of the oxide surface such as through interlocking fingers with the underlying balance of the coating thus the presence of hfo 2 increases the stability of the al 2 o 3 generally resulting in at least a two fold improvement in coating life the type of keying or interlocking arrangement which results from the use of hafnium in connection with the present invention is shown by the typical photomicrograph in fig1 at 500 magnifications after 850 hours exposure at 2100 u00b0 f 1150 u00b0 c in air that portion of the coating generally indicated as a is the outer surface portion or oxide scale with b being the aluminide coating portion of the type described in the above mentioned u s pat no 3 667 985 diffused into c the substrate portion of a ni base superalloy sometimes referred to as rene 120 alloy and consisting nominally by weight of 0 17 c 9 cr 4 ti 0 015 b 4 3 al 7 w 2 mo 10 co 3 8 ta 0 08 zr with the balance essentially ni and incidental impurities the irregular interlocking relationship between the oxide scale portion a and the aluminide coating portion b can be seen at the interface between those two portions referring to fig2 in which like primed letters identify like portions the same aluminide coating but without the inclusion of the element hf as in.