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

Abstract

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 ͑NMR͒, while surface morphologies ͑with and without smut film͒ were investigated by scanning electron microscopy ͑SEM͒. 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.

Importance of Etch Film Formation During AC Controlled Pitting of Aluminium

“…A further increase of the pH above 9 will result in oxidation of aluminium to AlO -2 ions (reaction 5); these will eventually precipitate as aluminium hydroxide when the pH of the layer close to the surface decreases rapidly [Pourbaix, 1974, Terryn, 1987, Laevers, 1995, Buytaert, 2006, Wilson et al, 2008, Hülser, 1955. The decrease of the pH can be due to protons (H + ) coming from the bulk electrolyte and/or oxygen evolution during the abovementioned reactions.…”

“…An interesting study on the influence of the additives during the electrograining was done by Wilson et al, (2008) who evaluated through NMR studies the differences induced by additives in a hydrochloric acid electrograining solution regarding the smut and the pitting morphology. The addition of DPPD or citric acid was proven to deteriorate or enhance respectively the pit uniformity.…”

“…The addition of acetic acid is shifting the pH in lower values thus producing a smut with very small pores [Wilson, 2008]. The porous smut layer increases the amount of aluminium dissolution or pit initiation resulting in a final surface morphology with less plateau areas [Wilson, 2008].…”

“…The addition of acetic acid is shifting the pH in lower values thus producing a smut with very small pores [Wilson, 2008]. The porous smut layer increases the amount of aluminium dissolution or pit initiation resulting in a final surface morphology with less plateau areas [Wilson, 2008]. On the other hand it was shown that the addition of DPPD shifts the interfacial pH to higher values, reducing the dissolution rate of aluminium; therefore a pitting morphology with few but large pits surrounded by large plateau areas is observed [Wilson, 2008].…”

“…The porous smut layer increases the amount of aluminium dissolution or pit initiation resulting in a final surface morphology with less plateau areas [Wilson, 2008]. On the other hand it was shown that the addition of DPPD shifts the interfacial pH to higher values, reducing the dissolution rate of aluminium; therefore a pitting morphology with few but large pits surrounded by large plateau areas is observed [Wilson, 2008]. Dimogerontakis et al, (2006) noticed that by increasing the current density the amount of produced smut increases as a result of the high concentration of aluminium inside the pits during the anodic half period and of the local increase of the pH during the cathodic half cycle.…”