Do Salt Generators damage limestone/masonary

[chem geek]

craiga,

It's very interesting that we are only now starting to see complaints of corrosion in SWG pools on this forum, but when it rains it pours. Look at this post for an example of very fast corrosion of stainless steel in an indoor pool with an SWG, but also not using any CYA. I have also started a controversial and technical thread here in The China Shop to try and understand what is going on (if anything, since this may be a general corrosion issue not related solely to SWG).

Unfortunately, we don't have definitive and absolute answers to this, but what can be generally said is that higher disinfecting chlorine (i.e. hypochlorous acid, HOCl) levels are more corrosive (as would any strong oxidizer), that higher salt levels (i.e. salinity) and/or higher conductivity are more corrosive, that acidic conditions (i.e. low pH) are more corrosive (even high pH can be corrosive, too), higher dissolved oxygen levels (oxygen is an oxidizer) are more corrosive, having dissimilar metals touching is very corrosive to them (due to galvanic corrosion), having high water flow rates is more corrosive (due to physical corrosion), etc.

[EDIT] The previous discussion about factors influencing corrosion are about corrosion of METAL, typically iron, steel (including stainless steel) and copper. Corrosion of stone surfaces probably does not have some of these factors such as galvanic corrosion so may not be affected as much by conductivity. However, the other factors are still relevant as most stone resellers will tell you not to use acid around some stones (even acid from juices such as lemon), to be careful of oxidants (including chlorine bleach) and to avoid salty environments (especially heavily salty environments such as by the ocean). Remember that plaster/gunite/grout can be corroded "dissolved" just by plain water alone (water is sometimes called "the universal solvent") and requires calcium carbonate to be saturated in the water. So every type of stone, cement or other coping/hardscape surface is probably a little different in its susceptibility to corrosion and what makes it corrode faster. Also different stones have different abilities to be sealed and therefore protected from water and different sealants have different susceptibility to oxidants. I'm sorry I don't have definitive answers for you, but at least by asking the right questions we can get ourselves on the right track. The only personal experience I have with the sensitivity to stone is with our travertine shower where after less than a year we started seeing darker shading to the stone and smelled mildew. It turned out that our stone installer used a cheap sealant that only lasts 6 months. We dried out and re-sealed with a high-quality sealant that should last 3 years and so far after 2 years have had no problems. [EDIT]

Spilling concentrated chlorine onto stone surfaces is most likely going to be a problem, so if your corrosion is limited to the area where you are pouring chlorine (bleach) into your pool, then that is most likely what is going on. If instead the corrosion is mostly where the salty water in your pool is getting splashed out or dripped, say near the steps where you get in and out, then the salinity (salt) of the water is the more likely culprit (though the chlorine in the pool could still be a factor here as well -- you would have to compare your situation with similar stone to others without SWG systems and lower salt-level pools). If the corrosion is generally occurring and not localized, then the corrosion is just natural and this particular stone should probably be sealed on a regular basis (which is probably a good idea to do anyway). In my own pool (without an SWG) our concrete "made-to-look-like-flagstone" hardscape is recommended to be sealed annually and we do so and have seen no signs of corrosion after 3 years (with the exception of one small area of grout far away from the pool that probably just had some imperfection to start with that has pitted a bit).

What your pool builder said is bogus, at least as far as the type of chlorine you are using in your pool. If he meant you spilled concentrated bleach (or had it splash when you added it to your pool), then he has a point, but if he just means the general pool water having chlorine in it that came from SWG generation vs. bleach vs. Cal-Hypo vs. Trichlor vs. Dichlor, then he's absolutely, positively wrong. No matter what the source of chlorine is, once it is in your pool it is the same substance. Having CYA in your pool will lower chlorine's effectiveness and that also means its corrosive capability, but that CYA will affect all sources of chlorine the same (since the chlorine part in water is identical from all sources). If there is a problem unique with having an SWG, then it is most likely the increased salt level more than anything else (at least, in my opinion). If you have a good reseller of stone materials who really knows and understands your particular stone (both coping and hardscape), you could ask them if there is anything you can do to reduce corrosion, such as using a sealer or, in the worst case with a stone that is very susceptible to salt and cannot be easily sealed, that you can't use a salt pool with that stone.

Let us know where this corrosion is occurring -- that will be interesting for us to help figure this out as to what the main factor is (since there are generally multiple factors contributing). You mentioned that it is the coping, but is it the underside of the coping that gets splashed from water all around the perimeter of the pool and is it also on the top surface where you get in and out of the pool?

[EDIT] Finally, do you have a full set of water balance numbers you can post (pH, TA, CYA, CH, TDS or salt, Temperature)? Limestone is mostly calcite which is calcium carbonate. So if your water balance was corrosive in the calcium carbonate saturation sense, then that could be a problem. I have found that the traditional Langelier Saturation Index incorrectly accounts for TDS and temperature (both use a "log" relationship in the formula but that does not reflect the actual relationship) but that the error for a 3000 ppm salt pool is only 0.1 which isn't much at all. My guess is that your water balance is OK, but it's worth checking nevertheless. Also, do you live in an area with acid rain? [END-EDIT]

Richard