Corrosion of metallic materials can generally be classified into the following types:
I. Dry corrosion and wet corrosion.
II. Chemical corrosion, electrochemical corrosion and physical corrosion.
III. Pitting corrosion and stress corrosion fracture.
1. FACTORS AFFECTING THE CORROSION RESISTANCE OF COATED STEEL SHEETS
The corrosion resistance of coated steel sheets is usually expressed by the degree of corrosion damage, such as loss of gloss, discoloration, chalking, fine cracking, cracking, blistering and rust stains.
There are many factors that can affect the corrosion resistance of coated steel sheets, including the type of substrate of the coated steel plate, the type of paint, the number and thickness of coating layers, the environment of service of coated steel plate and machining, transportation and storage in the application of coated steel plate.
Based solely on the principle analysis of the coating characteristics of the coated steel plate, the fundamental factors determining the corrosion resistance of coated steel sheets are the structural characteristics of the coating and the corrosion resistance of the substrate. The protection of the coating under the atmospheric corrosion environment is mainly determined by the corrosion resistance or aging resistance of the coating.
Although the organic coating has the ability to isolate the steel from the external corrosive environment, there are usually small holes in the organic coating through which external moisture, steam, oxygen and various ions can penetrate into the interface between the steel plate. steel and coating. In this situation, it may form a local battery and cause corrosion under the film.
The organic coating will bubble when corrosion occurs beneath the coating film. The bubbling position is consistent with the position of the anode and cathode of the local battery, where the anode area is acidic and the coating tends to become brittle. The anode area is alkaline, and the coating gradually softens and expands, which worsens adhesion. At the same time, due to water intrusion into the cathode domain, the bubbling is prone to growth and rupture, which eventually leads to large area corrosion.
The corrosion rate of the coated steel sheet is not only related to the physical and chemical properties of the organic coating, but also depends on the gas, liquid and ion resistance of the organic coating. The corrosion reaction of steel in contact with water and air is the result of the local action of the battery, which is the so-called electrochemical corrosion. The organic coating has the function of isolating the steel from the external corrosive environment.
When the ions pass through the coating and penetrate the interface between the steel and the coating to form a local battery, the high resistance of the organic coating is equivalent to the resistance of inserting a resistor between the two poles of the local battery, which which hinders local current flow, which hinders local current flow, which hinders corrosion of steel.
Therefore, it can be seen that the anti-corrosion performance of the coating is closely related to the composition, thickness, uniformity of the coating and the bonding strength between the coating and the steel. Increasing the thickness of the coating and the number of layers will reduce the actual small holes in the coating, which will also improve the corrosion resistance of the coating. For example, they can be used twice of coating and baking processes, or three times of coating and baking processes to obtain excellent corrosion resistance coated steel plate.
The anti-corrosion mechanism of coated steel plate is to isolate the substrate from the corrosive medium by the coating and prevent the chemical or electrochemical reaction from occurring on the surface of the substrate. Therefore, the anti-corrosion problem that we finally need to study needs to be integrated into the chemical corrosion and electrochemical corrosion of the substrate surface of the coated steel plate.
2. HOW TO CONTROL CORROSION OF COATED STEEL SHEETS
Based on the metal corrosion mechanism mentioned above, people have been searching for effective metal corrosion control methods. And after long-term practice, the following measures have basically been formed:
When selecting metal materials, it is necessary to choose those that are resistant to atmospheric corrosion;
When designing the structure, it is necessary to avoid cracks, grooves and other parts that are easy to accumulate water and cause stress corrosion cracking;
Change the composition and add a corrosion inhibitor to inhibit the corrosion process;
Do anti-rust treatment before packaging, and keep it dry and clean during storage. Use the surface protective coating.
In order to improve the corrosion resistance and special surface properties of ordinary steel sheets, various color coated steel sheets have been developed on the basis of ordinary steel sheets.
The coating of steel plate can be roughly divided into three types: organic coating, inorganic coating and composite coating. Among them, the development of organic coating steel plate is the fastest. Organic coatings can be formulated in various colors and patterns, which is why they are generally called color coated steel plates. The original color coated steel plate plate generally adopts hot rolled steel plate and galvanized steel plate. The organic coating is polyvinyl chloride, polyacrylic acid resin, epoxy resin, alkyd resin, etc. The combination of coating and steel plate can be divided into film lamination method and paint coating method.
Color coated steel plate can also be made into profiled plate, whose shape and cross-section size are basically similar to aluminum alloy profiled plate. This kind of profiled board has the advantages of good durability, beautiful appearance and convenient construction, so it is now widely used in the industry. The base of the coated steel plate is made of galvanized steel sheet, and both the back and front are painted to ensure corrosion resistance. The first layer on the front side is primer, usually epoxy primer, which has strong adhesion to metal. The back is also coated with epoxy or acrylic resin. In the past, alkyd resin was used for the surface layer, but now it is mainly polyester paint and acrylic resin paint.
