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RE: Stainless Steel Rust

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Here are some example that I have witnessed on the staining/rusting of stainless steel.
Seen at several waste water treatment facilities.
The bends have residual stresses caused by fabrication, cause some of the austenitic phase to change to a ferrite phase. One ends up with local galvanic corrosion cells (each phase has a different electrical chemical potential).
Chlorides are present. Attaching the SS, causing intergranual corrosion, and breaking the protective chromium oxide layer. ( Most any halide ion will break down the protective layer.)
Ozone may be present. ( I witnessed 304L SS 2" sch 80 pipe fail in about 4 hours in a stream with a high amount of ozone at a WWTP). This is why glass piping is generally used.
Sulphuric acid and sodium/potassium hydroxide may be present used to adjust the PH.
Cold working the SS will generally result in an increase rate of attack in acidic environments.
Some single cell organisms excrete iron sulphide/oxides that cause staining.
At another pharmaceutical facility I observed steel wool being use to clean the SS. Looked good until it got wet. (When I pulled up in the parking lot the next morning with the overnight dew, ... well you can imagine what it looked like.)
In petro-chemical & refinery operations,  I general take a small bottle of Nitric acid to test the material to see if it is Carbon Steel or Stainless Steel ( they look the same after a short period in these atmospheres).
In another application (Cryogenic freezers) the liner is generally type 304 Stainless steel.  The decontamination process of using a bleach solution caused corrosion. 
In another case, the rust staining was caused by water with iron ions dripping on the SS.  As the H2O evaporated , it left the ions behind, cause the stain.  The paint on the overhead structural steel  had failed.
Improper heat treatment can cause the SS to become susceptible to intercrystalline corrosion. ( I have seen cases where individual crystals popped out.) 
Just remember Stainless Steel only looks shiny in relative benign environments.  Stainless steel does corrode, something quite catastrophically. Corrosion (primary SCC) is The Number One cause of chemical plant equipment failure. Ninety-five percent (95%) of chemical equipment failure is caused by corrosion of the 18-8 family of Stainless Steel. 

Richard M. Beldyk, PE, NSPE, CWEng, CPE
Professional Engineer - Delaware 6788, Maine 9987, Maryland 26594, Michigan 6201048734, Ohio 66691, Oklahoma 20523, Pennsylvania PE061240, Virginia 37138
Registered Member - International Registry of Professional Engineers - Certificate IR101
AWS Certified Welding Engineer - Certificate  0009Eng
AFE Certified Plant Engineer - Certificate 4582