The HKU team’s answer was a strategy called “sequential dual-passivation.” Instead of relying only on the usual chromium oxide barrier, SS-H2 forms a second protective layer.

The first layer is the familiar Cr2O3 based passive film. Then, at around ~720 mV, a manganese based layer forms on top of the chromium based layer. This second shield helps protect the steel in chloride containing environments up to an ultra high potential of 1700 mV.

That is what makes the finding so striking. Manganese is usually not viewed as a friend of stainless steel corrosion resistance. In fact, the prevailing view has been that manganese weakens it.

“Initially, we did not believe it because the prevailing view is that Mn impairs the corrosion resistance of stainless steel. Mn-based passivation is a counter-intuitive discovery, which cannot be explained by current knowledge in corrosion science. However, when numerous atomic-level results were presented, we were convinced. Beyond being surprised, we cannot wait to exploit the mechanism,” said Dr. Kaiping Yu, the first author of the article, whose PhD is supervised by Professor Huang.

This is really impressive! I wonder how the manganese get past the chromium oxide, if it forms on top of it?