EFFECT OF CHLORIDE ON ss 316 l

What is the pH of the water?

Look for previous threads about this problem. Basically, SS is terrible for any low pH chlorides; Inconel works much better. Welding only exacerbates the problem. This is what plastics are really good for.

Not surprising, chloride attack on stainless steel will cause the metal to crack and welding the cracks often leads to further cracking making repair difficult to impossible. Pitting of the surface is the first indicator. Sounds like you have a textbook example of chloride attack of stainless steel.

You may want to look at copper or one of the copper alloys for this service. (Bronze, admiralty, etc.) Those tend to stand up better in salt water service than the stainless steels.

Are there any other minerals or metals in the water? That could have a galvonic reaction also.

usually stainless steel works "well" in a water with 270 ppm of chlorides. A safe limit is 500-1000ppm, but it all depends also on the temperature and the possibility of crevice, deposits where there could be a concentration of chloride. if the corrosion is located in welds then it could be a problem of sensitization of the weld materials.