X-RAY PHOTOELECTRON-SPECTROSCOPY AND AUGER-ELECTRON SPECTROSCOPY STUDIES ON THE PASSIVATION BEHAVIOR OF PLASMA-NITRIDED LOW-ALLOY STEEL IN NITRIC-ACID

Nitrided SAE 4140 steel has been passivated by concentrated nitric acid. The resulting film was characterized using a combination of surface-analytical techniques, such as X-ray photoelectron spectroscopy (XPS) to evaluate the chemical composition of the passive film. Auger electron spectroscopy (AES) combined with ion etching was used to determine the composition depth profiles of nitrided surface. It was found that preferential dissolution of iron leads to enhanced nitrogen and chromium concentrations within the oxynitrided layer. A dense protective oxynitrided layer was found to be formed on the nitrided surface when the concentration of nitric acid was as high as 8 M. The results of X-ray diffraction, XPS and AES analyses conclude that the protective nitride layer is composed of (Fe, Cr)4N, (Fe, Cr)2-3N and CrN in the inner layer, Fe2O3, Cr2O3 and remnant nitrides in the middle layer and nitrides accompanying Cr(OH)3.H2O and gamma'-FeOOH in the outermost layer.