Cleaning before sandblasting and spraying - electrochemical degreasing

       The process of immersing the metal parts hanging on the cathode or anode in alkaline electrolyte and passing direct current to separate the grease from the workpiece is called electrochemical degreasing. The speed of electrochemical degreasing is much higher than that of chemical degreasing, and the degreasing is thorough and the effect is good.



The basic principle of chemical degreasing is to use a metal part as an electrode and immerse it in an alkaline electrolyte. When direct current is applied, the interfacial tension of the metal solution interface is reduced due to polarization, and the solution can easily penetrate into the parts under the oil film. surface, and a large amount of hydrogen or oxygen is precipitated. When they emerge from the solution, they will have a strong stirring effect, violently impact and tear the oil film, so that the oil film adsorbed on the surface of the part is broken into small oil droplets, which are quickly separated from the workpiece and become opaque after entering the solution. liquid, so as to achieve the purpose of degreasing.



According to the polarity of the parts, electrochemical degreasing can be divided into two categories: cathodic degreasing and anodic degreasing. When the workpiece is connected to the cathode, it is called cathode degreasing, and the workpiece is connected to the anode, which is called anode degreasing. Electrochemical degreasing is the final degreasing process.



Factors affecting electrochemical degreasing include sodium hydroxide concentration, emulsifier selection, current density, electrolyte temperature, and degreasing method. Sodium hydroxide is a strong electrolyte, and its aqueous solution has strong conductivity. The higher the concentration, the stronger the conductivity, the higher the current density, and the faster the oil removal rate. Sodium hydroxide has a passivation effect on the surface of steel and can prevent corrosion of steel parts during anode degreasing. Sodium hydroxide has a strong corrosive effect on metals such as aluminum and zinc, and is generally not used for degreasing of these metals.



The role of emulsifiers has been relegated to a secondary role in electrolytic degreasing. Generally, surfactants such as OP-10, sodium alkyl sulfate, detergents 6501 and 6502, and soap are not used because of their strong foaming ability.



If the electrolyte contains the above components, during operation, the liquid surface will be covered by a large amount of foam of hydrogen and oxygen mixed gas, and an explosion will occur in case of sparks generated by poor electrode contact. Therefore, in the electrolyte of electrochemical degreasing, usually only emulsifiers with weak foaming ability such as trisodium phosphate and sodium silicate are added. The soap produced by the saponified grease in the process of degreasing also has a strong foaming ability. When there is a lot of foam on the liquid surface, the power should be cut off first, and then the parts should be put in or taken out to avoid explosion.



Increasing the current density can increase the oil removal rate and improve the oil removal quality in deep holes. However, when the current density is too high, a large amount of alkali mist will be formed, polluting the air in the workshop and possibly corroding parts. Not only will corrosion occur during anode degreasing, but aluminum and its alloy parts will be corroded due to the increase of pH value of the solution in the cathode area if the current density is too large during cathode degreasing; Increased hydrogen permeation results in embrittlement. Appropriate current density must ensure that enough bubbles can be precipitated to improve the oil removal effect, generally 5~10A/dm2.



Increasing the temperature can reduce the resistance of the electrolyte, increase the current density, promote saponification and emulsification, speed up oil removal, improve oil removal efficiency, and save electricity. However, if the temperature is too high, it will not only consume a lot of heat energy, pollute the air in the workshop, and deteriorate the working conditions, but also may corrode parts such as aluminum and zinc. The appropriate temperature is 70~90℃. Electrochemical oil removal mainly relies on electrolysis to remove oil, and the temperature of the electrolyte can be slightly lower than that of chemical oil removal.



When the metal part is connected to the anode, the oxidation process is carried out on the surface, and oxygen is released. The reaction process is as follows:



4OH -4e=O2↑+4H2O



Therefore, when the anode is used for degreasing, there is no danger of hydrogen embrittlement, and the corrosion residue on the surface of the part and some metal films, such as tin, zinc, lead, chromium, etc., can be removed. However, the anodic degreasing rate is lower than the cathodic degreasing, and aluminum, zinc, tin, lead, copper and their alloys are subject to corrosion. Steel parts can also suffer from spot corrosion when the solution has low alkalinity, low temperature and high current density, especially when the electrolyte contains chloride ions.



When the metal part is connected to the cathode, the reduction process is carried out on its surface, and hydrogen gas is released. The reaction process is as follows:



4H2O+4e = 2H2↑+40H-



The hydrogen evolved at the cathode is twice as much as the oxygen evolved at the anode. Therefore, the cathodic degreasing speed is fast and generally does not corrode parts. However, the hydrogen precipitated on the cathode easily penetrates into the steel parts and causes hydrogen embrittlement, especially high-strength steel or spring steel, which is easily damaged by hydrogen embrittlement. Parts degreased by the cathode are prone to foaming during electroplating due to the influence of hydrogen permeation; if the electrolyte contains a small amount of zinc, tin, lead and other impurities, sponge-like precipitates will be produced on the surface of the parts, which will also affect the subsequent plating quality. In order to overcome the shortcomings of the above two methods, it is best to use the combined electrochemical degreasing method.