Formation and structure evaluation of optimized slurry aluminide coating by nickel-cobalt electroplating layer on nickel-base superalloy

In this research, formation and structure evaluation of optimized slurry aluminide coating by applying a pre-coating of Ni/Ni-Co electroplated layer on Ni-base superalloy were studied. First, some pure Ni and Ni–45Co (at. %) layers were deposited on Ni-base superalloy, Hastelloy-X, samples separately by electroplating process at the specified conditions and 22-23 μm thickness. Subsequently, the slurry aluminizing process was carried out on the pre-plated and a not-plated samples via the specified heat treatment. The obtained coatings with 142-148 μm thickness were investigated by X-ray diffractometer and scanning electron microscope equipped by energy-dispersive X-ray spectroscope, and elemental mapping. According to the obtained results, β-NiAl desirable phase in all three samples and β-CoAl in the Ni-Co pre-plated sample were formed, definitely. The pre-plated layer reduced the diffusion of Cr and Fe, up to 4.5 times of magnitude and Mo, up to 2.5 times of magnitude to the upper parts of coatings by forming a diffusion barrier state. Present Ni in the pre-plated layer decreased the consumption of present Ni in the substrate alloy, too. Accordingly, the composition of substrate alloy in the pre-plated samples was less affected by diffusion coating process. Also, the present Co in the pre-plated layer of the Ni-Co pre-plated sample increased its amount, up to 7 times of magnitude in upper parts of the coating. This subject improved the possibility of formation of cobalt aluminide phases which confirmed by the formation of β-CoAl in upper parts of the coating.