Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Simultaneous deplating and plating
Reexamination Certificate
2002-01-25
2003-07-01
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Simultaneous deplating and plating
C204S164000, C204S206000, C204S22400M, C204S232000, C204S237000, C204S239000, C204S241000, C204S269000, C204S275100, C205S088000, C205S098000, C205S118000, C205S138000, C205S148000, C205S705000, C205S722000
Reexamination Certificate
active
06585875
ABSTRACT:
The present invention relates to an improved process and apparatus for cleaning and/or coating metal surfaces using electro-plasma technology.
Metals, notably, steel in its many forms, usually need to be cleaned and/or protected from corrosion before being put to their final use. As produced, steel normally has a film of mill-scale (black oxide) on its surface which is not uniformly adherent and renders the underlying material liable to galvanic corrosion. The mill-scale must therefore be removed before the steel can be painted, coated or metallised (e.g. with zinc). The metal may also have other forms of contamination (known in the industry as “soil”) on its surface including rust, oil or grease, pigmented drawing compounds, chips and cutting fluid, and polishing and buffing compounds. All of these must normally be removed. Even stainless steel may have an excess of mixed oxide on its surface which needs removal before subsequent use.
Traditional methods of cleaning metal surfaces include acid pickling (which is increasingly unacceptable because of the cost and environmental problems caused by the disposal of the spent acid); abrasive blasting; wet or dry tumbling; brushing; salt-bath descaling; alkaline descaling and acid cleaning. A multi-stage cleaning operation might, for example, involve (i) burning-off or solvent-removal of organic materials, (ii) sand- or shot-blasting to remove mill-scale and rust, and (iii) electrolytic cleaning as a final surface preparation. If the cleaned surface is to be given anti-corrosion protection by metallising, painting or plastic coating, this must normally be done quickly to prevent renewed surface oxidation. Multi-stage treatment is effective but costly, both in terms of energy consumption and process time. Many of the conventional treatments are also environmentally undesirable.
Electrolytic methods of cleaning metal surfaces are frequently incorporated into processing lines such as those for galvanising and plating steel strip and sheet. Common coatings include zinc, zinc alloy, tin, copper, nickel and chromium. Stand-alone electrolytic cleaning lines are also used to feed multiple downstream operations. Electrolytic cleaning (or “electro-cleaning”) normally involves the use of an alkaline cleaning solution which forms the electrolyte while the workpiece may be either the anode or the cathode of the electrolytic cell, or else the polarity may be alternated. Such processes generally operate at low voltage (typically 3 to 12 Volts) and current densities from 1 to 15 Amps/dm
2
. Energy consumptions thus range, from about 0.01 to 0.5 kWh/m
2
. Soil removal is effected by the generation of gas bubbles which lift the contaminant from the surface. When the surface of the workpiece is the cathode, the surface may not only be cleaned but also “activated”, thereby giving any subsequent coating an improved adhesion. Electrolytic cleaning is not normally practicable for removing heavy scale, and this is done in a separate operation such as acid pickling and/or abrasive-blasting.
Conventional electrolytic cleaning and plating processes operate in a low-voltage regime in which the electrical current increases monotonically with the applied voltage. Under some conditions, as the voltage is raised, a point is reached at which instability occurs and the current begins to decrease with increasing voltage. The unstable regime marks the onset of electrical discharges at the surface of one or other of the electrodes. These discharges (“micro-arcs” or “micro-plasmas”) occur across any suitable non-conducting layer present on the surface, such as a layer of gas or vapour. This is because the potential gradient in such regions is very high.
PRIOR ART
GB-A-1399710 teaches that a metal surface can be cleaned electrolytically without over-heating and without excessive energy consumption if the process is operated in a regime just beyond the unstable region, the “unstable region” being defined as one in which the current decreases with increasing voltage. By moving to slightly higher voltages, where the current again increases with increasing voltage and a continuous film of gas/vapour is established over the treated surface, effective cleaning is obtained. However, the energy consumption of this process is high (10 to 30 kWh/m
2
) as compared to the energy consumption for acid pickling (0.4 to 1.8 kWh/m
2
).
SU-A-1599446 describes a high-voltage electrolytic spark-erosion cleaning process for welding rods which uses extremely high current densities, of the order of 1000 A/dm
2
, in a phosphoric acid solution.
SU-A-1244216 describes a micro-arc cleaning treatment for machine parts which operates at 100 to 350 V using an anodic treatment. No particular method of electrolyte handling is taught.
Other electrolytic cleaning methods have been described in GB-A-1306337 where a spark-erosion stage is used in combination with a separate chemical or electro-chemical cleaning step to remove oxide scale; in U.S. Pat. No. 5,232,563 where contaminants are removed at low voltages from 1.5 to 2V from semi-conductor wafers by the production of gas bubbles on the wafer surface which lift off contaminants; in EP-A-0657564, in which it is taught that normal low-voltage electrolytic cleaning is ineffective in removing grease, but that electrolytically oxidisable metals such as aluminum may be successfully degreased under high voltage (micro-arc) conditions by acid anodisation.
The use of jets of electrolyte situated near the electrodes in electrolytic cleaning baths to create high speed turbulent flow in the cleaning zone is taught for example in JP-A-08003797 and DE-A-4031234.
The electrolytic cleaning of radioactively contaminated objects using a single jet of electrolyte without overall immersion of the object, is taught in EP-A-0037190. The cleaned object is anodic and the voltage used is between 30 to 50V. Short times of treatment of the order of 1 sec are recommended to avoid erosion of the surface and complete removal of oxide is held to be undesirable. Non-immersion is also taught in CA-A-1165271 where the electrolyte is pumped or poured through a box-shaped anode with an array of holes in its base. The purpose of this arrangement is to allow a metal strip to be electro-plated on one side only and specifically to avoid the use of a consumable anode.
DE-A-3715454 describes the cleaning of wires by means of a bipolar electrolytic treatment by passing the wire through a first chamber in which the wire is cathodic and a second chamber in which the wire is anodic. In the second chamber, a plasma layer is formed at the anodic surface of the wire by ionisation of a gas layer which contains oxygen. The wire is immersed in the electrolyte throughout its treatment.
EP-A-0406417 describes a continuous process for drawing copper wire from copper rod in which the rod is plasma cleaned before the drawing operation. The “plasmatron” housing is the anode and the wire is also surrounded by an inner co-axial anode in the form of a perforated U-shaped sleeve. In order to initiate plasma production the voltage is maintained at a low but unspecified value, the electrolyte level above the immersed wire is lowered, and the flow-rate decreased in order to stimulate the onset of a discharge at the wire surface.
Whilst low voltage electrolytic cleaning is widely used to prepare metal surfaces for electro-plating or other coating treatments, it cannot handle thick oxide deposits such as mill-scale without an unacceptably high expenditure of energy. Such electrolytic cleaning processes must normally be used, therefore, in conjunction with other cleaning procedures in a multi-stage operation.
WO-A-97/35052 describes an electrolytic process for cleaning electrically conducting surfaces using an electro-plasma (arc discharge) in which a liquid electrolyte flows through one or more holes in an anode held at a high DC voltage and impinges on the workpiece (the cathode) thus providing an electrically conductive path. The system is operated in a regime in which the electrical current decreases or remains su
Brooks & Kushman P.C.
CAP Technologies, LLC
King Roy
Leader William T.
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