Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 118 of 707

has a low hydrogen overvoltage, which can reduce electroplating efficiency. Extended immersion in the acid will further reduce this efficiency. Mass finish- ing may be a better option to remove rust and scale. Gold and Gold Alloys Brightening 1. Alkaline soak clean. 2. Anodic treatment at 6-12 V in a solution consisting of 20 oz/gal sodi- um citrate and 5 oz/gal Rochelle salts, 80-90O F (27-32O C). Gold may be recovered by dummying at low current density onto stainless steel cathodes, also using stainless steel anodes. High Carbon and Case-Hardened Steels High carbon steel can be heavily scaled. Both metals are susceptible to hydrogen embrittlement. In addition pickling high carbon steels may generate heavy sur- face smuts. Solutions containing sulfuric acid, fluorides, organic acid complex- ors, inhibitors, pickle aids, and wetting agents are preferred to process these met- als. The acid bath surface tension is reduced to promote superior wetting of the substrate. Once the scale or smut is removed an inhibitor film forms to prevent additional pickling. This prevents hydrogen embrittlement of critical parts, such as spring steel, and repeated smutting of high carbon steels. In some process cycles a follow-up electrocleaner is required to remove the inhibitor film following the acid treatment. The final preplate dip should be a dilute, 3-5% v/v sulfuric acid solution to neutralize alkaline films and provide a clean, water- break-free surface. Sometimes the parts can be cleaned in an off-line eletropol- ishing or mass finishing step before transfer to the actual process line. 1. Soak clean or preclean as described previously. 2. Anodically clean in an alkaline descaler. 3. Acid dip as described (see appropriate tables for suggested chemical con- stituents and operating conditions). In a double cleaning cycle the first acid may be an anodic electropolishing treatment. Low Carbon Steel 1. Alkaline soak clean. 2. Anodically electroclean (moderate caustic, silicated, complexed, wetted blend). Use at standard operating parameters for rack and barrel. 3. Acid dip in either hydrochloric acid at 5-25% v/v or sulfuric acid at 3-10% v/v. Depending on surface condition, wetters, inhibitors, or deflocculents may be required. Use at standard operating parameters. Magnesium Alloys Removal of corrosion, heavy oxides, and mill scale: 1. Alkaline soak clean. 2. Acid immersion treatment in either of the following solutions: A. Hydrofluoric acid at 20-25% v/v, 75-90O 85% phosphoric acid, 2-3% v/v of 42O F (24-32O C) or B. 20 oz/gal of Bé nitric acid, and 0.2-0.3 oz/gal 117

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