Oxidation resistance of TiAl improved by a CoCrAl coating

Shigeji Taniguchi; Toshio Shibata; Naoyuki Asanuma; Hanyi Luo; Fuhui Wang; Weitao Wu

doi:doi:10.1051/jp4:1993942

Proceedings of the 3rd International Symposium on High Temperature Corrosion and Protection of Materials
Actes du 3ème Colloque International sur la Corrosion et la Protection des Matériaux à Haute Température

J. Phys. IV France 03 (1993) C9-403-C9-410
DOI: 10.1051/jp4:1993942

Oxidation resistance of TiAl improved by a CoCrAl coating

Shigeji Taniguchi¹, Toshio Shibata¹, Naoyuki Asanuma², Hanyi Luo³, Fuhui Wang³ and Weitao Wu³

¹ Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565, Japan
² Graduate School of Engineering, Osaka University, Japan
³ Institute of Corrosion and Protection of Metals, The Chinese Academy of Sciences, Wencui Road 62, 110015 Shenyang, China

Abstract
The oxidation behavior of TiAl coupons coated with a fine-grained Co-30Cr-4Al (mass %) layer of about 30 µm thickness was studied at 1100-1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and acceleration stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The parabolic rate constants agree well with the reported values for alumina-former alloys. Voids are developed mainly at the scale/coating and coating/substrate interfaces as the oxidation proceeds. This is attributable to a slightly loose structure of the coating, its recrystallization during oxidation, and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of microcracks in convoluted scales. At 1400 K the growth of rutile on the scale seems to be additionally contributing to the acceleration.