Investigations of burning phenomena during the hard anodising of aluminium by local in-operando temperature measurements

The local heat evolution during sulfuric acid hard anodizing with and without provoked burning is investigated using a custom build setup for local in-situ temperature measurements. Local heating rates are computed from the measurements and compared to the average Joule heating and the heat change of the electrolyte. The development of temperature gradients during anodizing and burning is discussed in view of the formation of thermo-mechanical stress and strain and the influence on the oxide properties and its microstructure. Comprehensive SEM-mapping of the sample surface subjected to burning is performed and the results are analyzed in view of local temperatures as well as thermo-mechanical stress effects during anodizing. Based on the local in-situ temperature measurements an appropriate frequency range for pulse anodizing is defined that may effectively avoid the formation of large temperature differences, prevent burning and guarantee homogenous oxide properties throughout the entire surface. The formation of stress and strain in the oxide layer due to thermal expansion during anodizing is discussed in view of the resulting oxide properties and its microstructure. Finally, the results of this study are discussed in view of industrial anodizing processes including the homogeneity ofoxide properties and the prevention of burning.