The iron oxide scales (FeO, Fe3O4, Fe2O3) on the steel surface are all oxides that are insoluble in water. When they are immersed in an acid solution, these oxides will undergo a series of chemical reactions with the acid.

Because the iron oxide scale on the surface of carbon structural steel or low-alloy steel has a loose, porous and cracked structure, and the iron oxide scale undergoes repeated straightening, tensioning, and conveying along with the steel during pickling, these pores are cracked. Further increase and expand. Therefore, while the acid solution chemically reacts with the iron oxide scale, it can also chemically react with the iron of the steel matrix through the penetration of cracks and pores. In other words, at the beginning of pickling, the chemical reactions between all three oxides and metallic iron and the acid solution are carried out at the same time. Therefore, the process of pickling includes the following three aspects:

Dissolution

Various iron oxides in the iron oxide scale on the surface of the steel chemically react with acid to form water-soluble iron salt and dissolve in the acid solution. When pickling with hydrochloric acid or sulfuric acid, iron and ferrous chloride or sulfate that can be dissolved in the acid solution are generated, thereby removing the iron oxide scale from the surface of the strip steel. This effect is generally called dissolution.

Mechanical peeling

In addition to various oxides of iron, the iron oxide scale on the steel surface is also mixed with some metallic iron, and the iron oxide scale is porous, so the acid solution can pass through the pores and cracks of the iron oxide scale and the iron in the iron oxide scale or The matrix iron acts, and correspondingly produces a large amount of hydrogen. The expansion pressure generated by this part of the hydrogen can peel off the iron oxide scale from the surface of the steel. This effect of peeling off the oxide scale through the expansion pressure of hydrogen generated in the reaction is generally called mechanical peeling.

Reduction

When metallic iron reacts with acid, hydrogen atoms are first produced. Part of the hydrogen atoms combine with each other to form hydrogen molecules, which promotes the peeling of the iron oxide scale. The other part of the hydrogen atoms relies on its chemical activity and strong reducing ability to reduce high-valent iron oxides and high-valent iron salts into low-valent iron oxides and low-valent iron salts that are easily soluble in acid solutions.