Understanding passive film degradation and its effect on hydrogen embrittlement of super duplex stainless steel-Synchrotron X-ray and electrochemical measurements combined with CalPhaD and ab-initio computational studies

The passive film stability on stainless steel can be affected by hydrogen absorption and lead to microstructure embrittlement. This work shows that the absorption of hydrogen results in surface degradation due to oxide reduction and ionic defect generation within the passive film, which decomposes and eventually vanishes. The passive film provides a barrier to entering hydrogen, but when hydrogen is formed, atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution, as shown by synchrotron x-ray diffraction data. The vacancy concentration and hence the strains increase with increasing electrochemical cathodic po-larization. Under cathodic polarization, the surface oxides are thermodynamically unstable, but the complete reduction is kinetically restrained. As a result, surface oxides remain present under excessive cathodic polari-zation, contesting the classical assumption that oxides are easily removed. Density-functional theory calculations have shown that the degradation of the passive film is a reduction sequence of iron and chromium oxide, which causes thinning and change of the semiconductor properties of the passive film from n-type to p-type. As a result, the surface loses its passivity after long cathodic polarization and becomes only a weak barrier to hydrogen absorption and hence hydrogen embrittlement.

Automated summary

This work demonstrates that hydrogen absorption can lead to the degradation of the passive film on stainless steel, resulting in microstructure embrittlement. The absorption of hydrogen causes the reduction of oxides and generation of ionic defects within the passive film, leading to its decomposition. Atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution as observed in synchrotron X-ray diffraction data.