1 Problems

Corrosion resistance of phosphate coating according to GB11376-1989 for the neutral salt spray test, 90min not rust. Automotive fasteners its failure rate of resistance to neutral salt spray test 45%, phosphate film appearance, color inconsistencies, crystal thick, more than Dust.

2 factors

2.1 Poor solution composition ratio

Solution composition and the ratio of phosphate have a great impact on the quality, the use of self-adjustment of calcium phosphate solution for production, the formation of phosphate film surface crystallization of thick, multi-hanging gray appearance of the product of poor quality, the program phosphate solution prepared black precipitate produced when production is relatively large, basically need to play every day flood sediment. Phosphate solution by a variety of comparison, use PF-MIAM and PF-MIAR phosphate solution. Phosphate solution to adjust the simple, fine crystalline phosphate film, the car has been greatly improved corrosion resistance of fasteners and small parts of Dust, less precipitation solution. The phosphate effect can basically meet the requirements of automotive steel fasteners black phosphate.

 2.2 Acid ratio of is not correct

Acid than is the free acidity (Tb) and total acidity (Ta) of the relationship. Free acidity is too high, and the role of iron and steel parts quickly, will a lot of hydrogen, phosphate is not easy to make the interface layer saturated, leading to nucleation difficulties, thick film crystallization, loose more holes out corrosion performance degradation, and co-phosphate time extension; free acidity is too low, thin phosphate coating, phosphate coating did not even generate. Total acidity is too high too thin film, the total acidity is too low will film loose rough. Therefore, the total acidity is generally controlled at 85 ~ 140 points.

2.3 The impact of phosphate pre-treatment methods

Small fasteners basket mounted, easily accessible to the work piece, the process can not be allowed in full phosphate phosphate, the contact area is difficult to form a complete phosphate coating, so the resistance to neutral salt spray performance is poor; or allow the use of mounted between the work piece no contact, so that it can adequately respond it to improve the resistance to neutral salt spray performance have a multiplier effect.

2.4 Solution temperature 

Phosphate solution temperature can increase phosphate binding force, hardness, corrosion resistance. But the temperature should not be too high, otherwise it would be oxidation of Fe2 +Fe3 +, and to precipitate increased, the solvent to evaporate quickly, leading to an unstable solution.



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