Revisiting the Influence of Sn in Cu-Al alloys: A Third Element Effect Enabling Stainless Steel Type Aqueous Passivation Behavior
The influence of Sn alloying additions on the aqueous passivation behavior of Cu-Al alloys was revisited and found to function as a new third element effect in acidified 0.1 M Na2SO4 solution. The role of each element during the process of aqueous passivation was investigated using electrochemical and surface-sensitive ex-situ and in-operando spectroscopic techniques. The connection between passivation and the atomic arrangements of atoms in the solid solution was supported by first principles based cluster expansion calculations and Monte Carlo simulations probing the chemical short-range order in the Cu-Al-Sn system. High purity Sn, like high purity Cu, did not passivate in the test environment, whereas high purity Al formed a passive film with a stable passive current density of 0.01 mA/cm^2. Cu-xAl-Sn solid solution alloys where x greater than 18 at.%, containing less than 3 at.% Sn additions exhibited lower corrosion rates than Cu-xAl alloys, brought by Al(III) and Sn(IV, II) unidentified complex oxides formation on the surface. A strong influence of Sn on Al(III) passivation was observed, i.e., strongly suggesting a third element effect type behavior. Possible governing processes explaining the stainless steel type corrosion behavior are discussed, providing insights for exploring novel synergies in the design of corrosion resistant alloys.