The corrosion resistance of stainless steel generally increases with increasing chromium content. The basic principle is that when there is enough chromium in the steel, a very thin to dense oxide film is formed on the surface of the steel, which prevents further oxidation or corrosion. An oxidizing environment can strengthen the film, while a reducing environment inevitably destroys the film, causing corrosion of the steel.
(1) Corrosion resistance in various environments
The atmospheric corrosion resistance of stainless steel is basically a function of the amount of chloride in the atmosphere. Therefore, corrosion of stainless steel near the ocean or other chloride sources is extremely important. A certain amount of rainwater is important only if it acts on the chloride concentration of the steel surface.
Rural environment 1Cr13, 1Cr17 and austenitic stainless steel can be used for a variety of purposes, and there will be no significant changes in appearance. Therefore, stainless steel exposed in rural areas can be selected based on price, market availability, mechanical properties, processability and appearance.
Industrial environment In an industrial environment without chloride pollution, 1Cr17 and austenitic stainless steel can work for a long time, basically keep no rust, and may form a foul film on the surface, but when the dirt film is removed, it still maintains the original Bright appearance. In industrial environments with chlorides, stainless steel will be rusted.
Marine environment 1Cr13 and 1Cr17 stainless steel will form a thin rust film in a short period of time, but will not cause obvious dimensional changes. Austenitic stainless steels such as 1Cr17Ni7, 1Cr18Ni9 and 0Cr18Ni9 may appear when exposed to the marine environment. Corrosion. Corrosion is usually shallow and can be easily removed. 0Cr17Ni12M02 molybdenum-containing stainless steel is basically corrosion-resistant in the marine environment.
In addition to atmospheric conditions, there are two other factors that affect the resistance of stainless steel to atmospheric corrosion. That is, the surface state and the manufacturing process. The finishing level affects the corrosion resistance of stainless steel in chloride-containing environments. Matte surfaces (matte) are very sensitive to corrosion. That is, normal industrial finishing surfaces are less sensitive to corrosion. Surface finishing levels also affect the removal of dirt and rust. It is easy to remove dirt and rust from highly finished surfaces, but it is difficult to remove from a matte surface. For matte surfaces, frequent cleaning is required if the original surface condition is to be maintained.
2. Various acidic water
When stainless steel is used in a halide solution, especially a chloride solution, it should be considered that even if the corrosion rate is generally low, pitting and/or stress corrosion cracking may occur under certain conditions. Although there are many excellent effects in the use of stainless steel in the presence of chlorides (such as food processing equipment and seawater flowing under relatively low temperature conditions), various uses must be considered separately. Whether pitting or stress corrosion cracking occurs depends on many factors and factors such as the environment and equipment design and operation.
As mentioned earlier, the excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on the surface of the steel, making it passive. The passivation film is formed as a result of the reaction of the steel with oxygen when exposed to the atmosphere or due to contact with other oxygen-containing environments. If the passivation film is destroyed, the stainless steel will continue to corrode. In many cases, the passivation film is only destroyed on the metal surface and in local places. The effect of the corrosion is to form fine holes or pits, resulting in irregularly distributed small pit-like corrosion on the surface of the material.
2. Factors causing pitting
Pitting corrosion is likely to be the presence of chloride ions in combination with depolarizers. Pitting corrosion of passive metals such as stainless steel is often caused by localized destruction of the passivation film by some aggressive anions, and protection of passive states with high corrosion resistance. An oxidizing environment is usually required, but this is also the condition for pitting. The medium in which pitting occurs is a heavy metal ion such as FE3+, Cu2+, Hg2+ or a chloride solution containing Na+, Ca2+ alkali and alkaline earth metal ions of H2O2, O2 or the like in a C1-, Br-, I-, Cl04-solution.
The pitting rate increases with increasing temperature. For example, in a solution having a concentration of 4% to 10% sodium chloride, the weight loss caused by pitting corrosion is maximized at 90 ° C; for a more dilute solution, the maximum value occurs at a higher temperature.
3. Methods to prevent pitting
- Avoid concentration of halogen ions.
- To ensure the uniformity of the oxygen or oxidizing solution, stir the solution and avoid small areas where the liquid does not flow.
- Either increase the concentration of oxygen or remove oxygen.
- Increase the pH. Compared to neutral or acidic chlorides, the apparently alkaline chloride solution causes less pitting or is completely absent (the hydroxide ions act as an anticorrosive).
- Work at the lowest possible temperature.
- Add a passivating agent to the corrosive medium. Low concentrations of nitrate or chromate are effective in many media (preventing ions from preferentially absorbing on the metal surface, thus preventing chloride ions from damaging and causing corrosion).
- Using cathodic corrosion protection. There is evidence that stainless steel cathodically protected with low carbon steel, aluminum or zinc does not cause pitting in seawater.
Austenitic stainless steel containing 2%-4% molybdenum has good pitting resistance. The use of molybdenum-containing austenitic stainless steels can significantly reduce pitting or general corrosion, such as sodium hydride solution, seawater, sulfurous acid, sulfuric acid, phosphoric acid and formic acid.
3. Intergranular corrosion
Unstable austenitic stainless steels containing less than 0.03% carbon (titanium-free or niobium-free grades) are prone to intergranular corrosion in certain environments if not properly heat treated. Intergranular corrosion occurs when these steels are heated between 425 and 815 ° C or slowly cooled through this temperature range. Such heat treatment causes carbides to precipitate at the grain boundaries (sensitization) and causes the chromium depletion in the nearest region to make these regions susceptible to corrosion. Sensitization can also occur during welding, causing localized corrosion in the heat affected zone of the weld.
The most common method for checking the sensitivity of stainless steel is the 65% nitric acid corrosion test method. During the test, the steel samples were placed in a boiling 65% nitric acid solution for a period of 48 hours for a total of 5 cycles, and the weight loss was measured in each cycle. Generally, the average corrosion rate for 5 test cycles should be no more than 0.05mm/month.
Intergranular corrosion of austenitic stainless steel welded structures can be prevented by:
- Use low carbon grade 00Cr19Ni10 or 00Cr17Ni14Mo2, or stable grade 0Cr18Ni11Ti or 0Cr18Ni11Nb. Use these grades of stainless steel to prevent the precipitation of carbides during welding to cause harmful effects.
- If the face structure is small and can be heat-treated in the furnace, heat treatment may be performed at 1040-1150 ° C to dissolve the chromium carbide, and rapid cooling in the interval of 425-815 ° C to prevent rumination.
Welded ferritic stainless steel may also exhibit intergranular corrosion in certain media. This is caused by precipitation of carbides or oxides and metal lattice strain when steel is rapidly cooled from above 925 ° C. Stress-relieving heat treatment after welding can eliminate stress and restore corrosion resistance. Adding more than 8 times the carbon content of titanium to 1Cr17 stainless steel generally reduces the intergranular corrosion of the welded steel structure in some media. However, the addition of titanium is not effective in concentrated nitric acid.
4. Stress corrosion crack
Stress corrosion cracking is a combination of static stress and corrosion that causes cracks and metal embrittlement. Only tensile stress causes this form of damage. In fact, all metals and alloys (except for very few metals) are prone to stress corrosion cracking in certain environments. The damage to certain metals is either “stress corrosion” or “hydrogen embrittlement” (eg high There are also some different opinions on the cracking of strength steel in hydrogen sulfide. For the sake of discussion, all such damage caused by the external environment is included in the stress corrosion cracking.
Hardened (quenched and tempered) martensitic stainless steels are sensitive to stress corrosion cracking in solutions containing chlorides, thermal hydroxides or nitrates, or hydrogen sulfide. For austenitic stainless steels, the hydroxide solution of concentrated chloride is the main medium causing stress corrosion cracking. It has been shown 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 caused cracks.
Sensitized austenitic stainless steels are sensitive to intergranular forms of stress corrosion cracking. If the sensitivity is severe and/or the stress is high, this form of crack may be produced in an environment considered to be weak. Sensitized and austenitic stainless steels must not be used for stress conditions unless sufficient tests are performed to demonstrate that the environment encountered does not cause intergranular stress corrosion cracking.
The environment in which stress corrosion cracking occurs is often quite complex. E.g. The stress involved is usually not just the working stress, but the combination of residual stresses in the metal that are produced, welded, or heat treated. This situation can often be mitigated by the method of stress relief after fabrication. In the same gear, as mentioned above, the corrosive medium causing the crack is often only an impurity in the product being processed. In the overall solution, the amount of corrosive medium present may not be sufficient to cause cracking, but at the crack or in the splash zone above the liquid, the local concentration of the medium may cause damage.
Although there are several general methods for preventing stress corrosion cracking, the best method is to use materials that are resistant to stress corrosion cracking in this environment. Therefore, 0Cr18Ni13Si4 (American AISLX M15) or ferritic stainless steel should be used in the hot chloride environment. The use of ferrite and austenitic stainless steels in a hydrogen sulfide environment is generally suitable, and hardened martensitic stainless steels cannot be used.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)