US3563867A

US3563867A - Anodising of aluminium and its alloys

Info

Publication number: US3563867A
Application number: US598966A
Authority: US
Inventors: James M Kape; William Berwyn Hannaby
Original assignee: Acorn Anodising Co Ltd
Current assignee: Acorn Anodising Co Ltd
Priority date: 1965-12-09
Filing date: 1966-12-05
Publication date: 1971-02-16
Anticipated expiration: 1988-02-16
Also published as: CH472506A; FR1505680A; GB1114463A; DE1496721A1

Abstract

A METHOD OF ANODISING ALUMINUM AND ITS ALLOYS IN AN ELECTROLYTE COMPRISING 3% TO 30% BY VOLUME OF AN AQUEOUS SOLUTION OF SULPHURIC ACID COMPRISING ADDING BETWEEN 0.1% AND 2% BY VOLUME OF NITRIC ACID TO THE ELECROLYTE BEFORE OR DURING THE CARRYING OUT OF THE ANODISING OPERATION MAINTAINING THE ELECTROLYTE AT A TEMPERATURE BETWEEN 0*C. AND 25*C. AND CARRYING OUT THE ANODISING OPERATION AT AN APPLIED VOLTAGE BETWEEN 10 AND 60 AND AT A CURRENT DENSITY BETWEEN 60 AND 120 AMPERES PER SQUARE FOOT.

Description

United States Patent Oifice US. Cl. 204--58 1 Claim ABSTRACT OF THE DISCLOSURE A method of anodising aluminium and its alloys in an electrolyte comprising 3% to 30% by volume of an aqueous solution of sulphuric acid comprising adding between 0.1% and 2% by, volume of nitric acid to the electrolyte before or during the carrying out of the anodising operation maintaining the electrolyte at a temperature between C. and 25 C. and carrying out the anodising operation at an applied voltage between 10 and 60 and at a current density between 60 and 120 amperes per square foot.

This invention relates to a method of anodising aluminium and its alloys, for example, in the form of wrought or cast articles.

The anodising of aluminium. and its alloys in electrolytes including sulphuric acid solutions is well known. The concentration of sulphuric acid employed ranges from 5% by volume up to over 20% by volume and the .anodic coatings produced range in thickness from 2 microns or less up to over 25 microns, the current densities ranging from to 60 amperes per square foot and the applied voltage varying between 12 and 20 volts. The temperature of 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 5 C., usually 0 C., but the voltages required to produce the current densities of 10 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, HIS, 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 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 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.

3,563,867 Patented Feb. 16, 1971 According to this invention, a method of anodising aluminium and its alloys in an electrolyte comprising a sulphuric acid solution comprises adding nitric acid to the electrolyte before or during the carrying out of the anodising operation.

The amount of nitric acid added may vary between 0.1% and 2% both being percentages by volume, but the preferred addition is in the region of 1% by volume of the electrolyte. The electrolyte may comprise 3% to 30% by volume of an aqueous solution of sulphuric acid and preferably comprises 7.5% by volume of such solution.

Some examples of the way in which this invention may be carried into practice will now be described.

Two 1 foot square pieces of an alloy containing 99.0% of aluminium were anodised in separate baths each containing a 7.5% by volume aqueous solution of sulphuric acid at 20 0., one bath also containing 1% by volume of nitric acid (specific gravity 1.42).

The voltages applied and the current densities produced are shown in the following table:

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

Film thickness (microns) 7.57 mso'i Volt- Tempera- 7.5% plus 1.0% Alloy 1 age ture, 0. H2804 HNO HE9 17 21. 0 30 27 NS3 20 21. 5 27 27 NE 19 21. 5 27 27 N84 17 21. 0 20 27 N821 18 21.0 15 17.5

British Standards nomenclature.

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 a consequent 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 7.5% by volume solution of sulphuric 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 microns.

An exactly similar sample of the alloy HE9 was anodised at the same current density and temperature for one hour in an aqueous electrolyte containing 7.5%

3 by volume of H 50; and 1.0% by volume of HNO (specific gravity 1.42). The voltage only rose from 20 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 7.5% by volume of sulphuric acid and the other containing 7.5% by volume of sulphuric acid and 1% by volume 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 of British Standard 1615: 1961, 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 obtained on different alloys, at the same voltages and temperatures (peak voltage of about 60 volts and temperature of 0 C.) and a current density of about 36 amperes per square foot, when using either a conventional hard anodising electrolyte (17.5% volume H 80 and 1.0% by weight oxalic acid) or an electrolyte according to this invention (7.5% by volume H 80; and 1.0% by volume HNO Film thickness (microns) Sulphuric Convenacid-nitric tional acid elecelectrolyte trolyte 1 British Standards nomenclature.

Although in the examples given above the electrolyte had a sulphuric acid concentration of 7.5% by volume it has been found that this may vary between 3% and 30% by volume.

With the use of sulphuric acid-nitric acid electrolytes it' has been found 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 dilficult, 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 nitric acid is added to the sulphuric acid solution before anodising commences it will be understood that nitric acid could be added to a sulphuric acid electrolyte during the anodising operation.

We claim:

1. A method of anodising aluminium and its alloys in an electrolyte comprising 3% to 30% by volume of an aqueous solution of sulphuric acid comprising adding between 0.1% and 2% by volume of nitric acid to the electrolyte before or during the carrying out of the anodising operation maintaining the electrolyte at a temperature between 0 C. and 25 C. and carrying out the anodising operation at an applied voltage between 10 and and at a current density between 60 and amperes per square foot.

References Cited UNITED STATES PATENTS 2,231,086 2/1941 Mi'lller 204--58 FOREIGN PATENTS 396,743 8/1933 Great Britain.

JOHN H. MACK, Primary Examiner 45 R. L. ANDREWS, Assistant Examiner