Chromate coating is a common chemical conversion coating, which is widely used in the field of metal surface treatment, especially in improving the corrosion resistance, hardness and aesthetics of metal materials. It is a kind of protective film formed by the chemical reaction between the metal and the chromium compound in the chromate solution, which can be used for anti-corrosion and has a certain decorative effect. In this paper, the properties, applications and preparation processes of chromate coatings will be discussed in detail.
Chromate coatings are a type of chemical conversion coatings that share similarities with phosphate coatings. The main difference between the two is the reaction mechanism and the nature of the coating formed. The formation of a chemical conversion coating is the reaction of atoms on the surface of the metal with the anions in the solution to form a protective film that is firmly bonded to the base metal. In chromate coatings, the metal surface reacts with the chemical components in a solution of chromic acid, chromate or dichromate to produce a coating with trivalent and hexavalent chromium compounds as the main components.
Corrosion resistance
One of the most striking properties of chromate coatings is their excellent corrosion resistance. Through the formation of the chromate coating, the metal surface is effectively protected and the direct contact between the corrosive medium and the base metal is avoided. Especially on steel materials, the anti-corrosion performance can be greatly improved after chromate treatment. The corrosion resistance of the coating is mainly due to its "isolation shielding" effect, that is, the coating can isolate the base metal from the corrosive medium and prevent the metal from oxidizing. In addition, the hexavalent chromium in the coating can be slowly dissolved in solution to form a passivation layer, which further enhances the anti-corrosion properties.
Passivation and ornamentality
The chromate coating not only provides protection, but also provides a decorative effect to the base metal. The chromate coating formed on the surface of copper and its alloys has good passivation properties, which can effectively improve the corrosion resistance of metals. On aluminium and aluminium alloys, the chromate coating gives a smooth and transparent appearance and can be further dyed to add decorative properties.
Hardness and abrasion resistance
Chromate coatings are generally less hard, especially in wet conditions, and the coating is less resistant to wear. As the coating dries, its hardness increases, but it is still lower than other types of coatings. In order to improve the wear resistance of chromate coatings, the wear resistance of the coating can be improved by adjusting the processing process parameters (such as increasing the pH of the solution, controlling the rotation speed of the workpiece, etc.).
adhesion
The chromate coating has strong adhesion, mainly due to the fact that the coating is chemically directly bonded to the base metal to form a strong bond. The chromate coating has a certain ductility, so the coating can maintain good adhesion during the metal stamping process.
Coating color
The color of a chromate coating is affected by a number of factors, including the thickness of the coating, the surface condition of the base metal, the composition of the bath fluid, etc. By adjusting the coating thickness and reaction conditions, the color of the chromate coating can change from colorless and transparent to milky white to yellow, golden yellow, green, etc., and even to different shades such as olive or brown.
The chromate coating of aluminium is mainly formed by an acidic oxidation process. In this process, the aluminum surface is first corroded, hydrogen gas is generated and reacts with chromium ions in the chromate solution to form a chromate coating. The thickness and color of the coating are affected by factors such as the pH of the solution, the concentration of chromic anhydride, etc.
The chromate coating of zinc is usually formed by treatment in an acidic dichromate solution. The key to this process is to control the pH of the solution, as the rate at which zinc dissolves and the rate at which chromate films are formed varies under different pH conditions. Proper pH and treatment time contribute to the formation of a uniform chromate coating with excellent corrosion resistance.
The main components of chromate coatings are compounds of trivalent chromium and hexavalent chromium, which work together to form the skeletal structure of the coating. In chromate coatings, trivalent chromium compounds provide the mechanical strength of the coating, while hexavalent chromium compounds contribute to the passivation of the coating, so that the coating has better anti-corrosion properties.
The chromate coating preparation process for aluminum usually includes multiple steps such as mechanical polishing, chemical degreasing, corrosion, oxidation treatment, etc. During the oxidation process, the aluminum matrix reacts with the chromate solution to form a chromate oxide film. During the treatment, components such as phosphoric acid, chromic anhydride and ammonium bifluoride in the solution work together to determine the quality and appearance of the coating.
For galvanized or cadmium-plated steel workpieces, the preparation process of chromate coating is relatively simple and usually includes steps such as plating, rinsing, chromate treatment, and baking. Chromate treatment temperature and timing need to be precisely controlled to ensure that the color and corrosion resistance of the coating meet the desired requirements.
In practice, in order to improve the quality and efficiency of coatings, it is often necessary to optimize the process conditions. This includes adjusting the solution formulation, controlling the temperature and time of treatment, selecting the appropriate treatment equipment, and more. By optimizing the process, the adhesion, hardness and corrosion resistance of the coating can be improved to meet the needs of different industrial applications.
As an important surface treatment technology, chromate coating has a wide range of applications in many fields. By properly controlling the composition, structure and preparation process of the coating, it is possible to obtain a chromate coating with excellent properties. With the continuous advancement of technology, the application prospect of chromate coating will be broader.