Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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produced by 100 ml of a 50% solution of acetic acid in water over a small, open dish. A typical successful exposure is 20 hr above the acetic acid without the appear- ance of green (nickel) corrosion, loss of adherence of the organic coating, or tar- nishing of the brass or copper plate. Sulfur Dioxide/Kesternich (ASTM B 605) A few coatings are so stable that normal corrosion-resistance testing results in unwieldy exposure hours. To yield results in a realistic amount of time, an acidic attack mechanism is used. Two of these tests are the sulfur dioxide (ASTM B 605) and the Kesternich (Volkswagen) test, Volkswagen specification DIN 50018. These procedures normally are limited to tin-nickel and similar inert coatings, or to coatings where a quick detection of a fault or surface imper- fection is required. Both procedures use sulfur dioxide gas at elevated temperatures and humid- ity levels, forming sulfurous acid on the surface of the test specimen. Triple Spot (ASTM A 309) Galvanized steel is protected from corrosion by the sheer thickness of the zinc coating. An estimate of the corrosion resistance can, therefore, be obtained by mea- suring the average thickness of the zinc coating on the steel. The thickness is mea- sured using the "weight-area method" and an inhibited acid for stripping. This indirect method for measuring corrosion resistance of galvanically pro- tected steel is limited to thick coatings (galvanize). Relatively thin coatings, such as electroplate, are not as easily correlated to corrosion performance, because local variation in thickness will yield service failures that are not "pre- dicted" by this method. Electrographic and Chemical Tests for Porosity Pores and cracks in chromium over nickel or nickel over steel can be detected using absorbent paper soaked in chemicals that react with the substrate, such as dimethylglyoxime for nickel substrates and potassium ferricyanide for steel substrates (commonly called the ferroxyl test). The test specimen is covered by the soaked paper and, with pressure from a stainless steel cathode, a small current is passed (the test specimen is anodic). The chemicals react with the substrate at cracks and pores, thereby forming col- ored traces in the paper. Correction to actual service life is difficult. Pores in hard chromium deposits over steel can be detected by applying an acidified copper sulfate solution (50 g/L with a pH 2) for approximately 30 sec- onds. Galvanically deposited copper will appear at areas with pores present. 504

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