Electrolytic polishing process for stainless steel cages

Time : Marzo 8, 2024

Abstract: The principle of electric polishing for a stainless steel cage is introduced, and the polishing process conditions and factors affecting polishing quality are analyzed. Actual use shows that this process can meet production needs, and the polished stainless steel cage obtains a uniform bright layer.

Keywords: Rolling bearings; Retainer; Stainless steel; Electric polishing

1. Principles of Electric Polishing

The process of putting a workpiece into a specific solution for anodic electrolysis to make the metal surface smooth and produce a metallic luster is called electrolytic polishing or electro polishing. It removes the surface layer generated during the mechanical polishing process and generates a metal oxide layer with high corrosion resistance and reflectivity on the surface, while also reducing surface stress and friction coefficient. Electrolytic polishing is the anodic electrochemical corrosion process on the surface of stainless steel. When using stainless steel workpieces as the anode for electrolysis, the first step is the electrochemical and chemical dissolution of the surface oxide layer and metal, as well as the physical erosion of oxygen bubbles that precipitate on the workpiece, causing the surface layer to detach from the surface of the workpiece and be removed.

Electrochemical polishing may undergo the following reactions:

(1) Metal atoms lose electrons and transform into metal ions entering the solution

M → M2++2e

(2) Formation of oxide film

M+H2O → MO+2H++2e

(3) Oxygen precipitation

2H2O → O2+4H++4e

• Anodization of other substances in the solution.

The process of electrolytic polishing can be explained by mucosal theory as follows: when using stainless steel workpieces as anodes for electrolytic polishing, if the dissolution rate of the anode is greater than the diffusion rate of the anodic dissolution products from the anode surface to the depth of the electrolyte, the dissolution products will accumulate near the anode surface. A viscous liquid film with high resistance is formed, and its distribution on the anode surface is uneven. The mucosa at the protrusion is thin and has low electrical resistance. High current density, high oxygen precipitation, easy solution renewal, and fast dissolution rate; The concave mucosa is thick, with high resistance, low current density, and slow dissolution. As the polishing time continues, the protrusions on the anode surface are gradually flattened, making the surface smooth and smooth.

2. Electrolytic polishing process

2.1 Pre polishing treatment

Austenitic stainless steel and martensitic stainless steel are commonly used materials for stainless steel cages. Most austenitic stainless steel cages have relatively clean surfaces and can be polished directly without pre-treatment as needed. Martensitic stainless steel cages require heat treatment such as quenching and tempering, and their surface has a thick layer of oxide skin and oil stains. If not removed, it will seriously affect the polishing quality and can be removed in a pre-treatment solution. The pre-treatment solution formula is based on volume fraction: H2SO4 20% -25%, HCI 5% -8%, HNO3 3% -7%, glycerol 1% -2%, and residual distilled water.

H2SO4, HCI, and HNO3 in the above pre-treatment solution can remove the oxide scale and oil stains on the surface of the cage. Glycerol can prevent corrosion. Soak the workpiece in this solution for 3-8 minutes and stir appropriately until the oxide scale is removed. After cleaning with flowing water, remove the water and set it aside for use.

2.2 Electrolytic polishing

The polishing solution formula is based on a mass fraction of H3PO4 50% -60%, H2SO4 10% -18%, CrO3 4% -8%, glycerol 14% -18%, and distilled water residue.

• Preparation process. Dissolve CrO3 in distilled water first; Then add H3PO4 in proportion and stir evenly; Slowly add H2 SO4 and stir evenly; Stabilize for 10 minutes, then add a small amount of glycerol for reaction. Glycerin must be gradually added to the solution in small amounts multiple times; The newly prepared polishing solution needs to be pre treated by powering on for 24 hours under current conditions of 1-2A.

• Insert an appropriate number of retainers into the polishing groove using a stainless steel spindle, and continuously shake the workpiece to ensure good contact with the electrode and even polishing; The temperature is controlled between 80-95 ℃, the current density is 5-10A/dm2, and the time is 5-10 minutes. The cathode is made of lead plate.

• Wash the workpiece in a 5% dilute hydrochloric acid solution, then rinse the workpiece with flowing tap water for 1 minute, and then add 5% Na2CO3. Neutralize in the solution and clean with flowing water; Wash with anhydrous ethanol to remove water, dry on a dryer, and then package.

The appearance of the front and rear axle bearing brackets before polishing is shown in Figure 1: the middle black represents before polishing, and the surrounding light represents after polishing.

Figure 1 Polishing the appearance of the front and rear holders

3. Factors affecting polishing quality

3.1 Effects of electrolyte composition

(1) Phosphoric acid. During the electrolysis process, it can promote the formation of a barrier film on the polished surface, which has a certain inhibitory effect on the dissolution of stainless steel, thereby improving anode polarization and achieving mirror gloss on the polished surface. When the content is high, the resistance of the tank liquid increases, and the viscosity increases, resulting in a higher required voltage, which slows down the leveling speed; The content is too low and the activation tendency is small, resulting in uneven corrosion on the surface of the workpiece.

• Sulfuric acid. It can promote the rapid dissolution of the anode and is the main material for polishing when combined with phosphoric acid in a certain proportion. When the content is high, the polishing speed is fast and the workpiece has good brightness, but the activation tendency is large, which is easy to cause corrosion on the polishing surface and dense pitting; When the content is too low, the polishing speed is slow, the brightness of the workpiece is poor, and serious uneven corrosion occurs.

• Chromic anhydride. Chromic anhydride is a strong oxidizing agent that can generate a passivation film on the surface of the workpiece, and flatten the surface of stainless steel during the polishing process to obtain a bright surface. Chromic anhydride is hydrolyzed into dichromate ions in acidic electrolytic polishing solution.

When its content is too low, the surface of the polished workpiece is not bright; When the content is too high, it will reduce the polishing speed.

(4) Glycerol. Glycerol can adsorb onto the surface of the anode and have a certain inhibitory effect on anodic dissolution. It can also form a C3H5 (OH) 2PO4 complex with phosphoric acid, which can form a stronger blocking film on the anode surface, preventing the dissolution of the anode and making the polished surface very bright and fine. When its content is too low, the polished surface is bright but rough; When the content is high, the polished surface is bright and fine, but when the content is too high, too much foam affects the operation.

3.2 Impact of operating conditions

3.2.1 Current density

In most cases, the anode current density is almost proportional to the amount of dissolved metal. Only by selecting a good anode current density and controlling it within a certain anode potential range can good polishing quality be achieved. There is a suitable current density for any metal electro polishing system. Generally speaking, if the current density is too low and the electrode is in an activated state, the surface will undergo etching due to the anodic dissolution of the metal, resulting in a rough surface; When the current density is too high, a large amount of oxygen will precipitate, covering the local surface of the anode and causing poor conductivity. It may also cause local overheating of the anode surface, resulting in excessive corrosion of the metal surface.

3.2.2 Tank liquid temperature

The temperature of the polishing solution should be maintained within the specified process range to ensure the normal polishing and leveling speed of the stainless steel surface, in order to effectively reduce the viscosity of the electrolyte, reduce the thickness of the anode mucosa, accelerate the diffusion of anodic dissolution products, accelerate solution convection, facilitate the detachment of trapped bubbles on the anode, and avoid the formation of spots and pitting. If the temperature is too high, it can cause the solution to overheat, accelerate the transition from hexavalent chromium to trivalent chromium (Cr6++3e → Cr3+), and easily cause surface corrosion. The generated gas and vapor may push the polishing solution away from the metal surface, thereby reducing the polishing effect. If the temperature is too low, the viscosity of the solution will increase, and the surface mucosa of the anode will thicken, which is not conducive to the diffusion of anodic dissolution products and significantly reduces the polishing and leveling effect.

3.2.3 Polishing time

The polishing time depends on the following factors: the original surface state of the metal product, the current density and temperature used, the composition of the electrolyte, and the properties of the metal. During the initial period of electro polishing, the leveling speed Z of the anode surface is faster, and then it becomes slower and even increases in surface roughness after a certain period of time. Therefore, the polishing time should take into account the above factors comprehensively. In general, as the current density increases and the temperature increases, the polishing time should be shortened. When the original surface quality of the workpiece is good and the requirements are high, the polishing time should be shortened.

3.2.4 Anode movement

Anode movement accelerates the diffusion of anodic dissolution products, playing a stirring role, effectively eliminating bubbles trapped on the anode surface, avoiding the generation of streaks in the generated airflow, and preventing local overheating from causing surface corrosion; And it can update the polishing solution near the anode surface, making the temperature of the polishing solution more uniform, preventing local overheating of the metal surface, and accelerating the dissolution rate of the mucosa; Anode movement helps to increase anode current density and improve the electrochemical polishing quality of stainless steel parts.

3.2.5 Cathode materials

The cathode for electrolytic polishing generally chooses lead plate. From the perspective of current efficiency, increasing the cathode area is beneficial, but increasing the cathode area will accelerate the reduction rate of Cr6+to Cr3+. The cathode area should be controlled within 1/2-1/3.5 of the anode area to prevent the growth of trivalent chromium. Excessive trivalent chromium content can easily cause the polishing solution to age.

4. Solution maintenance and process requirements

(1) Stainless steel workpieces must be thoroughly degreased before polishing to prevent oil contamination from contaminating the polishing solution.

• It is necessary to regularly measure the relative density of the polishing solution during use. If the relative density is less than the specified value in the formula, it indicates that the polishing solution contains too much water. The solution can be heated to 80 ℃ by evaporation to remove excess water, and the insufficient volume can be supplemented with phosphoric acid and sulfuric acid according to the formula ratio. Before the workpiece enters the polishing groove, Z should drain or blow dry the moisture attached to the workpiece. If the relative density is too high and exceeds the specified value in the formula, it indicates insufficient moisture. It is necessary to supplement water appropriately to reduce the relative density to the specified value.

• Aging of the solution. Due to the dissolution of the stainless steel surface during the polishing process, the content of iron, cobalt, and nickel in the solution will gradually increase, and the solution will gradually lose its polishing ability. No matter how high the temperature or current is, it will not help restore the polishing ability. Analyze the solution. If the iron content exceeds 60g/L and the trivalent chromium content exceeds 25g/L, it indicates that the solution has aged. For the regeneration of polishing solution and the restoration of polishing ability, a portion of the solution can be replaced. Keep 20% of the old solution and replenish 80% of the new solution. By using less or no power treatment, normal polishing can be achieved quickly.

• Clean the cathode lead plate. During the polishing process, a thick layer of impurities such as iron and nickel will deposit on the surface of the cathode lead plate, affecting the conductivity of the cathode surface, leading to a decrease in current and a decrease in anode current density on the polished surface, seriously affecting the polishing quality. Therefore, the sediment on the cathode plate should be removed in a timely manner; If a hard thick film is formed, use strong tapping to remove it, then rinse it clean after Z to keep the entire circuit unobstructed.

• The distance between the cathode and anode is too large, resulting in an increase in resistance and energy consumption. The solution is prone to heating up, which affects the polishing quality. If the distance is too small, it is easy to cause short circuits, ignition, and blackening of the workpiece. The distance between the anode and cathode should be between 100-300mm.

• The hazards of chloride ions. Active chloride ions are not allowed to exist in the electrochemical polishing solution, as chloride ions can damage the protective mucosa formed on the surface during electrochemical polishing, causing the formation of excessively corrosive pitting on the surface of stainless steel. Chlorine ions can be oxidized at high current density on the anode to form chlorine gas and escape for removal. The source of chloride ions may be brought into the tank solution after being pickled with hydrochloric acid, or the raw materials may contain impure chloride ions.

• The optimal ratio of hexavalent chromium and trivalent chromium is Z. In electro polishing, hexavalent chromium has oxidizing properties and has a passivation effect on stainless steel. Trivalent chromium plays a role in maintaining electro polishing. The newly prepared electro polishing solution cannot achieve a good polishing surface without producing trivalent chromium through chemical reactions. Only when a certain amount of trivalent chromium is present in the solution can an ideal polishing surface be achieved. That’s why electrolytic polishing can only be carried out after electrical treatment, and the best polishing quality is in the middle stage of the electrolytic process. In a solution containing chromic anhydride, adding gelatin or glycerol can cause a strong reduction reaction with chromic anhydride, with some hexavalent chromium reduced to trivalent chromium. Hexavalent chromium is yellow, trivalent chromium is green, and it appears yellow green in the electrolyte. During the production process, the electrolyte should remain yellow green. At this point, according to chemical analysis data, the ratio of hexavalent chromium to trivalent chromium is Cr6+30%, Cr3+70%. If the solution is dark green, dissolved chromic anhydride can be added appropriately. If it is yellow, glycerol can be added appropriately.

• Pictorial cathode. For polishing complex workpieces, a pictorial cathode can be made to maintain a uniform distribution of anode current, especially for inner cavity workpieces. Proper placement of pictorial cathodes in the inner cavity is necessary to ensure consistent polishing of all parts of the inner cavity.

5. 5.Conclusion

Through practical verification, the introduced process has a good polishing effect on stainless steel cages, with a bright and uniform surface, which can meet the quality requirements of electric polishing for stainless steel bearing cages. Especially for heat-treated martensitic stainless steel cages, it can effectively solve the problems of uneven polishing and localized corrosion, and improve the surface smoothness and corrosion resistance of the workpiece.

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