Corrosion of alumina/titanium diboride composites in neutral and acidic chloride solutions

This research concerns the study of the corrosion behaviour of monolithic titanium diboride and the composites 50%Al2O3/50%TiB2 (50/50) and 70%Al2O3/30%TiB2 (70/30) in 3.5% NaCl solutions, at pH 7 and 3 (adjusted by HCl additions). The test temperature is 45 ± 1°C. Both electrochemical and chemical techniques are applied. Scanning electron microscope (SEM) observations are also used to investigate the corrosion attack morphology. Polarization curve recording evidences that at short immersion times both composites exhibit a passive behaviour. The stability of these passive conditions decreases at increasing volume fractions of titanium diboride in the material and is minimum in the monolithic material. The analyses of the aggressive solutions during 30 days of immersion of 50/50 under free corrosion conditions are in agreement with the hypothesis that at both pH values passivity is due to the presence of a titanium-containing insoluble surface film, probably constituted by hydrated titanium oxide. SEM observations evidence the presence of a third phase in the composites, beside titanium diboride and alumina, that is constituted by an aluminum borate, formed during sinterization. This phase undergoes a chemical attack during exposures to the aggressive solutions, under free corrosion conditions. The relative importance of chemical towards electrochemical corrosion, as assessed by coupling potentiostatic tests and chemical analyses, increases when passive conditions are maintained.