1. Existing problems
According to gb11376-1989, neutral salt spray test was carried out for the corrosion resistance of phosphating film, and no corrosion was found in 90min. The unqualified rate of neutral salt spray resistance test of automobile fastener is up to 45%, the appearance and color of phosphating film are inconsistent, the crystal is coarse and there is much ash hanging.
2. Influencing factors
2.1 poor solution distribution ratio
The composition and proportion of the solution have a great influence on the quality of phosphating. When the self-adjusting calcium phosphating solution is used for production, the phosphating film formed on the surface of the workpiece has coarse crystal, more ash hanging, and poor appearance quality of the product. The black phosphating solution prepared in this plan produces relatively more precipitates during production, and basically needs to hit the dregs every day. Pf-miam and pf-miar phosphating solutions are used in comparison with other phosphating solutions. The phosphating solution has the advantages of simple adjustment, fine crystallization of phosphating film, greatly improving the corrosion resistance of automobile fastener, less dust hanging on workpiece and less solution precipitation. The phosphating effect can basically meet the requirements of Black Phosphating for automobile steel fasteners.
2.2 incorrect acid ratio
Acid ratio refers to the relationship between free acidity (TB) and total acidity (TA). If the free acidity is too high, the reaction with iron and steel parts will be fast, and a large amount of hydrogen evolution will occur, which will make the phosphate in the interface layer not easy to saturate, resulting in the formation of crystal nucleus difficult, the crystal of the film layer is coarse, loose and porous, the corrosion performance is reduced, and the phosphating time is prolonged; if the free acidity is too low, the phosphating film is thin, or even there is no phosphating film. Too high total acid will make the film too thin, too low total acid will make the film loose and rough. Therefore, the total acidity is generally controlled at 85-140.
2.3 effect of phosphating pretreatment
The small fasteners are installed in baskets, so the workpieces are easy to contact, so they can not be fully phosphated during the phosphating process, and it is difficult to form a complete phosphating film on the contact parts, so the neutral salt spray resistance is poor; if the hanging or non-contact between workpieces is adopted, so that they can fully react, it will have a half power effect on improving the middle salt spray resistance.
2.4 effect of solution temperature
When the temperature of phosphating solution is increased, the adhesion, hardness and corrosion resistance of phosphating can be improved. However, the temperature should not be too high, otherwise, Fe2 + will be oxidized to Fe3 +, and the precipitates will increase, the solution will volatilize quickly, resulting in the instability of the solution.
3. Causes of poor corrosion resistance and Countermeasures
Causes of poor corrosion resistance of phosphatized fasteners and Countermeasures
(1) poor distribution ratio of solution: improve the formula of phosphating solution and use pl-vm phosphating solution.
(2) incorrect acid ratio: the free acid is too high, which can be adjusted by adding manganese carbonate, etc.; the total acid is too low, which can be adjusted by adding the main agent, and the total acid is controlled above 85 points.
(3) the main components of the solution are low: add the main solution.
(4) the main components of the solution are not correct: after the solution is settled, the tank temperature is controlled at about 95 ℃.
(5) the phosphating time is too short: the phosphating time is controlled within 10-15min.
(6) high Fe2 + content in the solution: add oxidant such as hydrogen peroxide to adjust Fe2 + content, or replace the tank solution.
(7) there is oxide in the solution: change the bath solution, or strengthen the cleaning after pickling and pretreatment of the workpiece.
(8) the pre-treatment is not clean: sand blasting or fasteners are used to keep a good surface condition before phosphating.
(9) too much load: hang or reduce the load to keep a certain space for fasteners to fully respond.
4. Conclusion
(1) according to different fasteners, after acid pickling or sandblasting, Phosphating for 10-15 min can basically meet the requirements of neutral salt spray resistance test for 90 min.
(2) according to the shape and size of fasteners, different mounting methods are used for Phosphating for 10-15 minutes, which can basically meet the requirements of neutral salt spray resistance test.
(3) the composition of the self-made phosphating solution is not well controlled, the solution is difficult to adjust, there are many precipitates, and the workpiece is easy to be excessively corroded in the process of phosphating. With pl-vm phosphating solution, the adjustment of solution parameters is simple, there is less sediment, the workpiece is hung with ash, and the film is crystallized with fine honey. The time of resistance to neutral salt spray can be improved, and the corrosion degree of workpiece is small after neutral salt spray test.
