Downsides to salt pools

[chem geek]

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]

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]

Brad,

I answer your question about chlorine/CYA relationships at this post so as not to get off-topic from corrosion here. Please respond over in that thread.

Richard

[chem geek]

I received this PDF file 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]

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

[steveinaz]

This is our first season with a SWCG (QuikChlor) and I haven't noticed any corrosion or damage to our Shasta Deck (similar to Kool deck). The only metal present in our pool is the light bezel, and I've seen no evidence of corrosion there either.