Abstract
Corrosion of long-distance oil and gas pipelines is an important subject in the research of oil and gas storage and transportation safety technology, and the leakage problem of oil and gas caused by easy corrosion and failures of flanges of oil and gas pipelines can not be ignored. Through comprehensive analysis and identification of influencing factors of flange corrosion, deeply look into the corrosion mechanism and find the main factors affecting flange corrosion and failures. Applying system safety principles, comprehensively proposing prevention technologies and measures, and adopting more optimized means and methods to improve anticorrosion level and ability of long-term oil and gas pipeline transportation, guide the safety prevention and management of oil and gas storage and transportation.
 
The stability and safety of long-distance oil and gas pipelines play a vital role in the overall economic construction and safety of people's lives and properties, and it also seriously affects the operation effect of oil and gas transportation. The corrosion mechanism and protection technology of oil and gas pipelines have been widely studied by many scholars at home and abroad, but there are few reports on the corrosion characteristics and protection technology of flanges in pipelines. In the long-distance oil and gas pipeline system, the importance of safety of the flange part is actually the same as that of the pipeline. Although the conveying medium is different, the leakage at the flange connection will directly change stability of the entire pipeline, causing great safety hazards and sometimes catastrophic damage to the environment. Compared with pipelines, anticorrosion measures of flange parts are actually more difficult. On the one hand, it is inevitable that the original mechanical processing stress balance of the flange will be destroyed when the flange is subjected to anticorrosion measures. Tiny gaps will appear, and when the flange is in contact with the atmosphere, corrosion will occur quickly. On the other hand, because of the special geometrical shape of the flange and the dissimilar metal connection between the flange and the bolt, galvanic corrosion will occur to a certain extent. More importantly, various cyclic stresses will exist in the pipeline due to construction errors and different transportation volume, which will cause erosion corrosion of the flange, inevitably accelerate corrosion of the flange and further affect safety of the pipeline. The research shows that there are areas with irregular planes in the pipeline, and their are significantly more affected by corrosion than other areas of the pipeline. The comparison of the cyclic stress of each area in the pipeline shows that the cyclic stress suffered by the flange is the greatest, and the probability of damage to the pipeline is also the highest.
 
Taking into account the complex and changeable geographic environment of long-distance oil and gas pipelines, the factors affecting flange corrosion can be divided into external corrosion factors and internal corrosion factors according to different corrosion ways. The summary of each factor is shown in the Table 1.
 
Table 1 Influencing factors of flange corrosion

 
1 Corrosion analysis of flanges
1.1 Corrosion caused by external environment
The external environmental factor that causes flange corrosion is the interaction between the flange gap and the medium which may cause corrosion. When no external protective measures are taken, the outer surface of the flange and the corresponding parts are directly exposed to water and atmosphere. Corrosion will occur rapidly. When the temperature between day and night changes greatly, there will be a accumulation of condensed water in the flange gap, which will generate anode and cathode on the flange surface to cause electron transfer, and then cause electrochemical corrosion. Factors such as oxygen content, water content and temperature can all have an impact on the degree of corrosion of the flange in the atmosphere. In addition, human factors are also one of the causes for corrosion. For example, the flange stress may be unbalanced due to the loosening of the bolts that may be caused when the flange is bridged, which will cause the leakage of the medium and cause corrosion. Because the construction process was not carried out in accordance with the safety technical specifications, there is a lack of the necessary protective layer and outer surfaces of pipelines and valves is damaged by lifting equipment. Once there is no timely repair and maintenance and direct construction, hidden dangers will occur and corrosion rates of pipelines and valves will be accelerated.
 
1.2 Electrochemical corrosion
The medium conveyed by long-distance oil and gas pipelines, no matter liquid or gas, often contains various corrosive substances such as hydrogen sulfide and sulfur dioxide. Acidic media containing hydrogen sulfide will especially form acid after being chemical combined with hydration, which will accelerate corrosion rates of flanges and pipelines. The corrosion products and reaction equations of different substances are shown in the Table 2.
 
The main cause for electrochemistry is the gap structure at the flange. In actual construction, gaskets will be installed at the flange to make up for the lack of sealing, but even so, there will still be a certain gap between the gasket and flange wall, which will cause the accumulation of media and cause electrochemical corrosion on the flange plane. Because of the airtight nature of the flange gap, the fluid in the gap is stagnant. After continuous consumption of the O2 in the medium, a primary cell with a difference in oxygen concentration is formed. The anode is inside the gap and the cathode is outside the gap. In the gap Fe→ Fe2++2e is the anode, and outside the gap O2+2H2O+4e→4OH is the cathode. At the same time, because of the excess Fe2+ in the gap, it will attract the outside cathode of the gap to enter, so that the internal and external charges can reach a balanced state. At this time, the high concentration of chloride in the gap is hydrolyzed, which causes the acidification of the medium in the gap and accelerates the dissolution of the anode. The dissolution of the anode will attract more Cl-, and make chloride continue to be hydrolyzed, which in turn make the medium in the gap acidify. The process is repeated again and again.
 
In addition, when the conveying medium is crude oil and the conveying speed is lower than a certain value, the crude oil will be in the state of diversion layers, especially in the pipe section with flanges, which will form accumulated water. Generally speaking, pipe sections with oil and water layers and water accumulation have a greater probability of corrosion. The smaller the pipeline transportation volume is, the easier the oil-water layer will appear, which will inevitably accelerate the speed of electrochemical corrosion.