SURFACE-TOPOGRAPHY AND WETTABILITY OF CHROMIUM

The rate of spreading of lubricating oil on chromium plating is shown to conform to a logarithmic relationship with time and to be affected markedly by the surface topography. The interstices between large nodules provide a capillary action and increase the wettability of the surface, provided they are of a similar dimension to the thickness of the liquid front. Anodic etching of the chromium in the plating bath increases the width of the pre-existing cracks, which enables the oil to penetrate them and improve the wettability of the surface. Extremely fine nodular surfaces contain nodules too small to influence the movement of the liquid front directly but increase wettability by increasing the specific surface energy. The results show that the wettability of chromium plating can be varied over a wide range through a knowledge of topographical effects and control of the deposition conditions.