What Is Acid Cleaning?

The method of using acid solution to remove scale and rust on steel surface is called pickling. It is a way to clean metal surfaces. It is usually performed with a pre-passivating treatment.

The pickling process used at this stage removes the thick oxide scale on the stainless steel surface. The high temperature and long pickling time lead to high processing costs. At normal temperature, the oxide scale removal process has a good effect on removing thin oxide scale and has no effect on thick oxide scale. Ideally, the black dross at the weld is almost impossible to remove. In view of this situation, based on the successful experience of stainless steel pre-treatment, the author adopted a two-step pickling comprehensive treatment technology to study the rapid pickling process of stainless steel at room temperature. This process can quickly remove the thick oxide scale on stainless steel surface at room temperature, especially the black skin and inclusions at the weld. It has a good removal effect. The surface is clean and bright after pickling, which is not easy.

Acids commonly used for pickling include inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, hydrofluoric acid, and sulfamic acid, and organic acids such as citric acid and EDTA. However, since acid has a corrosive effect on metal equipment, especially the corrosion of inorganic acid is more serious, at the same time, the released hydrogen will diffuse to the inside of the metal, causing hydrogen embrittlement of the washed equipment. In addition, a large amount of acid gas precipitated will worsen the working conditions. Therefore, a corrosion inhibitor should be added during pickling to suppress the corrosion of metals in acidic media, reduce the amount of acid used, improve the pickling effect, and extend the service life of thermal equipment. When pickling, not only the acid's ability to dissolve iron should be considered, but also factors such as scale components, metal materials, and waste liquid treatment methods should be considered. Therefore, choosing a good quality corrosion inhibitor is an important part of pickling, and understanding the corrosion inhibition performance of various corrosion inhibitors can better perform anticorrosive work.

Sulfuric acid pickling inhibitor

Sulfuric acid is often used as a cleaning agent in the acid leaching and rust removal of metal materials with a large amount of acid pickling, and is used in acid pickling boilers when the content of calcium compounds in the boiler fouling is very low. Due to the high concentration and high density of sulfuric acid, the industrial sulfuric acid used to wash a boiler is only one-fourth the volume of hydrochloric acid under the same amount of cleaning conditions, and concentrated sulfuric acid hardly corrodes steel, which brings chemical cleaning This greatly facilitates the system of storage, transportation and acid distribution. There are two main types of corrosion inhibitors used in sulfuric acid solutions. One is the corrosion inhibitor of nitrogen compounds, such as amines, pyridines, and pyridine bases and quinones, 2-methylpyridine, pyridine iodide compounds, and pyridine halides. Species are inhibitors of sulfur compounds, such as thiourea and thiourea derivatives. The latter is more effective in inhibiting the corrosion of carbon steel in sulfuric acid.

Hydrofluoric acid pickling corrosion inhibitor

Hydrofluoric acid is a weak inorganic acid that volatilizes in the air, and its steam is strongly corrosive and toxic. However, it has a fast rate of dissolving oxides and has the special properties of dissolving silica scale (oxide of silicon). In 1968, hydrofluoric acid pickling was successfully used for the first time in a post-operational supercritical pressure boiler in West Germany. Since then, 40% of West German boilers have been pickled with hydrofluoric acid. Although the use of hydrofluoric acid pickling has the disadvantages of unsafe operation, high price and environmental pollution. However, considering the characteristics of hydrofluoric acid, after using the new type of corrosion inhibitor, its application range is still very wide.

Nitric acid pickling corrosion inhibitor

Nitric acid has strong solubility to boiler scale and metal oxides, so nitric acid sometimes replaces hydrochloric acid pickling. Nitric acid is a very oxidizing acid, and most of the corrosion inhibitors are organic substances, which are prone to redox reactions. Therefore, there are fewer types of nitric acid corrosion inhibitors.

Sulfamic acid pickling inhibitor

Sulfamic acid is a powdery medium acid inorganic acid. During the pickling process, it is not easy to volatilize, has good solubility in water, does not cause the phenomenon of salt precipitation and precipitation, and does not contain halogen ions. It is suitable for cleaning carbonates and hydroxides with large amounts of calcium and other metals. Dirt. However, its ability to pick iron oxide is poor, and decomposition occurs above 60 degrees. Therefore, it is rarely used for pickling of large boilers, and is generally used for pickling of copper pipes ^[1] .