Use of PVD deposited TiN coating in retarding high temperature sulphidation

TiN coated Inconel 600 and Nimonic PE11 alloys were exposed to an atmosphere comprising a high sulphur potential (PS₂ ∼ 10^-1Pa) and a low oxygen potential (pO₂ ∼ 10-18 Pa) at 750°C for periods up to 72 h. The sulphidation kinetics, determined by a discontinuous gravimetric method, demonstrated that the TiN coating greatly enhanced the sulphidation resistance of the substrates, particularly during the early stages of exposure. For TiN coated Inconel 600 the post-exposure analysis by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and glancing angle X-ray diffraction, showed the formation of an outer layer containing Ni₃S₂ on the surface of the TiN coating while an inner layer consisting of Cr₂S₃ developed at the coating/substrate interface. The double layered scale formed on uncoated Inconel 600 consisted of Ni₃S₂ and Cr₂S₃. For the TiN coated Nimonic PE11, sulphide nodules consisting of three layers - (Fe,Ni)₉S₈ (outmost) Cr₂S₃/MoS₂ (innermost) were observed to develop and the TiN coating was sandwiched between (Fe,Ni)₉S₈ and Cr₂S₃. The portion of the TiN coating which was enveloped by those sulphide nodules became unstable after long-term exposure and subsequently dissociated thereby causing the loss of environmental protection. Moreover, the coating was damaged mechanically by the growth of the sulphide nodules. The scale formed on the uncoated Nimonic PE11 showed the formation of a similar structure in a similar sequence.