Corrosion accidents caused by poor process operation and maintenance

Thread Content

Corrosion accidents caused by poor process operation and maintenance: The operation and maintenance of equipment play a very important role. Whether the process parameters are appropriate, the operation is stable, and the maintenance is proper all affect the corrosion resistance and service life of the equipment. Because the process operation parameters are the parameters of the corrosive environment in which the equipment is located. If the environmental conditions exceed the range of corrosion resistance of the equipment materials, corrosion incidents are inevitable. This is exactly why it is said that “corrosion occurs on the equipment, but its root cause lies in the process.” Example: A large tower in a factory was equipped with titanium internals. During the shutdown, the operators took it upon themselves to clean it with hot concentrated hydrochloric acid. As a result, the titanium components were severely corroded and almost completely dissolved. Analysis: This is a typical example of equipment corrosion and damage resulting from a lack of basic knowledge about corrosion. We have heard of similar things as well. For example, the hydrochloric acid storage tanks designed by a certain factory are made of stainless steel, and some workers use stainless steel buckets to transport hydrochloric acid, and so on. The mistake here is assuming that titanium and stainless steel are excellent corrosion-resistant metal materials, and therefore will not corrode in any medium. The relationship between corrosion and the environment: One of the most fundamental principles in corrosion knowledge is that the corrosion behavior of metals is closely related to environmental conditions; therefore, it is impossible to discuss a metal’s corrosion resistance without taking those environmental conditions into account. For a metal, not only the type of medium but also factors such as its concentration, temperature, gas presence, impurities present, and flow velocity can have a significant impact on the pattern and rate of corrosion of the metal. The type, concentration, and temperature of the medium constitute the main characteristics of the corrosion environment. With the medium concentration on the horizontal axis and temperature on the vertical axis, the points where the corrosion rate of the metal equals a certain value are connected to form a curve; this curve is referred to in the literature as an \"isochor line\". The number of regions between two equal corrosion lines indicates the range of medium concentration and temperature within which the metal’s corrosion rate varies. This is what is known as a “corrosion map”. Corrosion maps clearly show the distribution of corrosion rates of metals in a certain medium, providing a very useful tool for us to select materials appropriately and determine the suitable environmental conditions for their use. ⑴Corrosion resistance of titanium: Titanium exhibits a low corrosion rate only when the hydrochloric acid concentration is below 10% and the temperature is below 40°C. That is, titanium can only be used in dilute hydrochloric acid at room temperature. At higher temperatures, the hydrochloric acid concentration must be lower. The corrosion rate of titanium in hot concentrated hydrochloric acid is very high. At 60°C in 20% hydrochloric acid, the erosion rate of titanium reaches 20 mm/y. No wonder some people said about the above examples, “There is no better way to dissolve titanium!” ⑵ Corrosion resistance of stainless steel: Ordinary stainless steel is also not resistant to hydrochloric acid corrosion. Because, like titanium, the corrosion resistance of stainless steel depends primarily on an oxidizing environment, whereas hydrochloric acid is a non-oxidizing acid. For example, Cr18Ni9 stainless steel has a corrosion rate of around 10 mm/year in 5% hydrochloric acid at room temperature, which is similar to that of carbon steel.