The use of coating technology is the main method of anti-corrosion in oilfield pressure vessels and storage tanks. In order to ensure the safe and reliable operation of the container, the type of coating should be selected reasonably, the construction process should be strictly regulated, and the quality of the coating should be accurately detected. Through literature research and on-site analysis of working conditions, the anti-corrosion mechanism and failure mechanism of coatings, the characteristics and application conditions of commonly used paints in oilfields, and the applicability evaluation methods of coatings are summarized. At the same time, methods for quality control of coating construction are proposed, and commonly used inspections are clarified. The inspection items corresponding to the instrument have analyzed the problems in the construction process from four important links including tank inspection, construction preparation, surface treatment and on-site painting, and proposed solutions for various problems. Accordingly, certain theoretical and technical support will be provided for the application of internal coatings in oilfield pressure vessels and storage tanks, and the life and reliability of the coatings will be significantly improved.

Keywords: anti-corrosion; internal coating; applicability; quality control; testing pressure vessels and storage tanks are widely used in all aspects of industrial production, especially in oil and gas purification units, refining and chemical units, and storage and transportation units in oil fields; Therefore, improving the reliability of pressure vessels and storage tanks is of great significance to the safe production and economic benefits of oil fields. At present, the main anti-corrosion methods used for pressure vessels and storage tanks in service include adding corrosion inhibitors, controlling the temperature of the medium, regularly removing sludge and liquid, strengthening maintenance, and spraying paint, etc. However, the effective and economical method is the internal coating technology. [1]. This article studies the anti-corrosion mechanism and failure mechanism of the coating, selects the appropriate type of coating, combines with the test method to evaluate the applicability of the coating, controls the quality of the coating construction process, and proposes solutions to existing problems, aiming to improve the coating The construction quality is improved, the service life of the coating is prolonged, and the safe and reliable operation of the container is ensured.

1 Coating's anti-corrosion mechanism and failure mechanism 1.1 Coating's anti-corrosion mechanism The anti-corrosion mechanism of coating has not been fully analyzed yet. The following 5 functions are generally recognized by people [2-5].

1) Shielding effect. A dense coating with a certain thickness can directly isolate the contact between metal and corrosive media, and prevent water, oxygen and other ions from penetrating and corroding the battery.

2) Corrosion inhibition and passivation. Some vegetable oleic acid contained in the coating reacts with alkaline pigments to obtain soap compounds with corrosion inhibition function; at the same time, the chromate, phosphate and lead salt in the anti-rust pigment will dissolve after the coating absorbs water. Passivation reaction with base metal.

3) Sacrificial anode protection. Add zinc powder and other metals that are more active than the base metal in the paint. Once electrochemical corrosion occurs, the active metal such as zinc powder will be used as the anode to be dissolved first, which indirectly protects the base metal.

4) The role of adhesion. The adhesion between the coating and the metal surface can prevent the coating from bubbling or peeling, thereby avoiding the increase of the osmotic pressure of corrosive media such as water and oxygen in the coating caused by bubbling or peeling and accelerating the penetration of corrosive media. 5) Resistance effect. The good electrical insulation properties of the coating prevent the formation of corrosion batteries, and can effectively inhibit the discharge of the cathode and the dissolution of anode metal ions during the electrochemical corrosion process. 1.2 Coating failure mechanism Coating failure is due to the deterioration of the coating due to the aging of the coating on the one hand; on the other hand, due to the penetration of water, oxygen and ions, the cathode area and the formation of defects such as micropores and unevenness of the coating are formed. Anode area, caused by electrochemical reaction [6-7]. The typical phenomena of coating failure caused by corrosion are bubbling and shedding.

1) Coating bubbling. The volatilization of the gas contained in the coating or the absorption of water by the coating will cause the coating to bubbling, and as the corrosive medium penetrates into the coating, it will cause the corrosion of the base metal. As corrosion products accumulate, the increase in internal pressure can also cause bubbling. The coating bubbling is shown in Figure 1.

2) The coating falls off. When the wet adhesion of the coating is reduced, water molecules will pass through the paint film to form a water layer on the surface of the metal substrate, and the oxygen passing through the paint film will dissolve in the water. With water and oxygen, the steel will have corrosion conditions. This will cause the coating to fall off or damage in a large area [8]. The coating peeling off is shown in Figure 2. 2 Coating selection The anti-corrosion coatings of oil field pressure vessels and storage tanks mostly use epoxy coatings, epoxy glass flake coatings, epoxy conductive coatings and non-toxic epoxy coatings. Most of them are heavy-duty anti-corrosion coatings and organic coatings. The composition is high molecular material or polymer, with good acid and alkali corrosion resistance. The SY/T0319-2012 [9] standard also stipulates the types of paints that can be used for different media and operating temperature ranges. For some new coatings, when they are not sure about their performance, they should first conduct test evaluation, or require the manufacturer to issue a formal report of the relevant test conducted by a third-party inspection agency. Each coating should provide detailed instructions, including composition, main characteristics, use, basic parameters (color, gloss, solid content, dry film thickness, theoretical dosage, flash point and drying time, etc.), overcoating interval time, surface Treatment, front and back matching paint, construction conditions, construction methods, construction parameters, packaging and storage, packaging specifications, precautions, health and safety, etc. 2.1 Solvent-free epoxy coating Solvent-free epoxy coating is a two-component reaction curing type anticorrosive coating, which is synthesized from low-viscosity epoxy resin (bisphenol A or bisphenol F), pigments and additives, etc. . The added curing agent is a low-viscosity modified amine, which is an insulating coating