Electropolishing is a popular surface finishing process of alloys and metals that have gained popularity from the 1950s. This process is still in demand and is largely implemented to achieve the desired surface finishes over a wide range of materials. The process helps improve the aesthetic appeal of the material as well as achieve low roughness values of surfaces. It is referred to by several names such as electrochemical polishing, electrolytic polishing, and anodic polishing. This white paper introduces you to electro polishing process and its effectiveness.
Why is Electropolishing Needed?
Many machining processes such as grinding, blanking, lapping, broaching, and milling may leave certain imperfections on machined parts. These imperfections may affect the aesthetic appeal, form, and safety of these parts. Microburrs are one of the common imperfections which if ignored may lead to contamination. Many such surface imperfections can be easily removed by employing the process of electropolishing.
What is Electropolishing?
The electrochemical surface finishing process is where a thin layer of material is removed from the metal surface. This process generally produces a smooth, shiny, and ultra-clean surface finish.
Electropolishing is often compared to reverse electroplating. In electroplating, a thin layer of positive charged metal ions is added on the surface. However, in electropolishing, a thin layer of metal ions is dissolved in electrolyte solution by passing electric current.
Electropolishing is often confused with passivation. However, these processes differ in terms of their application. Both processes are similar in many ways such as they are chemical, non-mechanical, and above all used to improve the surface finish of metals. Electropolishing and passivation both help improve the corrosion resistance properties of certain metals.
How Electropolishing Produces Better Surface Finishes?
The electropolishing process follows the principles of Faraday’s Law. It is performed using the electropolishing equipment. This equipment comprises anode, cathode, power supply, and electrolyte. Many components such as cathode and anode bars, heaters, bussing, DC rectifiers, and so on are much similar to equipment used in electroplating process.
During the process, the metal part acts as the source of positively charged ion – anode. This workpiece is connected to the power supply’s positive terminal. The cathode or negatively charged ion is connected to the power supply’s negative terminal. The negative ion is usually made of zirconium or stainless steel.
Both cathode and anode are immersed in an electrolyte solution, which is temperature-controlled. This solution generally comprises a mixture of viscous acids and their combination creates a high level of viscosity. The electric current passes from the positive terminal to the negative terminal through electrolyte solution. During this, metal ions that are on the surface of the metal part get oxidized and dissolve in the electrolyte solution.
During this process, several surface imperfections such as burrs are made to erode. This erosion is technically known as anodic levelling. Generally, these surface imperfections attract high electrical current density, which makes the erosions easy. This anodic leveling leaves smooth metal surfaces.
What Are the Different Steps Involved in Electropolishing?
Most electropolishing systems are designed to work as per the ASTM B912 standard which outlines the Electropolishing process for Stainless Steel alloys. There are mainly three steps followed by these systems.
- Surface Preparation: In this step, the metal work piece is prepared for surface finishing. Any oil and grease is removed from the surface to assure good results and uniformity after the application of electric current. This surface preparation involves several micro steps such as vapor degreasing, rinsing, detoxification, acid pickling, or descaling, and rinsing.
- Electropolishing: In this step, the cleaned part produced in the surface preparation step is electro polished using Electropolishing systems. The treatment time for parts may vary between 10 and 20 minutes.
- Post Dip: In this step, any residual components that may be deposited on the metal parts are removed. The part is further dried to prevent staining at any later phase of use. Generally, the electropolishing process produces byproducts such as sulfates and phosphates of heavy metals. If these byproducts are allowed to remain on the parts, they may develop white spots later on.
What Factors Affect Electropolishing?
The surface removal of a metal work piece is controlled using the following factors.
- Amount of time exposure of the surface to the electric current
- The composition of metal or alloy undergoing electropolishing
- Electrical current density ( this is dependent on the electrolyte)
- Electrolyte temperature
- Chemical composition of the electrolyte
How to Prepare Parts for Electropolishing?
Although electropolishing systems are designed to assure effective results, part preparation is a long process. Electropolishing is usually performed at the end of manufacturing so the parts must be carefully prepared for the process. The following are a few steps involved in parts preparation.
- Remove macro burrs if they exist on parts. This is because electropolishing is more appropriate for removing micro burrs. The effectiveness of burr removal through electropolishing will depend on the width of burrs.
- Remove any scratches on the surfaces of work parts because electropolishing will highlight them. Electropolishing is a process of micro-material removal, so any deep scratches that are already existing on the surface part will be highlighted after the process. The scratches can be easily removed using different mechanical methods.
- Remove coatings and adhesives from work parts before sending them for electropolishing. This is because the adhesives or coatings will produce patterns on work parts. It will generate uneven finishes during electropolishing.
- Remove any lubricants and soaps from the surfaces of the workpiece before sending it for electropolishing. Soaps and lubricants contain silicates that will adhere to parts and form patterns. These patterns will lead to uneven surface finishes.
- Ensure all welds are uniform and air-tight because tiny holes can entrap fluids within them when dipped in an electrolyte solution.
- Remove any scaling mark from the part that may be caused due to heat treating. This is because scales on the parts will create patterns and also affect their conductivity.
How Much Material Can Electropolishing Equipment Remove?
The amount of material removed by electropolishing systems will vary as per the settings. This material removal depends on the current passed through the parts surface and the run time of the equipment. Generally, electropolishing is performed once the parts machining finishes. Most electropolishing systems can remove 0.0003″ to 0.0007″ for polishing and deburring applications. The variables of the machine can be easily controlled to produce a consistent surface finish. The material removal in amounts ranging from 0.002″ to 0.005″ can be easily achieved during long process cycles. High amount of material removal can be achieved on the part’s surface if the current is applied for a long duration of time. Electropolishing systems are generally used over material parts that may have protrusions of 0.0015″ from the material part surface.
What Type of Acids Are Used for Electropolishing?
The mixture of phosphoric and sulfuric acid is used for electropolishing. This mixture also features dissolved salts and agents. Removed material forms sludge on the bath and forms a chemical equilibrium. This sludge or electrolyte is removed periodically to maintain a certain specific gravity of the solution.
Which Metals Are Compatible with Electropolishing?
Electropolishing is mainly performed on stainless steel. Most metals are compatible with this process; however, 400 and 300 series stainless steel are the most compatible. The metal parts made of 400 series have shown improved strength and hardness after electropolishing. Carbon steels, brass, aluminum, cobalt chrome, nickel and copper alloys, nitinol, and titanium are a few common metals that are most compatible with electropolishing.
What Are the Benefits of Electropolishing?
Electropolishing is performed using electropolishing systems and they assure excellent results. This process is much preferred over other surface finishing processes due to the following reasons:
- It produces brilliant surface finishes against abrasive polishing, which produce fine lines on parts. Electropolished metal possesses a shiny finish and high aesthetic appeal.
- This process helps remove cracks and fissures on the surface of metal parts. Once they are removed, the chances of developing corrosion also minimize.
- Electropolishing systems help improve the stability of parts that are required to withstand high bending and flexure.
- Electropolishing is good for metal parts with complex part geometries as they can reach and polish areas that are generally not accessible through other polishing methods.
- Like complex parts, this technique is also suited for polishing fragile parts, which may be damaged when mechanical polishing techniques are applied.
- The process helps remove discoloration from parts caused due to oxidation and heat tints.
- Most electropolishing systems can access even tiny areas on complex components.
- Electropolishing is perhaps one of those surface finishing techniques that helps achieve ultra-clean finishes on parts. Electropolished machined parts are widely used in food and medical industries, where sterilization is performed. This process helps remove components that will be deeply embedded in the parts and minimize the chances of contamination. The smooth surface finishes make sterilization much easier.
- Size control is another major benefit of this process. Although material removal is in micro sizes, the process helps achieve desired part sizes.
- Electropolishing process helps reduce the friction of metal parts.
What Are the Common Applications of Electropolishing?
The electropolishing process is widely used across industries where parts with smooth surface finishes are preferred. The following are some common applications of electropolishing.
- Sterilization is one of the most important features of electropolishing and many parts used in the pharmaceutical and food and beverage processing industries benefit from this.
- Electropolished fuel lines and gears are used in automobiles and they help improve their performance and lifespan.
- Many critical components in the aerospace industry are electro polished and this helps reduce their friction, as well as increase their performance.
- In the semiconductor industry, electropolishing is performed on fragile parts.
The electropolishing process is one of the most effective surface finishing processes widely employed across industries. It is effective for removing micro burrs where surface material is removed from parts in precise amounts within tight tolerances. The process itself assures the highest level of quality surface finish and unlike other surface finishing techniques, it requires no secondary cleaning. However, the effectiveness of the process will depend on the electropolishing systems chosen by you. HPI Pro engineers and fabricates electro polish finish lines for different parts configurations. Our electro polish finishing equipment is used in aerospace, pharmaceutical, medical, and military industries. These equipment can be easily integrated into your existing workflow.