Corrosion prevention measures for steel structures

Corrosion prevention measures for steel structures

1Weathering steel: the steel whose corrosion resistance is better than the steel of general structure is called weathering steel, which generally contains phosphorus, copper, nickel, chromium, titanium and other metals so that the metal surface forms a protective layer to improve the corrosion resistance. Its low-temperature impact toughness is also better than the general structural steel.

2 Hot dipping zinc: hot dipping zinc is to immerge the steel member after rust removal into the molten zinc solution at about 600℃ so that the surface of the steel member is attached with zinc layer. The thickness of the zinc layer shall not be less than 65μm for the thin plate below 5mm, and not less than 86μm for the thick plate. Thus play the purpose of anti-corrosion. The advantages of this method are long durability, high degree of industrialization and stable quality. Therefore, it is widely used in outdoor steel structures which are seriously corroded by the atmosphere and are not easy to maintain. Such as a large number of transmission towers, communication towers and so on. In recent years, a large number of light steel structure systems in the profiled plate. Also more use of hot-dip zinc corrosion prevention. The first process of hot dipping zinc is pickling for rust removal, followed by cleaning. These two processes are not thorough will give corrosion prevention risks. So it has to be dealt with thoroughly. For the designers of steel structures, should avoid the design of components with laminating surfaces, so as to avoid the gap in the laminating surface is not completely pickled or acid washing is not clean. Causes the galvanizing surface to flow yellow water phenomenon. Hot dipping zinc is carried out at high temperatures. Tubular members should be left open at both ends. If both ends are closed, the air in the pipe will expand and the head plate will burst, thus causing safety accidents. If one end is closed, zinc liquid flow is not smooth, easy to accumulate in the tube.

3 Thermal spraying aluminum (zinc) composite coating: this is a kind of long-term corrosion prevention method which is equivalent to hot-dip zinc corrosion prevention effect. The specific approach is to make sandblasting on the surface of the steel component to remove rust, so that the surface of the metallic luster and hair. Then use the heat source of thermal spraying equipment (acetylene-oxygen combustion flame, arc, plasma arc, etc.) to melt the aluminum (zinc) wire constantly sent out, and blow it to the surface of the steel member with compressed air to form a honeycomb-like aluminum (zinc) spraying layer (thickness of about 80μm~100μm). Finally, the capillaries are filled with epoxy resin or neoprene paint to form a composite coating. This method cannot be applied on the inner wall of the tubular member, so both ends of the tubular member must be air-tight sealed to prevent corrosion of the inner wall. The advantages of this process are strong adaptability to the size of the component, and the shape and size of the component are almost unlimited. Another advantage is that the thermal effects of this process are localized and constrained, so thermal deformation is not generated. Compared with hot dipping zinc, this method is less industrialized and the labor intensity of sandblasting aluminum (zinc) is higher.

4 Coating method: coating method corrosion prevention is generally not as effective as long-term corrosion prevention method (but the current fluorocarbon coating corrosion prevention number of years or even up to 50 years). Therefore, there are many indoor steel structures or outdoor steel structures that are relatively easy to maintain. It is low cost once, but high maintenance cost when used outdoors. The first step in the construction of the coating method is derusting. High-quality coating depends on thorough derusting. Therefore, the high requirements of the coating are generally used sandblasting shot peening, exposing the luster of the metal, remove all rust and oil stains. The coating applied in the field can be removed by hand. The choice of coating should take into account the surrounding environment. Different coatings have different resistance to different corrosion conditions. Coatings generally have primer (layer) and finish (layer). The primer contains more powder and less base material. Rough film-forming, strong adhesion with steel, good adhesion with finish paint. Topcoat is the base material, the film is glossy, can protect the primer from atmospheric corrosion, and can resist weathering. There are compatibility problems between different coatings, before and after the selection of different coatings should pay attention to their compatibility. The construction of the coating should have an appropriate temperature (between 5~38℃) and humidity (relative humidity is not greater than 85%). The construction environment of the coating should be less dust, and there should be no condensation on the surface of the component. Do not get wet within 4 hours after painting. The coating is generally done 4~5 times. The total thickness of dry film is 150μm for outdoor engineering and 125μm for indoor engineering, and the allowable deviation is 25μm. In the sea or sea or in the atmosphere with strong corrosion, the total thickness of the dry film can be increased to 200~220μm.

5 Cathodic protection method: the addition of more active metal on the surface of steel structure to replace the corrosion of steel. Often used in underwater or underground structures.

Surface finish before the paint

In the process of processing, transportation and storage of the workpiece, the surface is often with oxide scale, rust molding sand, welding slag, dust and oil and other dirt. To make deep can firmly attached on the surface of the workpiece, the workpiece surface before coating must be clean, otherwise, not only affects the bonding strength of coating and base metal and the corrosion resistance but also can make the matrix metal under even with protective coating can also continue to corrosion, the coating peeling, affect the mechanical properties and service life of the workpiece. Therefore, the surface treatment of the workpiece before painting is an important guarantee and measure to obtain a good quality protective layer and extend the service life of the product.

Common surface treatment methods are:

1.Handwork: such as scraper, wire brush or grinding wheel, etc. Manual can remove the surface of the workpiece rust and oxide skin, but manual processing labor intensity, low production efficiency, poor quality, cleaning is not thorough.

2.Chemical treatment: mainly the use of acid or alkaline solution and workpiece surface oxide and oil chemical reaction, make it dissolved in acid or alkaline solution, in order to achieve the removal of the workpiece surface rust oxide skin and oil purpose. Chemical treatment is suitable for the cleaning of thin plates, but the disadvantage is that if the time is not properly controlled, even if the corrosion inhibitor is added, the steel can also produce an over-corrosion phenomenon. For more complex structure and porous parts, after acid pickling, immersed in crevices or residual acid in the cavity is difficult to wipe out, if not handled properly, will become the hidden trouble of the workpiece after corrosion, chemicals and volatile, high cost, chemical discharge after processing work is difficult, if not handled properly, will cause serious pollution to the environment. With the increase of people's awareness of environmental protection, this treatment method is being replaced by mechanical treatment.

3.Mechanical treatment method: mainly including shot blasting and shot peening method. Shot blasting cleaning method is to use centrifugal force to accelerate the projectile, projectile to the workpiece for rust cleaning method. But the flexibility of shot blasting is poor, limited by the site, some blindness when cleaning the workpiece, in the inner surface of the workpiece is easy to produce less than the dead corner. Equipment structure is complex, wearing parts, especially blades and other parts wear fast, maintenance hours, high cost, one-time investment.

4.Shot peening is divided into shot blasting and sandblasting. Shot peening for surface treatment, striking force, cleaning effect is obvious. But the processing of shot peening on sheet workpiece, easy deformation of components, and steel shot hit the surface (no matter blasting or peening) make the metal base material deformation, because Fe3o4 Fe2o3 and plastic, no broken after stripping, deformation and the oil film and substrate, so with the oil of artifacts, shot blasting, shot peening can not thoroughly remove grease. In the existing workpiece surface treatment methods, the best cleaning effect is also sandblasting cleaning. Sandblasting is suitable for the surface of the workpiece requires higher cleaning. But China's current general sandblasting equipment is mostly composed of hinge dragon, scraper, bucket elevator and other original heavy sand transport machinery. Users need to build a deep pit and do waterproof layer to install machinery, construction costs are high, maintenance workload and maintenance costs are great, sandblasting process produces a large number of silica dust can not be removed, seriously affect the health of operators and pollution of the environment.