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The acids and auxiliary chemicals that dissolve mill scale and rust from hot-rolled steel, clean it before galvanizing, coating or cold rolling, and restore the passive surface of stainless steel after welding and heat treatment. Pickling lines at cold rolling mills, galvanizing plants and stainless fabricators are the main users.
Hot-rolled steel leaves the mill covered in iron oxide scale. Before it can be cold rolled, galvanized, drawn or coated, the scale must be removed, either mechanically (shot blasting, brushing) or chemically by pickling in acid. Hydrochloric acid is the standard for carbon steel strip: it pickles fast at moderate temperature, gives a clean surface, and the spent liquor can be regenerated — roasted to recover hydrochloric acid and iron oxide — in an acid regeneration plant beside a large pickling line. Sulphuric acid, used hot, survives in some wire and tube plants.
Inhibitors are added to the acid so that it attacks the oxide but not the base metal, reducing metal loss and the hydrogen that can embrittle high-strength steel. Degreasers come first, rinses follow, and in a hot-dip galvanizing plant the pickled steel passes through a zinc ammonium chloride flux before it is dipped.
Stainless steel forms a tougher chromium-rich oxide when it is annealed or welded, and a chromium-depleted layer beneath it. It is pickled in mixtures of nitric and hydrofluoric acid — in tanks on annealing and pickling lines, or as pastes, gels and sprays on welds — and then passivated, usually in nitric or citric acid, so that the chromium oxide film that makes it stainless reforms evenly. ASTM A380 and A967 are the standards fabricators work to.
These acids are hazardous: HF in particular is toxic through the skin, and fumes, spills and spent liquor are regulated. In the GCC hydrochloric acid is produced by the region's chlor-alkali plants and supplied in bulk to pickling and galvanizing lines; specialist inhibitors, pastes and passivants are imported and sold by chemical distributors.
| Region | Standard | What it covers |
|---|---|---|
| USA | ASTM A380/A380M ASTM |
Practice for cleaning, descaling and passivation of stainless steel parts, equipment and systems |
| USA | ASTM A967/A967M ASTM |
Chemical passivation treatments for stainless steel parts — nitric and citric acid methods and tests |
| USA | ASTM B912 ASTM |
Passivation of stainless steels using electropolishing |
| International | ISO 1461 ISO |
Hot-dip galvanized coatings on fabricated iron and steel articles — the coating that pickling prepares for |
Grade equivalents and the standards bodies: Steel grades & standards
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