Zinc is relatively stable in dry air and is not easy to change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form oxide or alkaline zinc carbonate film, which can prevent the zinc from continuing to be oxidized and play a protective role.
Zinc is easily corroded in acids, alkalis, and sulfides. The galvanized layer generally needs to be passivating treated. After passivation in chromic acid or chromate solution, the formed film is not easy to interact with humid air, and the anti-corrosion ability is greatly enhanced.
Advantages: low cost, convenient processing, and good effect.
Application: commonly used in atmospheric conditions and other good environments. But not suitable for friction parts
Cadmium coating is softer than zinc coating, it has less hydrogen embrittlement and strong adhesion; under certain electrolytic conditions, the obtained cadmium coating is more beautiful than the zinc coating. But the gas produced when cadmium melts is poisonous, and soluble cadmium salts are also poisonous.
Application: mainly used to protect parts from the atmospheric corrosion of sea water or similar salt solutions and saturated sea water vapor. Many aviation, marine and electronic industrial parts, springs, and threaded parts use cadmium plating. Can be polished, phosphated and used as a primer, but cannot be used to food wares.
very stable in humid atmosphere, alkali, nitric acid, sulfide, carbonate solutions and organic acids, and is easily soluble in hydrochloric acid and hot concentrated sulfuric acid.
The chromium layer has strong adhesion, high hardness, 800~1000V, good resistance, strong light reflectivity, and high heat resistance. It does not change color below 480℃, starts to oxidize above 500℃, at 700℃ hardness decreased significantly. The disadvantage of chromium is hard, brittle, and easy to fall off, which is more obvious when subjected to alternating impact loads.
Application: it’s not suggested to do chromium plating singly as an anti-corrosion layer. Generally, multi-layer electroplating (ie copper plating → nickel → chromium) can achieve the purpose of rust prevention and decoration. At present, it is widely used to improve the abrasive resistance of parts, repair size, light reflection and decorative lights.
Nickel has good chemical stability in the atmosphere and lye, and not easy to change color. It will be oxidized only when the temperature is above 600°C. It dissolves very slowly in sulfuric acid and hydrochloric acid, but easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid therefore has good corrosion resistance.
The nickel coating has high hardness, easy to be polished, has high light reflectivity so it looks bright. The disadvantage is that it has porosity. So it’s suggested to do a multilayer metal coating, nickel to be the intermediate layer.
Application: Usually in order to prevent corrosion and get good looking, it is generally used to protect decorative coatings. Nickel plating on copper products is ideal for anticorrosion, but because nickel is more expensive, copper-tin alloy plating is often used instead of nickel plating.