US 3887447 A
In a process for electrograining aluminium by subjecting the aluminium to alternating current whilst immersed in an aqueous electrolyte containing 2 - 20 gms/litre hydrochloric acid the electrolyte is stabilised against the presence of sulphate ion contamination and the pit size is controlled to a value below about 10 microns by the addition to the electrolyte of defined quantities of phosphoric acid or chromic acid. Both for operational reasons and for avoidance of health hazards it is preferred to employ phosphoric acid in an amount of about 5 - 10 gms/litre.
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Description (OCR text may contain errors)
United States Patent [191 Sheasby et al.
[ June 3, 1975 PROCESS OF ELECTROGRAINING ALUMINIUM  Inventors: Peter Geoffrey Sheasby; Alan Martin Smith, both of Banbury, England  Filed: Sept. 27, 1973  Appl. No.: 401,257
 US. Cl. 204/129.4; 204/l29.75; 204/129.95  Int. Cl C23b 3/02  Field of Search 204/1294, 129.9, 129.95,
 References Cited UNITED STATES PATENTS 2,721,835 10/1955 Axtell 204/12995 3,477,929 11/1969 Namikata et al. 204/12995 3,755,116 8/1973 Teral et al. 204/129.95
FOREIGN PATENTS OR APPLICATIONS 1,202,030 8/1970 United Kingdom 204/l29.95
831,954 4/"1960 United Kingdom 204/1294 1,441,928 5/1966 France 529,755 6/1955 Italy 204/ 129.4
Primary ExaminerT. Tung Attorney, Agent, or Firm-Cooper, Dunham, Clark, Griffin & Moran ABSTRACT In a process for electrograining aluminium by subjecting the aluminium to alternating current whilst immersed in an aqueous electrolyte containing 2 20 gms/litre hydrochloric acid the electrolyte is stabilised against the presence of sulphate ion contamination and the pit size is controlled to a value below about 10 microns by the addition to the electrolyte of defined quantities of phosphoric acid or chromic acid. Both for operational reasons and for avoidance of health hazards it is preferred to employ phosphoric acid in an amount of about 5 1O gms/litre.
2 Claims, No Drawings PROCESS OF ELECTROGRAINING ALUMINIUM The present invention relates to improvements in the process of electrograining (electroetching) aluminium.
Aluminium is commonly used for the production of lithographic printing plates. For this purpose it is desirable that it should have a roughened surface characterised by the presence of very fine pits.
One known way of producing a desirable fine, uniformly pitted surface with randomly oriented pits is by an electrograining technique, in which the aluminium is subjected to alternating current when immersed in dilute hydrochloric acid. The hydrochloric acid electrolyte generally incorporates 2-20 gms hydrochloric acid per litre.
In the alternating current electrograining of aluminium sheet in hydrochloric acid as a pretreatment for the production of lithographic plates it is conventional to employ a current density in the range of 20 to 60 amps/sq. ft. (2 to 6 amps/dm for a period of 4 to 15 minutes. The charge input involved is generally in the range of 15,000 50,000 coulombs/sq. ft.
The above conditions are typical for batch processes. Where a continuous process is employed, i.e., where moving strip material is moved past stationary electrodes, current densities of the order of 100 1,000 amps/sq. ft. (l0 100 amps/dm may be employed for treatment times of /2 3 minutes.
Whilst coarsely-pitted electrograined surfaces can readily be obtained with such electrolyte, it is found that considerable care must be exercised in order to obtain a finely pitted surface (pits below microns in diameter). In particular, it is found that the presence of sulphate ion in the electrolyte will usually lead to a coarsely pitted surface, in which the pits are larger than is desirable for optimum performance for lithographic purposes. Although it is possible to start up the process with electrolyte made up with deionised water, it is inevitable that the electrolyte will become increasingly contaminated with sulphate and other ions carried over from earlier stages of the process.
As conventionally performed the preparation of aluminium for use in lithographic printing comprises a three-stage process involving degreasing and pretreatment (usually etching in sodium hydroxide-based solutions) followed by electrograining and anodising. As explained above, electrograining is conventionally carried out by applying alternating current to the aluminium whilst immersed in dilute aqueous hydrochloric acid. Since it is economically necessary to wash the aluminium in normal tap water (deionised water being too expensive for wash purposes) after the pretreatment stages, it is inevitable that the electrograining electrolyte will become progressively contaminated by whatever ions may be present in the wash water, carried over on the surface of the aluminium. Most natural waters contain more than parts per million sulphate ion.
It has been shown by practical printing trials that very finely grained surfaces lead to extended plate life and better image definition in lithographic printing. We have however found that contamination of the hydrochloric acid electrograining electrolyte by more than about 10 15 parts per million sulphate ion leads to a much more irregular and coarsely pitted surface, with the result that lithographic printing plates of relatively poor quality result from electrolyte made up from tap present in an amount of 1.5 l5 gms/litre, whilst chromic acid (calculated as CrO may be present in an amount of L5 2.0 gms/litre.
It is greatly preferred to employ phosphoric acid to chromic acid, for reasons of economics, lesser criticality of concentration and far lower health hazards involved.
With 'both materials it is found desirable to hold down the quantity of alkali metal ion, since this is also found to lead to the formation of large, irregular pits during electrograining, although it is tolerable in much larger proportions than is sulphate ion. A very suitable concentration of orthophosphoric acid is 5 l0 gms/litre.
The invention is hereinafter further described with reference to the following examples:
EXAMPLE 1 A hydrochloric acid, containing 6 gms hydrochloric acid per litre, electrolyte was made up with deionised water 1 litre) and 5 gms orthophosphoric acid per litre was added. mm X 50 mm (3 in. X 2 in.) panels of 99.7 percent aluminium were electrograined in this solution at room temperature with a 75 mm X 75 mm (3 in. X 3 in.) graphite counterelectrode placed 50 mm (2 in.) away from it. An ac voltage of 8 volts was applied for 6 minutes and a finely pitted surface was produced with the average pit size about 3 microns. Varying the hydrochloric acid concentration between 6 gms and 9 gms per litre had no effect on the fineness of the surface produced, although the current passing varied from about 4 amps to 5 amps under these conditions (about 70 amps/sq. ft. at the treated face of the panel).
EXAMPLE 2 A 8 gms per litre hydrochloric acid electrolyte was made up, using tap water containing about ppm. of sulphate ion, and 8 gms per litre phosphoric acid was added. When panels of 99.7 percent aluminium were processed under the same conditions as Example 1, a finely pitted surface with pits ranging from l-5 microns in diameter were produced. A similar electrolyte was then made up without the addition of phosphoric acid and this produced a relatively irregular and coarsely pitted surface with pit sizes ranging from about 10 to 30 microns.
EXAMPLE 3 Trials were carried out in a 50-gallon tank containing 8 gms/litre hydrochloric acid electrolyte made up with tap water containing about 100 ppm. of sulphate ion. A 99.7 percent aluminium sheet 480 mm X mm (19 in. X 5 in.), electrograined at room temperature at a constant current of amps (about 200 amps/sq. ft. at the treated face of the panel) with the applied ac. voltage rising from 10 to 12 volts over the 2-minute electrograining period, produced a surface with pits up to 20 by 30 microns in size. After addition of 8 gms/litre orthophosphoric acid to the same electrolyte, processing a similar panel under the same con- 3 ditions gave a surface with pits ranging from 1 5 microns in size.
In these tests the aluminium plates were connected as one electrode of the system with a graphite electrode. However both electrodes may be formed of aluminium plates to be electrograined. The process may also be carried out on a continuous scale. A continuous strip of aluminium is drawn through a two-compartment cell, in each of which is a counterelectrode connected to an a.c. source, so that current flows between the counterelectrodes through the aluminium strip which is passed through an aperture in the partition between the two compartments and through the electrolyte between the electrodes and the adjacent portion of the strip. By using this method, which has long been known for continuous anodising, contact between the strip and an electrode is avoided and arcing problems are avoided.
1. In the process of electrograining aluminum for the production of lithographic printing plates which comprises subjecting the aluminum to alternating current while immersed in an aqueous electrolyte containing 2 20 gms/liter hydrochloric acid the improvement which comprises the addition to the electrolyte of L5 l5 gms/liter phosphoric acid whereby to maintain the pit size below about l0 microns in the presence of contaminant sulphate ion in the electrolyte.
2. The process of claim 1 wherein the aqueous electrolyte contains 5 l0 gms/liter phosphoric acid.