Difference Between Electropolishing and Passivation

The key difference between electropolishing and passivation is that electropolishing gives a superior finish and removes discolouration that passivation would leave behind.

Electropolishing is an electrochemical process that removes material from a metallic surface in order to reduce surface roughness while passivation is the process of coating a surface of a metal in order to reduce the chemical reactivity. Compared to passivation, electropolishing ultimately creates long-lasting results. That means; electropolishing results in a long-lasting coat on the surface of the substrate.

CONTENTS

1. Overview and Key Difference
2. What is Electropolishing 
3. What is Passivation
4. Side by Side Comparison – Electropolishing vs Passivation in Tabular Form
5. Summary

What is Electropolishing?

Electropolishing is the process of removing material from a metal surface to reduce surface roughness. We can do this by levelling micro-peaks and valleys. Thus, this process improves the surface finish. Moreover, electropolishing is useful for polishing, passivation and deburring metal parts. It is often described as the reverse of the electroplating.

In the mechanism of electropolishing, the substrate (the material on which the electropolishing is required) is immersed in an electrolyte as the anode. The electrolyte should be a temperature-controlled bath. Since this substrate is the anode, it is connected to the positive end of the DC power supply. The cathode is generally stainless steel, copper or lead. Moreover, the current passing through the anode oxidizes the metal on the surface of the substrate and dissolve the metal ions in the electrolytic bath. Then, these ions reach the cathode, and the reduction reaction occurs. Thus, this is how the surface roughness of the substrate is reduced in electropolishing.

The following section discusses the pros and cons of electropolishing:

Pros

• Easy operation and aesthetically pleasing finish
• Effective in polishing irregularly-shaped objects
• Useful to the sterile substrate
• Improves corrosion resistance
• Removes native oxide layers from metal surfaces; ex: TiO₂ layer on Ti metal

Cons

• Cannot remove very rough defects
• Electropolishing multiphase alloys may cause roughening

What is Passivation?

Passivation is the process of coating a surface of a metal in order to reduce the chemical reactivity. Therefore, a substrate that undergoes passivation is less affected by corrosion by the environment. In fact, a passivated surface can survive for a long time without undergoing metal leaching. Moreover, a passivation layer can be organic or inorganic in nature. The major use of this process is the restoration of the corrosion resistance of a contaminated stainless steel part.

Furthermore, the two major methods of passivation are nitric acid passivation and citric acid passivation. Earlier, nitric acid was used to passivate stainless steel. However, citric acid is the chemical we now use for this process since it is safer and more effective.

The following section discusses the pros and cons of both nitric acid passivation and citric acid passivation:

Pros and Cons of Nitric Acid Treatment Method

Pros: Low cost, require less contact time, the same nitric acid solution can be used several times,

Cons:  Hazardous effects of nitric acid, can dissolve heavy metals, which are toxic

Pros and Cons of Citric Acid Treatment Method

Pros: Not hazardous, dissolves only iron (does not dissolve heavy metals), keep the iron dissolved after neutralization, biodegradable end product, etc.

Cons: Expensive, if the solution has a low concentration, we need to heat it up to 80°C.

What is the Difference Between Electropolishing and Passivation?

Electropolishing is the removal of material from a metal surface to reduce the surface roughness while passivation is the process of coating a surface of a metal in order to reduce the chemical reactivity. Furthermore, the key difference between electropolishing and passivation is that electropolishing gives a superior finish and removes discolouration, which passivation would leave behind.

Moreover, electropolishing mainly includes immersing the substrate as the anode in an electrolytic solution and passing a DC current while passivation process includes steps such as alkaline cleaning, sanitization (strong oxidation), rinsing, drying and preservation. Moreover, another significant difference between electropolishing and passivation is that electropolishing is mainly done on nickel, tin and other metal alloys, whereas passivation is mainly used for stainless steel.

The following infographic summarizes the difference between electropolishing and passivation:

Summary – Electropolishing vs Passivation

Electropolishing is an electrochemical process that removes material from a metallic surface in order to reduce surface roughness while passivation is the process of coating a surface of a metal in order to reduce the chemical reactivity. In conclusion, the key difference between electropolishing and passivation is that electropolishing gives a superior finish and removes discolouration that passivation would leave behind.

Reference:

1. P. Allain, M. Echeverry-Rendón, in Hemocompatibility of Biomaterials for Clinical Applications, 2018.
2. Qing-Hua Lu, Feng Zheng, in Advanced Polyimide Materials, 2018.

Image Courtesy:

1. “Electropolishing principle” By LaurensvanLieshout – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Pre and post passivation” By Kees08 – Own work (CC BY-SA 4.0) via Commons Wikimedia