1.3 Erosion corrosion
The solid impurities such as tiny sand and gravel contained in the pipeline flush the pipe wall and cause erosion corrosion in the transportation process, which weakens and destroys the anticorrosion coating in the pipe wall, accelerating corrosion of inner walls of the pipe and forming erosion corrosion. Erosion corrosion is more harmful at the place where the pipeline structure changes, for example flanges. 
Table 2 Corrosion products and reaction formulas of different substances

 
From the actual transportation and corrosion situation, we might as well assume that the solid impurities in the pipeline are sand and gravel. Then, if the flow velocity (m/s1) increases, the amount of sand and gravel flowing into the gap will also increase, which will impact the flange. The corrosion rate will be faster and faster. When the flow rate increases to a certain value, sand and gravel will be taken along with the medium and flow through the flange gap quickly due to the physical properties of the medium. At this time, the greater the velocity is, the less the amount of gravel flow into the gap. The corrosion rate will also decrease. When the mass flow rate (kg/s-1) of sand and gravel increases, the corrosion rate at the flange will also increase. When the mass flow exceeds the specified value, the corrosion rate tends to be stable and no longer increases with the increase of the mass flow. This is mainly because the fact that after the mass flow is increased to a certain value, impact times between the solid impurities, for example sand and the flange gap are saturated, and there is no room to continue to rise. The corrosion rate cannot increase continually. In fact, in the transportation process, erosion corrosion always damages the pipe wall and flange together with electrochemical corrosion, increasing the corrosion rate.
 
1.4 Galvanic corrosion
Galvanic corrosion is often caused by the use of dissimilar metals in the connection of flanges or pipes, resulting in high and low potential differences and corrosion. In actual engineering technology and construction, the dissimilar metal connection is often an indispensable construction step, such as connections of stainless steel and carbon steel, stainless steel and duplex stainless steel, etc., which almost covers most of the machines and metal components. Therefore, galvanic corrosion is very easy to generate. In the medium, when two metals or alloys are in contact through electric conduction, the dissimilar metal connection has the conditions for galvanic corrosion. The corrosion of low potential metal accelerates, while the corrosion of high potential metal slows down, or even corrosion does not occur. In the flange connection structure, galvanic corrosion generally occurs between the flange and bolt, which will aggravate other corrosion such as crevice corrosion and corrosive pitting. Under the combined influence of various factors, the flange sealing surface will eventually fail and a leakage accident will occur.
 
1.5 Accelerated corrosion by internal pressure
In the process of medium transportation, the pipeline will exert certain pressure on the medium to be transported so that the transportation of the medium can reach a certain speed. However, the action of force also causes stress on the inner surface of the pipeline. This is also the main reason why the connecting part of pipe walls and flanges of the long-distance oil and gas pipeline suffers from stress corrosion damage. Then, under the combined influence of stress, chemical corrosion and galvanic corrosion, corrosion of the flange joints in the pipeline accelerates, making the flange corrode and crack due to the force and causing the flange to leak.
 

2. Anti-corrosion measures of flanges 

2.1 Optimization of flange materials
The simplest way to prevent corrosion of flanges is to avoid some known corrosion that may occur through the selection and application of materials. The variety and characteristics of raw materials determine the corrosion resistance of the flange. The corrosion resistance of the flange material is related to the service life of the flange, use environment and probability of corrosion. The commonly used flange materials for long-distance oil and gas pipelines are carbon steel such as 20#, A105, Q235A, etc., alloy steel such as 12Cr1MoV, 16MnR, 15CrMo and so on. In the actual application process, rationally choose flange materials for the industry according to the different conveying media and environmental conditions, and even formulate relevant standards to avoid known corrosion through mandatory requirements. At the same time, the conveying need should be met and reduce costs as much as possible.
 
2.2 Laying protective layers
For flange corrosion caused by external factors, the construction of anti-corrosion coatings on the outside of the flange is a common anti-corrosion measure for flanges of long-distance oil and gas pipelines. The coating can separate the outer surface of the flange from the external corrosive medium, slow down or prevent the interaction between the outer surface of the flange and the external corrosive medium, and is an effective means of anti-corrosion treatment on the outer surface of the flange. In the existing classification of coatings, they mainly include inorganic coatings and organic coatings. Epoxy coatings, petroleum and asphalt coatings, polyurethane coatings and polyene coatings are organic coatings among them. This type of coating is represented by the epoxy coating which has advantages of high density, smooth surfaces and good adhesion and is commonly used for flange anti-corrosion coatings. Ceramic coatings, enamel coatings and thermal spray glass coatings belong to inorganic coatings. These coatings have the characteristics of aging resistance, high temperature resistance, corrosion resistance and wear resistance, but their costs are high and their processes are more complicated. Because the coating is in long-term contact with the outside world, comprehensively consider the adhesion and mechanical strength when it comes to the choice of materials and the difficulty of construction.  It is necessary to not only avoid falling of the coating when it is in contact with external forces, but also prevent the difficulty of repair once it falls.