Electropolishing

The Reverse of Plating — Controlled Material Removal

Electropolishing is the electrochemical reverse of electroplating. Instead of depositing metal onto a cathode, the workpiece is made the anode and material is selectively dissolved from the surface. Peaks and micro-burrs dissolve preferentially, producing a smooth, bright, passivated finish. The process requires precisely controlled DC current at relatively high current densities — typically 5–30 A/dm² — to maintain the viscous film (anodic film) that governs selective dissolution. If current density falls below the critical threshold, pitting occurs; if it exceeds the plateau region, gas evolution disrupts the film and produces an orange-peel texture.

Surface Finish and Passivation Quality

Pharmaceutical, semiconductor, and food-processing industries demand electropolished surfaces for hygiene, corrosion resistance, and cleanability. Surface roughness targets of Ra < 0.4 μm are common, with some applications requiring Ra < 0.2 μm. Achieving these finishes consistently requires a rectifier that holds current density within a narrow window throughout the process cycle. Ripple in the DC output causes periodic fluctuations in the anodic dissolution rate, creating micro-waviness that increases Ra values and reduces the chromium-enriched passive layer thickness that gives electropolished stainless steel its superior corrosion resistance.

Process Control for Consistent Results

Electropolishing baths operate at elevated temperatures (40–80°C) with concentrated phosphoric-sulphuric acid electrolytes. As the bath heats during operation, conductivity changes and the rectifier must compensate automatically to maintain constant current density. Programmable current ramp-up prevents flash attack on thin sections, and timer functions ensure repeatable process times. For production environments, RS485 and TCP/IP interfaces allow the rectifier to integrate into automated handling systems where immersion time, current, and bath temperature are managed centrally.