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

Abstract

Corrosion is the main factor limiting the lifetime of metallic materials, and a fundamental understanding of the governing mechanism and surface processes is difficult to achieve since the thin oxide films at the metal–liquid interface governing passivity are notoriously challenging to study. In this work, a combination of synchrotron‐based techniques and electrochemical methods is used to investigate the passive film breakdown of a Ni–Cr–Mo alloy, which is used in many industrial applications. This alloy is found to be active toward oxygen evolution reaction (OER), and the OER onset coincides with the loss of passivity and severe metal dissolution. The OER mechanism involves the oxidation of Mo4+ sites in the oxide film to Mo6+ that can be dissolved, which results in passivity breakdown. This is fundamentally different from typical transpassive breakdown of Cr‐containing alloys where Cr6+ is postulated to be dissolved at high anodic potentials, which is not observed here. At high current densities, OER also leads to acidification of the solution near the surface, further triggering metal dissolution. The OER plays an important role in the mechanism of passivity breakdown of Ni–Cr–Mo alloys due to their catalytic activity, and this effect needs to be considered when studying the corrosion of catalytically active alloys.

Operando Surface Optical Reflectance Microscopy Study of Corrosion Film Growth on a Ni–Cr–Mo Alloy During Anodic Polarization

“…The cracks in the corrosion product scales are likely formed during drying in air and in the vacuum of the SEM, while in the electrolyte, the buildup of corrosion products forms a continuous film with no cracks visible in the 2D-SOR images. Similar scales have been reported previously on Ni alloys, , and we have previously observed scales on this class of Ni alloys under similar conditions . In a tilted SEM view, the average thickness of the scales was quantified to 1.1 ± 0.1 μm, as shown in Figure b, in good agreement with the 2D-SOR microscopy data presented above.…”

“…The frequency of the oscillations increases as the potential is increased. The oscillations are caused by constructive and destructive interference between the light reflected from the metal/film interface and the surface of the growing corrosion product film, as previously observed . For this, the film needs to be semitransparent to the wavelength of the light used.…”

“…Studying the anodic transpassive corrosion from the electrochemical current alone is difficult for catalytically active alloys toward OER . The studied Ni–Cr–Mo alloy is active toward OER, which also triggers the passivity breakdown . Therefore, to study the corrosion of alloys at high anodic potentials, standard electrochemical techniques used in the corrosion community have been combined with complementary techniques such as neutron reflectivity, X-ray diffraction, ambient pressure XPS, , and inductively coupled plasma-mass spectrometry (ICP–MS) .…”

“…We have previously studied the development of the passive film using ambient pressure X-ray photoelectron spectroscopy (XPS) combined with an electrochemical cell in 0.1 M NaCl electrolyte and have shown that the thickness of the native oxide film increases up to 700 mV vs Ag/AgCl and concurrently become Mo 6+ enriched . At higher potentials, the oxygen evolution reaction (OER) occurs, accompanied by the dissolution of the Ni alloy into the electrolyte and the redeposition of corrosion products on the electrode surface, which complicates the understanding of the surface processes significantly …”

Regulating Lattice Oxygen of Co3O4/CeO2 Heterojunction Nanonetworks for Enhanced Oxygen Evolution

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In recent years, pressing issues of environmental degradation and depletion of fossil fuels have necessitated the development of renewable clean energy sources. [1,2] Electrocatalytic water splitting has emerged as a promising strategy to address both environmental and energy issues simultaneously. [3,4] However, electrochemical efficiency of oxygen evolution reaction (OER) is limited by considerable overpotential required to complete the four-electron transfer steps involved in this reaction.

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