Protective-decorative chromium plating: 0.5~ 1 micron, which can be increased to 2 microns if wear resistance is needed.
Hard chromium plating is generally 5~80 microns, especially up to 300 microns, and the maximum size of the workpiece used for repair can reach 1000 microns, that is, 1 mm thick.
Milky chrome plating: 30~60 microns, and the thickness of bright wear-resistant layer is generally 50~ 150 microns.
Porous chromium plating: it can be less than 100 micron or more than 150 micron.
Chromium plating layer has high hardness, which can be changed in a wide range of 400 ~ 1200 HV according to bath composition and process conditions. Chromium plating layer has good heat resistance. When heated below 500℃, its luster and hardness have no obvious change. When the temperature is above 500℃, it begins to change color by oxidation, and when the temperature is above 700℃, the hardness begins to decrease.
The friction coefficient of chromium plating layer is small, especially the dry friction coefficient, which is the lowest among all metals. Therefore, the chromium plating layer has good wear resistance.
Extended data:
Compared with other single metal plating solutions, the composition of chromium plating solution is simple, but the chromium plating process is quite complicated and has the following characteristics.
① The main component of chromium plating solution is not metal chromium salt, but chromium oxychromic acid, which belongs to strong acid plating solution. In the electroplating process, the cathodic process is complicated, and most of the cathodic current is consumed in two side reactions: hydrogen evolution and reduction of hexavalent chromium to trivalent chromium, so the cathodic current efficiency of chromium plating is very low (10% ~ 18%).
And there are three abnormal phenomena: the current efficiency decreases with the increase of chromic anhydride concentration, and L decreases with the increase of temperature; It increases with the increase of current density.
② A certain amount of anions, such as SO _ 42-,SIF _ 62-and F-, must be added to the chromium plating solution to realize the normal deposition of chromium.
③ The dispersion ability of chromium plating solution is very low. For parts with complex shapes, pictographic anode or auxiliary cathode should be used to obtain uniform chromium plating. The requirements for hangers are also very strict.
(4) Chromium plating requires a high cathode current density, usually above 20A/dm2, which is 10 times higher than that of common electroplating seeds. Because the cathode and anode release a large amount of gas, the resistance of plating solution is large and the cell pressure is increased, which requires high power supply for electroplating. Power supply greater than 12V is required, and power supply less than 8V can be used for other plating species.
⑤ Insoluble anode is used for chromium plating instead of metal chromium. Lead, lead-antimony alloys and lead-tin alloys are usually used. Chromium consumed in the plating solution due to deposition or other reasons needs to be supplemented by adding chromic anhydride.
⑥ There is a certain dependence between chromium plating operation temperature and cathode current density, and chromium coatings with different properties can be obtained by changing the relationship between them.
References:
Baidu encyclopedia-chrome plating