USE OF PVD DEPOSITED TIN COATING IN RETARDING HIGH-TEMPERATURE SULFIDATION

TiN coated Inconel 600 and Nimonic PE11 alloys were exposed to an atmo sphere comprising a high sulphur potential (pS(2) similar to 10(-1) Pa ) and a low oxygen potential (pO(2) similar to 10(-18) Pa) at 750 degr ees 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, parti cularly during the early stages of exposure. For TiN coated Inconel 60 0 the post-exposure analysis by scanning electron microscopy, energy-d ispersive X-ray analysis, X-ray diffraction and glancing angle X-ray d iffraction, showed the formation of an outer layer containing Ni3S2 on the surface of the TiN coating while an inner layer consisting of Cr2 S3 developed at the coating/substrate interface. The double layered sc ale formed on uncoated Inconel 600 consisted of Ni3S2 and Cr2S3. For t he TiN coated Nimonic PE11, sulphide nodules consisting of three layer s - (Fe,Ni)(9)S-8 (outmost) Cr2S3/MoS2 (innermost) were observed to de velop and the TiN coating was sandwiched between (Fe,Ni)(9)S-8 and Cr2 S3. The portion of the TiN coating which was enveloped by those sulphi de nodules became unstable after long-term exposure and subsequently d issociated thereby causing the loss of environmental protection. Moreo ver, the coating was damaged mechanically by the growth of the sulphid e nodules. The scale formed on the uncoated Nimonic PE11 showed the fo rmation of a similar structure in a similar sequence.