Downsides to salt pools

[chem geek]

Brad,

I can only refer you to this same EPA document that I referenced before where it says that the 316 stainless steel is more resistant to chlorine pitting than 304. However, it does show in at least some studies that 316 is superior to 316L, but I would find it surprising that salt levels above 500 ppm are simply corrosive to 316L. The chlorine appears to be far more corrosive to stainless steel than the salt level, though the salt level may accelerate such corrosion. So my take would be that using CYA would cut down the corrosion from chlorine to the point where the higher salt levels of an SWG pool wouldn't be a problem with 316 type steel.

However, this is all just speculation on my part based on the fact that we haven't had any reports of stainless steel corrosion in SWG pools that have been using CYA (and haven't had horribly high chlorine levels). We did have the one report of the indoor pool with 3-5 ppm FC, but that had no CYA.

So if your customer is up to it, they can bring up the CYA level in their pool to at least the 10-20 ppm range (if not 30 ppm) and then try the SWG (with the 3000 ppm salt). My prediction is that they won't see corrosion. If they had 304 stainless steel, then they *might* see corrosion eventually, but even then I'd bet it would take a few years.

Now having CYA in an indoor pool is going to present a different set of problems since indoor pools tend to get combined chlorines that do not break down as readily as outdoor pools (due to the lack of sunlight) so you might need to periodically use a non-chlorine shock (potassium monopersulfate) for the indoor pool. First try monitoring (or have the customer monitor) the combined chlorine level -- if it's not a problem, then you're done. If not, then use the non-chlorine shock.

By the way, you mentioned that the customer saw corrosion issues with the walls and floor of the pool. What does that mean? Isn't the pool plaster/gunite? Is he now talking about corrosion as in roughness of the plaster finish? Or is this pool made out of stainless steel?! Or do the walls have some stainless steel bars and by "the floor" he means metal floor drains? I'm confused.

That's my two cents (again, for what it's worth),
Richard