Protective ability of cerium-containing conversion films on aluminium

Abstract

The options for electroless deposition of conversion films of lanthanum group metal oxides, in particular cerium, as an alternative to chromate conversion films (containing Cr6+) have been studied on aluminium. The main task of the study was to establish the influence of copper ions as a component of electrolytes for conversion treatment on the processes of formation and on the corrosion protective ability of cerium oxide films.It has been ascertained that the electrochemical activation of the aluminium surface in the presence of Cu2+ in the electrolyte, leads to formation of oxide layers richer in cerium and possessing a better protective ability.The presence of active cathodic sections of electroless, contact-deposited Cu has been shown to promote the corrosion activity of the system, due to the possibility of the appearance of micro-galvanic couples Al/Cu. A model experiment has been carried out to establish the inclination of the system to pitting corrosion. It has been shown that at the studied concentration of Cu2+ in the electrolyte the character of the corrosion does not change observably but there is a catalysing effect of the copper ions in the working electrolyte. The copper islands on the Al surface also have a catalysing effect, which is dominating in the processes of cerium oxide film formation. The chemical state and the composition of the conversion films have been defined by XPS measurements.