Steam oxidation resistance of Ni-Cr thermal spray coatings on 9Cr-1Mo steel. Part 2: 50Ni-50Cr

The present work is focused on the steam oxidation resistance of 50%Ni-50%Cr metallic coatings produced using atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF) spray processes on 9Cr-1 Mo type steel substrate. Thickness of the coatings obtained in the HVOF and APS processes were about 60 and 40 mum respectively. The steam oxidation resistance of the coatings was evaluated at four different temperatures viz., 600, 650, 700 and 750degreesC. The results showed that the thick and denser HVOF coating yielded a better steam oxidation resistance than the thin and porous APS coatings. At prolonged aging (1000 h), the HVOF coating showed the best protection in all tested steam temperatures. APS coating performed satisfactorily well till the 100 h of test duration. But, it started the scale initiation at the coating interface and incorporation of scales occurred at 1000 h of steam oxidation test. The reason for the protectiveness by HVOF coating and failure of APS coatings at prolonged aging are discussed in detail.