INFLUENCE OF PLASMA-SPRAYED MO COATING ON SULFIDATION BEHAVIOR OF INCONEL-600 AND NIMONIC PE11 ALLOYS

Sulphidation is a serious problem in many energy conversion systems. S ulphidation attack is particularly severe in environments of low oxyge n (P-O2 similar to 10(-18) Pa) and high sulphur (p(S2) similar to 10(- 1)-(10-3) Pa) potentials at temperatures above 700 degrees C. Recogniz ing that refractory metals have a high resistance to sulphidation in r educing environments, their use as overlay coatings in sulphur-contain ing environments deserves serious consideration. In the adoption of su ch an approach, Mo has been recognized as a highly sulphidation-resist ant metal with a Kp value of 10(-12) g(2) cm(-4) s(-1) in an atmospher e of p(S2) similar to 10(-1) Pa at 750 degrees C. In this study, Mo wa s deposited on two superalloys, Inconel 600 and Nimonic PE11, using ai r plasma spraying. The coated specimens were tested at 750 degrees C f or up to 168 h in an environment comprising p(S2) similar to 10(-1) Pa and p(O2) similar to 10(-18) Pa. The sulphidation kinetics (this is n ot real through-layer kinetics, since the specimens had open edges) we re determined by a discontinuous gravimetric method. The exposed sampl es were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni3S2 layer and an inner Cr2S3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers - an outer (Fe ,Ni)(9)S-8 layer, a mid Cr2S3 layer and an inner MoS2 layer. However, after prolonged exposure, the outer layer contained both (Fe,Ni)(9)S-8 and Ni3S2 on the surface of the uncoated Nimonic PE11. The kinetics r esults demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substra tes, 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 o f Ni3S2 for Inconel 600 and (Fe,Ni)(9)S-8 for Nimonic PE11. A MoS2 lay er constituted the inner layer for both materials. After prolonged sul phidation, a Cr2S3 layer gradually developed between the outer layer a nd the inner MoS, layer. It is apparent that the use of Mo coating hin dered the formation of the Cr2S3 layer.