I. Introduction

Pressure vessel is the place where various reactions occur in chemical production, and most chemical reactions are carried out in high temperature and high pressure environment. In order to ensure the normal chemical reaction and the safety of chemical production, we must attach great importance to the service life and self-integrity of pressure vessels. There are two factors that lead to the shortening of service life and integrity damage of pressure vessels, one is mechanical rupture, and the other is environmental rupture. Mechanical fracture refers to the injury caused by external impact; Environmental rupture refers to corrosion. Mechanical rupture mostly comes from outside, which is easy to find and deal with; Environmental cracking (corrosion) mostly occurs in pressure vessels, which is difficult to detect and treat. This paper discusses the types, causes and preventive measures of corrosion of chemical pressure vessels.

Second, the common corrosion types of chemical pressure vessels

Common corrosion types of chemical pressure vessels include physical corrosion, chemical corrosion, electrochemical corrosion and stress corrosion.

1, physical corrosion

At normal temperature and pressure, except for a few active metals, most other metals will not react. In the environment of high temperature and high pressure, when the metal with low melting point becomes liquid, it will physically dissolve the solid metal with high melting point, which is pure physical erosion and dissolution, so it is called physical corrosion. For example, the dissolution and erosion of steel containers by liquid zinc is a typical evil physical corrosion phenomenon.

2. Chemical corrosion

The cause of chemical corrosion is chemical reaction. The process of rearranging and combining the atoms of a substance to form a new substance is called a chemical reaction. Chemical reactions are often accompanied by luminescence, fever, discoloration, precipitation and other phenomena. Whether a new substance is produced is the basis for judging whether a reaction is a chemical reaction. The chemical raw materials in the chemical pressure vessel and the inner wall of the vessel generate new substances through displacement reaction, decomposition reaction, synthesis reaction and redox reaction, which changes the material of the chemical pressure vessel and weakens the pressure resistance and temperature resistance strength of the vessel.

3. Electrochemical corrosion

Electrochemical reaction refers to the difference of anion and cation concentration between the metal on the inner wall of chemical pressure vessel and the liquid chemical raw materials in the vessel, resulting in potential difference and current loop, so that the metal atoms on the inner surface of the vessel are stripped of electrons continuously and the structure of the metal atoms is destroyed. Finally, the inner wall metal is continuously peeled off to form corrosion. In chemical pressure vessels, the most common reaction between metal vessels and chemical raw materials is electrochemical reaction, and electrochemical corrosion is the most common corrosion phenomenon.

4. Corrosion caused by metal overstress

Chemical raw materials are not static inside the chemical pressure vessel, but constantly flow, rotate and churn under the action of stirring and reaction. Chemical raw materials mixed with various substances or even viscous substances, whether adhered or extruded, will have a scouring effect on the inner wall of the container, which is collectively called stress action, and the resulting corrosion is called stress corrosion. Stress corrosion develops rapidly, with no obvious change in appearance, which is not easy to detect and very destructive.

Three, chemical pressure vessel corrosion causes and preventive measures

1, the cause of corrosion

(1) The container itself. The reasons for the corrosion of the container itself are: ① the chemical composition of the metal. Metal materials have the characteristic of chemical reaction with chemical raw materials in containers. There are many impurities in metal materials, and the alloy composition, proportion and processing technology of metal materials are unqualified. (2) container manufacturing process. During stamping, heat treatment and welding, the internal stress of container metal materials will change. (2) Environmental reasons. In chemical production, on the one hand, pressure vessels have to withstand the test of high temperature and high pressure, on the other hand, they have to overcome all kinds of corrosion of chemical raw materials, which are conditional and limited. If they exceed the allowable limit, corrosion will inevitably occur. When different chemical raw materials with different properties, such as acid and alkali, are added, the composition, PH value, concentration and oxidation ability of chemical reactants in the container will inevitably deepen corrosion if the formula ratio is not strictly followed. In the environment of high temperature and high pressure, the corrosion resistance of metals will drop rapidly. The higher the temperature and pressure, the worse the corrosion resistance. When the chemical raw materials flow faster, the impact wear and cavitation wear will be greater, and the peeling speed of the protective film on the inner wall of the container will be accelerated due to stress.

2. Measures to prevent and control corrosion

(1) Select qualified pressure vessel materials. In the production, installation and use of chemical pressure vessels, it is necessary to strictly implement national standards, select suitable vessel materials according to the use and installation environment of the vessels, add different alloys such as nickel, copper, titanium and chromium to the vessel steel according to different conditions, and make different oxidation protection films on the surface of the vessels. In addition, the installation environment of pressure vessels should be fully considered, and ancillary facilities and equipment such as anti-virus, explosion-proof and fire prevention should be configured. (2) Select the appropriate corrosion inhibitor. The function of corrosion inhibitor is to slow down the corrosion rate of equipment. The corrosion resistance of the inner wall of pressure vessel can be effectively protected by adding only a few thousandths or one ten thousandth of the total amount of raw materials in proportion, and the normal chemical reaction will not be affected. This is an economical and practical anti-corrosion method. (3) Improve the welding quality of containers. Chemical pressure vessels are relatively large in size and need to be spliced by welding technology. Arc welding and argon arc welding are generally used. The selection, welding method, welding quality and post-weld heat treatment of covered electrode should be strictly implemented and carefully checked according to the requirements of production process and drawings, and intergranular corrosion test, ultrasonic flaw detection and radiographic flaw detection should be adopted to prevent tiny defects. The ultimate goal is to maintain the metallographic structure and properties of the welding zone, completely eliminate the residual stress of the weld and prevent cracks. (4) electrochemical protection. According to the electrochemical principle, sacrificial anode protection method or impressed current method is adopted, and sacrificial anode, sacrificial aluminum, zinc, alloy and other impressed anodes and impressed direct current protection cathode are used to prevent or slow down the corrosion of pressure vessel metal. (5) Use anticorrosive materials. Anticorrosive coatings are generally made of nonmetallic materials, such as synthetic resin, rubber, vegetable oil and slurry solvent. When they are evenly coated on the inner wall of pressure vessel, a film can be formed after drying, which weakens the penetration of chemical raw materials and reduces the corrosion current, thus achieving better anti-corrosion effect. This method is the most widely used anti-corrosion method at present. (6) Laying protective lining. For chemical raw materials with strong corrosiveness, it is impossible to effectively prevent and control them only by the above methods, so it is necessary to lay protective lining at this time. The protective lining is made of stainless steel, titanium alloy, special ceramics, PTFE, FRP and synthetic rubber. Protective lining has simple applicability and high cost, and is generally only used under special circumstances. (7) Inner wall covering method. The inner wall covering method is to plate a layer of corrosion-resistant metal or ceramic on the inner wall of the container, or to form a dense protective film on the inner wall of the container metal through chemical reaction. So as to isolate the contact between chemical raw materials, water and oxygen and container steel and prevent corrosion. Inner wall covering method is a cheap and universal protection method. (8) Strengthen daily maintenance. Effective prevention is the fundamental way to prevent the corrosion of chemical pressure vessels. Therefore, enhancing the sense of responsibility of employees and strengthening daily inspection and maintenance are the most effective anti-corrosion management measures. Strictly implement the relevant use and maintenance procedures of pressure vessels, regularly patrol inspection and sampling, make records of pressure vessel operation files, analyze the corrosion situation in detail, do a good job in active response, slow down or inhibit corrosion hazards, and ensure the safe operation of equipment.

Four. conclusion

In a word, the relevant personnel of chemical enterprises should attach great importance to the corrosion phenomenon of pressure vessels, always be vigilant, find and deal with the corrosion phenomenon of pressure vessels in time, so as to inhibit or slow down the corrosion damage, prolong the service life of equipment and ensure the safe operation of chemical equipment.

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