- Electropolishing (electrochemical polishing)
- Stainless Steel
- Passivation or rust prevention
- Surface Refinement
- Hydrogen Removal
- Advantages of Electrochemical Passivation
Electropolishing (electrochemical polishing) is the most effective treatment for the polishing and surface protection of stainless steel. It is a scientific process conducted in an electrolytic tank – the tank is filled with Metaglo electrolyte through which direct electric currents are passed (positive and negative) creating a strong magnetic field. The resultant force, combining with the electrolyte, produces a chemical reaction of sufficient magnitude to remove from the surface of the stainless steel a microscopic layer of metal which is basically iron. The result is a bright chrome surface, clean and very resistant to rust or tea-staining.
Stainless steel is available commercially in two grades, namely 304 and 316, the difference being the higher nickel and chrome content in 316 plus molybdenum. There are other varieties available for specialised applications and the reason for the stainless quality is the film of chromium monoxide created on the surface.
Stainless steel will rust under certain conditions and the following list indicated the causes of breakdown of the protective film:
- Surface defects produced during fabrication including embedded steel particles, weld spatter, grind marks, scratches, heat tint, arc strikes, paint and marking crayons. All of these initiate corrosion and will be accentuated in polluted environments.
- Heat ( welding ) tint. Welding creates a film of heavy oxide ( scale ) which corrodes quickly in acidic or polluted areas.
- Weld flux has the same effect as 2.
- Seaside locations. Ocean salt carried by sea spray contains chlorine and when deposited on the stainless steel very quickly corrodes the iron on the surface – this is the most common example of tea-staining.
The resistance of stainless steel to corrosion is due to the thin passive film covering the surface and is basically chromium monoxide. A breakdown of the film occurs by machining, hand or machine polishing, fabrication work or atmospheric influences, causing some form of localised corrosion such as pitting, crevice corrosion, intergranula corrosion and the modern form of tea-staining.
For complete and lasting protection, thorough passivation is necessary and can be achieved only by electropolishing.
Scale or discolouration caused by welding, annealing, nitriding or carburising, is easily removed by Metaglo processing. This carbonaceous smut in the weld is completely eradicated as well as iron embedded in the surface. Metaglo processing also removes the skin that remains after forging. Aside from improving the surface quality and appearance of the metal, this allows subsequent machining operations to be performed with less time, effort and tool wear.
Metaglo processing electrochemically erodes roughness. The high spots or “peaks” are attacked more rapidly than the low spots or “valleys” .This is crucial when a given dimension must be maintained while refining the surface roughness.
The presence of hydrogen in metals is accountable for three major problems:
- Bacterial Growth
- Metal Fatigue
- Explosive Conditions
Metaglo processing is a most effective method for removing hydrogen from stainless steel. Detail testing has proven that only this process has such thorough results as the hydrogen is not only removed at the surface, but throughout the metal as well. This is of paramount significance.
This is a very important issue as stainless steel requires a regular maintenance program to protect the chromium monoxide layer. Washing removes contaminants, such as salt, dirt and dust, which cause corrosion and is necessary to avoid tea-staining.
Tea-staining will occur on any surface because particles of iron are contained in dust and dirt and react with moisture and sea-spray (containing salt) to create an irregular rusty pattern. The brown stain on windows and car duco is tea-staining and, if left on the stainless steel not electropolished, will attack the iron in the surface of the metal.
Rain washing the surface is helpful in reducing tea-staining, so design the job to take advantage of the rain, but ensure good drainage. The stainless steel should also be washed when cleaning of the surrounding area takes place. Comparison may be made with motor vehicle duco, window panes and high gloss paint finishes and regular servicing is essential, depending on the environment.
For best results wash with soap or mild detergent and warm water, followed by rinsing with clean cold water. The appearance of the surface can be further improved if the washed surface is wiped dry. It may be necessary every two weeks.
The question often asked is, why is electrochemical passivation of stainless steel so far superior to the traditional methods using nitric acid and hydrofluoric acid?
Studies undertaken at the Bendix Corporation of the US by two senior engineers, La Roux K Gillespie and Jack C Bollinger have established that the electrochemical passivation processes give a result 6000 times better than methods employing nitric acid.
The major attributes of electrochemical passivation are:-
- The process gives rise to an increased ratio of chromium to ferrous, at the surface of the substrate.
- The electropolishing action removes contaminates from the metal that may have resulted from machining, welding or other manufacturing and fabricating procedures.
- The electrochemical process renders a thicker and more effective oxide surface, layer.
- The resultant surface is more aesthetic in that it gives a chrome-like appearance and is substantially smoother, reducing the possibility of aggressive material adhering to the surface.
- The METAGLO PROCESS eliminates the use of noxious chemical materials and hence the absence of any toxic waste.
With the increased use of stainless steel, fabrication procedure is demanding the exploitation of the latest technologies. METAGLO recognises this and has now put in place systems which will allow stainless steel fabricators to conform to the highest standards.