How to maintain the electrochemical polishing solution for stainless steel tube

Electrochemical polishing is the same as Electropolishing. Before electropolishing, stainless steel tube must be thoroughly degreasing and scrubbed with decontamination powder to prevent the oil from polluting the polishing bath. It is necessary to frequently measure the relative density of the electropolishing solution during use. If the relative density is less than the specified value of the formula, it indicates that the electropolishing solution contains too much water. The evaporation method can be used to heat the solution to above 80°C to remove the excess water. The insufficient volume can be supplemented with phosphoric acid and sulfuric acid according to the formula ratio. Before the stainless steel pipe enters the electrochemical polishing tank, it is best to drain or blow dry the water attached to the pipe. If the relative density is too high and exceeds the specified value of the formula, it means that the moisture is too low. A small amount of water should be added appropriately to reduce the relative density to the specified value. If possible, it is best to analyze the solution periodically and make timely adjustments based on the results.

Due to the dissolution of the surface of the stainless steel tube during the polishing process, the iron, nickel, and chromium content in the solution will gradually increase. At this time, the solution gradually loses its polishing ability. No matter how high the temperature or the large current is turned on, it will not help to restore the polishing ability. Analyze the solution, if the iron content exceeds 60g/L, and the trivalent chromium content exceeds, the solution has aged. There are two methods for regenerating polishing solution and restoring polishing ability. One method is to properly dilute the solution with water to reduce the acidity. Impurities such as iron, chromium, and nickel can be partially precipitated as salts, remove the sediment at the bottom of the tank, and then heat and evaporate to remove the water and restore the original relative density. This method is more cumbersome to operate. Need to consume more energy and time. Another method is to replace part of the solution. It is best to keep 20% of the old solution and add 80% of the new solution. It can be treated with less or no electricity, and normal polishing can be realized soon.

During the polishing process, a thick layer of iron, nickel and other impurities will be deposited on the surface of the cathode lead plate, which will affect the surface conductivity of the cathode, resulting in a decrease in current, and the cathode current density of the polished surface will not rise, which seriously affects the polishing quality. Therefore, it is necessary to remove the deposits on the cathode plate in time, sometimes forming a hard thick film, which can only be removed by strong beating, and finally rinsed to keep the entire circuit unobstructed.

The area ratio of the cathode to the anode is controlled at 1/2 of the anode area. In this case, the growth of trivalent chromium can be prevented, and excessive trivalent chromium is oxidized to hexavalent chromium on the anode surface. Too much trivalent chromium can easily age the polishing liquid. The distance between the cathode and the anode is too large, the resistance increases, the power consumption increases, and the solution is easy to heat up, which affects the polishing quality. If the distance is too small, it is easy to cause short circuit and ignition and burn black products. The distance between the cathode and the anode is preferably 100 to 300 mm. For polishing some complex large-diameter stainless steel tubes, pictograph cathodes can be made to maintain uniform anode current distribution. Especially for the inner surface of stainless steel tubes, a proper pictograph cathode is placed in the inner hole to make all parts of the inner hole uniformly polished.

During electrochemical polishing, due to the high current density and high current supply, the stainless steel tube must be cut off when entering and exiting the polishing tank. Do not hang or remove the fixture with electricity to prevent the generation of electric sparks, causing electrolysis, and The mixture of hydrogen and oxygen accumulated on the surface of the tank exploded.

Since a strong current passes through the bath liquid, it will heat up the bath liquid. In continuous operation, cooling measures must be taken, and a refrigerator is used to cool the continuously warming bath liquid. The temperature of the polishing solution should be appropriately maintained within the specified process range, so that the polishing and leveling rate of the stainless steel tube surface is maintained at a normal rate, so as to effectively reduce the viscosity of the electrolyte, reduce the thickness of the anode mucosa, accelerate the diffusion of anode dissolved products, and accelerate the convection of the solution. It is beneficial to the desorption of air bubbles remaining on the anode and avoids spots and pits. Too high a temperature will cause the solution to overheat, accelerate the conversion of hexavalent chromium to trivalent chromium, and easily cause surface corrosion. If the temperature is too low, the viscosity of the solution will increase and the mucosa on the anode surface will thicken, which is not conducive to the diffusion of anode dissolved substances and significantly reduces the polishing and leveling effect.

When the stainless steel tube is electrochemically polished, the anode current density is almost in proportion to the dissolution of the metal. Only by choosing the anode current density and controlling it in a certain anode potential range can you obtain good electrochemical polishing quality. The best value of anode current density should be based on different electrolyte formulations, through actual polishing, and observation of the best polishing results. The value is OK. Under the appropriate anode current density, according to the theory of mucosa, the protruding parts of the microscopic surface are preferentially dissolved, which is beneficial to leveling and finishing the surface. If the anode current density is too small, general anode dissolution occurs on the surface of the stainless steel tube, and the polishing effect cannot be achieved. The anode current density is too large, the mucosa is broken down, oxygen is violently precipitated in a gas flow, and the surface is overheated, which causes the electro-polishing solution to spread more rapidly, the mucosa is destroyed, no longer exists, and electrochemical corrosion occurs. Therefore, the anode current density must be controlled at the optimal value during the electropolishing process, that is, within the determined process range.

The movement of the anode accelerates the diffusion of the anode dissolved product, which plays a role of stirring, effectively removing the bubbles on the surface of the anode, avoiding the generation of streaks in the air flow, and preventing the surface of the stainless steel tube from being over-corroded by local overheating. The anode movement helps to increase the anode current density and improve the electrochemically polished surface quality of the stainless steel tube. Active chlorine ions are not allowed in the electrochemical polishing solution. Chloride ions can destroy the protective mucosa formed on the surface during electrochemical polishing and cause corrosive pitting on the surface of the stainless steel tube. Chloride ions can be oxidized at the high current density of the anode to become chlorine gas and escape and be removed. The source of chloride ions may be brought into the bath without washing after pickling with hydrochloric acid, or the introduction of impurity chloride ions in the raw materials.

The best ratio of hexavalent berkelium and trivalent chromium, hexavalent chromium is oxidizing in the electrochemical polishing solution, and it has a passivation protection effect on the surface of the stainless steel tube. The trivalent chromium has an effect on the maintenance of electropolishing. If there is no chemical reaction to produce trivalent chromium in the newly prepared electropolishing liquid, a good polishing surface cannot be obtained. Only when a certain amount of trivalent chromium is electrolyzed in the solution can an ideal polishing surface appear. If chromic anhydride is added to the formula, that is, hexavalent chromium will generate hydrogen on the cathode to reduce part of the hexavalent chromium to trivalent chromium through the electrolytic reaction; if there is no chromic anhydride in the formula, the trivalent chromium must be dissolved in the stainless steel tube by the anode Of chromium. This is why the newly prepared electrolytic polishing liquid must be fully electrolyzed before normal polishing can be performed. In the solution containing chromic anhydride, add gelatin or glycerin, they can have a strong reduction reaction with chromic anhydride, and part of the hexavalent chromium is transformed into trivalent chromium. Hexavalent chromium is yellow and trivalent chromium is green. They make the solution yellow-green in the electrolyte. This is why electrolytic polishing can only be done after electrification treatment. The best polishing quality is in the middle stage of the electrolytic process, and the electrolytic polishing solution should remain yellow-green during the production process. At this time, according to the chemical analysis data, the ratio of hexavalent chromium to trivalent chromium is 3:7. In order to maintain this ratio, the color of the electrolyte can be observed. If the color is yellow, it indicates that the electrolyte contains hexavalent chromium. High, you can add proper amount of gelatin or glycerin to reduce part of hexavalent chromium to trivalent chromium, or produce trivalent chromium through electrolysis with large cathode and small anode. If the color is dark green, it indicates that the electrolyte contains high trivalent chromium. Add an appropriate amount of chromium anhydride solution dissolved in water according to the proportion, or electrolyze the polishing solution with a large anode and a small catholyte to partially convert the trivalent chromium into hexavalent chromium. Can improve the polishing quality of the solution from time to time.

If the anode current density is 20 mA during electrolytic polishing and the time is 4 hours, observe with a tool metallographic microscope, the metal removal amount of the inner diameter of the stainless steel pipe thread is about 0.001mm per minute, and the metal removal amount of the outer diameter of the thread is 0.002 mm, the tooth profile is basically unchanged, only the top of the tooth is slightly blunt. The anode current density increases, and the metal thrown away increases proportionally. For the size of precision-sized stainless steel pipes, the amount of metal removal after electrochemical polishing should be considered.

Electrochemical polishing of the parts after electric welding or heat treatment. The stainless steel pipe after electric welding or heat treatment is carried out twice during the electrochemical polishing. The first time it enters the groove and polishes 3 to 5 minutes and takes it out to oxidize the loose welding slag and heat treatment. Use a wire brush to brush it off the skin, or knock it off with a small hammer, and then enter the groove for the second time and polish it for 3 to 5 minutes to get better results.

After electrochemically polished stainless steel pipes, if no subsequent processing, such as electroplating, coloring and other processes, must be passivated and neutralized. The role of neutralization is to fully eliminate the acidic substances adsorbed on the surface after electrochemical polishing and passivation. Neutralization is generally carried out in a 30g/L solution of sodium sodium carbonate. There is a uniform passivation film on the surface of the electrochemically polished stainless steel tube, so there is no need for passivation treatment. After electrochemical polishing, the stainless steel tube is cleaned in warm water at 40°C, then cleaned in cold water, neutralized and cleaned, and then dried with compressed air to effectively prevent the residual acid from corroding the polished surface.