Phosphating heat treatment technology

Phosphating heat treatment is a process of chemical and electrochemical reaction to form phosphate chemical conversion film, which is called phosphating heat treatment film. The main purpose of phosphating heat treatment is to provide protection for the base metal and prevent the metal from being corroded to a certain extent. Used as a primer before painting to improve adhesion and corrosion resistance of film layer; Antifriction and lubrication are used in metal cold working process.

basic principle

The phosphating heat treatment process involves chemical and electrochemical reactions. The reaction mechanism of different phosphating heat treatment systems and different materials is complex. Although scientists have done a lot of research in this area, it is not fully understood. Long ago, a chemical reaction equation was used to simply describe the film formation mechanism of phosphating heat treatment:

8Fe+5Me(H2PO4)2+8H2O+H3PO4 Me2Fe(PO4)2·4H2O(membrane)+Me3(PO4)·4H2O(membrane)+7FeHPO4(sediment)+8H2↑

Me is Mn, Zn, etc. Machu et al. believe that steel soaked in a high temperature solution containing phosphoric acid and dihydrogen phosphate salt will form a crystalline phosphating heat treatment film composed of phosphate precipitation and produce iron mono-hydrogen phosphate sediment and hydrogen gas. The explanation of this mechanism is relatively rough, and can not fully explain the film forming process. With the gradual deepening of the research on phosphating heat treatment, nowadays, various scholars agree that the film forming process of phosphating heat treatment is mainly composed of the following four steps:

(1) The concentration of H+ on the surface of matrix metal was decreased by acid etching

Fe - e - Fe2 + 2

2 H2 + 2 e - H2 (1)

(2) accelerator (oxidant) acceleration

+ - > [R] + H2O

Fe2 + + - > Fe3 + + [R]. (2)

Where, is the accelerator (oxidant), and [R] is the reduction product. As the accelerator oxidizes the hydrogen atoms generated in the first step reaction, the reaction (1) speed is accelerated, which further leads to a sharp decrease in the concentration of H+ on the metal surface. It also oxidizes Fe2+ in solution to Fe3+.

Multistage dissociation of phosphate

H3PO4 H2PO4-+H+ HPO42-+2H+ PO43-+3H- (3)

Due to the sharp decrease of H+ concentration on the metal surface, the phosphoric acid dissociation equilibrium at all levels moves to the right and finally becomes PO43-.

Phosphate precipitation crystallizes into phosphating heat treatment film

Phosphate precipitates are formed when the dissociated PO43- from the metal surface reaches the solubility product constant Ksp for the metal ions in the solution (such as Zn2+, Mn2+, Ca2+, and Fe2+)

Zn2 + + Fe2 + + PO43 - + H2O - Zn2Fe (PO4) 2 · 4 H2O left (4)

3 zn2 + + 2 po43 - h2o + 4 = Zn3 (PO4) 2 · 4 h2o left (5)

Phosphate precipitation and water molecules form phosphating heat treatment crystal nucleus together, and the crystal nucleus continues to grow into phosphating heat treatment grains, and countless grains are compacted into phosphate heat treatment film.

The adverse reaction of phosphate precipitation will form phosphating heat treatment sediment

Fe3 + + PO43 - = FePO4 (6)

The above mechanism can not only explain the film forming process of zinc, manganese and zinc calcium phosphating heat treatment, but also guide the formulation and design of phosphating heat treatment process. It can be seen from the above mechanism that appropriate oxidant can improve the reaction speed of (2); The lower H+ concentration can make the dissociation of phosphate (3) move more easily to the right to dissociate PO43-. If there is active point surface binding on the metal surface, the precipitation reaction (4) and (5) can form phosphate precipitation crystal nucleus without too much supersaturation. The production of phosphating heat treatment sediment depends on reaction (1) and reaction (2). The concentration of solution H+ is high, and the accelerant strength increases the sediment. Accordingly, in the actual phosphating heat treatment formula and process implementation, the surface is: appropriate strong promoter (oxidant); High acid ratio (relatively low free acid, i.e., H+ concentration); By adjusting the metal surface to the active point, the reaction speed of phosphating heat treatment can be increased and the film can be formed rapidly at lower temperature. Therefore, the formulation design of rapid phosphating heat treatment at low temperature generally follows the above mechanism and selects strong accelerators, high acid ratio and surface adjustment procedures.