1Anti corrosion measures for steel structure

(1) Weathering steel: the steel with better corrosion resistance than general structural steel is called weathering steel, which generally contains phosphorus, copper, nickel, chromium, titanium and other metals to form a protective layer on the metal surface to improve corrosion resistance.Its low temperature impact toughness is also better than that of general structural steel.The standard is weathering steel for welded structure (gb4172-84).

(2) Hot dip galvanizing: hot dip galvanizing is to immerse the derusted steel members in the molten zinc solution at a high temperature of about 600 ℃, so that the surface of the steel members is attached with a zinc layer. The thickness of the zinc layer shall not be less than 65 μ m for the thin plate below 5mm and 86 μ m for the thick plate.So as to prevent corrosion.The advantages of this method are long service life, high degree of industrialization and stable quality.Therefore, it is widely used in steel structure which is seriously corroded by atmosphere and is not easy to maintain.Such as a large number of transmission towers, communication towers, etc.In recent years, there are a lot of profiled steel plates in light steel structure system.Hot dip galvanizing is also widely used to prevent corrosion.The first process of hot dip galvanizing is pickling and derusting, and then cleaning.These two processes are not complete will leave a hidden danger to the anti-corrosion.So we have to deal with it thoroughly.For steel structure designers, it is necessary to avoid designing components with joint surface, so as to avoid incomplete pickling or unclean acid solution in the joints.The phenomenon of yellow water flowing on the surface of galvanized steel is caused.Hot dip galvanizing is carried out at high temperature.For tubular members, both ends should be open.If the two 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, the liquid zinc will not flow smoothly and easy to accumulate in the tube.

(3) Thermal spraying aluminum (zinc) composite coating: This is a long-term anti-corrosion method equivalent to hot-dip galvanizing.The specific method is to remove rust on the surface of steel members by sand blasting, so as to expose metal luster and roughen the surface.The aluminum (zinc) wire is melted by the heat source of thermal spraying equipment (acetylene oxygen combustion flame, arc, plasma arc, etc.) and blown to the surface of steel members by compressed air to form honeycomb aluminum (zinc) spraying layer (thickness about 80 μ m ~ 100 μ m).Finally, the pores were filled with epoxy resin or neoprene paint to form composite coating.This method can not be used in the inner wall of the tubular member, so the two ends of the tubular component must be sealed to prevent the inner wall from corrosion.The advantage of this process is that it has strong adaptability to the size of components, and the shape and size of components are almost unlimited.Shiplocks as large as Gezhouba are also constructed in this way.Another advantage is that the heat effect of this process is local and constrained, so it will not produce thermal deformation.Compared with hot-dip galvanizing, the industrialization degree of this method is lower, and the labor intensity of sand blasting aluminum (zinc) is higher.

(4) Coating method: the corrosion resistance of coating method is generally not as good as that of long-term anti-corrosion method (but the anti-corrosion life of fluorocarbon coating can even reach 50 years).Therefore, there are more indoor steel structures or outdoor steel structures which are relatively easy to maintain.It has low cost at one time, but high maintenance cost when used outdoors.The first step of coating method is to remove rust.A good coating depends on thorough rust removal.Therefore, the coating with high requirements is usually derusted by sandblasting and shot peening to expose the luster of metal and remove all rust and oil stains.The coating applied on site can be derusted by hand.The surrounding environment shall be taken into account in the selection of coating.Different coatings have different resistance to different corrosion conditions.The coating is generally divided into primer (layer) and topcoat (layer).The primer contains more powder and less base material.The film is rough, has strong adhesion with steel and good adhesion with topcoat.Topcoat is based on many materials, film-forming luster, can protect the primer from atmospheric corrosion, and can resist weathering.The compatibility of different coatings should be paid attention to when choosing different coatings.Proper temperature (5 ~ 38 ℃) and humidity (relative humidity no more than 85%) should be adopted for coating construction.The construction environment of coating should be less dust and there should be no condensation on the surface of components.No rain within 4 hours after painting.The coating is generally made 4-5 times.The total thickness of dry paint film is 150 μ m for outdoor engineering and 125 μ m for indoor engineering, and the allowable deviation is 25 μ M.The total thickness of dry paint film can be increased to 200 ~ 220 μ m on the sea or in the strong corrosive atmosphere.

(5) Cathodic protection: the corrosion of steel structure by adding active metal instead of steel.Often used in underwater or underground structures. 

2. key points of steel structure installation

(1) Friction coefficient: where f is the force that causes the initial slip of the specimen measured by the anti slip test, NF is the number of friction surfaces, and is the sum of the measured values of the pre tension of the high strength bolt corresponding to F. 

(2) Torque coefficient: where D is the nominal diameter of high strength bolt (mm), M is the applied torque value (n · m), and P is the bolt preload.The average value of torque coefficient K must be 0.110-0.150 for grade 10.9 high strength large hexagon bolt connection.The standard deviation should be less than or equal to 0.010. 

(3) Initial tightening torque: in order to reduce the influence of steel plate deformation during bolt tightening, secondary tightening can be used to reduce the interaction between successively tightening bolts.The first tightening of high strength bolt is the initial tightening, so that its axial force should reach 60% – 80% of the standard axial force.

(4) Final tightening torque: the final tightening torque of high-strength bolt is final tightening torque.Considering the loss of prestress, the final tightening torque is generally 5% – 10% larger than that calculated theoretically according to the design pretension.