Yes, stainless steel does corrod under specific conditions, and its corrosion resistance is not absolute, but is affected by multiple conditions such as material composition, environmental factors and processing technology. The following is an extension of the content you provide:
The reason why stainless steel has good corrosion resistance is mainly due to a dense chromium oxide film (Cr₂O₃) that can be formed on its surface. This layer of oxide film acts as a barrier, which can effectively isolate the direct contact between the external corrosive medium and the metal matrix, thereby protecting the stainless steel from corrosion. However, under certain circumstances, this passivation film may be damaged, leading to corrosion of stainless steel. These situations mainly include:
Passivation film rupture: When the stainless steel surface is exposed to high concentrations of chloride ions (Cl⁻) or subjected to mechanical damage, the passivation film may be destroyed. Once the passivation film is destroyed, the metal matrix is exposed to the external environment, thus increasing the risk of corrosion.
Element depletion: In certain forms of corrosion, such as intergranular corrosion, chrome-poor zones (i.e., areas with less than 12% chromium content) are formed due to the precipitation of carbides at grain boundaries. These chromium-poor zones lose the protection of the passivation film and are therefore more susceptible to corrosion.
Environmental synergy: Factors such as high temperature, acidic medium or stress environment will accelerate the corrosion process of stainless steel. In these environments, even if the passivation film on the stainless steel surface is intact, it is difficult to resist corrosion.
Intergranular corrosion
Mechanism: In the temperature range of 450-850℃ (such as welding heat affected zone), carbon and chromium in stainless steel formed chromium carbide (Cr₂₃C₆), resulting in chromium poverty at the grain boundaries. This chromium deficiency weakens the corrosion resistance at the grain boundaries, making stainless steel more vulnerable to corrosion.
Case: 304 stainless steel welded pipe fittings without solid solution treatment are prone to intergranular corrosion in an acidic environment. This is because the heat generated during the welding process will cause chromium carbide to precipitate at the grain boundaries, forming a chrome-poor zone.
Preventive measures: In order to prevent intergranular corrosion, low-carbon stainless steel (such as 304L) can be used, the addition of titanium/niobium (Ti/Nb) and other stabilizing elements to reduce the formation of chromium carbide, or the solid solution annealing treatment at 1050℃ to eliminate chromium carbide.
pitting
Mechanism: Chloride ions (Cl⁻) can penetrate the passivation film on the surface of stainless steel and form corrosion pits in local areas. Especially in stagnant liquids or crevices, pitting corrosion is more obvious.
Case: 316 stainless steel in Marine environment is prone to pitting holes due to long-term exposure to salt spray. This is because the concentration of chloride ions in the Marine environment is high, and it is easy to penetrate the passivation film on the surface of stainless steel.
Preventive measures: In order to prevent pitting, you can choose molybdenum (Mo) containing 316L stainless steel to enhance the ability to resist chloride ions, or through surface passivation treatment to further improve the corrosion resistance of the stainless steel surface.
Stress Corrosion Cracking (SCC)
Mechanism: When tensile stress and corrosive media (such as chloride ions, high temperature water, etc.) work together, cracks will be caused on the surface of stainless steel and make it expand. Such cracks usually develop along grain boundaries and eventually lead to the fracture of the material.
Case: 316L stainless steel pipes in chemical plants are prone to stress corrosion cracking in media containing chloride ions. This is because the media in chemical plants usually contain corrosive substances, and the pipeline will be subjected to a certain tensile stress during operation.
Preventive measures: In order to prevent stress corrosion cracking, residual stress can be removed (such as through heat treatment processes), use high nickel (Ni) austenitic steel or duplex stainless steel to improve the stress corrosion resistance of the material.
Crevice corrosion
Mechanism: When a metal is in contact with a non-metal, electrochemical corrosion is formed between the two due to the difference in oxygen concentration. This corrosion usually occurs in crevices or narrow Spaces, so it is called crevice corrosion.
Case: Stainless steel bolts at flange joints are corroded due to fluid accumulation under sealing gaskets. This is because the fluid accumulation under the sealing gasket forms an anoxic environment, resulting in electrochemical corrosion between the stainless steel bolt and the sealing gasket.
Preventive measures: To prevent crevice corrosion, the design can be optimized to reduce the presence of crevices, or high chromium-nickel alloys (such as 254SMO) can be used to improve the resistance of the material to crevice corrosion.
Full corrosion
Mechanism: In a strong acid (such as concentrated sulfuric acid) or strong alkali environment, the passivation film on the surface of stainless steel will completely dissolve, resulting in corrosion of the entire material.
Case: 304 stainless steel in concentrated sulfuric acid due to the inability to form a stable oxide film and rapid corrosion. This is because concentrated sulfuric acid is very oxidizing and corrosive, and can quickly destroy the passivation film on the surface of stainless steel.
Preventive measures: In order to prevent comprehensive corrosion, you can use molybdenum, copper (Cu) containing super austenitic steel (such as 904L) to improve the material's resistance to comprehensive corrosion.
Material composition:
Chromium (Cr) : Chromium is one of the most important alloying elements in stainless steel, and its content should reach at least 12% to form a stable passivation film. Stainless steel with high chromium content (such as 310S containing 25% Cr) can significantly improve its corrosion resistance at high temperatures.
Molybdenum (Mo) : The addition of molybdenum can significantly enhance the ability of stainless steel to resist chlorine ion pitting. For example, 316L stainless steel contains 2.5% molybdenum, which gives it better corrosion resistance in an environment containing chloride ions.
Nickel (Ni) : Nickel can improve the austenitic stability of stainless steel and reduce its sensitivity to stress corrosion cracking (SCC). In addition, nickel can improve the toughness and weldability of stainless steel.
Environmental Conditions:
Chloride ion concentration: When the chloride ion concentration in the environment exceeds 200 ppm, the corrosion resistance of 304 stainless steel will be significantly reduced. This is because chloride ions are able to penetrate the passivation film and form corrosion pits beneath it.
Temperature: For every 10 ° C increase in temperature, the corrosion rate of stainless steel usually increases 1-2 times. This is because the chemical reaction rate is accelerated at high temperatures, and the stability of the passivation film is also reduced.
Processing technology:
Welding: Improper welding operations can lead to an increased risk of intergranular corrosion in the heat affected zone. This is because the heat generated during the welding process will precipitate chromium carbide at the grain boundary, forming a chrome-poor zone.
Surface treatment: electrolytic polishing or passivation treatment can enhance the densification and integrity of the passivation film on the surface of stainless steel, thereby improving its corrosion resistance. These treatments can form a more uniform and stable oxide film on the stainless steel surface.
Material selection optimization:
Marine environment: When used in the Marine environment, 316L stainless steel containing molybdenum (Mo) or duplex stainless steel 2205 should be preferred. These materials have good resistance to chloride ion corrosion and high strength.
High-temperature acidic environment: When working in an environment with high temperature and acidic media, 310S stainless steel or Hastelloy containing high chromium (Cr) and nickel (Ni) should be selected. These materials maintain good corrosion resistance in extreme environments.
Process control:
Solid solution treatment after welding: Solid solution treatment of welded stainless steel parts (usually quench between 1050-1100℃) can eliminate the precipitation of carbides and restore the corrosion resistance of the material.
Avoid residual stress from cold working: Residual stress created during cold working increases the risk of stress corrosion cracking. Therefore, an appropriate annealing process should be used to eliminate the residual stress and improve the toughness of the material.
Environmental Management:
Control the chloride ion concentration in the medium: by adding corrosion inhibitors and other ways to control the chloride ion concentration in the medium, you can slow down the corrosion rate of stainless steel.
Clean the surface of the equipment regularly: Cleaning the surface of the equipment regularly can prevent the accumulation of pollutants that cause crevice corrosion and other types of corrosion problems.
Enquiry telephone：
Scan and consult wechat customer service