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Analysis of anti corrosion status and Countermeasures of special shaped parts such as bolts in marine environment

In the marine environment, bolts and other special-shaped parts are very common, which are the key basic parts in the mechanical industry. Due to the influence of salt and relative humidity in the marine environment, bolts and other special-shaped parts cause serious corrosion problems, security risks and huge economic losses. At present, there are many ways to solve the corrosion of special-shaped parts such as bolts. This paper briefly analyzes the basic measures and ideas of corrosion protection of special-shaped parts around the anti-corrosion solutions of special-shaped parts. Starting from the surface treatment technology, it discusses the anti-corrosion measures such as coating protection, surface oiling protection, coating material anti-corrosion and protective sleeve protection, and analyzes the characteristics and effects of various protective measures, Finally, the development prospect of anti-corrosion technology for special-shaped parts such as bolts in marine environment in China is prospected.

Introduction

Marine environment is one of the most severe corrosive environments in nature. Metal materials are prone to serious corrosion in marine atmospheric environment or seawater. Many long-term exposure to the marine atmospheric environment, such as bolts and other special-shaped parts of serious corrosion, resulting in part of the bolts and other special-shaped parts disassembly difficulties, the connection strength between structural parts is reduced, at the same time, it brings new problems to the normal service and daily maintenance of structural parts.
The corrosion of special-shaped parts is common and has not attracted people’s attention for a long time. However, designers usually pay more attention to the changes of mechanical properties such as stress-strain condition, fatigue life, impact load and fracture failure of special-shaped parts, and ignore the possible problems caused by corrosion of special-shaped parts. At the initial stage of corrosion, the surface of special-shaped parts will produce rust, which will affect the appearance. At the same time, if the corrosion is not treated in time, it will even lead to corrosion fracture of structural parts, which will not only affect the safe use of structural parts, but also cause safety accidents. Bolts and other special-shaped parts are found and valued only when the corrosion is very serious. The bolt corrosion and failure of some equipment may even affect the function of structural parts, which may lead to equipment accidents and casualties, resulting in immeasurable losses. Therefore, we should pay more attention to the corrosion and anti-corrosion measures of special-shaped parts such as bolts.

Corrosion status and cause analysis of special-shaped parts in marine environment

According to statistics, the annual economic loss caused by corrosion of steel members in China is as high as 150 billion yuan, accounting for about 4% of the GDP in that year, and the corrosion loss of marine environment accounts for about one third. Improving the corrosion resistance of steel structure in marine environment is a comprehensive problem, which needs to be considered from the aspects of design, construction and maintenance.
The corrosion of metal steel members in the marine environment is mainly affected by the acid gas in the atmosphere, the relative humidity in the atmosphere, the salt content in the atmosphere and so on. The marine atmospheric corrosion resistance is poor, which seriously affects the normal service life of the structural members. Compared with the terrestrial atmospheric environment, the marine atmospheric environment has higher water vapor content, higher salt content and more severe metal corrosion. In addition, in the marine atmospheric environment, the metal surface is easy to attach fungi and molds, which further increases the moisture content of the metal surface and aggravates the corrosion. It is reported that under natural conditions, more than 50% of the corrosion is caused by atmospheric corrosion. In addition, seawater is also a typical corrosion electrolyte solution. There are three peak points of metal corrosion in the actual marine environment. The first peak point is the spray splash area, which usually occurs above the average high tide line. It is the most serious corrosion area of metal components and the most severe marine corrosion environment; The second peak occurs about 0.5 ~ 1m below the average low tide line. In this area, the dissolved oxygen content is high, the flow rate is fast, the water temperature is high, and there are a lot of marine organisms attached and propagated, which is easy to produce corrosion; The third peak usually occurs below the junction of sea mud and sea water, resulting in corrosive cells [2].
Bolt is an important part of connecting and fixing steel structure, but it is often the area with the most serious corrosion. The designer’s anti-corrosion requirements for bolts are generally limited to the ex factory transportation and storage protection of bolts. In engineering application, due to the protection difficulties caused by small clearance and complex structure, the design usually does not put forward specific protection scheme, which often leads to the integrity of other parts, But the bolts have begun to corrode. If no further measures are taken, the corrosion will be intensified, which will directly lead to the reduction of the connection strength of steel members, and there will be serious security risks. At the same time, due to the special structure of the cable, welding parts of steel members, spherical joints of offshore equipment, flanges, edge plates of storage tanks and other special-shaped parts of the sea crossing bridge, there may be many defects, edges and gaps in the process of corrosion protection, which lead to the local unevenness of the structural parts, easy to produce water, and then stress corrosion, crevice corrosion and corrosion Intergranular corrosion and galvanic corrosion behavior, the corrosion problem of special-shaped parts is often very serious, which is a short plate for corrosion protection of steel members in marine environment [3-4].

Analysis of anti-corrosion measures for special-shaped parts such as bolts

Materials and surface treatment process

The steel used in the production of bolts and nuts are rolled or forged, usually pearlite, ferrite or austenite. In addition, the content of sulfur and phosphorus in raw materials has a certain impact on the anti-corrosion performance of products. In addition to the special components such as sulfur and phosphorus, in order to adapt to different use environment, the content of other components will be limited in the production raw materials. The chemical composition of bolts, nuts and other structural parts is not the main factor to improve the anti-corrosion performance, and the main protection means is through the external coating protection [5-6].
The surface treatment processes of special-shaped parts such as bolts usually include surface bright treatment, hot dip coating, electroplating, electroplating + special anti-corrosion coating and bluing treatment. In addition, there are some special treatment processes such as Dacromet process, stainless steel treatment, Teflon process, alloy co infiltration and so on [7-8].

Coating protection

Coating protection is the most commonly used anti-corrosion measures in the marine atmospheric environment. Its main advantages are low cost and simple process. However, the requirements for the surface treatment of the substrate are very high. Because of the irregular shape of bolts and other special-shaped parts, the surface treatment is difficult to meet the requirements. In addition, the anti-corrosion coating is easy to cause sagging, missing coating and other phenomena in the construction process, which makes it difficult to achieve the required film thickness in the local area or edge area of the special-shaped parts. Therefore, the surface coating in these areas is prone to blistering and peeling, and due to the short protection time, it is difficult to remove the surface coating and rust layer in the later maintenance construction, resulting in high maintenance cost. At present, it is generally considered to choose high-density coating, such as epoxy, polyurethane, rubber and so on.

Rubber coating

The rubber coating has excellent anti-corrosion performance and is suitable for corrosion protection of special-shaped parts such as bolts. The product is a kind of two-component room temperature vulcanized rubber viscous liquid, which is a medium hard elastomer after vulcanization. It mainly uses the room temperature rapid vulcanization technology of liquid rubber and the bonding technology suitable for most common solid surfaces, and can form a solid combination with non rubber materials. At present, it has been applied in special-shaped parts anti-corrosion (valve well, flange, valve), concrete base, pipe support, sweating pipe, corroded bolt and nut, offshore steel pile and other parts, but considering the economic problems, it has not been widely applied in the field of special-shaped parts protection [9-10]. The rubber coating can be applied to special-shaped parts, and the main performance advantages are as follows:

  • (1) It has high chemical stability, excellent anti-corrosion performance, easy disassembly and good weather resistance. It is suitable for special-shaped parts such as bolts and flanges that need to be disassembled frequently;
  • (2) It has good adhesion to corroded steel surface, can maintain high adhesion for a long time, and can be re coated. After re coating, the adhesion between new and old anti-corrosion coatings is excellent. For the corrosion of severely corroded metal components, considering the loose structure of the rust layer, it is usually necessary to brush twice. The first brushing should use the low viscosity anti-corrosion elastic adhesive, which can exhaust the air of the rust layer; When brushing for the second time, a dense and non porous anticorrosive coating can be formed. For smooth metal components, one coat of anti-corrosion paste is enough;
  • (3) After curing, the adhesive is a kind of elastomer material, which is as elastic as the rubber skin, so as to ensure that the coating will not crack and peel during the change of thermal expansion and cold contraction due to the change of outdoor temperature difference.

20210606211908 76739 - Analysis of anti corrosion status and Countermeasures of special shaped parts such as bolts in marine environment

High build polyurethane coatings

In the marine environment, the heavy-duty anti-corrosion system of special-shaped parts is epoxy zinc rich epoxy intermediate layer polyurethane surface layer. The traditional polyurethane coating is usually low solid content, high gloss, high-pressure airless spraying, and the one-time film thickness can only reach 30 μ m. Otherwise, sagging may occur, which can not meet the requirements of heavy-duty corrosion protection in marine environment. It is suitable to use high build polyurethane coating. The high build polyurethane coating is composed of hydroxyl resin, polyaniline nanocomposite, titanium dioxide, mica powder, titanium phosphate powder, drying agent, antirust pigment, etc. component A and component B is HDI Trimer. The thick paste polyurethane has the advantages of low viscosity, high solid content, high theoretical coating rate, and the wet film can reach 200 ~ 250 at one time μ m. Smooth appearance, no bubbles, good adhesion, wear resistance, chemical resistance and other properties. It is widely used in steel structure facilities, gas gathering stations, chemical facilities, various storage tanks, bridges, motors, trains, automobiles, offshore oil service platforms and structural parts.

Strippable coating

At present, the strippable coatings are mainly divided into hot-melt coating and solvent coating. The hot-melt coating is mainly composed of acetate butyric acid fiber and ethyl cellulose as the main film-forming materials. The resin material and other auxiliary materials are mixed and dissolved by heating. The hot-melt type is tough plastic at room temperature, which needs to be heated to 140 ~ 180 ℃ before coating. The construction operation is complex, the cost is high, and the construction size is limited; Solvent based coatings are prepared by dissolving perchloroethylene resin, low molecular weight polystyrene, nylon copolymer, polyvinyl alcohol, polyacrylic acid resin and SBS copolymer in organic solvent and adding some auxiliary materials. Solvent based strippable coatings are widely used because they are easy to use and can be dip coated at room temperature. Compared with other coatings, the strippable coating is suitable for the protection of special-shaped parts such as bolts that need to be removed frequently due to its strippability [11-12].
The main performance characteristics of the strippable coating are as follows [13]:

  • (1) Strippability: it can adhere to the brushing surface, has a certain adhesive force, and can completely peel off with a small force, without residue and damage to the base surface;
  • (2) Corrosion resistance: good adhesion with metal substrate, which can effectively prevent corrosive medium from invading the surface of substrate, with excellent corrosion resistance;
  • (3) Moisture resistance: it can effectively prevent the condensation and invasion of water vapor, and ensure that the protected interface is clean;
  • (4) Applicability: wide range of application, can play a comprehensive protection role in a variety of alkali and different environments;
  • (5) Weather resistance: strong chemical inertia, stable property and long protection period;
  • (6) Environmental protection: it does not contain or produce harmful VOC substances. The construction process is environmental friendly, non-toxic and harmless. The products will not cause harm to the environment in the process of use and post-treatment;
  • (7) Mechanical properties: the peelable coating has good ductility and high tensile strength, which can effectively prevent surface abrasion and scratch.

20210606212058 93731 - Analysis of anti corrosion status and Countermeasures of special shaped parts such as bolts in marine environment

Oiling protection

Oil coating is mainly to coat a layer of special grease on the surface of special-shaped parts such as bolts (carbon steel bolts usually use grease, stainless steel bolts use graphite lithium base grease), so as to achieve anti-corrosion effect by isolating air. Oiling protection construction is convenient and fast, without special equipment. But there are some limitations. When the special-shaped parts are protected, the local position can not be completely covered, and the anti rust grease will produce emulsion reaction when contacting with water for a long time, and it is easy to dry in the dry environment. Therefore, regular inspection and maintenance measures should be taken to prevent the failure of protective measures caused by the drying, emulsification and volatilization of antirust oil. Antirust oil will also oxidize in the air and generate acidic substances. On the one hand, it will accelerate the corrosion of metal components, and at the same time, it will destroy the structure of protective grease, which will reduce the dropping point, increase the viscosity of base oil, and deteriorate the fluidity. In addition, the water, dust and harmful gas in the use environment will also lead to the deterioration of antirust oil [14].
At present, the protection of bolts and other special-shaped parts of some sea crossing bridges adopts oil coating protection, the surface of bolts and nuts is smeared with grease, and PVC (polyvinyl chloride) protective cover is added outside [15], so as to prevent the direct contact of external moisture and air with the components, so as to achieve the protective effect. But the marine corrosion environment is bad, the protective cover is easy to deteriorate and fall off, and the butter is liquefied and lost, which leads to serious corrosion of steel members in the later stage, and seriously affects the safety service.

Anti corrosion of coating materials

The coating material anti-corrosion technology uses three layers of protective layer, which are anti-corrosion paste, anti-corrosion belt and external protective agent from inside to outside. In addition, anti-corrosion mortar is used to fill the local voids. The anti-corrosion paste is the most important part of the coating material anti-corrosion. The anti-corrosion paste directly contacts with the metal surface, and its rust conversion components can transform the rusted part of the surface into a black protective film. The formation of protective sealing layer can prevent the further oxidation and corrosion of the steel substrate, playing a dual role of rust removal and rust prevention. The anti-corrosion belt is made of non-woven fabric soaked in anti-corrosion solution. It is relatively soft and can be pasted on all kinds of complex structures. After construction, the outer surface of the anti-corrosion belt will be oxidized and polymerized to become dry, and the inner surface will be in close contact with the steel substrate to maintain the soft state and achieve the best anti-corrosion performance. At the same time, the anti-corrosion tape impregnated with specially prepared anti-corrosion materials has good sealing performance, which can separate the substrate surface from water, salt, air and other corrosive media, so as to achieve the best protection performance. In addition, the anti-corrosion belt has good flame retardancy and weather resistance. The outermost protective agent is a curable surface protective material, which can be directly applied on the surface of the anti-corrosion belt. The outer surface protective agent can be oxidized in the air for a short time to form a protective film with excellent weather resistance, good sealing performance, toughness and integrity, which can effectively prevent the invasion of corrosive media and natural aging [16].
The coating material has long-term corrosion resistance and good protection effect, which can usually protect for 15 ~ 20A. However, at present, the cost of coating material is high, and it does not have the conditions for large-scale application. In addition, the coating material anti-corrosion technology is not suitable for the flange with frequent internal leakage.

Protective sleeve

According to the shape and specification of special-shaped parts such as bolts, the standard protective sleeve is processed, and the sealant is added in the protective sleeve to effectively prevent air and moisture from invading the surface of the substrate.
The disassembly of the protective sleeve is very simple, and does not affect the maintenance and disassembly of the equipment. It is an effective protection method, which is suitable for mass standardized construction to increase the beauty. However, the bolt protection sleeve is not suitable for medium immersion or water flowing environment. The protective sleeve can be made of polypropylene (PP), polyvinyl chloride (PVC), polycarbonate (PC), silica gel (MQ) and other materials, and can be filled with polysulfide, epoxy, polyurethane adhesive sealant. The specific operation steps are as follows:

  • 1) Surface derusting: use grinding wheel or sandpaper to remove the rust on the outside of bolts and base. The derusting grade is Sa2.5. Then clean the bolts with solvent (acetone or alcohol is recommended) to remove rust, oil and other impurities to ensure that the bolts are clean and dry;
  • 2) Apply sealant: apply the sealant evenly on the exposed part of the bolt and the contact part with the base; The recommended coating thickness is 2 ~ 2.5mm, and the sealant thickness shall be applied evenly without exposed part, so as to ensure the sealing effect. The whole process should be completed within the application period of sealant;
  • 3) Appearance beautification: after the anti-corrosion sealant is completely cured (generally within 24 ~ 48h), paint polyurethane finish on the outside of the sealant to enhance the anti-corrosion effect and ensure beautiful appearance.

Polysulfide + protective sleeve

Polysulfide sealant as a high-grade sealing material, its main component is liquid polysulfide rubber, supplemented by reinforcing agent, curing agent, plasticizer and so on. Due to the existence of “s” atom in the main chain of liquid polysulfide rubber, the saturated – S-S -, – C-S chemical chain structure is formed after curing, so polysulfide sealant has excellent oil resistance, aging resistance, solvent resistance, acid and alkali resistance, good water resistance and heat resistance, low air permeability, certain elasticity, and good adhesion with metal and non-metal. Usually, the two-component polysulfide sealant needs to mix a and B components evenly in a certain proportion in order to achieve good sealing effect; One component polysulfide is a kind of solvent-free room temperature curing sealant, which mainly uses the curing reaction between sealant and trace moisture contained in the air, and reacts with the surface active groups of the matrix to form a bonding effect. Polysulfide sealant has good adhesion and sealing performance and long service life. Because of its low price, it is recommended to be used preferentially.

Epoxy coal tar + protective sleeve

In the 1960s, epoxy coal tar pitch has become a widely used high-performance anticorrosive coating. Epoxy coal tar pitch is commonly used for anticorrosion of buried and underground steel pipelines, ballast tanks of oil tankers, steel members and concrete structures. At present, epoxy coal tar coating is widely used in the anti-corrosion engineering of buried steel pipelines and steel storage tanks in petrochemical and urban construction departments in China [17].
Epoxy coal tar pitch has excellent salt water resistance, chemical resistance, water resistance, oil resistance, microbial corrosion resistance, and the film has excellent impact resistance, wear resistance, adhesion and other physical properties, which is suitable for the protection of special-shaped parts in marine environment.

UV curing coating + protective sleeve

UV curable coating is a kind of coating that can be crosslinked and cured under UV irradiation. It has been widely used in the protection of metal, plastic, glass, ceramics and other substrates because of its high efficiency and environment-friendly characteristics. There are many kinds of UV curable coatings with excellent performance, and their components mainly include reactive oligomer (prepolymer), active diluent (monomer), photoinitiator, various fillers and additives [18].
UV curable coating is a kind of seamless sealing fireproof anti-corrosion insulation sealing material, which has excellent temperature resistance, chemical corrosion resistance and high service life. It has certain resistance to general acid, alkali, salt, a variety of organic solvents and oils, sea water and soil, and can completely hinder the penetration of internal and external media or air, so as to ensure the long-term protection of the substrate, It can provide long-term anti-corrosion and strengthening effect, and is suitable for anti-corrosion, anti rust and anti-seepage of various bolts and other special-shaped parts under heavy salt fog, heavy acid fog and other heavy corrosion environment [19].

Conclusion

  • (1) With the implementation of the marine power strategy, China pays more and more attention to the corrosion problem in the marine environment. Although bolts and other special-shaped parts are only small parts of the project, they are mostly exposed on the surface. If there are problems such as corrosion, it will affect the appearance and the strength of the whole component. There are many protection measures for special-shaped parts, such as bolts. Usually, a single anti-corrosion measure is difficult to meet the requirements of long-term protection. In order to continuously improve the long-term protection ability of special-shaped parts, we must adopt a variety of effective schemes and develop new methods and technologies;
  • (2) For the protection of special-shaped parts, the service time, protection characteristics, cost and other aspects can be comprehensively analyzed according to the service conditions. Small scale pilot can be adopted first, and standardized construction scheme can be gradually formed;
  • (3) It is an urgent problem to study the protection materials and construction technology of special-shaped parts under water and rust conditions, and develop long-term protection materials suitable for special-shaped parts, high temperature resistance, easy to clean and maintain.

Authors: Shi huibin1, Guan HUA1, Liu zhoufeng, Wen jiejing, Qiu yunpeng, Zhang yao

Source: Network Arrangement – China Bolts Supplier: www.epowermetals.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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