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    Fastener Technology of Throat Hoop

    Fastener Technology: Improving Corrosion Resistance of Phosphating Fasteners

    1. Problems

    The corrosion resistance of phosphating film was tested by neutral salt spray test according to GB11376-1989, and no corrosion occurred for 90 minutes. The unqualified rate of neutral salt spray test for automobile fasteners is 45%. The appearance and color of phosphating film are inconsistent, the crystallization is coarse and the hanging ash is more.

    2. Influencing factors

    2.1 Poor composition of solution

    The composition and proportion of the solution have a great influence on the quality of phosphating. When the self-adjusting calcium salt phosphating solution is used for production, the phosphating film formed on the surface of the workpiece has coarse crystallization, more ash hanging and poor appearance quality. The black phosphating solution prepared by this scheme produces relatively more precipitation during production, and basically needs waterlogging sediment every day. PF-MIAM and PF-MIAR phosphating solutions were used after comparing various phosphating solutions. The phosphating solution has the advantages of simple adjustment and fine crystallization of phosphating film, which greatly improves the corrosion resistance of automobile fasteners, and less ash hanging and solution precipitation. Its phosphating effect can basically meet the requirements of Black Phosphating for automobile steel fasteners.

    2.2 acid ratio is incorrect

    The acid ratio refers to the relationship between free acidity (Tb) and total acidity (Ta). Free acidity is too high, it acts quickly with iron and steel parts, and it will release a lot of hydrogen, which makes the phosphate in the interface layer not easy to saturate, and leads to the difficulty of nucleation. The crystallization of the film layer is coarse, loose and many holes, the erosion performance is reduced, and the phosphating time is prolonged; the free acidity is too low, the phosphating film is thin, or even no phosphating film is formed. High total acidity will make the film too thin, and low total acidity will make the film loose and rough. Therefore, the total acidity is generally controlled at 85 to 140 points.

    2.3 Effect of Phosphating Pretreatment

    Small fasteners are equipped with baskets, and the workpiece is easy to contact. During the phosphating process, the workpiece can not be fully phosphated. It is difficult to form a complete phosphating film on the contact part, so the performance of neutral salt spray resistance is poor. If the workpiece is mounted or not contacted, it can fully react, it will have a multiplier effect on improving the performance of neutral salt spray resistance.

    Effect of 2.4 Solution Temperature

    The bonding strength, hardness and corrosion resistance of phosphating can be improved by increasing the temperature of phosphating solution. But the temperature should not be too high. Otherwise, it will oxidize Fe2+ to Fe3+, increase precipitates and volatilize quickly, which will lead to unstable solution.

    3. Reasons for poor corrosion resistance and Countermeasures

    Causes of poor corrosion resistance of phosphating fasteners and Countermeasures

    (1) The composition of the solution is not good: PL-VM phosphating solution is used to improve the formula of phosphating solution.

    (2) incorrect acid ratio: too high free acidity, can be adjusted by adding manganese carbonate, etc. Total acid is too low, can be adjusted by adding main agent, total acid control is above 85 points.

    (3) The main components of the solution are low: the main solution is added.

    (4) The main components of the solution are incorrect: after the solution is settled, the tank temperature is controlled at about 95 C.

    (5) The phosphating time is too short: the phosphating time should be controlled in 10-15 minutes.

    (6) The content of Fe2+ in solution is high: adding oxidant such as hydrogen peroxide to adjust the content of Fe2+ or changing the bath fluid.

    (7) There are oxides in the solution: change the tank fluid, or strengthen the cleaning of the workpiece after pre-treatment and pickling.

    (8) Pretreatment is not clean: Sandblasting or fasteners are used to maintain good surface condition before phosphating.

    (9) xcessive loading: use mounting or reduce loading, so that the fastener to maintain a certain void, so that it can fully respond.

    4. conclusion

    (1) According to the different fasteners, after pickling or sandblasting, Phosphating for 10-15 minutes can basically meet the requirements of 90 minutes of neutral salt spray test.

    (2) According to the shape and size of fasteners, Phosphating for 10-15 minutes with different mounting methods can basically meet the requirements of neutral salt spray test.

    (3) The composition of the self-made phosphating solution is not well controlled, the adjustment of the solution is difficult, the precipitation is more, and the workpiece is easy to cause over-corrosion in the process of phosphating. With PL-VM phosphating solution, the solution parameters are simple to adjust, the sediment is small, the workpiece hangs ash, and the film crystallizes fine honey. The neutral salt spray resistance time of the workpiece can be improved, and the corrosion degree of the workpiece after neutral salt spray test is small.

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