Corrosion Resistance, Composition, and Stratification of Passive Films: Ni-22Cr and Ni-22Cr-6Mo Alloys Passivated and Exposure Aged in Acidic Chloride Solutions

Abstract

Ni-Cr-based super-alloys have exceptional corrosion resistance, which is improved with Mo alloying. The correlation between passive layer performance and composition was studied to gain a deeper understanding of the role of Mo by comparing the behavior of Ni-22Cr to Ni-22Cr-6Mo (wt%) alloys. The passive layers were formed using galvanostatic holds to create fast and slow growth conditions using high and low current densities. A potentiostatic hold was added to initiate exposure aging. The passive film was characterized using electrochemical impedance spectroscopy, linear sweep voltammetry, atomic emission spectro-electrochemistry, and X-ray photoelectron spectroscopy (XPS). Combined electrochemical and XPS characterization offered insight in composition and stratification, bonding (oxide, hydroxide), and layer modulation as a function of electrochemical conditions and performance. Most importantly, Mo addition enhanced Cr(III) bound in oxide, fast growth conditions resulted in less corrosion resistant films, and exposure aging increased Cr-enrichment and reduced stratification of Mo-cations. The correlation between passive film performance and Cr, Ni, and Mo oxidation states, bonding, oxide-hydroxide contributions, and stratification is discussed. Generally accepted correlations, such as Cr-cation concentration and performance of the passive layer, have to be reexamined to account for the complex chemical make-up of the passive layer.