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The resistance of stainless steel to atmospheric corrosion generally varies with 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, 1 Cr 17, 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. The basic principle behind the corrosion resistance of stainless steel is that when there is sufficient chromium in the steel, a very thin and dense oxide film forms on its surface; this film prevents further oxidation or corrosion, and its effectiveness generally increases as the chromium content rises. 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, 1 Cr 17, 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 rust formation. A film of dirt may form on the surface, but once this film is removed, the steel retains its 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 size. Austenitic stainless steels such as 1 Cr 17Ni7, 1 Cr 18Ni9, and 0 Cr 18Ni9 may develop some corrosion when exposed to the marine environment. Rust is usually superficial and can be easily removed. 0 Cr 17 Ni 12M 02 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, namely 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; in contrast, normally industrially finished surfaces are less susceptible 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 difficult to do so from dull surfaces. For matte surfaces, more frequent cleaning is required to maintain their original condition. ②Freshwater 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, whose corrosiveness 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, it should be noted that in some cases, 1Cr13 may be susceptible to moderate pitting in fresh water. However, pitting can be completely avoided using cathodic protection methods. 1Cr17 and austenitic stainless steels can almost completely resist fresh water corrosion at room temperature (ambient temperature). ③Acidic water refers to polluted natural water that is leached from ores and coal; due to its high acidity, its corrosiveness is much greater than that of natural fresh water. Due to the leaching of sulfides contained in ores and coal by water, acidic water usually contains large amounts of free sulfuric acid. Furthermore, this water contains large amounts of ferric sulfate, which has a significant effect on the corrosion of 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, especially since their corrosion film poses little resistance to heat conduction. That is why stainless steel pipes are widely used in heat exchange applications.