Metal Finishing Guide Book

2012-2013

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Table IV. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel by Immersion Acid % v/v Temperature, OF Hydrochloric 20-50 Sulfuric 20-40 Phosphoric 25-45 Fluorides, oz/gal Wetters, oz/gal Inhibitors, oz/gal 80-120 3-5 0.01-0.03 0.002-0.004 90-150 1.5-3.0 0.01-0.03 0.002-0.004 90-150 0.5-2.0 0.01-0.03 0.002-0.004 lic 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 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 mini- Table V. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel Cathodically Sulfuric Acid, % v/v 5-10 Temperature, O F Fluorides, oz/gal Chlorides, oz/gal Wetters, oz/gal Inhibitors, oz/gal 90-120 1.5-3.0 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 1. Sulfuric or 2. Hydrochloric 84 Acid Concentration, % v/v Temperature, O F 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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