Anti-corrosion technology of oil and gas pipeline's flanges and bolts (Part Two)

1.2 Molybdenum disulfide or grease
The anti-corrosion technology of applying molybdenum disulfide or grease (calcium based grease) is one of the common ones used in anti-corrosion for metal. Because of the adhesion and lubricity of Molybdenum disulfide or grease, it can effectively prevent corrosion. Therefore, it is suitable for daily maintenance of valves and other storage and transportation equipment. The main construction process is to remove rust on the metal surface that needs to be painted, and then evenly coat a layer of molybdenum disulfide or grease on the surface. In order to achieve a good anti-corrosion effect, the requirements for the treatment level of the metal's surface are also high. When the condition permits, it is recommended that the sandblasting process should be used. The surface should be sandblasted to level Sa2 or above. In addition, the physical and chemical properties of molybdenum disulfide or grease are easily affected by changes in environmental factors and fail especially due to high temperature or rain, resulting in losing protection capabilities. Therefore, it should not be applied to areas with heavy rainfall or high temperatures.
 
1.3 Flange gaps filling with grease and aluminum foil tapes or PVC tapes
This method is an improved method of applying molybdenum disulfide or grease anti-corrosion technology. After applying grease to the flange gap, a protective layer is wrapped. The metal surface treatment level of this technology is slightly lower than the above two anti-corrosion technologies. The end flange connection surface and bolts are subjected to simple surface treatment. Generally, the minimum requirement is level St2. This technical requirement only requires to inject grease or its mixture into the gap of the flange connection after surface treatment; apply grease on the connecting bolts and nuts, and then use aluminum foil tape or PVC to wrap and seal. This technology can effectively extend the service life of flanges and their accessories such as gaskets and bolts. This technology is economical, and the construction process is simple; the flange and its accessories being exposed to the weather can be avoided. However, the grease cannot fill the entire gap due to its poor cold flow characteristics; the sealing adhesion of the aluminum foil tape or PVC tape will be affected by the service time and sunlight, causing the sealing performance to gradually decrease, which promotes the penetration of corrosive media like moisture, reduces the anti-corrosion effect of the grease and shortens the service life of the device.
 
1.4 Flange gaps filling with mineral cement and winding petrolatum tape for corrosion protection
Petrolatum tapes and petrolatum mortar are anticorrosive products made from mineral oil extracted from petroleum, which have good sealing and waterproof performance. In the construction process of this anti-corrosion technology, the surface treatment level of the metal to be protected is relatively low; the metal surface can be treated directly by basic techniques such as manual sanding; a layer of anti-corrosion primer is applied; inject the mineral cement into the gap.  After the gap is filled with mineral cement, wind a layer of petrolatum tape on the gap and the flange end face. At the same time, in order to solve the problem of easy aging and worse performance of petrolatum tape under high temperatures, an outer protective tape is laid on the surface of the petrolatum tape; the waterproof material and process identification color are painted. This anti-corrosion technology has low surface treatment requirements, good adhesion between the material and the surface of the substrate, good anti permeating performance, and is not  solidified, hardened and cracked, well resistant to acids, alkalis, salts and microorganisms, resistant to water, water steam and other gases, which is suitable for the anti-corrosion of metal and steel structure with complex geometric structure.
 
However, there are two main shortcomings. First, the applicable temperature range is small, which is not more than 55℃. The temperature of the above-ground oil and gas pipelines and flanges may exceed the operating temperature of the petrolatum tape after being exposed to the sun, resulting in petrolatum mortar peeling off the metal surface and reducing the adhesion. Because petrolatum is a kind of mineral oil extracted from petroleum, it is easy to cause the petrolatum to melt and drip, emit toxic gases, and cause certain pollution to the station. Moreover, chemical changes easily happen at high temperatures. The petrolatum tape deteriorates and ages, which affects the anti-corrosion performance. Second, the cold flow characteristics of petrolatum mortar are poor, part of the flange gap cannot be filled, and gas gaps are prone to appear in the gap. When the petrolatum tape is not tightly wound or is aging and loosened, infiltrated corrosive media such as condensed water or rainwater easily accumulate in the holes, accelerating the corrosion rate of metals and affecting the overall anti-corrosion performance.
 
1.5 Enviropeel anti-corrosion
The Enviropeel is a kind of thermoplastic anti-corrosion material added with corrosion inhibitor and can be sprayed by using equipment. The main component of the coating is cellulose acetate. It is currently used in a variety of industrial occasions, especially in the bolt connection system corroded by galvanic couple. The main steps of the Enviropeel anti-corrosion system are relatively simple and quick. Because of the lower requirements for the metal surface treatment level, the surface can be polished by hands or power tools. The treatment level can be as low as St2, and then the ENVIROPEEL material is sprayed. The anti-corrosion principle of the Enviropeel anti-corrosion is mainly to block corrosive media such as dust and water through the sprayed coating so as to isolate the corrosive environment and fill the connection gaps that cannot be reached by the painting process and prevent galvanic corrosion, pitting and atmospheric corrosion between metals.
 
Moreover, inhibitors in the material are added and percolate through the various crevice structures of the metal with the coating to prevent corrosion. The the Enviropeel anti-corrosion system is not restricted by the environment and the structure and shape of the object, and it has the advantages of being recycled many times, non-toxic and harmless, easy to peel off, and not containing volatile organic compounds. However, there are some disadvantages, which are mainly as follows: first, the tools and equipment are dedicated. Dedicated spraying equipment is required. Second, the adhesion between the anti-corrosion layer and the pipeline is poor. Corrosive media such as moisture may still enter through the gaps. Repeated spraying and maintenance are required. Third, impurities such as dust are easy to adhere to the surface of the anti-corrosion layer. The effect of repeated spraying is not good.