What is Chrome Plated
Chromium is a microstrip blue silver white metal, chromium metal in the air easily passivated, forming a thin layer of passive film on the surface, which shows the nature of precious metals.
Chromium layer has a high hardness, according to bath composition and process conditions, the hardness can vary within a wide range of 400 ~ 1200HV. The chrome layer has better heat resistance, heating below 500 ℃, the gloss and hardness were no significant changes in temperature above 500 ℃ began to oxidize, more than 700 ℃ hardness began to decrease. Chromium layer friction coefficient is small, especially the dry friction coefficient, in all metals is the lowest. So chrome layer has a good wear resistance.
The chrome layer has good chemical stability and does not act in bases, sulfides, nitric acid and most organic acids, but is soluble in hydrochloric acid (such as hydrochloric acid) and hot sulfuric acid. In the visible range, the reflection of chromium is about 65%, between silver (88%) and nickel (55%), and due to chromium does not change color, the use of long-term to maintain its reflectivity and superior to silver and nickel .
Chrome Plated Characteristic
Chromium from the commonly used chrome plating solution, compared with other single metal plating solution, although the composition of chrome plating solution is simple, but the chrome plating process is quite complex, and has the following characteristics.
① The main component of chromium plating solution is not metal chromium salt, but chromium oxygen acid - chromic acid, is a strong acid bath. During the electroplating process, the cathodic process is complicated, and most of the cathode current is consumed in the two side reactions of hydrogen evolution and hexavalent chromium reduction to trivalent chromium. Therefore, the cathodic current efficiency of chrome plating is very low (10% ~ 18%). And there are three anomalies: current efficiency decreases with increasing concentration of chromic anhydride l decreases with increasing temperature; increases with the current density.
② in the chrome plating solution, you must add a certain amount of anions, such as SO42-, SiF62-, F-, in order to achieve the normal deposition of chromium metal.
③ chrome plating liquid dispersion is very low, the shape of complex parts, the need to use a pictographic anode or auxiliary cathode, in order to get a uniform chrome layer. The requirements of the hanger is also more stringent.
④ chrome plating need to use a higher cathode current density, usually more than 20A / dm2, plating than the average more than 10 times. Due to the large amount of gas precipitated from the cathode and the anode, the resistance of the plating solution is large, the cell pressure is increased, and the requirement for plating power is high, and the power larger than 12 V is required.
⑤ chrome-plated anode without metal chromium, and the use of insoluble anode. Commonly used lead, lead-antimony alloy and lead-tin alloy. Chromium in the bath due to deposition or other reasons to be consumed by adding chromium anhydride to supplement.
⑥ chrome-plated operating temperature and cathode current density have a certain dependence, changing the relationship between the two can get different performance chrome plating.
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