The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles

Abstract

The Fe-Cr and Ni-Cr alloys have excellent corrosion resistance in many aggressive environments due to the Cr enriched passive film that forms on their surface protecting the alloy from the environment. Although referred to as a “passive” film, it is actually quite dynamic, forming and dissolving as a material interacts with its environment. In previous research, the direct measurement of elemental dissolution rates by atomic emission spectroelectrochemistry (AESEC) during passivation has proven useful to understand selective dissolution and passivation for Fe-Cr and Ni-Cr alloys [1-6]. In particular, by applying a mass balance to the elemental dissolution rates, the time resolved elemental surface enrichments of the different alloying elements were obtained. For example, the formation and dissolution of the passive film on stainless steel was followed by monitoring the Cr enrichment as a function of time [2,3], and the methodology extended to measure simultaneous Cr and Si enrichments for a Si rich stainless steel [4]. In this work, the kinetics of elemental dissolution were monitored during electrochemical experiments for a commercial Ni-Cr-Mo alloy (Hastelloy C-22) [5] and for binary Ni-Cr alloys, ternary Ni-Cr-Mo alloys in 2 M H2SO4. In particular, we present an in situ determination of the growth and dissolution of the passive film during cyclic passive active cycles for a series of Ni-Cr and Ni-Cr-Mo alloys. The methodology also allows us to follow passive film formation during spontaneous reaction at open circuit. The mechanism of passive film formation and the specific role of Mo will be discussed based on these kinetic results. The figure gives some highlights for the commercial alloy. An overview of the electrochemistry of the alloys is available from the AESEC polarization curves with elemental dissolution rates (Fig, left). The upper curve gives the conventional log /je/ vs. E profile while the lower curves show the elemental dissolution rates (jM). Note that jΣ is the sum of all elemental currents. A nearly 100% faradaic yield is indicated by je ≈ jΣ in the later passive domain. The active peak of the polarization curve for Hastalloy C-22 was not associated with a significant elemental dissolution, indicting an important difference as compared to the previously investigated austenitic stainless steel (Fe-Cr-Ni alloys) in the same electrolyte. Rather elemental dissolution occurred before the active peak and during the passive domain. Mo dissolution however only occurred in the later stages (E>0) of the passive domain indicative of a selective enrichment of Mo in the active and early passive domain. This observation complements a recent observation of Mo enrichment during the transpassive domain and Mo release during the passive domain in neutral chloride electrolyte [6]. The direct observation of passive film growth and dissolution is shown to the right of the figure. The upper curve gives the potential – time sequence showing three cycles of activation (A, E = -0.8 V), spontaneous passivation (SP1, Eoc), passivation (P, E = +0.3 V), and spontaneous passivation (SP2, Eoc). The lower curves give the excess Cr and Mo based on the elemental dissolution rates of the alloy components and a mass balance calculation. The formation and dissolution of a Cr enrichment was observed during cyclic potentiostatic passivation (+0.3 V) and activation (-0.8 V) respectively, and during spontaneous passivation at the open circuited potential. Interestingly, Mo enrichment was only observed during spontaneous passivation (≈ -0.2 V). Under anodic polarization, partial Mo dissolution was observed, which then dissolved completely during the activation cycle from high potential. These results will be discussed in terms of the thermodynamic stability and solubility of the Ni, Cr and Mo oxides and will be extended to include pure binary Ni-Cr and ternary Ni-Cr-Mo alloys.