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A measuring method for thin inorganic coatings on metals has been developed, based on the capacitance of the coating, using an electrical conductive rubber probe as the electrode, pressed against the surface and the substrate as the counter electrode. The reciprocal value of the capacitance is displayed in Angstrom units as the dielectric thickness and by multiplication with the dielectric constant the geometrical thickness is found. If calibration samples, with known thickness, are used, the variable resistance is set to the value of the sample. Coatings of equal nature can then be measured in terms of the geometrical thickness. The optimal range of the instrument is between 500 and 5000 Å units. Below 500 Å, the meter tends to indicate higher values than measured by other means like electro microscopy. Coatings in excess of 5000 Å can be measured provided a high dielectric constant of the coating exists. Most of the work was carried out on aluminum samples on which oxide layers of various thickness values were created.
Barrier layer anodic oxide layers could be measured from 50 Å (3.5V), and except for the very low values a good correlation with the expected coating thickness was obtained. The thickness of hydrated oxide layers formed in hot water is a good example of the application of the method, finding the critical temperature and immersion time at which the bondability is affected. Clear conversion coatings in Alodine 1000 has been successfully measured and the influence of concentration, temperature, and process time can be studied without the need of laborous gravimetrical methods. Results of measurements of phosphoric acid anodic oxide coatings and the influence of voltage and time on coating formation are shown. The range above 5000 Å can be used for chromic acid and sulfuric acid anodized material although hydration differences on samples with varying thicknesses will interfere with accurate readings of the geometrical thickness.
inorganic coatings, metals, measurements, thin coatings, aluminum
Director, Pilotti S. A. Ltd., Birkenhead, Auckland,