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Corrosion's Natural Enemy: Why Are Chromic Acid Conversion Coatings So Effective on Zinc and Aluminum?
Chromic acid conversion coatings, also known as Arotin coatings, are primarily used to passively treat steel, aluminum, zinc and other metal alloys to prevent corrosion. Its primary function is not only to act as a corrosion inhibitor, but it also improves the adhesion of coatings and adhesives, provides an aesthetically pleasing decorative appearance, and even maintains the conductivity of certain metals. This coating is often applied to products such as screws, hardware and tools, and gives these usually white or grey metal items a unique iridescent green-yellow colour in appearance.
The composition and structure of chromic acid conversion coating is very complex, including chromium salts and other chemical components. Changes in these components will affect the corrosion resistance of the final product.
Application process of chromic acid conversion coating
The process of applying a chromic acid conversion coating typically involves immersing the metal part in a chemical bath until the desired coating thickness is achieved. This process is usually carried out at room temperature, and the parts are removed after soaking for a few minutes, rinsed and allowed to dry. Direct immersion provides stability compared to other methods and ensures uniformity and stability of the coating. When the coating is applied it is initially soft and gelatinous but dries and becomes hydrophobic over time and hardens within 24 hours.
Coating Chemical Characteristics
The chromic acid coating process is based on the red oxygen reduction reaction. In the case of aluminum, hexavalent chromium reacts with aluminum to form trivalent chromium and aluminum oxide hydroxide. These hydroxides further combine under the right conditions to form colloids of small particles and ultimately form nanoscale solid frameworks. This framework shrinks during the drying process, thereby increasing the strength and stability of the membrane.
During the drying process, the coating will form microscopic cracks similar to "dried mud". The metal in these cracks will still react with the solution retained within, maintaining the overall protective effect.
Application effects of specific metals
Chrome effect of zinc
The weather resistance of the zinc coating is significantly improved after chromic acid treatment. The color of the chromate coating changes from clear or blue to orange, gold, olive green and black. The color change also reflects the thickness of the coating. Generally speaking, the darker the color, the stronger the corrosion resistance. .
Application of aluminum and aluminum alloys
For aluminum, the process of converting the chromic acid bath is relatively simple. It only requires a process tank at ambient temperature, and the entire process is completed within 1 to 5 minutes, making it easy to operate. The common commercial formula of Arodin 1200s contains some chemicals such as chromic acid. After treatment, the aluminum film appears golden yellow and the thickness is generally between 200 and 1000 nanometers.
The formation of this chemical film not only improves the corrosion resistance of aluminum alloy, but also makes it more durable in application.
Safety considerations
While chromic acid conversion coatings offer superior protection, the associated hexavalent chromium compounds have become a focus of health risks. Due to the carcinogenicity of hexavalent chromium and its potential harm to those exposed to it, many new commercial formulations strive to replace hexavalent chromium with trivalent chromium while complying with EU ROHS and REACH guidelines to reduce the health threat to workers and consumers. .
Future Development Direction
With the strengthening of environmental awareness, the market requirements for chromic acid conversion coatings are also increasing, and future technological developments will focus more on safe and environmentally friendly alternatives. Whether safer and more effective alternative technologies can be developed to meet industrial needs will become an important issue in the future?
