Effect of Additives on Smut-Layer Formation and Pitting during Aluminum Etching in Hydrochloric Acid

The structure of the smut film and underlying surface morphology of aluminum AA1050 electrograined in hydrochloric acid with additions of acetic acid, citric acid, or disodium phenyl phosphate dihydrate was studied. In addition, a series of titration experiments was undertaken to determine changes to the buffering effect of the electrolyte due to the inclusion of the additives. Characterization of the chemical and structural properties of the etch films was performed by both solid state and solution nuclear magnetic resonance (NMR), while surface morphologies (with and without smut film) were investigated by scanning electron microscopy (SEM). Experimental results indicate that modifications to electrolyte composition by different types of additive can influence the resulting smut-layer formation. Furthermore, the effect of these changes to the chemical nature and smut morphology by the inclusion of additives is to alter the final pitted surface structure of the underlying aluminum. These results clearly demonstrate the importance of smut-film formation on pitting and the combination of NMR and SEM as a useful investigative tool for these types of processes.