Instability of anodically formed TiO2 layers (revisited)

On the occasion of the 4th Gerischer Symposium, a remarkable Ti anodising experiment first reported in 1978 by Di Quarto, Doblhofer and Gerischer has been reproduced. The key observation of the original experiment was the presence of a fast cell voltage decay occurring when the current during galvanostatic Ti anodising was decreased below a critical, low threshold value. In order to reveal the origin of this instability, which at the time was suggested to be mechanical in nature, chronopotentiometric data have been combined with high resolution in situ curvature measurements, the latter allowing to monitor the internal stress evolution during anodising. Our synchronised in situ electrochemical and curvature measurements have confirmed for the first time the previously assumed sequence of phenomena underlying the reported instability (O2-evolution, oxide swelling, oxide delamination and oxide regrowth) through their mechanical consequences (curvature noise, electrostriction jumps and growth stress transitions). However, no evidence could be obtained as to a possible mechanical origin of the instability itself. Instead, based on detailed SEM and TEM inspection, a link with the microstructural evolution has been establised, involving the presence of anatase micro-crystals in the early stages of anodising. Local gas evolution at these anatase crystals is then believed to lead to the gradual appearance of open, crater-like defects on the oxide surface, which in turn allow for electrolyte penetration during the voltage decay when gas evolution at these surface features has stopped.