Zinc plated
Features: Zinc is relatively stable in dry air and not easy to change color. In water and humid atmosphere, it interacts with oxygen or carbon dioxide to form oxides or alkaline zinc carbonate films, which can prevent zinc from continuing to be plating and oxidized and play a protective role.
Zinc is very vulnerable to corrosion in acids, bases and sulfides. The galvanized layer is generally passivated. After passivated in chromic acid or chromate solution, the formed passivation film is not easy to interact with moist air, and the anti-corrosion ability is greatly enhanced. For spring parts, thin-walled parts (wall thickness <0.5m) and steel parts requiring high mechanical strength, hydrogen must be removed, and copper and copper alloy parts must not be dehydrogenated.
The standard potential of zinc galvanization is low, easy to process and effective, so the zinc coating is anode for many metals.
Application: Universal use of galvanized in atmospheric conditions and other good environments. But it is not suitable for rubbing parts.
Cadmium plating
Features: Parts in contact with the oceanic atmosphere or seawater and hot water above 70°C, the cadmium coating is relatively stable, corrosion resistance, good lubrication, and dissolves slowly in dilute hydrochloric acid, but it is very easy to dissolve in nitric acid, insoluble in alkali, and its oxides are insoluble in water. Cadmium coating is softer than zinc coating, the hydrogen brittleness of the coating is small, the adhesion is strong, and under certain electrolysis conditions, the cadmium coating obtained is more beautiful than the zinc coating. However, the gas produced by cadmium melting is toxic, and soluble cadmium salts are also toxic.
Under general conditions, cadmium is a cathodic coating on steel and an anode coating in oceanic and high-temperature atmospheres.
Application: It is mainly used to protect parts from atmospheric corrosion of seawater or similar salt solutions and saturated seawater vapor. Many aviation, nautical and electronic industrial parts, springs and threaded parts are cadmium plating. It can be polished, phosphated and painted as the bottom layer, but it cannot be used as a food utensil.
