Where did you hear that most salt manufacturers recommend installing zinc anodes? If any do, then that would not imply that the corrosion comes from the electrolysis in the SWG -- the SWG installations also require higher salt levels so go hand in hand and you can't assume it's the SWG. If there were more pools with 3000 ppm salt levels without an SWG, then comparisons could be made; otherwise, the assumption may not be valid.
It is well known that high chlorine levels (especially in indoor pools without CYA) are corrosive to metals and that high chloride levels accelerate stainless steel corrosion (see
this post). Those are known mechanisms with the only issue being the rate of such corrosion in 3000 ppm salt pools. It has been proposed that stray currents may also be to blame, but no clear mechanism for that has yet been found and that would not explain corrosion of the metal parts in a pool cleaner, for example, since there is no current flow from the electrolysis in the salt cell possible in that case -- the metal is just exposed to salty, chlorinated water and is not electrically connected (and such corrosion in non-SWG pools is still in salty, chlorinated water, but with lower levels of salt). If the bonding wire and all metal components were to have a positive voltage applied to them, then there is no question that metal corrosion would be greatly accelerated.
Using a zinc block as a sacrificial anode in what is called cathodic protection is the same as putting a negative charge on the bonding wire to protect all metal components attached to that wire (and protecting the wire itself). And this would protect metal from corrosion regardless of whether it comes from the oxidation from chlorine (and oxygen) in pool water that is accelerated by the increased conductivity from salt (and by the higher chloride level in the case of stainless steel and probably aluminum) OR whether it comes from any stray currents or positive voltages.
The only zinc anodes I've heard recommended are with some automatic pool cover companies that have found corrosion with their leading edge poles for vanishing covers where the pole is in the water (and I believe the pole is made of aluminum which apparently is more susceptible to corrosion than stainless steel typically used in pools). I think either recommending zinc anodes or otherwise placing a constant negative (current limited) voltage on the bonding wire is a good idea for any pool, but especially those with higher levels of salt or disinfecting chlorine (hypochlorous acid).
By the way, I used a paper clip to hold my pool cleaner bag shut and it showed signs of corrosion in a matter of days (discolored and pitting) and my pool has < 800 ppm salt and around 3-5 ppm FC with 25-30 ppm CYA. There is clearly no external electrolysis (i.e. SWG) involved. Corrosion IS an electrochemical process, but it does not require an external applied voltage or far-away electrolysis to take place. Another similar paper clip in a glass of salt water took a lot longer to corrode then a few days (I experimented with that to see what would happen) -- the chlorine is the primary oxidizer (corrosive agent). The metal gets oxidized by the chlorine and the chlorine gets reduced (turned into chloride). If I ever get some spare time, I can try testing different salt and chlorine and CYA levels since the paper clip is apparently a rather rapidly corroding metal. Anyone is welcome to try the experiment and report the results.
Richard
