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
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