Deposition of a low-activity type cobalt-modified aluminide coating by slurry aluminizing of a pre-Co-electroplated Ni-based superalloy (IN738LC)

Abstract

Incorporating the Co element into the β-NiAl phase as a third element enhances the hot corrosion and oxidation properties of aluminide coatings. The deposition of Co-modified aluminide coatings is limited to co-deposition by the pack cementation method or a two-step pack cementation process of Co and Al, so far. This study investigates the possibility of depositing a low-activity type of Co-modified aluminide coating by combining the electroplating process with the slurry aluminizing method. To accomplish this objective, different thicknesses of Co-electroplated layers, deposited by a Watts bath, ranging from 5 to 20 μm are slurry aluminized using a slurry consisting of Al particles as the Al source and a PVA aqueous solution as the binder at 1100 °C for 2 h. Formation mechanisms are discussed through slurry aluminizing of 5, 10, and 20 μm thick Co-electroplated layers at the temperature range of 700–1100 °C with 100 °C intervals at zero time. FE-SEM surface morphology, BSE-SEM cross-sectional observation, surface XRD characterization, and EDS elemental analysis are utilized to characterize the coatings. Results show a critical thickness of the Co layer is required to deposit a defect-free low-activity type of Co-modified aluminide coating, which is determined to be ∼10 μm in this study. The final aluminide coating thickness decreases from ∼140 to ∼65 μm by increasing the Co-electroplated layer thickness from 5 to 20 μm. The identified phases on the surface differ from β-NiAl to β-CoAl by increasing the Co-electroplated layer thickness up to critical value. The Void formation occurs by the Kirkendall effect in coatings with more Co-electroplated layer thickness than the critical value, segregating the coatings within the β-CoAl phase.