Microstructure and hot corrosion behavior of slurry silicon-aluminide coating modified by chromizing and chromium plating on superalloy Rene-80

In this article, the microstructural characteristics of silicon-aluminide coatings with and without Cr applied on Rene-80 nickel-based superalloy substrates are presented and analyzed. To add Cr to the aluminide coating, enriching the surface layer of the alloy with Cr was performed in two ways: (1) chrome plating and (2) pack cementation chromizing. In addition, diffusion annealing of the plated Cr layer was performed with the aim of increasing the depth of the chromium-rich layer. Investigations showed that the addition of Cr to the silicon-aluminide coating causes the presence of Cr in the upper part in the form of chromium silicide precipitates. To provide resistance to hot corrosion, at least 8 wt% of Cr and 10-13 wt% of Si are required in the upper layer of the coating. The amounts of Cr and Si elements in the upper part of both CrSiAl-P and CrSiAl-E coatings were relatively similar and in the range of 12-14 and 13-14 wt%, respectively. The hot corrosion test was carried out for up to 20 thermal cycles, each cycle consisting of placing samples soaked in Na₂SO₄ salt mixture for 5 h at 900 °C and then exposed to air atmosphere. To understand the mechanism of hot corrosion, after 10, 30, 70, and 100 h of hot corrosion, structural and phase analysis was done on the samples. The results showed that adding Cr to the slurry silicon aluminide coatings is more effective than adding it in increasing the hot corrosion resistance of the coatings. Cr-modified silicon aluminide coating in the form of CrSiAl-E plating showed the best high-temperature hot corrosion behavior.