Hot Corrosion Evaluation of Co--Cr--Al Coatings Modified by Active Elements \Extended abstract

Proc. Int. Conf. Metallurgical Coatings, San Diego, California, U.S.A., Apr. 1980. Effect of addition elements on hot corrosion resistance was investigated for electron beam--physical vapour deposited Al2O3-forming Co--Cr--Al coating compositions on Ni- and Co-base superalloys. Tests were conducted for 500 h at 677 deg C in an environment of diesel fuel modified with 2% S with an air:fuel ratio of 30:1 and contg. 10 ppm sea salt. Specimens were cycled to ambient every 25 h and examined every 75 h, when type 2 hot corrosion attack was found to occur. The following observations were made : type and level of active element addition could influence low temp. hot corrosion resistance; expected beneficial effect of a high Cr level was not observed; up to 2% Yt was beneficial with 0.2% being optimum; 0.4% Hf was beneficial and equivalent to 0.2-1% Yt, but 1.5% Hf was detrimental; 1.6% Si was beneficial; Zr was detrimental except at very low levels; Ti was detrimental at all levels studied.--E.J.S.