The effect of silicon on the oxidation of a Ni-6 at.% Al alloy in 1 atm of pure O2 at 900°C

The oxidation behavior of a binary Ni-6Al alloy and of three ternary Ni-xSi-6Al alloys containing 2, 4 and 6 at.% Si has been studied at 900 degrees C under 1 atm of pure O-2. The addition of 2 at.% Si to Ni-6Al increases the short-time oxidation rate of Ni-6Al, which is subsequently reduced and becomes similar to that of the binary alloy. However, the presence of this silicon level is already able to stop after some time the coupled internal oxidation of Al+Si by forming a healing oxide layer rich of alumina at the front of internal oxidation. The addition of 4 at.% Si to the same alloy permits a more rapid inhibition of the internal oxidation and the formation of a steady-state, inner alumina-rich scale. Finally, the addition of 6 at.% Si prevents the internal oxidation completely and leads to an earlier growth of a protective oxide layer in contact with the alloy as well as to a further reduction in the scaling rate. The role of Si in promoting the formation of protective scales in comparison with the binary alloy is examined on the basis of an extension to ternary alloys of a criterion proposed by Wagner for the transition between the internal and external oxidation of the most reactive component in binary alloys.