2. Corrosion mechanism of sprayed metal.
Generally speaking, the corrosion of metals is the result of many factors, some of which play an important role in the corrosion process. Paint is sprayed on the metal surface to form a coating protection layer. The area where the metal is corroded is the interface area between the coating film and the metal surface, and the corrosion continues to extend to the depth of the metal matrix.
If the paint coating on the metal surface can effectively isolate the penetration of water, oxygen, electrons and stray ions, the corrosion of the coated metal can be greatly slowed down or avoided. If the isolation effect is not good, the coating retaining layer will not protect the metal from corrosion. The production practice shows that the permeability of spraying protective layer to water seriously affects the adhesion of metal spraying surface, while the permeability of oxygen greatly affects the corrosion performance of metal. There are many forms of corrosion of sprayed metal, but the root cause and occurrence of corrosion are closely related to chemical and electrochemical effects.
3. Analysis of corrosion factors of sprayed metal and preventive measures
3. 1 metal materials and other factors.
The corrosion of sprayed metal is closely related to the corrosion resistance of metal material itself. Metals used for spraying include steel, aluminum alloy, copper alloy or magnesium alloy. Undoubtedly, different metal materials have different corrosion resistance and corrosion resistance. Due to the different surface conditions of metal materials, the anti-corrosion and anti-corrosion protection effects of the coating after spraying are obviously different. For example, steel plate parts purified by sandblasting and steel plate parts of the same brand corroded naturally are protected by the same kind of painting. Due to the adverse effects of corrosion, the corrosion rate of naturally corroded steel plate parts is dozens of times higher than that of sandblasted steel plate parts, and its anti-corrosion protection effect is obviously lower than that of the latter. Defects on the metal surface, such as inclusions, microcracks and stresses, and adsorption of water and active ions (Cl-, Br-, etc.). ) will affect or even accelerate the corrosion of sprayed metal to varying degrees.
Pretreatment before painting, such as purification, degreasing, activation and rust removal, and the application of surface treatment technology can effectively improve the corrosion resistance of coated metal. Production practice has proved that the corrosion resistance of sprayed metal has great influence on the pretreatment quality of substrate before painting. Effective degreasing, derusting or sandblasting before coating the surface of metal (especially casting) can purify and activate the surface, ensure good adhesion between paint film and base metal, and is very beneficial to improve the corrosion resistance of sprayed metal.
Phosphating treatment in the pretreatment process of steel coating is widely used as the bottom layer of painting, which is understandable for improving the adhesion of coating and the corrosion resistance of coated metal.
Phosphating, chemical oxidation and anodic oxidation of aluminum alloy can be used as the bottom layer of painting, which is undoubtedly excellent for improving and improving the corrosion resistance of coated metal.
In a word, a good surface treatment process of metal substrate is an important part and reliable foundation to improve the corrosion resistance of sprayed metal.
3.2 Metal Spraying Construction Technology and Environmental Impact
The cleanliness of metal surface seriously affects the adhesion of coating. Pinholes, nodules, peeling or poor adhesion often occur if the water, oil stains and other foreign bodies remaining or adsorbed on the product surface are not removed.
After phosphating of iron and steel parts, chemical oxidation or anodic oxidation of aluminum alloy parts, chemical oxidation of magnesium alloy parts, etc. Paint coating should be carried out within 24 hours, and paint coating should also be carried out within 6 hours after sand blasting and shot blasting of steel parts. Practice has proved that this process is a good way to improve the adhesion between coating and substrate and improve corrosion resistance.
The oil-water separation in compressed air for spraying is not sufficient and complete, which makes the compressed air contain excessive water and oil, which will inevitably affect the quality of spraying film and produce various faults and defects. Therefore, cleaning and replacing the filters in oil-water separation facilities regularly and keeping the compressed air clean are beneficial to improve the performance of paint coatings.
The temperature and humidity at the spraying site have obvious influence on the quality of spraying film. Attention should be paid to keeping the temperature of the paint construction site within the range of 10 ~ 30℃ and the relative humidity not exceeding 80%. The construction site must be kept clean and orderly.
If the temperature is too high and the relative humidity is high, the paint layer will turn white and appear orange peel-like, while if the temperature is too low, the paint layer will sag easily, which will adversely affect the coating quality of metal surfaces.
Air spraying is usually used for painting metal surfaces. Pay attention to keep the distance between the spray gun and the sprayed surface within the range of 200 ~ 300 mm, and control the compressed air pressure within the range of 2 ~ 4 kg/cm ㎡, so as to effectively ensure the coating quality.
3.3 Influence of coating performance
The performance of the coating has a great relationship with the corrosion resistance of the coated metal, and a high-quality coating can effectively protect the metal even under harsh conditions. Different types of coatings have different characteristics. The factors that affect the performance of coating protective layer are mainly determined by synthetic resin, filler, crystallinity and solubility.
According to the coating performance, applicable environment and other conditions, the types of coatings are selected pertinently, so that the anti-corrosion protection function of sprayed paint film can be more effectively played.
The selection of paint base, solvent and additives should be appropriate, and the proportion must be strictly in accordance with the technological requirements to ensure full and uniform stirring and dilution to the specified working viscosity.
In recent years, the extensive application of anti-rust coatings has further improved the application scope and anti-corrosion ability of sprayed metals, so it is necessary to vigorously promote and improve them.
4. Conclusion
In a word, it is undoubtedly an effective measure to improve the anticorrosion performance of sprayed metal by strengthening the specification of pretreatment and surface treatment process of metal spraying coating, strengthening the control and management of coating process, ensuring good adhesion between sprayed protective layer and parent material, overcoming and avoiding pinhole, bubbling, wrinkling, cracking and shedding of coated protective layer, and making sprayed protective layer an excellent barrier to inhibit the invasion of corrosive media.