Downsides to salt pools

[Waterworks]

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]

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]

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]

I see no reason for this thread to be stickied. I have un-stuck it.

[chem geek]

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 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]

I found the following graph or similar variations of it in several documents talking about corrosion.

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 (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]

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]

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. 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]

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.