Electropolishing: What is it, and how does it work?
Electropolishing is simply electroplating in reverse. Instead of depositing a coating of another material on a surface, the process of electropolishing is to remove a surface layer, typically 20 – 40 micrometres in depth, in the case of stainless steel.
Electropolishing requires a source of electrical current and a rectifier to convert from alternating current (A.C.) to direct current (D.C.). With the aid of busbars, the D.C. is transmitted to the job and cathode bars suspended over a tank containing an electrolyte solution, normally a mixture of acids.
By electrolytically removing metal from a metal object’s surface, ion by ion, electropolishing is a chemical surface finishing process.
As a result, the primary objective is to minimise microroughness, thereby reducing the likelihood of dirt adhering and enhancing the cleanability of surfaces.
Deburring, brightening, and passivating can also be accomplished with electropolishing. This process exposes a metallurgically clean, undisturbed surface. The unwanted effects of mechanical surface treatment – mechanical and thermal stress, particle embedding, and surface roughening – are avoided or reversed.
The inherent corrosion-resistance of a given stainless steel grade is fully optimised. The corrosion-resistance and cleanability of stainless steel have made electropolishing a common treatment.
It is typically used in the pharmaceutical, biochemical, medical and food-processing industries. Due to the fact that electropolishing is a non-mechanical, thermal, or chemical process, small and mechanically fragile parts can be treated. The electropolishing process can be applied to almost any shape or size of part.
A guide to chemical surface treatments: when to use them and why
Chemical surface treatments are available in a variety of forms. Although they all contribute to the production of a clean metallic surface, their purposes are distinct:
Pickling
The process of pickling removes oxides, particularly heat tint (from welding) and other discolourations or corrosion products. By creating a clean metallic surface, it creates the conditions for stainless steel to undergo natural self-passivation. The most common compositions include nitric acid and hydrofluoric acid. Depending on the temperature and contamination level of the solution, the treatment time will vary.
Passivating
In the presence of oxygen from air or water, stainless steel self-passivates spontaneously. However, it can take a couple of days for the passive layer to reach its full thickness. Passivation treatment speeds up the formation of the passive layer, ensuring that it builds up quickly to its full strength under controlled conditions. Diluted nitric acid is used for passivation, with treatment times ranging from 15 minutes to an hour.
Decontamination or acid cleaning
This removes ferrous particles which would corrode if left on the stainless steel surface – for example after being exposed to ferrous contaminants (grinding dust, rust particles from adjacent carbon steel work, abrasion from tools, etc.)
Electropolishing
Electropolishing is applied to clean metallic surfaces, to reduce micro-roughness and produce a number of other highly desirable effects, described in this publication. Electrolysis involves a flow of current and a solution of mainly sulphuric acid and orthophosphoric acid as the electrolyte. The treatment usually lasts between two and twenty minutes.
If you have a project you’d like to discuss, get in touch with the Anopol team today.