Metal Finishing Guide Book

2012-2013

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Table I. Operating Parameters and Applications for Acid Dips Acid Type Concentration Range Temperature Time Agitation Liquid 5-8% v/v 75-110OF 1-3 minutes Solution movement Powder 4-24 oz/gal (30-180 g/L) 75-110OF (24-43OC) 1-3 min 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 hydrofluoric acids. Most brass parts have been formed, machined, or stamped; therefore, 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 metalTable II. Acid Dip Operating Parameters for Steel & Stainless Steel Acid % v/v Wetters, oz/gal Deflocculents, oz/gal Inhibitors, oz/gal Hydrochloric 5-15 0.01-0.03 0.06-0.09 0.002-0.004 Sulfuric 7-20 0.01-0.03 0.06-0.09 0.002-0.004 Phosphoric 10-20 0.01-0.03 0.06-0.09 0.002-0.004 Table III. Acid Dip Operating Parameters for Nonferrous Metals Metal Sulfuric Acid Concentration, % v/v Fluoride, oz/gal Persulfate, oz/gal Wetters, oz/gal Brass 3-5 Copper and Alloys 2-4 0.2-0.4a ��� 0.01-0.03 ��� 1-3 Zinc Alloys and Castings 1-2 0.01-0.03 0.1-0.3 ��� 0.01-0.03 a Fluoride source is typically fluoroboric acid or ammonium bifluoride. Sulfamic acid may also be added at 1-2 oz/gal. 82

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