The difference between pickling, phosphating, and passivation

In an effort to prevent corrosion or simply improve appearance, engineers will often specify an additional finishing treatment process for a portion, or all, of a metal part’s exterior surface after it has been machined or fabricated.

No matter what the reason, each of these metal surface treatment finishing processes plays an important role in the manufacturing process:

Pickling

The definition of pickling

Acid is used to remove oxide scale chemically according to a certain concentration, temperature, and speed, which is called pickling.

Pickling classification

• According to the type of acid: sulfuric acid pickling, hydrochloric acid pickling, nitric acid pickling, hydrofluoric acid pickling. Pickling must choose different media according to the material of the steel, such as pickling carbon steel with sulfuric acid and hydrochloric acid, and pickling stainless steel with mixed acid of nitric acid and hydrofluoric acid.
• According to the shape of steel: wire pickling, forging pickling, steel plate pickling, strip steel pickling, etc.
• According to the type of pickling equipment: tank pickling, semi-continuous pickling, full continuous pickling, tower pickling.

The principle of pickling

Pickling is the process of chemically removing the oxide scale on the metal surface, so it is also called chemical pickling. The iron oxide scale formed on the surface of the steel pipe is an alkaline oxide that is insoluble in water. When they are immersed in an acid solution or sprayed on the surface of the acid solution, these alkaline oxides can undergo a series of chemical changes with the acid.

Because the iron oxide scale on the surface of carbon structural steel or low-alloy steel is loose, porous and cracked, and the iron oxide scale undergoes repeated straightening, stretching, and conveying along with the strip steel on the pickling line, these pores and cracks further increase and expand, so while the acid solution chemically reacts with the iron oxide scale, it also reacts with the base iron of the steel through the cracks and pores. That is to say, at the beginning of pickling, three kinds of chemical reactions between iron oxide scale and metallic iron and acid solution are carried out at the same time.

1. The iron oxide scale reacts chemically with acid and is dissolved (dissolution).
2. Metal iron reacts with an acid to generate hydrogen, which mechanically peels off the iron oxide scale (mechanical peeling action).
3. The generated atomic hydrogen reduces iron oxides to ferrous oxides that are easy to react with acid, and then react with acid to be removed (reduction).

Passivation

The Principle of passivation

The passivation mechanism can be explained by film theory. That is to say, passivation is due to the action of metal and oxidizing substances, which produces a thin and dense passivation film on the metal surface with good covering performance, and can also be firmly adsorbed on the metal surface. Passivation film firmly adsorbed on the metal surface. This film exists as a separate phase, usually a compound of oxidized metal. It plays the role of completely separating the metal from the corrosive medium, preventing the metal from contacting the corrosive medium, so that the metal basically stops dissolving and forming a passive state to achieve the role of anti-corrosion.

Advantages of passivation

1. Compared with the traditional physical sealing method, the passivation process has the characteristics of absolutely no increase in the thickness of the workpiece and no change in color, which improves the precision and added value of the product, and makes the operation more convenient; 　　
2. Since the passivation process is carried out in a non-reactive state, the passivation agent can be added and used repeatedly, so the life is longer and the cost is more economical.　　
3. Passivation promotes the formation of the oxygen molecular structure passivation film on the metal surface, the film layer is dense, the performance is stable, and it has a self-repairing effect in the air at the same time, so compared with the traditional method of applying anti-rust oil, the passivation formed The passivation film is more stable and corrosion-resistant.　　

Most of the charge effects in the oxide layer are directly or indirectly related to the thermal oxidation process. In the temperature range of 800-1250℃, the thermal oxidation process with dry oxygen, wet oxygen, or water vapor has three continuous stages. First, the oxygen in the ambient atmosphere enters the formed oxide layer, and then the oxygen diffuses into the inside through the silicon dioxide. When it reaches the Si02-Si interface, it reacts with silicon to form new silicon dioxide. In this way, the oxygen entry-diffusion-reaction process continues to occur, so that the silicon near the interface is continuously converted into silicon dioxide, and the oxide layer grows into the silicon wafer at a certain rate.

Phosphating

Phosphating treatment is a kind of chemical reaction to form a layer of film (phosphating film) on the surface of a surface treatment process.

The phosphating process is mainly used on the metal surface. The purpose is to provide a protective film for the metal surface to isolate the metal from the air and prevent it from being corroded. It is also used for the primer before the painting of some products. With this layer of phosphorus, The chemical film can improve the adhesion and anti-corrosion ability of the paint layer, improve the decoration and make the metal surface look more beautiful, and can also play a lubricating role in the cold working of some metals.

After phosphating treatment, the workpiece will not be oxidized and rusted for a long time. Therefore, the application of phosphating treatment is very wide, and it is also a commonly used metal surface treatment process, which is more and more used in automobiles, ships, machinery manufacturing, and other industries.

Classification and application of phosphating

Normally, a kind of surface treatment will show one color, but phosphating treatment can show different colors by using different phosphating agents according to actual needs. This is why we often see phosphating The processing is gray, colored, or black.

Iron-based phosphating: After phosphating, the surface will appear iridescent and blue, so it is also called color phosphor. The phosphating solution is mainly made of molybdate, which will form an iridescent phosphate film on the surface of steel materials. It is also mainly used for coating the bottom layer to achieve the anti-corrosion ability of the workpiece and improve the bonding force of the surface coating.