I read the liked article and there is a discussion about rusting with many comments saying it's bunk. Yet this article:
Pool School - Salt Water Chlorine Generators
states:
Adding salt to the pool does very slightly increase the risk of corrosion. Hardly any SWG owners experience a problem with corrosion. Because the exact risks for some of the materials used in pools are not well understood, there has been some debate in the industry about the use of SWGs.
The two most common situations where damage has occurred are in indoor pools, where the FC level has been allowed to get way too high, and when you have one of the softer kinds of natural stone (sandstone, limestone, etc) above the water line, and the stone gets splashed regularly, for example in a waterfall. Even in these two situations, problems are rare.
I also read somewhere on TFP that brass screws should be used for in-pool ladders so wonder if that's true. The reason I'm asking is I'm considering switching myself in the next year and wonder about this issue. For me it probably NBD as there is no metal near the pool and no ladder.
Salinity affects corrosion in two distinct ways for metals. First, it increases the TDS (conductivity) of the solution thus allowing galvanic corrosion to occur. Pure water by itself is not a very good conductor but, once you start adding "stuff" to it, it can conduct currents. Secondly, salinity will mean greater chloride ion (Cl-) concentrations and chloride increases the likelihood of pitting corrosion in steels and stainless steels because it interferes with the reformation of passive films (mostly in the form of chromium oxide) when low pH situations arise. However, it must be stated clearly that pH is the
dominant factor in the initiation of corrosion. You must have low pH conditions (or very anodic voltage) to initiate corrosion.
As for stone materials, salinity plays a role in the degradation (not really "corrosion" as is stated above) of stone materials. Essentially, water mixed with salts drives these salts into the porous spaces of stone materials (like soft limestone). When the water evaporates, it leaves behind concentrated salt solutions that begin to deposit salt inside the stone matrix. The recrystallization pressure form the salt coming out of solution causes a lot of stress on the stone material and begins to fracture it. Eventually these micro-cracks and fissures will start to multiply and connect together and that will cause spalling of the stone material. Chlorides can cause this but you typically need very high concentrations of chlorides to see an effect. Sulfate salts (like those you get from using dry acid or non-chlorine shock) are much, MUCH worse. Sulfates typically have much larger recrystallization pressures associated with them and will more easily fracture stone materials.
As for brass in a pool, that would depend on the type of brass used. Straight Cu/Zn brasses would not be any better than steels and could lead to copper contamination. Marine brasses that contain aluminum and tin as additives are much better in saline environments. However, as stated above, everything is driven by pH. Once the pH drops too low, or if any anodic currents are present, then metals will corrode. So I really see no advantage to using brass over stainless steel. The first order of business is to always maintain good water quality (pH, TA, etc., all in ranges that keep the saturation index near zero).
