Phosphate coating is a coating that is widely used in the anti-corrosion treatment of metals, especially on the surface of steel. In this article, the properties, formation mechanisms, preparation methods, and advantages and disadvantages of phosphate coatings will be discussed in detail.
Different types of corrosion resistance
Manganese phosphate film – excellent corrosion resistance for high-strength applications.
Zinc phosphate film – also has good corrosion resistance and is suitable for a variety of environments.
Ferrous phosphate film - poor corrosion resistance and easy oxidation, resulting in a decrease in the adhesion of the film to the substrate.
Environmental impact- Phosphate coatings exhibit excellent corrosion resistance in atmosphere, mineral oils and vegetable oils, but poor corrosion resistance in acids, alkalis and water vapors.
The paintability of phosphate films depends on the thickness and structure of the film, and generally thinner film thicknesses do not significantly affect the adhesion of the film.
The addition of a sealing layer (such as organic coatings) can significantly improve the corrosion resistance and achieve the effect of 1+1>2.
The zinc phosphate film begins to lose weight at temperatures above 106°C, gradually changes color above 250°C, and peels off at 350°C.
Manganese phosphate membranes lose weight at 160-170 °C and crystal water decreases at 200-218 °C.
The main disadvantages of phosphate coatings include low hardness, insufficient mechanical strength, and poor toughness.
In the case of zinc phosphate coating, steel parts are immersed in an acidic aqueous solution containing zinc dihydrogen phosphate to form a phosphate film.
Phosphate coatings exhibit a porous structure, typically 1-50 microns thick, and the color and density of the membrane vary depending on the composition of the phosphate solution and the treatment process.
High temperature type (80-99°C) – high film thickness, but high energy consumption.
Medium temperature type (50-75°C) – short processing time, good corrosion resistance of the membrane, suitable for large-scale applications.
Ambient (10-40°C) – low energy consumption, simple operation, but long processing time.
The phosphating process is divided into impregnation and spraying, the former is suitable for processing a variety of shapes, and the latter is suitable for large area workpieces.
Temperature, free acidity and total acidity all affect the quality of the phosphate film and need to be kept within the appropriate range.
The ion concentration in the solution also needs to be controlled to ensure the compactness and uniformity of the membrane.
The treatment of phosphating slag is an important issue in environmental protection. Some manufacturers at home and abroad have begun to try to recycle and reuse phosphating slag to reduce environmental pollution.
The inspection of phosphate coatings includes visual inspection, corrosion resistance inspection, thickness and weight inspection, etc.
Corrosion resistance can be assessed by immersion and drip methods.
As a high-performance coating material, phosphate coating is widely used in metal surface treatment to significantly improve its corrosion resistance and adhesion. Its porous structure not only provides the coating with excellent mechanical strength, but also improves heat resistance. In the process of preparation and application, environmental factors and related inspection standards also need to be paid attention to to ensure the performance and sustainable development of materials.