Self-Healing Properties of High Entropy Alloys:Insights from Element Resolved Electrochemistry
Authors: Ogle Kevin , Xie Chenyang , Sun Fan , HAN Junsoo 
Published: December 2024

Multi-principal element alloys such as the high entropy alloy family are promising for diverse applications. Navigating the large number of possible element combinations however represents a serious obstacle for the optimization of alloy composition. Progress will require a clear understanding of what each elemental component will add to the overall behavior of the alloy. Corrosion resistance is a particularly interesting case in that the usual techniques of electrochemistry measure only the rate of electron transfer. Element resolved electrochemistry (or AESEC, atomic emission specroelectrochemistry) provides information by (1) allowing us to measure directly and quantitatively the contribution of each element to the electrochemical behavior [1], and (2) allowing us to measure directly the open circuit corrosion rate. These measurement are invaluable for the investigation of complex alloys as it gives us specific information on the role of each alloying component.

In this work we will discuss the phenomenon of repassivation - the ability of a material to reform the passive film following damage to the film. At open circuit, the reaction is driven by the oxidizing strength of the environment. This is an essential property that will often determine the corrosion resistance of the material under a variety of conditions. However, it is difficult to assess by conventional electrochemical methods as no electrochemical current is measured. AESEC provides a direct measurement of the repassivation rate as well as information on which elements are enriching on the surface [2]. In addition to the open circuit corrosion measurements, the chemistry of the alloying elements may be investigated over a wide range of potential using element resolved polarization curves.

A series of alloys were prepared in-house based on the Cantor alloy composition, CoCrFeMnNi. The rate of repassivation in a sulfuric acid electrolyte was determined following the removal of the passive film by electrochemical means. Firstly, we will present results as a function of Cr content and Mn content representing the alloying components that are well known to either enhance (Cr) or degrade (Mn) the formation of the passive film. The we will present results for a series of alloys replacing Mn with an alternative element, including Mo, V, Ti, Al, W and Cu.

The extent to which each alternative element enhances or degrades the spontaneous passivation was investigated quantitatively. Insight into the mechanism of passivation (or degradation) was revealed by determining the extent of selective dissolution or surface enrichment during spontaneous reaction with the electrolyte and as a function of potential during conventional linear polarization experiments. This family of elements covers a wide range of electrochemical behavior and consequently we anticipate that each element will affect the repassivation in a unique manner. In this way, this work may serve as a compendium of mechanisms that may be generalized to other elements showing similar properties.

1) “Atomic emission spectroelectrochemistry: Real time rate measurements of dissolution, corrosion and passivation”, K Ogle, Corrosion,
75
(2019)1398-1419. (Open access)

2) “Spontaneous passivation of the CoCrFeMnNi high entropy alloy in sulfuric acid solution: the effects of alloyed nitrogen and dissolved oxygen”, X Li, P Zhou, H Feng, Z Jiang, H Li, K Ogle, Corrosion Science, (2022)110016.