Previous work has shown that a low level of sulphate impurity in chromic acid can significantly change the growth rate and the morphology of porous anodic films formed on aluminium at a constant voltage. The changes were associated with incorporation of sulphate into the films. The present study employs electron microscopy to reveal the growth of larger pores and cells at longer times of anodizing than previously used, leading to a duplex film morphology, with finer pores in the outer region. The change in pore size correlates with a reduction in the concentration of sulphate in the film. However, the behaviour of a tungsten tracer indicates that the sulphate does not alter the mechanism of pore generation significantly. From the results of sequential anodizing in sulphate-containing and sulphate-depleted electrolytes, it is suggested that incorporated sulphate ions lead to the generation of a space charge layer, which has an important role in determining the current density.
Elabar, D., Hashimoto, T., Qi, J., Skeldon, P., & Thompson, G. E. (2016). Effect of low levels of sulphate on the current density and film morphology during anodizing of aluminium in chromic acid. Electrochimica Acta, 196, 206–222. https://doi.org/10.1016/j.electacta.2016.01.230