The metal workpiece is immersed in the phosphating tank, so that the chemical reaction and physical action on the surface to form a layer of chemical transformation film with special structure and function is called the phosphating process, and the formed chemical transformation film is called the phosphating film.
The porous crystalline structure of the phosphating film determines that the surface area of the workpiece is much larger than the surface area after shot blasting and sandblasting, so the adhesion of the coating is much larger than that of the physical pretreatment. The electrical conductivity of phosphating film is poor, which plays a good inhibitory effect on electrochemical corrosion, and the anti-rust time after phosphating is greatly extended.
The mechanism of phosphating is a complicated mixed reaction process of electrochemical and chemical reactions. To put it simply, after the iron is in contact with the phosphoric acid solution, the iron surface will first undergo a slow reduction reaction with the H+ generated by phosphoric acid ionization to generate ferrous ions. The ferrous ions then react with the dihydrogen phosphate ions ionized from the phosphate solution. Due to the high concentration of dihydrogen phosphate, the reduced ferrous ions immediately react with it to form phosphate, forming a phosphate layer on the surface of the particles. Phosphating process generally includes: degreasing, washing, surface adjustment, phosphating, post-treatment and so on.
The mechanism is simple to understand:
After the sample leaves the main phosphating solution, a large amount of phosphating solution remains on the surface of the sample, which is mainly concentrated in the pores of the phosphating film. If the residual phosphating solution is washed immediately at this time, it is bound to interrupt the continued film formation of the phosphating solution in the pores, making it easy to form microbatteries in the pores and accelerate the corrosion process.
If it is dried naturally, the phosphating liquid remaining in the pores will continue to form a film, making the phosphating deposition process continue in the pores, and the phosphating film thickens in the pores, filling the pores and sealing the phosphating film.
At this stage, the permanent magnet of the drive motor mostly uses the phosphating process, and Baotou Jinshan Magnetic Material has done a process parameter optimization on the phosphating of magnetic steel: DOE of three variables, concentration, temperature and time.
The conclusions are as follows: It is feasible to use phosphoric acid pickling before phosphating of sintered NdFeb magnets. Due to the weak acidity of phosphoric acid, little damage to the matrix, good color consistency after phosphating, and easier to control than nitric acid, it is feasible from the perspective of process. With phosphoric acid pickling, the phosphoric acid time can be appropriately long, although with the increase of time, the irreversible loss value of magnetic flux will increase, but the time does not exceed 3-5min, the irreversible loss value of magnetic flux is acceptable. However, if the time is longer, the irreversible loss value of the magnet magnetic flux will also increase greatly. However, the temperature of the pickling liquid should be strictly controlled during pickling, in principle, it should not exceed 40°C, otherwise it will lead to a significant increase in the irreversible loss of magnetic flux.
There are three common phosphating treatment methods: high temperature phosphating, middle and low temperature phosphating and room temperature phosphating.
With the continuous improvement of science and technology, the phosphating process is becoming more and more mature, the production quality of normal temperature phosphating solution and oxidation accelerator is also getting higher and higher, and the market sales price is getting lower and lower. All these lay a good foundation for the promotion and application of the normal temperature phosphating process. However, the performance parameters of phosphating process at different temperatures also have obvious differences.
GB/T 1376-2020: phosphating films for metals and other inorganic coatings of metals. This standard determines the method of phosphating film requirements.
GB/T 6807-2001: Technical conditions for phosphating of steel workpiece before coating. This standard specifies the classification, technical requirements, inspection methods and acceptance rules of phosphating films before coating steel workpieces.
HB/Z 5080-1996: phosphating process for steel parts, HB5067461-2005: Hydrogen embrittance test for plating process - Part 1 Mechanical method.
At present, the common problems in domestic phosphating technology are: high phosphating temperature, short service life of phosphating solution, difficult washing and water pollution. It not only brings burden to environmental governance but also consumes a lot of water, phosphate and other resources. Therefore, the development direction of phosphating technology is mainly to improve the quality and reduce pollution, energy saving, non-toxic environmental protection, and further clean phosphating, phosphating liquid should have the characteristics of saving materials and energy, reasonable function, and the product does not harm human health and damage the ecological environment during and after use.
No nickel phosphating
Nickel is a common additive in phosphating solution, which has the function of refining grain, filling void and improving corrosion resistance, etc. Phosphate coating is usually applied by phosphating solution containing nickel. The nickel deposited during this process (either in elemental form or as an alloy component such as Zn/Ni) provides the appropriate electrical conductivity for the coating during subsequent electrocoating. However, due to its high toxicity and environmental hazards, its use should be avoided or at least reduced whenever possible.
Nitrite free phosphating
Nitrite can consume hydrogen produced on the interface of phosphating reaction, promote the formation of film layer, is a good accelerator, but nitrite is toxic, harmful to human health, and at the same time there are more sediment, poor corrosion resistance and other shortcomings, inconvenient to use. At present, the main alternatives used are hydrogen peroxide, hydroxy-amino compounds and organic nitro compounds.
Low temperature phosphating
Most of the existing processes are medium or high temperature phosphating. At higher temperatures, the transformation film crystal of phosphate is poor in both uniformity and density, resulting in the weakening of its corrosion resistance. In addition, the higher phosphating temperature is bound to increase energy consumption, which will lead to environmental pollution and other problems. Therefore, although the film forming speed of low-temperature phosphating technology is extremely slow and the production efficiency is low, it is undeniable that this is still the direction of phosphating technology and process development. Low temperature phosphating reactions usually require the addition of accelerants to improve efficiency.
No phosphating
The traditional phosphating process will produce a large number of sediment and phosphorous emissions, the environmental pollution is more serious, and the traditional phosphating process to oil removal, rust removal, surface adjustment, phosphating, passivation and other processes, the process is complex, high energy consumption, and in the treatment project is also prone to produce some toxic and harmful substances. With the attention of people to environmental protection, the traditional phosphating process will be more and more strictly limited, and the non-phosphating technology has been widely concerned. At present, there are two kinds of research: one is the nano ceramic membrane treatment agent with fluozircon acid as the main film forming substance, and the other is the silane treatment agent.