Corrosion resistance of stainless steel

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The corrosion resistance of stainless steel generally improves as the chromium content increases. The basic principle is that when there is sufficient chromium in the steel, a very thin and dense oxide film forms on its surface, which prevents further oxidation or corrosion. An oxidizing environment can strengthen this film, whereas a reducing environment will inevitably destroy it, leading to the corrosion of steel. 　(1) Corrosion resistance in various environments 　　1. Atmospheric corrosion 　　The resistance of stainless steel to atmospheric corrosion generally varies depending on the chloride content in the atmosphere. Therefore, proximity to the ocean or other sources of chloride pollution is extremely important for the corrosion of stainless steel. A certain amount of rainwater is only significant when it affects the chloride concentration on the steel surface. 　　In rural environments, 1Cr13, 1Cr17, and austenitic stainless steels can be used for various purposes without any significant change in their appearance. Therefore, stainless steel used in rural areas can be selected based on price, market availability, mechanical properties, workability, and appearance. 　　Industrial environments: In industrial environments free from chloride contamination, 1Cr17 and austenitic stainless steels can operate for extended periods of time with little to no corrosion. A film of dirt may form on their surface, but once this film is removed, they retain their original shiny appearance. In industrial environments with chlorides, it will cause stainless steel to rust. 　　In marine environments, 1Cr13 and 1Cr17 stainless steels form a thin rust layer in a short period of time, but this does not result in any significant changes in their dimensions. Austenitic stainless steels such as 1Cr17Ni7, 1Cr18Ni9, and 0Cr18Ni9 may experience some degree of corrosion when exposed to marine environments. Rust is usually superficial and can be easily removed. 0Cr17Ni12M02 molybdenum-containing stainless steel is essentially corrosion-resistant in marine environments. 　　In addition to atmospheric conditions, there are two other factors that affect the resistance of stainless steel to atmospheric corrosion. That is, the surface condition and manufacturing process. The finish level affects the corrosion resistance of stainless steel in chloride-containing environments. Non-reflective surfaces (rough surfaces) are highly sensitive to corrosion. That is, normal industrially finished surfaces are less sensitive to rusting. The surface finishing level also affects the removal of dirt and rust. It is easy to remove dirt and rust from highly polished surfaces, but it is difficult to do so on dull surfaces. For matte surfaces, regular cleaning is necessary to maintain their original condition. 　　2. Freshwater 　Freshwater can be defined as water that is not acidic, saline, or brackish, and comes from rivers, lakes, ponds, or wells. 　　The corrosivity of fresh water is influenced by the pH value of the water, oxygen content, and tendency to form scale. Scaley (hard) water. Its corrosivity is primarily determined by the amount and type of scale formed on the metal surface. The formation of this scale is due to the minerals present and the temperature. Non-scaling (soft) water, which is generally more corrosive than hard water. Its corrosivity can be reduced by increasing the pH value or reducing the oxygen content. 　　1Cr13 stainless steel is significantly more resistant to fresh water corrosion than carbon steel, and it exhibits excellent properties when used in fresh water. This steel is widely used in applications such as shipyards and dams, where high strength and corrosion resistance are required. However, certain situations should be taken into account. 1Cr13 may be sensitive to moderate pitting in fresh water. However, pitting can be completely avoided through cathodic protection methods. 1Cr17 and austenitic stainless steels can almost completely resist fresh water corrosion at room temperature (ambient temperature). 　　3. Acidic water Acidic water refers to polluted natural water extracted from ores and coal; due to its high acidity, its corrosiveness is much greater than that of natural fresh water. Due to the leaching effect of water on the sulfides contained in ores and coal, acidic water usually contains large amounts of free sulfuric acid. In addition, such water contains significant amounts of ferric sulfate, which has a very strong corrosive effect on carbon steel. 　　Carbon steel equipment exposed to acidic water is usually corroded very quickly. Results of tests using various materials exposed to acidic river water show that austenitic stainless steels exhibit high corrosion resistance in such an environment. 　　Austenitic stainless steels exhibit excellent corrosion resistance in fresh water and acidic river water. In particular, their corrosion layer poses little obstruction to heat conduction; therefore, stainless steel pipes are widely used in heat exchange applications. 　　4. Saline water The corrosion characteristic of saline water is that it often manifests in the form of pitting corrosion. In the case of stainless steel, it is largely due to saline water that causes local damage to the passivation film, which provides corrosion resistance. Other reasons for pitting in these steels are that the barnacles and other marine organic substances attached to the stainless steel equipment can form electrochemical concentration cells. Once formed, these batteries are highly active and cause significant corrosion and pitting. In the case of high-speed flow in saline water, such as in pump impellers, corrosion of austenitic stainless steel is usually very minimal. 　　For condensers using stainless steel tubes, it is necessary to maintain a water flow rate of more than 1.5 m/s to minimize the accumulation of seawater organic matter and other solids within the tubes. When designing the structure of stainless steel equipment for treating saline water, it is advisable to minimize gaps and use thick-walled components. 　　5. Soil Metal buried in soil is in a complex state of constant change, depending on weather and other factors. Practice has shown that austenitic stainless steels generally exhibit excellent resistance to corrosion in most soils, whereas 1Cr13 and 1Cr17 suffer from pitting corrosion in many soils. 0Cr17Ni12Mo0 stainless steel can fully resist pitting corrosion in tests conducted in all types of soil. 　　6. Nitric acid: Ferritic and austenitic stainless steels containing at least 14% chromium exhibit excellent resistance to corrosion by nitric acid. 1Cr17 stainless steel is widely used in processing equipment in nitric acid plants. However, since 0Cr18Ni9 generally has good formability and weldability, it has largely replaced 1Cr17 stainless steel in the aforementioned applications. The resistance of other austenitic stainless steels to nitric acid corrosion is similar to that of 0Cr18Ni9. 0Cr17 stainless steel generally has a slightly higher corrosion rate than 0Cr18Ni9, and higher temperatures and concentrations have a significant adverse effect on it. 　　If the heat treatment applied to steel is inadequate, hot nitric acid will cause intergranular corrosion in austenitic and ferritic stainless steels; therefore, this type of corrosion can be prevented by applying appropriate heat treatment, or by using stainless steels resistant to such corrosion. 　　7. Sulfuric acid: Standard stainless steel grades are rarely used in sulfuric acid solutions, as their range of applicability is very limited. At room temperature, 0Cr17Ni12Mo2 stainless steel (the standard grade with the best resistance to sulfuric acid corrosion) is suitable for sulfuric acid concentrations below 15%. It is corrosion-resistant when it is 85% or higher. However, in higher concentration ranges, carbon steel is usually used. Martensitic and ferritic stainless steels are generally not resistant to corrosion by sulfuric acid solutions. 　　As is the case with nitric acid, sulfuric acid can cause intergranular corrosion if stainless steel is not properly treated. For welded structures that cannot be heat-treated after welding, low-carbon grades such as 00Cr19Ni10 or 00Cr17Ni14M02, or stabilized grades such as 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steels should be used. 8. Phosphoric acid: Austenitic stainless steels exhibit good resistance to corrosion in phosphoric acid solutions, and are therefore widely used in equipment for the production and processing of phosphoric acid. It exhibits effective corrosion resistance at various concentrations at temperatures up to 107°C. Equipment made of 0Cr17Ni12M02 stainless steel can handle (phosphoric acid) at levels of \"over 100% H3PO4\" well, even at temperatures as high as approximately 95°C. 　　It should be noted that trace impurities of fluoride or chloride salts are sometimes present in phosphoric acid produced by wet-process methods. The presence of these halides in the acid may have a detrimental effect on the corrosion resistance of stainless steel. 　　Martensitic and ferritic stainless steels have significantly poorer resistance to phosphoric acid corrosion than austenitic stainless steels, and therefore are generally not used in this acid. 　　9. Hydrochloric acid: Even at room temperature, hydrochloric acid solutions of various concentrations rapidly corrode stainless steel. Therefore, it is impossible to use stainless steel in such acids. 　　10. Other inorganic acids: Austenitic stainless steels generally exhibit good resistance to corrosion by boric acid, carbonic acid, chloric acid, and chromic acid at almost all concentrations and temperatures, with the exception of 100% chloric acid. The corrosion resistance of 1Cr13 and 1Cr17 stainless steels to chromic acid is significantly inferior to that of austenitic stainless steels, but they exhibit relatively good resistance to boric acid and carbonic acid corrosion. 　　11. Acetic acid: Austenitic stainless steels generally exhibit excellent resistance to acetic acid corrosion, whereas martensitic and ferritic stainless steels are not suitable for most applications requiring resistance to acetic acid corrosion. Austenitic stainless steels can fully resist corrosion by acetic acid of various concentrations at room temperature; at higher temperatures, 0Cr17Ni12Mo2 and 0Cr19Ni13Mo3 exhibit better resistance to acetic acid corrosion than other austenitic stainless steels. 　　12. Formic acid: At room temperature, formic acid can be completely treated using any austenitic stainless steel. However, when it is hot formic acid, it can rapidly corrode stainless steel without molybdenum; therefore, 0Cr17Ni12M02 and 0Cr19Ni13M03 must be used. At various temperatures, formic acid rapidly corrodes martensitic and ferritic stainless steels. 　　13. Oxalic acid: Under normal conditions, at room temperature and with a concentration of at least 50%, stainless steel exhibits good resistance to corrosion by oxalic acid. However, at higher temperatures, the oxalic acid solution, just as at room temperature at a 100% concentration, causes considerable corrosion to all stainless steels. 　　14. Lactic acid: 0Cr18Ni9 stainless steel can be used for equipment used in the storage of lactic acid at temperatures up to approximately 38°C. At higher temperatures, molybdenum-free austenitic stainless steels suffer from pitting; therefore, 0Cr17Ni12M02 and 0Cr19Ni13M03 are preferred. Maraging and ferritic stainless steels generally have low resistance to lactic acid corrosion. 　　15. Bases Stainless steel generally exhibits good resistance to corrosion by weak bases, such as ammonium hydroxide. For strong bases such as sodium hydroxide and potassium hydroxide, austenitic stainless steels exhibit good corrosion resistance at temperatures of up to about 105°C and concentrations of up to about 50%; however, at higher temperatures and concentrations, the corrosion rate can become significant. Austenitic stainless steels develop stress corrosion cracks when the temperature is above the boiling point at atmospheric pressure (and at slightly lower temperatures, near 50% concentration). 　　16. Hydrochloric acid solution: Apart from halide solutions under certain conditions, stainless steel generally exhibits excellent resistance to corrosion by hydrochloric acid solutions. Regarding acidic salts, the corrosion resistance of stainless steel is inevitably affected to some extent by the special acids formed as a result of salt hydrolysis. For acidic salt solutions at higher temperatures, molybdenum-containing austenitic stainless steels (0Cr17Ni12Mo2 and 0Cr19Ni13Mo3) generally exhibit better corrosion resistance than other grades of stainless steel. 　　When stainless steel is used in halide solutions, especially chloride solutions, it should be taken into account that although the corrosion rate is generally low, pitting and/or stress corrosion cracking may occur under certain conditions. Although stainless steel achieves excellent results in many applications involving chlorides (such as food processing equipment and seawater flowing at relatively low temperatures), it is necessary to consider each application separately. Whether pitting or stress corrosion cracks occur depends on many factors such as the environment, equipment design, and operation. (II) Corrosion phenomena 1. Pitting As mentioned earlier, the excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on its surface, which renders it in a passive state. The formation of this passivation film is due to the reaction of steel with oxygen when it is exposed to the atmosphere, or as a result of contact with other oxygen-containing environments. If the passivation film is damaged, the stainless steel will continue to corrode. In many cases, the passivation film is only damaged on the metal surface and in localized areas; corrosion causes the formation of small pores or pits, resulting in irregularly distributed pitted corrosion on the surface of the material. 　　2. Factors causing pitting Corrosion often occurs due to the presence of chloride ions that combine with depolarizers. Pitting in passive metals such as stainless steel is usually caused by certain aggressive anions that damage the passivation film locally. A oxidative environment is required to maintain the passive state, which provides high corrosion resistance; yet this same environment is also the condition that leads to pitting. The media that cause pitting are chloride solutions containing heavy metal ions such as Fe3+, Cu2+, and Hg2+ in the presence of C1-, Br-, I-, and ClO4- ions, or chloride solutions of Na+ and Ca2+ alkaline and alkaline-earth metal ions that contain substances such as H2O2 and O2. 　　The pitting rate increases as the temperature rises. For example, in solutions with a sodium chloride concentration of 4%-10%, the maximum weight loss due to pitting occurs at 90°C ； For more dilute solutions, the maximum occurs at higher temperatures. 　　3. Methods to prevent pitting 　　① Avoid the accumulation of halide ions. 　　②Ensure the uniformity of the oxygen or oxidizing solution, stir the solution, and avoid areas where no liquid is flowing. 　　③Either increase the oxygen concentration or remove oxygen. 　　④Increase the pH value. Compared to neutral or acidic chlorides, significantly alkaline chloride solutions cause less pitting, or none at all (hydroxide ions act as a corrosion inhibitor). 　　⑤Operate at the lowest possible temperature. 　　⑥Add a passivator to the corrosive medium. Low concentrations of nitrates or chromates are effective in many media (by inhibiting the preferential adsorption of ions on metal surfaces, thereby preventing corrosion caused by the adsorption of chloride ions). 　　⑦Cathodic protection is used. There is evidence that stainless steel protected by electro-synodic cathodic protection using low-carbon steel, aluminum, or zinc does not suffer from pitting in seawater. 　　Austenitic stainless steels containing 2%-4% molybdenum exhibit good pitting resistance. The use of molybdenum-containing austenitic stainless steels can significantly reduce pitting or general corrosion in corrosive media such as sodium hydride solutions, seawater, sulfurous acid, sulfuric acid, phosphoric acid, and formic acid. 　　4. Intergranular corrosion Unstable austenitic stainless steels with a carbon content of over 0.03% (grades that do not contain titanium or niobium) are prone to intergranular corrosion in certain environments if not heat-treated properly. These steels suffer from intergranular corrosion when heated between 425–815°C, or when cooled slowly through this temperature range. Such heat treatment causes carbides to precipitate at the grain boundaries (sensitization), and it leads to chromium depletion in the adjacent areas, making those areas susceptible to corrosion. Sensitization can also occur during welding, causing subsequent local corrosion in the weld heat-affected zone. 　　The most common method for testing the sensitivity of stainless steel is the 65% nitric acid corrosion test. During the test, the steel specimen was placed in boiling 65% nitric acid solution for a period of 48 hours per cycle, for a total of 5 cycles, with the weight loss measured at the end of each cycle. Generally, the average corrosion rate over 5 test cycles should not exceed 0.05 mm/month. 　　Intergranular corrosion in welded austenitic stainless steel structures can be prevented by the following methods: ① Use low-carbon grades such as 00Cr19Ni10 or 00Cr17Ni14Mo2, or stable grades such as 0Cr18Ni11Ti or 0Cr18Ni11Nb. The use of these grades of stainless steel helps to prevent the precipitation of carbides in amounts that could have harmful effects during welding. 　　②If the structural components of the metal part are small and can be heat-treated in a furnace, heat treatment at 1040–1150°C can be carried out to dissolve chromium carbide, followed by rapid cooling in the range of 425–815°C to prevent the precipitation of sigma phase. 　　Welded ferritic stainless steels can also experience intergranular corrosion in certain media. This is caused by the precipitation of carbides or oxides and strain in the metal lattice when steel is cooled rapidly from above 925°C; stress relief heat treatment after welding can eliminate this stress and restore corrosion resistance. Adding titanium in an amount more than 8 times the carbon content to 1Cr17 stainless steel can generally reduce intergranular corrosion of welded steel structures in certain media. However, adding titanium in concentrated nitric acid is not effective. 　　5. Stress corrosion cracks Stress corrosion cracks result from the combined effect of static stress and corrosion, which leads to metal embrittlement. Only tensile stress causes this type of failure. In fact, all metals and alloys (with very few exceptions) are prone to stress corrosion cracking in certain environments. There are also differing opinions regarding whether the failure of some metals is due to \"stress corrosion\" or \"hydrogen embrittlement\" (such as the cracking of high-strength steels in hydrogen sulfide). For the purpose of discussion, all such damages caused by external environments are included under the category of stress corrosion cracks. 　　Hardened (quenched and tempered) martensitic stainless steels are susceptible to stress corrosion cracking in solutions containing chlorides, thermal hydroxides or nitrates, or hydrogen sulfide. For austenitic stainless steels, hydroxide solutions of concentrated chlorides are the primary medium causing stress corrosion cracks. It has been proven that several other environments can also cause stress corrosion cracking in austenitic and martensitic stainless steels. However, it should be noted that in many such environments, the presence of impurities may have already caused cracks. 　　Sensitized austenitic stainless steels are susceptible to stress corrosion cracking in the intergranular mode. If sensitivity is high and/or stress is high, this type of crack may occur in what would otherwise be considered a weak environment. Sensitized and austenitic stainless steels must never be used in stressful applications unless sufficient testing has been conducted to prove that the encountered environment will not cause intergranular stress corrosion cracking. 　　The environments in which stress corrosion crack failure occurs are usually quite complex. For example. The stresses involved are usually not just service stresses, but rather a combination of residual stresses generated in the metal as a result of manufacturing, welding, or heat treatment. This situation can often be alleviated by using methods to relieve stress in the manufactured equipment. In the same category, as mentioned above, the corrosive agent that causes cracks is often merely an impurity present in the product being processed. In the overall solution, the amount of corrosive agents present may not be sufficient to cause cracks, but at the cracks or in the splashing area above the liquid, the local concentration of these agents can lead to damage. 　　Although there are several general methods to prevent stress corrosion cracking, the best approach is to use materials that are resistant to stress corrosion cracking in that environment. Therefore, in hot chloride environments, 0Cr18Ni13Si4 (American AISI LX M15) or ferritic stainless steel should be selected. In hydrogen sulfide environments, ferritic and austenitic stainless steels are generally suitable, while hardened martensitic stainless steels should not be used. The properties of various stainless steels are shown in Tables 2-5-1 and 2-5-4.