THE SIGNIFICANCE OF VELOCITY EXPONENTS IN IDENTIFYING EROSION-CORROSION MECHANISMS

The modes of erosion-corrosion are diverse and may vary from being ''e rosion-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 oxi de 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 e xponents. The critical factors which affect velocity exponents in thes e environments are identified, and some general principles and proviso s are outlined when attempting to use such a technique to identify the mechanism of erosion-corrosion on the material surface. It is shown t hat the velocity exponents derived for ''erosion-dominated'' condition s are similar to those evaluated for ''ductile'' erosion processes. Ho wever, for ''corrosion-dominated'' conditions the exponents are signif icantly lower than those derived for ''brittle'' erosion processes at room temperature. For ''erosion-corrosion-dominated'' conditions the s ituation is more complex with velocity exponents being strongly depend ent on temperature, alloy composition and relationship between velocit y and particle flux. It is concluded that velocity exponents may be us ed only in very specific cases to identify erosion-corrosion mechanism s as the relationship between erosion-corrosion rate and velocity is c omplex and is a function of a wide range of parameters.