The oxidation of three ternary Ni-6Si-xAl (x=6, 10, 15, atomic fraction,%) alloys has been studied at 900 degrees C in 0.1 MPa O-2. The three binary Ni-Al alloys formed external NiO scales associated with an internal oxidation of aluminum. On the contrary, all the ternary alloys formed a continuous external scale of alumina directly in contact with the metal substrate, which inhibited the internal oxidation of aluminum and reduced considerably the corrosion rate. Thus, the presence of silicon is very effective to reduce the critical aluminum content needed for the formation of exclusive alumina scales with respect to binary Ni-Al alloys. The third-element effect due to silicon has been interpreted on the basis of an extension of Wagner's criterion for the transition from the internal to the external oxidation of the most reactive component in binary alloys.
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