Repair Joints in Nickel-Based Superalloys With Improved Hot Corrosion Resistance

Abstract

Sample repair joints in the nickel-base superalloys Inconel IN-713 and IN-738 were tested in the laboratory for Type I high temperature hot corrosion (HTHC) resistance at 900°C. The joints were produced using a conventional “wide-gap” brazing process, having a composition similar to IN-718, and a novel powder metallurgy repair technique LPM™ which in this study had a composition similar to alloy IN-738. Metallographic analysis of the resulting structures showed that the IN-718 based repairs, with and without simple aluminide coatings, had suffered extensive intergranular attack of the braze joints. However, the HTHC resistance of cast IN-718 was found to be excellent under identical test conditions. A comparison of the uncoated LPM™ repair joints and cast IN-738 revealed only subtle differences in the morphology of the corrosion products; the maximum depths of attack were similar in each case. Silicon modified aluminide coatings provided a slight reduction in the rate of attack for the IN-738 alloy, while simple aluminide coatings were less resistant to HTHC than the base alloy. Similar results were found for the LPM™ joints, however localized coating penetration was observed in the vicinity of boride particles embedded in the coatings. These differences in behaviour were interpreted with reference to the chemical and structural changes brought about by the use of varying levels of boron as a melting point depressant in the repair layers.