Moisture induced environmental embrittlement of intermetallics

This paper reviews recent investigations of the kinetic aspects of moisture induced environmental embrittlement of intermetallic alloys. The main kinetic steps of the embrittlement include surface adsorption, oxidation to form atomic hydrogen, diffusion of dissolved hydrogen [H] from surface to bulk, and hydrogen induced embrittlement. The first part of the paper reviews the chemical aspects of surface adsorption and oxidation reactions using thermodynamic approaches and kinetic equations. The second part of the paper reviews the aspects of hydrogen diffusion in fcc and bcc related ordered structures and the hydrogen concentration profile at room temperature. Then, three possible cases (diffusion rate limited, adsorption reaction rate limited, and mixed rate limited) and their related kinetic equations are discussed. The influence of certain important kinetic factors on the hydrogen concentration profile is also considered. The beneficial effect of B additions on environmental embrittlement is illustrated by using the kinetic approach. The third part of the paper reviews experimental results of the environmental embrittlement in aluminides, silicides, and other intermetallics. The temperature and strain rate dependence of embrittlement, the susceptibility of various intermetallics to environmental embrittlement, as well as the effects of alloy composition and test environment, are discussed. The last part of the paper describes alloy design principles used for alleviating moisture induced environmental embrittlement at ambient temperatures. As an example, the steps taken to design FeAl alloys resistant to environmental embrittlement are briefly mentioned. (C) 2001 loM Communications Ltd and ASM International.