Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
1999-08-06
2001-07-31
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S721000
Reexamination Certificate
active
06267870
ABSTRACT:
Rolled aluminium alloy sheet is extensively used as lithographic plate substrate, for which purpose it is finally processed by tension levelling and cleaning. On being electrograined in nitric acid, surface defects may arise which show up as ungrained mirror-like areas, typically 1-2.0 mm in diameter, in a matt grained surface and which lead to large rejection rates. One such area per 20 m
2
of rolled sheet can lead to rejection of the strip. This is an increasing problem because inspection is becoming more rigorous and graining is lighter.
This invention results from the inventors' discovery that these surface defects result from the presence of particles more noble than Al on the surface of the Al workpiece. Such particles most usually contain copper or consist of copper. The actual quantity of copper-containing metal deposited overall is very small and is extremely difficult to detect in the rolling production stages. Other contaminant metal particles are possible. This invention addresses the problem of surface defects in Al sheet by removal of metal particles contaminating the surface thereof. Removal of such particles is preferably effected at a late stage in production, after any likely sources of contamination have been passed. Of course, rolled Al sheet is cleaned, particularly for lithographic use but also for all other purposes; but it has been found that cleaning techniques in current use may not be effective to remove surface metal particles.
The invention provides a method of treating an Al workpiece to improve a surface thereof, which method comprises removing noble particles, e.g. Cu-containing particles present on the surface. Preferably removal is effected by subjecting the Al workpiece to electrolytic treatment, e.g. anodising the Al workpiece in an electrolyte capable of dissolving the metal particles. Preferably the Al workpiece is anodised at a current density of at least 2 kAm
−2
.
The same particles may initiate corrosion in rolled sheet intended to be painted for architectural or automobile use; and in rolled sheet to which anodic oxide films or organic coatings are intended to be applied.
The workpiece is preferably rolled sheet or strip. The term Al is herein used to denote pure aluminium metal and alloys containing a major proportion of aluminium. While the invention is believed applicable to Al alloys generally, it is of particular importance in relation to 1000 and 3000 series alloys (of the Aluminum Association Inc. Register) intended for use as lithographic substrates, and also 5000 and 6000 series alloys intended for architectural or vehicle or other use.
The electrolyte, which needs to be capable of dissolving the metal particles, may be acidic or alkaline. Caustic soda and caustic potash are possible. Sulphuric acid is a possible electrolyte, optionally containing HF or other additives as in the cleaning fluid marketed by Henkel under the trademark Ridolene 124/120E. Preferred electrolytes are based on phosphorus oxyacids. This family of acids includes orthophosphoric acid H
3
PO
4
; metaphosphoric acid and pyrophosphoric acid based on P
2
O
5
; and also phosphorous or phosphonic acid H
3
PO
3
; hypophosphorous or phosphinic acid H
3
PO
2
; and perhaps others. As electrolytes with dissolving power for Cu (and for aluminium oxide) they all have generally similar properties.
Contamination of the sheet can occur at any stage in the rolling or handling process but is most likely to occur during hot rolling. The process according to the invention is preferably carried out after hot rolling has been completed. Lithographic sheet is normally cleaned after cold rolling to final gauge. The present treatment can be applied at that stage. However, there are practical advantages to removing contamination by cleaning at an earlier stage either on completion of hot rolling or at an intermediate stage in the cold rolling, for example after an intermediate anneal. Cleaning at this earlier stage has the following advantages:
1. The contaminating particles are less likely to be firmly rolled into the surface and therefore are more easily removed.
2. A portion of each contaminating particle becomes smeared over the surface as cold rolling proceeds and this smear increases the size of each resulting area to be removed.
3. As the sheet is cold rolled to progressively thinner gauge, the surface area to be cleaned increases resulting in increased cost of cleaning.
Cleaning at an earlier stage in the process does increase the risk of contamination arising later in the process remaining in place. However, this risk may be outweighed by the advantages listed above. Of course, the cleaning process can be repeated at later stages in the process and may in any case be followed by a conventional lighter cleaning operation.
The method involves anodising the Al workpiece, using either direct current or more preferably alternating current. When a.c. is used, it is supposed that electrolysis of the metal particles occurs when the Al surface is anodic. In addition, when the Al surface is made cathodic, copious quantities of hydrogen gas are formed all over the surface and blow loose debris off. The anodic action can also help to loosen particles of detritus by undercutting the surrounding Al substrate.
The a.c. wave form may be sinusoidal or not as desired. The a.c. current may be biased in either the cathodic or anodic direction. The a.c. frequency is at least several cycles per second and is preferably the commercial frequency.
Alternatively d.c. anodising may be used. While this is effective to loosen or dissolve metal particles, there is some risk that particles may be re-deposited. This risk can be avoided by causing the electrolyte to flow across the surface of the workpiece or by rapidly removing the workpiece from the electrolyte. Alternatively, d.c. anodisation for a period sufficient to loosen metal particles on the surface of the Al workpiece, can be followed by making the workpiece cathodic for a short period sufficient to generate a burst of hydrogen gas and blow the loosened particles away from the surface. Preferably the workpiece is removed from the bath under anodic conditions.
The concentration of phosphoric acid, or other electrolyte, is preferably from 5-30%, particularly 10-25% and more particularly 15-25% e.g. 20%. At low concentrations, the power of the acid to dissolve or loosen metal particles may not be sufficient. At high concentrations, the electrolyte may be so viscous that carry-over of electrolyte becomes a problem, particularly in continuous operations involving immersion for short periods.
The electrolyte temperature is preferably maintained at 50-100° C. Below 50° C., the dissolving power of the electrolyte may be too low. Although there is no theoretical upper limit of temperature, it is in practice inconvenient to heat phosphoric acid or other electrolytes to temperatures above 100° C. The preferred temperature for a phosphoric acid electrolyte is 80-100° C. e.g. 90° C. At temperatures of 70° C. and above, anodising can be performed under conditions to remove an aluminium oxide film from the surface of the workpiece, thus effectively cleaning the workpiece, and the treatment to remove metal particles according to this invention can thus be carried out in conjunction with cleaning. At temperatures in the range 50-80° C. (preferably 50-70° C. for Mg containing alloys) anodising can be performed under conditions to create or maintain an anodic aluminium oxide film, and this may increase the surface resistance of the Al workpiece and favour a current path through the metal particles. Thus operating under conditions to create, rather than remove, anodic aluminium oxide helps to remove the metal particles by electrolysis. The anodic film may be completely or partially dissolved if the strip is left in the electrolyte away from the influence of the electrodes.
A relatively high current density of at least about 2 kAm
−2
is preferred to remove metal particles. This is higher than the current densities ordinarily used when anodisi
Amor Martin Philip
Ball Jonathan
Alcan International Limited
Cooper & Dunham LLP
Phasge Arun S,.
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