Classification according to the mechanism of corrosion
According to the mechanism of corrosion, it can be divided into three categories: chemical corrosion, electrochemical corrosion and physical corrosion.
1. Chemical Corrosion
Chemical corrosion refers to the damage caused by the direct pure chemical interaction between the metal surface and the non-electrolyte. Sulfur corrosion of metals in high temperature gas and high temperature oxidation of metals are chemical corrosion.
2. Electrochemical Corrosion
Electrochemical corrosion refers to the damage caused by the electrochemical reaction between the metal surface and the ionically conductive medium. Electrochemical corrosion is the most common and common corrosion, such as the corrosion of metals in the atmosphere, seawater, soil and various electrolyte solutions.
3. Physical Corrosion
Physical corrosion refers to the destruction of metals due to pure physical dissolution. Its characteristics are: when the low melting point metal is dissolved into the metal material, it will have a "slicing" effect on the metal material. Since the strength of the metal with low melting point is generally low, it will preferentially fracture under the stress state, thus becoming the crack source of the metal material. It should be said that this kind of corrosion is rare in engineering.
Classification according to corrosion form
According to the type of corrosion, it can be divided into three categories: General corrosion, Local corrosion and Stress corrosion.
1. General corrosion
Also known as uniform corrosion, is basically the same degree of corrosion that occurs over a larger area of the pipeline. Uniform corrosion is the least dangerous type of corrosion.
① In engineering, it is often necessary to give enough corrosion allowance to ensure the mechanical strength and service life of the material.
② Uniform corrosion is usually evaluated by the corrosion depth of the corrosive medium to the metal material or the reduction of the wall thickness of the metal component (called the corrosion rate) per unit time. The SH3059 standard stipulates that the material whose corrosion rate does not exceed 0.05mm/a is a fully corrosion-resistant material; the material whose corrosion rate is 0.05-0.1mm/a is a corrosion-resistant material; the material whose corrosion rate is 0.1-0.5mm/a is a high-corrosion material Corrosion-resistant materials; materials whose corrosion rate exceeds 0.5mm/a are non-corrosion-resistant materials.
2. Local Corrosion
Local corrosion, also known as non-uniform corrosion, is far more harmful than uniform corrosion, because uniform corrosion is easy to detect and easy to fortify, while localized corrosion is difficult to predict and prevent, often causing sudden damage to metal components without warning. , resulting in serious fire or personal injury. Local corrosion is very common. According to statistics, uniform corrosion accounts for 17.8% of the entire corrosion, while local corrosion accounts for about 80%.
(1) Pitting
① The deep corrosion concentrated on individual small points on the global surface is called pitting corrosion, also known as pitting corrosion. The pit diameter is equal to or less than the depth.
② Pitting corrosion is one of the most destructive hidden corrosion forms of pipelines. Austenitic stainless steel pipes are most prone to pitting corrosion when transporting media containing chloride ions or bromide ions. If the outer wall of stainless steel pipes is often wetted by seawater or natural water, pitting corrosion will also occur, because seawater or natural water contains a certain amount of chloride ions.
③ The pitting corrosion process of stainless steel can be divided into two stages: the formation of pits and the development of pits.
The incomplete parts of the passivation film (outcropping dislocations, surface defects, etc.) are used as pitting corrosion sources, which are in an active state for a certain period of time, the potential becomes negative, and a micro-battery is formed between it and its adjacent surface, and has a large cathode and a small anode area ratio, so that the metal at the source of the pitting corrosion is rapidly dissolved, and the pit begins to form.
The etched pits that have formed progress as the corrosion continues. Excessive positive charges accumulate in the pores, causing the influx of external Cl- to maintain electrical neutrality, followed by an increase in the chloride concentration in the pores. Acidification of the solution in the pores due to chloride hydrolysis further accelerates the dissolution of the anode in the pores. As a result of this autocatalysis, the pits continue to develop in depth.
④ The solution retention is prone to pitting corrosion; increasing the flow rate will reduce the pitting corrosion tendency, and sensitization treatment and cold working will increase the pitting corrosion tendency of stainless steel; solution treatment can improve the pitting corrosion resistance of stainless steel. The pitting corrosion resistance of titanium is higher than that of austenitic stainless steel.
⑤ Piping corrosion also occurs in carbon steel pipes, usually in steam systems (especially low-pressure steam) and hot water systems, which are corroded by dissolved oxygen, and the temperature is most serious between 80 and 250 °C. Although the steam system is deoxygenated, it is difficult to ensure that the amount of dissolved oxygen does not exceed the standard due to the lack of strict operation control. Therefore, pitting corrosion of carbon steel pipes caused by dissolved oxygen often occurs.
(2) Crevice Corrosion
When the material transported by the pipeline is an electrolyte solution, crevice corrosion will occur in the gaps on the inner surface of the pipeline, such as flange gaskets, incomplete penetration of single-sided welding, etc. Some passive metals such as stainless steel, aluminum, titanium, etc. are prone to crevice corrosion.
The mechanism of crevice corrosion is generally considered to be the principle of concentration corrosion cells, that is, due to the difference in oxygen concentration or metal ion concentration between the solution in the crevice and the surrounding solution. Crevice corrosion occurs in many media, but it is most serious in chloride-containing solutions. The mechanism is not only the action of oxygen concentration cells, but also autocatalysis such as pitting corrosion.
(3) Corrosion of welded joints
Usually occurs in stainless steel pipes, there are three forms of corrosion.
① The welding meat is corroded into a spongy shape, which is the selective corrosion of δ ferrite that occurs in austenitic stainless steel.
In order to improve the welding performance, austenitic stainless steel usually requires the weld to contain 3% to 10% ferrite structure, but in some strongly corrosive media, delta ferrite selective corrosion will occur, that is, corrosion only occurs in The delta ferrite phase (or further decomposition into the sigma phase) results in a spongy appearance.
② Corrosion in the heat affected zone. The reason for this corrosion is that the temperature here is just in the sensitization zone during the welding process, and there is sufficient time to precipitate carbides, resulting in intergranular corrosion.
(4) Wear and corrosion
Also known as erosion corrosion. When the corrosive fluid suddenly changes direction at the bends such as elbows and tees, it will cause mechanical erosion and damage to the passive film or corrosion product layer on the metal and metal surfaces, and at the same time, it will produce intense electrical shock on the fresh surface of the metal that is constantly exposed. Chemical corrosion, resulting in more severe corrosion damage than other parts. This damage is the detachment of the metal from the metal surface with its ions or corrosion products, rather than as a solid metal powder as in pure mechanical wear.
Wear corrosion is most likely to occur when air bubbles or suspended solids are trapped in the fluid. The passive film of stainless steel has poor wear and corrosion resistance, while titanium is better. The steam system and H2S-H2O system have serious wear and corrosion on carbon steel pipe elbows and tees.
(5) Corrosion of condensate
For hot corrosive gas pipelines containing water vapor, condensation will occur on the inner wall where the insulation layer is terminated or damaged because the local temperature drops below the dew point, resulting in condensate corrosion, that is, dew point corrosion.
(6) Local atmospheric corrosion at the damaged coating
For carbon steel lines in chemical plants, this corrosion can sometimes be severe because the atmosphere in chemical plant areas often contains acid gases that are much more corrosive than the natural atmosphere.
3. Stress Corrosion
The fracture failure of metal materials under the combined action of tensile stress and specific corrosive media is called stress corrosion cracking. The time for stress corrosion cracking to occur is long or short, some crack after a few days, and some only crack after several years, which shows that stress corrosion cracking usually has a long or short incubation period.
Stress corrosion cracks are dendritic and generally develop in the direction perpendicular to the tensile stress. The microscopic morphology of cracks includes transcrystalline, intercrystalline (along-crystalline) and a mixed type of both. The source of stress, for the pipeline, the residual stress during welding, cold working and installation is the main one.
Not all metals and media work together to cause stress corrosion cracking. Among them, stress corrosion cracking occurs in metal materials only in certain specific corrosive environments.
