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The discussion on the causes of yellowing in steel plate phosphating, as conducted by Zhengxiong Machinery Co., Ltd. located in Zhengzhou, 45001, and authored by Wang Gefa, presents a comprehensive analysis of the chemical processes involved in the formation of phosphating films. The phosphating process is highly complex, involving multiple reactions such as ionization, hydrolysis, displacement, neutralization, precipitation, complexation, and redox reactions. These reactions are often catalyzed by nitrates or nitrites, and their complexity increases with the presence of various additives like film-forming agents.
One of the key factors influencing the phosphating reaction is the use of accelerators, which can be either thermal or chemical. Heating the solution facilitates the introduction of electropositive metals into the reaction, promoting the formation of a more uniform and dense phosphating film. For example, adding a small amount of copper salt to the phosphating solution initiates a displacement reaction, leading to the deposition of copper on the steel surface. This creates micro-cathodes that enhance electrochemical reactions, thereby accelerating the formation of the phosphating layer.
Depolarizers play an important role in this process by preventing hydrogen from covering the crystal nuclei. When an oxidizing accelerator is present, hydrogen atoms are directly oxidized into water, allowing for better ion diffusion and film formation. Additionally, introducing crystalline nuclei before phosphating helps ensure even distribution of the phosphating crystals, leading to a more compact and protective coating.
Yellowing during phosphating can occur due to several reasons. At low catalyst concentrations, the metal corrosion reaction progresses slowly, resulting in insufficient formation of the phosphating film. This leaves the metal exposed to air, causing oxidation and subsequent yellowing. Similarly, high total acidity in the phosphating solution can hinder the formation of a proper film. The buffer system in the solution may not effectively reduce the acidity at the workpiece surface, leading to incomplete phosphating and yellow discoloration.
Another cause of yellowing is the failure of surface adjustment. If the steel plate is not properly prepared before phosphating, active centers necessary for crystal nucleation may be absent, leading to uneven film formation and yellow spots. In some cases, improper application of degreasing agents or incorrect adjustments in the phosphating tank can also lead to similar issues.
Spray phosphating systems are particularly sensitive to nozzle failures. If a nozzle is blocked or not functioning properly, the affected area may not receive enough phosphating solution, resulting in localized yellowing. Adding appropriate salts like copper or nickel to the solution can improve the quality of the phosphating film, as observed through scanning electron microscopy.
In addition to phosphating, the development of multifunctional cleaning agents has been explored to improve surface preparation before phosphating. A new oil-based metal cleaning agent was formulated, containing surfactants, co-solvents, rust inhibitors, and bactericides. It demonstrated excellent performance in terms of penetration, emulsification, and anti-rust properties. The product is suitable for various applications, including painting, electroplating, and phosphating, and offers advantages over both water-based and oil-based cleaners in terms of efficiency and environmental impact.
Finally, the phosphating time is critical in achieving a high-quality film. Insufficient time can lead to incomplete growth of crystal nuclei, resulting in a weak and porous film that fails to protect the underlying metal. Proper control of process parameters, including temperature, concentration, and dwell time, is essential to prevent defects like yellowing.
Overall, understanding the chemical mechanisms behind phosphating and addressing the root causes of defects such as yellowing is crucial for improving the quality and durability of the final coated surface.