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White Rust Military Application

 

I have white rust forming between a Bolt/washer and a plate which are mechanically assembled. The washer is standard steel and zinc passivated IAW ASTM B633-85, SC#, TYPE II with an Olive Drab Chromate conversion coating (military application). The plate is stainless steel made in the UK to BS 970 304S15 specs and then blackened to RA/PS 122 C. What is the white rust caused by. Will this lead to failure over time. The two parts have been in contact for only two months.

Michael Fedoruk
- Calgary, Alberta, Canada

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White rust usually indicates that the zinc is corroding after the chromate conversion coating is no longer offering protection. If the olive drab coating is still visible on the outside of the washer and bolt, then I would say that the bolt and washer have become the anode in contact with the black oxided Stainless steel, and this will lead to premature failure of the coating system.

  
Tom Pullizzi
Falls Township, Pennsylvania
 

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You are experiencing Galvanic corrosion. When two different types of metals come into contact, you set up a voltage and current flowing between the two metals that encourages rust. The bolt is corroding sacrificially and providing extra protection to the stainless steel. The white corrosion is zinc oxide, which is rusted plating. The next thing you will eventually see is iron oxide, which is your part's base iron rusting. Is it possible to change the design so that the bolt is made of the same stainless steel?

Tim Neveau
Rochester Hills, Michigan