Intergranular Corrosion of Pure Aluminium in Relation to the Behaviour of Grain-Boundaries during Melting^*

Abstract

IN two previous papers^1,2 we showed in the case of pure aluminium recrystallized by the ‘critical deformation' method that the rate of etching of the grain boundaries by hydrochloric acid was more influenced by the relative orientations of adjoining crystals than by impurities in the metal itself. Thus the grain boundaries in two aluminium specimens of 99·99 and 99·998 per cent purity were corroded at the same rate in the case of crystals oriented at random, whereas the boundaries separating crystals of nearly identical relative orientations, or straight twin boundaries², were not corroded at all, regardless of the duration of etching and the concentration of the reagent. We have already reported a similar effect³ when studying the micrographic etching of boundary lines in high-purity aluminium.