So full disclaimer, I am new to distilling. I haven't even run my first run yet, but I know something (quite a bit actually) about this topic of metals and salt(electrolyte) interaction because I am a sailor and licensed sea captain.make sure you use ordinary non-iodized salt not iodized. Virtually all salt sold in NZ in containers for domestic consumption is iodized. Also be aware that salt is sodium chloride and that chlorides attack stainless steel and can bad pit it. It can also and will generally shorten your element life.
The process that causes salt or any electrolyte to attack metal is called galvanization. It is most often caused when two pieces of dissimilar metal are electrically connected and submerged in an electrolytic compound (salty water for instance). What happens is one of the metals becomes a cathode and another becomes an anode. Much like the way a battery works, (a basic battery is two plate of different metals submerged in acid (an electrolyte)) the anode will be stripped of electrons that travel through the salty water to the cathode creating an electrical current.
For this to happen you have to have two metals that are dissimilar. This is key to understand the next part.
If your still is getting pitted then it mean the metal is likely of poor quality or wasn't actually "stainless steel". Different parts of the still are dissimilar enough from other parts that this galvanization is allowed to occur. Where the pits are happening, the metal is giving up electrons the the parts of the still that are not pitting (it doesn't actually accumulate, it just ends up in solution) The metal that makes stainless steel truly stainless is chromium, yet for some reason there is a lot of "stainless" steel that doesn't include any chromium. You know how some stainless eating utensils are 18/10 stainless? Well this is cheap stainless, it's got iron and nickel but no chromium and that's why your knives pit after years of being washed in the dishwasher. It's a little better than the kind of stainless you get from home depot or lowes when you buy stainless screws, yet they still rust red rust and rot away just as fast as carbon steel, but without any of the benefits of being a hard steel. It damn junk and it really pisses me off.
Real stainless grades of 304 or 316 will not behave this way. 304 has no chromium, but the alloy has been mixed well. On sailboats we use this stuff above the waterline. 316 has chromium, but it's expensive stuff. It's used below the waterline...that means it's in ocean water (a lot saltier than your mash) pretty much it's entire lifetime. Seeing as how still boilers are not constantly submerged in seawater, there is no good goddamned reason for your stainless to pit unless it's cheap junk. Pitting means the metals were dissimilar, meaning it wasn't mixed to true homogeneity (it's the same no matter where you look).
It may be fine for people to have cheap for a while, but if you REALLY want a still that lasts a lifetime, and maybe a few lifetimes, it would be good to ensure that you are buying REAL stainless steel to build your boilers. Mirror finish doesn't mean anything, that just means it was polished.
I am not sure how to tell you how to verify what the steel is that's been used to make your still, but maybe asking where they source their steel from and asking that source would be a good start.
I hope this helps.