Influence of plasma-sprayed Mo coating on sulphidation behaviour of Inconel 600 and Nimonic PE11 alloys

Abstract

Sulphidation is a serious problem in many energy conversion systems. Sulphidation attack is particularly severe in environments of low oxygen ( p O2~10 −18 Pa) and high sulphur ( p S2 ~ 10 −1_10−3 Pa) potentials at temperatures above 700°C. Recognizing that refractory metals have a high resistance to sulphidation in reducing environments, their use as overlay coatings in sulphur-containing environments deserves serious consideration. In the adoption of such an approach, Mo has been recognized as a highly sulphidation-resistant metal with a Kp value of 10 −12 g 2 cm −4 s −1 in an atmosphere of p S2 ~ 10 −1 Pa at 750°C. In this study, Mo was deposited on two superalloys, Inconel 600 and Nimonic PE11, using air plasma spraying. The coated specimens were tested at 750°C for up to 168 h in an environment comprising p S2 ~ 10 −1 Pa and p O2 ~ 10 −18 Pa. The sulphidation kinetics (this is not real through-layer kinetics, since the specimens had open edges) were determined by a discontinuous gravimetric method. The exposed samples were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni 3S 2 layer and an inner Cr 2S 3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers—an outer (Fe, Ni) 9S 8 layer, a mid Cr 2S 3 layer and an inner MoS 2 layer. However, after prolonged exposure, the outer layer contained both (Fe, Ni) 9S 8 and Ni 3S 2 on the surface of the uncoated Nimonic PE11. The kinetics results demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substrates, Mo coating led to the development of two sub-scale layers at the early stages of exposure (e.g. up to 5 h). The outer layer consisted of Ni 3S 2 for Inconel 600 and (Fe, Ni) 9S 8 for Nimonic PE11. A MoS 2 layer constituted the inner layer for both materials. After prolonged sulphidation, a Cr 2S 3 layer gradually developed between the outer layer and the inner MoS 2 layer. It is apparent that the use of Mo coating hindered the formation of the Cr 2S 3 layer.