Electro-polishing is the electrochemical removal of a metal surface where the part being treated isconnected to the positive pole of a D.C. power source, whilst being immersed in a suitableelectrolyte. Electro-polishing operates anodically and is basically the reverse of electroplating. Metalremoval takes place preferentially at the peaks of the surface profile resulting in a smoothing andde-burring of the micro-profile whilst the macro shape of the surface is not altered. (Fig 1 ) Thissmoothing generally produces a bright reflective finish but as only a small amount of metal (30-40μm) is removed, the process will not remove grinding or machining marks and other visiblesurface irregularities. Careful choice of the electrolyte allows electro-polishing to be carried out onmost common metals, but it is in the treatment of stainless steel that it finds its greatest commercialapplication.

When stainless steel sheet is manufactured at the mill, the surface is irregular as a result of therolling mill process. This irregularity is a result of differential distribution of the component materials,this result also traps swarf and other contaminate particles that get ingrained in the surface of thematerial. (See photomicrograph Fig 2.)

Mechanical finishing of stainless steel produces a surface that is not necessarily clean or smooth.Anyone who has seen mechanical polishing being carried out will know that the operative quicklybecomes covered with polishing compound, the work-piece is just as dirty, with polishing compoundtrapped in the surface (see photomicrograph Fig 3.). Blasting does not always removecontamination and can merely hide or even peen it further into the surface. Any mechanical finishapplied is liable to introduce surface contamination and this is particularly important with stainlesssteel where the corrosion resistance of the metal is dependant upon the purity ofthe surface thatallows the formation of an uninterrupted passive layer.

Electro-polishing, in contrast, removes surface contaminants and micro-smoothes the surfaceproduces an excellent passive chromium rich surface and thereby enhances the corrosionresistance. Other advantages offered by electro-polishing include reduced total surface area,surface cleanliness, ease of cleaning, reduced friction and de-burring. (see photomicrograph Fig 4.)

Before stainless steel components can be electro-polished they must undergo pre-treatmentprocesses to remove any grease, dirt and weld discolouration. Degreasing and cleaning can becarried out by alkaline or other proprietary methods with the weld discolouration being removed byeither chemical or electrochemical pickling. Enclosure parts are mounted on jigs and are processedon the Cooper Crouse-Hinds automated electro-polishing plant.

The end result is a highly corrosion resistant chromium enriched enclosure fabricated using high-grade materials. An example of a NEXT enclosure can be seen in below in Fig 5.

The main benefits of electro-polishing of enclosure can be summarized as below:

• Improved resistance to corrosion due to chromium enrichment of the surface.
• Increased durability, and therefore longevity.
• Reduced dirt adhesion for easier cleaning and maintenance.
• Reduction of surface roughness, and therefore friction (circa 50 % in Ra)
• Removal of sharp edges (burrs, corners) sometimes referred to as chemical milling.
• Results in a bright, reflecting surface.
• Removal of H2 (hydrogen) from alloy: degassing (hydrogen embrittlement).
• Easier cleaning, reduced build-up of products on the surface, reduced bacterial growth.
• Reduced radioactive contamination and improved decontamination behavior.