Letter 31037

In need of expert Metallurgical help ASAP  

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I have run into a situation regarding Stainless Steel and Passivation that I am trying to resolve and would like to talk to a metallurgist about. I have some stainless steel that is welded to carbon steel that was passivated and then the tank around it was blasted with steel grit and coated. I now have some pitting and corrosion and would like to find out possible causes.

David Keehan
Manufacturer - Dallas, Texas

Can you provide a picture. Some times after grit blasting we immerse the part in 50% nitric acid or disimilar metal corrosion or so many reasons. Would you please provide more information.

Cair Shishani
aircraft maintenance - Abu Dhabi, UAE

David, did you write that you are passivating the carbon steel? That might explain some of the corrosion, as carbon steels are attacked by most passivation solutions. We certainly know it immediately when a material mixup causes us to put some alloy steel in the passivation tanks!

Lee Gearhart metallurgist East Aurora, NY

If the corrosion is along the weld and up to maybe a couple of inches away you most likely have galvanic corrosion from mating the dissimilar metals.

If the rust is general, blotchy, is on the stainless, and lesser on the carbon steel it may be iron contamination embedded into the material from blasting. Passivation does fix this but only if sufficient time is given.

If the corrosion is almost all on the carbon steel then it's probably either the pickle or environmental.

What is the environment? If this is a tank is the stainless above or below the liquid level? What is the temperature?

Jeff Watson - Pearland, TX

Lots of possible answers here. All of the carbon steel (passivated?) in the system, the weld heat affected zone, the steel grit blast all could cause your problem. Passivation may not be easy with an assembly of this type. A picture would help. If the unit is in a water solution of any kind you can expect corrosion.

Lee Kremer Stellar Solutions, Inc. McHenry, Illinois

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