Basic principle and classification of phosphating

Phosphating is a process of chemical and electrochemical reaction to form phosphate chemical conversion film, which is called phosphating film The purpose of phosphating is 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 anti-corrosion ability of the paint film; it can be used for antifriction and lubrication in the metal cold working process 1 Basic principle 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, it has not been fully understood In a long time ago, a chemical reaction equation was used to describe the mechanism of phosphating film: 8fe + 5me (H2PO4) 2 + 8H2O + H3PO4 me2fe (PO4) 2.4h2o (film) + me3 (PO4) · 4H2O (film) + 7fehpo4 (sediment) + 8h2 ↑ me as Mn and Zn According to Machu et al., soaking steel in a high temperature solution containing phosphoric acid and dihydrogen phosphate will form a crystalline phosphating film composed of phosphate precipitates, and produce ferrohydrogen phosphate sediment and hydrogen The explanation of this mechanism is rather rough, and it can not completely explain the film forming process With the development of phosphating research, scholars agree that the process of phosphating film formation mainly consists of four steps as follows: ① acid etching reduces the H + concentration on the surface of base metal; ② accelerant (oxidant) accelerates [O] + [H] → [R] + H2O Fe2 + + [O] → Fe3 + + [R] Where [O] is an accelerant (oxidant) and [R] is a reduction product As the accelerant oxidizes the hydrogen atom produced in the first step of the reaction, the speed of reaction (1) is accelerated and the H + concentration on the metal surface drops sharply At the same time, Fe2 + in the solution is oxidized to Fe3 + (3) due to the sharp decrease of H + concentration on the metal surface, the dissociation equilibrium at all levels of phosphate shifted to the right, and finally to PO43 - (4) phosphate precipitates and crystallizes into phosphating film When the dissociated PO43 on the metal surface and the metal ions (such as Zn2 +, Mn2 +, Ca2 +, Fe2 +) in the solution (metal interface) reach the solubility product constant KSP, the phosphate precipitates Zn2 + + Fe2 + + PO43 - + H2O → zn2fe (PO4) 2.4h2o ↓ (4) 3zn2 + + 2po43 - + 4H2O = zn3 (PO4) 2.4h2o ↓ (5) Phosphate precipitates together with water molecules to form phosphating crystal nucleus, and the crystal nucleus continues to grow into phosphating crystal Numerous crystal grains are closely piled up to form phosphating film The side reaction of phosphate precipitation will form the mechanism of phosphating sediment Fe3 + + PO43 - = FePO4 (6) and above, which can not only explain the phosphating film forming process of zinc system, manganese system and zinc calcium system, but also guide the design of phosphating formula and phosphating process It can be seen from the above mechanism that the proper oxidant can improve the speed of reaction (2); the lower H + concentration can make the dissociation equilibrium of phosphate radical dissociation reaction (3) more easily move to the right and dissociate PO43 -; if there is active point surface combination on the metal surface, the precipitation reaction (4) (5) can form phosphate precipitated crystal nucleus without too much supersaturation; the generation of phosphate sediment depends on In reaction (1) and reaction (2), the high concentration of H + in solution and the strong accelerant make the sediment increase Accordingly, in the actual phosphating formulation and process implementation, the surface is: appropriate strong accelerant (oxidant); high acid ratio (relatively low free acid, i.e H + concentration); adjusting the metal surface to have the active point can improve the phosphating reaction speed, and can quickly form film at a lower temperature Therefore, in the formulation design of low-temperature rapid phosphating, the above mechanism is generally followed, such as strong accelerant, high acid ratio, surface adjustment process, etc About phosphating sediment Because phosphating sediment is mainly FePO4, the amount of Fe3 + must be reduced if the amount of sediment is to be reduced That is to say, there are two ways: reducing the H + concentration (low free acidity) of phosphating solution and reducing Fe2 + oxidation to Fe3 + The phosphating mechanism of zinc and aluminum is basically the same The phosphating speed of zinc material is faster, the phosphating film is only composed of zinc phosphate, and there is little sediment The phosphating mechanism of aluminum is basically the same as above There are many classification methods of phosphating, but they are generally classified according to phosphating film forming system, thickness of phosphating film, temperature of phosphating application and accelerator type 2.1 according to the classification of phosphating film system, the phosphating film system is mainly divided into six categories: zinc system, zinc calcium system, zinc manganese system, manganese system, iron system and amorphous phase iron system The main components of zinc phosphating bath solution are: Zn2 +, h2po3 -, NO3 -, H3PO4, accelerant, etc The main components of the phosphating film (iron and steel parts): zn3 (PO4) 2.4h2o, zn2fe (PO4) 2.4h2o The phosphating grains are dendritic, acicular and porous It is widely used in priming, anti-corrosion and antifriction lubrication before painting The main components of zinc calcium phosphating bath solution are: Zn2 +, Ca2 +, NO3 -, H2PO4 -, H3PO4 and other additives The main components of phosphating film (iron and steel parts): zn2ca (PO4) 2.4h2o, zn2fe (PO4) 2.4h2o, zn3 (PO4) 2.4h2o The phosphating grains are compact grains (sometimes large acicular grains) with few pores It is used for priming and anti-corrosion before coating The main body of Zn Mn phosphating bath solution consists of Zn2 +, Mn2 +, NO3 -, H2PO4 -, H3PO4 and other additives The main components of the phosphating film are zn2fe (PO4) 2.4h2o, zn3 (PO4) 2.4h2o and MnFe) 5h2 (PO4) 4.4h2o The phosphating grains are of particle needle dendrite mixed crystal type with few pores It is widely used for priming before painting, anti-corrosion and antifriction lubrication in 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 iron and steel parts: (MnFe) 5h2 (PO4) 4 · 4H2O The thickness of the phosphating film is large, the pores are few, and the phosphating grains are dense It is widely used in anti-corrosion and antifriction lubrication in cold working The main composition of the iron phosphating bath solution: Fe2 +, H2PO4, H3PO4 and other additives Main composition of phosphating film (iron and steel workpiece): fe5h2 (PO4) 4 · 4H2O phosphating film is thick, phosphating temperature is high, treatment time is long, there are many pores in the film, and the phosphating grains are granular It is used for anti-corrosion and antifriction lubrication in cold working The main components of amorphous iron phosphating bath solution are Na +, NH4 +, H2PO4, H3PO4, MoO4 - (ClO3 -, NO3 -) and other additives Main composition of phosphating film (iron and steel parts): Fe3 (PO4) 2 · 8H2O Fe2O3 phosphating film is thin, and the microstructure of the film is plane distribution of amorphous phase, which is only used for priming before painting 2.2 according to the thickness of phosphating film, according to the thickness of phosphating film (weight of phosphating film), it can be divided into four categories: sub lightweight, lightweight, sub heavyweight and heavyweight The weight of the sub lightweight film is only 0.1-1.0g/m2, which is usually amorphous iron phosphating film It is only used for priming before painting, especially for the large deformation workpieces Lightweight film with a weight of 1.1-4.5g/m2 is widely used for primer before paint, but less used in anti-corrosion and cold processing industries The thickness of the sub heavyweight phosphating film is 4.6-7.5g/m2 Because of the heavy film, the film is thick (generally > 3 μ m), and less used as the primer before painting (only as the primer before painting for steel parts which are basically not deformed), it can be used for anti-corrosion and cold working antifriction and lubrication Heavy duty film with a weight of more than 7.5g/m2 is not used as primer before painting, and is widely used for anti-corrosion and cold working 2.3 according to the phosphating treatment temperature, it can be divided into four categories: normal temperature, low temperature, medium temperature and high temperature Phosphating at room temperature is non heating phosphating The general treatment temperature of low temperature phosphating is 30-45 ℃ Phosphating at medium temperature is generally 60-70 ℃ High temperature phosphating is generally higher 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 generally follow the above division method 2.4 according to the type of accelerant, as there are only a few accelerants for phosphating, it is beneficial to understand the bath solution according to the type of accelerant According to the type of accelerant, the phosphating treatment temperature can be generally determined For example, the NO3 accelerant is mainly the middle temperature phosphating Accelerators are mainly divided into: nitrate type, nitrite type, chlorate type, organic nitride type, molybdate type and other major 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 type includes: 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, such as steel parts, aluminum parts, zinc parts and mixed parts.