That's interesting about the copper not showing signs of corrosion while the stainless steel did. I wonder if the chlorine levels are better maintained in his spa or if copper is more resistent to any potentially salt-amplified chlorine corrosion. Copper is more resistant than steel so perhaps even though chloride ion might interfere with any protective oxide layer that gets formed, that once removed the underlying steel in stainless steel corrodes faster than the underlying copper under any thin copper oxide protective layer (if any). This is all speculation on my part.

On a separate topic, I ran into this article about the damage to plaster from high levels of CYA. What is interesting is that the drop in CYA over time seems to indicate that even levels of CYA not much lower than 100 ppm can be detrimental to plaster. If you look at the line that starts with 200 ppm, it drops rather quickly down to around 130 ppm and then continues to drop more slowly to around 80-90 ppm. This may indicate that there is some plaster deterioration effect even at 80 ppm though it is clearly much slower. This would be another reason for SWG manufacturers to take a look at seeing if they can't figure out a way to operate more efficiently at lower CYA levels.

Richard