THE DEVELOPMENT OF SURFACE-FEATURES ON NICKEL DURING EROSION-CORROSION

The bombardment of metal surfaces under conditions of erosion, or corr osion-affected erosion where the corrosion component is small compared with erosion, leads to substantial physical rearrangement of that sur face. It has been found that impingement normal to the surface induces a random pattern of hills and valleys, whereas impingement at an angl e to the surface produces a pattern of ripples, aligned across the dir ection of the erosive stream. Recently, it has been found that, after long times at high temperature, a hill and valley pattern produced by normal erosive impact transforms into a ripple pattern by alignment of the hills and valleys in each metal grain. In this, the individual gr ains show patterns of ripples that lie at different orientations in th e different grains, suggesting that this is determined by the slip sys tem of the crystal. Currently, this phenomenon is being studied using polycrystalline nickel and a nickel bicrystal in order to determine th e mechanism by which the ripple pattern evolves and which determines t he orientation of the ripples.