An electronoptical study of the conversion coating formed on aluminium in a chromate/fluoride solution

The development of conversion coatings during the immersion of ‘bare’ aluminium, and aluminium covered by a barrier-type anodic film in a chromate/fluoride solution has been studied by transmission electron microscopy of stripped films and ultramicrotomed sections. The conversion coating produced on aluminium is always relatively uniform in thickness and grows at a rate that decreases rapidly with time. Fine randomly sited pathways through the coating apparently have an important role during film growth. During immersion of the barrier film non-uniform dissolution exposes the substrate metal and at sites where the metal is bared conversion coating formation occurs. Consequently, metal reaction and rapid conversion coating growth occur at these sites before all the adjacent barrier film is dissolved. This leads to a roughening of the surface.