Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Figure 2. Metallographic microscope: overview (A); configuration of the lenses (B); inverted stage (C); and, epoxy-mounted paint film (D). purchase microscopes that have the capability of taking digital micrographs and viewing the pictures on an adjacent monitor. In addition, microscopes have a built-in scale that allows one to immediately measure the length or width of any section of the micrograph. This is particularly valuable when measuring the dry film thickness (DFT) of paints on a variety of substrates, including non-flat sur- faces, such as stucco, concrete, and weathered wood. EXAMPLES OF FAILURE ANALYSIS The following sections are some examples of failure analysis. Each question is im- mediately followed by a scenario setting the scene/tone for the question. 1. Did the contractor comply with the paint specification? A Customer Specification calls for a fabricator to galvanize a steel product, ap- ply an epoxy primer, and followed by a polyurethane top coat. The product was to be exposed to an aggressive marine environment, but after only a few years, the product failed prematurely due to extensive corrosion. The fabricator denied he had done anything wrong and insisted he had followed the Customer's Specifi- cation to the letter. Samples of the failed steel were cross-sectioned and analyzed using a metal- lographic microscope. Figure 3 is a representative image. The microscopic analy- sis shows the fabricator had not followed the Customer Specification. The gal- vanizing was not a uniform, coherent layer of zinc, which may have been due to poor processing procedures. The organic coating system started by applying the 272

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