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topic 5544

Alternate means of measuring conversion coatings


Dear Sir or Ma'am, I am a quality engineer with a sheet metal manufacturing company. We have run across an issue with one of our customers regarding measuring chemical conversion coating on aluminum alloys. Their manufacturing specification is calling for the coating thickness to be between 0.00002-0.00005 inch. From the information that I have and the knowledge of the coating process which is limited to say the least I am questioning if this thickness is even possible. ASTM B449-93 [link by ed. to spec at TechStreet] is referring to coating mass in grams/meter square.

We have an instrument to measure paint and plating thickness (Fisher DualScope) but the ASTM spec it references to is the EDDY Current method and specifically states that chemical conversion coatings are not measurable under this standard. My questions are, is it possible to achieve a coating thickness desired by our customer spec. and can the thickness be measured without having to run test panels and weighing before and after.

Joseph M. Parker
- Smithfield, NC USA


You are quite right; it is not possible to measure such thicknesses without very specialised equipment - electron microscope viewing prepared cross sections I think. The customer may be worried about fine tolerances (?). Chromate coating weights are measured by the strip and weigh method. If the panels are not hot air dried you can strip the coating using 30-40% nitric acid cold. Prepaint coatings are typically 0.3 - 0.8 g/sq metre (c. 30-80 mg/sq ft. R

Roger Bridger
- Croydon UK

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