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