In the realm of fastener surface treatment processes, several methods, including electroplating, electrophoresis, painting, oxidation blackening, and phosphating, are commonly employed. This article will focus on surface phosphating treatment, shedding light on its purpose, key factors influencing its effectiveness, and the advantages it offers.

What is Surface Phosphating Treatment?

Surface phosphating treatment is a chemical and electrochemical process that generates a phosphate chemical conversion film on metal surfaces. This film is referred to as a phosphating film and is primarily used on iron and steel surfaces, although it can also be applied to non-ferrous metals like aluminum and zinc.

Purposes of Surface Phosphating Treatment:

Enhancing Paint Adhesion and Corrosion Resistance: Phosphating treatment acts as a primer before painting, improving the adhesion and corrosion resistance of the paint layer. The porous nature of the phosphating film allows paint to penetrate into its structure, creating a strong bond with the metal surface.

Protecting Base Metal: It provides a protective layer on the base metal, reducing susceptibility to corrosion. The phosphating film serves as a non-metallic, non-conductive isolation layer, preventing the formation of micro-batteries on the metal's surface and effectively safeguarding the underlying metal from corrosion.

Anti-Friction Lubrication: Surface phosphating treatment can function as a lubricant in metal cold working processes, reducing friction and wear.

Creating a Clean Surface: Phosphating film only forms on clean, oil-free, and rust-free metal surfaces. Consequently, metal workpieces subjected to phosphating treatment exhibit a clean, grease-free, and rust-free surface.

Factors Influencing Phosphating Effectiveness:

Temperature: The temperature plays a significant role in the phosphating process. Lower temperatures result in thinner phosphating layers with finer crystallization, while higher temperatures lead to thicker layers with coarser crystallization.

Free Acidity: The concentration of free hydrogen ions in the phosphating solution, known as free acidity, affects the process. Excessive free acidity accelerates corrosion on steel surfaces, hindering phosphating film formation and reducing corrosion resistance. Conversely, low free acidity results in slow corrosion reactions and difficulties in film formation.

Total Acidity: Total acidity, the sum of phosphates, nitrates, and acids in the solution, should ideally be controlled within the specified range. Maintaining higher total acidity accelerates phosphating reactions and produces finer and more uniform film grains.

pH Value: The pH value of the solution is crucial. Manganese phosphating solutions are typically kept between pH 2-3 to prevent powder formation on the workpiece surface. For iron systems, the pH is generally controlled between 3-5.5.

Ion Concentration: The concentration of ions in the solution, such as Fe2+ and Zn2+, also affects phosphating outcomes. Excessive Fe2+ can hinder film formation and reduce corrosion resistance, while high levels of Zn2+ result in coarse-grained films and increased brittleness.

In conclusion, surface phosphating treatment is a valuable process with multiple benefits, including improved paint adhesion, corrosion protection, lubrication, and surface cleanliness. To achieve optimal results, it is essential to consider and control factors such as temperature, acidity levels, total acidity, pH value, and ion concentration during the surface treatment process.