US3562121A

US3562121A - Anodising of aluminium and its alloys

Info

Publication number: US3562121A
Application number: US694810A
Authority: US
Inventors: James M Kape; William B Hannaby
Original assignee: Acorn Anodising Co Ltd
Current assignee: Acorn Anodising Co Ltd
Priority date: 1967-11-22
Filing date: 1968-01-02
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09
Also published as: GB1215314A

Abstract

THIS INVENTION RELATES TO A METHOD OF ANODISING ALUMINIUM AND ITS ALLOYS, FOR EXAMPLE, N THE FORM OF WROUGHT OR CAST ARTICLES IN A SULPHURIC ACID ELECTROLYTE TO WHICH HAS BEEN ADDED EITHER DURING OR BEFORE ANODISING A SUBSTANCE WHICH WILL IMPART NITRATE IONS TO THE ELECTRLYTE. SUCH SUBSTANCES INCLUDE NITRIC ACID AND SOLUBLE NITRATES AND NITRITES.

Description

United States Patent O US. Cl. 204-58 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of anodising aluminium and its alloys, for example, in the form of wrought or cast articles in a sulphuric acid electrolyte to which has been added either during or before anodising a substance which will impart nitrate ions to the electrolyte. Such substances include nitric acid and soluble nitrates and nitrites.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of an application for Letters Patent Ser. No. 598,966 filed on Dec. 5, 1966 by J. M. Kape et al.

BACKGROUND OF THE INVENTION The anodising of aluminium and its alloys in electrolytes including sulphuric acid solutions, thus producing sulphate ions in the solution, is well known. The concentration of sulphuric acid employed ranges from 92 gm. per litre up to over 370 gm. per litre and the anodic coatings produced range in thickness from 2 microns or less up to over 25 microns, the current densities ranging from 10 to 60 amperes per square foot and the applied voltage varying between 12 and 20 volts. The temperature f the electrolyte must be maintained at between 15 C. and 25 C. for optimum results.

Very hard anodic coatings have also been produced on aluminium alloys at temperatures below C., usually at 0 C., but the voltages required to produce the current densities of to 60 amperes per square foot are higher than those just referred to and may rise to as high as 100 volts for the production of hard coatings of a thickness of over 25 microns on some alloys. The higher the voltage required to produce a given current density the greater the amount of cooling needed to maintain the electrolyte at the required temperature.

In addition some aluminium alloys, particularly those containing more than 1% of copper, nickel or silicon or admixtures of these elements, are very diflicult to anodise or hard anodise in a sulphuric acid electrolyte using conventional techniques to produce an anodic coating of sufficient thickness to have a commercial value. Such alloys include, for example, those known under the nomenclature of British Standards Specifications 1470 and 1490 as LM6, H15, RR250 and LM4.

It is an object of the present invention to provide an improved method of anodising aluminium and its alloys whereby the voltage required to produce a given current density is lowered and the voltage rise during anodising 3,562,121 Patented Feb. 9, 1971 "ice is reduced, in relation to those of normal processes, yet with the production of anodic coatings having properties equivalent to those of the coatings produced in these normal processes and with considerable savings in electrical energy and in the amount of cooling required to maintain the required temperature of the electrolyte. A further object of the invention is to enable acceptable anodic coatings to be formed upon aluminium alloys hitherto found difficult to anodise.

SUMMARY OF THE INVENTION According to the invention there is provided a method of anodising aluminium and its alloys in an electrolyte containing a sulphuric acid solution comprising adding to the electrolyte before or during the anodising operation a soluble compound to produce nitrate ions in the electrolyte, the amount of the compound being such as to enable an anodised surface to be formed on aluminium or its alloy.

Preferably, the amount of nitrate ion forming compound does not exceed the equivalent of 45 gm. per litre of nitric acid in water.

The compounds used can be any water soluble compound containing nitrate or nitrite groups which, when in solution, will provide the requisite nitrate ions. Examples of such compounds are: ammonium nitrate, cupric nitrate, urea nitrate, guanidine nitrate, m. nitro-benzoic acid, m. or p. nitrophenol, aniline nitrate.

The electrolyte may be an aqueous solution containing 56 to 560 gm. per litre of sulphuric acid and preferably is an aqueous solution containing 140 gm. per litre of sulphuric acid.

The anodising operation may be carried out at an applied voltage between 10 and 60. The current density used may be between 60 and 120 amperes per square foot.

The electrolyte temperature may be between 0 C. and 25 C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some examples of the way in which this invention may be carried into practice will now be described. The initial examples are of the use of nitric acid as the ion producing compound, since this is the most basic compound providing the nitrate ions. Thereafter will be described the use of other compounds.

Current density in amperes per square foot Sulphuric Sulphuric acid-niacid electric acid trolyte electrolyte It should be noted that the voltage-current density relationship varies considerably with the aluminium alloy used.

T'wo samples of each of a number of different wrought aluminium alloys (identified below by the nomenclature of British Standards 1470 and 1490) were separately anodised in the two baths referred to above at the same voltage and temperature, a sample of a given alloy being anodised at a given voltage and temperature for 1 hour in the sulphuric acid electrolyte and an exactly similar sample being anodised for half an hour (at the same voltage and temperature) in the sulphuric acid-nitric acid electrolyte. The thicknesses of the anodic coatings were then measured and are tabulated here:

Film thickness (microns) 140 gm./ litre 140 141128047 gm. .2 gm. Alloy (Brit1sh Stand- ,Tempcralitre litre ards nomenclature) Voltage ture, C. H2804 +HNOa It can be appreciated from these results that the addition of nitric acid to the electrolyte resulted in a much higher current density for the same applied voltage, with consequently a more rapid growth of the anodic coating.

A sample of the alloy HE9 was anodised at a current density of 60 amperes per square foot in an electrolyte consisting of a 140 gm. per litre solution of sulphuric Film thickness (microns) Sulphuric Convenacid-ni- Alloy (British Standards tional trie aeid nomenclature) electrolyte electrolyte The coating formed on the alloy HE with the electrolyte according to this invention, showed a substantial improvement in abrasion resistance over the hard anodic coating produced on this alloy by the conventional meth od. It will also be noted that this table includes the alloy LM6, referred to above and that the electrolyte according to the invention enables an anodic coating of commercially acceptable thickness to be obtained.

Although in the examples given above the electrolyte had a sulphuric acid concentration of 140 gm. per litre it has been found that this may vary between 56 and 5 60 gm. per litre.

Further examples of the use of nitric acid in various quantities for hard anodising aluminium alloys known as H15 and H30 are given in the following table:

Current Anod- Film Temper- Voltdensity ising thickature, age (amps/ time ness Electrolyte 0. range sq.ft.) (min.) (mil) Alloy 280 gmjlitre H 80 plus 14 gm./litre HNO3 3 20-90 48 28 2. 3 H80 3 -60 48 60 3. 0 H15 1 22-55 48 60 4. 0 H15 280 gm./litrc H SOl plus 2.1 gm./litre HNO3 1 20-90 48 37 2. 4. H 1 20-50 48 3.0 H15 280 g1n./litre H280 plus 28 gm./litre HNO; 1 19-60 48 25 2. 2 H30 -3 22-53 48 60 4. 2 H15 280 grnJlitre H SO; plus 35 gm./litre HNOa 0 18-90 48 31 2.0 H30 0 21-45 48 60 3. 8 H15 280 gm./litre H 80 plus 42 gin/litre HNO3 2 17. 590 48 33 2. 4 H30 2 18-55 48 60 4. 2 H15 280 gm./litre H2804 plus 49 gnrllitre HNO;

acid maintained at 20 C. After one hour the voltage had risen from 20 to over 50 volts and the average thickness of the anodic coating produced was 75 microns.

An exactly similar sample of the alloy HE9 was anoldised at the same current density and temperature for one hour in an aqueous electrolyte containing 140 gm. per litre of H 80 and 14.2 gm. per litre of HNO (specific gravity 1.42). The voltage only rose from 2 0 to 22 volts and the average thickness of the anodic coating produced was 100 microns.

Two samples of the alloy HE9 were separately but similarly anodised to produce anodic coatings 25 microns thick in respective electrolytes, one containing 140 gm. per litre of sulphuric acid and the other containing 14.2 gm. per litre of nitric acid. The specific abrasion resistance of both the coatings, when measured at an air flow rate of 70 litres per minute by method 0 of the British Standard 1615: 196 1, was 2.0 grammes per micron of film thickness.

The sulphuric acid-nitric acid electrolyte is particularly useful for producing thick hard anodic coatings. In the following table are set out the film thicknesses ob- Any soluble compound which will produce nitrate ions in solution (i.e. soluble compounds containing nitrate or nitrite groups) may be substituted for the nitric acid. The following table gives the equivalent amounts of a number of such compounds in relation to 14 gm. per litre of nitric acid in order to give the same results:

The following table sets out experimental results obtained using theabove compounds for hard anodising aluminium alloys known under the British Standards Nomenclature H.15 and H30. The first example in the table indicates the effect of an electrolyte containing only sulphuric acid for comparison purposes.

Current Anod- Film Temper- Voltdensity ising thickature, age (amps/ time ness Electrolyte range sqit.) (min.) (mil) Alloy 280 gin/litre H280 plus 280 gmJlitre I-IgSOi 4 22-60 48 31 2. 4 H30 3 22-24 24 H15 230 gmJIitre HQSO; plus gm./litre NH NO 1 20-58 48 60 3. 2 H

280 gnL/litre H 804 plus 22.9 gm./lit1'e NH4N03 1 -90 48 41 2. 2 H30 1 20-70 48 60 3. 5 H15 280 gmJlitre H2504 plus 45.8 gmJlitre NH4NO3 0 20-54 48 60 3. 2 H15 280 gm./litre HzSOr plus 35.2 gmflitre urea nitrate 2 24-00 48 33 2. 1 H30 2 18-50 48 39 3. 5 H15 280 gin/litre H2804 plus gnL/litre guanidine nit-rate 4 20-90 48 33 2. 2 H 4 23-51 48 46 3. 7 H15 280 gmJlitre H280 plus 25 gmJlitre p-nitrophenol -1 20-50 48 60 3. 1 H15 3 24-90 48 28 1. 5 H30 280 gmJlitre H2804 plus 50 gm./litre m-nitrobenzene sulphonic acid (Na, salt) +3 27-35 48 60 3. 5 H15 +7 22-90 48 2. 3 H30 280 gm [litre H280; plus gm./litre m-nitrobenzoie 2 18-75 48 3.0 H30 acid (Na salt).

280 gm./litre I'IZSO4 plus 25 gum/litre Cu (N002 2 22-30 48 55 1. 4 H15 280 gmJlitre H2804 plus 50 gm/litre C11(N03)2 0 23-37 8 54 4. 0 H15 0 15-42 48 25 2. 5 H30 280 gm./litre H280 plus gm. flitre C11(N0a)2 4 15-60 43 3 5 H30 With the use of sulphuric acid-nitrate ion electrolytes it has been tound possible to produce much thicker anodic coatings than can normally be produced on certain aluminium alloys, notably those containing appreciable amounts of copper or silicon, which are frequently required to be hard anodised.

Moreover, the use of lower voltages made possible by the choice of this electrolyte considerably facilitates the continuous anodic oxidation of aluminium wire and strip. In addition the present invention enables anodic coatings of acceptable commercial thickness to be produced on aluminium alloys hitherto regarded as diflicult, such as those containing more than 1% of copper, nickel or silicon or admixtures of these elements.

Although as described above it is assumed that the nitrate ion producing compound is added to the sulphuric acid solution before anodising commences it will be understood that nitrate ion producing compound could be added to a sulphuric acid electrolyte during the anodising operation.

We claim:

1. A method of anodising aluminium and its alloy comprising the steps of making up an electrolyte containing 140 gm. per litre of sulphuric acid, adding to the electrolyte before or during the anodising operation a soluble compound to produce nitrate ions in the electrolyte, the amount of such a compound being the equivalent of between 1 gm. and 45 gm. per litre of nitric acid, and carrying out the anodising operation at a temperature between 0 C. and 55 C., at an applied voltage of between 10 and 60, and at a current density of between 60 and amperes per square foot.

2. A method of anodising aluminium and its alloys in an electrolyte containing a sulphuric acid solution comprising the step of adding to the electrolyte before or during the anodising operation a soluble compound, to produce nitrate ions in the electrolyte, the amount of the compound being such as to enable an anodised surface to be formed on aluminium or its alloy, wherein the amount of nitrate ion producing compound is the equivalent of between 1 gm. and 45 gm. per litre of nitric acid in the solution, wherein the nitrate ion producing compound is selected from the group consisting of nitric acid, ammonium nitrate, cupric nitrate, urea nitrate, guanidine nitrate, m. nitrobenzoic acid, In. or p. nitrophenol and aniline nitrate, wherein the electrolyte is an aqueous solution comprising between 56 and 560 gm. per litre of sulphuric acid, wherein the anodising operation is carried out at an applied voltage between 10 and 60, and wherein the anodising operation is carried out at a current density between 60 and 120 amperes per square foot.

3. A method according to claim 2, wherein the anodising operation is carried out at an electrolyte temperature between 0 C. and 55 C.

References Cited UNITED STATES PATENTS 2,231,086 2/1941 Miiller 20458 FOREIGN PATENTS 396,743 8/1933 Great Britain.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner