Coatings have been used for thousands of years as a simple and cost-effective means of preserving corrosion. Since the industrial revolution, large-scale production of metal materials such as steel has taken many measures to reduce the problem of metal corrosion, but organic coatings have so far been the most effective, economical and most common. At present, the most common and most important anticorrosive and shop primer for steel structures is the zinc-rich primer. The development of modern industrial technology not only provides better technical material conditions for the development of steel structure anti-corrosion coatings, but also provides a broader market, and will also make anti-corrosion coatings mature and perfect.
Zinc-rich paint
Zinc-rich paint is mainly used in the coating system for the corrosion protection of various steel structures in the form of zinc-rich primer. Because its formula is rich in a large amount of electrochemical anti-rust pigment, zinc powder, it is called zinc-rich. coating. Zinc-rich paint film is hard, good adhesion, strong anti-rust ability, excellent three-proof performance and anti-oxidation and anti-corrosion performance, as well as excellent cathodic protection and static conduction, no adverse effect on welding performance, and can be large Some coatings have been used together, and have become the most common and important primer in ferrous metal anticorrosive applications such as steel. There are few competitors in the field of atmospheric and marine anti-corrosion. Only when the material to be coated is in an acid-alkaline corrosive medium, since these media can penetrate and easily react with the zinc powder to generate hydrogen, other anti-rust primers are used. Zinc-rich primers can be divided into two major categories in production and application, namely inorganic zinc-rich primers using inorganic substances such as silicates as binders and organic zinc-rich primers using epoxy resins as binders. When used in the field of anti-corrosion, they must meet the following requirements: 1Because the zinc powder in the primer should be in close contact with the steel to exert its electrochemical protection, the pretreatment of the substrate surface is strict, and the substrate is coated. Sandblasting must be carried out to ensure the quality of construction. 2 Zinc-rich primer paint film is usually thin and susceptible to external environmental media, so it is generally not used alone, it must be matched with appropriate intermediate paint and top coat, and the specified film thickness can be achieved. Because zinc-rich primers have good corrosion resistance and weldability, they are also commonly used in shop primers, also known as steel pretreatment primers or maintenance primers. The steel plate or steel structural parts coated with the maintenance primer do not corrode during pretreatment, cutting and welding. When the steel structural components are installed, no need to spray sand. Just remove the dirt on the surface of the coated objects with water and directly carry out the lowering. Painting of primer or topcoat.
Requirements for zinc powder
Zinc powder is the most important electrochemical anti-rust pigment. Compared with other metals, it has the following characteristics: lighter than iron, good ductility, and alloyed with iron. The most important is its electrochemical activity, its standard electrode potential is -0.76 V, which is more active than iron -0.44 V, and is "self-sacrificing". Zinc is melted and processed into fine particles of high-purity zinc powder. It is used as an important anti-rust pigment in anti-corrosion coatings. When the paint film is eroded, zinc powder is first corroded as an anode, and the substrate steel is used as a cathode. protection. Since the oxidation product formed by the zinc powder as a sacrificial anode seals the coating, the protective effect of the coating on the substrate is enhanced. Zinc powder is gradually consumed in protecting the substrate from erosion, but at a very slow rate. The formation of corrosion-resistant products reduces the potential difference between the coating and the substrate. When the paint film is damaged, new metal zinc is exposed, and the potential difference will immediately increase, resulting in a new round of strong Cathodic protection, so the corrosion of zinc-rich paint will not spread from the damage of the paint film to the surroundings. In order to ensure that the zinc powder in the zinc-rich primer can be combined with the steel to function as a conductive and sacrificial anode, to ensure the working quality of the zinc-rich primer, the content of the zinc powder contained in the zinc-rich coating accounts for the total dry film quality. The percentage has been stipulated internationally. For example, the American Steel Structure Painting Association standard "SSPC - paint 20" specifies: inorganic zinc-rich paint is not less than 74%, organic zinc-rich paint is not less than 77%. The reason why the zinc content in the organic zinc-rich paint is higher than that of the inorganic zinc-rich paint is that the conductivity is inferior to that of the inorganic zinc-rich paint. The long-acting anti-corrosion inorganic zinc-rich coating products with many single coats have a zinc powder content of 75% in water-soluble paints and over 82% in solvent-based paints. The requirements for the thickness of the zinc-rich primer are also different for different coating applications, and the requirements for the size and shape of the zinc powder are also different, as shown in Table 1.
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