Pickled steel

Zinc and Zinc Alloys. Zinc metal is highly reactive in acid solutions such as sulfuric, hydrochloric, and nitric dissolving rapidly at acid concentrations normally used to pickle steel and aluminum. Dilute (1—4%) solutions of these acids can be used with caution to remove zinc oxides. 

Although phosphoric acid can be used for pickling steel, it is seldom used simply for scale removal since it is so expensive and slow in action. Steel plates are often initially descaled in sulphuric acid and then, after rinsing, immersed in 2% phosphoric acid containing 0-3-0-5% iron at 85°C for 3-5 min. The plates are then allowed to drain and dry without further... 

Acid pickling. Steel products are immersed in heated acid solutions to remove surface scale during pickling operations. This generates wastewater from three sources ... 

Ferric chloride is largely produced as a by-product of pickling steel and of titanium dioxide pigment production. It can also be produced by direct methods. The traditional method, known as direct chlorination, produces anhydrous ferric chloride and involves the reaction of dry chlorine with red-hot iron. 

Sulfuric acid is number one in synthetic chemical production. It is used to produce phosphate fertilizer, high octane gasoline, and a wide variety of inorganic and organic chemicals. Large quantities are consumed to pickle steel (cleaning and removal of surface oxides) disposal of spent pickling liquor can be a problem. 

Rolled or pickled steels corrode faster than machined or polished steels. The mill scale on the sample must be removed before exposure to avoid the possibility of mill scale acting as cathode. The mill scale may initially give protection against atmospheric corrosion, but scale tends to crack and spall under shock conditions, resulting in bare spots on the sample exposed to the atmosphere. The vertical plates are attacked more slowly than plates at 45°. 

Steel pickling to remove oxides and scales is accomplished through solutions of H2SO4, HCl, or hydrofluoric acid. The pickled steel is then rinsed with water and coated with oil before proceeding to the next step in the process. 

Great quantities of sheet, plate, coil stock, and other forms of metal are pickled in production mills or by metal recyclers. On steel products, the goal is the removal of the mill scale formed on the surface during hot processing (> 575°C). The bath is an aqueous hot hydrochloric, sulfuric, or less frequently phosphoric add solution containing an inhibitor. The inhibitor must allow uniform attack on the metal, as little metal loss as possible, and when pickling steel, must leave a clean surface free of carbon smut. 

Oxide Removal. A li tly oxidized joint area may also be deaned by brushing with a stainless steel wire brush or by draw filing. When weld corrosion resistance is important, however, these dean-ing methods should be followed immediately by add pickling. Steel wool or abrasives should never be used, because of the danger of contamination. 

These specifications cover alkyd painting systems for commercial blast cleaned or pickled steel. These systems are suitable for use on parts or structures exposed in Environmental Zones lA (interior, normally dry) and IB (exterior, normally diy). The color of the finish paint must be specified. 

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