AC electrograining of aluminium in hydrochloric and nitric acid: a comparative study

This paper focuses on the comparison of ac electrograining of aluminium in hydrochloric and nitric acid. Several aspects are considered: the final mor phology obtained as a function of the current density and the acid concentr ation for both systems, the development of the morphology as a function of the applied charge density, the formation of the etch products cmd the anod ic current response obtained in the acid solutions containing chloride or n itrate anions. It is concluded that for both systems. in a current density - acid concentration diagram three different regions with characteristic mo rphologies can be recognised. Some remarkable differences are noted between the two systems. At relatively high concentrations, hydrochloric acid lead s to a highly deep-etched surface while more unattacked surface areas ave o btained in nitric acid. The development of the morphology as a function of the applied charge, the current density and the acid concentration is very complex and different for the two systems. Convection has an effect on the location of the regions of the different morphologies, meaning that the rea ctor type may have a major influence on the graining morphology obtained El ectrochemical measurements, represented in the form of anodic chronoamperom etric diagrams, show that the basic mechanism of anodic pitting is differen t for the two types anions.