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Corrosion mechanism of multilayer electroplating
The use of intermediate multilayer electroplating to make a protective-decorative chromium coating on a steel substrate is primarily based on corrosion considerations.The potential of iron is negative, which is-0.44V, whereas the potential of Cr in activated state is-0.4 ~ 0.6 V.Chromium plating is easily blunt in the atmosphere, which makes the potential of chromium suddenly jump from negative to +0.8 V ~ + 1V and become positive potential.Therefore, when chromium plating is plated on the surfaces of aluminum, zinc, iron and nickel with negative potential, chromium plating is cathodic plating.
Generally, chromium coatings are porous and full of cracks, and may penetrate through the coating directly to the substrate.In this case, the chromium coating obviously does not play an anti-corrosion role, but will accelerate the corrosion of the substrate.Especially, the potential difference between chromium and steel is very large, so in the process of electrochemical corrosion, although chromium will not corrode, iron will suffer serious corrosion and rust.This kind of phenomenon often occurs in decorative chrome plating products.
Protective-decorative chromium plating to achieve the purpose of high corrosion resistance, it is hoped that the intermediate coating must have the following requirements:
1.The intermediate coating should have good adhesion between the substrate and the surface coating, so copper plating and copper alloy are widely used in industry;
2, so that the substrate metal potential is close to the surface coating potential.Reduce the potential difference between coatings;
3.The coating is required to be semi-bright and full-bright;
4.It is required that the coating should have no porosity or micropores, microcracks or various micro-discontinuous chromium plating layers;
5, the coating should have good ductility and easy polishing;
6.The quality of the coating shall meet various requirements in the standard.
In order to improve the protective-decorative ability of chromium plating layer, nickel + chromium and copper + nickel + chromium protective decorative system are commonly used as intermediate coatings.
High corrosion resistance chromium plating systems, such as double-layer nickel, triple-layer nickel, nickel seal, high stress nickel, wrought nickel, nickel-iron alloy and nickel-cobalt alloy, have been developed by using the knowledge of corrosion mechanism.It has been proved in production that the above plating coatings can form microporous and microcracked chromium coatings on iron and steel parts to meet the requirements of high corrosion resistance.But also can reduce the thickness of nickel layer and reduce the cost.

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