Summary of Anti-corrosion Methods for Surface of Lifting Appliances

In recent years, with the development of domestic engineering construction, the lifting appliances is more and more widely used in modern production such as ships, aerospace, electric power, base materials, metallurgy, bridges and railways. The increase in the number of cranes has brought opportunities to manufacturing enterprises, as well as many challenges. According to authoritative statistics, while the number of cranes in China is increasing year by year, the number of cranes dismantled and scrapped each year is also increasing significantly, of which more than 80% of cranes are scrapped caused by corrosion failure of their metal structures. Because of the high frequency of use of lifting appliances and the relatively harsh working environment, it is often laid aside in the open air or humid and corrosive environment, and the protective layer of its skin paint often loses its protective function due to damage, thus causing the metal structure being corroded. In Zaozhuang City, there have been two cases in which the main stress components of gantry cranes were severely corroded, and the users were forced to use the cranes under overload in violation of regulations, resulting in the damage of metal structures and the accidents of machine damage and human death. The situation abroad is also not optimistic. According to statistics of the developed country America, about 50 people are killed by crane accidents every year. Corrosion of metal structure materials of lifting appliances not only easily causes safety accidents, but also causes huge waste of metal materials. Article 3.9 of Safety Rules for Lifting Appliance - Part 1: General Provisions (GB6067.1-2010) clearly stipulates that when the main stressed components of cranes are corroded, they shall be inspected and measured. When the cross-section corrosion of the main stressed component reaches 10% of the design thickness, it shall be scrapped if the repair effect is poor.

Therefore, how to improve the anti-corrosion performance of crane metal structure and reduce the energy consumption of crane in harsh environment is an urgent problem to be solved at present. In addition, the reference[6] mentioned that the metal structure of the lifting appliances serving in harsh environment such as stone processing base and strong chemical corrosion environment can hardly reach the expected life if ordinary paint are used for their metal structure, which further shows that the paint can hardly meet the anti-corrosion requirements of cranes in harsh environment. In conclusion, it is of great significance to study the applicable surface protection treatment technology of lifting appliances, delay its corrosion rate and prolong its service life.

Based on the analysis of the protection methods of crane metal structures at home and abroad and related research, this paper proposes a new protection technology for metal structures of heavy machinery, which can effectively prolong the service life of crane metal structures, save the production cost of replacing cranes, and at the same time promote the innovation ability of crane manufacturers in this field.

1. Corrosion and Cause Analysis of Metal Structure of Lifting Appliances

The main girder and other main parts of the lifting appliances are mainly made of ordinary carbon steel Q235, and Q235B, Q235C and Q235D are required for the important load-bearing components of the crane metal structure. For the general lifting appliances metal structure, boiling steel Q235F is allowed when the design temperature is not lower than -25°C. Corrosion forms of ordinary carbon steel can be divided into uniform corrosion, pore corrosion and intergranular corrosion. The harmfulness of uniform corrosion is low. Because metal components have a certain cross-sectional size, slight uniform corrosion generally does not significantly reduce the mechanical properties of metals. However, if a layer of "skin" is removed from the inner cavity surface of the box-shaped metal structure (box beam, box leg, box arm, etc.) of lifting appliances, the web plate and arm plate structure will become thinner, which will easily lead to safety accidents. The uniform corrosion is shown in Fig. 1a. Pore corrosion and intergranular corrosion are the local corrosion of metal bodies. These two kinds of corrosion will reduce the effective cross-sectional area of components, and make the parts easily break suddenly. These two kinds of corrosion behaviors are much more harmful, as shown in Fig. 1b and Fig. 1c. The research shows that intergranular corrosion is mainly caused by the residual stress inside the material or the stress exerted by the outside, which causes the material to be damaged by the combined action of stress, strain and corrosion, and this kind of corrosion will lead to extremely serious damage and failure of metal structures.

The corrosion mechanism of metal structure of lifting appliances mainly includes chemical corrosion and electrochemical corrosion. Chemical corrosion refers to the destruction of materials caused by pure chemical corrosion between materials and non-conductive media directly, while electrochemical corrosion is the most common and important corrosion type of metal materials caused by electrochemical reactions. Under normal environment, the surface of lifting appliances steel structure generally generates iron rust Fe₃O₄ and Fe₂O₃. However, at high temperature, iron oxide FeO is easily formed in steel structures. In addition, Fe₃C structure in iron and steel easily reacts with gas at high temperature as follows

Fe₃C+ O₂=3Fe+ CO₂↑

Fe₃C+C O₂=3Fe+ 2CO↑

The generated gas escapes from the steel surface, and the decarburized layer is formed on the surface of the steel structure, thus affecting the service performance of the lifting appliances. In harsh environment, the three conditions necessary for electrochemical corrosion are easy to be met: existence of potential difference, electrolyte solution, and contact. As long as the above three conditions are met at the same time, electrochemical corrosion can be formed, thus destroying the metal structure of lifting appliances.

2. Analysis of Metal Structure Protection Methods of Lifting Appliances

At present, the main anti-corrosion methods of crane metal structure involve metal structure surface coating and sacrificial anode protection method. Sacrificial anode protection method usually adds filler (such as zinc) with higher activity than steel in coating, which can protect metal structure from corrosion by sacrificial anode through electrochemical principle. Although this method does not need external power supply, it requires extremely high quality of anti-corrosion coating, consumes non-ferrous metals, and requires periodic anode replacement, resulting in high cost and complicated process. Surface coating methods are mainly divided into corrosion-resistant metal coating method and nonmetal coating method. Corrosion-resistant metal coating method generally includes electroplating method, cladding method, hot plating method, infiltration plating method and spraying method, etc. These methods feature high technological requirements and high cost, and are suitable for small workpieces. However, for large-scale lifting appliances that has been put into practical application, these methods are beyond the reach of the above methods, so they are not widely used. Nonmetal coating method is to smear the metal surface with basic antirust paint. This method is of cost efficiency and easy to operate. Although it is widely used for crane corrosion prevention, a single paint film can not completely impede the technological difference between water and oxygen, which will lead to a poor long-term effective protection of the paint. In the research of nonmetal coating method, Wang Hongfeng, et al. analyzed the specific causes of corrosion of metal structures of lifting appliances, and put forward a new paint coating process, but this method can only increase the corrosion resistance life of lifting appliances to about 5a. It can be known that the protection methods mentioned above have their own advantages and disadvantages. At present, it is impossible to find an effective anti-corrosion method for the metal structure of lifting appliances.

3. Application and Analysis of Spray Polyurea Elastomer Technology

Thermal spraying technology has developed rapidly, and has been widely used in many engineering fields as a material surface improvement technology. By using this method, the prepared material is added into spraying equipment, heated to semi-molten or molten state, and then sprayed onto the selected substrate surface with a spray gun to form a coating film. Part of properties of this coating are better than those of the substrate, so it can effectively improve the surface properties of the substrate material, or provide the substrate material with several more coating film-like structures. Spray polyurea elastomer (SPUA) technology is one of them. SPUA technology is a new solvent-free and pollution-free green construction technology developed abroad in recent ten years, which meets the current national policy requirements on ecological environment protection, energy saving and emission reduction. The appearance of SPUA technology provides a new material and construction technology for the anti-corrosion field, and also proposes a new chose for engineering application. Polyurea elastomer is a new type of universal coating material, which integrates plastics, rubber, coatings, glass fiber reinforced plastics, etc. Compared with other traditional coatings, it has obvious performance advantages: if it does not contain catalyst, it can be cured quickly, and it will not produce flowing phenomenon when sprayed on any surface; Good construction performance, low cost, suitable for large-area coating; The coating is dense and seamless, resistant to long-term corrosion by medium such as acid, alkali, salt and oil, as well as UV aging; It will have no cracking after long-term outdoor use. Its advantages and disadvantages compared with traditional coating technology are shown in Table 1.

In the research of spray polyurea elastomer (SPUA), Fedotova, a Soviet scholar, studied the synthesis of polyurea in 1960s. By the mid-1980s, Primeaux, a famous chemical expert, first successfully developed SPUA technology. This technology was put into use in some States and districts of the United States in the early 1990s. Because of its excellent comprehensive performance, it was widely welcomed by enterprises and users. Then, South Korea and Japan also introduced SPUA technology and put it into research and application successively. China introduced this spraying equipment in 1997, and developed a series of products with independent intellectual property rights, such as wear-resistant, anti-corrosion and flexible anti-collision materials. At present, domestic research mainly focuses on the practicability of polyurea coating. Sun Zhiheng, et al. put forward the polyurea elastomer technology, which has excellent anti-corrosion, wear-resistant and anti-seepage effects and has been widely applied water conservancy construction projects in China. The exploration and research on many key technologies such as polyurea coating technology of lifting appliances in China are still not perfect, and the related research reports are very few. The related research has only been done on the mechanical properties of crane metal structure after spraying polyurea coating. The results show that when the thickness of polyurea coating is 0-0.6mm, the tensile strength, maximum stress, area shrinkage and surface hardness of polyurea coating gradually increase. However, when the thickness is greater than 0.6mm, the surface hardness of the coating is basically unchanged, and the tensile strength and maximum stress gradually decrease. If polyurea coating is to be industrially applied in the field of lifting appliances, domestic experts and scholars should also make further research on the corrosion resistance and degradation behavior of the coating.

Table 1 Advantages and Disadvantages of Polyurea and Other Coating Technologies

4. Conclusion

With the progress of the times and the development of science and technology, according to the data of Analysis Report on Market Demand Forecast and Transformation and Upgrading of China's Crane Manufacturing Industry in 2015-2020 released by Preoperative Industry Research institute, after 2015, the whole crane manufacturing market gradually has been picked up, with a compound annual growth rate of 6.96%. By 2020, with the aging of the previous wave of cranes and the opening of various domestic large-scale projects, the sales volume of cranes will be close to 44,000 units. Besides, the research on the surface protection treatment technology suitable for lifting appliances to improve the service life of crane metal structure can not only save the production cost of replacing cranes, but also improve the competitiveness of crane manufacturers in the field of cranes. By analyzing the corrosion mechanism of lifting appliances and the shortcomings of current anti-corrosion methods, this paper puts forward a new type of metal structure protective coating for lifting appliances, and points out the current status and development direction for domestic and foreign research. According to the domestic energy saving and emission reduction policy, it is imperative to improve the anti-corrosion performance of cranes and promote the technological progress of domestic crane industry. SPUA anti-corrosion technology will have broad prospects in the crane industry.