Multimodal in Situ synchrotron Analysis Reveals the Effect of the Oxygen Evolution Reaction on Corrosion of a Ni-Cr-Mo Alloy
Authors: Larsson Alfred , Grespi Andrea , Abbondanza Giuseppe , Eidhagen Josefin , Gajdek Dorotea , Simonov Konstantin , Yue Xiaoqi , Lienert Ulrich , Hegedues Zoltan , Scardamaglia Mattia , Shavorskiy Andrey , Merte Lindsay , Pan Jinshan , Lundgren Edvin 
Published: February 2024

Nanometer-thin and spontaneously formed oxide films, or passive films, govern the corrosion resistance of many advanced alloys [1]. Our current understanding of the structure and chemistry of the passive film is primarily based on ex situ surface analysis using X-ray Photoelectron Spectroscopy (XPS) [2] and Scanning Tunneling Microscopy (STM) [3]. XPS has historically been limited to Ultra High Vacuum (UHV) conditions, so the results do not represent the true passive film/electrolyte interface. To observe corrosion initiation and progression and obtain a fundamental understanding of corrosion mechanisms, there is a need for techniques that can combine in situ capabilities with detailed chemical and structural information from the surface region. We have previously used Ambient Pressure (AP) XPS to study passive film growth [4] [5] and a combination of X-ray Reflectivity (XRR), Grazing Incidence X-ray Diffraction (GI-XRD), X-ray Fluorescence (XRF) to study the passivity breakdown of duplex stainless steel [6].
Ni-based alloys are known for their excellent mechanical properties and corrosion resistance and are used in many demanding industrial environments. Ni is commonly alloyed with Cr and Mo, where Cr is known to improve the corrosion resistance due to the formation of a Cr2O3 oxide film on the surface, while the role of Mo is debated. Commonly used electrochemical techniques for studying passivity breakdown of other alloys may not be applicable to Ni-Cr-Mo alloys because the measured electrochemical current is not only due to corrosion reactions [7]. Ni and Mo are good catalysts for the Oxygen Evolution Reaction (OER) [8, 9], which adds another dimension of complexity to the material system since OER is known to be coupled with dissolution and degradation in other metallic systems [10]. A fundamental holistic understanding is missing regarding how the chemistry and structure of the passive film on Ni-Cr-Mo alloys evolve in realistic aqueous conditions and how that correlates with the onset of dissolution, which determines the breakdown of passivity.
Here we present a comprehensive study combining several synchrotron-based techniques to study the surface region of a Ni-Cr-Mo alloy in NaCl solutions in situ during electrochemical polarization, as shown in Figure 1 a). XRR and AP-XPS were used to investigate the thickness and chemistry of the passive film. GI-XRD was used to determine the change in the metal lattice underneath the passive film. XRF was used to quantify the dissolution of alloying elements. X-ray Absorption Near Edge Structure (XANES) was used to study the chemical state of the dissolved species in the electrolyte and the chemical state of corrosion products formed on the surface. The combination of these techniques allowed us to study the corrosion process and detect the passivity breakdown in situ and correlate it to the onset of OER.
Growth of the passive film and enrichment of Mo6+ oxide was observed in the passive range below 800 mV vs. Ag/AgCl, followed by a drastic increase in the electrochemical current coupled with the formation of a thick film of MoO3, and Cr(OH)3, as seen from the AP-XPS data in Figure 1 b). The current increase at potentials above 800 mV vs. Ag/AgCl coincided with the dissolution of Ni2+, Cr3+, and Mo6+, as seen in Figure 1 c). Quantitative analysis revealed that a substantial part of the measured current was due to oxygen evolution, as shown in Figure 1 d). The experimental techniques and the unique information they provide will be discussed, as well as the role of OER on the passivity breakdown of the Ni-Cr-Mo alloy.

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Figure 1