A new device for checking surface contamination based upon electrical measurement

Abstract

Surface treatments, such as plating, painting, marking or assembling with adhesives, require reliable surface cleanliness. Industrial workshops cannot use sophisticated techniques, and there is a need for fast and reliable tests. Very simple ‘drop’ or ‘marker’ tests exist, but they provide only ‘go — no go’ information. Measurements of electrical surface potential decay have been found to be convenient, and a commercial device for metal surfaces is now available, based upon this principle. Positive electrical charges are deposited upon the metal through corona discharge, and the resulting surface potential is measured by an electrostatic probe; its decay is followed over 5–10s. Usually, the relationship between the logarithm of the potential and the decay time is linear; a microcomputer calculates the two parameters of this law, which are related to the nature and thickness of the contaminating layer. Flatness of the surface is not required, making it possible to control parts with various geometries. It is easy to compare the contamination of a surface with a standard, and to decide whether the surface can be considered sufficiently clean. The device is rather small, inexpensive and very simple to operate; it is designed for quality control in industry. Developments for plastic parts are under progress, and valuable results have already been obtained, e.g. for ageing of epoxy parts.