Downsides to salt pools

[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