Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 112 of 707

chemical surface preparation PICKLING AND ACID DIPPING BY STEPHEN F. RUDY HUBBARD-HALL, WATERBURY, CONN. Acid treatment identifies a process whereby the base metal is subjected to mild, moderate, or aggressive etching. It's reasonable to assume the solution pH is below 2.0. What happens chemically can be illustrated by the reaction between metal, oxide, and acid: Metal Oxide+Acid=Metal Salt+Water Metal+Acid=Metal Salt+Hydrogen (proton) Hydrogen (proton)+Hydrogen (proton)=Hydrogen (gas) The metal, as is, contains an oxide surface layer before immersion in the acid bath. This condition was probably accentuated by a previous reverse electro- cleaning step. The oxide layer hurts the prospective finish two ways: adhesion of electrodeposits to the base metal will be poor, and the metal surface in an oxide condition is a poor conductor. The oxide must be totally and cleanly removed. Depending on the degree of surface acid treatment, this can be done in a single immersion, double immersion, or cathodically in an electrified acid. Acid for- mulations, available in liquid or powder, are available in three common forms: Single constituent, mineral, inorganic type. Combination of two or more acids. May consist of inorganic and organic acids. Single or multiacid combinations, which also contain surfactants, dispersants, and inhibitors. The acids dissolve oxides and smuts. They also remove scales and rusts. Sulfuric acid is perhaps the cheapest of the acids used and it's much less fuming. Hydrochloric acid provides a much better rate of pickling action at lower tem- peratures but does generate more fumes. Hydrochloric acid is also widely used for stripping chrome- and zinc-plated deposits off parts and rack tips. Phosphoric acid must be heated to achieve acceptable pickling activity, and for- mation of light iron phosphate films on the surface may be detrimental if elec- troplating is part of the cycle. Surfactants and wetting agents lower the solution surface tension, permitting the bath's active agents to more readily penetrate into and attack undesirable surface coatings and films. Other functions of wetting agents and surfactants include emulsification of residual oils and grease (not removed in previous alkaline cleaning or carryover) and formation of a thin surface foam blanket to help retard the corrosive effects of fumes and mists. Dispersants prevent redeposition of removed soils. Inhibitors provide two-fold effects: controlled pickling, which prevents excessive surface action, and prevention of immersion deposits in aged, contaminated baths. A review of the acid dips available and their respective activities should help to clarify the solution selection process. This issue of the Metal Finishing Guidebook contains additional discussions, ref- erences, and suggestions for activation and pickling, as well as more detailed infor- mation regarding rinsing, analysis, testing, and related subjects. The chapter "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing 111

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