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Discoloration of Stainless Steel

I am looking for the reason which stainless steel discolors and rusts. Especially I do not understand well about the reason to discolor(it looks blue or black). Is discoloration a chemical change? Or Does it only look blue? Even if analyzed by EDX, only iron, carbon, and chromium were detected.

Atsushi Nishikawa
- Nakajuku, Echigawa-cho, Echi-gun, Shiga, Japan

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Atsushi: We need some more details before we can answer your questions appropriately. What grade of stainless steel is it, and when does the discoloration appear- immediately or after a time? What treatments are you giving to the stainless steel? The discoloration is definitely a chemical change, but it is hard to tell what it is without more information. The change to blue-black color can be a serious problem that is causing the rusting that you mention. If you prefer to talk to us you can call me or email me at the number or address on our Web Page. Lee Kremer Stellar Solutions, Inc. McHenry, Illinois   Colors on the stainless... is it PVD titanium plated or INCO coated colors? Michael Liu Taylor specialty stainless steel distributor - Dallas, Texas

I happened to see recently a 304 stainless steel tank with light blue discoloration stains (on the cylindric wall, on the top hemisphere and on the mixer blades). This tank contains usually milky products and undergoes CIP after each shift.

Do you know the reason for the phenomenon and how to avoid it?

Best regards,

Shraga Yaron
- Hod Hasharon, Israel