when and why clorides deposit on ss pipe?

Question

clorides in water

Answer 1

The chlorides do not deposit on the stainless steel pipe, they attack the microstructure of the metal, and all the stainless materials, both the austenitic stainless steels and the ferritic stainless steels react differently to the attack by chlorides.
See:
http://en.wikipedia.org/wiki/Stainless_steel#Intergranular_corrosion
For a better explanation of what happens.

Also do a search for "intergranular corrosion of stainless steel pipe" and "stress corrosion cracking of stainless steel pipe", and see what you come up with.

Answer 2

While Stainless Steel (SS) is resistant to oxidation because of the chromium, all grades of SS are not resistant to chloride attack. Molybdenum containing grades are resistant to chloride attack. Chloride ions and water form hydrochloric acid which is a reducing acid and thus the chromium oxide film doesn't offer any protection. Ferric Chloride and Chromium Chloride etc. are formed resulting in corrosion.