GB1562586A

GB1562586A - Aluminium etching

Info

Publication number: GB1562586A
Application number: GB1924778A
Authority: GB
Original assignee: Pennwalt Corp
Current assignee: Pennwalt Corp
Priority date: 1978-05-12
Filing date: 1978-05-12
Publication date: 1980-03-12

Description

(54) ALUMINIUM ETCHING (71) We, PENNWALT CORPORA TION, a corporation organised and existing under the laws of the State of Pennsylvania, United States of America, of Pennwalt Building, Three Parkway, Philadelphia, Pennsylvania 19102, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for etching aluminium and aluminium alloy and compositions for use therein and is a modification of our U.K. Patent No. 1,434,894.

In the alkali etching of aluminium extrusions and sheets, industrial metal finishers are periodically troubled by the occurrence of an undesirable etch finish known as 'galvanized etched finish'. This undesirable and unpredictable etched finish may be described as displaying a highly reflective and grossly crystalline appearance. Ofen the galvanizing is so severe that the aluminium must be scrapped. In less severe cases, the aluminium must be mechanically polished before it can be reprocessed. Such losses and/or multiple processing contribute greatly to the cost of aluminium etching.

In our U.K. Patent No. 1,434,894 it is disclosed that the problem of galvanizing of aluminium or aluminium alloy during etching can be alleviated by etching with a chromate-free aqueous etching solution comprising: from 15-200 gms/litre of alkali metal hydroxide; from 0.5-15 gms/litre of a chelating agent effective to suppress the precipitation of alumina; and from 0.0001-1.0 gm/litre of an oxidising agent, at a temperature of from 75-190"F; said oxidising agent concentration being maintained throughout the etching process.

In accordance with the present invention it has been found that substantially higher amounts of oxidising agent, specifically, sodium chlorate can be used in such solutions with beneficial results.

In accordance with one aspect of the present invention, therefore, there is provided a process according to claim 1 of U.K. Patent No. 1,434,894, modified in that the etching solution is nitrate and nitrite free and contains, as the oxidising agent, sodium chlorate in an amount in excess of 1 gm/litre, but not exceeding 20 gm/litre, more usually not exceeding 10 gms/litre. Preferably, in accordance with this modification the sodium chlorate is present in an amount of from 2-3 gms/litre.

In accordance with the second aspect, this invention provides and etching solution according to claim 6 of U.K. Patent No.

1,434,894, modified in that it is nitrate and nitrite free and contains, as the oxidising agent, sodium chlorate in an amount in excess of 1 gm/litre, but not exceeding 20 gms/litre, more usually not exceeding 10 gms/litre, and preferably in an amount of from 2-3 gms/litre.

The other ingredients of the etching solution used in accordance with this modification, are as disclosed in U.K. Patent No.

1,434,894, viz 15-200, preferably 30-75 gms/litre alkali metal hydroxide, and 0.5-15 gms/litre chelating agent.

Any alkali metal hydroxide may be used in the process of this invention, but sodium hydroxide or potassium hydroxide are preferred, with sodium hydroxide being most preferred since it is cheap and readily available. Likewise, any chelating agent capable of chelating alumina may be used. Preferred chelating agents are sorbitol, gluconic acid, glucoheptonic acid, mannitol, ascorbic acid, sorbose, tannic acid, ethylenediamine tetraacetic acid, sodium chrom glucosate, diglycolic acid, picolinic acid, aspartic acid, dithiooxamide, d-gluconolactone, and 1-rhamnose.

In the etching process the aluminium metal should be carefully prepared for etching by first removing any cutting, rolling or stamping oils and greases. This is usually done by cleaning with an inhibited mild alkaline cleaner. The aluminium metal is then rinsed with water and then deoxidized by immersion or spray application in an inhibited acidic deoxidizer. Following another water rinse, the aluminium metal is ready for etching.

The etching process of this invention is usually performed by immersion of the workpiece in the etching solution contained in a steel tank. The tanks are equipped with coils for heating and/or cooling the etching bath. Agitation of the bath is desirable to maintain a uniform concentration of the etching solution, particularly at the interface with the workpiece. Good ventilation is required to dispel the caustic fumes and by-product hydrogen gas.

Spraying of the workpieces with the etching solution is another way of practicing the invention.

The temperature of the etching solution will generally be in the range of 75C to 1900F. A preferred temperature range is from 140"F to 1700F. The preferred temperature range gives a good etching speed without producing excessive fumes at higher temperatures.

The time at which the workpiece is in contact with the etching solution will vary with the condition of the workpiece, that is, the extent of machining marks and other surface irregularities which are desired to be removed. An etching time of about 2 to about 15 minutes covers most of the conditions encountered. Most etching is done in about 3 to about 7 minutes and this is a preferred time range.

The sodium chlorate, either in a solid form as a granular or powdered material or as a liquid in aqueous solution can be combined with the sodium hydroxide and chelating compound. A typical solid concentrate would be alkali metal hydroxide - 95%, chelating agent - 4% and sodium chlorate 2-3%.

The alkali metal hydroxide can also be supplied as an aqueous concentrate in which the chelating compound and the sodium chlorate are to be dissolved therein.

In addition to the alkali metal hydroxide, chelating compound and sodium chlorate the etching baths used in this invention may contain other conventional additives such as, detergents to improve rinsing, to provide a foam blanket on the bath, or to extend the bath life, brightening additives to increase the reflectivity, and fluorides to increase the grain size.

Examples of surfactants used in alkaline etching baths are shown in U.S. 3,557,000.

The modification of the present invention is illustrated by the following example.

EXAMPLE In an etching bath using sodium hydroxide etchant at 45 gm/liter and sorbitol chelating compound at 4.35 gm/liter, after 24 hours of operation, analysis of the bath showed 60.0 gm/liter of dissolved aluminum. Analysis of the bath for oxidant was negative. A section of 6063 aluminum alloy extrusion was prepared for etching by immersion in a mild inhibited alkaline cleaner, rinsing in cold water, immersing in an inhibited deoxidizer solution and finally rinsing in cold water. It was then etched in the above solution for 5 minutes at 71"C.

The etched finish was shiny, silver colored, had fair hide of surface defects and was completely covered with a fine grained "galvanized" etch.

At this time 1.0 gm/liter of sodium chlorate was added to the bath. Another section of 6063 aluminum was prepared as above and etched in the bath for 5 minutes at 71 CC. The etched surface produced had a reduced amount of "galvanized" etch but the "galvanized" etch was still visible.

The concentration of the sodium chlorate in the bath was raised to 3 grams per liter and another section of 6063 aluminum, prepared as above, was etched in this bath for 5 minutes at 710C. The surface produced by this etching was completely free of "galvanized" etch.

WHAT WE CLAIM IS: 1. A process according to claim 1 of U.K. Patent No. 1,434,894, modified in that the etching solution is nitrate and nitrite free and contains, as the oxidising agent, sodium chlorate in an amount in excess of 1 gm/litre, but not exceeding 20 gms/litre.

2. A process according to claim 1, wherein the etching solution contains sodium chlorate in an amount not exceeding 10 gms/litre.

3. A process according to claim 2, wherein the etching solution contains sodium chlorate in an amount of from 2-3 gms/litre.

4. A chromate-free aluminium etching solution comprising a nitrate and nitrite free aqueous solution containing 15-200 gms/litre of alkali metal hydroxide 0.5-15 gms/litre of chelating agent effective to suppress the precipitation of alumina; and sodium chlorate in an amount in excess of 1 gm/litre but not exceeding 20 gms/litre.

5. A solution according to claim 4, wherein said solution contains sodium chlo

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. dithiooxamide, d-gluconolactone, and 1-rhamnose. In the etching process the aluminium metal should be carefully prepared for etching by first removing any cutting, rolling or stamping oils and greases. This is usually done by cleaning with an inhibited mild alkaline cleaner. The aluminium metal is then rinsed with water and then deoxidized by immersion or spray application in an inhibited acidic deoxidizer. Following another water rinse, the aluminium metal is ready for etching. The etching process of this invention is usually performed by immersion of the workpiece in the etching solution contained in a steel tank. The tanks are equipped with coils for heating and/or cooling the etching bath. Agitation of the bath is desirable to maintain a uniform concentration of the etching solution, particularly at the interface with the workpiece. Good ventilation is required to dispel the caustic fumes and by-product hydrogen gas. Spraying of the workpieces with the etching solution is another way of practicing the invention. The temperature of the etching solution will generally be in the range of 75C to 1900F. A preferred temperature range is from 140"F to 1700F. The preferred temperature range gives a good etching speed without producing excessive fumes at higher temperatures. The time at which the workpiece is in contact with the etching solution will vary with the condition of the workpiece, that is, the extent of machining marks and other surface irregularities which are desired to be removed. An etching time of about 2 to about 15 minutes covers most of the conditions encountered. Most etching is done in about 3 to about 7 minutes and this is a preferred time range. The sodium chlorate, either in a solid form as a granular or powdered material or as a liquid in aqueous solution can be combined with the sodium hydroxide and chelating compound. A typical solid concentrate would be alkali metal hydroxide - 95%, chelating agent - 4% and sodium chlorate 2-3%. The alkali metal hydroxide can also be supplied as an aqueous concentrate in which the chelating compound and the sodium chlorate are to be dissolved therein. In addition to the alkali metal hydroxide, chelating compound and sodium chlorate the etching baths used in this invention may contain other conventional additives such as, detergents to improve rinsing, to provide a foam blanket on the bath, or to extend the bath life, brightening additives to increase the reflectivity, and fluorides to increase the grain size. Examples of surfactants used in alkaline etching baths are shown in U.S. 3,557,000. The modification of the present invention is illustrated by the following example. EXAMPLE In an etching bath using sodium hydroxide etchant at 45 gm/liter and sorbitol chelating compound at 4.35 gm/liter, after 24 hours of operation, analysis of the bath showed 60.0 gm/liter of dissolved aluminum. Analysis of the bath for oxidant was negative. A section of 6063 aluminum alloy extrusion was prepared for etching by immersion in a mild inhibited alkaline cleaner, rinsing in cold water, immersing in an inhibited deoxidizer solution and finally rinsing in cold water. It was then etched in the above solution for 5 minutes at 71"C. The etched finish was shiny, silver colored, had fair hide of surface defects and was completely covered with a fine grained "galvanized" etch. At this time 1.0 gm/liter of sodium chlorate was added to the bath. Another section of 6063 aluminum was prepared as above and etched in the bath for 5 minutes at 71 CC. The etched surface produced had a reduced amount of "galvanized" etch but the "galvanized" etch was still visible. The concentration of the sodium chlorate in the bath was raised to 3 grams per liter and another section of 6063 aluminum, prepared as above, was etched in this bath for 5 minutes at 710C. The surface produced by this etching was completely free of "galvanized" etch. WHAT WE CLAIM IS:

1. A process according to claim 1 of U.K. Patent No. 1,434,894, modified in that the etching solution is nitrate and nitrite free and contains, as the oxidising agent, sodium chlorate in an amount in excess of 1 gm/litre, but not exceeding 20 gms/litre.

5. A solution according to claim 4, wherein said solution contains sodium chlo

rate in an amount not exceeding 10 gms/litre.

6. A solution according to claim 5, wherein said amount of sodium chlorate is from 2-3 gms/litre.