Abstract
In this research, microstructure and hot corrosion behaviour of a modified aluminide coating were studied. The coating was formed via a two-step process, Ni/Ni-Co electrodeposition on Ni-base superalloy, Hastelloy-X, followed by slurry aluminising. The obtained coatings with 140 ± 5 μm thickness were characterised by an X-ray diffractometer (XRD) and scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscope (EDS) and elemental mapping before and after the corrosion tests. The results showed the β-NiAl for simple and pure Ni pre-plated, and the β-(Ni,Co)Al for NiCo pre-plated aluminised samples were formed definitely as a predominant phase. The initial electrodeposited layer performed as a Ni reservoir to reinforce aluminide formation. Besides, it reduced outward diffusion of alloying elements such as Cr, Fe, and Mo from substrate alloy as a diffusion retardant. Accordingly, the chemical composition of substrate alloy in pre-plated samples remained almost constant rather than its simple aluminised counterpart during the diffusion coating process. After the hot corrosion test carried out in the salt mixture of Na2SO4/NaCl environment, the results indicated that the presence of Co was beneficial and improved corrosion resistance of the obtained coating even in the Cl-containing salt mixture.
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