Phosphating is a process of chemical and electrochemical reaction to form a phosphate chemical conversion film, which is called phosphating film. The purpose of phosphating is mainly to provide protection for the base metal and prevent the metal from corrosion to a certain extent; It is used for priming before painting to improve the adhesion and corrosion resistance of the paint film; It is used for antifriction lubrication in metal cold working process.
1. Rationale:
The phosphating process includes chemical and electrochemical reactions. The phosphating reaction mechanism of different phosphating systems and materials is complex. Although scientists have done a lot of research in this area, they have not yet fully understood it. A long time ago, the phosphating film formation mechanism was simply described by a chemical reaction equation:
8Fe+5Me (H2PO4) 2+8H2O+H3PO4Me2Fe (PO4) 2 · 4H2O (membrane)+Me3 (PO4) · 4H2O (membrane)+7FeHPO4 (sediment)+8H2 ↑
Me is Mn, Zn, etc. Machu, etc. believed that steel immersed in a high-temperature solution containing phosphoric acid and dihydrogen phosphate will form a crystalline phosphating film composed of phosphate deposits, and produce phosphate iron hydrogen sediment and hydrogen. The explanation of this mechanism is rather rough and cannot completely explain the film forming process. With the gradual deepening of phosphating research, today, scholars agree that the phosphating film forming process mainly consists of the following four steps:
① Acid etching reduces the H+concentration on the surface of the base metal
Fe – 2e→ Fe2+
2H2－+2e→2[H] (1)
H2
② Accelerating agent (oxidant)
[O]+[H] → [R]+H2O
Fe2++[O] → Fe3++[R]
In the formula, [O] is the accelerator (oxidant), and [R] is the reduction product. Because the accelerator oxidizes the hydrogen atom generated in the first step of the reaction, the speed of reaction (1) is accelerated, which further leads to a sharp drop in the H+concentration on the metal surface. At the same time, Fe2+in the solution is oxidized to Fe3+.
③ Multistage dissociation of phosphate
H3PO4 H2PO4－+H+ HPO42－+2H+ PO43－+3H－ （3）
Due to the sharp drop of H+concentration on the metal surface, the dissociation equilibrium of phosphate at all levels moves to the right, and finally PO43 -.
④ Phosphate precipitates and crystallizes into phosphating film
When PO43 - dissociated from metal surface reaches the solubility product constant Ksp with metal ions (such as Zn2+, Mn2+, Ca2+, Fe2+) in solution (metal interface), phosphate precipitation will be formed
Zn2++Fe2++PO43－+H2O→Zn2Fe(PO4)2·4H2O↓ （4）
3Zn2++2PO43－+4H2O=Zn3(PO4)2·4H2O↓ （5）
Phosphate precipitation and water molecules together form phosphating crystal nucleus, which continues to grow into phosphating grains, and countless grains are closely stacked to form a phosphating film metaphysically.
The side reaction of phosphate precipitation will form phosphating sediment
Fe3++PO43－=FePO4 （6）
The above mechanism can not only explain the phosphating film forming process of zinc series, manganese series and zinc calcium series, but also guide the design of phosphating formula and process. From the above mechanism, it can be seen that appropriate oxidants can improve the speed of reaction (2); Lower H+concentration can make the dissociation equilibrium of phosphate dissociation reaction (3) more easily move to the right to dissociate PO43 -; If there is active point surface binding on the metal surface, the precipitation reaction (4) (5) can form phosphate precipitation nuclei without too much supersaturation; The generation of phosphating sediment depends on reaction (1) and reaction (2). High concentration of H+in solution and strong accelerator will increase the sediment. Accordingly, in the actual phosphating formula and process implementation, the surface is: an appropriate strong accelerator (oxidant); High acid ratio (relatively low free acid, i.e. H+concentration); Adjusting the metal surface to have an active point can improve the phosphating reaction speed, and can quickly form a film at a lower temperature. Therefore, the above mechanism is generally followed in the design of low temperature rapid phosphating formula, and strong accelerator, high acid ratio, surface adjustment process, etc. are selected.
About phosphating sediment. Because the phosphating sediment is mainly FePO4, the amount of Fe3+must be reduced in order to reduce the amount of sediment. That is, two methods are adopted: reduce the H+concentration of phosphating solution (low free acidity) to reduce the oxidation of Fe2+to Fe3+.
The phosphating mechanism of zinc and aluminum is basically the same as above. The phosphating speed of zinc material is fast, and the phosphating film is only composed of zinc phosphate, and there is little sediment. Generally, more fluorine compounds are added to aluminum phosphating to form AlF3 and AlF63 -. The mechanism of aluminum phosphating step polymerization is basically the same as above.
2. Phosphating classification
There are many classification methods for phosphating, but they are generally classified according to phosphating film forming system, phosphating film thickness, phosphating temperature and accelerator type.
2.1 Classification according to phosphating film system
According to the phosphating film forming system, it is mainly divided into six categories: zinc system, zinc calcium system, zinc manganese system, manganese system, iron system and amorphous iron system.
The main components of zinc phosphating bath solution are: Zn2+, H2PO3 -, NO3 -, H3PO4, accelerant, etc. The main composition of the formed phosphating film (steel parts): Zn3 (po4) 2 · 4H2O, Zn2Fe (PO4) 2 · 4H2O. Phosphated grains are dendritic, acicular and porous. It is widely used for priming before painting, anti-corrosion and cold working antifriction lubrication.
The main components of zinc calcium phosphating bath solution are: Zn2+, Ca2+, NO3 -, H2PO4 -, H3PO4 and other additives. The main composition of phosphating film (steel parts): Zn2Ca (PO4) 2 · 4H2O, Zn2Fe (PO4) 2 · 4H2O, Zn3 (PO4) 2 · 4H2O. Phosphated grains are compact granules (sometimes with large needle like grains) with few pores. It is used for priming and anti-corrosion before painting.
The main composition of zinc manganese phosphating bath solution: Zn2+, Mn2+, NO3 -, H2PO4 -, H3PO4 and other additives. The main composition of the phosphating film: Zn2Fe (PO4) 2 · 4H2O, Zn3 (PO4) 2 · 4H2O, (Mn, Fe) 5H2 (PO4) 4 · 4H2O. The phosphating grains are in granular needle dendritic mixed crystal form with few pores. It is widely used for priming before painting, anti-corrosion and antifriction lubrication during cold working.
The main composition of manganese phosphating bath solution: Mn2+, NO3 -, H2PO4, H3PO4 and other additives. The main composition of phosphating film formed on steel parts: (Mn, Fe) 5H2 (PO4) 4 · 4H2O. The phosphating film is thick with few pores, and the phosphating grains are dense. It is widely used in anti-corrosion and cold working antifriction lubrication.
The main composition of iron phosphating bath solution: Fe2+, H2PO4, H3PO4 and other additives. The main composition of the phosphating film (steel workpiece): Fe5H2 (PO4) 4 · 4H2O. The phosphating film is thick, the phosphating temperature is high, the treatment time is long, the film has many pores, and the phosphating grains are granular. It is used for anti-corrosion and cold working antifriction lubrication.
The main components of amorphous iron phosphating bath solution: Na+(NH4+), H2PO4, H3PO4, MoO4 - (ClO3 -, NO3 -) and other additives. The main composition of phosphating film (steel parts): Fe3 (PO4) 2 · 8H2O, Fe2O3. The phosphating film is thin, and the micro film structure is planar distribution of amorphous phase, which is only used for priming before painting.
2.2 Classification according to the thickness of phosphating film
According to the thickness of phosphating film (weight of phosphating film), it can be divided into four types: sub lightweight, lightweight, sub heavyweight and heavyweight. The weight of the secondary lightweight film is only 0.1~1.0g/m2. Generally, it is amorphous iron system phosphating film, which is only used for priming before painting, especially for large deformed workpieces. The lightweight film weighs 1.1~4.5 g/m2, and is widely used for priming before painting, but less used in anti-corrosion and cold processing industries. The thickness of the sub heavy phosphating film is 4.6 ～ 7.5 g/m2. Due to the large film weight, the film is thick (generally>3 μ m) It is less used as the primer before painting (only used as the primer before painting for basically non deformed steel parts), and can be used for corrosion prevention and cold processing to reduce friction and lubrication. The heavy film weighs more than 7.5 g/m2 and is not used as primer before painting. It is widely used for anti-corrosion and cold working.
2.3 Classification according to phosphating treatment temperature
According to treatment temperature, it can be divided into normal temperature, low temperature, medium temperature and high temperature. Normal temperature phosphating is no heating phosphating. The general treatment temperature of low temperature phosphating is 30-45 ℃. Medium temperature phosphating is generally 60~70 ℃. High temperature phosphating is generally greater than 80 ℃. The temperature division method itself is not strict. Sometimes there are sub medium temperature and sub high temperature methods, depending on the wishes of each person, but the above division method is generally followed.
2.4 Classification according to accelerator type
Since there are only a few kinds of phosphating accelerants, it is helpful to understand the bath solution according to the type of accelerants. The phosphating treatment temperature can be generally determined according to the type of accelerant, for example, NO3 accelerant is mainly medium temperature phosphating. Accelerators are mainly divided into nitrate type, nitrite type, chlorate type, organic nitride type, molybdate type and other main types. Each accelerator type can be used together with other accelerators, and there are many branch series. Nitrate type includes: NO3 - type, NO3 -/NO2 - (autogenous type). Chlorate types include: ClO3 －, ClO3 －/NO3 －, ClO3 －/NO2 －. Nitrite includes: nitroguanidine R - NO2 -/ClO3 -. Molybdate type includes MoO4 -, MoO4 -/ClO3 -, MoO4 -/NO3 -.
There are many ways to classify phosphating, for example, it can be divided into steel parts, aluminum parts, zinc parts and mixed parts by material.
2、 Pretreatment before phosphating
In general, phosphating treatment requires that the workpiece surface should be clean metal surface (except for two in one, three in one and four in one). Before phosphating, the workpieces must be pretreated to remove grease, rust, oxide skin and surface adjustment. In particular, the phosphating for priming before painting requires surface adjustment to make the metal surface have certain "activity", so as to obtain a uniform, fine and dense phosphating film, and meet the requirements of improving the adhesion and corrosion resistance of the paint film. Therefore, phosphating pretreatment is the basis for obtaining high-quality phosphating film.
1. Degrease
The purpose of grease removal is to remove the grease and greasy dirt on the surface of the workpiece. Including mechanical method and chemical method. Mechanical method mainly includes manual brushing, sand blasting and shot blasting, flame burning, etc. Chemical method mainly includes solvent cleaning, acid cleaning agent cleaning, strong alkaline solution cleaning and low alkaline cleaning agent cleaning. The following describes the chemical degreasing process.
1.1 Solvent cleaning
The solvent method is generally used to remove grease by non flammable halohydrocarbon vapor method or emulsification method. The most common method is to use trichloroethane, trichloroethylene and perchloroethylene vapor to remove grease. Steam degreasing is fast, efficient, clean and thorough, and has a very good removal effect on all kinds of oil and grease. Adding a certain amount of emulsion into chlorinated hydrocarbons has good effect both in soaking and spraying. Due to the toxicity of chlorinated halogens and high vaporization temperature, as well as the appearance of new water-based low alkaline cleaning agents, solvent steam and lotion degreasing methods are rarely used now.
