September 4, 2023
Chromium is a silvery white metal with a slight sky blue tinge. Although the electrode potential is very negative, it has a strong passivation performance, and it is quickly passivated in the atmosphere, showing the properties of noble metals, so the chrome-plated layer of iron parts is the cathode coating. The chromium layer is very stable in the atmosphere and can maintain its luster for a long time. It is very stable in corrosive media such as alkali, nitric acid, sulfide, carbonate and organic acids, but it is soluble in hydrohalic acid such as hydrochloric acid and hot concentrated sulfuric acid.
The chromium layer has high hardness, good wear resistance, strong reflective ability, and good heat resistance. There is no significant change in gloss and hardness below 500℃, when the temperature is greater than 500℃, it begins to oxidize and discolor; when the temperature is greater than 700℃, it begins to soften.
Due to the excellent performance of the chromium plating layer, it is widely used as a protective outer layer and functional coating of a decorative coating system.
Characteristics of chromium plating: 1. Chromium plating uses oxygen-containing acid as the main salt. Chromium has a strong affinity with oxygen. Electrolysis is difficult and the conduction efficiency is low. 2. Chromium is a price-changing metal and contains oxygen-containing acid radicals, so the cathode reduction process is very complicated. 3. Although chromium plating has a large polarization value, the degree of polarization is very small, so the dispersion and coverage capabilities of the plating solution are still poor. Auxiliary anodes and protected cathodes are often used. 4. Chromium plating requires high current density, but the current efficiency is very low. A large amount of hydrogen is released, resulting in a large ohmic voltage drop in the plating solution, so the voltage of chromium plating is relatively high. 5. Chrome anodes cannot be used for chromium plating. Pure lead, lead-tin alloy, lead-antimony alloy and other insoluble anodes are usually used.
Special phenomena of the chromium plating process: 1. As the concentration of the main salt chromic anhydride increases, the current efficiency decreases. 2. As the current density increases, the current efficiency increases. 3. As the temperature of the plating solution increases, the current efficiency decreases. 4. As the plating solution temperature increases, the current efficiency decreases. The stirring is strengthened and the current efficiency is reduced, and even chromium plating cannot be performed.