The significance of velocity exponents in identifying erosion-corrosion mechanisms

The modes of erosion-corrosion are diverse and may vary from being "erosiondominated", 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 evaluates the velocity exponents. The critical factors which affect velocity exponents in these environments 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 relationship between velocity and particle flux. It is concluded that velocity exponents may be used only in very specific cases to identify erosion-corrosion mechanisms as the relationship between erosion-corrosion rate and velocity is complex and is a function of a wide range of parameters.