Method of removing oil on the surface of square tube
（1） Organic solvent cleaning
It mainly uses organic solvents to dissolve saponified and unsaponified oil to remove oil stains. The commonly used organic solvents include ethanol, cleaning gasoline, toluene, carbon tetrachloride, trichloroethylene, etc. The more effective solvents are carbon tetrachloride and trichloroethylene, which will not burn and can be used for oil removal at higher temperatures. It should be noted that after oil removal by organic solvent, supplementary oil removal must also be carried out. When the solvent volatilizes on the surface of the rectangular tube, there is usually a thin film left, which can be removed in the following processes such as alkali cleaning and electrochemical oil removal.
（2） Electrochemical cleaning
Cathode oil removal or alternate use of anode and cathode is more commonly used. The hydrogen gas separated from the cathode or the oxygen gas separated from the anode by electrochemical reaction is mechanically stirred by the solution on the surface of the rectangular tube to promote the oil stain to escape from the metal surface. At the same time, the solution is continuously exchanged, which is conducive to the saponification reaction and emulsification of the oil. The remaining oil will be separated from the metal surface under the influence of the continuously separated bubbles. However, in the process of cathodic degreasing, hydrogen often permeates into the metal, causing hydrogen embrittlement. In order to prevent hydrogen embrittlement, the cathode and anode are usually used to remove oil alternately.
（3） Alkaline cleaning
A cleaning method based on the chemical action of alkali is widely used because of its simple use, low price and easy availability of raw materials. Since the alkali washing process depends on saponification, emulsification and other functions, a single alkali can not be used to achieve the above performance. A variety of components are usually used, and additives such as surfactants are sometimes added. The alkalinity determines the degree of saponification reaction, and the high alkalinity reduces the surface tension between oil and solution, making oil easy to emulsify. In addition, the cleaning agent remaining on the surface of the rectangular tube can be removed by water washing after alkali washing.
（4） Surfactant cleaning
It is a widely used oil removal method by using the characteristics of surfactant such as low surface tension, good wettability and strong emulsifying ability. Through the emulsification of surfactant, an interfacial facial mask with certain strength is formed on the oil-water interface to change the state of the interface, so that the oil particles are dispersed in the aqueous solution to form an emulsion. Or through the dissolving action of surfactant, the oil stain insoluble in water on the rectangular tube is dissolved in the surfactant micelle, so as to transfer the oil stain to the aqueous solution.