Chromium layer has a high hardness, according to the bath composition and process conditions, the hardness can be a wide range of 400 ~ 1200HV changes. Chromium layer has good heat resistance, heating at 500 ℃ below its gloss, hardness, no significant change in temperature greater than 500 ℃ began to oxidative
discoloration, greater than 700 ℃ hardness began to decrease. The friction coefficient of the chrome-plated layer, especially the dry friction coefficient, is the lowest among all the metals. So the chrome layer has a good wear resistance.
Chromium plating from the commonly used chromic acid plating, compared with other single metal plating, chrome-plated liquid although the composition is simple, but the chrome plating process is quite complex, and has the following characteristics.
① chrome-plated liquid is not the main component of chromium metal, but chromium-containing acid - chromic acid, is a strong acid bath. During the electroplating process, the cathodic process is complicated, and most of the cathodic current is consumed in the two side reactions of hydrogen evolution and hexavalent chromium reduction to trivalent chromium, so the cathodic current efficiency of chrome plating is very low (10% ~ l8%). And there are three anomalies: the current efficiency with the increase in the concentration of chromium anhydride l decreases with increasing temperature; with the current density increases.
② In the chrome plating solution, must add a certain amount of anions, such as SO42-, SiF62 a, F first, in order to achieve the normal deposition of metal chromium.
③ chrome-plated liquid dispersion is very low, for the complex shape of the parts, need to use the pictographic anode or auxiliary cathode, in order to get a uniform chrome layer. The requirements of the hanging is also more stringent.
④ chrome need to use a higher cathode current density, usually more than 20A / dm2, higher than the average plating more than l0 times. As a result of the cathode and anode a large number of precipitation of gas, so that the resistance of the bath larger, increased tank pressure, high demand for plating power, need to use more than 12V power supply, and other plating use 8V power supply can be.
⑤ chrome-plated anode without metal chromium, and the use of insoluble anode. Usually lead, lead-antimony alloy and lead-tin alloy. The chromium consumed in the plating solution due to deposition or other reasons is supplemented by the addition of chromic anhydride.
⑥ chrome plating operating temperature and cathode current density have a certain dependence, change the relationship between the two can get different properties of chromium coating.
Chromium plating from the commonly used chromic acid plating, compared with other single metal plating, chrome-plated liquid although the composition is simple, but the chrome plating process is quite complex, and has the following characteristics.
① chrome-plated liquid is not the main component of chromium metal, but chromium-containing acid - chromic acid, is a strong acid bath. During the electroplating process, the cathodic process is complicated, and most of the cathodic current is consumed in the two side reactions of hydrogen evolution and hexavalent chromium reduction to trivalent chromium, so the cathodic current efficiency of chrome plating is very low (10% ~ l8%). And there are three anomalies: the current efficiency with the increase in the concentration of chromium anhydride l decreases with increasing temperature; with the current density increases.
② In the chrome plating solution, must add a certain amount of anions, such as SO42-, SiF62 a, F first, in order to achieve the normal deposition of metal chromium.
③ chrome-plated liquid dispersion is very low, for the complex shape of the parts, need to use the pictographic anode or auxiliary cathode, in order to get a uniform chrome layer. The requirements of the hanging is also more stringent.
④ chrome need to use a higher cathode current density, usually more than 20A / dm2, higher than the average plating more than l0 times. As a result of the cathode and anode a large number of precipitation of gas, so that the resistance of the bath larger, increased tank pressure, high demand for plating power, need to use more than 12V power supply, and other plating use 8V power supply can be.
⑤ chrome-plated anode without metal chromium, and the use of insoluble anode. Usually lead, lead-antimony alloy and lead-tin alloy. The chromium consumed in the plating solution due to deposition or other reasons is supplemented by the addition of chromic anhydride.
⑥ chrome plating operating temperature and cathode current density have a certain dependence, change the relationship between the two can get different properties of chromium coating.
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