XPS study of the role of nitrogen in oxyanion formation during aqueous corrosion of molybdenum and chromium nitride

A dual electrode electrochemical approach, in combination with variable angle X-ray Photoelectron Spectroscopy, was used to determine the role of nitrogen in the generation of the oxyanions, MoO₄^2- and CrO₄^2-, from Mo₂N and CrN samples. During simultaneous anodic polarization of a coupling of Fe and either pure Mo, Mo₂N or CrN in deaerated 0.1 M HCl, the presence of nitrogen was found to enhance the formation of molybdate and chromate oxyanions. These oxyanions deposited back onto the nitride surfaces as insoluble salts formed with cations released from the iron electrode. These results are found to be in agreement with earlier published work on polarization of surface nitrided stainless steel alloys. The increased formation of oxyanions is postulated to be the result of deprotonation of electrolyte in contact with the nitride coating and a subsequent shift in pH to higher values. In addition to acting as a kinetic barrier, the oxyanions act as an electrostatic barrier to the ingress of the Cl^- anions which cause pitting. By analogy, this also provides a guide to the corrosion behavior of Mo and Cr nitride coatings.