The difference between phosphating and passivation treatments in metals lies in their purposes and mechanisms.

Phosphating is an essential method for corrosion prevention in metal materials. Its objectives include providing corrosion protection to the base metal, serving as a primer before painting, enhancing the adhesion and corrosion resistance of coating layers, and acting as a lubricant in metal processing. Phosphating can be categorized into three types based on its applications: 1) coating phosphating, 2) cold extrusion lubrication phosphating, and 3) decorative phosphating. It can also be classified by the type of phosphate used, such as zinc phosphate, zinc-calcium phosphate, iron phosphate, zinc-manganese phosphate, and manganese phosphate. Additionally, phosphating can be categorized by temperature: high-temperature (above 80 ℃) phosphating, medium-temperature (50–70 ℃) phosphating, low-temperature (around 40 ℃) phosphating, and room-temperature (10–30 ℃) phosphating.

On the other hand, how does passivation occur in metals, and what is its mechanism? It's important to note that passivation is a phenomenon caused by interactions between the metal phase and the solution phase or by interfacial phenomena. Research has shown the impact of mechanical abrasion on metals in a passivated state. Experiments indicate that continuous abrasion of the metal surface causes a significant negative shift in the metal potential, activating the metal in a passivated state. This demonstrates that passivation is an interfacial phenomenon occurring when metals come into contact with a medium under certain conditions. Electrochemical passivation occurs during anodic polarization, leading to changes in the metal's potential and the formation of metal oxides or salts on the electrode surface, creating a passive film and causing metal passivation. Chemical passivation, on the other hand, involves the direct action of oxidizing agents such as concentrated HNO3 on the metal, forming an oxide film on the surface, or the addition of easily passivatable metals such as Cr and Ni. In chemical passivation, the concentration of the added oxidizing agent should not fall below a critical value; otherwise, it may not induce passivation and could lead to faster metal dissolution.

Post time: Jan-25-2024