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This post was last edited by B0SS on 2019-9-26 at 12:06. After being used for a long time or for many years, gas cylinders are inevitably subject to corrosion caused by rainwater, air, or humid atmosphere. This corrosion is resulting from the action of substances such as water, oxygen, and acidic pollutants in the atmosphere. The rusting of steel under natural atmospheric conditions is one of the most common forms of atmospheric corrosion. There are three types of atmospheric corrosion: ① dry atmospheric corrosion. At this time, there is virtually no water vapor in the atmosphere, and ordinary metals form an invisible oxide layer at room temperature. The surface of the steel remains shiny. ②Corrosion caused by humid atmosphere. It refers to the corrosion that occurs in a thin film layer of metal that is invisible to the naked eye. At this time, water vapor is present in the atmosphere; when the concentration of water vapor reaches the critical humidity (65% for iron and nearly 100% for copper), a thin layer of water forms on the metal surface, leading to uniform corrosion. If pollutants such as CO2, H2S, and SO2 are present in the atmosphere, corrosion accelerates significantly. The corrosion of steel under atmospheric conditions is essentially electrochemical corrosion in the presence of a water film. ③Corrosion of the gas storage tank under liquid film conditions can be observed. It refers to corrosion that occurs when the relative humidity in the air is around 100%, or in rain and other aqueous solutions. At this point, water forms droplet aggregates on the metal surface, resulting in a visible water film. Factors affecting atmospheric corrosion ① The influence of water. Among the substances that cause corrosion in steel in atmospheric conditions, water is the main factor (generally, the higher the humidity, the stronger the corrosive effect). The principle of corrosion is as follows: a) Water is an electrolyte, and it can also dissolve a large number of ions, thereby causing metal corrosion. b. Water can dissociate into H+ and OH—; different pH values have a significant impact on the dissolution and corrosion of metals and oxides. ②The impact of SO2. In areas polluted by industrial exhaust gases, SO2 has the most severe impact on the corrosion of steel. The exhaust gases generated from the use of oil, natural gas, and coal as fuels contain large amounts of SO2, and the corrosion rate of steel plates increases as the concentration of SO2 in the atmosphere rises. ③Marine-atmospheric influences. The atmosphere near the ocean contains a certain amount of salt, with NaCl being its main component. Cl– is highly corrosive, and as a result it can accelerate corrosion. The further away from the ocean, the less salt is present in the atmosphere, and thus the less corrosion occurs. ④Other impacts. In the atmospheric environment of oil production, there may be large amounts of harmful substances such as Cl2, NH3, H2S, and solid dust particles; the corrosion of the steel in gas storage tanks also increases as the concentration of these substances rises. The synergistic effect of several substances will lead to accelerated corrosion of steel.