Why Stainless Steel is Stainless

Q: Why doesn't stainless steel corrode? Or does it? Is there something about the alloys used that inhibit corrosion?
Harry, Albuquerque, New Mexico

A: Chromium keeps stainless steel shiny in much the same way aluminum stays bright-- chromium forms a tough oxide film that stops more corrosion. The film is self-healing if oxygen, even in small amounts, is present. Scratch or cut stainless steel and a new film quickly forms over the cut.

You asked if stainless steel corrodes. Yes it does if the oxygen content of the surrounding air is low. Then the oxide film cannot form and the steel corrodes especially if exposed to such conditions for a long time. Intense heat, for example welding a joint, can discolor stainless steel.

Stainless steels come in various types, some more resistant to corrosion than others. Those with the lowest percentage of chromium and the highest percentage of carbon corrode the most easily. Metallurgists have tinkered with alloys through the years discovering useful properties. Austenitic stainless, 16 to 26 percent chromium and up to 35 percent nickel, resists corrosion the best and is nonmagnetic. Duplex steels, 18 to 28 percent chromium and 4.5 to 8 percent nickel, resist stress corrosion cracking and chloride ion attack and so find uses in salt-water applications.

Further Reading:
The Stainless Steel Information Center of the Specialty Steel Industry of North America