The effects of ion implantation on the initial stages of hot corrosion attack of cast Co22Cr11Al at 700 °C

Abstract

The initiation and early propagation stages of low temperature hot corrosion (LTHC) attack of CoCrAlY-type overlay coatings is under study so that improved coatings can be developed through an understanding of the deterioration mechanisms. In an attempt to resolve the role played by the active element addition ( i.e. yttrium, hafnium etc.), the LTHC of both a cast Co22Cr11Al alloy and ion-implanted Co22Cr11Al alloys was studied. In this paper, results for yttrium-implanted and cobalt-implanted Co22Cr11Al are presented. Implanted and unimplanted alloy surfaces were pre-oxidized for 1 h at 700 °C before exposure to conditions that simulate the LTHC attack that is typically observed on hot-end components of gas turbines operated in a marine environment. Scanning electron microscopy and energy-dispersive X-ray analysis were used to characterize the oxide scales both prior to and after LTHC exposures. The LTHC attack was markedly more severe in the yttrium-implanted surfaces than in the unimplanted or the cobalt-implanted surfaces. Moreover, little difference was observed in the LTHC response between the cobalt-implanted and the unimplanted alloy. Both for yttrium-implanted and cobalt-implanted alloys and for the unimplanted alloy, the β-CoAl matrix phase was preferentially attacked whereas the α-Co solid solution phase showed little attack; the relative degree of LTHC attack on the alloy surfaces increased with exposure time. Mechanisms are proposed to explain the enhanced LTHC attack observed in the yttrium-implanted CoCrAl alloy.