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Therefore, it is necessary to perform an oxidation treatment on the outer surface. Oxidation blackening is also a common surface treatment method. This treatment prevents the screw fasteners from being loosened in a strong oxidizing chemical solvent. After a period of time, a layer is more beautiful. The dense black iron oxide film also has anti-corrosion function.
Another method is to blacken the phosphating. When the screw fasteners are put in the phosphating solution, they will accumulate on the surface of the screw to form a water-insoluble crystalline phosphate conversion coating. The corrosion function is much stronger than the previous two corrosion resistance functions.