Preferential Grain Etching of AlMgSi(Zn) Model Alloys

摘要

Preferential grain etching, also referred to as "grainy appearance", "galvanizing" or "spangling", is caused by the presence of Zn in the alkaline etching bath. As little as only a few ppm Zn dissolved in the bath is sufficient to produce the effect (1). The problem may occur in production lines where alkaline etching is used prior to anodizing. A common practice to avoid preferential grain etching is to precipitate zinc by adding sulphides or chromates to the etching bath (1).rnKoroleva et al. (2) investigated preferential grain etching of high purity aluminium on neighbouring grains with surface normals <334>, <225> and <119>. Rounding off to the nearest low-index planes, they found that the largest height difference existed between the {001} plane and the deeper {111} plane. The {112} plane was etched to about 1/3 of this distance from the higher {001} plane. They also measured the open circuit potential of the three grains and found an increasingly negative potential with grain depth after