Galvanized Plating
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 zinc from continuing to be oxidized and play a protective role.
Zinc is very susceptible to corrosion in acids, alkalis and sulfides. The galvanized layer is generally passivated. After passivation 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 removal must be carried out, and copper and copper alloy parts may not be hydrogen removed.
The standard potential of zinc is relatively negative, so the zinc coating is an anodic coating for many metals.
Application: Makes galvanizing commonly used in atmospheric conditions and other favorable environments. But not for friction parts.
Cadmium plating
Parts in contact with marine atmosphere or seawater and in hot water above 70 ℃, the cadmium coating is relatively stable, has strong corrosion resistance, good lubricity, dissolves very slowly in dilute hydrochloric acid, but is very easy to dissolve in nitric acid. , insoluble in alkali, and its oxides are not soluble in water.
The cadmium coating is softer than the zinc coating, the hydrogen embrittlement of the coating is small, and the adhesion is strong, and under certain electrolytic conditions, the obtained cadmium coating is more beautiful than the zinc coating. But the gas produced when cadmium melts is toxic, and the soluble cadmium salt is also toxic. Under normal conditions, cadmium is a cathodic coating on steel and an anodic coating in marine and high temperature atmospheres.
Application: It is mainly used to protect parts from the atmospheric corrosion of seawater or similar salt solutions and saturated seawater vapor. Many aviation, marine and electronic industrial parts, springs, and threaded parts are plated with cadmium. Can be polished, phosphatized and used as a paint primer, but cannot be used as tableware.
Chrome Plating
Chromium is 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.
Under the action of direct current, if the chromium layer is used as the anode, it is easily soluble in caustic soda solution.
The chromium layer has strong adhesion, high hardness, 800~1000V, good wear resistance, strong light reflectivity, and high heat resistance. Decreased significantly. The disadvantage of chromium is that it is hard, brittle and easy to fall off, which is more obvious when it is subjected to alternating shock loads.
At the same time,The chrome is porous. Metal chromium is easily passivated in the air to form a passivation film, thus changing the potential of chromium. Chromium on iron thus becomes a cathodic coating.
Application: It is not ideal to directly plate chrome on the surface of steel parts 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 in improving the wear resistance of parts, repairing dimensions, light reflection and decorative lights.
Nickel plating
Nickel has good chemical stability in the atmosphere and lye, is not easy to change color, and is only oxidized when the temperature is above 600°C. It dissolves slowly in sulfuric acid and hydrochloric acid, but is easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid and thus has good corrosion resistance.
Nickel plating is hard, easy to polish, has high light reflectivity and adds aesthetics. Its disadvantage is its porosity, in order to overcome this disadvantage, multi-layer metal plating can be used, and nickel is the intermediate layer. Nickel is a cathodic coating for iron and an anodic coating for copper.
Application: Usually used to prevent corrosion and increase aesthetics, so it is generally used to protect decorative coatings. Nickel plating on copper products is ideal for anti-corrosion, but because nickel is more expensive, copper-tin alloys are often used instead of nickel-plating.
Post time: Oct-11-2022