Basic knowledge of metal pipeline corrosion protection

1 What is metal corrosion? Metal corrosion is a kind of phenomenon that metal and surrounding medium are damaged by chemical, electrochemical or physical action 2 What kinds of common corrosion of metal pipes can be divided according to their action principle? According to its action principle, the common corrosion of metal pipeline can be divided into chemical corrosion and electrochemical corrosion 3 What are the commonly used anti-corrosion measures? Common anticorrosive measures include coating, lining, electric protection and corrosion inhibitor 4 What is chemical corrosion? Chemical corrosion refers to the damage caused by pure chemical interaction between metal surface and non electrolyte Chemical corrosion can be divided into gas corrosion and non electrolyte corrosion 5 What is electrochemical corrosion? Electrochemical corrosion refers to the phenomenon of damage caused by electrochemical reaction between metal and electrolyte 6 How does crevice corrosion occur? Many metal components are connected by screws, rivets, welding and other ways There may be a narrow gap at the defects of these connections or welded joints The width of the gap (generally 0.025 ~ 0.1mm) is enough to allow electrolyte solution to enter, so that the metal in the gap and the metal outside the gap form a short-circuit primary battery, and strong corrosion occurs in the gap This local corrosion is called crevice corrosion 7 What is point corrosion? Pitting corrosion refers to the pitting corrosion which is concentrated in the local area of the metal surface and penetrates into the metal interior 8 What are the characteristics of pitting and pitting? Pitting: the hole diameter is less than the depth; pitting: the hole diameter is greater than the depth 9 What is stress corrosion? According to the corrosion mechanism, stress corrosion can be divided into several types? The process of material failure caused by the combination of strain and corrosion caused by residual or external tensile stress is called stress corrosion According to the corrosion mechanism, stress corrosion can be divided into: (1) anodic dissolution (2) hydrogen induced cracking 10 Definition of corrosion fatigue? Corrosion fatigue refers to the damage of materials caused by the synergistic action of metal and alternating stress in corrosive environment 11 How does oxygen concentration corrosion occur? The most common corrosion phenomenon of underground pipeline is oxygen concentration cell Due to the different concentration of oxygen in different parts of the pipeline and the low natural potential (non-equilibrium potential) of the pipeline in the oxygen poor part, it is the anode for corroding the primary battery, and its anode dissolution speed is obviously higher than that of the other surfaces, so it suffers from corrosion When the pipeline passes through the joint of different soil properties, the clay section is poor in oxygen and prone to corrosion, especially at the joint of two kinds of soil or the part near the excavated end of the buried pipeline For oil storage tank, the oxygen concentration difference is mainly manifested in poor contact between tank bottom plate and sand base, as well as the difference of air permeability around the tank and in the center of the tank The low oxygen concentration in the center of the tank makes the anode corroded 12 What is bacterial corrosion? How did it come about? Bacterial corrosion is the phenomenon that when metals are corroded in the soil containing sulfate, the hydrogen of cathodic reaction will reduce the sulfate to sulfide, and the sulfate reducing bacteria will use the energy of reaction to propagate and accelerate the metal corrosion When sulfate is contained in some anoxic soil, sulfate reducing bacteria will breed They need hydrogen or some reducing substances to reduce sulfate into sulfide in the process of metabolism SO42 - + 8h → S2 - + 4H2O due to the presence of sulfate and other H +, the cathodic reaction of metals in the process of corrosion in the soil is produced by atomic hydrogen When it is attached to the metal surface in the soil, it can not continuously become a bubble to escape, and cathodic polarization will occur, which significantly slows down the corrosion process However, there are still bacteria in sulfuric acid, which just find a way for atomic hydrogen to reduce SO42 - to S2 -, and then combine with Fe2 + to form black FES deposit When the pH value of the soil is 5-9 and the temperature is 25-30 ℃, it is most beneficial to the propagation of bacteria 13 How does galvanic corrosion occur? When two metals or alloys with different electrode potentials contact each other and are in electrolyte solution, the metals with negative electrode potentials are corroded continuously, while the metals with positive electrode potentials are protected This kind of corrosion is called galvanic corrosion.