A NEW DEVICE FOR CHECKING SURFACE CONTAMINATION BASED UPON ELECTRICALMEASUREMENT

Surface treatments, such as plating, painting, marking or assembling w ith adhesives, require reliable surface cleanliness. Industrial worksh ops cannot use sophisticated techniques, and there is a need for fast and reliable tests. Very simple 'drop' or 'marker' tests exist, but th ey provide only 'go - no go' information. Measurements of electrical s urface potential decay have been found to be convenient, and a commerc ial device for metal surfaces is now available, based upon this princi ple. Positive electrical charges are deposited upon the metal through corona discharge, and the resulting surface potential is measured by a n electrostatic probe; its decay is followed over 5-10 s. Usually, the relationship between the logarithm of the potential and the decay tim e is linear; a microcomputer calculates the two parameters of this law , which are related to the nature and thickness of the contaminating l ayer. Flatness of the surface is not required, making it possible to c ontrol parts with various geometries. It is easy to compare the contam ination of a surface with a standard, and to decide whether the surfac e can be considered sufficiently clean. The device is rather small, in expensive and very simple to operate; it is designed for quality contr ol in industry. Developments for plastic parts are under progress, and valuable results have already been obtained, e.g. for ageing of epoxy parts.