THE OXIDATION MECHANISM OF PT20NI30AL50

Additions of platinum to aluminium oxide forming alloys are known to improve their high temperature corrosion resistance. This is known as the ''precious metal effect''. In this study the effect of platinum on the oxidation mechanism was investigated by comparing the oxidation mechanism of beta-NiAl and Pt20Ni30Al50. This composition agrees with the average composition of a platinum modified aluminide coating. The alloys were oxidized at temperatures from 1000 to 1200-degrees-C. The growth of the oxide scale on the NiAl alloy proceeds both by aluminium and by oxygen diffusion through the scale resulting in growth within the scale. However, on Pt20Ni30Al50 the growth of the scale is limited to the oxide/gas interface due to a predominant aluminium transport through the scale. Therefore the growth stresses are reduced. Another difference is the extensive void formation at the beta-NiAl/oxide interface. This was not observed on the Pt20Ni30Al50 samples. The phenomena mentioned above are the two likely reasons for the improved oxide scale adherence.