ELECTROCHEMICAL CHARACTERIZATION OF NEW STAINLESS CU-AL-SN ALLOYS

Electrochemical characterization of new Cu-Al-Sn alloys shows that a threshold value of the aluminium content exists, above which the corrosion behaviour is markedly improved. The polarization resistance at the corrosion potential rises and the current density within the passive range decreases significantly. Alloying with tin is indispensable to achieve these effects: the tin alloying efficiency depends on the Sn/Al atomic ratio. Long term immersion investigations show that the corrosion resistance of alloys with a sufficient Al content and a proper Sn/Al atomic ratio is very strongly improved as a function of time, with no visual change in the colour of the surface. The excellent corrosion resistance in neutral solution was found to be due to a gradual formation of an alumina-type oxide film. In acidic and alkaline solutions, where AL oxides are not stable, the behaviour of the Cu-Al-Sn alloys becomes similar to pure Cu. Although the explanation of the apparent effect of alloying with tin requires further studies, the electrochemical investigations and Auger analysis suggest that the beneficial role of tin addition is not due to the formation of a tin oxide layer but rather to a synergistic action of tin on the alumina-type oxide film.