Contribution to the Study of Localised Corrosion of Aluminium and its Alloys: III. Influence of organic protective coatings

Following previous work, the anodic behaviour of Al in 0·5 M NaCl at pH 2 has been studied in the presence of organic protective coatings. The specimens were chemically or mechanically pretreated in order to obtain different degrees of surface roughness. The organic coatings were applied by dipping or by electrodeposition, the porosity, adhesion and uniformity of the coatings varying according to the type and number of layers of the coating. On the assumption, previously reported, that the slope of the straight line, I = f(E), defines the electrolytic resistance, R_E, which is inversely related to the reactive (exposed) anodic area, the protective capacity of the coatings was found by comparing values of R_E/R′_E where R′_E is the electrolytic resistance of a similarly pretreated bare electrode. The dissolution mechanism remained unchanged in the presence of the organic coatings. The values of R_E were shown to be related to the thickness of the layers, the adhesion, the distribution of pores and the roughness of the metal surface. The passage of the anodic current did not materially modify or damage the coating. The properties of coatings applied by dipping and by electrodeposition were compared.