Velocity is a critical test variable in erosion, and can easily overshadow changes in other variables, such as target material, impact ลฝ . angle, etc. The effect of velocity on erosion rate was studied in 70-30 brass cold worked and annealed and Fe-C martensite ลฝ . as-quenched and tempered and it was found, as previously shown, that erosion rate is dependent on velocity by a power law, given by ER s kV n . However, the velocity exponent n was found to be target material independent and is governed by test conditions, including particle characteristics and the erosion test apparatus. In addition, n is not dependent on the erosion mechanism. Results from tested as-quenched martensite and tempered martensite showed that the exponent is approximately 2.9 for both materials, even though martensite eroded by a brittle cracking mechanism, while tempered martensite eroded by a plastic deformation mechanism. No difference in the erosion rate relationship was found between the fully annealed and the 70% cold worked brass. The exponent n was found to change over time with nominally the same erosion test conditions, indicating that n is very sensitive to slight changes in erodent particles andror the test apparatus, and that it must be measured periodically if erosion results generated at different times are to be compared. 1