Ni-Cr-based alloys are considered amongst the most corrosion-resistant when utilized in aqueous marine applications. The superior corrosion resistance of these alloys is known to depend on the presence of a homogeneous solid solution and the desirable attributes of their oxide films. However, the scientific basis for excellent passivation and protection against localized corrosion initiation remains uncertain and insights are often based on short term laboratory testing under limited conditions even though exposures are long-term. The engineering basis for alloy choice often relies on lab testing, field exposures or empirical metrics such as the PREN that do not even consider exposure time. Both highly engineered legacy and emerging alloys lack information on the precise attributes of protective passive films after long-time exposures. However, relevant field exposure periods are nearly infinite in time.

The focus of the current work was on solid solution Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W alloys. Potential step passivation studies in 0.1 NaCl pH 4 solutions in the passive range were investigated over 10s to 10 days at select potentials. To characterize the formation and exposure aging of oxide films, electrochemical methods were used as well as XPS and three-dimensional APT. Improvements in passive films protectiveness was correlated Cr³⁺ enrichment and aliovalent cation accumulation (Figure 1), and minimal morphological roughening. There is possibility of either oxide doping and/or phase separation promoted by aliovalent cations associated with alloying with Mo and W. Ni²⁺ rich oxides and hydroxides were preferentially dissolved with time. The most corrosion resistant alloys possess passivating films that are efficacious not only just after formation, but that self-healing capabilities and improve over long exposure times as evident from impedance spectroscopy and metastable pit studies. Such alloys were observed to be the most resistant when interrogated for local corrosion after various periods of exposure compared to other alloys whose impedance response degraded over exposure time. The latter alloys eventually arrived at conditions favoring breakdown.

This work contributes to the scientific understanding of the long-term passivation process in Ni-based alloys in acidic chloride-containing environments. The presence of Mo and W as minor alloying elements on aqueous passivation and localized corrosion resistance initiation and stabilization correlated with certain features of electrochemically grown oxide films. Oxide exposure aging time, potential, and oxide “transformation” diagrams are introduced and shown to provide valuable insights on alloy. Passivated films may form as solute captured solid solutions after short term passivation or when grown quickly but are gradually enriched in Cr, Mo and W over long time. Can such Cr enrichment continue during exposure without limits? Factors affecting and limiting Cr enrichment are discussed as time allows.

This work was supported by the Office of Naval Research under MURI “Understanding Atomic Scale Structure in Four Dimensions to Design and Control Corrosion Resistant Alloys” through Northwestern University #SP0028970-PROJ0007990-MURI ONR N00014-16-1-2280.