GB2237032A

GB2237032A - Plating aluminium

Info

Publication number: GB2237032A
Application number: GB9022266A
Authority: GB
Inventors: Eugene F Yarkosky; Patricia A Cacciatore
Original assignee: Enthone Inc
Current assignee: MacDermid Enthone Inc
Priority date: 1989-10-12
Filing date: 1990-10-12
Publication date: 1991-04-24
Anticipated expiration: 2010-10-12
Also published as: IT9067773A1; ES2027496A6; MX171954B; DE4032232A1; NL9002225A; FR2653138B1; GB2237032B; IT1241638B; CH684275A5; FR2653138A1; JPH03236476A; IT9067773A0; GB9022266D0; NL194398B; DE4032232C2; NL194398C

Abstract

A method of producing smooth zinc coatings on aluminium substrates comprises forming a zinc or zinc alloy deposit on the substrate and then employing a nitric acid bath containing a source of Group VIII metal ions, eg ferric ions as a zinc remover. Optionally the step of forming the zinc or zinc alloy deposit on the substrate is repeated. The zinc deposit is preferably formed by electroless deposition. In an alternative embodiment a method of forming a metal coating, other than zinc, on an aluminium substrate having a zinc or zinc alloy coating comprises forming a metal coating by electroless plating using a metal plating composition comprising a source of metal ions, a source of cadmium ions and a reducing agent. The coated product may be used as a memory disc.

Description

1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

28 29 30 31 32 33 METHOD OF PLATING ALUMINI This invention relates to the metal plating of zincated aluminium and, more particularly, to providing an enhanced adhesive and smooth plating by employing an improved double zincating process and, preferably, in combination with a unique electroless metal plating procedure, utilizing a specially formulated electroless metal plating bath.

Metal plating of aluminium is of considerable commercial interest and one application is the preparation of memory disks which are used in a variety of electronic applications such as computer and data processing systems. Aluminium is the preferred substrate for the disk although other suitable metals may be employed. In general, a relatively thin layer of nonmagnetic electroless nickel is applied on the aluminium followed by a thin layer of a magnetic material such as cobalt. A signal is stored on the disk by magnetizing the cobalt layer to represent the signal at a selected moment in time.

Typical alloys used for memory disks are Aluminium Associated Numbers 5086 and 5586. These disks contain magnesium in an amount of about 4% by weight. Generally, the aluminium disks are about 1.25 to 5 mm. thick and contain, by weight, about 4% to 4.90% magnesium, 0.01% to 0.40% copper, 0. 01% to 0.40% zinc, chromium, nickel, iron, silicon and the balance aluminium and inevitable impurities.

A 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 I The completed metal plated disk must be extremely 2 smooth and uniform so as to prevent "crashing" against magnetizing head of the device which flies extremely close (generally 5-8 microinches) to the disk surface. While the starting aluminium substrate must itself be extremely smooth and flat as described in U.S. Patent No. 4,825,680, the metal plating of the disk must likewise be smooth and uniform so that the final disk product meets the exacting specifications required of these type products.

Unfortunately, however, metal plating of a substrate, and even electroless metal plating, does not necessarily produce a smooth coating. Plating voids, inclusions, bridging and the like are only some of the plating problems which can cause a rough disk surface which is unacceptable.

Aluminium and its alloys also present additional plating problems because of the rapidity with which they form an oxide coating when exposed to air. As a result, special treatments must be employed when plating on aluminium. These treatments include mechanical treatments; chemical etches, especially acid etches containing iron, nickel, and manganese salts; alkaline displacement solutions, especially those depositing zinc, brass, and copper; anodizing, especially in phosphoric, sulphuric or chromic acids; and electroplating with zinc at low current densities for a few seconds. of these treatments, the alkaline displacement solutions are generally the most successful commercially.

1 i a 1 3 1 2 3 4 5 6 7 a 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 While many metals can be deposited on aluminium by displacement. zinc is the most common. In this case, the process is known as the zincate process.

Over the years a number of improvements have been made in the conventional zincate formulation and zincating process, with most of them aimed at accelerating the rate of film formation, and the degree of adhesion and uniformity of the zinc coating produced. A detailed summary of the zincating process may be found in Loch, U.S. Patent No. 4,346,128, and Saubestre, U.S. Patent No. 3,216,835.

In Loch, U.S. Patent No. 4,346,128, the process involves soaking the zincated work piece in nitric acid f or f rom one to three minutes (instead o f the usual 20-30 seconds) used to strip the zincate coating. This procedure purportly produces a thin uniform oxide coating on the substrate that serves to further reduce zinc deposition rates and to thereby provide better zincate adhesion for the final (second) zincate coating.

In the conventional zincating process, the aluminium is prepared by alkaline cleaning to remove organic and inorganic surface contaminations such as oil and grease, followed by a cold water rinse. The cleaned aluminium is then sufficiently etched to eliminate solid impurities and alloying constituents which might create voids resulting in bridging of subsequent deposits. After a water rinse, the aluminium is de-smutted to remove metallic residues and aluminium oxides remaining on the surface. Thorough rinsing is 1 It 4 1 required and then the zincate coating is applied using an immersion zinc bath to prevent re-oxidation of the cleaned surface.

2 3 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 The zinc coating is obtained by immersion of the aluminium part in an alkaline solution containing zinc ions. The amount of zinc depends on the time and type of immersion bath used, the aluminium alloy, temperature of the solution and pretreatment process. The zinc coating bath also functions as an etching solution and any oxides reformed during the transferring operations are dissolved by the alkaline zincate while depositing zinc onto the aluminium.

The general procedure now followed by industry is to double zincate whereby the first zinc film is removed using nitric acid followed by application of a second immersion zinc deposit. Double zincating is a preferred method for plating aluminium and is especially useful on certain difficult-to-plate aluminium alloys to ensure better adhesion of the final metal layer deposit.

Double zincate methods for preparing aluminium for metal plating are wellknown in the art as discussed. in general, any aluminium alloy may be treated using the present invention and exemplary alloys are 5086, 5586 and CZ-46. The aluminium may be wrought or cast.

While the specific double-zincate method employed may vary according to the alloys treated and the claimed results, all the methods use a HN03 acid dip to remove the first z incate f ilm and this is the step that the present invention is concerned with.

1 a 1 2 3 4 5 6 7 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 The use of cadmium ih electroless nickel plating baths is described in U. S. Patent No. 2,929,742. Cadmium is disclosed as affecting the hydrogen overvoltage and is considered beneficial to improving the brightness of the deposit. Amounts of up to 100 mg/l cadmium chloride are shown.

Despite the acceptance and effectiveness of the double zincating process, the need still exists for an improved process providing both enhanced adhesion and smoothness of the metal plating on the zincated aluminium substrate. Without being limited to theory, it is believed that the properties of the metal plate are directly related to the thickness, uniformity and continuity of the zincate coating with thinner coatings generally providing a smoother and more adhesive metal plating.

The present invention seeks to provide a method f or preparing aluminium substrate articles that may have extremely smooth metal plated coatings.

The present invention also seeks to provide an improved double zincating process for the metal plating of aluminium, which improved process provides a thinner, more uniform, and continuous zincate coating and produces enhanced adhesive metal plating deposits and metal plating smoothness.

The invention additionally seeks to provide an improved electroless metal plating composition and plating method for coating zincated aluminium substrates with extremely smooth coatings.

1 0 6 1 2 3 4 5 6 7 a 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 31 32 33 34 Other objects and advantages may become apparent from the following description.

Thus according to a first aspect of the present invention there is provided a method of forming a metal coating on an aluminium substrate, the method comprising:

(a) forming a zinc or zinc alloy deposit on the substrate; (b) contacting the substrate with a zinc removing composition containing a source of Group VIII metal ions; and (c) optionally repeating (a).

It is preferred that (c) is not optional; that is to say, the zincating stages (a) and (c) are both included: this is known as a Mouble zincating11 process.

In one, or both, of the zincating stages (a) and/or (C) the zinc or zinc alloy is preferably formed by contacting the substrate with a zinc or zinc alloy plating composition. Such a composition suitably comprises a source of zinc ions, an alkali and optionally a source of metal alloy ions.

Here the term "aluminium", in relation to a substrate, encompasses substrates made only of aluminium, mostly aluminium (Al is the major component) and aluminium alloys (e.g. with any combination of Mg, Cu, Zn, Cr, Ni and/or Fe). Other elements may be present in the substrate e.g. silicon. Preferred substrate are aluminium disks (e.g. memory disks).

1 1 7 1 2 3 4 6 7 a 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 A preferred process according to the present invention comprises:

(a) optionally cleaning an aluminium substrate, such as by contacting the aluminium substrate with an alkaline (and suitably non-etch) cleaner, for example a non-silicated alkaline cleaner; (b) optionally etching the aluminium substrate, such as by contacting the aluminium substrate with a (preferably acidic) etchant; (c) optionally de-smutting the aluminium substrate such as by contacting the aluminium substrate with a (preferably acidic) de-smutting agent, for example one that contains nitric acid and/or sulphuric acid and optionally a fluoride salt; (d) forming a zincate coating on the aluminium substrate (such as by forming a zinc or zinc alloy deposit) such as by using a zinc or zinc alloy plating composition comprising a source of zinc ions, an alkali (preferably a 'hydroxide) and optionally a chelating agent, a wetting agent (e.g. anionic) and/or a metallic additive (such as a source of metal ions other than zinc, such as iron (e.g. Fe3+)); (e) contacting the aluminium substrate with a (zinc removing) composition comprising nitric acid and a source of Group VIII ions (such as ferric ions); (f) optionally repeating stage (d), to apply a zincate coating on the aluminium substrate; (g) forming a metal coating, preferably other than zinc, (such as a nickel or nickel alloy deposit) on the aluminium substrate, preferably by electroless plating, such as for example using a metal (e.g. nickel or nickel alloy) electroless plating composition 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 comprising a source of metal (e.g. nickel) ions, a source of cadmium ions. a reducing agent and preferably a complexing or chelating agent; and (h) optionally repeating (g) to provide an additional metal coating, which is preferably thicker than the coating formed in (g).

It has been found that extremely smooth metal plated aluminium substrates, e.g., memory disks, can be made by preferably employing a special double zincating process in conjunction with an electroless metal plating bath containing an effective amount of cadmium. The double zincating process for preparing aluminium and aluminium alloys for metal plating can be improved by utilizing a specially formulated HN03 bath to strip the first zincate film from the aluminium. The stripped aluminium may then be water rinsed and coated with a second zincate film. Broadly stated, the HN03 bath used to strip the zincate coating preferably comprises a source of at least one Group VIII ion, preferably a source of ferric ions (in an effective amount) for example, present at from 0.1 g/1 to 2 g/1 and HN03 in an amount of from 250 or 350 to 600 g/1 or higher.

Following the zincate procedure, an electroless metal, e.g., nickel, plating of the aluminium can be improved by employing an electroless metal plating bath containing an effective amount of a source of cadmium (ions) to provide an extremely smooth metal coating. Broadly stated, the electroless metal bath preferably comprises:

9 1 2 3 4 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 (1) a source of metal (eg nickel) ions; (2) a reducing agent such as a hypophosphite and/or an amine borane; (3) an acid or alkali (eg hydroxide) pH adjuster to provide the required pH; (4) a complexing agent for the metal ions sufficient to prevent their precipitation in solution; and (5) an effective amount of cadmium ions (to provide the extremely smooth coating of the invention).

Suitably the cadmium level is from 0.1 to 1 mg/1, with a preferred level being from 0.4 to 0.7 mg/1.

The invention may thus be thought of as, in one aspect, broadly residing in the use of a cadmium containing electroless plating bath as hereinbefore described to provide an enhanced smooth coating on a zincated aluminium substrate.

It has been found that the consumption of the cadmium can occur rapidly in the first stages of plating, but at about 10 minutes and thereafter the consumption can be very slow and the presence of the cadmium may not be important during further plating. Thus a preferred mode of operation is to start the plating of the zincated aluminium, substrate in an electroless bath containing an effective amount of cadmium, for example from 0.1 to 1 mg/l, and not to replenish the cadmium until a new zincated substrate is to be plated in the bath.

The electroless metal plating bath suitably also contains a source of metal ions; a reducing agent; a f 1 2 3 4 6 7 8 9 chelator, and/or a pH adjuster. These components are conventionally replenished by measuring the concentration of the component and adding more of the component,, as needed, to maintain the level between desired operating limits. Present plating methods that use automatic controllers which continually measure and replenish the bath components are suitable (a continuous dynamic replenishment process). Other methods, such as manual measuring and replenishment at certain intervals, e.g.,, hourly,, etc., may also be employed.

11 12 13 14 16 17 is 19 21 22 23 24 The invention may also be realised in a method f or depositing a smooth electroless metal coating on a 26 zincated aluminium, substrate comprising coating a thin 27 f irst layer of the metal on the substrate using an 28 electroless metal plating bath containing cadmium, 29 followed by the use of a different electroless metal 30 plating bath to plate the substrate with a second metal 31 coating to the desired thickness, with the thickness of 32 the second metal coating being thicker than the first 33 metal coating.

The invention also relates to a double zincating process for preparing aluminium and aluminium alloys for metal plating wherein the aluminium after pretreatment is zincated by immersion in a zincating bath, the zincated aluminium is then dipped in a nitric bath to remove the zincate coating, f ollowed by immersion in a zincating bath to coat the aluminium and metal plating of the zincated aluminium, the improvement comprising employing in the nitric acid bath an effective amount of a Group VIII ion.

11 1 1 The present invention additionally encompasses a method 2 for depositing a smodth electroless metal coating on a 3 zincated aluminium substrate wherein the zincated 4 substrate is immersed in an electroless metal plating bath to plate a coating of a desired thickness and the 6 bath contains metal ions and a reducing agent which 7 components are controlled within desired operating 8 limits by periodic replenishment the improvement 9 comprising utilizing an effective amount of cadmium in the bath at the start of the plating operation and not 11 replenishing the cadmium to the desired operating level 12_ until the plating operation is finished and/or new, 13 unplated zincated aluminium substrates are immersed in 14 the bath for plating.

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 In a highly preferred embodiment, multiple plating baths can be employed whereby a thin coating of nickel is provided on the zincated surface from an electroless bath containing a source of cadmium ions followed by plating a thicker, final coating (such as also of nickel) from a second (e.g. conventional) electroless plating bath. This preferred process is similar to the process described in U.S. Patent No. 4,567,066 granted to P.B. Schultz and E.F. Yarkosky.

A preferred process of the present invention is described as follows. It should be understood that water rinses are preferably employed after each stage.

A f irst stage in the process is usually to clean the aluminium substrate surface, of grease and oil with an alkaline (non-etch) cleaner, such as ENBOND (R) NS-35 sold by Enthone, Incorporated, West Haven, Connecticut.

12 k_ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 ENBOND NS-35 is a nonsilicated mildly alkaline cleaner. This is suitable for use (i.e. contact with the aluminium substrate) over a temperature range of from 490C to 660C, e.g. 60-630C preferably for 1 to 5 or 6 minutes.

Etching of the cleaned aluminium substrate may then be performed using etchants such as ACTANE (R) E-10, ENBOND E-14 or ENBOND E-24, all of which are sold by Enthone. These materials are either acidic or alkaline. However, an acid etchant is generally preferred, particularly when surface dimensions, tolerances and integrity are important. The etchants are preferably used (i. e. contacted with the aluminium substrate) at elevated temperatures, such as from 490C to 660C, e.g. 60-630C, preferably for from 1 to 3 minutes.

De-smutting of the aluminium substrate may be performed using a HN03 solution (for example 50 by volume) or mixtures of HN03 and H2S04 alone (e.g. a 50/50 solution) or in combination with ACTANE 70 sold by Enthone. ACTANE 70 is an acidic, fluoride salt product containing ammonium bifluoride. A preferred de-smutting solution contains 25 by volume H2S04, 50 by volume HN03 and 1 lb/gallon ACTANE 70 in water. This may be performed at about room temperature for from 30-90 seconds, e.g. about 1 minute.

It is at this point that a zincate coating can be applied to the aluminium by immersion in a zincate bath as described in Saubestre, U.S. Patent No. 3,216,835, supra. A preferred bath, due to its demonstrated 1 13 2 31 32 33 1 effectiveness, is ALUMON (R) EN sold by Enthone. ALUMON EN contains an alkali metal hydroxide, a zinc compound or salt (such as zinc oxide and/or zinc sulphate), a chelating agent and optionally an anionic wetting agent and/or a metallic additive (e.g. Fe3+ ions). The composition is preferably at about room temperature when contacted with the aluminium substrate.

3 4 6 7 8 9 An article by D.S. Lashmore entitled "Immersion 11 coatings on Aluminium", Plating and Surface Finishing, 12 67, pages 36-42 (1980) shows the use of iron (e.g., 13 ferric chloride) in the zincate solution to deposit 14 iron along with the zinc and produce a more adhesive zincate coating which can be very resistant and 16 comparatively insoluble in HN03. ALUMON (R) EN and 17 other commercial zincate solutions contain iron.

is 19 20 21 22 23 24 25 26 27 28 29 30 Generally, a double zincate process involves immersion of the aluminium substrate in a (dilute) zincate plating composition or bath (such as an ALUMON (R) EN) for a period of from 20-50 seconds, suitably followed by a thorough cold water rinse, a zinc removing operation with nitric acid, preferably a further cold water rinse, and a second zincate immersion and advantageously a subsequent rinse.

In contrast to the Loch procedure, the present invention involves the use of a source of Group VIII ions, e.g., ferric ions, in the nitric acid bath. This accomplishes a similar result of reducing zinc deposition rates while providing a zincate coating which may be very adhesive, uniform and continuous and 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 on which an extremely smooth metal plating may be coated.

The nitric acid composition used to strip the (e.g. first) zincate coating is preferably a 50% by volume solution. This may have a concentration range of from 350 to 600 g/1, and preferably from 450 to 550 g/1. The composition also contains a source of Group VIII ions, preferably a source of ferric ions. This is suitably provided in an amount of from 0.1 g/1 to 1 or 2 g/1, preferably 0.3 g/1 to 0.8 g/1 and most preferably 0.4 g/1 to 0.6 g/1. At levels below about 0.1 g/1, minimal effects may be obtained while at levels below about 0.1 g/1, minimal effects may be obtained while at levels above about 2 g/1, the surface topography may be severely affected.

The nitric acid zinc removing composition may be employed (that is to say contacted with the aluminium substrate) at any suitable temperature, although it is preferred that this is from 200C to 250C or higher but preferably 210C to 230C. Contact (suitably immersion) times suitably range from 30 to 90 seconds, preferably from 40 to 60 seconds.

Exemplary Group VIII ions which may be employed include iron, nickel and cobalt. Ferric ions are particularly preferred.

It will be understood by those skilled in,the art that the concentration, composition temperature and contact (immersion) time are interrelated and that, in general, the higher the temperature and concentration, the 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 shorter the contact (immersion) time necessary to achieve the desired surface ef fect, and the use of Group VIII ions in the bath may provide enhanced adhesion and smoothness of the metal plating.

While other metals may now be plated on the specially prepared zinc coated aluminium substrate, the following description, although applicable to any suitable metal, may specifically reference nickel, as illustrative because of its commercial importance.

Electroless nickel plating compositions f or applying nickel coatings (that is to say, nickel and nickel alloy deposits) are well known in the art and plating processes and compositions are described in numerous publications. For example, compositions for depositing electroless nickel are described in U. S. Patent Nos. 2,690,401; 2,690,402; 2,762,723; 2,935, 425; 2,929,742; and 3,338,726. Other useful compositions for depositing nickel and nickel alloys are disclosed in the 35th Annual Edition of the Metal Finish Guidebook for 1967, Metal and plastics publications Inc., Westwood, N.J., pages 483-486.

Preferred electroless metal, e.g. nickel, electroless, plating compositions are aqueous and suitably comprise at least four ingredients dissolved in a solvent, typically water. They are:

(1) a source of metal, e.g. nickel, ions; (2) a reducing agent such as a hypophosphite, borohydride, hydrazine or an amine borane; (3) an acid or hydroxide pH adjuster to provide the required pH; 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 is 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 (4) preferably a complexing or chelating agent for the metal ions, suitably in a sufficient amount to prevent metal ion precipitation in solution; and (5) a source of cadmium ions.

A large number of suitable complexing agents for electroless nickel solutions are described in the above noted publications. It will be appreciated by those skilled in the art that the nickel, or other metal being applied, will usually be in the form of an alloy with other materials present in the bath. Thus,, if hypophosphite is used as the reducing agent, the deposit will usually contain nickel and phosphorous. Similarly, if an amine borane is employed, the deposit will usually contain nickel and boron. Thus, use of the term "nickel" includes the other elements, such as S and P, normally deposited therewith.

The source of metal, e.g. nickel, ions may be provided by any suitable soluble salt such as a sulphate, chloride, acetate and mixtures thereof. The concentration of the metal, such as nickel, in solution may vary widely and is preferably from 0.1 to 100 g/1, such as from 2 to 50 g/1, e. g. 2 to 10 or 30 g/1.

The reducing agent, especially when the aluminium substrate is a memory disk, is usually the hypophosphite ion. This may be supplied to the bath by any suitable source such as an alkali metal (e.g. sodium or potassium) ammonium, and/or metal, eg, nickel, hypophosphite. Other reducing agents such as amine boranes, borohydrides and hydrazine may also suitably be employed. The concentration of the reducing agent is preferably in excess of the amount 1 17 1 2 3 4 5 6 7 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 sufficient to reduce the amount of metal ions, e.g. nickel present in the'bath.

The metal plating composition can be acid, neutral or alkaline. An acid or alkaline pH adjuster may be selected from a wide range of materials such as hydroxides (e.g. ammonium hydroxide or alkali metal, e.g. sodium, hydroxide) and mineral acids, e.g. hydrochloric acid, and the like. The pH of the bath may range from 2 to 12; acids baths are preferred. A pH range of 4 to 5, e. g., 4. 3 to 4. 6, is preferred. both for the cadmium contained in the bath when depositing the metal coating on the zincate layer, such as depositing a final layer of nickel when the cadmium containing bath is used to provide only a thin, strike coating.

The complexing (or chelating) agent may be selected from a wide variety of materials such as carboxylic acids, lactic acid, malic acid and those containing anions such as acetate, citrate, glycollate, pyrophosphate and the like, with mixtures thereof also being suitable. Ranges for the complexing agent, based on the anion, may vary widely, for example, from 1 to 300 g/1, preferably from 5 to 50 g/1.

The electroless metal plating compositions may also contain other ingredients known in the art such as buffering agents, bath stabilizers, rate promotors, brighteners, etc. Stabilizers such as lead, antimony, mercury. tin and oxy compounds such as iodate may also be employed.

18 1 A suitable plating composition may be formed by 2 dissolving the ingredients in water and adjusting the 3 pH to the desired range.

4 The zinc coated aluminium substrate may be plated with 6 the electroless metal, e.g. nickel-cadmium, to the 7 desired final thickness. Preferably, the substrate is 8 contacted with the plating composition by immersion in 9 the composition. One may then plate a thin metal, e.g.

nickel, coating adequate to provide a suitable base for 11 the extremely smooth thick deposits of a final metal, 12 e.g. nickel, plate, for example using a different 13 electroless metal, e.g. nickel, plating composition.

14 The metal deposited using the metal plating composition 16 (or first metal coating if there are two) preferably 17 has a thickness range up to about 0.1 mil, or higher, 18 with from 0.005 to 0.08 mils, e.g. from 0.01 to 0.05, 19 being preferred. An immersion time of 15 seconds to 15 20 minutes will usually provide the desired coating, 21 depending on composition parameters. A temperature 22 range of from 250C to boiling, e. g., 100oC, may be 23 employed, with a range of from 300C to 950C being 24 preferred.

26 A next stage in the preferred procedure is to complete 27 the metal plating to the desired thickness and physical 28 characteristics by immersing the metal, e.g. nickel 29 coated substrate in another electroless nickel plating 30 bath, (which may be the same or different, from that 31 used to form the first metal coating) which is 32 preferably maintained at a temperature ranging from 33 300C to 1000C, e.g. boiling, preferably from 800C to 1 1 19 1 2 3 4 5 6 7 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 950C. A thickness up to 5 mils, or higher may be achieved, with a range of from 0. 1 to 2 mils being preferable for most applications. When this "strike bath" process is used, it is preferred not to rinse the strike coated aluminium substrate before immersion of the substrate in the next, i.e. second and usually final plating composition. The cadmium ions may be provided by the use of any soluble cadmium source

such as a cadmium salt, e.g. cadmium sulphate. It is important to control the cadmium concentration to obtain the extremely smooth coatings of the invention and a suitable effective level is from 0.1 to 1 mg/1, preferably 0.3 to 0.8 mg/1 and most preferably 0.5 to 0.7 mg/1. Amounts as high as 2 or 3 mg/1 or higher may be used f or certain applications not requiring as smooth a surface as for memory disks.

It will be appreciated by those skilled in the art that the rate of plating may be influenced by many factors including:

(1) pH of the plating solution; (2) concentration of reductant; (3) the temperature of the plating composition; (4) concentration of soluble metal, e.g. nickel; (5) ratio of the volume of composition to the area plated; (6) presence of soluble fluoride salts (rate promoters); and (7) presence of wetting agent and/or agitation, and that the above parameters are only provided to give general guidance for practising the invention.

1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 A second aspect of the present invention relates to a method of forming a metal coating on an aluminium substrate, the method comprising forming a metal (other than zinc) coating on an aluminium substrate having a zinc or zinc alloy coating, the metal coating being formed by electroless plating using a metal plating composition comprising a source of metal ions, a source of cadmium ions and a reducing agent.

The aluminium substrate, preferably has the zinc or zinc alloy coating formed by a method of the first aspect, and so preferred features and characteristics of the first aspect are applicable to the second aspect mutatis mutandis.

The method of forming the metal coating may be repeated more than once, for example, twice. In this latter case it is preferred that the second metal coating is thicker than the first.

Although the cadmium may be replenished periodically, it is not replenished (such as to a desired operating level) until the plating (operation) is completed and/or new aluminium substrates (zincated ones) are contacted with the composition (such as by immersion) for electroless plating.

A third aspect of the present invention relates to an aluminium substrate having a metal coating formed according to a method of the first and/or second aspect. Preferred features and characteristics for the third aspect are as for the first and/or second aspect mutatis mutandis.

21 1 The invention may be illustrated by way of example with 2 reference to the accompanying drawings, in which:

3 4 FIGURES IA and 2A (PRIOR ART) are photomicrographs at 5 magnification 50OX of electrolessly nickel plated 6 aluminium substrates which were prepared for plating 7 using a conventional, prior art double zincate process; 8

9 FIGURES IB and 2B are photomicrographs at 10 magnification 50OX of electrolessly nickel plated 11 aluminium substrates which were prepared for plating 12 using a double zincate process according to the present 13 invention; 14 FIGURE 3 is a graph showing that a nitric acid 16 stripping solution according to the present invention 17 (containing Fe... ions) removes more zinc from a is zincated aluminium substrate than a conventional, prior 19 art zihcate nitric acid solution; 21 FIGURE 4 is a graph showing that aluminium substrates 22 prepared according to the present invention have less 23 zinc on the surface to be metal plated (a thinner 24 coating) than substrates prepared using a conventional, 25 prior art, double zincate process;

26 27 FIGURE 5A is a photomicrograph at magnification 50OX 28 of an electrolessly nickel plated aluminium substrate 29 using a conventional, prior art, zincating and plating 30 procedure; and

31 32 FIGURES 5B, 5C and 5D are photomicrographs at 33 magnification 50OX of an electrolessly nickel plated 22 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 aluminium substrata using a zincating and/or plating process according to the present invention.

The invention will now be described by way of example, with reference to the following Examples, which are provided for the purposes of illustration and are not intended to be construed as being limiting on the present invention. In the following specific examples (which are illustrative and in no way limitative) all parts and percentages are by weight and temperatures in degrees centigrade unless otherwise specified.

COMPARATIVE EXAMPLE IA AND EXAMPLE IB IA. CZ-46 aluminium alloy disks were double zincated and plated with electroless nickel using the following procedure (a cold water rinse followed each of the steps):

(1) Immerse in ENBOND NS-35 (alkaline cleaner) for 3 minutes at 60OC; (2) Immerse in ACTANE E-10 (etchant) for 1 minute at 60OC; (3) Immerse in 50% by volume HN03 for 1 minute at room temperature (to de- smut); (4) Immerse in ALUMON EN (first zincate) for 35 seconds at room temperature; (5) Immerse in 50% by volume HN03 (zinc remover) for 1 minute at room temperature; (6) Immerse in ALUMON EN (second zincate for 16 seconds at room temperature; (7) Immerse in ENPLATE ADP-300 (nickel plating composition) for 1 hour at 84-870C (pH 4.5 0.1).

7 23 f 2 3 4 5 6 7 a 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 ENPLATE ADP-300 is ap acidic based (pH4.6) electroless nickel plating bath containing, in g/1, nickel sulphate hexahydrate (26), sodium hypophosphite (20), sodium lactate (60%) (71), malic acid (11.8),, sodium hydroxide (4.6), potassium iodate (0.015), lead nitrate (0.0003) and an anionic surfactant (0.02).

FIGURE 1A shows the nickel surface resulting from using the above conventional, prior art, double zincating procedure.

IB. When the same procedure was used except that ferric ions (as ferric chloride) were added to the HN03 in step (5) at a level of 0.5 g/1 Fe..., a markedly smoother nickel surface was obtained as shown in FIGURE 1B.

COMPARATIVE EXAMPLE IIA AND EXAMPLE IIB IIA. A procedure similar to COMPARATIVE EXAMPLE I was conducted on 5586 aluminium alloy disks as follows:

(1) Immerse in ENBOND NS-35 (alkali cleaner) for 5 minutes at 630C; (2) Immerse in ACTANE E-10 (etchant) for 2 minutes at 630C; (3) Immerse in 50% by volume HN03 for 1 minute at room temperature (to de-smut); (4) Immerse in ALUMON EN (first zincate) for 45 seconds at room temperature; (5) Immerse in 50% by volume HN03 (zinc remover) for 30.seconds at room temperature; (6) Immerse in ALUMON EN (second zincate) for 15 seconds at room temperature;.

1 24 1 2 3 4 5 6 7 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 (7) Immerse in ENPLATE ADP-300 (nickel plating composition) for 2 hours at 84-870C (pH 4.5 + 0.1).

FIGURE 2A shows the nickel surface resulting from using the above conventional, prior art, double zincating procedure.

IIB. When the same procedure was used except that f erric ions were added to the HN03 in step (5) at a level of 0. 5 g/1. a markedly smoother nickel surface was obtained as shown by FIGURE 2B.

COMPARATIVE EXAMPLE IIIA AND EXAMPLE IIIB IIIA. The procedure of COMPARATIVE EXAMPLE I (steps (1) - (4) was followed to prepare a number of zincated CZ-46 aluminium alloy disks.

The disks were chosen at random and a total of 40 ft2 was stripped at room temperature for each HN03 bath tested. The control HN03 was 50% by volume.

I1IB. Here an HN03 composition of the present invention was used which was 50% by volume and contained 0.50 g/1 ferric ions (supplied as ferric chloride).

FIGURE 3 shows the amount of zinc coating removed per square feet of disk stripped and the results clearly show that the HN03 composition containing ferric ions of the invention removes more of the zinc coating than a conventional HN03 solution. This is important because less zinc is introduced to the plating solution.

1 2 3 4 COMPARATIVE EXAMPLE IVA AND EXAMPLE IVB IVA. This Example demonstrates that less zinc is deposited on the substrate to be metal plated when using the double zincate method of the invention as 6 compared to the conventional zincate process.

7 a 9 10 11 12 13 14 is 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 CZ-46 aluminium alloy disks were treated using steps (1) - (4) of the procedure of COMPARATIVE EXAMPLE I. one group of the disks were randomly selected and immersed in a conventional, prior art, KN03 solution (50% by volume) for 1 minute at room temperature.

IVB. The other group of disks were immersed in a 50% by volume HN03 solution containing 0.5 g/1 ferric ions, according to the present invention (supplied as ferric chloride) for the same length of time and temperature.

Both groups of disks were then immersed in a second zincate bath (as in step (6) of COMPARATIVE EXAMPLE I) for either 10, 20, 30, 40, 50 or 60 seconds at room temperature. The zincated disks were then stripped in 50% by volume HN03 and the amount of zinc deposited on the disk determined by Atomic Absorbance Spectrophotography.

FIGURE 4 shows that less zinc was deposited on the disks when using the method of the invention as compared to the conventional double zincate method. This is important because there is less surface disturbance and consequently a thinner, but denser coating of zinc is obtained. The iron apparently acts as an inhibitor thus retarding and thereby controlling 26 1 2 3 4 5 6 7 a 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 the dissolution of aluminium by zinc. Furthermore, the thinner deposits of zinc will not contaminate the subsequent electroless nickel bath as rapidly.

COMPARATIVE EXAMPLE VA AN12 EXAMPLE VB VA. Alumium 5586 alloy disks were plated using the procedure of COMPARATIVE EXAMPLE I to a thickness of about 0.40 mils. All the tests were run under the same plating conditions of 840C, pH 4.6, a work load of 0.31 ft2/gal, continuous filtration and plating time of 2 hours. The nickel, pH and sodium hypophosphite were continuously replenished over the 2 hour plating time using an automatic controller.

FIGURE 5A represents the nickel surface resulting from using the above conventional zincating and plating procedure.

VB. FIGURE 5B represents the nickel surface resulting from using the above procedure with the exception that 0.5 g/1 ferric ions (as ferric chloride) were added to the nitric acid (step (5)).

FIGURE SC represents the nickel surface resulting from the above procedure with the exception that 0.75 ng/1 cadmium was added to the nickel plating bath before plating and not replenished during the 2 hour plating time.

FIGURE 5D represents the nickel surface resulting from using the above procedure with the exception that 0.5 g/1 ferric ions (as ferric chloride) were added to the Q j A 27 1 2 3 4 5 nitric acid (step 5) and 0.75 ag/1 cadmium was added to the nickel plating bath and not replenished during the two hour plating time.

As can clearly be seen from the figures, the conventional process produces a rough surf ace having many nodules. Figures 5B and 5C show the beneficial effects of using ferric ions and cadmium, respectively and Figure 5D shows the extremely smooth surface provided using the preferred method of the invention.

6 7 8 9 11 12 13 14 15 16 17 is 19 21 22 23 24 26 27 28 29 30 31 32 33 1 1 1 i 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Claims

CLAIM$

1. A method of forming a metal coating on an aluminium substrate. the method comprising:

(a) forming a zinc or zinc alloy deposit on the substrate; (b) contacting the substrate with a zinc removing composition containing a source of Group VIII metal ions; and optionally repeating (a).

2. A method as claimed in claim 1 wherein (c) is not optional.

3. A method as claimed in claim 1 or 2 wherein in (a) and/or (c) the zinc or zinc alloy is formed by contacting the substrate with a zinc or zinc alloy plating composition.

4. A method as claimed in claim 3 wherein the plating composition suitably comprises a source of zinc ions, an alkali and optionally a source of metal alloy ions.

5. A method as claimed in any of claims 1 to 4 wherein the zinc removing composition contains nitric acid.

6. A method as claimed in any of claims 1 to 5 where the Group VIII metal is ion, cobalt or nickel.

R 4 A 1 k 29 1 2 3 4 5 6

7 8 9 10 11 12 13 14 is 1,6 17 is 19 21 22 23 24 25 26 27 28 29 30 31 32 33 7. A method as cl ' aimed in any of claims 1 to 6 wherein the Group VIII metal ion is ferric ion.

8. A method as claimed in any of claims 1 to 7 wherein the Group VIII metal ion is provided in an amount of from 0.1 to 1.0 g/1.

9. A method as claimed in any of claims 1 to 8 wherein the zinc removing composition comprises 50% by volume nitric acid.

10. A method as claimed in any of claims 3 to 9 wherein the plating composition contains an hydroxide.

11. A method as claimed in claim 10 wherein the hydroxide is an alkali metal hydroxide.

12. A method as claimed in any of claims 3 to 11 wherein the plating composition contains an anionic wetting agent.

13. A process as claimed in any of claims 1 to 11 comprising:

(a) optionally cleaning an aluminium substrate with a non-etch cleaner; (b) optionally etching the aluminium substrate; (c) optionally de-smutting the' aluminium substrate; 4 A 1 2 3 4 6 7 a 9 10 11 12 13 14 15 16 17 is 19 20 21 22 23 24 25 26 27 28 29 31 32 33 (d) forming a zinc or zinc alloy deposit on the aluminium substrate; (e) contacting the aluminium substrate with a zinc removing composition containing nitric acid and a source of Group VIII metal ions; (f) forming a zinc or zinc alloy deposit on the aluminium substrate; (g) forming a metal coating (other than zinc) on the aluminium substrate for electroless plating using a metal plating composition comprising a source of metal ions, a source of cadmium ions and a reducing agent; and (h) optionally repeating (g) to provide a different metal coating, which is preferably thicker than the coating formed in(g).

14. A method as claimed in claim 13 wherein the non-etch cleaner is alkaline.

15. A method as claimed in claim 13 or 14 wherein the non-etch cleaner is a non-silicated alkaline cleaner.

16. A method as claimed in any of claims 13 to 15 wherein the etchant is acidic.

17. A method as claimed in any of claims 13 to 17 wherein the desinutting agent contains nitric acid and/or sulphuric acid and optionally a fluoride salt.

. 0 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 18. A process as claimed in any of claims 13 to 17 wherein (a), (b), (c) and (g) are not optional.

19. A method -of f orming a metal coating on an aluminium substrate, the method comprising forming a metal coating (other zinc) on an aluminium substrate having a zinc or zinc alloy coating, the metal coating being formed by electroless plating using a metal plating composition comprising a source of metal ions, a source of cadmium ions and a reducing agent.

20. A method as claimed in claim 19, wherein the cadmium is provided at a level of from 0.1 to 1.0 mg/1.

21. - A method as claimed in claim 19 or 20 when the aluminium substrate has a zinc or zinc alloy coating is prepared according to a method of any of claims 1 to 17.

22. An aluminium substrate having a metal coating prepared according to a method as claimed in any of claims 1 to 18 or prepared according to a method as claimed in any of claims 19 to 21.

23. An aluminium substrate having a metal coating formed according to a method substantially as herein described with reference to any of the Examples Ib, IIb, IIIb, M or Vb.

24. A method of forming a metal coating on an aluminium substrate substantially as herein described with reference to the Examples lb, IIb, IIIb, M or Vb.

Published 1991 atIbe Patent Office. State House. 66/71 High HoIborn. Landon WCIR 47P. Further copies may be obtained from Sales Branch, Unit 6, Nine Mile Ploint. Cwmfelinfach, Cross Keys, Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray. KenL