Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table I. Operating Parameters and Applications for Acid Dips Acid Type Concentration Range Liquid 5-8% v/v Temperature 75-110O F Powder 4-24 oz/gal (30-180 g/L) 75-110O F (24-43O Time 1-3 minutes C) 1-3 min Agitation Solution movement Solution movement Applications" describes specific acid treatments that supplement the information given in this chapter. ACID DIP This is the simplest type because the requirement is not complicated. The parts are conditioned as follows: alkaline cleaner film is neutralized and light oxide and/or rust is attacked and removed. Many parts fall into this category. This bath contains an inorganic acid, either hydrochloric or sulfuric. It may also include a surfactant to improve wetting of parts by lowering surface tension, accelerate the activation process, and emulsify trace oils and grease. Acid dips generally meet the operating criteria shown in Tables I and II. In the case of processing steel, brass, and copper together, the solution may contain a special inhibitor, preventing immersion copper deposits on steel. For brass activation the acid typically contains a blend of sulfuric and hydro- fluoric acids. Most brass parts have been formed, machined, or stamped; there- fore, up to 4% lead can be incorporated into the metal to facilitate mechanical work. Fluoride is the best additive to dissolve lead smuts, leaving a clean surface for plating. (See Table III.) Copper and copper alloys (beryllium, tellurium, etc.) activate well in formulas containing sulfuric acid and persulfates. This solution provides the preferred light etch to activate and desmut. Most zinc alloys respond well to activation in sulfamic or sulfuric acid blends containing fluorides (preferably ammonium bifluoride). Fluoride effectively dissolves smuts, while the inorganic/organic acid mixture activates the surface. ACID PICKLE This process bath is most readily used to condition steel and stainless steel parts. It can be used as an immersion dip or cathodically. (See Tables IV to VI.) Some metal- Table II. Acid Dip Operating Parameters for Steel & Stainless Steel Acid Hydrochloric Sulfuric Phosphoric % v/v 5-15 7-20 10-20 Wetters, oz/gal 0.01-0.03 0.01-0.03 0.01-0.03 Deflocculents, oz/gal 0.06-0.09 0.06-0.09 0.06-0.09 Table III. Acid Dip Operating Parameters for Nonferrous Metals Sulfuric Acid Metal Brass Copper and Alloys Fluoride source is typically fluoroboric acid or ammonium bifluoride. Sulfamic acid may also be added at 1-2 oz/gal. Zinc Alloys and Castings a 112 Concentration, % v/v 3-5 2-4 1-2 Fluoride, oz/gal 0.2-0.4a — 0.1-0.3 Inhibitors, oz/gal 0.002-0.004 0.002-0.004 0.002-0.004 Persulfate, Wetters, oz/gal oz/gal — 1-3 — 0.01-0.03 0.01-0.03 0.01-0.03

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