Stress-affected and stress-affecting instabilities during the growth of anodic oxide films

The objective of the current paper is to (re-)address the question whether internal stress is a fundamental parameter driving some generic cases of growth instabilities commonly encountered during the growth of anodic oxide films, namely breakdown and pore initiation. This has been done by unraveling possible correlations between a key electrochemical characteristic of the instability event and the internal stress evolution, the latter being measured in situ during the very same anodising experiment. As such, we have been able to make more conclusive statements as compared to the merely speculative arguments in the literature whether these instabilities have a mechanical origin or not. In the case of breakdown, the two well-documented types of breakdown events encountered during galvanostatic Zr anodising were both found to be stress-affected: instantaneous compressive internal stresses were identified as the driving force for both the densifying phase transformation responsible for type-I breakdown, as well as for the buckling-induced delamination events observed during type-II breakdown. Pore initiation in anodic Al2O3 on the other hand was found not be stress-affected. Instead, pore formation is rather believed to induce itself a modification in the mechanical behaviour, and was therefore classified as stress-affecting.