Application of thermal spraying for long-term corrosion protection in foreign steel structure bridges
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The application of thermal spraying in bridges: In Europe, 40 million pounds of zinc wire are used each year for thermal spraying purposes to prevent corrosion, while in the United States, 2 million pounds of zinc wire are used annually for this purpose； The corrosion resistance life of conventional thermal spray composite coatings is typically 20 to 40 years or more. Many bridges in the UK have over 25 years of experience with zinc plating for corrosion protection. Based on their experience with zinc spraying on large structures, the engineers responsible for bridge maintenance believe that for bridges in harsh environments, especially those with areas that are difficult or impossible to access, zinc or aluminum spraying is an effective treatment method. The UK cited examples of large steel bridges across the country where thermal spraying has been used for corrosion protection, all of which have achieved long-term corrosion resistance with no need for maintenance for over 25 years. Among them, the Fourth Queensferry Crossing Bridge was the largest steel bridge in the world in the 1970s; it required 20,000 tons of steel, and 1 million pounds of zinc wire was used for coating, which provided excellent corrosion protection – it functioned without any repairs for over 20 years. The George Street Bridge in Newport, South Wales, was built in 1964 and is located in an area with high levels of industrial pollution; it was protected against corrosion through zinc plating. By 1985, after 21 years without any maintenance, the condition of its coating was found to be good, and it was determined that no maintenance would be required for at least another 5 years. The fourth road bridge in Qucesferry, Scotland, with a main span of 1000 meters, is located in a windy marine environment. It was protected by zinc spraying followed by a sealing treatment; after 13 years without any maintenance, its protective coating still retained its original color and the structure remained in perfect condition.
Three common methods for preventing corrosion in steel structure bridges, both domestically and internationally, are introduced. Currently, there are roughly three such methods, which are briefly described as follows: 1. Corrosion prevention method using heavy-duty coatings. Using coatings for corrosion protection not only results in an attractive appearance but also involves a simple application process; this method has been widely used for many years. With the development of coating technologies, heavy-duty anti-corrosion coatings have become the mainstream for protecting steel bridges. In heavy-duty anti-corrosion coatings, both domestically and internationally, the anti-corrosion coating processes and the types of coatings used are quite similar; that is, the coating process involves a multi-layer system consisting of a primer, an intermediate coat, and a topcoat ； The paint types all consist of an epoxy (inorganic) zinc-rich primer, an epoxy mica-iron oxide intermediate coat, and an epoxy polyurethane or epoxy colored top coat, or a chlorinated rubber top coat, etc. Mechanism of action of heavy anti-corrosion paint: 1. Shielding effect: The paint coating mechanically separates steel from the corrosive environment. 2. Passivation and corrosion inhibition: In a painting system, the first shop primer provides passivation and corrosion inhibition effects for steel, enhancing the adhesion of the paint layer; however, its anti-corrosion effect is quite weak. 3. Cathodic protection effect: The addition of zinc powder to anti-corrosion primers (such as zinc-rich primers) provides cathodic protection for steel. However, with the advancement of anti-corrosion technologies and the continuous deterioration of the human living environment, the environmental impact of coatings has gradually come to be recognized; as a result, heavy anti-corrosion coatings are no longer used extensively on steel bridges abroad. The anti-corrosion lifespan of heavy-duty anti-corrosion coatings is generally 8 to 10 years, as demonstrated by numerous application examples around the world. British Standard BS5493 specifies that, in any environment, thermal spraying with zinc or aluminum is recommended for corrosion protection with a service life of 10 to 15 years or more. II. Hot-dip galvanizing for corrosion protection: This method is suitable for the guardrails, escalators of steel structure bridges, as well as prefabricated small steel bridges. The thickness of the hot-dip zinc coating generally does not exceed 80 μm, and it can provide effective protection for around 10 years in a typical industrial atmospheric environment. However, the hot-dip galvanizing anti-corrosion process requires immersing the workpieces in a bath of molten zinc metal; due to the limitations of the bath’s capacity, it is not possible to apply this anti-corrosion treatment to steel bridge components of larger sizes. Secondly, hot-dip galvanizing for corrosion protection cannot be carried out on-site; it can only be done in specialized factories, which inevitably results in high costs associated with transportation back and forth. Furthermore, once the hot-dip galvanized coating is damaged, it cannot be repaired on its own. Therefore, the hot-dip galvanizing method has significant limitations when it comes to providing comprehensive corrosion protection for steel structure bridges. III. Thermal spraying anti-corrosion method: Thermal spraying technology is a method that uses a heat source to heat spraying materials such as metals, non-metals, and ceramics, atomizing and spraying the molten particles which are then deposited on the surface of the substrate to form a special surface coating. There are mainly three types of heat sources used for spraying: the first is the arc heat source, the second is the plasma heat source, and the third is the gas heat source. Thermal spraying is thus divided into arc spraying, plasma spraying, and flame spraying; flame spraying and arc spraying are the main ones used in the field of corrosion protection. Compared to flame spraying, arc spraying uses electrical energy instead of gas combustion, which significantly improves work efficiency and safety. Especially with the advent of mechanized equipment for arc spraying, this technology is now fully capable of meeting the requirements of bridge construction timelines. Moreover, the arc temperature is much higher than that in flame spraying, and the adhesion of the coating is also much stronger; as a result, the quality of the coating is sufficient to ensure long-term corrosion resistance. Due to high labor costs in the United States, the cost of using arc spraying for corrosion protection is even lower than that of heavy-duty anti-corrosion paints. Decades of testing have proven that thermal spraying technology is the best method for long-term corrosion protection of steel structure bridges, and this conclusion has been recognized by many government agencies and engineering communities around the world.