Bolt corrosion In addition to the conventional method, here is a new method.

0 Introduction
Bolted connection is a common connection method in offshore projects, its structure is simple, reliable, easy to construct, easy to remove, so the use of large. However, because of its installation location, installation methods, the degree of attention and other factors, bolt corrosion protection is often overlooked.
bolts are prone to damage to their corrosion protection during installation and are difficult to repair

. During the project maintenance phase, bolts also need to be checked and replaced regularly. The bolts have been disassembled many times, and the original anti-corrosion layer is easily damaged. In the ocean environment, bolts are often surrounded by water vapor trapped in bolt holes. In this way, the bolts and the surrounding metal structure will form a corrosion primary battery, the extreme corrosion of large cathode anode. As a bolt for a small anode, it corrodes very quickly.
corrosion, which occurs in bolt holes, is barely visible from the bolt surface in the middle and early stages, and only at a later stage will there be a significant corrosion product on its surface. Therefore, the corrosion of bolts has a strong concealment, which will produce great safety hazards.
1 commonly used bolt corrosion protection
common forms of bolt corrosion protection are electroplating, thermal leaching, chemical surface treatment, paint, ceramic protective film and so on.
1.1 electroplating
is the use of electrolytic principle, on the surface of the bolt attached to a uniform and well-adhered coating. The coating is mostly zinc, chromium and other materials that can be used as sacrificing anodes. When the coating is intact, the coating can isolate the bolt from the corrosion environment and inhibit the occurrence of corrosion, and when the coating is broken, the coating can act as a sacrifice anode to protect the bolt electrochemically.
1.2 Thermal immersion
Thermal immersion is a process of dipping bolts into a molten coated metal liquid and then removing them for cooling so that their surfaces form a metal coating. The anti-corrosion principle of thermal immersion plating is similar to plating, but it is thicker than plating, and the binding force between layers is better and the corrosion resistance is stronger. The common form of hot dip plating is hot dip galvanizing.
1.3 Chemical Surface Treatment
Chemical surface treatment is the placement of bolts in specific chemical solutions that alter the surface of the bolts and transform the surface of the bolts to form a protective covering layer on their surfaces. It can be divided into phosphate passivation, chromate treatment and oxidation treatment. Chemical surface treatment is mostly used for stainless steel bolts. Chemical surface treatment is irreversible and, once broken, difficult to repair.
1.4 Coatings
coatings are one of the most common and widely used corrosion-resistant methods, which can be applied to the surface of the bolts using different construction methods to form a solid film that adheres firmly to a certain strength and is continuous. The film isolates the bolt from its surroundings and acts as cathode protection if the coating contains elements such as zinc.
1.5 Ceramic protective film
the principle of ceramic protective film is to use the ceramic melting point high, high hardness, wear resistance, corrosion resistance, good insulation characteristics, the use of spray coating method of the molten state of ceramic material particles sprayed on the surface of the bolt, forming a ceramic film. However, the construction of ceramic protective film relative to several other anti-corrosion methods of the construction difficulty and cost are high, it is difficult to popularize in large numbers on the project.
2 Bolt anti-corrosion method commonly used in marine engineering
In the field of marine engineering, bolts are used in large quantities as a common connection, so that when selecting the corrosion-proof form of bolts, not only corrosion protection, but also costs such as maintenance. At present, in the field of marine engineering in China, the most widely used form of corrosion of carbon steel bolts is generally zinc seepage.
zinc seepage is a method of preparing bolts buried in zinc alloy powders that remain at a certain temperature for a period of time, which will produce a metallurgical diffusion between zinc/iron. Due to the special porousness of the surface of the zinc seepage layer, the surface of the zinc seepage layer needs to be closed within 2h after the zinc seepage is completed. Zinc seepage relative to galvanizing, zinc atom diffusion seeped into the bolt surface, changed the surface content and structure, so that the zinc seepage layer is very uniform and dense, strong adhesion, by impact and other difficult to peel off.
but bolts in the transport, storage, installation process, it is inevitable to the surface of the zinc seepage layer irreversible damage. The damaged zinc seepage layer is usually repaired with a zinc-rich primer before installation. However, the corrosion resistance of the repair site is much worse than that of the original zinc seepage layer, so that after the bolt is installed, it is easy to form a large cathode anode corrosion original battery, corrosion often occurs here. And these repairs only apply to the installation before, during the installation process, the bolts need to meet a certain tightening torque, so during the installation process, the zinc seepage layer is prone to breakage, and these breakages can not be repaired. This poses a hidden danger to the corrosion of late bolts.
3 Bolt of an anti-corrosion recommended practice
combined with the special service environment of marine engineering, the use of zinc seepage coating and closed paint anti-corrosion mode, its anti-corrosion performance is strong, good resistance to mechanical impact, but closed paint is used to close the gap in the zinc seepage coating, can not afford to anti-corrosion effect, zinc seepage coating itself is a cost-effective anti-corrosion coating, in the marine environment will gradually dissolve, late corrosion. Therefore, a new recommended practice is proposed, that is, the original zinc seepage and closed paint corrosion form unchanged on the basis of painting. Apply the screws before installing the bolts and reserve them at both ends. Apply both ends and nuts of the screw after bolting.
basic anti-corrosion requirements for 3.1 bolts
Bolts are found to be zinc seepage and paint in accordance with standard requirements before they are shipped, and installation and coating work is completed upon completion of the site.
3.2 Surface Treatment
Before working on a primer on a closed paint coating, all surface contaminants such as oil, grease, lubricants, etc. on the bolt surface should be removed, and the removal method can be selected for solvent wipe.
3.3 Construction method
can be sprayed or brushed. In view of the particularity of bolt coating, it is recommended to use brushing to reduce unnecessary loss of paint.
3.4 Coating system
bolt nut coating system needs to refer to the surrounding service environment, if the high temperature environment, the use of high temperature paint coating system.
3.5 Construction recommendations
for the coating of bolts, it is recommended to follow the following steps.
3.5.1 Single-head nut bolt
(1) covers the position of the thickness of the 2 nuts.(2) the rest of the area is coated with primer.(3) contact area is coated with medium paint, finish paint.(4) remove the cover, install the nut, remove the surface grease, the remaining parts of the designated coating system (bottom, medium, surface are done).3.5.2 Double-head nut bolt
(1) covers the position of the thickness of the 2 nuts.(2) the rest of the area is coated with primer.(3) the contact area is coated with medium and finish paint.(4) in addition to the cover, install the nut, remove the surface grease, the remaining parts of the designated coating system (bottom, medium, surface are done).3.6 Advantages
With this anti-corrosion practice, an anti-corrosion layer can be formed on the surface of the closed paint to resist some corrosion before the zinc seepage coating begins to consume, greatly reducing the consumption rate of the zinc seepage coating. At the same time, because the exposed part of the nut is installed after coating, can play a certain role in blocking the bolt hole, reduce the entry of water vapor. The double-layer protection of coating zinc seepage can effectively reduce corrosion and reduce late maintenance.
4 Conclusion
In summary, this new bolt corrosion recommendation, although compared to other forms of corrosion protection to increase the field construction coating process, in the project construction phase increased labor and material costs. However, compared with the huge investment, long service time, high reliability and high safety characteristics of the entire marine engineering project, from the overall project point of view, is completely feasible and recommended.
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