Observations on the dissolution tests for sealing quality of anodic coatings on aluminium

An approach is made towards a better understanding of the influence of some variables on the immersion tests currently in favour for determination of the sealing quality of anodic coatings on aluminium. The paper first discusses the dissolution mechanism of unsealed oxide coatings and that of coatings with different degrees of sealing. Data are then given which show the effects of the dissolved aluminium and of sodium acetate and acetic acid additions on the dissolving power of the so-called “acetic bath” as well as of sodium sulphite and sulphuric acid additions on the so-called “acidified sulphite bath”. The influence of a nitric acid pretreatment on the results obtained from both baths is also discussed. Finally, the effect of ageing on the oxide coating is commented upon.