The high temperature corrosion behaviour of Hf modified chromo-aluminised coatings produced by a single step process

In this paper the introduction of Cr, Al and Hf into the surface of the RENE 80, INC718 and 2.25Cr-1Mo steel by pack cementation in a single step process has been described. The coated alloys were subject to cyclic hot corrosion and cyclic oxidation test at 870 and 875 degrees C. The surface treated alloys were also sulphidised at 750 degrees C in an environment of H-2/H2S/H2O yielding P-S2 = 10(-6) atm. and P-O2 = 10(-23) atm. The coated and exposed specimens were characterised by optical microscope, SEM, EPMA, EDX and XRD. Cyclic hot corrosion and cyclic oxidation tests showed that the presence of the Cr-Al-Hf on the surface of the alloys enhanced their corrosion resistance. The rare earth element modified chromium aluminide coatings showed superior protectivity. However the Cr-Al-Hf coating on INC718 delayed the formation of the refractory metal sulphides in sulphidising atmosphere and thereby reduced its sulphidation resistance whilst the coating on 2.25Cr-1Mo steel only slightly increased its resistance to sulphidation attack.