Metal Finishing Guide Book

2013

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Table IV. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel by Immersion Acid % v/v Temperature, OF Fluorides, oz/gal Wetters, oz/gal Inhibitors, oz/gal Hydrochloric Sulfuric Phosphoric 20-50 20-40 25-45 80-120 90-150 90-150 3-5 1.5-3.0 0.5-2.0 0.01-0.03 0.01-0.03 0.01-0.03 0.002-0.004 0.002-0.004 0.002-0.004 hydrochloric, sulfuric, hydrofluoric, and phosphoric; and deflocculents, wetting agents, and inhibitors. In some immersion or electrolytic applications scales are seen to literally peel off in sheets and rusted surfaces "whiten." The acid pickle, when used in doublecleaning cycles (immersion or electrolytic), is usually the first acid step, followed by the second electrocleaner and a final acid dip. DESCALING ACID This process bath is a step above the acid pickle in regard to chemical strength and application. The general focus is removal of heavy deposits: rusts, scales (hot forging or rolling, casting), and other heat treatments. The bath constituents are similar to the acid pickle, but contain higher levels of accelerators such as fluorides and chlorides. Operating parameters: concentration, temperature, and time, may be increased when compared to the acid pickle. Descaling acids can be used by immersion or electrolytically. These baths are sometimes used to pretreat parts off line before processing in a standard finishing line. The pickling requirement is a critical factor of time, temperature, and concentrations of the components. Times may range from 5 to 30 minutes. This makes the incorporation of an inhibitor very important. Surface pickling can be accomplished with minimal attack on the base metal, while also greatly minimizing hydrogen embrittlement. Stainless steels may be passivated as follows: 40% v/v of 42OBé nitric acid at 75-145OF (24-63OC). Table V. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel Cathodically Sulfuric Acid, % v/v Temperature, Fluorides, F oz/gal O 5-10 90-120 1.5-3.0 Chlorides, oz/gal Wetters, oz/gal Inhibitors, oz/gal 3.0-4.0 0.01-0.03 0.002-0.004 4-6 V. 30-50 A/ft2. Use chemically pure lead anodes or carbon. For any cathodic acid application using carbon anodes in the presence of fluorides, the best resistant grade of carbon to fluoride attack should be used. Table VI. Operating Parameters and Applications for Descaling Steel and Stainless Steel by Immersion Acid Acid Concentration, % v/v 1. Sulfuric or 2. Hydrochloric 80 Temperature, OF Fluorides, oz/gal Chlorides, oz/gal Wetters, oz/gal Inhibitors, oz/gal 7-12 90-160 4.0-6.0 6.0-8.0 0.01-0.03 0.002-0.004 7-12 90-160 4.0-6.0 — 0.01-0.03 0.002-0.004

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