Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 501 of 707

Crevice corrosion: When specimens with complicated geometric shapes are allowed to contact corrosive liquids (water, salt solution), the sharply recessed areas (pores) in the surface of the specimen contact less oxygen than the remaining sur- face due to differences in oxygen diffusion. The metal inside the pores becomes anodic to the bulk. An example would be the crevice underneath the head of a bolt or screw when these are tightened to a nut or other surface. Sandwich corrosion: The gap between joining flat surfaces becomes anodic to the exterior surface. The corrosion products tend to push the joined surfaces apart. An example is the gap between riveted aluminum sheets comprising an aircraft wing or body panel. Poultice corrosion: Accumulated soil on a corrodible surface acts as a "poultice," holding thousands of pockets of electrolyte (water, salt, etc.) onto the surface. Differences in oxygen concentration then act to corrode the surface. An example is the dirt that accumulates on the underside and wheel wells of an automobile. Filiform corrosion: This type of oxygen cell corrosion is peculiar to organic coatings (paints, lacquers, etc.) that are subjected to chipping. The chipped area contacts more oxygen than the metal covered by coating. As the covered metal that is some distance from the chip corrodes and precipitates hydroxides under the paint, a wormlike appearance in the coating develops as the coating is lifted off the base metal. The "worm" traces take straight lines under constant temperature and twist under variant temperature. Chemical Attack The chemical nature of acids and certain chemicals is that they attack (dis- solve) metals. The performance of a coating in resisting attack is determined by subjecting the test specimen to varying concentrations of acids, acid-forming gas- es, or chemical solutions. Acid attack, chemical attack, and electrochemical techniques can also be used to determine the porosity of a coating, which can sometimes be related to service life. Examples of such techniques are the ferroxyl test, electrographic printing, and the copper sulfate test (on hard chrome over steel). TEST METHODS The following are commonly performed accelerated corrosion and porosity tests. Table I summarizes applicability and denotes the most common test used on a given coating/substrate combination. Salt Spray (ASTM B 117) This is the most widely specified corrosion test. It has broad applicability. The salt solution that is utilized closely simulates the corrosive effects of outdoor expo- sure on automotive hardware (except some decorative nickel-chromium appli- cations; see CASS and Corrodkote in Table I). Test results normally are obtained in a few hours for lesser protective systems (phosphate, oil, hard chromium plating) and it may take many days (40-80) for superior systems such as tin-nickel plating, heavy galvanize, and galva- nize/paint combinations. The corrosion mechanisms employed are oxygen concentration cell and gal- vanic effects, accelerated by use of an electrolyte with a chloride content of 5% weight (more has been shown to be unnecessary); elevated temperature; inclination of test specimen; and utilization of a fine-fog mist. 500

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