Automobile fasteners are often treated for rust prevention by galvanizing or phosphating. The former offers good protection, but during passivation, Cr6+ is produced which can cause cancer and also leads to hydrogen embrittlement of the workpiece, requiring post-treatment to remove hydrogen.
The latter basically does not change the mechanical properties and geometric dimensions of the workpiece, and the film thickness can be increased as required. Immersing in anti-rust oil can enhance the protective performance, meeting the requirements for large-scale production of automobile fasteners.
Phosphating process
At present, there are high-temperature phosphating and normal-temperature phosphating systems. However, the corrosion resistance of normal-temperature phosphating is relatively low, and strictly speaking, it is not very suitable for fasteners, especially for automotive fasteners.
High-temperature phosphating is relatively common, carried out at 90 to 95℃ for 10 to 15 minutes. The phosphating solution at high temperature can obtain a thicker film layer, with better corrosion resistance, adhesion, hardness and heat resistance; however, it has the disadvantages of high energy consumption, large solution evaporation, rapid changes, and the phosphating film is prone to contain sediment and uneven crystal size. For fasteners, zinc-manganese phosphating is the main type.
Process flow
High-temperature alkaline solution degreasing (80 – 90℃, 10 – 15 min) → Loading → Rinsing → Boiling water scalding → Pre-rust removal (old acid solution, 5 – 10 min) → Rust removal (new acid solution, 3 – 5 min) → Rinsing → Surface conditioning (0.5 – 1.0 min) → Phosphating → Rinsing → Hot saponification (70 – 80℃, 1 – 3 min) → Unloading → Drying → Oil immersion → Spinning dry.
Key Points of the Process
- Pre-treatment
a. Oil removal
After heat treatment or mechanical processing, the surface of the fasteners is covered with a lot of oil stains, which are removed in a plastic tank with a degreaser.
b. Rust removal
If the surface oxide scale of fasteners is severe, shot blasting or sandblasting can be used. These methods can remove the oxide scale quickly, and the sandblasted surface is uniform. However, thoroughly removing the oxide scale is a crucial step.
For low-intensity ≥ 8.8 grade, concentrated hydrochloric acid can be used (with better fuming effect). Advantages: It has a strong chemical dissolution effect on metal oxides, and its rust removal capacity is almost directly proportional to the concentration.
Under the same concentration and temperature, the corrosion rate of hydrochloric acid is 1.5 to 2.0 times faster than that of sulfuric acid. After corrosion, there is less residue on the surface of the fasteners, and the acid-washed surface is uniformly grayish-white and smooth, which is convenient for phosphating.
- Surface conditioning
Surface conditioning is an indispensable process in zinc-based systems. The current surface conditioning process adopted can further enhance the quality of the phosphating film (with a dense and fine texture and good rust resistance), which is particularly important for phosphating of automotive fasteners.
- Oil immersion
After saponification, the phosphating film is immersed in oil, which can enhance its resistance to neutral salt spray (for example, when the phosphating film weighs 16g/m2, the immersion in oil can increase the resistance to medium-temperature salt spray to 550 hours).
The traditional hot-dip base oil (such as mechanical oil, spindle oil or transformer oil soaked at 105-110℃ for 5-10 minutes) has very poor rust prevention performance. Only by adding rust inhibitors can the rust prevention performance of the base oil be improved.
The greatest advantage of rust preventive oil is its good resistance to salt water, salt spray, moisture, acid neutralization and water displacement. Adding rust inhibitors to the base oil can produce excellent dehydrating rust preventive oil.
- Spin dry
Fasteners immersed in anti-rust oil can be dried by a stirring centrifuge to avoid excessive oil.
Quality Control
a. Pretreatment quality
During mass production, due to the varying skills of operators and the significant influence of human factors, when the loading capacity is too large, the mobility is poor. Therefore, it is necessary to carefully check whether the rust prevention is thorough.
Also, pay attention to thorough rinsing between processes to avoid cross-contamination and the resulting instability of the bath solution, which could otherwise reduce the quality of phosphating.
b. The Influence of Material Quality
Sometimes in production, it is found that under the same environmental conditions, the phosphating effect of fasteners produced by different manufacturers varies. That is, some can be phosphated to a black color, while others have a lighter phosphating color.
This is related to the material of the fasteners. For chromium steel 40Cr and SCM435 steel, the color is relatively light, while carbon steel ML35 and 35K is darker. In addition, different heat treatment conditions will result in different crystal orientations, and the difference in crystal orientation affects the phosphating effect.
In addition, the depth of the color of the phosphating film is also related to the composition of the phosphating agent (such as the type and concentration of metal ions, etc.). It is necessary to select the appropriate phosphating agent through experiments, and the color can also be deepened by changing the pickling method.
c. Optimize process parameters
Acidity: Different phosphating agents should be selected with different total acidity (TA) and free acidity (FA). The higher the TA, the faster the phosphating reaction, the more fine and uniform the crystalline film layer, and the phosphating temperature can also be reduced.
However, if it is too high, there will be more sediment, and powder-like attachments will appear on the surface of the workpiece. If the TA is too low, the phosphating process will be slow and the crystals will be coarse. For high-temperature phosphating, TA should be 40 to 60.
When FA is normal, the film-forming property is good and the film is thick. If it is too high, it will be difficult to form a film (the dissolution rate of the film layer is greater than the formation rate of the film layer), the film will be loose and yellowish, with poor corrosion resistance, and sometimes the reaction will not stop (bubbles will continuously form in the solution).
If it is too low, the film-forming property will be poor, the film will be in powder form, easily wiped off, and there will be more sediment. FA should be 1 to 5.
The higher the temperature, the faster the film forms, the thicker the film, and the better the rust prevention performance, but it accelerates the dissolution of iron, increasing Fe3+ and causing more sediment, and the workpiece surface adheres to white ash.
If the temperature is too low, the film forms slowly, the crystals are coarse, the rust prevention performance decreases, and the Fe3+ content in the solution increases, turning the phosphating solution into a soy sauce color and rendering it unusable. It is necessary to keep the solution temperature as uniform as possible.
If you are looking for quality fasteners, please visit us here https://hktl-fastener.com/.
