Analysis helps you solve the troubles of pinholes and pitting in the chrome plating layer!

The hard chromium layer has extremely high hardness and wear resistance. Hard chromium plating is widely used in the surface treatment of various parts. The quality of the coating directly affects the service life and work efficiency of the parts, but pitting and pinholes are also difficult problems that often appear in the coating and are not easy to overcome. After long-term exploration, combined with production practice, the causes and preventive measures of pitting and pinholes were initially found.

1 Causes of pinholes and pitting

1. The cause of pinholes. Pinholes are tiny holes from the surface of the coating to the underlying covering layer or base metal. It is mostly caused by gas (hydrogen) staying on the surface of the plated parts. Pinholes can also be caused by pits in the base metal. Therefore, the causes of pinholes are more complicated. (1) The metallographic structure of the base material is not uniform or the internal stress is not eliminated enough. These parts are not plated with chromium during electroplating, and pinholes are formed. For this kind of pinholes, the coating is cracked, and the distribution of pinholes is often irregular. (2) The oil stains on the surface of the plated parts are not thoroughly cleaned, and the oil stains during electroplating are not conductive or have poor conductivity, and the gas tends to stay on these parts and cause pinholes. The pinholes formed by this factor are locally dense and irregular. (3) There are deep rust spots on the plated parts before electroplating, especially in summer, when the climate is humid, the plated parts are easy to rust. When this kind of rusted plated parts are electroplated, the rusted parts are not conductive, resulting in pinholes. (4) The high sulfate content in the plating solution causes the content of trivalent chromium to increase rapidly, and the tendency to form alkali chromate on the cathode surface increases, which leads to deterioration of the coating quality and the formation of pinholes. (5) Where the surface is rough or severely scratched, hydrogen is likely to evolve. After hydrogen is deposited on the cathode, it often adheres to the surface of the electrode in the form of bubbles, causing insulation there, so that metal ions cannot discharge where hydrogen bubbles are attached, but can only discharge around these bubbles. If hydrogen bubbles always stay in one place during the electroplating process, pinholes will be formed in the plating layer. (6) There are too many particles and iron impurities in the plating solution, and the suspended matter adheres to the inner wall of the plating; or the plating solution has high oil content, high viscosity, poor fluidity, gas is not easy to escape, and pinholes are easy to produce.

2. The cause of the pitting The pitting is a small pit formed on the surface of the metal during the electroplating and processing process. Although there is a coating on it, the defect cannot be plated flat. (1) Due to the raw material or pre-plating treatment, there are small pits or small defects on the substrate. (2) The leveling effect before plating fails to meet the requirements. Under the condition of high current density, nodules will appear, and pitting will be left at the nodules after polishing. (3) The parts after deplating are directly electroplated, which is prone to pitting. (4) For parts with slight rust spots, when weakly corroded, small pits will be formed after the rust spots are corroded. Small pits will be formed in these places during electroplating. (5) Due to the improper selection of grinding force or oilstone, the oilstone is crushed during the grinding process, squeezing the raw material into small pits, thereby forming pits.

2Precautions Once pinholes or pits are formed, it is difficult to make up for them. Therefore, this article can only propose the following preventive measures for the reasons of the above analysis.

1. Preventive measures for pinholes (1) Ensure that the components in the plating solution are within the process range, so that the mass concentration ratio of chromic anhydride to sulfate is within the range of about 100:1. In the traditional, single medium concentration In the process of chrome plating, when the mass concentration of sulfate exceeds 2.8g/L, serious pinholes will occur. At the same time, try not to bring harmful ions into the plating solution, otherwise it will be difficult to remove. Maintain the plating solution and clean it regularly. When the oil pollution and impurities exceed the requirements seriously, it must be updated. (2) Check the surface of the plated parts before entering the tank to ensure that the surface technical parameters of the plated parts entering the process meet the process requirements. (3) Control the quality of raw materials. For the raw materials entering the process, it is necessary to ensure the uniformity of the metallographic structure and eliminate the stress. (4) Plated parts with severe corrosion should be returned to the previous process, reprocessed, and plated again after being qualified. (5) Adding a wetting agent to the plating solution helps to overcome the generation of pinholes. (6) Generally, the lower the surface roughness of the substrate, the more favorable it is for the deposition of the coating. Therefore, the surface roughness should be reduced as much as possible. At the same time, appropriate oilstone and processing technical parameters should be selected for the grinding and polishing process to avoid scratches as much as possible. (7) Full stirring of the plating solution, movement of the cathode, or regular filtration of the solution are all conducive to the escape of hydrogen.

2. Preventive measures for pitting (1) After the unqualified parts are removed from the hard chromium layer, they are subjected to fine grinding and polishing and then electroplating. (2) Control the quality of raw materials to make the surface of the substrate as flat as possible. (3) When grinding, the cutting force and oilstone should be selected appropriately to avoid crushing the oilstone during the machining process and causing pitting. (4) Anti-rust treatment should be done for parts that are not electroplated immediately. (5) When the surface roughness of the parts cannot meet the requirements, the current density can be appropriately reduced during electroplating to reduce pitting. The above-mentioned measures have been tested in production practice and are effective and have achieved good results.