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At present, pickling is mostly used to remove scale from boilers and water-cooled heat exchange equipment. During operation, the pickling solution not only dissolves scale, but also causes strong corrosion of the base metal. People always hope that scale can be removed quickly and completely without the base metal being corroded. To achieve this purpose, corrosion inhibitor technology was developed. Many corrosion inhibitors currently used in industry can basically achieve this: It does not affect the scale dissolution rate, but can reduce the corrosion weight loss of the base metal to an allowable level, which is usually said to have a high corrosion inhibition rate. But it is not enough to do this alone. The following two issues must also be paid attention to during pickling. One problem is corrosion hydrogenation. Boilers and general water-cooled heat exchange equipment are usually made of carbon steel or low alloy steel. Their corrosion in the pickling solution is mainly hydrogen evolution corrosion, that is, the cathodic process of corrosion is the reduction of hydrogen ions. The corrosion is very strong, not only causing rapid dissolution of the metal, but also the large amount of hydrogen produced at high speed will penetrate into the metal. The increase in the hydrogen content inside the metal will cause its mechanical properties to become brittle, that is, hydrogen embrittlement. Therefore, it is actually not allowed to pickle equipment (especially pressure vessels) without adding corrosion inhibitors. However, although the addition of corrosion inhibitors can significantly reduce the dissolution rate of metal in the pickling solution and significantly reduce the amount of hydrogen evolution, it does not mean that the penetration of hydrogen into the metal has also been reduced. The evolution of hydrogen during corrosion is thought to consist of two main steps, namely: The first step - hydrogen ion discharge H++e→H The second step - hydrogen atoms compound into hydrogen molecules H+H→H2 If the added corrosion inhibitor mainly inhibits the above second step, then the concentration of nascent hydrogen atoms on the metal surface will increase and diffuse into the metal with a large gradient, which will accelerate hydrogen penetration. For example, some corrosion inhibitors containing arsenic, selenium, etc. will accelerate hydrogen penetration in acidic solutions. Therefore, when selecting a pickling corrosion inhibitor, it is very important to not only look at its corrosion inhibition efficiency, but also its inhibitory effect on hydrogen penetration. Another problem is corrosion caused by Fe+++. During pickling, Fe+++ will be introduced into the pickling solution due to the dissolution of scale. It is a substance that promotes corrosion. In acidic media, the standard potential of the cathodic reduction of Fe+++ is very positive, making it easy to carry out the cathodic reduction reaction Fe++++e—→Fe+++. The reduction of Fe+++ promotes the anodic dissolution reaction Fe—→Fe+++2e, which increases the corrosion rate of metal, and is especially prone to pitting corrosion. Generally, when pickling large boilers, they often contain about 0.1% Fe+++. Therefore, an excellent pickling corrosion inhibitor should have good ability to inhibit Fe+++ corrosion. If the selected corrosion inhibitor has poor ability to inhibit Fe+++ corrosion, some corresponding measures should be taken, such as adding some reducing agents to reduce Fe+++ to Fe++ ； Add certain complexing agents to "hide" Fe++ and reduce the effective concentration of Fe+++, thereby reducing the adverse effects caused by Fe+++. Inhibiting the corrosion-promoting effect of Fe+++ (and Cu++) is an issue that cannot be ignored during pickling.