Anti-corrosion Technology of Oil and Gas Pipeline's Flanges and Bolts (Part One)

Abstract
In the operation of oil and gas pipelines, flanges and bolts are often damaged by metal corrosion, which seriously affects their overall safety. Six mainstream anti-corrosion systems and their basic construction methods at home and abroad are introduced and analyzed, and their advantages and problems exposed in engineering practice are systematically explained based on relevant theories and engineering application experience. The five main factors should be considered comprehensively when oil and gas pipeline flanges and bolts anticorrosion systems are selected, that is, environmental factors, cost expenditure, anticorrosion effect, service life and later maintenance.
 
During the construction of oil and gas pipelines and storage and transportation equipment, the flanges and connecting bolts are exposed to the atmosphere and are corroded by oxygen, moisture, and microorganisms, causing metal corrosion and other damage and gradually losing their original performance. At the same time, users often fail to design specific anti-corrosion schemes for flanges and connecting bolts, which leads to problems such as difficult disassembly of bolts and nuts during later maintenance, failure of bolts and nuts after disassembly, and early failure of flanges and bolts and nuts, greatly shortening the service life of the flange and its part's structure, reducing the safety of the service, and bringing certain safety hazards to the production and operation of the enterprise. The corrosion of flanges and connecting bolts has complex and diverse forms, but the main reasons for the corrosion are as follows:
 
(1) During the construction of oil and gas pipelines in the station yard, flange surfaces, connecting bolts, nuts and gasket surfaces are not painted with anti-corrosion measures such as paint layers, and wrapped moistureproof layers. The parts are directly exposed to the atmosphere after being produced in the factory. The metal surface is in direct contact with the corrosive medium in the atmosphere to form chemical corrosion.
(2) The environment of oil and gas pipelines and various oil and gas supply facilities is relatively complex, especially with the increase in the volume of imported and exported crude oil and oil product transfer in recent years, and coastal depots have gradually developed into an important link in the oil and gas storage and operation industry. Therefore, metals are inevitably corroded by ocean salt spray and seawater during use. When the metal is exposed to the sunlight with a temperature of 60℃, it will be corroded and damaged and the normal use of facilities will be influenced, coupled with other unpredictable effects. Moreover, in the chemical park, the content of corrosive media such as H2O, SO2, NO2 and H2S in the atmosphere is high, which further accelerates the corrosion of metals.

(3) If the flange base body is made from stainless steel, at the welding seam, it will be in the sensitization temperature range due to the heat effect of the next welding on the metal of the previous welding during processing and manufacturing, and crystal will appear. It is depleted in chromium but not resistant to corrosion, resulting in intergranular chromium depletion, poor corrosion resistance, and intergranular corrosion.

(4) After the flanges, gaskets and bolts are connected, there is still a certain gap. Under the action of corrosive media, crevice corrosion is formed between the metals, and the metal surface corrosion between the gaps is intensified.
(5) Flanges, gaskets, bolts, nuts, and other oil and gas supply facilities which are made from different materials are connected to the atmosphere or non-metallic materials such as graphite to form a closed loop, resulting in galvanic corrosion. The corrosion of metals such as carbon steel with lower potential is accelerated under the action of uneven potential. It can be seen that the corrosion of the oil and gas pipeline's flanges and bolts has a greater impact on the safety of the pipeline's operation. Therefore, it is extremely necessary to have effective anti-corrosion measures on oil and gas pipeline's flanges and bolts.
 
1. Anti-corrosion technology systems
At present, the commonly used flange and bolt's anti-corrosion technology systems at home and abroad mainly include painting, molybdenum disulfide or grease, flange gaps filling with grease and aluminum foil tapes or PVC tapes, flange gaps filling with mineral cement and winding petrolatum tape for corrosion protection, enviropeel anti-corrosion and viscoelastic anti-corrosion systems. Because of the differences in application and key points, a mature and unified flange and bolt anti-corrosion technology system has not been formed. Therefore, the following six types of flange and bolt anti-corrosion systems commonly used at home and abroad are introduced and analyzed to provide a reference for companies and choose a suitable and economical anti-corrosion system.
 
1.1 Painting
Painting is currently one of the most commonly used anti-corrosion technologies for most metals, and it is also the most simple, and economical anti-corrosion technology. Spray paint on the metal surface after surface treatment to form a coating. Shielding effects, wet adhesion, corrosion inhibition effects of pigments and fillers play the role of anti-corrosion and being waterproof so as to prevent or delay the occurrence of damage, for example, corrosion. However, in order to ensure a good anti-corrosion effect, higher requirements are put forward for the metal's surface treatment, and the basic requirement for the blast cleaning is above level St2. There should be no visible oil, grease, dirt, oxide skins, rust, coatings and exogenous impurities on the surface when it is observed without magnification. However, when simple manual surface treatment is used, it is difficult to remove grease, dirt, rust and other stains in the flange gap and thread gap, and the requirement for corresponding surface treatment cannot be reached. After being coated with paint, it is easier to fall due to external factors in the later period. It is difficult to achieve the effect of long-term good anti-corrosion, and the painting needs to be maintained from time to time, which is time-consuming and labor-intensive. Therefore, in order to make the coating and metal surface have greater adhesion, it is generally required to use a sandblasting process to treat the metal surface, and it is required to be sandblasted to a level above Sa2'. However, this kind of treatment will cause a certain degree of damage to the metal structure of the equipment due to excessive processing. In addition, for safety reasons, excessive processing on surfaces such as sandblasting cannot be performed on the operating oil and gas pipelines, especially the flanges built in a relatively narrow and humid environment, for example, underground. The constraints of the environment and factors are not conducive to sandblasting of flanges and bolts.