Downsides to salt pools

[chem geek]

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, 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 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]

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]

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

[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.