Compatibility of aluminide-coated Hastelloy X and Inconel 617 in a simulated gas-cooled reactor environment

Abstract

Commercially prepared aluminide coatings on Hastelloy X and Inconel 617 substrates were exposed to controlled-impurity helium at 850 and 950 °C for 3000 h. Optical microscopy, scanning electron microscopy (SEM), electron microprobe profiles and SEM X-ray mapping were used to evaluate and compare exposed and unexposed control samples. Four coatings were evaluated: aluminide, aluminide with platinum, aluminide with chromium, and aluminide with rhodium. With extended time at elevated temperature, nickel diffused into the aluminide coatings to form ε phase (Ni 3Al). This diffusion was the primary cause of porosity formation at the aluminide-alloy interface. Aluminide coatings with chromium (HI-15) showed the lowest coating-substrate interface porosity and intermetallic phase growth. The presence of excess chromium in the HI-15-aluminide-coated Hastelloy X sample is believed to have compensated partially for the nickel loss by its diffusion into the nickel-depleted coating-substrate transition zone. None of the other coating additions had an obvious effect on the overall behavior of aluminide-coated Hastelloy X or Inconel 617 in the impure helium environment. The exterior of the aluminide coatings was not visually affected by the impure helium. Carbide precipitation below the coating-substrate interface, resulting from carbon ingress from the helium environment, was observed in uncoated control samples. Carburization of aluminide-coated substrates was less extensive than that of uncoated materials and was related to the degree of porosity at the coating-alloy interface.