Stainless steel is a cornerstone material of modern industry. It is valued for its excellent corrosion resistance and durability. However, even this strong material faces challenges such as pitting and crevice corrosion, which can compromise the integrity and safety of the structure.
Understanding the causes, development, and prevention of these types of corrosion is essential to maximizing the performance and service life of stainless steel.
What is Stainless Steel
Stainless steel is a widely used metal material, known for its excellent corrosion resistance. Its main components include about 1.2% carbon, about 35% nickel, 5% molybdenum and more than 10.5% chromium. Of this, chromium reacts with oxygen to form an additional protective film, which greatly enhances stainless steel’s ability to resist corrosion, allowing it to be used in a variety of challenging environments. In the oil and gas industry, stainless steel is used in large quantities to manufacture such as pipelines, heat exchangers, oil platforms and storage containers and other key components, a strong guarantee of these facilities in the harsh chemical environment of stable operation.
What is pitting and crevice corrosion
Pitting corrosion
Pitting corrosion is a very covert form of corrosion. When pitting corrosion occurs in stainless steel, tiny holes will appear on its surface, these holes are often difficult to detect in the initial stage, but will cause serious damage to the material. With the development of corrosion, these small holes will gradually deepen into the material, so that the strength of the material is constantly reduced, seriously affecting its structural integrity.
Crevice corrosion
Crevice corrosion occurs between various surface gaps in stainless steel. When these gaps in the accumulation of water or other mixtures, and the oxygen content is reduced, it will form an acidic environment, thereby triggering corrosion. This corrosion will rapidly weaken the protective layer of stainless steel and negatively affect the performance of the material.
How Stainless Steel Corrosion Occurs
The essence of corrosion is a series of electrochemical reaction processes. Take the iron in the metal pipe as an example. During the corrosion process, the iron is oxidized at the anode. How to lose electrons to become Fe₂⁺ ions dissolved in water. At the same time, the electrons generated in the oxidation process participate in the reduction reaction at the cathode, turning the O₂ dissolved in water into OH- ions.
Taking stainless steel as an example, the chromium it contains can form a protective oxide film to resist corrosion. However, when the environmental conditions are unfavorable, such as the presence of high concentrations of chloride ions, high temperatures, low pH values, or mechanical damage to the surface of the material, this protective film will be destroyed. Once the protective film is destroyed and cannot be re-formed in the solution in which it is located, the corrosion reaction will rapidly intensify, leading to corrosion of the stainless steel.
Difference between pitting and crevice corrosion
Morphology and location
Pitting usually forms obvious small holes on the exposed surface of stainless steel. These small holes may be randomly distributed and may be isolated small spots in the initial stage, but they will gradually increase and deepen over time. Crevice corrosion mainly occurs in closely connected parts. Such as between pipes and pipe supports or fixtures, and at the joints of adjacent pipes. The location of dirt and deposits under the crevice is difficult to directly observe.
Triggering conditions
Pitting corrosion occurs mainly due to the local rupture of the chromium-rich oxide protective film on the surface of stainless steel. The metal underneath becomes fragile in the corrosive solution and is susceptible to continuous attack by chemicals such as chloride ions. In an environment with high chloride concentration. Especially in areas of high-salt residues formed by evaporation of seawater, and in high-temperature environments, the risk of pitting corrosion will increase significantly.
A key factor in crevice corrosion is the lack of oxygen supply in the crevice. When electrolyte solution accumulates in the crevice. As oxygen is consumed, an oxygen-deficient, chlorine-rich and acidic environment is formed inside the crevice, which triggers corrosion. This type of corrosion may also occur at lower temperatures. Because the energy required to form corrosion in the crevice geometry is lower than that of pitting corrosion.
Corrosion development process
Once pitting begins, it can develop relatively quickly. The small holes will continue to deepen and may even penetrate the pipe wall, causing leakage, environmental pollution and safety hazards. And it may be caused by the generation and expansion of cracks in tensile parts.
Crevice corrosion will form relatively wide and shallow corrosion pits in the crevice. And it will spread rapidly along the crevice, posing a serious threat to the overall integrity of stainless steel. Because the existence of crevice is relatively common and difficult to avoid, the harm of crevice corrosion to stainless steel structural parts cannot be underestimated.
How to Prevent Pitting Corrosion and Crevice Corrosion
Material selection
When selecting stainless steel materials, the application environment and the required corrosion resistance should be fully considered. For example, 316 stainless steel contains an appropriate amount of molybdenum and has good corrosion resistance. In contrast, 304L stainless steel has relatively weak pitting and crevice corrosion resistance.
For harsher environments, such as high chloride concentrations or high temperature environments, super austenitic stainless steels (such as 6Mo or 6HN) or super duplex stainless steels (such as 2507) are better choices. Because they have higher corrosion resistance and mechanical strength, they can meet the higher requirements of application scenarios.
Surface treatment and protection
Coating, paint or other surface treatment methods can be used to enhance the corrosion resistance of stainless steel. These protective layers can form an additional barrier on the surface of stainless steel. Thereby reducing its direct contact with corrosive substances and reducing the risk of corrosion.
Environmental management
When using stainless steel, try to avoid contact with high concentrations of corrosive chemicals. For example, control the chloride ion concentration in the environment and maintain appropriate temperature and pH. For environments where water or moisture may accumulate, good drainage and ventilation conditions should be ensured. Prevent moisture and corrosive substances from accumulating on the surface or in the gaps of stainless steel.
Design optimization
During the equipment design stage, the formation of gaps should be minimized. For example, welding connections are used instead of bolt connections or rivet connections to avoid pipes directly leaning against the pipe wall or contacting each other.
For unavoidable gaps, sealants or other sealing materials can be used to seal them to prevent electrolyte solutions from entering the gaps and causing corrosion. At the same time, in the design of the pipeline system, it is necessary to ensure that the solution can flow smoothly, avoid the formation of dead zones, and reduce the possibility of corrosion.
By comprehensively understanding pitting and crevice corrosion in stainless steel and implementing effective preventive measures, industries can significantly extend the service life of their equipment and facilities. Proactive steps in material selection, design optimization, surface treatment, and environmental management not only ensure safety and operational stability but also minimize economic losses and potential risks. For practical applications, adopting a scientific approach tailored to specific conditions is key to overcoming the challenges of stainless steel corrosion and maintaining long-term performance.
