Downsides to salt pools

[nater]

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]

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 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 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 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 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 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, Hanna Instruments, and LaMotte 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]

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 http://www.poolforum.com/pf2/showthread.php?t=6237 (#5+)

I have also been in 2 threads discussing SS rusting (http://www.poolforum.com/pf2/showthread.php?t=5114 / http://www.poolforum.com/pf2/showthread.php?t=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 )


{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]

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]

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)

[Waterworks]

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]

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