Classification and characteristics of corrosion

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Classification and characteristics of corrosion 1. Pitting corrosion Pitting corrosion is also known as pit corrosion or hole corrosion. Pitting can vary in size; generally, the depth of pitting is much greater than its diameter. Pitting occurs on metals with a passivation film or protective coating on their surface. Due to the heterogeneity in metal materials, such as defects, impurities, and solutes, when a medium contains certain active anions (such as Cl‑), these anions are first adsorbed at certain points on the metal surface, thereby destroying the passivation film on that metal surface. Once this passivation film is damaged and the metal lacks self-passivation ability, corrosion occurs on the metal surface. This is because metal from the body can easily leak out at the defects on the metal surface, putting it in an activated state, while the passivation layer remains in a passive state. This creates an active-passive corrosion cell. Since the area of the anode is much smaller than that of the cathode, the current density at the anode is very high; as a result, corrosion progresses deeper, and the metal surface is quickly corroded to form small holes. This phenomenon is known as pitting corrosion. In the statistics of corrosion failure types in the petroleum and chemical industries, pitting accounts for approximately 20% to 25%. In a medium with poor flow and containing reactive anions, conditions conducive to the accumulation and concentration of reactive anions are easily established, promoting the formation of pitting corrosion. Rough surfaces are more prone to pitting corrosion than smooth surfaces. A decrease in pH and an increase in temperature both increase the tendency for pitting. Oxidizing metal ions (such as Fe3+, Cu2+, Hg2+, etc.) can promote the formation of pitting. However, certain oxygen-containing anions (such as hydroxides, chromates, nitrates, and sulfates) can prevent pitting. Although pitting results in a small loss of weight, the corrosion rate is high due to the very small area of the anode. In severe cases, it can cause holes in the equipment, leading to leaks of large amounts of oil, water, and gas; sometimes this even results in serious accidents such as fires and explosions, making it a highly dangerous phenomenon. Pitting accelerates intergranular corrosion, stress corrosion, and corrosion fatigue; in many cases, pitting is the origin of these types of corrosion. 2. Crevice corrosion: In an electrolyte, narrow gaps are formed between metal surfaces or between a metal surface and a non-metal surface. The movement of relevant substances within these gaps is hindered, resulting in a concentration cell that causes localized corrosion; this type of corrosion is known as crevice corrosion. Crevice corrosion often occurs at the joints of flanges in equipment, as well as at the areas where gaskets, linings, wraps overlap with metal. It can arise in different metals and in various corrosive environments, thereby causing serious disruptions to the proper operation of production equipment and even leading to damage. For titanium and titanium alloys, crevice corrosion is the most critical corrosion phenomenon to consider. In the medium, as the oxygen concentration increases, the amount of crevice corrosion increases ； As the pH value decreases, the anodic dissolution rate increases, and so does the amount of crevice corrosion ； An increase in the concentration of active anions raises the susceptibility to crevice corrosion. However, an increase in certain oxygen-containing anions reduces the amount of crevice corrosion. 3. Stress corrosion: The phenomenon of brittle cracking below the strength limit that occurs in materials when they are exposed to specific corrosive media and subjected to static tensile stress (including residual stresses caused by external loads, thermal stress, cold working, hot working, welding, etc., as well as wedge stresses resulting from corrosion products in cracks) is known as stress corrosion cracking. Stress corrosion cracking begins with the formation of tiny pits in the parts of the metal that are susceptible to corrosion; these pits give rise to elongated cracks, which spread rapidly and can cause severe damage in a short period of time. Stress corrosion cracking accounts for the highest proportion among the types of corrosion-induced failure in the petroleum and chemical industries, reaching up to 50%. There are two basic conditions for the occurrence of stress corrosion: one is that the material has a certain susceptibility to stress corrosion cracking in the presence of the medium ； Second, there is a sufficiently high tensile stress. The stress that causes stress corrosion cracking can come from service stress or from residual stresses generated during the manufacturing process. According to statistics, in stress corrosion cracking incidents, over 80% are caused by residual stresses, while those caused by service stresses account for less than 20%. The stress corrosion process can generally be divided into three stages. The first stage is the incubation period. During this stage, as a result of the localization of the corrosion process and the action of tensile stress, crack nucleation occurs ； The second stage is the period of corrosion crack growth; once cracks initiate, they expand under the combined effect of the corrosive medium and tensile stresses in the metal ； In the third stage, due to the local concentration of tensile stress, rapid crack growth leads to the failure of the part. When stress corrosion cracking occurs, there is no significant uniform corrosion, and even the amount of corrosion products is very small; sometimes it is difficult to detect them with the naked eye. Therefore, stress corrosion is a highly dangerous form of failure. Generally, an increase in the chloride concentration in the medium reduces the time required for stress corrosion cracking. The corrosive effect of different chlorides decreases in the order of ions such as Mg2+, Fe3+, Ca2+, Na1+, Li1+. The temperature at which stress corrosion occurs is generally between 50°C and 300°C. To prevent stress corrosion, measures should be taken to reduce corrosion and eliminate tensile stress. Mainly: first, try to avoid using materials sensitive to stress corrosion ； Second, when designing the equipment structure, it is necessary to strive for rationality, minimizing stress concentration and the accumulation of corrosive agents ； Third, when manufacturing processing equipment, attention should be paid to eliminating residual stresses. 4. Corrosion fatigue occurs under the combined action of a corrosive environment and cyclic stress. This reduction in corrosion fatigue resistance caused by corrosive agents is known as corrosion fatigue. The stress value for fatigue failure is below the yield point; fatigue failure occurs only when the stress exceeds a certain critical cyclic stress value (the fatigue limit, or fatigue life). Corrosion fatigue, on the other hand, can cause failure even under very low stress levels, which makes it highly dangerous. The main factors affecting material corrosion fatigue include the stress alternation rate, medium temperature, medium composition, material size, processing, and heat treatment. Increasing the loading cycle rate, lowering the pH value of the medium, or raising the temperature of the medium will all reduce the corrosion fatigue strength. Stress concentration resulting from damage on the material surface or lower roughness reduces the fatigue limit, thereby also decreasing the fatigue strength. 5. Intergranular corrosion: Intergranular corrosion is a type of localized corrosion damage in which metal materials are corroded along the boundaries between their grains in specific corrosive environments, resulting in a loss of cohesion between those grains. Equipment or parts affected by this type of corrosion may still appear intact and shiny on the outside, but since the bonds between the grains are broken, the material loses almost all of its strength; in severe cases, it loses its metallic sound and turns into powder upon gentle tapping. According to statistics, intergranular corrosion accounts for about 4% to 9% of corrosion-related failures in petroleum and chemical equipment, and it mainly occurs in containers and heat exchangers welded from rolled materials. It is generally believed that the depletion of alloying elements at grain boundaries is the main cause of intergranular corrosion. Intergranular corrosion can be prevented by improving the purity of the material, removing harmful trace elements such as carbon, nitrogen, phosphorus, and silicon or adding small amounts of stabilizing elements (titanium, niobium) to control the carbides that precipitate at the grain boundaries, as well as by employing appropriate heat treatment procedures and processing techniques. 6. Uniform corrosion: Uniform corrosion refers to corrosion that occurs at almost the same rate across the entire metal surface in contact with the environment. When using corrosion-resistant materials, resistance to uniform corrosion should be the primary criterion for assessing their corrosion resistance, with resistance to local corrosion being considered only in special cases. 7. Wear corrosion (erosion): The material degradation process resulting from the combined action of wear and corrosion is called wear corrosion. Wear corrosion can occur in fluid pipes with high-speed flow, as well as in pumps and pipes carrying fluids with suspended abrasive particles. In some components subject to flow, such as the valve disc (head) and seat in high-pressure pressure relief valves, the impeller of centrifugal pumps, and the blades in fans, the relative flow velocity of the corrosive medium is very high. This causes the passivation film on the surface of corrosion-resistant metal materials to be unable to recover due to excessive mechanical erosion, resulting in a significantly increased rate of corrosion. If solid particles are present in the corrosive medium, it will **exacerbate wear corrosion**. 8. Hydrogen embrittlement: Once metal materials, especially titanium, absorb hydrogen, brittle hydrides are formed, which reduces their mechanical strength. In corrosive media, hydrogen released due to corrosion reactions in the metal, as well as hydrogen absorbed during the manufacturing process, are the main sources of hydrogen in the metal. The surface condition of metals has a significant impact on hydrogen absorption. Studies have shown that titanium surfaces with a ground finish absorb the most hydrogen, followed by surfaces in their original state, while surfaces that have undergone vacuum annealing or pickling are the least capable of absorbing hydrogen. Oxidizing titanium in the atmosphere can effectively prevent hydrogen absorption.