Electropolishing
The electro-polishing , and electropolishing called, is one of the subtractive manufacturing processes . More precisely, it is assigned to the electrochemical removal process with an external power source . In this case, metal is anodically removed in an electrolyte specially tailored to the material , that is, the metallic workpiece forms the anode in an electrochemical cell .
Plasma polishing is similar to the process of electrolytic polishing, but works with ecologically regarded as harmless saline solutions . An important difference is the significantly higher voltage that leads to a plasma film around the workpiece, which is why another mechanism of action leads to the removal.
Electrolytes - active pairings
The electrolytes used differ depending on the metals to be processed . Common electrolytes today are often mixtures of mineral acids and water, and in some cases alcohols . Mixtures of phosphoric acid and sulfuric acid are used for the electropolishing of stainless steels and steels as well as aluminum alloys . Brass and copper can be processed in mixtures of phosphoric acid and alcohols.
For most stainless steels, aged, aqueous electrolytes with 45% by weight of phosphoric acid and 35% by weight of sulfuric acid at temperatures around 50 to 65 ° C and current densities above 5 A / dm² can be used. Mixtures of 55% by weight phosphoric acid and 35% by weight sulfuric acid are suitable for electropolishing aluminum. Aqueous electrolytes composed of 50% by weight phosphoric acid and 30% by weight alcohol, for example 2-propanol, are suitable for copper and brass .
In addition, surface-active substances are often added to the electrolyte baths. In most cases, the electrolytes are hazardous substances. Correspondingly, expert handling of the substances is necessary in order to avoid damage to health and the environment.
Electrolytes for the most common metals, which are no longer in use today because of the perchloric acid content, have the following composition:
- one part perchloric acid (concentrated), one part butylcellosolve (for buffering ) and seven parts ethanol (to keep the dissociation low) with a voltage of 45 V and a current density of 0.3 A / cm² or one part ortho - phosphoric acid
- one part perchloric acid (concentrated), one part glycerine with one part ethanol at a current density of 0.1 to 0.2 A / cm².
Electrochemical removal
The ablation usually takes place with direct current , but pulsed currents are also used . The workpiece is connected anodically. In industry, current densities are applied, which enable an erosion in the transpassive area of the current density- voltage curve. As a result, not only is metal removed, but oxygen is also produced on the anode, the workpiece.
Procedure
The surface roughness is reduced by electropolishing. If this is the goal of the machining, one can speak of electrochemical smoothing. Processing lowers the micro-roughness of the metallic surfaces. Roughness peaks are removed more quickly than roughness valleys, since a transport-limiting polishing layer forms in front of the surface during electropolishing in mineral acid mixtures, which promotes the removal of roughness peaks. The nano-roughness is also reduced. In this case it is electrochemically polished. The gloss is a result of the roughness in the range of fractions of the wavelength of visible light . Structures in the macro area are retained. Edges and corners are broken down more, which results in very fine deburring in the entire surface area. The method can therefore also be used for electrochemical deburring .
Pretreatment
The metallic workpieces must be cleaned and degreased prior to electropolishing. Electrolytic or electrochemical pickling may be required. If necessary, parts are also machined before machining. For example, they are ground, sanded or polished.
scope of application
Electropolishing is used for decorative purposes, for example for facade sheets and jewelry. It is used in pipeline and tank construction. Electropolishing is also used in medical technology, because electropolishing reduces the ability of germs to adhere. This reduces cross-contamination. That is why biotechnology in particular likes to use electropolished stainless steel containers. Surgical instruments, but also implants such as vascular supports ( stents ) are processed in this way. Samples are also electropolished for material studies. This type of electrochemical surface treatment takes place on sheet metal, large containers such as tanks, pipelines and the like in the food and chemical industries, as well as in microtechnology. Most often, chromium and chromium- nickel steels, especially stainless steels, are electropolished. The reason for this lies in the fact that highly polished surfaces of such steels are far more corrosion-resistant than untreated ones. Viewed microscopically, such a treatment reduces the surface considerably, which in turn offers fewer opportunities for attack by environmental influences. Examples are sewage treatment plants and the chemical industry, but they are also frequently used in the maritime sector. In addition, z. B. in the high vacuum industry often electropolished recipients and connectors are used in order to keep the surface and thus the amount of adhering adsorbates as low as possible. In aircraft construction, electrochemical removal is used for lightweight construction in order to partially reduce the thickness of the aluminum sheets.
Materials
Various metallic materials can be processed. The process is particularly common with stainless steels.
Individual evidence
- ↑ M. Buhlert: Electropolishing - electrolytic polishing, smoothing and deburring of stainless steel, steel, brass, copper, aluminum and titanium. Eugen G. Leuze Verlag, Bad Saulgau 2009, ISBN 978-3-87480-249-9 .
- ↑ M. Buhlert: Electropolishing and electrical structuring of stainless steel, brass and aluminum. Investigation of the transpassive removal process including undesirable side effects. (= Progress reports VDI. Series 2, No. 553). VDI-Verlag, Düsseldorf 2000, ISBN 3-18-355302-3 .