What is electropolishing? How does electropolishing work?

Electropolishing is an electrochemical finishing process that removes a controlled, thin layer of metal from a part’s surface, leaving it smooth, shiny, and ultra-clean.

Often called reverse electroplating, electropolishing dissolves microscopic surface irregularities rather than depositing material, producing a refined finish that improves both appearance and performance.

In electropolishing, the workpiece is connected as the anode (positive terminal) in a DC electrical circuit, while a cathode (usually stainless steel or zirconium) serves as the negative terminal. Both are submerged in a temperature-controlled electrolyte bath, typically a mix of sulfuric and phosphoric acids.

When current is applied, metal ions on the surface oxidize and dissolve into the electrolyte. Higher points on the surface draw more current and erode first—a phenomenon known as anodic leveling. The result is a surface with reduced roughness and improved uniformity, often measured in microns of material removal.

Key factors that control the process include:

• Electrolyte composition
• Bath temperature (typically 170–180 °F)
• Current density
• Exposure time
• Base alloy composition

While stainless steel is the most common choice, many alloys can be electropolished:

• Stainless steels: Both 300- and 400-series; electropolishing enhances corrosion resistance of the stronger but less corrosion-resistant 400-series.
• Other alloys: Aluminum, brass, carbon steels, copper alloys, nickel alloys, titanium, and nitinol.

Cast metals containing high silicon, sulfur, or carbon often yield limited results, and die-cast aluminum or zinc is generally unsuitable.

Specialized processes may be required for alloys such as titanium or nitinol.

Material Removal

Electropolishing removes metal at the micron level, making it ideal for precision applications. Typical removal is between 0.0003″ and 0.0007″, though in extended processes up to 0.005″ may be dissolved.

The process is best applied to micro burrs (less than ~0.0015″ tall), where controlled removal ensures consistent surface finishes.

Electropolishing provides a wide range of functional and aesthetic improvements, including:

• Decorative finish: Bright, reflective, uniform surfaces.
• Microscopic smoothness: Up to 50% reduction in surface roughness (Ra).
• Deburring: Removes sharp micro-edges and burrs (less than ~0.0015″ tall) from machining.
• Ultra-clean finish: Eliminates embedded contaminants and improves sterilization.
• Corrosion resistance: Removes surface defects where corrosion could start.
• Reduced friction: Improves performance of moving or threaded parts.
• Extended part life: Increases durability of components under stress.
• Complex geometries: Reaches internal or delicate features without mechanical damage.

Electropolishing of stainless steel is standardized under ASTM B912 – Passivation of Stainless Steels Using Electropolishing. The specification outlines three essential phases:

1. Surface preparation: Cleaning to remove oils, greases, and oxides that interfere with polishing.
2. Electropolishing: The controlled electrochemical removal process itself, with treatment times ranging from seconds to minutes.
3. Post dip: Removal of residual electrolytes and by-products, typically using nitric or citric acid, followed by rinsing and optional drying to prevent staining.

Following ASTM B912 helps ensure consistent results and is especially important for industries with stringent compliance requirements, such as medical devices and aerospace.