The significance of velocity exponents in identifying erosion- corrosion mechanisms

Abstract.- The modes of erosion-corrosion are diverse and may vary from being "erosion-dominated", where erosion of metal is the dominant process, to "corrosion-dominated", where erosion of oxide scale is the dominant process. The intermediate situation in which erosion of transient oxide is the predominant process is termed "erosion-corrosion-dominated " and describes the regime in which continual formation and removal of oxide occurs down to the scale/metal interface. This paper considers some of the recent erosion-corrosion data and evalu-ates the velocity exponents. The critical factors which affect velocity exponents in these environ-ments are identified, and some general principles and provisos are outlined when attempting to use such a technique to identify the mechanism of erosion-corrosion on the material surface. It is shown that the velocity exponents derived for "erosion-dominated " conditions are similar to those evaluated for "ductile " erosion processes. However, for "corrosion-dominated' ' conditions the exponents are significantly lower than those derived for "brittle " erosion processes at room temperature. For "erosion-corrosion-dominated " conditions the situation is more complex with velocity exponents being strongly dependent on temperature, alloy composition and relation-ship between velocity and particle flux. It is concluded that velocity exponents may be use