(Corrosion Division Morris Cohen Graduate Student Award) Evolution of Passivity and Passivity-Breakdown for Cr and Cr-Containing Alloys
Authors: Choudhary Sanjay , Thomas Sebastian , Birbilis Nick 
Published: February 2024

Among several corrosion prevention strategies for metallic alloys, alloying with so-called ‘passivating’ elements remains the most effective approach. For example, chromium (Cr) remains the key functional alloying element for Fe-based stainless steels, Ni-based Inconel® alloys and Co-based Stellite® alloys, which dominate the family of corrosion resistant alloys (CRAs) used in a wide range of applications. In addition to conventional alloys, recent exploration in the materials community has led to development of an emerging class of alloys, so-called high entropy alloys (known more broadly as multi-principal element alloys (MPEAs)), which contain multiple alloying elements in ‘principal’ fractions – and of which the corrosion performance has not been widely studied to date. It is however challenging to ascertain the key controlling factors responsible for the passivity in MPEAs using conventional electrochemical techniques due to the combination of low sensitivity and the lack of element resolved information. Moreover, the physical and chemical properties of the nano-metric thin passive film upon CRAs are challenging to evaluate using conventional microscopy and spectroscopy. Consequently, several fundamental aspects of passivity of Cr-containing CRAs remain open questions to be elucidated.

The work herein sought to investigate the passivity and dissolution of Cr and Cr-containing alloys, including stainless steel and several MPEAs; primarily using an in-situ electrochemical method called atomic-spectroelectrochemistry (ASEC). ASEC uses a modified flow electrochemical cell coupled with an atomic spectrometer, revealing the real-time and element-resolved dissolution rate during electrochemical testing. In addition to ASEC analysis, the project herein employed a range of electrochemical testing, electron microscopy, X-ray photoelectron spectroscopy, and thermodynamic calculations to explore Cr-based passivity.

Significance between composition and growth kinetics on the passivity of Cr and Cr-containing alloys was established. It was found that the passive films upon Cr and Cr-containing alloys (including MPEAs) obey a direct-logarithmic growth law, consistent with the assumptions of the point defect model. In addition, the results unveiled a complex interdependence of oxidation rate of material and dissolution rate of the passive film during anodic polarization, found to be an important parameter to evaluate barrier properties of passive films. The influence of microstructural features including grain boundaries and chemical heterogeneities on Cr-based passivity was explored for stainless steel 316L; whereas the role of alloying elements and electrolytic on the evolution of compositional morphology and thickness of the Cr2O3-based passive film was studied for MPEAs exhibiting single-phase microstructures. The origin of passive film breakdown and role of potential and chloride ions on the breakdown mechanism were elucidated for stainless steel 316L; whereas transpassive dissolution and origin of secondary passivation were studied for equiatomic CoCrFeNi and AlTiVCr MPEAs.

Such insights advance the knowledge of underlying mechanisms and associated rate controlling factors of passivity in Cr and Cr-containing alloys, aiding towards an ‘oxide-by-design’ paradigm, to guide design of new CRAs.