This paper investigates the relationship between the surface roughness parameter, Ra, and the erosion of the material surface. Jet impingement tests were carried out on the surface of mild steel plates at an impact angle of 30 degrees to the material surface. The erosion test rig utilised sand as the erodent at a concentration of 2%wt, with average particle size of 270 μm. The average surface roughness Ra, of the test specimens ranged from 0.07 μm to 1.14 μm. The erosion damage was assessed by a unique application of the surface profilometry technique. In addition, by using Fast Fourier Transforms to convert the surface profile scans into wave-number spectra, the changes in the material surface after erosion exposure were easily monitored. The investigation showed that the surface roughness parameter plays a significant role in the erosion process, particularly in the initial stages. The operating conditions of the impinging jet were associated with an equilibrium condition, both for the surface roughness and the eroded depth. As a result, materials having an initial surface roughness value greater than the equilibrium Rae value showed a decrease in surface roughness, whereas those having an initial value less than the equilibrium value showed an increase in surface roughness. This trend was confirmed by the changes in the amplitudes of the wave-number spectra for each plate, over the erosion period. The eroded depth was maximum for the plate having the smallest difference between the initial Ra value and the equilibrium value Rae. This subsequently decreased as the deviation from the equilibrium value was increased.

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