Metal surface phosphating treatment Derusting process Phosphating purpose provides protection to the base metal, to prevent metal corrosion to a certain extent; used for primer before painting, improve the adhesion and corrosion resistance of the paint layer; In the process, it reduces friction and lubrication.
Phosphating treatment of metal surface is a common pretreatment technology. In principle, it should be treated by chemical conversion membrane, which is mainly used for phosphating of metal surface. Phosphorification can also be applied to non-ferrous metals (such as aluminum and zinc).
Metal surface phosphating is a chemical and electrochemical reaction to form a phosphate chemical conversion film. The phosphate conversion film formed is called a phosphate film.
Metal surface phosphating is a simple, reliable, low-cost, and easy-to-use process for greatly improving the corrosion resistance of metal surfaces. Therefore, it is widely used in actual production.
The modern phosphating process is generally degreasing → washing → descaling → surface adjustment → phosphating → washing → drying.
This paper mainly explains the third process in the phosphating treatment of metal surface, the derusting process.
Before the coating, the rust is removed, and the oxide steel is oxidized to produce a hard and brittle oxide scale. For example, hot-rolled steel sheets, heat-treated parts, forgings, and welded parts may have scale. It can be seen that it is very important to remove the scale and yellow rust on the steel surface to effectively protect the coating. There are two types of rust removal methods, one is chemical method and the other is mechanical method.
1. General chemical law
Chemical descaling is generally a pickling method, and an alkali method is also used.
1, pickling corrosion inhibitor
When rust is removed by inorganic acid, hydrogen is precipitated, which leads to "hydrogen embrittlement". In order to prevent the occurrence of hydrogen embrittlement and reduce the corrosion of the substrate, a corrosion inhibitor should be used. The corrosion inhibitor is a polar molecule. It can selectively form a protective film on the active surface of the metal surface to prevent the acid and iron from continuing to react to achieve corrosion inhibition. The corrosion inhibitors currently used are:
1. Some corrosion inhibitors for the processing of scraps in Chinese factories
The composition is complicated, and there are many useless substances mixed in, so there is a problem in quality stability.
2, pyridine and its derivatives
The raw materials of such corrosion inhibitors are separated from the by-products of the coal tar or shellfish refining process, and the main component is a mixture of derivatives of pyridine and quinoline. .
3. Thiourea and its derivatives
Such corrosion inhibitors mostly use thiourea and di-o-tolylthiourea. Such corrosion inhibitors decompose at higher temperatures and have poor resistance to hydrogen embrittlement.
4. Aldehyde-amine polycondensates
This type of corrosion inhibitor is made by polymerizing formaldehyde and aniline in an acidic medium. The properties of this substance are unstable.
2) Pickling In pickling, inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, etc. are generally used, and acid corrosion inhibitors are commonly used to reduce the dissolution of the matrix metal. For special steel or non-ferrous metals, mixed acid or oxalic acid, chromic acid, citric acid, etc. are commonly used.
a) hydrochloric acid pickling
Hydrochloric acid is also a common acid in pickling. The commonly used concentration is 5~15%. The concentration of industrial hydrochloric acid is generally about 36 to 38%, which is a colorless transparent liquid. It is smoked when exposed to air and has a pungent sour taste. Therefore, it is generally diluted twice before use. Although the ferrous ion has a corrosion inhibition effect in hydrochloric acid, when the amount of dissolved iron is increased to 100 to 120 g/L, in order to maintain the pickling ability, an additional acid amount is required. Hydrochloric acid can not be used to clean stainless steel and aluminum alloy, because the chloride ion is a reactive ion that can partially destroy the passivation film, which is the main factor causing small pore corrosion, and the operation is convenient.
b) sulfuric acid pickling
In general, most of the steel is washed with sulfuric acid, and the concentration of 98% concentrated sulfuric acid is diluted and used. The concentration of the sulfuric acid is different, so the derusting ability is also different. In addition, the temperature has a great influence. The commonly used industrial sulfuric acid concentration is 98%, its specific gravity is about 1.84g/ml, concentrated sulfuric acid has strong oxidizing property, it is a colorless oily liquid, odorless, mixed with water to release a lot of heat, sulfuric acid can also be used Clean stainless steel and aluminum alloy parts that cannot be pickled with hydrochloric acid.
c) nitric acid
The general product concentration of concentrated nitric acid is 65~68%, the density is 1.40g/ml, colorless transparent liquid. Nitric acid has a strong corrosive effect on the human body. The rust removal of steel parts before painting is rarely used, but for stainless steel components, nitric acid cleaning is more suitable, and nitric acid is used to clean copper rust, and the effect is particularly good.
d) Phosphate pickling
The price of phosphoric acid is much higher than that of sulfuric acid and hydrochloric acid, so although there are many advantages, it is not commonly used in pickling. Moreover, the pickling effect is slower, so it is not used for descaling, but only for removing thin rust. The problem of hydrogen permeation is small in phosphoric acid pickling and can be ignored.
e) Organic acids
The use of organic acid pickling, higher cost, higher operating temperature, longer time, but also has its unique features, such as complexation. These organic acid cleaning relies mainly on complexation, and the iron ions are dissolved in a complexed form to achieve the purpose of rust removal.
f) hydrofluoric acid
Hydrofluoric acid is highly corrosive and can strongly corrode glass or silicon-containing compounds, and is generally sealed in a polyethylene plastic container. Even when the mass fraction is 0.1%, iron oxide can be dissolved well. The residual liquid is easy to handle and can be neutralized with lime.
2, mechanical method
1) Hand polished
It is carried out using simple tools or sanding materials, and this method is used for rust-removing operations with a small amount of work. The general method is: rust removal with a scraper; rust removal with a polishing paste; and rust removal with a wire brush.
2) Mechanical polishing and rust removal
Use a tool or material that polishes and removes rust to drive it to remove rust. For example, it is fixed on a fixed wheel by sand cloth, sandpaper, abrasive paste, wire brush, etc., and the wheel can be polished on the rust-removing surface when it is rotated close to the rust-removing surface.
3) Sandblasting, shot blasting
The sand or metal particles are sprayed onto the surface of the metal product with high pressure air to derust or descale.
4) Rolling and rust removal
The article and the abrasive are placed together in a drum to rotate or rotate, so that the article is in direct contact with the material, rubbed to remove rust and descaling.
[Article source: Hefei Xianghe Phosphate Coating Co., Ltd. http:// 】
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