Plasma-arc Cleaning Technology
The first problem for any metal product user is cleaning of product surface from oxides and other technological lubricants.
Roll scale consists of three layers. Pure metal surface directly contacts with a lay of FeO oxide (wustite), the next layer is Fe3O4 (magnetite) and the final outer layer is Fe2O3 (haematite). Chemical composition of alloyed steel scale is much more complicated than that of carbon steel: it also contains chrome oxides, nickel oxides and etc. – depending on chemical composition of the steel. Apart from oxides, on the surface there may be protective, preservative or technological lubricants, organic dyes, plastics or other contaminants.
Metal-roll, rod, wire, pipes, etc. delivered today from producing plants have roll scale, rust and other substances on their surfaces – because of using traditional but out-of-date cleaning methods, such as acid-base pickling, sand and shot blasting, mechanical cleaning with abrasives, steel brushes or milling tools.
All the methods listed above cannot be considered ecologically clean, universal and cheap, and they don’t comply with modern demands for cleaning quality. Moreover, all those methods have a serious disadvantage – after cleaning all products intensively corrode again in the humid air.
But mechanical engineers would like to use equipment that is cheap in price and in operation, universal, ecologically clean. That equipment should guarantee high adhesion ability and corrosion resistance of cleaned surface and should not require expensive consumables.
Development of plasma-arc cleaning method has become an answer to growing demands of mechanical engineers.
Plasma arc cleaning was first used to remove oxides and other contaminants from the surface of aluminium and certain aluminium alloys while developing a technology for argon-arc welding of aluminium alloy structures.
Metal surface cleaning occurs in the electric arc cathode spots that are chaotically “moving” on the surface cleaned. Due to high current density of explosive electron emission (1011-1012 À/m2) arising at surface micro asperities, where electric field intensity is quite high and work function of electron output is lowered because of Schottky effect, quick heating, smelting, and explosive evaporation of surface micro areas take place as well as ionization of cathode material vapour and surface contaminations. Almost fully ionized plasma is formed and energy with density about 1011 W/m2 is transported to the surface being cleaned, temperature runs up to (5-10)´103 K and pressure of oxide and other contaminant vapour reaches 107‑108
Researches have shown that the speed of cathode spot movement at current densities about 1010 A/m2 depends on the thickness of the oxide layer (be it composed of furnace scale, roll scale, rust or other contaminants), the pressure of product material and other contaminant saturation vapour, the thermal conductivity and temperature of the product, the configuration and relief of the surface, the pressure and chemical composition of the surrounding environment.
Most productivity and high quality of plasma arc cleaning is reachable when ambient pressure is dropped to 1,33´102 – 1,33 Pa relative to atmosphere pressure. Power inputs for cleaning of
Besides, the surface activated by plasma gains high adhesive ability to all existing protective coatings (metallic and polymeric) and to all types of lubricants (to soap during wire drawing). Experiments have shown that surface adhesive ability to protective coatings is 4 and more times higher than that after any existing method of cleaning or surface preparation and this fact is for scores of years a guarantee that a protective coating will not flake away and a product will not be destroyed because of corrosion.
Atoms of metals implanted back to the cleaned surface from the oxide vapour, form thin films – with thickness of several atomic layers. In case the product to be cleaned is steel, the films will consist of pure iron and will for years protect the cleaned surface from corrosion in the humid air and in some cases in the water.