Acid cleaning and pickling baths

It is normal, in non-automated acid cleaning and pickling baths, for there to be no filtration of the solutions, the solid contaminants being allowed to settle out of suspension. Batch filtration can, however, substantially improve acids life and simplify waste treatment. A typical array of equipment used for pickling solution recycling is shown in Figure 4.35, similar to most of the solution treatment processes described in this Section. 

Water rinses are used in surface preparation operations such as acid pickling, alkaline cleaning, and nickel deposition to remove any process solution film left from the previous bath. A water rinse may also follow the neutralization step. Another common water use is in the ball milling process, which uses water as the vehicle for the enamel ingredients, as a cooling medium, and for cleaning the equipment. Coating application processes normally use wet spray booths to capture oversprayed enamel particles. Water wash spray booths use a water curtain into which the enamel particles are blown and captured. 

The importance of this problem is underlined by the data in Table 4 concerning the behaviour of ARMCO iron in 1 N HCl solutions inhibited with the commercial product Borg P16 at a temperature of 75 °C. Borg P16 is a specific commercial inhibitor of the corrosion of iron and carbon steels in hydrochloric acid solutions and is a derivative of the commercial product Rodine 213. This inhibitor is used both for the acid cleaning of steam generators and for pickling baths. 

An effective surface treatment requires a clean surface. Metal surfaces are cleaned with an alkaline, neutral, or weakly acidic cleaner, an organic solvent, or by pickling with molten-salt baths [5,87]. Fluorinated surfactants in a pickling and descaling bath disperse scum, speed runoff of acid when metal is removed from the bath, and increase bath life [206,207]. The fluorinated surfactant inhibits nascent hydrogen formation and, therefore, prevents embrittlement by hydrogen [208]. 

Fabricated articles are less commonly hot-dip aluminised now than in the past. Coatings are applied after cleaning the work, e.g. pickling in hydrochloric acid in the case of steel and then preheating. The work is then immersed in a molten salt bath, a flux or reducing atmosphere, prior to... 

Pickling as a method of surface preparation is generally carried out by immersing the steel in an acid bath and then rinsing with clean water. It is essentially a works process because it must be carefully controlled. Site application of acid washes, etc, is not recommended. 

Electrolytic Tinplate. Much of the tin mill product is made into electrolytic tinplate (ETP). A schematic of an ETP cross section is given in Figure 1. The steel strip is cleaned electrolytically in an alkaline bath to remove rolling lubricants and dirt, pickled in dilute mineral acid, usually with electric current applied to remove oxides, and plated with tin. It is then passed through a melting tower to melt and reflow the tin coating to form the shiny tin surface and the tin-iron alloy layer, chemically treated to stabilize the surface to prevent growth of tin oxide, and lubricated with a thin layer of synthetic oil. 

Nitric acid is used by the steel industry to remove surface oxides (pickling) of stainless steels, to brighten and clean surfaces after salt-bath descaling and to prepare stainless steel surfaces for corrosion resistance (passivation). 

It is also used for the manufacture of soluble phosphate fertilizers (Chap. 21), of ammonium sulfate for use as a fertilizer, of other sulfates, and in the manufacture of many chemicals and drugs. Steel is usually cleaned of iron rust (is pickled ) by immersion in a bath of sulfuric acid before it is coated with zinc, tin, or enamel. The use of sulfuric acid as the electrolyte in ordinary storage cells has been mentioned (Chap. 14). 

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