Structural characteristics of steel bridges and the necessity of adopting heavy anti-corrosion technology

At present, steel bridges at home and abroad are very rich in structural forms, including large steel box girder structure, steel truss structure, steel pipe arch or steel box arch structure, and a variety of steel composite beam structures. Compared with the general concrete bridge, steel bridge has the advantages of large span capacity, high strength and short construction period.

Since the completion of Wuhan Yangtze River Bridge, the first steel truss bridge across the Yangtze River in 1955, large-scale steel structure bridges have been springing up, and the steel consumption of such bridges is generally more than 10000 tons or even more than 100000 tons.The surface corrosion, stress corrosion and corrosion fatigue are the main reasons for the appearance defects, life reduction and even loss of working capacity of the super large steel bridge members.

So far, the corrosion problem of steel structure is bringing huge losses to the national economy of all countries in the world.In some major industrial countries, the annual economic loss caused by steel structure corrosion accounts for 2% – 4% of the GDP.

In 1975, the economic loss caused by corrosion in the United States was about 70 billion US dollars, accounting for 4.2% of the GDP of that year, and as high as 126 billion US dollars in 1982; in 1969, the corrosion loss in Britain was 1.365 billion pounds, accounting for 3.5% of the GDP; in 1976, the corrosion loss of Japan was 9.2 billion US dollars, accounting for 1.8% of the GDP; according to the statistics of China in 1995, the corrosion loss was as high asMore than 150 billion yuan, accounting for about 4% of GDP.

At present, the economic loss caused by steel structure corrosion in the world has reached hundreds of billions of dollars every year.Moreover, the accidents caused by the corrosion of steel structure endanger the safe operation of the structure, and the catastrophic accidents caused by corrosion are common, especially for the welded steel structure and the steel structure under the condition of large stress, the corrosion will be greatly accelerated under the action of stress.

The corrosion protection of bridge steel structure has become a hot topic.Only in the design and construction, according to its own structural characteristics and environmental conditions, adopt the corresponding heavy-duty anti-corrosion technology, and reasonably consider the later coating maintenance, can we ensure the normal use and long-term life of the steel bridge.
▍ application of heavy anti-corrosion technology in bridge steel structure

2.1 Application trend of heavy duty anticorrosion technology

In Europe and the United States, the development process of steel bridge anti-corrosion is as follows: in the 1940s, paint anticorrosion was adopted; in the 1950s and 1970s, heavy-duty anti-corrosion coating, hot-dip galvanizing corrosion protection, flame spraying anti-corrosion and arc spraying anti-corrosion coexisted; after the 1980s, with the development of arc spraying technology, arc spraying anti-corrosion was widely used, mostly spraying zinc at the initial stage,Now arc spraying aluminum has become the trend of corrosion protection.

20 After the 1990s, with the increasingly frequent international exchanges, some advanced heavy-duty coatings brands from Europe and the United States have successively entered China, such as Akzo Nobel, Hempel of Denmark and sigma of the NetherlandsInternational well-known coating companies have successively built factories in China, which objectively promoted the development of domestic heavy-duty anti-corrosion technology, gradually improved the traditional mode of lead coating system in China, and formed heavy-duty anti-corrosion technology with metal spraying and zinc coating as the core.

2.2 List of heavy-duty anti-corrosion equipment for the exterior surface of bridge steel structure

2-1 Application examples of system number, variety and coating type

System 1

Aluminum spraying arc spraying aluminum sealing paint epoxy micaceous iron sealing paint intermediate coating epoxy micaceous iron intermediate coating polyurethane topcoat: Wuhan Yangluo Yangtze River Bridge, Wuhan Junshan Yangtze River Bridge, Hainan Qiongzhou bridge, Zhejiang Qiandao Hunan Pu Bridge, Zhoushan Taoyaomen bridge, Wanxian Yangtze River Bridge, etc

System 2

Primer – inorganic silicate zinc rich primer, sealer – Epoxy iron red sealer – intermediate paint, epoxy micaceous iron intermediate paint – topcoat polyurethane topcoat: Dongping Bridge in Foshan, Guangdong, Humen Bridge in Guangdong, Haicang Bridge in Xiamen, the Second Yangtze River Bridge in Nanjing, Yichang Yangtze River Bridge in Yichang, and Mingshi Strait Bridge in Japan

System 3

Primer – epoxy zinc rich primer – intermediate coat – epoxy micaceous iron intermediate paint – top coat: Shanghai Huangpu Bridge, Shanghai Yangpu Bridge, Wuhu Yangtze River Bridge, Dongguan Dafen North Bridge, etc

▍ heavy duty anticorrosion coating of bridge steel structure

3.1 Action mechanism and failure mechanism of heavy duty anticorrosive coatings

With the development of coating technology, heavy duty anti-corrosion coating has become the mainstream of steel bridge anti-corrosion (such as system 2 and system 3 above).In heavy-duty anticorrosive coatings, the anticorrosive coating process and coating varieties are very similar, that is, the coating design consists of primer, intermediate paint and topcoat.

The paint is composed of epoxy (inorganic) zinc rich primer, epoxy mica iron oxide intermediate paint and polyurethane or epoxy or chlorinated rubber topcoat.The mechanism is as follows.

1 The shielding coating mechanically separates the steel from the corrosive environment.

2Passivation and corrosion inhibition

In the coating system, the first shop primer has passivation and corrosion inhibition effect on steel and increases the adhesion of paint layer

3.3Cathodic protection

If zinc powder (such as zinc rich primer) is added to the anticorrosive primer,The cathodic protection coating for steel is mainly the isolation protection of mechanical shielding. The aging and pulverization of the coating weakens or loses its effect. The zinc powder for cathodic protection is the adhesion of film-forming materials in the coating and steel. With the cracking and aging of film-forming materials, zinc powder cannot combine with steel,In this way, the cathodic protection will disappear naturally.

Secondly, the paint coating itself has numerous micro pinholes. Under salt spray and humid environment for a long time, chloride ions and water molecules will corrode the base metal through the pinholes. At the junction of the paint layer and the base metal, the volume of steel corrosion products accumulates and expands, resulting in the peeling off of the paint layer, and the corrosion spreads rapidly around the peeling off area of the paint layer, leading to the failure of the whole anti-corrosion system.

3.2 Construction technology of heavy duty anticorrosive coating

3.3.1 Surface treatment before coating of heavy-duty anti-corrosion coating the surface treatment grade of heavy-duty anti-corrosion coating primer is generally sa2.5-sa3.0 and relative roughness is 25-70 μ M. sand blasting, shot blasting and other spray derusting methods are usually used for surface treatment, and local small area can be mechanically derusted to ST3 grade.

In special cases, such as water-borne inorganic zinc rich coating has higher requirements for surface treatment, sometimes its cleanliness must reach the surface treatment level of metal spraying.

3.2.2 High pressure airless spraying technology is widely used. At present, the most popular way at home and abroad is to use high pressure airless spraying technology.Its working principle is to pressurize the coating to 210 kg / cm2, atomize the coating into fine particles through the nozzle, and spray it directly to the surface of the coating.

Compared with the general air spraying method, the high pressure airless spraying has the characteristics of high efficiency, small coating loss, thick film-forming, high coverage rate and strong adhesion, which is well adapted to the large-scale coating of bridge steel structure anti-corrosion.

3.3.2 The coating and post curing of inorganic zinc rich coating inorganic zinc rich coating can resist the corrosion of water, sea water and chloride because silicate or silicic acid hydrolysate combines with zinc powder and reacts with steel to form zinc silicate iron, which forms strong chemical bond on steel surface.

The corrosion resistance and durability of inorganic zinc rich coatings are far better than that of epoxy zinc rich coatings. According to the National Aeronautics and Space Administration (NASA), among all kinds of zinc rich coatings, the water-borne inorganic zinc rich coatings have the best corrosion resistance, and their service life in marine atmospheric conditions is at least 25 years.

The anti rust mechanism of inorganic zinc rich coating is that zinc powder directly contacts with the steel surface of the substrate, and forms a strong chemical bond combination. When water penetrates into the paint film, it will form a battery composed of zinc powder and substrate steel plate. Due to the electrochemical activity of zinc (standard electrode potential is -0.7635) 449v, the current flows from zinc to iron, which makes the steel subject to cathodic protection.

Therefore, the surface treatment of the substrate must be strict, must reach SA 2.5 grade, and make the surface have a certain roughness, increase the surface area of the substrate, and ensure that zinc powder can be in close contact with steel. If the treatment is not complete and there is residual rust layer, the above anti rust mechanism can not be established.The following is the film forming mechanism of waterborne inorganic zinc rich coatings.

In the formula: me Na, K, LiH + – ① for post curing coatings, they are additional acidic compounds, such as dilute H3PO4; ② for self curing coatings, water-based (CO2 + H2O) water-based inorganic zinc rich coatings in the air, only through the evaporation of water can the wet film form a dense network structure paint film of zinc silicate, so this kind of coating must be at the specified temperature (surface construction temperature)Construction under the environment of above 5 ℃ and below 50 ℃ and humidity (relative humidity below 85%).At the same time, because the inorganic zinc rich coating must absorb the moisture in the air during curing, so the air relative humidity must be kept above 60% and the curing time should be 5-7 days. 

3.3 Application of metal spraying technology in bridge steel structure

At present, the anti-corrosion life of heavy-duty coatings is generally 10-15 years, which has been proved by a large number of application examples in the world.The British standard bs5493 stipulates that no matter what kind of environment, the anti-corrosion period is more than 15 years, and metal spraying (such as zinc spraying and aluminum spraying) is advocated.

3.3.1 Anticorrosion principle of metal spraying (arc spraying)

Among the metal spraying technologies, arc spraying is the most widely used, and its application prospect is also more extensive (such as the above system 1).The anti-corrosion principle of arc spraying is to use arc spraying equipment to heat, melt, atomize and spray two charged wires (such as zinc and aluminum) to form an anticorrosive coating, and an organic sealing coating is added to form a long-term anti-corrosion composite coating

It has a long corrosion resistance life, and its anticorrosion life can reach more than 50 years. At the same time, the anti-corrosion coating does not need any other anti-corrosion maintenance during the service life of 30 years; for the maintenance after 30 years, it only needs to brush the sealing coating on the arc spraying layer; it does not need to re spray to realize one-time anti-corrosion and the coating is durable and effective.

Arc spraying coating and metal substrate have excellent coating adhesion (up to 10MP). Metal spraying layer is combined with base metal by mechanical inlaying and micro metallurgy. Under slight bending, impact or collision, it can also ensure that the anti-corrosion coating will not fall off, peel off, bond firmly and effectively for a long time, which can not be achieved by any other surface anti-corrosion coating.

The anti-corrosion principle of arc spraying zinc and aluminum coating is cathodic protection. In corrosive environment, even if the anticorrosive coating is partially damaged, it still has the effect of sacrificing itself to protect steel substrate, and its protection effect and adhesion are far lower than that of arc spraying coating.

3.3.2 Technical advantages of arc spraying

Compared with flame spraying, electric energy is used to replace gas combustion, which greatly improves work efficiency and safety. Especially with the emergence of mechanized arc spraying equipment, arc spraying technology can fully meet the needs of bridge construction period, and the arc temperature is far higher than that of flame, and the coating adhesion is far greater than that of flame spraying, so the coating quality is also completely acceptableIn order to meet the needs of long-term corrosion protection.

Due to the high labor cost in the United States, the cost of arc spraying is even lower than that of Heavy-duty paint.After decades of testing, it has been proved that spraying technology is the best method for long-term corrosion protection of steel structures. This conclusion has been recognized by government departments and engineering circles in many countries.

3.3.3 Construction technology of arc spraying

The general arc spraying equipment consists of rectifier power supply, control device, spray gun, wire tray or wire feeding device, compressed air supply system, etc.Wire tray and compressed air supply system are the same as wire flame spraying.The process parameters of arc spraying zinc and aluminum are not only related to the spraying materials, but also depend on the equipment and production efficiency

3.3.4Arc spraying long-term anti-corrosion technology has been widely used in coal mine, railway, water conservancy, port and wharf, metallurgy, machinery, radio and television, medical, electric power, fire protection and other fields since the 1990s.

Such as Majishan Port Wharf steel pile of Baoshan Iron and Steel Group, rail functional parts of Shanghai Maglev express train, Three Gorges water control project of Yangtze River, steel box girder and bridge deck of Wuhan Junshan Yangtze River Bridge,As well as the steel structure bridges such as hunanpu bridge of Chun’an Qiandao, Huangbai River Bridge of Yangtze River, Xialaoxi bridge, Sanyuanli overpass of Guangzhou airport and Pizhou canal bridge of XuLian expressway, long-term arc spraying anticorrosion technology has been adopted for corrosion protection, and good anticorrosion effect has been achieved.China has been able to solve the problem of corrosion protection of national large steel bridges by using arc spraying long-term anti-corrosion technology.

Up to now, the heavy-duty anti-corrosion technology of bridge steel structure has gradually formed two major development trends. One is the rapid development of heavy-duty anti-corrosion system based on inorganic zinc rich coating; the other is the relatively mature heavy-duty anti-corrosion system based on metal spraying layer. These two systems have attracted the attention of various types of steel structure bridge anti-corrosion due to their excellent anti-corrosion performance and environmental adaptability.In the future decades, it will save a lot of steel bridge anti-corrosion maintenance costs, reduce environmental pollution, prolong the service life of steel bridges, and produce huge economic and social benefits.