As an efficient metal surface treatment technology, electropolishing plays a crucial role in on-demand production. This process can improve the metal surface finish and texture and enhance the corrosion resistance of the workpiece with electrochemical reactions. This article provides an overview of electropolishing.
What is Electropolishing?
Electropolishing is an electrochemical machining process that polishes a metal surface using anodic dissolution when the metal is immersed in a suitable electrolyte and subjected to a low current density. Electropolishing applies to most common metallic materials and, like chemical polishing, has recently been adopted for polishing semiconductor materials.
Electropolishing relies on the preferential dissolution of surface asperities to achieve a smooth and lustrous finish like chemical polishing. Electropolishing requires the application of an electric current. In the electropolishing bath, the workpiece being polished acts as the anode, and a non-dissolving metal acts as the cathode; a voltage is applied across these two electrodes.
Upon energizing the system, an oxidation reaction occurs on the anode surface: metal atoms lose electrons, becoming ions that enter the electrolyte. Crucially, the current density is highest at the surface peaks, causing the dissolution rate to be significantly faster than at the valleys. This ultimately results in the surface becoming level and smooth. Simultaneously, the electrolyte forms a passivation film (e.g., an oxide layer or a viscous salt film) on the sample surface. This passivation film is thicker in the valleys, further inhibiting dissolution and reinforcing the leveling effect. The result is a smooth, stress-free finish.
Electropolishing Conditions
Power Source: Direct current is used in principle, though alternating current can also be used for materials like steel and aluminum. Common DC power sources include selenium rectifiers, transformer rectifier units, or storage batteries.
Electrolytic Cell
The main tank body is typically constructed of steel with an internal lining. The lining material depends on the electrolyte composition and often includes glass, ceramics, lead, enamel, celluloid, stainless steel, plastic, steel, or copper. Electrolytes containing acetic acid or acetic anhydride cannot be used in lead tanks. Hydrofluoric acid electrolytes may use lead or plastic tanks.
Heating and Cooling
Depending on the type of metal, the bath temperature may need to be elevated. Heaters are usually placed directly in the solution, and jacketed cooling systems are commonly employed.
Cathode
The cathode material must be insoluble in the electrolyte. Low-resistance materials such as lead, copper, aluminum, graphite, and stainless steel are frequently used. To promote uniform dissolution, auxiliary cathodes are often shaped to conform to the workpiece’s geometry. These auxiliary cathodes are typically fabricated from easily workable lead.
Insulation
The areas of the anode (workpiece) that do not require polishing, along with the fixturing (jigs/racks), must be insulated to prevent their dissolution. Vinyl resin is a common insulating material for this purpose.
Rinsing
Post-polishing rinsing procedures are fundamentally the same as those for chemical polishing. After polishing, the parts should be thoroughly rinsed, dried, and oiled as quickly as possible.
Electrolyte
For electropolishing to succeed, a liquid or solid film must form on the metal surface, and the metal must dissolve through this film at a stable, diffusion-controlled rate. Therefore, the electropolishing solution must simultaneously possess the dual functions of dissolving the metal and forming a protective film. Additionally, it must exhibit exceptionally high conductivity and good throwing power. Throwing power, in this context, refers to the uniformity of the current density distribution across the workpiece surface.
Aluminum and its alloys: Both acidic and alkaline electrolytes can be used, such as alkaline, fluoride-based, and phosphoric-chromic acid systems. The most widely used system is the phosphoric-sulfuric-chromic acid blend.
Steel: Electrolytes include perchloric acid systems, phosphoric-sulfuric-chromic acid systems, and phosphoric-oxalic-glycerol systems. For electropolishing stainless steel, the phosphoric-sulfuric-chromic acid solution is the most common choice.
Copper and its alloys: Known polishing solutions include phosphoric acid systems and phosphoric-sulfuric-chromic acid systems. In a pure phosphoric acid solution, the formation of a viscous layer of saturated, difficultly soluble copper phosphate salt on the anode surface enhances the polishing effect. To maintain the integrity of this viscous layer, agitation must be performed at low temperatures.
Electropolishing Process
Mechanical polishing. The sample is first mechanically polished to reduce its thickness to a certain level (typically tens of micrometers) in preparation for subsequent electropolishing.
Electrolyte selection. An appropriate electrolyte is selected based on the sample material. For instance, a mixture of phosphoric acid and sulfuric acid is common for stainless steel, while a mixture of nitric acid and methanol may be used for aluminum.
Electropolishing setup. Specialized equipment is used. The sample is secured as the anode, the electrolyte serves as the conducting medium, and the cathode is typically made of an inert material.
Current and time control. An appropriate current and voltage are applied, and the electropolishing time is controlled. Both current density and time must be optimized based on the sample material and the required final thickness.
Sample inspection. The sample’s thickness and surface quality are regularly checked until the desired thinness and smoothness are achieved.
Electropolishing Considerations
Current Density Control
An excessively high current density may lead to over-dissolution, while a current density that is too low will result in a poor polishing effect.
Electrolyte Temperature and Treatment Time
For materials that are prone to passivation, a higher electrolyte temperature, typically above 60°C, is necessary. A higher temperature also broadens the effective current density range for achieving a bright finish. The duration of electropolishing is related to the accumulation of anodic products on the workpiece surface and the time required for a viscous film of a certain thickness to form. Once the polishing effect is established, further extending the polishing time is unnecessary and can sometimes lead to detrimental results.
Distance Between Electrodes
In general, the current tends to concentrate around the edges of the electrodes in electrolytic processes. Consequently, when processing large, flat materials, the edges are more likely to achieve a bright finish than the center. To mitigate this non-uniform current distribution, the anode area is typically made larger than the cathode area, and the distance between the electrodes is increased. Increasing the electrode distance, however, increases energy consumption. Therefore, depending on factors such as the electrolyte’s resistivity, temperature, and current density, the distance between electrodes is usually maintained between 10 and 60 cm.
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Properties of Electropolishing
Electropolishing effectively reduces surface roughness and is particularly suitable for complex geometries and internal surfaces, often improving the original roughness level by 1 to 2 grades.
A range of properties related to the metal’s surface state are altered after electropolishing. The process removes the work-hardened layer, leading to a reduction in the surface hardness of the workpiece until the hardness reaches a stable base value. This removal, however, can potentially lower the fatigue limit; the extent of this reduction depends on factors such as the metal’s properties, structure, and the degree and effect of prior work hardening.
Furthermore, electropolishing forms a passivation film, a thin, gap-free oxide layer, on the polished surface, thereby significantly enhancing corrosion resistance.
About Getzshape
Getzshape delivers globally compliant, precision machined parts and surface finishes, including electropolishing solutions—adhering to standards like ASTM B912 and ASME BPE, we can promise engineers superior corrosion resistance, best surface quality, and dimensional integrity for critical components worldwide. Contact us to explore our capabilities.
