Effect of Surface Condition on Steam Oxidation of Martensitic Steels and Nickel-Based Alloys

The steam oxidation behavior of inner tube surfaces in the as-manufactured condition for selected martensitic steels and nickel-based alloys was compared with that of standard laboratory test coupons that were ground to a 600 grit surface finish prior to the steam exposure. Exposures up to 10,000 h in the temperature range of 550°C to 700°C revealed that the as-delivered inner tube surfaces may exhibit substantial internal oxidation as a result of the (solution) heat treatment in the final stages of the manufacturing process. The internal oxides contained mainly chromia in the martensitic steels and alumina and/or titania in the nickel-based alloys. For the latter materials, the internal oxidation, especially of Ti, caused the steam-grown external chromia scales to be generally thinner than those observed on the specimens that were ground prior to oxidation testing. For the martensitic steels, the internal oxidation modified the incorporation of Cr (Mn, Si) into the surface scales, thus suppressing the formation of protective chromia-based surface scales. For all studied materials, the internal oxidation present in the as-received condition may affect the steam oxidation behavior at temperatures in the range of 550°C to 700°C up to 10,000 h exposure time.