As a special kind of special steel with excellent performance, stainless steel is widely used in the production and life of people. Usually the stainless steel material in molding process, most of them are subject to various heat treatment processing, this time such as annealing, quenching, welding and forging process, it will form a layer of black oxide on the surface of stainless steel, the oxide skin will not only affect the appearance quality of stainless steel, will have a negative impact the stainless steel machining process, so in the following processing prior to this layer of oxide skin removing.
The oxide scale of stainless steel is mainly composed of NIO2, Cr2O3, FeOOCr2O3, FeOOCr2OO3Fe2O3 and other dense spinel oxides. It is very firmly combined with the matrix of stainless steel and is hard to remove by ordinary acid washing. In addition, the difficulty of removal also varies with the steel and the manufacturing conditions. Therefore, it is of great practical significance to grasp the methods of various stainless steel oxide pickling and to choose the production process reasonably.
Below is a simple way to remove the acid washing of the oxide skin.
The first is the three acid washing method, the acid is sulfuric acid, nitric acid and hydrochloric acid. It is a more effective acid cleaning method for removing the stainless steel oxide skin. The proportion of the three acids is probably 15%-20%H2SO4, 5% ~ 7%HCl, 5% ~ 7%HNO3, and the pickling temperature of three acids should be kept at about 70-800 degrees. The specific pickling time should be decided by the composition and condition of stainless steel oxide scale, usually 20 to 50min. The three method can successfully remove acid stainless steel oxide, but because the pickling process will precipitate a lot of nitrogen dioxide, which will worsen labor conditions, environmental pollution. In addition, the acid washing process is easy to form the phenomenon of acid washing.
The second method is nitric acid and hydrofluoric acid pickling, this kind of acid washing is a better way of stainless steel pickling. The commonly used proportioning method is 4%-6% hydrofluoric acid and 8%-12% nitric acid. The temperature of the acid is 50 to 600 degrees. The pickling time depends on the oxide scale of the stainless steel surface, usually 20 to 40min. The operation process must be looked over diligently to avoid pickling. In fact, the acid pickling speed is faster, the operation is easy and the quality is good. However, hydrofluoric acid is highly toxic, and requires better ventilation and wastewater treatment.
The third is alkali cooking, also called the acid washing compound method. In order to solve the weakness of tricarboxylic acid method, the compound method of alkali boiling and pickling can not only optimize the quality of pickling, but also reduce the consumption of metals. Alkali cooking can loose oxide, then remove the oxide using acid washing. Most of the ingredients in the alkali boiled liquid are the mixture of sodium hydroxide and sodium nitrate, with a ratio of 8:2.
The temperature of alkali boiling is controlled at 500 to 550 degrees C. The time of alkali cooking is related to the steel, specification, quantity and oxidation skin state of stainless steel, usually from 30min to 1.5h. Stainless steel material after alkali boiling to immediately immersed tank burst to the scale, then the use of three acid washing to remove oxide.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)