Metal Finishing Guide Book


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Table I. Operating Parameters and Applications for Acid Dips Acid Type Concentration Range Temperature Liquid Powder 5-8% v/v 4-24 oz/gal (30-180 g/L) 75-110OF 1-3 minutes 75-110OF (24-43OC) 1-3 min Time Agitation Solution movement Solution movement 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 metallic smuts and scales do not respond well to the acid dip. A more aggressive treatment is supplied by the acid pickle. Some heat-treat scales, weld scales, rusts, and oxides (formed by buffing or polishing operations) are readily attacked and removed in an appropriate acid pickle bath. The solution will also perform the functions of the acid dip. A typical acid pickle contains inorganic acids, such as Table II. Acid Dip Operating Parameters for Steel & Stainless Steel Acid % v/v Wetters, oz/gal Deflocculents, oz/gal Inhibitors, oz/gal Hydrochloric Sulfuric Phosphoric 5-15 7-20 10-20 0.01-0.03 0.01-0.03 0.01-0.03 0.06-0.09 0.06-0.09 0.06-0.09 0.002-0.004 0.002-0.004 0.002-0.004 Table III. Acid Dip Operating Parameters for Nonferrous Metals Sulfuric Acid Metal Concentration, % v/v Brass 3-5 Copper and Alloys 2-4 Zinc Alloys and Castings 1-2 Fluoride, oz/gal 0.2-0.4a — 0.1-0.3 Persulfate, oz/gal — 1-3 — Wetters, oz/gal 0.01-0.03 0.01-0.03 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. 78

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