Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – Electrolyte composition or defined electrolyte
Patent
1997-10-16
1999-12-07
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
Electrolyte composition or defined electrolyte
205705, 205711, 205717, 205723, B23H 380
Patent
active
059977213
DESCRIPTION:
BRIEF SUMMARY
There is a considerable volume of data on the cleaning of aluminium workpieces prior to subsequent surface finishing treatments. Some of these are only suitable for batch production as a precursor to, for example, architectural anodising and are not fast enough for continuous high speed operation. A good overview is given in "The Surface Treatment and Finishing of Aluminium and its Alloys" by S Wernick, R Pinner and P G Sheasby, Finishing Publications Ltd, 1987, Teddington, U.K.
Generally aluminium surfaces are cleaned using acid or alkaline solutions. Alkaline etching solutions are faster than acid ones and tend to cope well with residual organics on the surface of the workpiece. Unfortunately, they do not dissolve the magnesium oxides left on the surface of magnesium containing alloys that have been thermally treated. They also often require an acidic desmutting step and very careful rinsing control, and deposits build up rapidly in the bath. The fastest acidic cleaners contain hydrofluoric acid plus another acid such as sulphuric acid. Such known treatments are capable of removing material at rates up to about 1 g/m.sup.2 /min.
In U.S. Pat. No. 3,718,547, W E Cooke et al. describe a high speed continuous electrolytic surface cleaning treatment of aluminium strip. In a preferred embodiment, the strip is made successively cathodic, anodic and finally cathodic again while being subjected to d.c. electrolysis in a sulphuric acid electrolyte at 90.degree. C. This treatment results in the formation of an anodic oxide film quoted as being 5 to 50 mg per 100 square inches (which corresponds to a film thickness of 30-300 nm assuming an oxide density of 2.5 g/cm.sup.3) and which forms an excellent base for lacquer.
In U.S. Pat. No. 4,097,342, W E Cooke et al describe an electrolytic cleaning treatment step which involves subjecting aluminium strip to d.c. anodising for a few seconds at high temperature and current density in a concentrated strong mineral acid electrolyte.
The present invention provides a method of cleaning an Al or Al alloy workpiece which method comprises anodising the workpiece using a chosen a.c. voltage X (expressed in rms V) in an acidic electrolyte capable of dissolving aluminium oxide and maintained at a temperature of at least 70.degree. C. under conditions such that the surface of the workpiece is cleaned with any oxide film thereon being non-porous and having a thickness Y (expressed in nm) wherein Y is not more than about half X, or a thickness of not more than about 20 nm. Preferably the cleaning treatment consists essentially of this step, i.e. without any other special steps being necessary. The following technical explanation may be of interest.
Anodising, whether a.c. or d.c., can produce a wide range of oxide film structures. The type of structure produced is generally dependent on the voltage applied across the film at the surface and the aggressiveness of the electrolyte. Thus in a non-aggressive electrolyte only a barrier film is grown that reaches a limiting thickness governed by the voltage applied, i.e. a limiting field is achieved that will no longer drive ions through the film. However, if the electrolyte can dissolve the film then, once the normal barrier film thickness is achieved, cells are formed on the surface that each have a pore in the centre. The oxide film at the base of these pores continues to grow into the metal and be dissolved rapidly at the electrolyte-film interface thus maintaining the barrier film thickness. Dissolution at the base of the pores is greatly enhanced over the normal chemical dissolution rate by the electric field which results in the columns of oxide between the bases of the pores being left unattacked or `growing` to form the cell walls. In an aggressive acid, such as sulphuric or phosphoric acid, the structure formed is strongly dependent on the temperature and acid concentration. Thus at room temperature the dissolution in the pore is so slow that low currents are used and films can be made many microns thick without the original outer
REFERENCES:
patent: 3929591 (1975-12-01), Cu et al.
patent: 4097342 (1978-06-01), Cooke et al.
patent: 4372831 (1983-02-01), Rosswag
Ball Jonathan
Davies Nigel Cleaton
Kumpart Armin
Limbach Peter Karl Ferdinand
Alcan International Limited
Phasge Arun S,.
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