Checking Quality of Seal

Question:

 We manufacture parts that have a Type II anodic coating that is 0.4–0.6 mil in thickness. We have devised a quick test for seal quality that is done with a water soluble “magic marker.” The marker is put on the part and then wiped off with water. The mark should disappear if the seal is acceptable. Is this test the best quick way to check quality of seal? M.E.

Answer:

 Your method may seem to work, but it is not the “official” quick test method for checking quality of seal on anodic coatings. The recognized test is the one contained in ASTM B136-84, “Modified Stain Test”. I quote a summary of the procedure from part of that document, “The method comprises contacting the test area of the anodized specimen with nitric acid solution and, after rinsing and drying, applying a special dye solution followed by rinsing and rubbing the test area with pumice powder, drying, and visual examination of the test area for retention of the dye stain. Coatings that exhibit no dye stain or change in color are considered to have passed the test.” For a copy of the standard, visit www.astm.com. There is a charge for these standards even if ordered on line.