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Reading a galvanic chart

 

I have been evaluating multiple galvanic corrosion charts and have found that I do not understand the difference between stainless steel (active) and stainless steel (passive). Can someone provide me with an explanation?

Thanks,

Scott J. Wibbenmeyer
- Festus, Missouri, USA

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Active stainless steel is a clean stainless steel surface free of any oxides.

If stainless steel is cleaned and left exposed to the air it will auto-passivate or you can accelerate the process by immersing the stainless in nitric acid. In each case the chromium on the surface of the stainless will combine with oxygen and form a chrome oxide layer which protects the stainless steel from corrosion. This is what makes stainless steel rust resisting.

You may have both active and passive surfaces coexisting on the same piece of stainless. If pitting is taking place, the bottom of the pits is usually active while the surrounding surface is passive. That is why the pits continue to develop.

Hope this helps,

John Holroyd
- Elkhorn, WI

What is a recommended quick and easy remedy for the pitting on an installed specimen of stainless steel?

J. Plante
- Naples, Florida