Basic principles and classification of phosphating

phosphating is a chemical and electrochemical process to form a phosphate chemical transformation membrane, the formation of phosphate conversion film is called phosphating membraneThe purpose of phosphating is mainly: to provide protection to the substrate metal, to a certain extent to prevent metal corrosion, to paint before the base, improve the adhesion and corrosion resistance of the paint film layer, in the metal cold processing process to reduce the use of lubrication1 basic principles
phosphating processes include chemical and electrochemical reactionsThe phosphating reaction mechanism of different phosphating systems and different materials is more complexAlthough scientists have done a lot of research in this area, it has not been fully clarifiedA long time ago, the phosphating membrane mechanism was simply expressed with a chemical reaction equation:8Fe 5Me (H2PO4) 2-8H2O-H3PO4Me2Fe (PO4) 2.4H2O (membrane) and Me3 (PO4) 4H2O (membrane) 7FeHPO4 (sink) , 8H2 ,Me for Mn, Zn, Machu, etc, Machu and so on, steel in a high temperature solution containing phosphoric acid and dihydrophosphate salt, will form a phosphate sediment composed of grainy phosphate film, and produce phosphate a hydrogen iron sink and hydrogenThis mechanism is rough to explain and cannot fully explain the membrane processWith the gradual deepening of the study of phosphatization, today, scholars agree that the phosphating membrane process is mainly composed of the following four steps:(1) acid erosion makes the surface of the substrate metal H-plus concentration reduced
Fe-2e-Fe2-
2H2-2e-H2(1)(2) promoter (oxidant) accelerationthe hydrogen atom produced by the first reaction of the promoter, the hydrogen atom produced by the promoter oxidizing the first step, further causing a sharp drop in the concentration of the metal surface H It also oxidizes the Fe2 plus in the solution into Fe3 plus multi-stage excital (3) phosphate root
H3PO4H2PO4 -H-HPO42-HPO42-2H-PO43-3H-3)
due to a sharp drop in the concentration of H-plus on the metal surface, resulting in the phosphoric acid root level of desodisbalance to the right to move, and ultimately PO43- (4) phosphate precipitation crystallized into a phosphating film
when the metal surface dissociated PO43-and the solution (metal interface) of metal ions (e.g Zn2 plus, Mn2 plus, Ca2 plus, Fe2 plus) reaches the solubility constant Ksp, the phosphate precipitation Zn?2 Fe2 PO 43-H2-H2-Zn2Fe (PO4)2.4H2O (4)
3Zn2 s 2PO43-4H2O (PO4)2.4H2O (PO4) 2.4H2O (5) phosphate precipitation and water molecules together to form phosphatized crystal core, the crystal core continues to grow into phosphating grains, countless grains close lying together to go to school into phosphating membrane The side reaction of phosphate precipitation will form phosphating sediment
The above mechanism of fe3-PO43-FePO4(6) can not only explain the process of phosphating of zinc, manganese and zinc calcium, but also guide the design of phosphating formulation and phosphating process From the above mechanism can be seen: the appropriate oxidizing agent can improve the speed of reaction (2), lower H-concentration can make the separation balance of phosphate root excitatization reaction (3) easier to move to the right to remove PO43-; Correspondingly, in the actual phosphating formula and process implementation of the surface is: the appropriate strong promoter (oxidant), a higher acid ratio (relatively low free acid, i.e H-concentration), so that the metal surface adjustment to the active point can improve the phosphating reaction speed, can quickly form a membrane at a lower temperature Therefore, in the design of low-temperature rapid phosphating formula generally follow the above mechanism, the choice of strong promoter, high acid ratio, surface adjustment process about phosphating sediment Because the phosphecification sediment is mainly FePO4 to reduce the amount of slag must reduce the production of Fe3 plus, that is, by two methods: reduce the pandering liquid's H-plus concentration (low free acidity) to reduce Fe2-plus oxidation into Fe3 zinc and aluminum phosphating mechanism is basically the same as above The speed of zinc material is fast, the phosphating film is only a single composition of zinc phosphate, and the sediment is very little Phosphorusification of aluminum is generally added to more fluorine compounds, so that it forms AlF3, AlF63-, aluminum phosphatization step gathering and the above mechanism is basically the same 2 phosphatization classification there are many classification methods, but generally according to the phosphating membrane system, phosphating film thickness, phosphating use temperature, promoter type classification 2.1 according to the phospanding membrane system classification
according to phosphating membrane system is mainly divided into: zinc system, zinc calcium system, zinc manganese system, manganese system, iron system, amorphous phase iron system six categories zinc phosphating tank fluid body into: Zn2, H2PO3-, NO3-, H3PO4, promoter and so on The formation of phosphating membrane body composition (steel parts): Zn3 (po4) 2.4H2O, Zn2Fe (PO4) 2.4H2O The phosphated grain is branch-like, needle-like and porous Widely used in painting before the bottoming, corrosion prevention and cold processing reduction lubrication the main components of zinc calcium phosphating tank fluid are: Zn2, Ca2, NO3-, H2PO4,H3PO4 and other additives Forming the main composition of phosphating film (steel parts): Zn2Ca (PO4) 2.4H2O, Zn2Fe (PO4) 2.4H2O, Zn3 (PO4) 2H2O Phosphated grains are tight-grained (sometimes large needle-like grains) with fewer pores Apply to the bottoming and corrosion prevention before coating zinc manganese phosphating tank fluid body consists of: Zn2, Mn2, NO3-, H2PO4,H3PO4 and other additives The main body composition of phosphating film: Zn2Fe (PO4) 2.4H2O, Zn3 (PO4)2.4H2O, (MnFe) 5H2 (PO4) 4H2O, phosphating grain is grain-needle-branch-type mixed crystal, with fewer pores Widely used in pre-paint bottoming, corrosion prevention and cold processing reduction lubrication manganese phosphating tank fluid body consists of: Mn2 plus, NO3-, H2PO4, H3PO4 and other additives The main body of phosphating film is formed on the steel parts: (MnFe) 5H2 (PO4) 4H2O The phospandering film is thick, the pores are small, and the phosphating grain is dense Widely used in anti-corrosion and cold processing reduce slower lubrication the main body of iron phosphating tank fluid: Fe2 plus, H2PO4, H3PO4 and other additives The main composition of phosphating film (steel workpiece): Fe5H2 (PO4) 4.4H2O phosphating film thickness, high phosphating temperature, long processing time, more membrane pores, phosphating grain is grainy Application for corrosion prevention and cold processing to reduce lubrication the main body composition of the amorphous-phase iron phosphating tank fluid: Na plus (NH4 plus), H2PO4, H3PO4, MoO4-(ClO3-, NO3-) and other additives The main composition of phosphated film (steel parts): Fe3 (PO4) 2.8H2OFe2O3 phosphated film thin, micromembrane structure is amorphous lying plane distribution, only used in pre-painted 2.2 according to the thickness of the phosphecin membrane classification
according to the phosphecin film thickness (phosphate film weight), can be divided into sub-lightweight, lightweight, sub-heavyweight, heavyweight four The sub-lightweight film weight is only 0.1 to 1.0g/m2 generally is anamorphic iron phosphating film, only used for pre-painting, especially the deformation of large workpieces before painting the bottoming effect is very good Lightweight film weight 1.1 to 4.5g/m2 is widely used in the paint before the bottom, in the corrosion prevention and cold processing industry less application The sub-heavyweight phosphecing film thickness 4.6 to 7.5g/m2 due to the larger film weight, the film is thicker (generally 3 m), less as the pre-paint bottoming (only as a basic non-deformation of the steel parts before the paint bottom), can be used to prevent corrosion and cold processing to reduce the smooth Heavyweight film is significant in 7.5g/m2 not as a pre-paint base, widely used in corrosion prevention and cold processing 2.3 according to the phospanding treatment temperature can be divided into normal temperature, low temperature, medium temperature, high temperature four categories Normal temperature phosphating is non-temperature phosphating The general treatment temperature of low temperature phosphating is 30 to 45 degrees C Medium temperature phosphating is generally 60 to 70 degrees C High temperature phosphatization is generally greater than 80 degrees C The temperature division method itself is not strict, and sometimes there is sub-temperature, sub-high temperature method, depending on the wishes of the individual, but generally follow the above-mentioned division method 2.4 by promoter type
because the phosphecin promoter is mainly only a few, according to the type of promoter is beneficial to the understanding of the tank fluid According to the type of promoter, the phosphating treatment temperature can be determined, such as NO3-promoter is mainly medium temperature phosphatization The promoters are mainly divided into: nitrate type, nitrite type, chlorate type, organic nitride type, niobium acid type and other major types Each promoter type can be used in conjunction with other promoters, with many branch series Nitrate types include: NO3-type, NO3-/NO2-(self-produced type) Chlorate types include: ClO3-, ClO3-/NO3-, ClO3-/NO2- Nitrites include: nitrosinRR-NO2-/ClO3- Palladium types include: MoO4-MoO4-/ClO3-MoO4-/NO3- there are many methods of classification, such as the material can be divided into steel parts, aluminum parts, zinc parts and mixed parts phosphating