1, cleaning ability: pickling stone is just a chemical cleaning method used by some stone friends, which can quickly and effectively remove dirt and impurities attached to stone skin. Different concentrations of oxalic acid (oxalic acid, chemical formula COOOH) and hydrochloric acid (aqueous solution of hydrogen chloride, chemical formula HCL) are used as cleaning agents. After ordinary washed stones and pebbles are soaked in oxalic acid or low-concentration hydrochloric acid, the dirt on the skin and impurities mixed in cracks and holes can basically be removed and shed quickly, and the stone skin washed with clean water is very clean and bright. Such pickling only removes dirt and scale and restores the essence of stone. Of course, it should not be considered as fraud.
2. Used for fraud:? Some people have found that some stones rich in calcium carbonate are easily corroded and dissolved by concentrated acid in the process of pickling, so the normal method of washing stones by pickling has gradually evolved into a means of counterfeiting.
The commonly used acids for pickling are 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, due to the corrosive effect of acids on metal equipment, especially inorganic acids, the released hydrogen will diffuse into the metal, making the cleaned equipment hydrogen brittle.
In addition, a large amount of acid gas will worsen working conditions. Therefore, corrosion inhibitors should be added during pickling to inhibit the corrosion of metals in acidic medium, reduce the consumption of acid, improve the pickling effect and prolong the service life of thermal equipment. When pickling, we should not only consider the iron dissolving ability of acid, but also consider the factors such as scale composition, metal materials and waste liquid treatment methods. Therefore, the selection of corrosion inhibitors with good quality is an important part of pickling, and understanding the corrosion inhibition performance of various corrosion inhibitors can better carry out anti-corrosion work.
1 sulfuric acid pickling inhibitor
Sulfuric acid is often used as the main cleaning agent in the process of acid leaching rust removal of metal materials with a large amount of acid pickling, and when the content of calcium compounds in boiler dirt is low. Because of the high concentration and density of sulfuric acid, the volume of industrial sulfuric acid used to clean a boiler is only 1/4 of that of hydrochloric acid, while concentrated sulfuric acid is almost non-corrosive to steel, which brings great convenience to chemical cleaning and greatly simplifies the storage, transportation and acid making system.
There are mainly two kinds of corrosion inhibitors used in sulfuric acid solution. One kind is nitrogen-containing compound corrosion inhibitor, such as amine, pyridine, pyridine base and quinone, 2- methylpyridine, pyridine iodine compound and pyridine halide; The other is the corrosion inhibitor of sulfur-containing compounds, such as thiourea and thiourea derivatives. The latter has high corrosion inhibition efficiency for carbon steel in sulfuric acid.
2 hydrofluoric acid pickling inhibitor
Hydrofluoric acid is a weak inorganic acid, which volatilizes in the air, and its vapor is highly corrosive and toxic. However, it has the special property of dissolving silicon scale (silicon oxide) because it dissolves oxides quickly. 1968 hydrofluoric acid pickling was successfully used for the first time in a running supercritical pressure boiler in west Germany. Since then, 40% boilers in West Germany have adopted hydrofluoric acid pickling. Although hydrofluoric acid pickling has some disadvantages such as unsafe operation, high price and environmental pollution. However, considering the characteristics of hydrofluoric acid, it is still widely used after the application of a new corrosion inhibitor.
3 nitric acid pickling inhibitor
Nitric acid has strong solubility in boiler scale and metal oxides, so nitric acid sometimes replaces hydrochloric acid for pickling. Nitric acid is a very oxidizing acid, but the corrosion inhibitors are mostly organic, which is prone to oxidation-reduction reaction. Therefore, there are fewer kinds of corrosion inhibitors for nitric acid pickling.
4 sulfamic acid pickling inhibitor
Aminosulfonic acid is a powdery inorganic acid with moderate acidity. In the pickling process, it is not volatile, has good solubility in water, does not appear salt precipitation phenomenon, does not contain halogen ions, and is suitable for cleaning scale containing a large number of carbonates and hydroxides of calcium and other metals. However, the pickling ability of 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.
References:
Pickling _ Baidu Encyclopedia