Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 348 of 707

plated deposits. Brush plated chromium is softer, since tank plated chromium is generally in the 750 to 1100 D.P.H. range. CORROSION PROTECTION: Brush plated cadmium, lead, nickel, tin, zinc, and zinc-nickel deposits on steel have been salt spray tested per ASTM B-117. When the results were compared with AMS and military specification requirements, the brush plated deposits met or exceeded the requirements for tank electroplates. ELECTRICAL CONTACT RESISTANCE: Brush electroplates are often used to ensure good electrical contact between mating components on printed circuit boards, bus bars and circuit breakers. A low contact resistance is the desired characteristic in these applications. HYDROGEN EMBRITTLEMENT: Cadmium and zinc-nickel plating solutions have been specifically developed for plating or touching up high-strength steel parts without the need for a post-plate hydrogen embrittlement relief bake. Additional testing has shown that brush plated Nickel High Speed likewise shows favorable HE performance. Hydrogen embrittlement testing over the past 20 years has become progressively more stringent. A no-bake, alkaline, brush plating cadmium deposit has passed an aircraft manufacturer's test, which is perhaps the toughest imaginable. The test consisted of the following steps: 1.Prepare six notched tensile samples from SAE 4340, heat-treated to 260- 280 Ksi with 0.010" radius notch. 2.Plate samples with 0.5 to 0.7 mil cadmium 3.Load the bars to 75% of ultimate notched tensile strength. 4.Maintain the load for 200 hours. The evidence acquired to date suggests that brush plating offers low levels of hydrogen contamination of base metals. A REPRESENTATIVE JOB To illustrate how brush plating is performed, consider, as an example, building up a 6" diameter by 2" long bore in a housing which is oversized 0.002" on the radius. The part is too large to be mechanically turned so the operation will be done manually. The part is set up in a position that will be convenient for the opera- tor and permit solutions and rinse to be controlled during processing. The area to be plated is precleaned to remove visible films of oil, grease, dirt and rust. Adjacent areas are solvent cleaned to assure that masking will stick. Areas next to the bore are masked for about two inches. The base material is identified and the supplier's manual is examined to determine the preparatory procedure. If the base material is cast iron, for exam- ple, three steps are involved: electroclean, etch and desmut. Several nickel and copper plating solutions are suitable for this application. If an alkaline copper solution is selected, a nickel preplate or "strike" is required to ensure adequate adhesion; therefore, a five-step preparatory and plating proce- 347

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