Downsides to salt pools

[Waterworks]

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]

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

[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.