Corrosion and protection of carbon steel bolts in LNG terminal
LNG terminals are mostly located in coastal areas, in the marine atmospheric environment with high humidity and high salt. According to ISO 12944-2, coastal and offshore areas with high salinity belong to high corrosive environment and are classified as C5. However, a large number of carbon steel bolts are used for natural gas pipelines and utility pipelines in LNG terminal. In this environment, ordinary carbon steel bolts will soon corrode, the bolt corrosion products will expand, and the threads will bite together. It is difficult to screw the nuts during maintenance, and they can only be cut off by gas cutting, which is labor-consuming, time-consuming and expensive, and with the extension of time, When the strength of a severely corroded bolt can not bear the shared pressure, it breaks. The pressure it bears is shared with other bolts, which is very easy to cause chain fracture of corroded bolts, flange leakage and unpredictable economic losses and personal injury. In order to avoid the above situation and effectively prevent the corrosion of carbon steel bolts, it is necessary to understand the causes of intensified corrosion of carbon steel bolts in LNG terminal.
Causes of intensified corrosion of carbon steel bolts in LNG terminal
The atmosphere contains water and oxygen. A thin electrolytic liquid film containing oxygen is formed on the surface of carbon steel bolts. The oxygen in the film acts as a depolarizer to corrode carbon steel bolts, which belongs to electrochemical corrosion. The LNG terminal is located in the coastal area. Due to the evaporation of seawater, water and salt diffuse into the atmosphere, forming a marine atmospheric environment with high humidity and high salt. In this environment, the surface water film of carbon steel bolts is thickened, more oxygen reaches the bolt surface through the water film, and the water film also contains salt diffused into the atmosphere. The water film becomes a highly corrosive electrolytic liquid film, which accelerates the corrosion of the surface of carbon steel bolts. Therefore, the reason is that the strong corrosive water film formed on the surface of carbon steel bolt intensifies its corrosion.
Anti corrosion measures for carbon steel bolts
The root cause of preventing the corrosion of carbon steel bolts is to prevent the formation of galvanic cell reaction environment and eliminate electrochemical corrosion. For bolts, due to the particularity and frequent operation of threads, the anti-corrosion measures shall not affect the normal operation of threads. This requires that the anti-corrosion measures should not only meet the requirements of mechanical properties, but also meet the requirements of process performance and environmental adaptability. The mechanical property requirements are the basic requirements, and the anti-corrosion measures cannot change the chemical composition and mechanical properties of the bolt body; Process performance requirements include requirements for coating thickness, screwing performance, corrosion resistance, bonding strength, wear resistance and hardness; The requirement of environmental adaptability is that the anti-corrosion measures meet the anti-corrosion requirements for various harsh environments. At present, surface coating technology is mainly used for corrosion protection of bolts. In addition, coating anti-corrosion measures can be adopted in places that are not often repaired. According to the characteristics of LNG terminal, several common anti-corrosion measures for carbon steel bolts are introduced below.
Anti corrosion measures for carbon steel bolt coating
For carbon steel bolts, there are many coating anti-corrosion measures, such as powder zincification technology, Dacromet coating technology, electro galvanization technology, fluorocarbon coating technology, Teflon coating technology, composite coating technology, etc. in LNG terminal, i.e. marine atmospheric environment, powder zincification technology, Teflon coating technology and composite coating technology are often used as coating anti-corrosion measures.
- (1) Powder zincification Technology: it is a chemical heat treatment process. Based on the principle of atomic penetration and diffusion, the zincification agent and steel parts are heated to a certain temperature. The active zinc penetrates from the metal surface to the inside, and the iron atoms diffuse to the outside, forming a uniform zincification layer on the metal surface. This technology is mature and basically pollution-free. The mechanical properties of the galvanized bolts will not change greatly, the coating thickness is uniform and can be accurately controlled, and the hardness is high and wear-resistant. The bonding strength between the zinc coating and the base metal is high, and the hydrogen atoms in the carbon steel have diffused and escaped in the heat treatment project, so there will be no harm of hydrogen embrittlement. In theory, because the galvanized layer is a zinc iron metal compound, the potential is lower than that of iron but higher than that of zinc. As anode sacrificial protection, it can protect the substrate carbon steel from corrosion and does not consume too fast as pure zinc, so the galvanized layer has strong corrosion resistance. Moreover, the tests of Luo Changhong and others have proved that the galvanized layer can protect carbon steel bolts in the marine atmospheric environment with high humidity and salinity. Based on the advantages of powder zincification, this anti-corrosion measure can be adopted for carbon steel bolts in LNG terminal. This anti-corrosion measure is adopted for carbon steel bolts on utility pipelines of a LNG terminal. However, this kind of galvanized bolt can not be used on the stainless steel flange. It will corrode the stainless steel flange, so pay attention to the specific occasion of using galvanized bolt.
- (2) Teflon coating technology: it adopts fluororesin electrostatic spraying process to form a coating on the substrate surface after spraying. Teflon coating is a fluorine coating based on polytetrafluoroethylene. Teflon coating has excellent heat and low temperature resistance, corrosion resistance, sliding resistance and moisture resistance. In particular, it has excellent sliding performance and low friction coefficient when contacting with most materials. It is especially suitable for bolt coating. Teflon coating has strong hydrophobicity and is hardly corroded by any medium. It can be used as a protective layer to isolate the corrosion electrolyte membrane from contact with the substrate and protect the carbon steel bolts from corrosion in the marine atmospheric environment. The coated bolt can be used on flanges of various metal materials, which is not limited by the material of the flange. However, this coating is easy to be damaged during transportation or construction, affecting its anti-corrosion effect. For example, in a LNG terminal in the south, Teflon coating technology was used as an anti-corrosion measure to prevent the corrosion of carbon steel bolts. It was found that some bolts with damaged coating were seriously corroded on site.
- (3) Composite coating technology: composite coating refers to the protective layer composed of two or more different coatings. Because the bolt has a clear limit on the thickness of the coating, the general composite coating only contains two kinds of anti-corrosion coatings to solve the deficiency of one coating in anti-corrosion. The two coatings should have good affinity and fusion, and their respective protective properties complement each other. Organic coating and inorganic coating have their own advantages and disadvantages, which meet the requirements of composite coating. At present, there are various types of composite coatings, and appropriate composite coatings shall be selected according to the requirements of LNG terminal for bolt anti-corrosion performance. Yfluoro coating is a composite coating used for carbon steel bolts of fire water pipeline in a LNG terminal in South China. Yfluoro coating consists of two kinds of coatings, one is the special treatment layer on the metal surface, and the other is the fluoropolymer coating coated on the surface treatment layer. Fluoropolymer coating isolates the contact between electrolyte liquid membrane and bolt carbon steel and eliminates the opportunity of electrochemical corrosion, so it can resist the corrosion of marine atmosphere.
Coating and anti-corrosion measures for carbon steel bolts
In addition to coating anti-corrosion measures, there is also a coating anti-corrosion measure for carbon steel bolts in LNG terminal. Coating anti-corrosion is a measure for anti-corrosion of flange, gasket and bolt. It includes the following two types: (1) viscoelastic anti-corrosion technology: it is a typical coating anti-corrosion measure. The structure is shown in Figure 1.
The core of viscoelastic anti-corrosion technology is viscoelastic. Viscoelastic is a high-performance polymer material with special adhesion. It will not flow and solidify at will within a certain temperature range; Generally, it is maintained in this state during the service life, and good water tightness and air tightness are maintained. Because the viscoelastic body contains water repellent agent, it has very good hydrophobicity and repels water. It is almost impossible for water to pass through the anti-corrosion coating to reach the metal surface, so it can completely eliminate water intrusion, truly inhibit the formation of corrosive environment, cut off corrosion from the root, achieve anti-corrosion effect, and is suitable for harsh corrosive environments such as marine atmosphere. Viscoelastic body can be directly used on irregular surfaces without special treatment, which is simple and convenient. Viscoelastic body has the characteristics of liquid state and has the function of self repair when it is damaged by stones. Even if the external environment changes, the anti-corrosion tape will not crack or peel off due to environmental changes, it can still maintain a close fit with the metal surface and hardly affect the sealing performance. And no cathodic protection stripping phenomenon, good chemical resistance, safety and environmental protection, non-toxic and no carcinogens. The coating measures solve the corrosion problem from the root, and the anti-corrosion effect is good. However, the bolts with such anti-corrosion measures are not convenient for disassembly and maintenance, and are not suitable for frequent disassembly and maintenance. Moreover, the price of this elastomer material is relatively expensive. It is generally used in places with heavy corrosion prevention and infrequent maintenance.
(2) Another coating measure is to treat the surface, apply preservative or grease, install polyethylene protective cap for protection, and seal the edge after installation to prevent moisture from entering. If the space is narrow, wax tape is used for coating and corrosion prevention. The structure is shown in Figure 2.
Although the coated bolt has no viscoelastic coated bolt and has good anti-corrosion performance, it is easier to disassemble, easier to repair and lower cost than the viscoelastic coated bolt. This anti-corrosion measure was adopted in a LNG terminal in the south to solve the corrosion problem caused by coating damage of carbon steel bolts on site.
The above anti-corrosion measures are applied in LNG terminal, each with advantages and disadvantages. According to the specific project requirements, appropriate anti-corrosion measures for carbon steel bolts shall be taken based on the comprehensive consideration of the characteristics, performance, manufacturing cost and other factors of anti-corrosion measures.
Source: Network Arrangement – Carbon Steel Bolts Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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