nater,
Thanks for the info. I did check the Wiki link a while ago, but I always try to find independent information since Wiki can sometimes be wrong (as it is modified by anyone), though usually it isn't wrong for too long. At any rate, you probably were still writing your post when I responded to Carl and gave this EPA PDF link which, out of the many many sources I've looked at, seems to distill the essence of stainless steel corrosion. It's focus is more on steel in dirt or atmospheric exposure near the sea (at least for some of its studies), but it also contains a wealth of information on the types of stainless steel and their relative corrosion resistance. Specifically, refer to the following sections:
II. Definitions of Alloys and Corrosion - talks about metal corrosion generally, not specific to stainless steel.
V. Seawater of Marine Environments - though this has higher salinity and other chemical and organic components compared to pool water, it still talks about various factors affecting corrosion rates.
VI. Types of Stainless Steels - the most useful section for understanding corrosion resistance of different types of stainless steel.
VII. General Corrosion of Stainless Steels - charts of specific corrosion measurements in multiple studies. This validates the general Class groupings described in section VI. It is this section that contains the following interesting paragraph:
Non-halide salts have little effect on stainless steels, but chlorides particularly tend to promote pitting, crevice corrosion, and stress-corrosion cracking. In some cases sulfates seem to aggravate the effects of chlorides. Chlorides present in amounts as little of 0.3% with sulfates present can produce severe corrosion. Even quite low concentrations of chlorides can cause corrosion when concentrated by occlusion in surface films. Oxidizing chlorides such as ferric or cupric chloride are specific for severe pitting, although halide salts can cause severe pitting and stress corrosion cracking. The austenitic stainless steels are, however, the most susceptible of all the stainless steels to “chloride” stress corrosion cracking.
I am not so concerned with stress corrosion cracking since that doesn't seem to be as applicable to the pool environment. It would be critical for a mountain climber, however (and see this link similar to the one nater gave above)! Note that the statement I put in bold above talks about 0.3% chloride which is 3000 ppm if the % chloride is measured as % salt (sodium chloride), but more likely this is literally % chloride which would be 3000 ppm chloride which is about 5000 ppm salt. Either way, it says that the chloride level close to what is found in salt pools can produce severe corrosion when sulfates are present (perhaps the 5000 ppm salt level is close to the 6000 ppm level reported in the SWG study, but I would be surprised if corrosion were truly "insignificant" in a 3000 ppm salt pool over more than one year, especially if there are sulfates in the water or if CYA is not used so that the chlorine level is too high). Unfortunately, it doesn't say what level of sulfates start to cause this problem, but be aware that dry acid (sodium bisulfate) and non-chlorine shock (potassium monopersulfate) both introduce sulfates into a pool so should probably be avoided in salt pools. It also means that fill water high in sulfates may make corrosion worse in salt pools.
IX. Copper and Copper Alloys - useful for understanding what might be found in a heat exchanger (in a gas-fired heater, for example).
XI. Specific Properties of Cast Copper Alloys - mentions how Copper combined with Nickel improves strength and corrosion resistance.
Richard
P.S.
I also found this study on the corrosion of Portland cement by salt (though at much higher levels of 5% which is 50,000 ppm -- higher than the sea, but with regular wetting and evaporation, could be achieved) and this study on Portland cement and blended concretes (at sea salt levels, probably around 35,000 ppm) in the presence of sulfate and though the sulfate did not make the initiation of corrosion start any faster, it did make the progression of corrosion (once initiated) faster. Also, magnesium sulfate was worse than sodium sulfate. Fortunately, dry acid has sodium while non-chlorine shock has potassium (which is chemically more similar to sodium than magnesium). However, fill water "hardness" typically has magnesium at about one-third to one-fourth the amount of calcium on a molar basis, but the bottom line is that the pool is mostly sodium and calcium, not magneisum (for posistive charged ions, aka cations). If the salt levels in these studies were closer to salt pool levels, then I'd probably fork over the money to get the full study to find out the sulfate levels, but it's not worth it when the salt level is so much higher. This link gives a decent overview of corrosion issues with concrete. I'm sure there's lots more, but what I am looking for is a valid scientific study that relates corrosion rates for specific materials to chloride and sulfate levels (if there's a study with actual pool water, that would be even better, of course, since calcium carbonate saturation *may* reduce corrosion rates for certain materials). With that kind of information, we can set some guidelines for the kinds of materials to be used, estimate their expected life, and make recommendations with regard to chlorine level (including CYA) and identify other risk factors (e.g. sulfates) and their impact. It would be nice if manufacturers would take up the slack in this area, but as we have seen from the "lack of full information" on the chlorine/CYA relationship (independent of salt pools), this is something we may have to do ourselves first.
Though Taylor does not appear to offer a sulfate test kit (they have a sulfite test, but that's not the same thing), there do appear to be test kits from Hach, Hanna Instruments, and LaMotte and probably others as well. Any data gathering that is done on pools to try and figure out causes of corrosion should probably test for sulfates in addition to all of the other standard water chemistry parameters (pH, TA, FC, CC, CYA, Salt, Borates, Temp).
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P.P.S.
I want to remind everyone that most people (and servicers/installers) on this forum are NOT reporting corrosion problems with salt (SWG) pools. I do not want people scared off of SWG just because I'm trying to investigate what is going on with a few reports of corrosion and some servicers/installers who believe they see more. I am trying to be as unbiased as possible and just want facts that can be disseminated as information so that people can make intelligent choices. That's all.
waste, Ben, and others who have experience servicing multiple pools (some with SWG, some without), please, please give us your feedback.
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