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.
Metal cathode (negative) sheets are suspended from the cathode bars into the electrolyte. The job to be electropolished is suspended from the anode (positive) bar into the electrolyte and adjacent to the cathode sheets.
When the current is switched on, metal removal from the component surfaces takes place in a controlled manner with micro-peaks being eroded preferentially. This results in a smoothing of the affected surfaces, whilst the macro-profile of the surfaces remains unaltered.
Careful selection of the process electrolyte solution allows electropolishing to be carried out on special metals such as titanium, copper and duplex materials, but it is in the treatment of stainless steel that it has found its greatest commercial application.
Hastelloy©, Inconel© and other Nickel-Chromium alloys have also proved suitable for electropolishing. The full list includes: Titanium, Tungsten, Molybdenum, Aluminium, Cobalt Alloys, Nickel & Nickel Alloys, Copper & Copper Alloys, Brass and Zirconium.
In many cases, electropolished stainless steel surfaces are bright and reflective. This results from the removal of an often contaminated surface layer and the electrochemical action of micro-smoothing.
The passive oxide layer, which is essential to prevent stainless steel from corroding, cannot be improved upon following electropolishing.
By reducing the total surface area, a result of micro-smoothing, unwanted products are less likely to adhere to an electropolished finish. In the same way, surfaces can be cleaned and kept clean more readily.
Friction is also reduced and the mechanism of preferentially removing surface high spots makes electropolishing suitable for eliminating fine burrs. The highly reflective bright finish is also suitable for a number of decorative applications, particularly where the shape of the item requiring polishing is extremely complex.
A clean stainless steel surface will generate its own passive oxide layer when exposed to oxygen. It is this oxide layer that gives stainless steels their corrosion resistant qualities.
However, it is possible to accelerate the formation of this layer by chemical passivation.
Correct Pickling, in a formulated solution of dilute acids, produces a uniform matt grey finish – the effect of chemical micro-etching.
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