Effect of plasma nitriding on hydrogen behavior in electroplated chromium coating

The phase composition and hydrogen behavior in coatings produced on Armco iron by electrochemical deposition of chromium (Cr) and subsequent plasma-assisted nitriding were investigated. At cathodic Cr deposition, no hydrogen charging of the iron substrate occurs. The amount of hydrogen absorption within the Cr coating (50 μm thick) was measured as high as 450 ppm. Plasma-assisted nitriding of a galvanic Cr coating caused the formation of a surface layer consisting of compact nitrides (Cr2N+CrN) and precipitates of nitride within the bulk of the Cr coating and a (Cr,Fe) alloy layer between the chromium coating and the steel substrate. The compact nitride layer prevented hydrogen escape from the Cr coating. The nitride precipitates served as hydrogen traps, and thus hampered hydrogen desorption. Hydrogen-assisted phase transformations in nitrided electroplated Cr coating were observed and are discussed.