Stainless steel rusty problem

Recently, We got consult "why stainless steel get rusted like following?"

Under normal conditions, stainless steel use for decoration will not get rusted. But this also depend on material kind, the application environment. 

For outdoor usage, such as stainless cladding or other stainless steel decoration, or the location is nearby sea, 316L stainless steel is suggested normally. If daily maintenance is right. If any polluted substance, which easy to make metal surface get rust, adhere to stainless steel surface.