I suspect that is not the case. (If anyone knows about this, please let us know).

As the electrolysis proceeds, the product concentrations build up at the generating plates (though there is water flow to help keep them lower) and this slows down the generating reactions. Note that the reaction with oxygen produces hydrogen ion so is acidic and that the production of chlorine AND dissolving of that chlorine in water also produces hydrogen ion so is also acidic. However, the oxygen reaction produces one hydrogen for each electron while the chlorine reaction produces half a hydrogen for each electron. This means that as the pH gets lowered near the plate, the chlorine reaction will become more favored, assuming that the chlorine gas is able to dissolve in water rapidly. Having CYA in the water helps make this happen because it combines with the hypochlorous acid that is produced so that the reaction of chlorine dissolving in water can continue at a rapid pace. So the lower buildup of products in the chlorine reaction, due to CYA binding with the generated hypochlorous acid, means that this reaction becomes even more favored compared to the oxygen generation.

This can be netted out as follows:

2Cl- --> Cl2(g) + 2e-
Cl2(g) + H2O --> HOCl + H+ + Cl-
HOCl + CYA --> Cl-CYA + H2O
----------------------------------------
Cl- + CYA --> Cl-CYA + H+ + 2e-

Each succeeding reaction above moves the products into another form and that takes them away from being able to go "backwards" to create reactants again (or to inhibit or slow down the creation of more products). It is almost as if one turned up the water flow with respect to sweeping away chlorine gas, but that this is done chemically.

Richard

giroup01
01-29-2007, 03:09 PM
Sure.


:eek:

Thanks! ...I'll have to chew on that, but yes, thank you!

When you say:

Having CYA in the water helps make this happen because it combines with the hypochlorous acid that is produced so that the reaction of chlorine dissolving in water can continue at a rapid pace. So the lower buildup of products in the chlorine reaction, due to CYA binding with the generated hypochlorous acid, means that this reaction becomes even more favored compared to the oxygen generation.


Would this explain why SWG manufacturers seem to specify a somewhat higher than normal CYA? To increase the ratio of chlorine to oxygen?

Gunslinger
01-29-2007, 03:42 PM
Richard, your explanations of SWG chemistry are truly awesome, but I don't believe for NY second that the reason for the manufacturers' 70-80ppm CYA recommendation (I've actually seen 80-100ppm recommended) has anything to do with real-world chemistry. Rather, I suspect it has more to do with a slightly different interpretation of the acronym "CYA", rooted in the early days of the SWG evolution when reliable operation was not up to par with current devices. In a loss-of-power event, for example, high CYA levels would help to ensure that all your residual FC doesn't disappear on a sunny day before you've noticed something is amiss. I believe there is no longer any rationale justification for this recommendation other than my suspicion that the manufacturers have no interest in pursuing the matter and are content to leave it unchanged.
Personally, I barely manage to keep my CYA level above 25ppm, and have experienced no consequences from that during the four seasons I have operated my Pool Pilot SWG -- although I do test my FC and Ph quite often.

chem geek
01-29-2007, 05:18 PM
Gunslinger,

You may be right, but users of this forum, including Evan (waterbear) noticed that the output efficiency of the salt cell fell dramatically at lower CYA levels. That is, you needed to increase the time and/or power of the salt cell to achieve the same FC level when the CYA was below the 70-80 recommended range and this effect was MUCH larger than the sunlight protection effect of the additional CYA (which is quite minimal at higher CYA -- see this graph (http://richardfalk.home.comcast.net/pool/HalfLife.gif)) . Now this specific CYA level probably varies by manufacturer, but this effect is absolutely real as it was reported by multiple users. Maybe your salt cell is one of the lucky ones that operates well at lower CYA, or maybe you never got to the 80 ppm CYA where the efficiency dramatically improves for some salt cells.

So at least some SWG manufacturers may be recommending higher CYA in order to have higher efficiency. The issue then becomes whether efficiency of chlorine generation is the only factor or whether the proportion of oxygen generation is also affected. I don't know the answer to that and was just speculating based on chemistry -- we don't have real-world data on SWG output (i.e. measuring the gasses) to know for certain.

giroup01,

I believe the high CYA recommendation is primarily for SWG cell efficiency as mentioned above. I do not know if the oxygen effect is real (there might be coatings on the plates to inhibit oxygen generation, if that's even possible) or if the manufacturers know about it if it is or if they recommend higher CYA partly for that reason. Basically, I don't know a lot. But if the chlorine generation rate goes down at lower CYA but the conductivity stays the same and there is the alternate reaction of generating oxygen available, then it seems reasonable to assume that more oxygen is generated at lower CYA. The extreme example of this occurs if you remove all of the chloride and just have a different electrolyte (sulfates, carbonates, etc.) in which case you get pretty much ONLY oxygen and hydrogen. I had fun as a kid making such concoctions and carefully exploding them (this is definitely a "don't try this at home" scenario where you must not use glassware and should wear glasses).

Richard

Waterworks
01-30-2007, 12:06 AM
I was just speaking with my customer that has the indoor Stainless Steel pool. (This is not the same pool that I spoke of last week with the massively rusted ladders and +100 ppm chlorine.) Once his pool was up and running with the SWG he quickly noticed corrosion issues with the walls and floor of the pool. He is now using pucks, and I had him add a little bit of extra CYA. The build up of CYA over time won't be an issue because he drains the pool periodically. He has not noticed any corrosion since getting rid of the SWG and switching to pucks.

I told him that I thought that high chlorine was the cause of the corrosion and that I thought that by adding some CYA and more closely monitoring chlorine levels we could avoid the corrosion while allowing him to use his SWG again. The pool is completely 316L grade Stainless and is grounded properly. When the initial corrosion happened he brought a metallurgist out to look at the problems. The metallurgist told him that 316L stainless could not handle sodium chloride in concentrations above 500 ppm. This is vastly different from everything I've read about 316L steel.

Let me know what you think.

Brad

chem geek
01-30-2007, 01:06 AM
Brad,

I can only refer you to this same EPA document (http://www.epa.gov/tri/lawsandregs/alloys/alloysreport.pdf) that I referenced before where it says that the 316 stainless steel is more resistant to chlorine pitting than 304. However, it does show in at least some studies that 316 is superior to 316L, but I would find it surprising that salt levels above 500 ppm are simply corrosive to 316L. The chlorine appears to be far more corrosive to stainless steel than the salt level, though the salt level may accelerate such corrosion. So my take would be that using CYA would cut down the corrosion from chlorine to the point where the higher salt levels of an SWG pool wouldn't be a problem with 316 type steel.

However, this is all just speculation on my part based on the fact that we haven't had any reports of stainless steel corrosion in SWG pools that have been using CYA (and haven't had horribly high chlorine levels). We did have the one report of the indoor pool with 3-5 ppm FC, but that had no CYA.

So if your customer is up to it, they can bring up the CYA level in their pool to at least the 10-20 ppm range (if not 30 ppm) and then try the SWG (with the 3000 ppm salt). My prediction is that they won't see corrosion. If they had 304 stainless steel, then they *might* see corrosion eventually, but even then I'd bet it would take a few years.

Now having CYA in an indoor pool is going to present a different set of problems since indoor pools tend to get combined chlorines that do not break down as readily as outdoor pools (due to the lack of sunlight) so you might need to periodically use a non-chlorine shock (potassium monopersulfate) for the indoor pool. First try monitoring (or have the customer monitor) the combined chlorine level -- if it's not a problem, then you're done. If not, then use the non-chlorine shock.

By the way, you mentioned that the customer saw corrosion issues with the walls and floor of the pool. What does that mean? Isn't the pool plaster/gunite? Is he now talking about corrosion as in roughness of the plaster finish? Or is this pool made out of stainless steel?! Or do the walls have some stainless steel bars and by "the floor" he means metal floor drains? I'm confused.

That's my two cents (again, for what it's worth),
Richard

Waterworks
01-30-2007, 09:21 AM
The pool is a one-piece shell made entirely of 316L stainless steel and was set in place by a crane; like a one-piece fiberglass pool, but made of steel. Don't ask how much it cost because I have no idea. I won't be able to convince him to try the SWG on that pool again because if I'm wrong it's an awfully expensive mistake. After the first round of corrosion the metalurgist applied some sort of clear corrosion proof sealant and somehow got rid of the original corrosion. I'm guessing that this was expensive as well.

Brad

chem geek
01-30-2007, 02:19 PM
Brad,

Yikes! I had no idea such a thing as an all stainless steel pool existed. You are absolutely right that you don't want to take any chances with that kind of pool. Not using salt and adding CYA would be wise, just to play it absolutely safe. Since the customer is using Trichlor tabs (pucks), you should remind him that it is important for the feeder (if it's a floating feeder) not to park itself near the side of the pool with the circulation off. Otherwise, the acidity from the Trichlor will corrode the stainless steel in the vicinity. This is what happened to me in my own pool with stainless steel bars along the side of the pool -- the bars didn't corrode, but the mountings closest to the floating feeder did.

Richard

Waterworks
01-30-2007, 05:21 PM
I don't think that Stainless Steel pools actually do 'exist' per se. As far as I know one of his companies builds SS tanks for agriculture or something and he had them build him this one as a one-of-a-kind thing.
He uses an off-line chlorinator, so the floating thing won't be a problem.

Brad

chem geek
01-30-2007, 06:14 PM
Brad,

OK, so with an off-line chlorinator he won't have the problem of the floating dispenser, but given this guy's history of not monitoring his water chemistry (which may have gotten the chlorine to a high level) he could now be in the position of getting his pH very low in his pool if he uses Trichlor exclusively and doesn't check the pH once and a while and add pH Up (Sodium Carbonate or Washing Soda). If the pH gets too low, then he can end up corroding his stainless steel again. Given his history, it would be a good idea to keep the TA on the high side, at least 120 ppm, as long as he's using Trichlor. Also keep in mind that with the CYA from the Trichlor he will have to maintain higher Free Chlorine levels as the CYA level rises in order to have proper disinfection and to prevent algae. See Ben's Best Guess CYA chart (http://www.poolforum.com/pf2/showthread.php?t=365) as a rough guide for how much FC is needed at various levels of CYA.

By the way, if you can get contact info for the metallurgist, then you can give that to me (via E-mail or Private Message) and I'll contact him to see where he got his 500 ppm salt limit for this type of stainless steel.

Thanks,
Richard

Poolsean
01-30-2007, 06:19 PM
There's a company in North Carolina, Bradford Industries, that makes these Stainless Steel pools and spas and cost into the Tens of thousands of dollars! Most of the ones they display and are on their brochures show most of the pool/spa being tiled. However they do have some very large custom built ALL Stainless Steel commercial projects.

Very unusual looking with all straight sides. We have one of these all stainless steel spas in Boca Raton Florida at what was formerly the Mission Bay Aquatic Complex. This is a non-salt chlorinated spa. They use sodium hypochlorite from a Stenner Chemical Feed Pump. Most of the corrosion appear at the welds.

Just the fact that this spa was already showing signs of corrosion was reason enough for me to not propose a salt system on it. I'm sure salt would have been blamed for any more damage.

Waterworks
01-31-2007, 01:02 AM
This guy also has four pure copper spas from Diamond Spas at the same residence. AFAIK these spas are similar to Bradford spas' units. There is some tile and some copper exposed. He uses inline chlorinators for these units and has not complained at all about corrosion.

Brad

chem geek
02-02-2007, 02:50 AM
That's interesting about the copper not showing signs of corrosion while the stainless steel did. I wonder if the chlorine levels are better maintained in his spa or if copper is more resistent to any potentially salt-amplified chlorine corrosion. Copper is more resistant than steel so perhaps even though chloride ion might interfere with any protective oxide layer that gets formed, that once removed the underlying steel in stainless steel corrodes faster than the underlying copper under any thin copper oxide protective layer (if any). This is all speculation on my part.

On a separate topic, I ran into this article (http://www.tricitypoolservice.com/tc-plaster_study.html) about the damage to plaster from high levels of CYA. What is interesting is that the drop in CYA over time seems to indicate that even levels of CYA not much lower than 100 ppm can be detrimental to plaster. If you look at the line that starts with 200 ppm, it drops rather quickly down to around 130 ppm and then continues to drop more slowly to around 80-90 ppm. This may indicate that there is some plaster deterioration effect even at 80 ppm though it is clearly much slower. This would be another reason for SWG manufacturers to take a look at seeing if they can't figure out a way to operate more efficiently at lower CYA levels.

Richard

Poolsean
02-02-2007, 12:34 PM
Richard,

It's not a matter of a salt system working efficiently at lower cya levels. Cya levels have nothing to do with efficiency. It has to do with retaining the chlorine in the water. Try the same with a bleach pool and low cya. The chlorine is quickly consumed by UV. Same thing with the chlorine produced by a salt system. UV.
We know, from trial and error more than anything, that the 60 -80 ppm range works. Since ORP controllers are affected by cya, 30 - 50 ppm is recommended.
But cya at lower than 25 ppm? You will drastically see an increase consumption of chlorine, salt generator or bleach, it doesn't matter the source.

chem geek
02-04-2007, 08:04 PM
Sean,

If the SWG manufacturer recommendation for CYA is solely based on retaining chlorine in the water (which is independent of whether you use an SWG or use another source of chlorine), then why did Evan (waterbear) and others see such a difference in efficiency going from 60 to 80 ppm? Yes, more CYA has the chlorine last longer, but it has diminishing returns and the difference between 60 and 80 isn't that high yet he (and others) saw a large difference -- that is, he was able to turn down his SWG a large amount.

The half-life of chlorine is determined by the separate half-lives of unbound chlorine (hypochlorous acid and hypochlorite ion) with a half-life of around 35 minutes AND the half-life of bound chlorine to CYA (i.e. the chlorinated isocyanurates) with a half-life of around 8.4 hours (some sources say 6 hours). The net result is shown in this chart (http://richardfalk.home.comcast.net/pool/HalfLife.gif) which shows that the bulk of the benefit from CYA occurs at relatively low levels of CYA. This chart is similar to and consistent with the infamous Kent Williams "Cyanurics - Benefactor or Bomb?" article (http://www.ppoa.org/pdfs/PrP_Cyanurics%20-%20Benefactor%20or%20Bomb.pdf). The half-life of chlorine is about 6 hours at 30 ppm CYA while at 80 ppm this only increases to a little over 7 hours. That's not a huge difference. The reason for the small change is that the vast majority of chlorine, even at 30 ppm CYA, is stored in the form of chlorinated isocyanurates (that is, bound to CYA) so adding more CYA only cuts down the remaining free, unbound and disinfecting chlorine that is already at such a small level that it doesn't contribute much to the total loss. At 30 ppm CYA the disinfecting chlorine level is at 1/30th the level it would be if there was no CYA or put another way, almost 97% of the chlorine is bound to CYA and only 3% is unbound and getting cut in half every half-hour. At 80 ppm CYA, almost 99% is bound to CYA and only 1% is unbound. Though 1% is certainly much smaller than 3%, it is still a small number in an absolute sense -- it's what happens to the 97% or 99% that drives the total chlorine loss more than anything else.

It was interesting that the rate constants for CYA combining with hypochlorous acid turned out to be about the same amount of time that water flows through an SWG cell when the CYA level was somewhere in the neighborhood of 50-60 ppm or so. That's what had me think that this was why the CYA recommendation was higher along with what Evan (waterbear) and others were seeing in terms of efficiency. Also, it seemed quite strange that most non-SWG CYA recommendations are 30-50 ppm while only some of the SWG manufacturers were saying to have it at 70-80 ppm with "80 ppm being ideal". Don't you find that strange?

Richard

mas985
02-05-2007, 11:20 AM
Last year I did an experiment with my pool and SWG and got different results than what others have experienced. I started out at 30 ppm of CYA and gradually increased it to 60 ppm and saw no increase in production of chlorine at all. My residual chlorine started out at 1 ppm and ended up at 1 ppm with the same pump run time and SWG setting. I know this is at odds with what others have experienced but I was fairly careful to do this during the spring and space out the CYA addition since it can take up to a week to fully dissolve.

Since you can only measure the chlorine residual which is a combination of SWG production and CYA retention, you cannot be sure that the increase in chlorine residual that some people experience is due to higher production. It could be that the UV exposure is high enough that they are simply increasing the retention of chlorine and thereby increasing the residual.


One thing I have noticed is a huge difference in production with water temp. This fall when the water temp dropped to 60 degrees I noticed that my production almost doubled. This is probably a combination of water temp and UV exposure but this seemed to have a much larger impact than CYA level did.

Since I have very high CH fill water, I am planning a refill soon while the CH is at it's lowest. This gives me another opportunity to repeat the same experiment and see if I get different results this time.

waterbear
02-05-2007, 12:31 PM
Getting back to the original topic of this thread, which is what are the downsides to salt pools, I am curious if Australia, which has been using salt pools for about 30 years now, has modified their construction techniques or has had problems with corrosion, expecially in some of the older pools with SWG's. Also, I believe that many of the pools there use a much higher salt concentration than we do in the U.S.
I think that finding out this info would be most useful.

chem geek
02-05-2007, 01:29 PM
Since I have very high CH fill water, I am planning a refill soon while the CH is at it's lowest. This gives me another opportunity to repeat the same experiment and see if I get different results this time.
Mark,

I remember when you posted these results before and waterbear commented how the largest gain was seen approaching 80 ppm CYA so if you do your experiment be sure to get the CYA up to that high a level. That would be a good test, especially if your SWG manufacturer is one of the ones that recommends the 70-80 ppm CYA level. We can then put this issue to rest one way or the other (and I'll apologize to gunslinger who's NY skepticism may have been correct; and to PoolSean) -- it's an important issue since it would be easy for us to recommend lowering the CYA to the more "normal" levels closer to 30 ppm for most pools which would give more disinfecting chlorine at the 3 ppm FC level (probably enough to keep away mustard/yellow algae which one user on this forum couldn't keep away without 5-6 ppm FC in his high CYA SWG pool). I just didn't want to recommend that if it had a negative effect on efficiency.

Thanks,
Richard

chem geek
02-05-2007, 01:40 PM
Getting back to the original topic of this thread, which is what are the downsides to salt pools, I am curious if Australia, which has been using salt pools for about 30 years now, has modified their construction techniques or has had problems with corrosion, expecially in some of the older pools with SWG's. Also, I believe that many of the pools there use a much higher salt concentration than we do in the U.S.
I think that finding out this info would be most useful.
I agree with you. In the P.S. to this post (http://www.poolforum.com/pf2/showthread.php?p=41555#post41555) I talk about a pool installer in Australia with whom I've had E-mail correspondence. This is what I wrote in that post:

I communicated with a pool installer from Australia who indicated that they see very little corrosion in their pools (SWG or non-SWG), but nearly all of their installations use the marine-grade 316 stainless steel, the paving is limestone or clay that is fired and that the supplier warrants for use in salt pools or it is coated to make it salt/sulfate resistant. They normally install heat pumps with titanium coils, rather than gas heaters (and such copper and cupro-nickel heaters do show corrosion). They also typically see higher TDS due to higher salt and sulfates in their tap water so even non-SWG pools have high salt levels, especially after seasons of chlorinating liquid usage.

Now since this is just one guy I talked with, this isn't a representative sample and it's based on his experience and opinion. It does seem that they take more care in selection of materials, but it also seems that they did this even before SWG pools were introduced due to their already very high salt (I'm guessing this is partly hardness, not just sodium chloride) and sulfates water.

If anyone else out there knows pool installers in Australia, it would be helpful to get some more information to confirm, counter, or expand upon what was said above. I did find this link (http://www.awqc.com.au/reports/2005DWQR.pdf) to South Australian water quality and it appears that in the cities (metropolitan areas) the water hardness is around 100 ppm and the TDS (presumed to be mostly salt after accounting for hardness) is around 330 ppm. However, in the country, the water hardness is much higher with most in the 200-500 ppm range while TDS was also higher with most in the 350-900 range but with some over 2000 ppm. This link (http://audit.ea.gov.au/ANRA/docs/fast_facts/fast_facts_23.html) describes how "Australia's ancient weathered landscapes and some surface waters are naturally salty." There are other links as well, but generally they indicate a saltier environment (though I should point out, not at SWG levels).

[EDIT] It is also true that older SWG systems recommended salt levels of around 5000-6000 ppm, but most newer SWG systems are around 3000 ppm. So it would be reasonable to assume that since Australia had SWG pools earlier, that more of them (using the older SWG cells) would be at higher salt levels. That would make any potential corrosion issues worse so is another reason they would be more inclined to use more chlorine/sulfate/chloride resistant materials. [END-EDIT]

With everything said up to this point, I'm not too concerned with metal corrosion in SWG pools that use CYA (i.e. most outdoor pools). I think what we have learned that may not have been known before is that indoor pools using an SWG with no CYA risk corrosion of their stainless steel, even at "normal" FC levels of 3-5 ppm. The study Sean gave us and some of Brad's customer experiences also show that any pool where the chlorine is not monitored and goes sky high is also at risk and that this tends to happen more with indoor pools (probably because there is no sunlight to limit how high the chlorine level can get). As for hardscape materials, that still seems to be an open question but I'm leaning towards recommending more diligence in care of such materials in an SWG environment (either stronger materials or regular rinsing/diluting and/or sealing) though stone materials should really be cared for in any environment. The other open issue is sulfates in SWG pools and at this point I would recommend minimizing the use of non-chlorine shock and dry acid in such pools, but this is just being cautious and not yet born out by a lot of experience (except PatL34's caution referred to by waste in this thread (http://www.poolforum.com/pf2/showthread.php?p=41642&postcount=23)).

Thanks,
Richard

waterbear
02-06-2007, 12:07 PM
Another thought that just occurred to me on this topic can best be summed up by "Location, Location, Location". Where you live might be (and most likely is) a factor in how your pool is constructed. I happen to live in Florida on a a 2 block wide barrier island between the Atlantic and the Intercoastal Waterway and salt spray is everywhere. We just accept it as a fact of life--one of the prices you pay for living by the ocean. Therefore, materials used in all construction tend to be possibly a bit more salt resistant than materials that might be used elsewhere. (This thought occurred to me as I was rinsing the salt off my car!:eek::rolleyes:) This goes along with what Chemgeek said about water quality in Australia and the material they use there as a result.
Perhaps a better title for this thread might be "Is your pool constructed of corrosion resistant material?" intstead of "Downsides to salt pools";).

chem geek
03-30-2007, 11:31 AM
There's been talk about various sources of electrochemical corrosion and stray voltages and currents so I want to address some of this so that we can try and figure out what is really going on. First, I want to distinguish between different types of corrosion sources for metal in conductive solutions or exposed to moist soil or moist air:

1) "Normal" corrosion. This is where there is a single metal exposed to some oxidizer (usually oxygen in air and dissolved oxygen in water, though in pools chlorine is by far the more powerful oxidizer). Such corrosion is usually slow unless some deep pits form at which point the corrosion begins to look more like an electrolytic cell with a lower concentration of oxidizer deep in the pit and with electron flow in the metal and ion flow in the solution between the surface and the bottom of the pit.

2) "Galvanic" corrosion. I use this term to refer to when dissimilar metals are touching or connected electrically (by a wire, or a common connection to ground, etc.). In this case, the more anodic metal will corrode and will generally do so much more quickly than with corrosion of a single metal. This technique is used intentionally to protect steel pipes from corroding by connecting them electrically to zinc "sacrificial anodes". Zinc will corrode much faster than steel and when connected together, it essentially imparts a net negative potential on the steel which slows down its corrosion.

3) "Electrolytic" corrosion. I use this term to refer to a forced potential difference that is applied to two metals in a conductive solution (or moist ground, etc.). The potential difference must be DC in nature in order for electrolysis to occur and it may come from stray voltages or may be intentional (as in an electrolytic cell). The corrosion rate is high for this type of corrosion and is related to the potential difference, but it is non-linear and the rapid corrosion rate will only start to occur after the appropriate potential difference is achieved, though this is typically only a volt or two.

4) "Passivity" corrosion. I use this term to refer to corrosion that occurs when there is interference with the formation of a passivity or protective layer that normally prevents or slows down corrosion. Stainless steel is the most common example and I've discussed elsewhere in this thread how chlorides (from salt) can interfere with the formation of the passivity layer and how sulfates may accelerate this process.

A valid question is whether the SWG cell itself can be a source of DC voltages, so to discuss this let me first show what normally goes on if there is no leakage of voltage/current:

< e- < e-
______________________ DC Voltage from ____________________
e- | Transformer/Diodes | ^
v | | e-
| |
Negative Plate releasing electrons Positive Plate absorbing electrons
2H2O + 2e- --> H2(g) + 2OH- Cl- --> HOCl + H+ + 2e-

Cl-, OH- > H+ + OH- --> H2O < HOCl, H+
Notice that what goes on is that there is a current flow of electrons in the wire between the plates and that this current is driven from a voltage source, typically stepped down AC voltage from a transformer that is then converted to DC voltage by a diode bridge or equivalent circuit. There is also an ion charge flow of negative ions moving from the left to the right and positive ions moving from the right to the left -- both of which represent a NET negative charge flow from left to right. I also show how chloride ions will diffuse from left to right since they are used up (converted to chlorine) on the right and how chlorine (HOCl) diffuses from the right to the left. What mostly goes on, however, is that the hydroxyl ion produced on the left combines with the hydrogen ion produced on the right to form water.

In the above cell, there is water flow (from the pump) and this will tend to push both the positive and negative ions (and neutral species as well) downstream possibly out of the cell and into the pipe. However, both positive and negative charges are in the flow so there is no NET charge flow and therefore no ion current (except between the plates as described above). If there were some sort of net charge flow down the pipe, then there would have to be some sort of net charge creation in the cell. This is possible, but only if there is a leak of electrons so that the circuit is complete. This is shown in the following diagram:

< e-
< e- < e- .........................
______________________ DC Voltage from _________________|__ :
e- | Transformer/Diodes | ^ :
v | | e- :
| | :
Negative Plate releasing electrons Positive Plate absorbing electrons :
2H2O + 2e- --> H2(g) + 2OH- Cl- --> HOCl + H+ + 2e- :
:
LESS LESS LESS LESS : ^
Cl-, OH- > H+ + OH- --> H2O < HOCl, H+ : e-
:
:
OH- ^ :
v Fe(2+) :
e- > :
Metal in Pool .........................................:
Fe(s) --> Fe(2+) + 2e-
Notice that now there is an electron current leak (it's a positively charged leak in the sense that it wants to "receive" electrons) and this causes less positively charged hydrogen ions to be produced at the plate so when all ions are pushed down the pipe with the water flow, there is a net negative charge due to an excess of hydroxyl ions (OH-) over hydrogen ions (H+). In addition, at the metal in the pool, the metal (iron, in this example) corrodes so forms ferrous ions in the water and these are excess positive charges that migrate back toward the cell (through the floor drain and skimmer since the ions won't generally diffuse "upstream" through the returns). So there is a net negative ion current flow from the salt cell to the metal in the pool, thus completing the circuit. In essence, the electrical connection between the wire connected to the positive plate in the salt cell and the metal in the pool effectively "extends" this plate to be partly in the cell and partly in the pool. The problem is that the metal in the pool isn't made out of platinum, titanium, graphite, or another material that does not corrode, so instead of making chlorine, the metal corrodes since that reaction is much more favored. The resistance of this long flow path is obviously much higher than between the plates so the current will be small, but even a current of 2.5 milliamps would result in 1.6 ounces of metal corrosion in a year of "on" time (25 milliamps would corrode 1 pound of metal in a year).

The question becomes, "can the plate or wire on one side of the salt cell (especially the positively charged side) become connected to metal in the pool?" There is a bonding wire and it seems as if it is intentionally connected to the SWG, but I do not understand why. I can see that for most metal that is in devices that carry electricity, such as the pump, that you want to bond it to other metal in the pool so that no potential difference exists (essentially shorting out an electrolytic circuit), but you obviously can't connect the two plates in the salt cell together or else they will short out and no electrolysis will occur. You really shouldn't connect one side either, for the reasons shown above in the diagram. Perhaps the salt cell is bonded in case the electrical housing gets wet, but that seems strange to me. Does anyone know how the bonding to the SWG is done? Has anyone looked inside their SWG power box to see what the bonding wire is connected to and if there is any way for it to be electrically connected to a wire going to a plate?

If DC voltages are measured at the pool, say between the pool water and some metal or the ground, then it should be trivial to turn off the SWG (perhaps even unplug it) and see if the DC voltages remain or go away (or measurably decline). If the SWG is not the source of DC voltages, then these could be traced back to the source. It is not uncommon to find AC voltage differences around a pool due to the electrical distribution system which shunts the neutral to ground in the power system. The neutral is not completely neutral so ground voltages (and currents) are possible. However, AC voltages do not cause electrolysis and therefore will not cause corrosion. Bonding all pool metal together (as well as a grounding grid in coping) removes this potential difference by shunting the current to the bonding wire instead of inside you!

Richard