There are many types of metal protective layers for springs. In terms of springs, the metal protective layer is generally obtained by electroplating gold. The electroplated protective layer can not only protect from corrosion, but also improve the appearance of the spring. Some electroplated metals can also improve the working performance of springs, such as increasing surface hardness, increasing wear resistance, improving thermal stability, and preventing radiation corrosion. But if it is purely for the corrosion of the spring, the electro-galvanized layer and the electro-cadmium layer should generally be used.
Zinc is relatively stable in dry air, hardly changes, and is not easy to change color. A white film of zinc oxide or carbon zinc carbonate is formed in humid air. This dense film prevents further corrosion. Therefore, the galvanized layer is used as the anti-corrosion protection layer of the spring under normal atmospheric conditions. All springs that are in contact with solutions such as sulfuric acid, hydrochloric acid, caustic soda, and working in humid air such as sulfur trioxide, should not be coated with zinc. Generally, passivation treatment is carried out after the galvanized layer, which can improve the protective performance of the coating and increase the appearance of the surface.
In oceanic or high-temperature atmospheres, springs in contact with seawater, and springs used in hot water at 70°C, cadmium is relatively stable and has strong corrosion resistance. Cadmium coating is brighter and more beautiful than zinc coating, softer, and has better plasticity than zinc. The coating has less hydrogen embrittlement and is most suitable for springs as a protective layer. But cadmium is scarce, expensive, and cadmium salt is highly toxic, which is very harmful to the environment. Therefore, it is restricted in use. Therefore, most of the springs used in aviation, navigation and electronics industries use cadmium plating as a protective layer.
The non-metal protective layer of the spring is dip-coated or sprayed with a layer of organic substances, such as paint, asphalt, plastic, etc., on the surface of the spring to protect the spring from corrosion. Non-metallic protective layer, thicker film, good chemical stability, good mechanical anti-corrosion effect, but low hardness, easy to be scratched and damaged, and the film is aging.
The paint used for the spring paint layer usually has the following types:
a. Asphalt paint Asphalt paint has good water resistance, moisture resistance and corrosion resistance, especially excellent acid resistance and good alkali resistance. But the adhesion, mechanical strength and decoration are poor.
b. Phenolic paint Phenolic paint is divided into primer and top coat. Phenolic primer has strong adhesion and good anti-rust performance, but the mechanical strength and gloss of the paint film are poor. The paint film of phenolic topcoat is hard and has good gloss, but the weather resistance is poor, and the paint film is easy to turn yellow.
In general, the paint layer can be used alone or as a colorant after phosphating. Sometimes, some springs are also painted with different colors to distinguish them in order to classify them according to the load. The specific type and brand of paint used should be determined according to the working environment. If necessary, it should be noted in the spring pattern.