Influence of Dichromate Ions on Corrosion Processes on Pure Magnesium

Abstract

The corrosion behavior of Mg is of interest because of its growing use as an alloy in the transportation\nindustry and also because it is a major component of some intermetallic phases in Al alloys, such as the deleterious\nS (Al2CuMg)-phase found in AA2024-T3. Pure Mg corrodes rapidly in a chloride-containing solution and even\ndissolves in water if the surface hydroxide is damaged by scratching the surface, for example. Uniform dissolution is\ndrastically reduced in NaCl solutions (from 0.01 to 0.5 M) with the addition of very dilute concentrations of\ndichromate (10-4 M). However, it is replaced by a strong localized attack in the form of fast filiform-like attack. On a\nlarge-grained sample with a defined defect structure, the attack can be seen to propagate at twin boundaries.\nOrientation imaging microscopy analysis found that corrosion was limited to planes near {0001} orientations with\npropagation being in prismatic directions. Auger electron spectroscopy analysis shows that interaction of chromate\nwith the Mg hydroxide results in incorporation of reduced chromium ions in the hydroxide surface layer. Formation\nof a more resistant surface film could explain the very local nature of the corrosion in this case. The interaction\nbetween dichromate ions and Mg hydroxide can also explain the higher corrosion resistance of S-phase particles in\nchloride solutions containing dilute dichromate, although differences in the surface film formed compared to pure\nMg are observed. Sputter-etching of the surface in order to assess the depth of the attack revealed that very hard or\nisolating corrosion products difficult to sputter are produced along the filiform path and that chromium compounds\nare not integrated in the corrosion products. Focused ion beam sectioning followed by scanning electron microscopy\ninvestigation of the sectioned area, demonstrates the presence of a continuous protective surface film. Adhesion\nbetween the Mg hydroxide and the metal is lost at the location of the corrosion filament, suggesting that the\nmechanism of propagation is similar to filiform corrosion under a coating. The depth of attack is a couple of\nmicrometers with large cracks present within the corroded area that could induce severe surface damage.