Iron and steel is easy to oxidize and corrode the metal, its surface generally exists oxide skin and rust.Common oxides on the surface of iron and steel are ferrous oxide (Feo, gray), ferric oxide (Fe,O, red), ferric oxide (Fe,O, H, orange) and ferric oxide (Fe,04).Blue and black) etc. Before hot dip galvanizing, iron rust on steel surface should be removed completely, otherwise it will affect the platingEffect, even produce leakage plating.A. Principle of pickling: the rust of iron and steel caused by atmospheric corrosion is usually ferrous hydroxideFerric hydroxide, the oxide skin produced by high temperature, is mainly ferric tetroxide and ferric trioxideIron.
Iron oxides are easy to dissolve with acid. Taking hydrochloric acid as an example, its reaction is as follows:Ferric tetroxide and ferric oxide are difficult to dissolve in sulfuric acid and hydrochloric acid solutions at room temperature.But when there is iron at the same time, the corrosion electrode is formed.Iron first dissolves and produces hydrogen gas, causing the oxide to fall off the surface of the steel. Hydrogen dissociated,The ferric oxide and ferric oxide were first reduced to iron oxide and dissolved in reaction.Hydrogen is precipitated out during pickling and rust removal, which has adverse effect on steel parts. Because hydrogen is diffusibleIn the metal, hydrogen embrittlement is caused, leading to the reduction of toughness, ductility and plasticity of the metal. The hydrogen pointsWhen the ions escape from the acid solution, they may easily cause acid fog and affect the environment. To overcome these shortcomings,In production, corrosion inhibitor, wetting agent and fog agent are often added into acid lotion to improve.B. Commonly used pickling method: hydrochloric acid is a kind of pickling agent widely used in hot dip galvanizing enterprises.Materials. It's usually 30-32% now on the market, which is the same amount345-372 - g/LHCL. There is an old method of acid washing, which is used directly after the introduction of hydrochloric acid. With the increase of iron content, the acid washing speed becomes slower and slower. At this time, new hydrochloric acid is added to the old acid.To increase the life of the acid. Most manufacturers now dilute hydrochloric acid by 15-20% after it reaches the plant.The rate at which the pickling starts is slightly slower. As the iron content increases faster and faster,When FeCl reaches 200-250g/L, the acid washing rate decreases.