The corrosion performance of shipboard equipment is one of the important factors affecting its service life. There are more and more "aging ships" caused by corrosion factors, and the coating process of ship-borne equipment is very important.
1 Anti-corrosion coating selection
The selection of anti-corrosion coatings is the key to the anti-corrosion design of the entire equipment. The environment and conditions at sea call for high-performance coatings. The coating should have the advantages of good corrosion resistance, high solid content, thick film type, good construction performance, and low shrinkage of the coating.
The overall coating can be divided into three parts: primer, middle coat and top coat. Primer plays an important role in supporting heavy-duty coatings. The primer needs to have good adhesion to play an excellent anti-rust and anti-corrosion effect.
Zinc-rich primer is one of the important coatings for shipboard equipment. It has excellent cathodic protection, excellent gas cutting and welding properties, and can be matched with a variety of upper coatings. It has excellent long-term anti-corrosion performance, excellent Wear resistance, heat resistance, buckling resistance. Zinc-rich primer can withstand high temperature of 400 ℃. The middle paint plays a connecting role in the overall coating. The middle paint for heavy-duty coatings must maintain good adhesion with the primer and topcoat, increase the total thickness of the coating, improve the shielding ability of the coating, and reduce costs. Epoxy mica iron intermediate paint is a very good intermediate paint for offshore equipment. The mica iron oxide contained in it has a good barrier effect on the penetration of corrosive media. The choice of topcoat in heavy-duty coatings must have high weather resistance. Organically modified polysiloxane topcoat is very suitable for offshore equipment, it has low viscosity, and easy to control the wet film thickness to reduce loss, and can be directly coated on the zinc-rich primer, omitting the intermediate paint, so it is Commonly used topcoat for offshore equipment.
2 Process defect control of coating process
The coating process of ship-borne equipment is very important. The subtle process technology in the whole operation process needs to be handled carefully to ensure the good anti-corrosion performance of the whole equipment. After the heavy-duty anti-corrosion coating is sprayed, various disadvantages formed by the paint film will seriously affect the quality of the coating and even the quality of the product. In addition to the quality of the paint itself, the defects of the paint film are mostly caused by factors such as incorrect paint construction. The main disadvantages and treatment methods of heavy-duty paint film are summarized as follows:
(1) hanging
After spraying, the paint film hangs or runs vertically down the surface. In severe cases, the coating migrates downward as if torn apart. At the defect, the phenomenon that the upper part is exposed and the lower part paint is crowded together. Solution: Choose nozzles with appropriate aperture and fog width, supplemented by adjusting pump pressure, and strictly control the amount of diluent added; use a wet film meter to detect film thickness, control the thickness of the sprayed wet film or reduce excessive spraying; Finally, during the drying and curing period of the coating, continue to ensure that the temperature and ventilation of the spray booth are sufficient. (Related instruments: Sag Tester)
(2) pinhole
Pinholes refer to pinpoint-like small holes on the surface of the paint film, which is the most taboo problem in painting. It is formed because the air bubbles mixed in the paint film are broken and cannot be leveled before the paint film dries and solidifies. Pinholes cause some parts to be blank and unpainted, which is a channel for corrosion. After the initial pitting corrosion is caused, the corrosion will gradually spread under the paint film, resulting in early failure of the entire paint film. Solution: When the paint is mixed and stirred, try to avoid air mixing. If there is air mixing, it can be set aside for a period of time to let the air mixed in be discharged before use; in the case of high ambient temperature, it is not suitable to apply too thick at one time. It is not advisable to add excessive thinner or unsuitable thinner in the interior; use sandpaper to grind off a layer of slight and small half-eye pinholes, dilute it with the original matching paint after cleaning, and repair a thin layer with a paint brush to make the paint easier Fill in the pinholes.
(3) dry spray
When the paint mist particles of the heavy-duty coating reach the surface of the steel to be coated , the liquid has dried up and cannot form a continuous coating film. In severe cases, it looks like sand particles, and the coating adhesion is poor, which will cause serious corrosion. Solution: When airless spraying is performed at a temperature exceeding 30°C, it is necessary to achieve the minimum pressure for construction, and the distance between the Spray Gun and the object to be coated should be appropriate. The correct operation of airless spraying is shown in Figure 1; arrange the construction sequence reasonably, Avoid unfavorable climatic factors to reduce waste, and properly shield adjacent parts; if spraying quick-drying paint, use appropriate thinner to slow down the volatilization speed. (Related instruments: Adhesion)
(4) shrinkage cavity
After the paint is sprayed, small bowl-shaped pits are formed on the paint film. There are often drops or strips of matter in the center of the pits, and the edges are raised. Solution: Before coating, carefully clean the surface of the equipment to be coated, and carefully check the film-forming and drying conditions of the previous coating, especially the cleanliness of the surface; for bright, smooth, and hard surfaces of the equipment to be coated, frosting is required.
(5) crack
Coating cracks can be divided into fine cracks, cracks and mud cracks.
Fine cracks are tiny cracks on the surface of the coating, but do not penetrate deep into the substrate; cracks are more severely damaged than fine cracks, and the cracks develop to penetrate the coating and expose the substrate; mud cracks cause serious damage to the coating, so that the coating is completely damaged. Loss of adhesion and falls off by itself. Solution: Strictly manage the film thickness, control the wet film, and not make the coating too thick; once the coating is thick, you should wait until the front coating is fully dry before re-coating. 30%~50% (as shown in Figure 2); if the film thickness requirement is high and the operation and construction are difficult to control, it can be sprayed in two times; design the correct coating package. Hard coatings should not be applied over soft, malleable coatings. (Related Instruments: Thickness Gauge)
(6) whitish
After the paint film dries, the phenomenon that the surface appears milky white is called whitish. The whitishness makes the paint film cloudy and dull, sometimes accompanied by a small amount of micropores and ripples, which reduces the mechanical properties of the coating. Solution: prevent the thinner from having a low boiling point and volatilize too quickly, and the surface of the coating film will quickly form a low temperature, causing the moisture to condense and turn white; prevent the low temperature and low humidity of the painting environment from dew condensation on the coated surface; prevent the oil-water separator from failing, and the paint mixed with water.
3 Conclusion
This paper analyzes the selection method and commonly used materials of shipborne equipment coatings, and classifies the process defects in the process of spraying heavy-duty anti-corrosion coatings, and summarizes the solutions to each type of process problems. Since the coating quality is greatly affected by many factors such as raw materials, construction methods, construction personnel, climatic conditions, and the environment, there are very important technical requirements for the anti-corrosion coating of ship-borne equipment. This article has a certain effect on the improvement and specification of anti-corrosion coating process for ship-borne equipment, and is of great help to improve the quality of ship-borne equipment.
