Effects of nitrogen on the electrochemical passivation of metal nitride coatings produced by ion beam assisted deposition

摘要

Hard nitride coatings produced by ion beam assisted deposition (IBAD) are candidates to replace electroplated chromium (EHC) in a number of tribological applications, but the differences in the corrosion behavior of EHC and IBAD nitrides have not been fully characterized. In this study, coatings of Cr-N have been synthesized with IBAD, using 1000 eV nitrogen ions and e-beam vapor deposition. The chemical composition of the coatings were examined using Rutherford backscattering spectrometry (RBS), the aqueous corrosion behavior of the alloys was studied by electrochemical techniques, and the chemistry of the passive oxide was examined using angle-resolved x-ray photoelectron spectroscopy (XPS). Passive films formed on Cr-N coatings in 0.1 M HCl contain large amounts of nitrogen as ammonium ion (NH_4~+). Ammonium ion was also found in passive films formed on pure Cr coatings, presumably from nitrogen gas dissolved in the electrolyte for deaeration. Passive films on nitrogen-containing coatings were found to be much thinner and more strongly bipolar than those formed on pure Cr, leading to a much more noble corrosion potential. Chromate production is enhanced by the presence of nitrogen, possibly by an acid-buffering effect of the ammonium ion. These chromate oxyanions may be responsible for the enhanced bipolarity of the passive film noted above, and may act as an electrostatic barrier to chloride or hydroxyl ingress. They may provide some protection against pinhole corrosion by replating onto an actively dissolving surface as an insoluble chromate salt.