VAPOR-PHASE ALLOY DESIGN OF CORROSION-RESISTANT OVERLAY COATINGS

This paper examines the design of hot corrosion-resistant Ni-Cr-Al-bas ed coatings by co-deposition from multiple sources. Initial studies ex amined the deposition of a wide range of ternary alloy compositions by magnetron sputtering or co-evaporation from multiple EB sources. In t he magnetron sputtering work a segmented target, consisting of pure Ni , Cr and Al, was used. For the co-evaporation studies, binary and pure element source materials were used. In both deposition procedures new coating compositions were produced by mixing these source materials i n the vapour phase. The new coatings were deposited onto pure Al2O3 su bstrates to ensure that no interaction between the coatings and substr ate would occur during the corrosion tests. Low temperature hot corros ion of the novel coating compositions was assessed using an 'ash recoa t' test procedure. Samples were coated in sodium chloride (at 0.75 Mg cm-2) every 20 h and exposed in a flowing air-1% SO2 equilibrated to a n air-SO2-SO3 environment, at 750-degrees-C for times up to 100 h. Pac k aluminised IN738 and uncoated IN738 were included in the test as ref erence materials and these exhibited classic type II hot corrosion mor phologies, as did some of the new coatings tested. Other new coatings resulted in similar deposit chemistries but showed a much lower tenden cy to form corrosion pits. By modelling the corrosion behaviour along quasibinary sections drawn through the Ni-Cr-Al ternary diagram, it ha s been possible to construct an iso-corrosion contour map at 750-degre es-C. These studies have highlighted a new coating composition capable of resisting type II hot corrosion, which on the basis of short time tests offers a six-fold improvement in hot corrosion resistance over c onventional Ni-Cr-Al-Y overlay coating compositions as measured using this 'ash recoat' test procedure.