Salt water generators voiding the AGP warranty

[chem geek]

Yes, I think you are right that it's just inferior (low chromium level) stainless steel and not zinc-plated/galvanized. True zinc turns black rather quickly in chlorinated pools (just take a paper clip into pool water to see the effect within a few days).

The warranty issue isn't just regarding AGP's, but also some gas heaters as I list in this post. Now realistically, the number of problems are small, but they are not zero.

I also know that one of the worst commonly used metals in a saltwater pool is aluminum where my own PB has said that "vanishing" pool covers that have headers touching the water require zinc anodes connected to the bonding wire to protect that aluminum from corrosion that he has seen if not protected. Cathodic protection, where you connect a sacrificial anode electrically to the metal you want protected, is very reasonable and generally works well. Anodic protection isn't a general solution since it essentially intentionally corrodes the metal in order to force a passivity layer, but some metals don't form good passivity layers while others do and as noted in the Wiki page, it's quite tricky to get right.

One can also provide cathodic protection by putting a direct negative voltage onto the metal to be protected (say, via a power supply of low DC voltage connected to the bonding wire with the positive end grounded and therefore sacrificed). That's essentially what the sacrificial anodes do -- they convert some metal to ions with the excess electrons providing a negative voltage that makes it harder for the protected metal to corrode, essentially increasing the over-voltage or activation energy hurdle required for such corrosion.

In some extreme cases with salt splash-out (see this post), even cathodic protection with sacrificial anodes would probably not be sufficient since the concentration of salt is simply too great.

And yes, all of these issues occur with lower salt levels as well. It's just that they take longer. Increasing the conductivity of the water by 2-6 times by increasing TDS from 500-1500 to 3000 is going to increase corrosion rates, period. The only question is whether that matters and the answer to that depends on the quality of materials being used and whether there is supplemental protection.

The salt (and chloride for stainless steel) levels vs. the quality of materials or supplemental protection is very similar to chlorine/CYA vs. algae. It is a race or reaction rate competition. In the case of salt vs. corrosion, it is between the factors causing corrosion such as conductivity and chloride ion level (as well as pH and active chlorine level) vs. the factors inhibiting corrosion such as the quality of materials (including passivity layer reformation from stainless steel) or use of cathodic protection. In the case of chlorine/CYA vs. algae, it is between the rate of killing algae from the active chlorine level roughly proportional to the FC/CYA ratio vs. the rate of algae growth that is limited by nutrient levels as well as sunlight and temperature (and type of algae). Just as some chlorinated cyanurate manufacturers say that "CYA doesn't matter", some salt system manufacturers (not Sean) say that "salt doesn't matter". Obviously, both statements are incorrect -- CYA does in fact matter just as salt does in fact matter. Any rational analysis of this situation will not point fingers at a single cause, but will look at these competing factors in order to properly balance them.

[Poolsean]

Ben,

We have not, as a company, experienced enough corrosion damage issues to commit to corrosion studies on AGP, so that is not on engineering radar screen.

In the past, I have suggested rinsing off the bottom and top rails, as these are places where water may collect. As the water evaporates, the salt concentration increases, and will become corrosive due to high levels of salinity. Keep in mind that Sodium Hypochlorite/Bleach usage will also introduce salt to a pool. The same reaction occurs as the water evaporates, yet no one complains of corrosion from the use of bleach. I realize that a salt chlorine generator operates at a higher concentration, the fact is, when the water drips out, and sits on the top cap or bottom rail, whether SCG or bleach, the concentration of salt will be the same as the water evaporates.

Some AGP manufacturers are using resin or polymer top rails, but the bottom rails, which are usually steel or aluminum, should still be occasionally rinsed off.