ALUMINIZING OF IRON NICKEL-BASE ALLOYS FOR RESISTANCE TO HIGH-TEMPERATURE GASEOUS ENVIRONMENTS

Several commercial alloys, 310 stainless, 321 stainless, Incoloy 800H and Incoloy 825, have been pack aluminized at 850, 900 and 970-degrees-C (in view of the potential benefits of Al2O3 scales in protecting alloys in sulphidizing gases at high temperature). The coated specimens have been oxidized in air for up to 1000 h at 750 and 950-degrees-C in order to ascertain their performance, with emphasis on the interdiffusional stabilities of the coatings with respect to the particular substrates, prior to evaluating their usefulness in more aggressive environments. Although several of the coatings were cracked, most of the aluminized systems were relatively flaw-free after the coating treatment. On subsequent oxidation at 750-degrees-C, there was relatively little interdiffusion between the coatings and the substrates and the oxidation rates were low. At the higher temperature, interdiffusion resulted in considerable changes in the underlying substrate, particularly for the 321 stainless systems, although the Al2O3 scales continued to provide oxidation resistance. The extent of interdiffusion increased with decreasing austenite stability in the alloy and decreasing alloy nickel concentration. The most effective systems were developed on Incoloy 825 where interdiffusion was relatively slow due to the more stable austenite matrix and to the presence of molybdenum in the interdiffusion layer between the coating and the substrate. This element can act as a barrier to aluminium diffusion across the layer.