Research on the construction technology of anti-corrosion coatings in steel structures.

Abstract: This paper introduces the principle
effect
anti-corrosion coating, and the main factors that affect the anti-corrosion effect. The construction process and quality control of anti-corrosion coatings in steel structure are described.
Keywords: steel structure, anti-corrosion coating, construction technology
0 Introduction
In the field of construction engineering, steel structure with its unique advantages, especially high-rise, ultra-high-rise, light steel structure, large-span space structure, etc., because of the beautiful architectural shape, good seismic performance, etc. have been more and more widely valued and applied. Although the steel structure has many advantages and is widely used in construction, it also encounters some problems in the application, the first problem is the corrosion prevention of the steel structure.
1. The common anti-corrosion method
1.1 weather-resistant steel
changes the composition of the metal structure, adding alloy elements such as copper, chromium and nickel in the steel refining process, so that the metal surface forms a protective layer to improve corrosion resistance.
1.2 cathode protection method
attach more active metal to the surface of the steel structure to prevent corrosion of the steel, often used in underwater or underground structures.
1.3 hot-soaked zinc
immerses the rusted steel part in the zinc liquid melted at a high temperature of 600 degrees C, so that the surface of the steel is attached to the zinc layer, thus playing a role in corrosion prevention. The advantages of this method are long durability, high degree of industrialization, stable quality, and a large number of outdoor steel structures that are severely corroded by the atmosphere and are not easy to maintain.
1.4 hot-sprayed aluminum (zinc) composite coating
which is a long-lasting corrosion-resistant method comparable to the corrosion-resistant effect of hot-soaked zinc. This is done by sand blasting the steel surface to remove rust, so that its surface reveals a metallic luster and hair. The aluminum (zinc) wire that is constantly sent is then melted with acetylene oxygen flame and blown to the steel surface with compressed air to form a honeycomb aluminum (zinc) spray coating (thickness of about 80 to 100 m). Finally, fill capillary holes with epoxy paint or butyl rubber paint to form a composite coating. The advantage of this process is that the dimensions of the components are adaptable, the shape and size of the components are almost unlimited, as large as the locks of Gezhou Dam are constructed in this way. Another advantage of the process is that the thermal effects of the process are local and do not produce thermal deformation. Compared with the hot-dip zinc method, the industrialization of this method is low, the labor intensity of blasting aluminum (zinc) is high, and the quality is susceptible to the influence of the operator.
1.5 coating method
coating method is also the use of anti-corrosion coating method, with low price, wide selection, strong applicability and other advantages, is the most commonly used anti-corrosion method. This paper mainly introduces the coating method.
2. Principles and effects of corrosion-resistant coatings
2.1 Anti-corrosion physics
The physical principle of corrosion-resistant coatings is to separate the protective material from the external corrosive substances through corrosion-resistant coatings. A dense anti-corrosion coating is formed by film forming to isolate the erosion of the protected material by corrosive substances, known as shielding. Lead-containing coatings, for example, react with oil to form lead soaps to ensure the tightness of corrosion-resistant coatings.
2.2 Anti-corrosion chemical principle
Anti-corrosion chemical principle is to neutral and neutral harmless substances harmful acid-base substances to protect the material in the corrosion-resistant coating from corrosive substances. Rust-proof coatings often add certain compounds, certain pigments or their reaction products to film or moisture, which can slow corrosion (including passivation) on metal substrates. For example, substances such as aluminum hydroxide, vanadium hydroxide and zinc oxide can easily be chemically mediated with harmful substances such as acids and alkalis to achieve corrosion-resistant effects.
2.3 Anti-corrosion electrochemical effects
anti-corrosion electrochemical effects refers to the addition of some special substances in
anti-corrosion coatings
, so that water and oxygen through anti-corrosion coatings will react to form corrosion-resistant ions, so that steel and other metals surface passivation, thereby preventing the dissolution of metal ions, to achieve the purpose of corrosion protection, that is, the so-called cathode protection. The electrode level of the paint film is lower than that of the metal substrate, which is "sacrificed" as an anode in the corrosion cell, thus protecting the metal substrate (cathode). Chromate is the most common of these special substances.
3. Factors affecting the anti-corrosion effect of anti-corrosion coatings
effects of 3.1 film-forming materials
Mainly to see whether it is easy to react with corrosive media under dry film conditions or break down into small molecules in the medium. Whether from the anti-electrochemical corrosion or from a simple isolation effect, the shielding effect of anti-corrosion coating depends on the structural pores and coating pinholes of the film-forming material. Water, oxygen, and ions pass through the paint film at different speeds. Water travels much faster than ions. Oxygen is more complex and has a lot to do with temperature. Water and oxygen pass through the paint film to form corrosion cells on the metal surface. Ions pass through the paint film less, regardless of their direct effect on the metal substrate, but will increase the conductivity of the paint film. When there are more official groups in the molecular structure of the film, the structure of the paint film has fewer pores, and in the process of film formation can form a mesh three-dimensional structure with high cross-link density, thus enhancing the corrosion resistance of the coating. The physical and mechanical properties of paint film greatly affect the anti-corrosion effect of anti-corrosion coatings. They are related to the relative molecular mass, chain section, side bond group, etc. of film-forming matter.
3.2
effects of paint fillerscoloring pigments in coatings, which are used to regulate the mechanical properties of the paint film or the fluidity of the coating. For corrosion-resistant coatings, in addition to pigments, there are two types of pigments for the purpose of preventing corrosion: one is to use its chemical properties to inhibit metal corrosion of anti-rust pigments, the other is flaky pigments, through physical action to improve the shielding of the coating. Rust-proof pigments can be divided into alkaline pigments, soluble pigments and metal powder pigments. Alkaline pigments, such as red dan, lead monoxide, lead cyanamide, lead chromate, calcium lead and lead sulfate, can react with oily film-like materials to produce metal soap. After metal soap is in contact with water, its decomposition can play a role in slowing corrosion. In addition, the paint film that produces metal soap improves the shielding effect on the environment. Alkaline pigments keep paint films, metals and interfaces slightly alkaline, and also blunt metal surfaces, thus acting as corrosion-resistant. Soluble pigments, usually chromate pigments. After contact with water, the pigment dissolves chromate ions, which have strong oxidation. In the case of metal powder pigments, the widely used metal powder pigments are zinc powders. The dry coating of the zinc-rich coating consists mainly of zinc powder, which maintains direct contact between the zinc powder particles and between the substrate and the zinc powder. When moisture invades the coating, a battery consisting of zinc powder and substrate steel plates is formed. The current flows from zinc to iron, protecting the substrate from cathodic protection. In addition, the corrosion products of zinc are deposited between zinc powders and on the steel surface, which enhances the shielding of the coating. Flaky pigments shield water, oxygen and ions from corrosion factors through the coating, cutting off capillary holes in the coating. Scale-like pigments that overlap parallel to each other have a maze effect on the coating, prolonging the way infiltration of corrosion media and improving the corrosion resistance of the coating. Commonly used flaky pigments are
mica powder
aluminum powder, mica iron oxide, glass scales, stainless steel scales and so on.
4. Construction process of anti-corrosion coatings
4.1 substrate treatment
substrate treatment is aimed at removing burrs, rust, oil and other attachments from the surface of the component, so that its surface reveals a metallic luster. The more thoroughly the substrate is treated, the better the coating attachment effect. Substrate treatment methods are manual mechanical treatment, chemical treatment, mechanical injection treatment, etc. , see Table 1.
table 1 substrate treatment method
4.2 supporting coating construction
coating generally has primer layer and finish layer. The primer contains more powder, less base material, rough film, strong adhesion with steel, and good bonding with the paint. The paint is more base material, the coating film has a luster, can protect the primer from atmospheric corrosion, and can resist weathering. There is a problem of compatibness between different coatings, when choosing different coatings should pay attention to their compatible. The form of the matching coating is primer-medium paint-finish, primer-finish and primer-in-lacquer. The primer mainly adheres to and anti-rust effect, the paint mainly anti-corrosion and anti-aging effect, the role of medium paint is between the base, the paint, and can increase the thickness of the paint film. They can only be used in a matching way to get the best results. Reasonable coating methods have a great effect on ensuring coating quality, construction progress, saving materials and reducing costs. Commonly used methods of painting are manual brushing, manual roller painting, immersion, air spray and airless spray.
control of the 4.3 coating construction
the first step in the construction of the coating is to remove rust. High-quality coatings rely on thorough rust removal, generally using blasting pellets to remove rust, exposing a metallic sheen and removing all rust and oil. Coatings for on-site construction can be removed by hand. The construction temperature (5 to 38 degrees C) and humidity (relative humidity of 85%) of the coating should be suitable, the dust of the construction environment should be less, the surface of the component should not be exposed, and the 4h shall not be rained after coating. General coating 4 to 5 channels. The total thickness of the dry paint film of outdoor engineering is 150 m, the total thickness of the interior engineering is 125 m, the deviation is allowed to be 25 m, and the total thickness of dry paint film is 200 to 220 m by sea, sea or in a highly corrosive atmosphere.
5
construction steel structures are susceptible to corrosion in air or humid environments, especially when air contains acid and alkaline media. Corrosion not only causes uneven corrosion of steel surface, but also causes stress concentration due to local corrosion, which causes the early destruction of steel structure, especially under the effect of repeated impact load, but also promotes the reduction of fatigue strength and brittle fracture. Corrosion of steel structures can endanger the safety of buildings and cause huge losses. Only when designed and built, is it effectively corrosion-resistant to ensure the long life of steel structures.