Passivation of Ni-Cr and Ni-Cr-Mo Alloys in Low and High pH Sulfate Solutions

Abstract

The kinetics of passive oxide film formation, its thickening, and composition on Ni-22Cr and Ni-22Cr-6Mo wt.% alloys were investigated selected anodic potentials. Experiments were performed in acidic and alkaline sulfate environments using a number of characterization techniques including a combination of potentiodynamic polarization, on-line atomic emission spectro-electrochemistry (AESEC), in situ potentiostatic passive film growth, along with in situ neutron reflectometry (NR) and ex situ X-ray photoelectron spectroscopy (XPS). The roles of solution pH and Mo on the passivation behavior were discussed in terms of thermodynamic and kinetic factors governing passivation. The pH was found to have an impact on the relative chemical compositions of passive film of the Ni-22Cr alloy but not noticeably for the Ni-22Cr-6Mo alloy. Ni-rich films formed early during the passivation process while Cr(III) enrichment was observed at longer times albeit less extensively than observed previously in Cl- solutions. The fraction of Cr(III) cations also increased with alloying of Mo at low and high pH demonstrating a strong effect of Mo on Cr(III) content during aqueous passivation in Ni based superalloys however a larger effect is seen in Cl-. This is a noteworthy finding as Mo is usually assumed to mainly influence pit and crevice stabilization.

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni–Cr–Mo Alloys

“…[32] NiO and MoO 2 are good catalysts for the oxygen evolution reaction (OER), [33][34][35][36][37][38][39] which adds another dimension of complexity to understanding the degradation of Ni-Cr-Mo alloys since OER is known to be coupled with dissolution and degradation in other related materials. [40][41][42][43][44][45] The oxidation and corrosion of Ni alloys have previously been studied with electrochemical measurements, [20,31] X-ray photoelectron spectroscopy (XPS), [23,[27][28][29][46][47][48] scanning tunneling microscopy, [21,30,49] time-of-flight secondary-ion mass spectrometry, [50,51] transmission electron microscopy and energy loss spectroscopy, [52,53] operando neutron reflectivity, [54,55] and inline inductively coupled plasma mass spectrometry. [56,57] providing valuable information of the electrochemical behavior, the passive film, and metal dissolution.…”

Editors’ Choice—AutoEIS: Automated Bayesian Model Selection and Analysis for Electrochemical Impedance Spectroscopy

“…67 Designing self-healing alloys is one of the mitigation routes traditionally followed by using elements such as chromium, and molybdenum. 68 These elements tend to quickly form their oxides/ hydroxides under aqueous environments such that a thin passive film forms between the alloy-environment interface, suppressing any further active corrosion. The challenge today is that progress may be nearly exhausted in the study and discovery of binary and ternary alloys with major solutes and one or two impurity elements that control corrosion resistance.…”

A High Throughput Aqueous Passivation Testing Methodology for Compositionally Complex Alloys Using a Scanning Droplet Cell

“…These exact details can be elucidated in follow-on high fidelity studies which can address many of these questions. 42,[44][45][46][47][48][49] Moreover, a similar HTp approach can be developed in sulfate and chloride with a better emphasis on chloride breakdown. Some indicative metrics might differ while others such as self-healing are likely of similar value toward guiding the discovery of improved alloys.…”