Chromium plating is a traditional surface plating technology that has been used for more than 70 years. The chromium plating layer has high hardness, wear resistance, corrosion resistance and can keep the surface bright for a long time, and the process is relatively simple and the cost is low.
Chromium plating is a traditional surface plating technology that has been used for more than 70 years. The chromium plating layer has high hardness, wear resistance, corrosion resistance and can keep the surface bright for a long time, and the process is relatively simple and the cost is low. For a long time, chromium coating is not only used as decorative coating, but also widely used as wear and corrosion resistant coating of mechanical parts. Hard chromium plating technology is often used to repair broken parts. However, electroplating hard chromium processes can cause serious environmental problems. It is not chromium itself that causes environmental problems, but the chrome-plating process. Chromium is an inactive element that can be safely used in everyday objects and even artificial joints. The main problem is that chromic acid solution used in chromium plating process produces acid mist and wastewater containing Cr(VI). Therefore, countries will be more and more stringent restrictions on chromium plating process. For example, the United States lowered the current air emission standard for hexavalent chromium from 0.1Mg/ m3 to 0.0050 ~ 0.0005Mg/ m3. The Navy department estimates that the U.S. Navy alone will need $22 million in one-time investment in equipment upgrades and $46 million a year in collection, disposal and other costs after implementing the standard. In addition, electroplating hard chromium plating has some other disadvantages:
(1) the hardness of hard chromium plating is generally HV 0.3 800 ~ 900, far less than some ceramics and metal ceramic materials of high hardness and wear resistance, and the hardness of hard Chromium plating at the temperature rise will be due to the release of its internal stress and rapidly reduced, its working temperature can only be lower than 427℃, Therefore, it is difficult to adapt to the modern mechanical high temperature, high speed work requirements.
(2) There are micro-cracks in the chromium plating layer, which will inevitably produce penetrating cracks (as shown in Figure 1), resulting in the corrosive medium penetrating from the surface to the interface and corroding the matrix, resulting in rust spots or even spaling on the surface of the coating.
(3) Electroplating process deposition speed is slow (about 25μm/h), plating 0.2-0.3mm thick coating often needs 2 ~ 3 shift time, is not good for the application of thick coating.
Therefore, people have been trying to find a coating process to replace electroplating hard chromium. With the continuous development of coating technology in recent years, some coatings have proved to be cleaner, more effective and even cheaper than electroplating hard chrome. These coating technologies including physical vapor deposition (PVD), chemical vapor deposition (CVD), such as laser coating technology and thermal spraying technology, the supersonic flame spraying (HVOF) the cost of the coating and plating hard chromium plating, some powder materials have than plating cost is low, and continues to decrease, and the plating hard chromium plating costs are rising, More importantly, thermal spraying coating technology has been successfully applied in textile machinery, paper making and aviation.
