Let’s talk about uniform corrosion
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The entire or most of the metal surface is corroded, with the degree of corrosion being roughly uniform. Generally, a layer of corrosion product film on the surface can slow down corrosion; high-temperature oxidation is an example of this. Similarly, easily passivated metals such as stainless steel, titanium, and aluminum form an extremely thin passivation film in an oxidizing environment, which provides excellent protection and essentially stops corrosion. The oxide film (rust) formed on iron in the atmosphere and water provides very low protection. Generally, uniform corrosion is quite severe. There are also cases of uniform corrosion that do not produce a surface film, such as iron dissolving rapidly and completely in dilute sulfuric acid or hydrochloric acid. Full-scale corrosion without a membrane is very dangerous, but it occurs rarely in real life, unless there is a serious mistake in material selection, such as using iron or aluminum equipment to store and transport hydrochloric acid. The degree of uniform corrosion can be expressed by the corrosion rate. Two units are commonly used: one is the weight lost per unit area per unit of time, expressed in g/(m2·h) ； The other is the average corrosion thickness per unit time, expressed in mm/year. The conversion relationship between the two is as follows: lmm/year = 8.76 g/(m2·h) × 1/d, where d is the density of the material.
General corrosion refers to the situation where almost all or most of the metal surface is corroded, with a relatively uniform degree of corrosion. This type of corrosion sometimes forms a layer of corrosion product film on the metal surface, which can slow down the corrosion process to a certain extent, such as the oxidation of metals at high temperatures. Certain metals such as stainless steel, titanium, and aluminum can form an extremely thin passivation film under specific conditions; this film provides excellent protection and can essentially stop corrosion. However, the protective capacity of oxide films (rust) formed on iron in the atmosphere or water is low, resulting in more severe corrosion. If a protective surface film is not formed, as in the case of iron corroding in dilute sulfuric acid or hydrochloric acid, the metal will dissolve rapidly; this is a very dangerous situation that usually occurs only when there are serious mistakes in the choice of material. The degree of uniform corrosion can be measured by the corrosion rate, which is often expressed as the weight loss per unit surface area per unit time (g/(m²·h)) or the average thickness reduction per unit time (mm/year). .
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