Oxygen concentration difference corrosion of the storage tank bottom plate
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Please refer to the description in red text below: After the storage tank is built, its bottom surface is in direct contact with the load-bearing foundation; moist air cannot reach the center of the bottom surface, and thus no oxygen deficiency can occur, preventing differential corrosion. However, after the storage tank is put into use, crude oil is continuously poured in or drawn out, causing the mass of the tank to change constantly. The bottom plate may bulge upward, and at this point, moist air can penetrate to the center of the lower surface of the bottom plate, resulting in the entire lower surface of the bottom plate coming into contact with oxygen. The oxygen concentration is higher at the edges of the bottom plate, forming an oxygen-rich area; the closer to the center of the bottom plate, the lower the oxygen concentration, with the center being an oxygen-poor area. Differences in oxygen concentration provide one of the conditions for differential oxygen-corrosion to occur. When the temperature drops, a physical phenomenon known as liquefaction occurs at the lower surface of the storage tank bottom and the upper surface of the load-bearing foundation, resulting in the formation of water droplets; this is also one of the conditions that lead to differential corrosion due to oxygen supply. At this point, when oxygen, moisture, and metal are all present, differential aeration corrosion occurs. My question is: does corrosion occur more easily in the edge plates with high oxygen content, or in the center of the tank bottom with lower oxygen content? What is the theoretical basis?
The basic principle is: the area where electrons flow in is protected, while the area where electrons flow out is corroded.
This is a concept related to cathodic protection: a difference in oxygen concentration leads to a potential difference, which in turn creates corrosion cells in the soil medium, accelerating the corrosion at the anode area. Metals that rely on an oxide film or passivation layer for corrosion resistance are particularly prone to this type of corrosion. Oxygen concentration cell corrosion occurs in many media, especially those containing oxygen. Oxygen concentration cell corrosion actually means that the edge plates with higher oxygen levels are less prone to corrosion, while the center of the tank bottom with lower oxygen levels is more susceptible to corrosion. This refers to the condition where oxygen, moisture, and metal are all present, resulting in differential corrosion under oxygen supply conditions. In fact, oxygen concentration cell corrosion is similar to crevice corrosion and under-scale corrosion protection. For example, in crevice corrosion, crevice areas form beneath the deposits; in the solution within these crevices, it is very difficult for oxygen to be replenished ； On the metal surface outside the gap, the supply of oxygen is sufficient; therefore, the area outside the gap is an oxygen-rich region that acts as the cathode, while the area inside the gap is an oxygen-poor region that acts as the anode. The corrosion sites caused by the oxygen concentration cell formed in the gap area are located within the gap or beneath the deposits. The reason is that Cl- is replenished when there is a lack of oxygen in the gap; at this time the pH value is acidic, which inevitably leads to metal oxidation and corrosion. When, on occasion, it is observed at the site that the metal surface at the center of the bottom surface of the tank is not severely affected, this is because such areas do not yet meet the conditions for electrochemical corrosion; usually, there is a lack of moisture in such cases.
This should be stray current corrosion.
Could you give me a link for free access?