NL9002225A - METHOD FOR PREPARING ALUMINUM MEMORY DISCS WITH A SMOOTH METAL COATING. - Google Patents

Description

Title: Process for the preparation of aluminum memory discs with a smooth metal cover.

This invention relates to galvanized aluminum sheet metal plating and, more particularly, to provide an improved adhesive and smooth metal sheet plating using an improved double galvanizing process and preferably in combination with a unique electroless metal plating method, using a specially formulated electroless metal plating bath.

Metal plating of aluminum is of considerable commercial interest and one application is in the manufacture of memory discs used in a variety of electronic applications such as computer and data processing systems. Aluminum is the preferred substrate for the disc, although other suitable metals can be used. Generally, a relatively thin layer of nonmagnetic electroless nickel is applied to the aluminum 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 display the signal at the chosen time.

Typical aluminum alloys used for memory disks have classification numbers 5086 and 5586. These disks contain magnesium in an amount of about 4% by weight. Generally, the aluminum discs are about 1.25 to 5 mm thick and contain about 4 to 4.90 wt% magnesium, 0.01 to 0.40 wt% copper, 0.01 to 0.40% zinc, chromium, nickel, iron, silicon and otherwise aluminum and unavoidable impurities.

The finished metal-plated disc must be extremely smooth and uniform to prevent "crashing" against the magnetizing head of the device which floats extremely close (generally 5-8 microinches) over the disc surface. Because the starting aluminum substrate itself must be extremely smooth and flat as described in U.S. Patent No. 4,825,680, the metal plating of a disc must likewise be smooth and uniform so that the final product meets the exact specifications required for this type of products.

Unfortunately, metal plating of a substrate, and even electroless metal plating, does not necessarily give a smooth coating. Plating cavities, inclusions, couplings and the like are just some of the plating problems that can cause a rough disc surface, which is not acceptable.

Aluminum and its alloys also exhibit additional plating problems due to the rate at which they form an oxide coating when exposed to air. As a result, special treatments must be used when plating on aluminum is to take place. These treatments include mechanical treatments; chemical etchants, especially acidic etchants containing iron, nickel and manganese salts; basic displacement solutions, especially those that precipitate zinc, bronze, and copper; anodizing, especially in phosphorus, sulfur and chromic acids; and electroplating with zinc at low current densities for a few seconds.

Of these treatments, the basic displacement solutions are the most commercially successful.

Although many metals can be deposited on aluminum by displacement, zinc is the most common. In this case, this process is known as the galvanizing process. Over the years, a number of improvements have been made over the conventional zincate formulation and the zincate process, most of which aimed to accelerate the degree of film formation, and the degree of adhesion and uniformity of the zinc coating produced. A detailed listing of the zincate process can be found in Loch, U.S. Patent No. 4,346,128 and Saubestre, U.S. Patent No. 3,216,835, which patents are hereby incorporated by reference. In the conventional zincate process, the aluminum is prepared by basic cleaning to remove organic and inorganic surface contaminants such as oil and grease, followed by a cold water rinse. The cleaned aluminum is then etched sufficiently to eliminate solid impurities and alloying constituents that may create cavities that result in subsequent deposits bridging. After a rinse with water, the aluminum is burned clean to remove metallic residues and aluminum oxides that still remain on the surface. Thorough rinsing is required and then the zincate coating is applied using an immersion zinc bath to prevent re-oxidation of the cleaned surface.

The zinc coating is obtained by immersion of the aluminum part in a basic solution containing zinc ions. The amount of zinc deposited is actually very small and depends on the time and type of immersion bath used, the aluminum alloy, temperature of the solution and the pretreatment process. The zinc coating bath also functions as an etching solution, and all oxides reformed during the transfer operations are dissolved by the basic zincate, depositing zinc on the aluminum.

The procedure now generally followed by industry is double galvanizing where the first zinc film is removed with nitric acid followed by the application of a second immersion zinc deposit. Double galvanizing is a preferred method of aluminum plating and is particularly useful on certain hard-to-coat aluminum alloys to ensure better adhesion of the final metal layer deposit.

Despite the acceptance and effectiveness of the double galvanizing process, there is still a need for an improved process that provides both improved adhesion and smoothness of the metal plating on the galvanized aluminum substrate. Without being limited to theory, it should be assumed that the properties of the metal coating 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.

It is an object of the present invention to provide a method of preparing aluminum substrate articles with extremely smooth metal plated coatings.

It is a further object of the present invention to provide an improved double galvanizing process for metal plating aluminum, which improved process provides a thinner more uniform and continuous zincate coating and produces an improved adhesion of metal plating deposits and metal plating smoothness.

Another object of the invention is to provide an improved electroless metal plating composition and plating method for coating galvanized aluminum substrates with extremely smooth coatings.

Other objects and advantages will become apparent from the following detailed description.

It has now been found that aluminum substrates with an extremely smooth metal plating, for example, memory discs can be made, preferably using a special double galvanizing process in conjunction with an electroless metal plating bath containing an effective amount of cadmium. The double galvanizing process for the preparation of aluminum and aluminum alloys for metal plating has been improved by using a specially formulated HNO3 bath to strip the first zincate film from the aluminum.

When conventional procedures are followed, the stripped aluminum is then rinsed with water and coated with a second zincate film. The metal is plated on this second zincate film. Broadly stated, the HNO3 bath used to strip the zincate coating includes Group VIII ions and, preferably Iron III ions, in an effective amount, for example, from about 0.1 to 2 g / L and HNO3 in an amount of about 250 or 350 to 600 g / l. or higher.

Following the zincate procedure, the electroless metal, for example nickel, plating of the aluminum is improved by using an electroless metal plating bath containing an effective amount of cadmium to provide extremely smooth metal coatings. Broadly stated, the electroless metal bath (1) contains a source of metal ions, (2) a reducing agent such as hypophosphite or an amine borane, (3) an acid or hydroxide pH adjuster to provide the required pH, (4) a complexing metal ion agent sufficient to prevent their precipitation in the solution and (5) an effective amount of cadmium ions to provide the extremely smooth coating of the invention. Generally, the cadmium level will be about 0.1 to 1 mg / l, with a preferred level of about 0.4 to 0.7 mg / l.

It has been found that the consumption of the cadmium occurs quickly in the first steps of the plating, namely 10 min., After which the consumption is very slow and the presence of the cadmium is not important during the further plating. A preferred form of operation consists of starting the plating of the galvanized aluminum substrate in an electroless bath containing an effective amount of cadmium of about 0.1 to 1 mg p / 1, and not replenishing the cadmium until new galvanized substrates are plated. should be in the bath. The electroless metal plating bath contains metal ions, reducing agents, chelators, etc., and 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 the desired processing limits. New plating methods use automatic controllers that measure continuously and replenish the bath components. Other methods such as measuring by hand and topping up at certain intervals, for example every hour etc. can also be used.

In a highly preferred embodiment, multiple plating baths are used where a thin nickel coating is provided on the galvanized surface of an electroless bath containing cadmium ions followed by plating a thicker, final coating from a second conventional electroless plating bath. This preferred method is similar to the method described in U.S. Patent No. 4,567,066 assigned to P.B. Schultz and E.F. Yarkosky, which patent is hereby incorporated by reference.

Figures 1A and 2A are micrographs of 500X of electroless nickel plated aluminum substrates prepared for plating by a conventional double zinc plating process.

Fig. 1B and 2B are micrographs of 500X of electroless nickel plated aluminum substrates prepared for plating by the double zinc plating process of the present invention.

Fig. 3 is a graph showing that the nitric acid stripping solution of the invention (containing iron III ions) removes more zinc from a galvanized aluminum substrate than a conventional zincate nitric acid solution.

Fig. 4 is a graph showing that aluminum substrates prepared according to the invention have less zinc on the surface to be plated with metal (have a thinner coating than substrates prepared using a conventional double zincate process).

Fig. 5A, 5B, 5C and 5D are micrographs of 500X of electroless nickel plated aluminum substrate using different galvanizing and plating procedures.

The double settling method of preparing aluminum for metal plating is well known in the art as discussed above. Generally, any aluminum alloy can be treated using the method of the invention, and preferred alloys are 5086, 5586 and CZ-46. The aluminum can be forged or cast.

While the specific double galvanizing method used may vary with the alloys to be treated and the claimed results, all of these methods use an HNO3 acid dip to remove the first zincate film and this is the step to which the present invention is directed. A typical procedure used in industry is as follows and it should be understood that water rinses are generally used after each process step.

The first step is generally to clean the aluminum surface of grease and oil and a basic non-etch cleaner such as ENBOND (R) NS-35 sold by Enthone, Incorporated, West Haven, Connecticut can be used appropriately. ENBOND NS-35 is a non-silicified mild base cleaner that can be used in a temperature range of about 49 to 66 ° C for 1 to 5 minutes.

Etching of the cleaned aluminum can 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 basic. The acid etchant is generally preferred especially when surface dimensions, tolerances and integrity are important. The etchants are generally used at elevated temperatures of about 49 to 66 ° C for 1 to 3 minutes.

The alloy burn-out can be performed using an HNO3 (e.g. 50% by volume) or mixtures of HNO3 and H2SO4 alone or in combination with ACTANE 70 sold by Enthone. ACTANE 70 is an acid fluoride salt product containing ammonium bifluoride. A typical clean burn solution contains 25 vol.% H2SC> 4, 50 vol.% HNO3 and 1 pound p / gallon ACTANE 70 in water.

Currently, a zincate coating is applied to the aluminum by immersion in a zincate bath as described in Saubestre, U.S. Patent No. 3,216,835 mentioned above. A bath preferred for its proven effectiveness is ALUMON (R) EN sold by Enthone. ALUMON EN contains an alkali metal hydroxide, a zinc salt (such as zinc oxide, zinc sulfate, etc.), a gelling agent, optionally anionic wetting agents and metallic additives. An article by DS Lashmore entitled "Immersion Coatings on Aluminum", Plating and Surface Finishing, 67, 36-42 (1980) shows the use of iron (eg, iron III chloride) in the zincate solution to precipitate iron along with zinc and a produce more adhesive zincate coating, which is very resistant and relatively insoluble in HNO3. ALUMONS other commercial zincate solution contain iron.

In general, the double zincate process involves immersion of the aluminum substrate in a dilute zincate bath such as an ALUMON (R) EN for a period of 20-50 seconds followed by a thorough rinse with cold water, a zinc stripping operation in nitric acid, another rinse with cold water , and a second zincate immersion and then rinse. As noted in Loch, U.S. Patent No. 4,346,128, cited above, the improved Loch process permeates the galvanized workpiece in nitric acid from 1 to 3 minutes instead of the usual 20 to 30 seconds used to strip the zincate coating. This procedure apparently produces a thin uniform oxide coating on the substrate which serves to further reduce zinc deposition amounts thereby providing better zincate adhesion for the final (second) zincate coating.

In contrast to the Loch procedure, the use of Group VIII ions, for example iron III + ions, in the nitric acid bath achieves the same result of reducing the zinc deposition rates, while providing a zincate coating which is very adhesive, uniform and continuous and which has a extremely smooth metal plating can be applied.

With regard to the improvement of the invention, the nitric acid solution used to strip the first zincate coating is generally a 50% by volume solution with a concentration range generally about 350 to 600 g / l and preferably about 450 to 550 g / 1. The improved solution also contains group VIII ions, preferably iron III ions, in the amount of about 0.1 to 1 or 2 g / 1, preferably 0.3 to 0.8 g / 1, and most preferably 0.4 to 0 , 6 g / 1. At levels below about 0.1 g / l, minimal effects are obtained, while at levels above about 2 g / l, the surface topography can be seriously affected.

The nitric acid solution can be used at any suitable temperature, usually about 20 to 25 ° C or higher, and preferably 21 to 23 ° C. Immersion times can range from about 30 to 90 seconds, and preferably about 40 to 60 seconds.

Examples of group VIII ions that can be used include iron, nickel and cobalt. Iron III ions are particularly preferred.

The person skilled in the art will understand that the concentration, solution temperature and immersion time are interrelated and that generally the higher the temperature and concentration, the shorter the immersion time will be necessary to obtain the desired surface effect, the invention being based on the use of group VIII ions in the bath to obtain the improved adhesion smoothness of the metal plating.

While other metals can now be plated on the specially prepared zinc-coated aluminum, the following description will focus specifically on nickel because of its commercial importance.

Electroless nickel plating compositions for applying nickel coatings are well known in the art, and plating methods and compositions are described in various publications. For example, electroless nickel precipitation compositions are described in U.S. Patent Nos. 2,690,401; 2,690,402; 2,7762723; 2,935,425; 2,929,742; and 3,338,726. Other useful compositions for the precipitation of nickel and its alloys are disclosed in The 35th Annual Edition of the Metal Finish Guidebook for 1967, Metal and plastics publications Ine., Westwood, N.J., 483-486. Each of the previous publications is hereby incorporated by reference.

Generally, electroless nickel deposition solutions comprise at least four ingredients dissolved in a solvent, typically water. These are (1) a source of the nickel ions, (2) a reducing agent such as hypophosphite or an amine borane, (3) an acid or a hydroxide pH counter to provide the required pH and (4) a metal ion complexing agent sufficient to prevent their precipitation in solution. Numerous suitable complexing agents for electroless nickel solutions are described in the above publications. Those skilled in the art will know that the nickel, or other metal to be applied, is usually in the form of an alloy with the materials contained in the bath. Therefore, when hypophosphite is used as a reducing agent, the precipitate will contain nickel and phosphorus. Similarly, when an amine borane is used, the precipitate will contain nickel and boron. Therefore, the term nickel includes the other elements which normally precipitate therewith.

The nickel ion can be provided using any soluble salt such as nickel sulfate, nickel chloride, nickel acetate and mixtures thereof. The concentration of the nickel in solution can vary widely and is about 0.1 to 100 gr.p / 1, preferably about 2 to 50 gr.p / 1, for example 2 to 10 gr.p / 1.

The reducing agent, especially for memory disks, is usually the hypophosphite ion supplied to the bath by any suitable source such as sodium, potassium, ammonium and nickel hypophosphite. Among reducing agents such as amine boranes, borohydrides and hydrazine can also be suitably used. The concentration of the reducing agents is generally sufficient in excess of the amount to reduce the nickel in the bath.

The baths may be acidic, neutral or basic, and the acid or basic pH adjuster may be selected from a wide range of materials such as ammonium hydroxide, sodium hydroxide, hydrochloric acid, etc. The bath pH may vary from about 2 to 12, with a acid baths are preferred. A pH range of 4 to 5, e.g. 4.3 to 4.6, is preferred for the cadmium containing bath which is used to precipitate the coating on the zincate layer. A range from 4 to 5, e.g. 4.3 to 4.6, is also preferred for the bath used to deposit the final nickel layer when the cadmium containing bath is used to provide only a thin level coating.

The complexing agent can be selected from a wide variety of materials such as lactic acid, maleic acid and those containing anions such as acetate, citrate, glycolate, pyrophosphate, etc., with mixtures thereof being suitable. Areas for the complexing agent based on the anion can vary widely, for example from about 1 to 300 gr.p / 1, preferably about 5 to 50 gr.p / 1.

The electroless nickel plating baths may also contain other ingredients known in the art such as buffering agents, bath stabilizers, rate enhancers, brighteners, etc. Stabilizers such as lead, antimony, mercury, tin and oxy compounds such as iodate can be used.

A suitable bath can be formed by dissolving the ingredients in water and adjusting the pH to the desired range.

The zinc coated aluminum part can be plated with the electroless nickel cadmium bath to desired final thickness. Preferably, the portion is immersed in the bath to plate a thin nickel coating sufficient to provide a suitable base for the extremely smooth thick deposits of the final nickel plating using a different electroless nickel bath. Thicknesses can range from about 0.1 mil, or higher, with 0.005 to 0.08 mil, for example 0.01 to 0.05 being preferred. An immersion time of 15 seconds to 15 minutes usually provides the desired coating, depending on the bath parameters. A temperature range from about 25 ° C to boiling, for example 100 ° C, can be used with a range from 30 to 95 ° C being preferred.

The next step in the preferred procedure is to complete the nickel plating to the desired thickness and physical characteristics by immersing the nickel plating portion in another electroless plating bath which is maintained above a temperature range of about 30 to 100 ° C, for example boiling, preferably 80 to 100 ° C 95 ° C. A thickness of up to 5 mils, or higher can be used with a range of about 0.1 to 2 mils being used for most purposes. When using this leveled bathing process, it is preferred not to rinse the leveled coated substrate before immersion of the substrate in the next final plating bath.

The cadmium ions can be provided using any soluble cadmium source such as cadmium sulfate. It is important to control the cadmium concentration and obtain the extremely smooth coatings of the invention and an effective level of about 0.1 to 1 mg / l, preferably 0.3 to 0.8, most preferably 0.5 to 0.7 can be used appropriately. Amounts up to 2 or 3 mg / 1 or higher can be used for certain applications that do not require as smooth a surface as memory discs.

The use of cadmium in electroless nickel plating baths is described in U.S. Patent No. 2,929,742. Cadmium is disclosed as an agent that produces the hydrogen overpotentially and is considered to be beneficial in improving the gloss of the precipitate. Amounts up to 100 mg / 1 cadmium chloride are shown.

Those skilled in the art will know that the plating rate can be influenced by many factors including (1) the pH of the plating solution (2) the concentration of the reducing agent (3) the temperature of the plating bath (4) the concentration of the soluble nickel (5) the ratio from bath volume to surface to be plated, (6) presence of soluble fluoride salts (rate influencers) and (7) presence of wetting agent and / or stirring, and that the above parameters are mentioned only to provide a general guide to practice the invention. to give; the invention which involves the use of a cadmium-containing electroless plating bath as described above to provide an improved smooth coating of a galvanized aluminum substrate.

The composition and process of the present invention will now be more fully illustrated by the following specific examples which are illustrative and in no way exhaustive and in which all parts and percentages are by weight and temperatures in degrees Celsius unless otherwise indicated indicated.

EXAMPLE I

CZ-46 aluminum alloy discs with a double galvanized and plated with electroless nickel using the following procedure (a cold water rinse followed each step: (1) Immersion in ENBOND NS-35 for 3 minutes at 60 ° C; (2) Immersion in ACTANE E-10 for 1 minute at 60 ° C; (3) Immersion in 50 vol.% HN03 for 1 minute at room temperature; (4) Immersion in ALUMON AND for 35 seconds at room temperature; (5) Immersion in 50 vol.% HN03 for 1 minute at room temperature; (6) Immersion in ALUMON AND for 16 seconds at room temperature; (7) Immersion in ENPLATE ADP-300 for 1 hour at 84-87 ° C (pH 4.5 ± 0.1).

ENPLATE ADP-300 is an electroless electroless nickel bath (pH 4.6) containing in g / 1, nickel sulfate hexahydrate (26), sodium hypophosphite (20), sodium lactate (60%) (71), maleic acid (11.8), sodium hydroxide (4.6), potassium iodate (0.015), lead nitrate (0.0003) and an anionic surfactant (0.02).

Fig. IA shows the nickel surface resulting from the use of the above conventional double galvanizing procedure. When the same procedure was used except that iron III ions (as iron III chloride) were added to the HNO3 in step (5) at a level of 0.5 g / l iron III +++, a considerably smoother nickel surface as shown in Fig. 1 was obtained. 1B.

EXAMPLE II

The procedure of Example I was essentially repeated on 5586 aluminum alloy discs as follows: (1) Immersion in ENBOND NS-35 for 5 minutes at 63 ° C; (2) Immersion in ACTANE E-10 for 2 minutes at 63 ° C; (3) Immersion in 50% by volume HNO3 for 1 minute at room temperature; (4) Immersion in ALUMON EN for 45 seconds at room temperature; (5) Immersion in 50% by volume HNO3 for 30 seconds at room temperature; (6) Immersion in ALUMON EN for 15 seconds at room temperature; (7) Immersion in ENPLATE ADP-300 for 2 hours at 84-87 ° C (pH 4.5 ± 0.1).

Fig. 2A shows the nickel surface resulting from the use of the above conventional double zincate procedure. When the same procedure was used except that iron III ions were added to the HNO3 in step (5) to a level of 0.5 g / l, a considerably smoother nickel surface was obtained as shown in Fig. 2B.

EXAMPLE III

The procedure of Example I (steps (1) - (4) was used to prepare a number of galvanized CZ-46 aluminum alloy disks.

The disks were chosen at random and a total of 40 ft 2 were stripped at room temperature by each HNO 3 bath tested. The control HNO- was 50% by volume and was compared with the HNO 3 according to the invention which was 50% by volume and contained 0.5 g / l iron III ions (supplied as iron III chloride).

Fig. 3 shows the amount of zinc coating removed per square foot from the stripped disc and the results clearly show that the HNO 3 containing iron III ions removed more of the zinc coating than a conventional HNO 3 solution. This is important because less zinc is introduced into the plating solution.

EXAMPLE IV

This example shows that less zinc is deposited on the substrate to be plated with metal when the double zincate method is used according to the invention compared to the conventional double zincate method.

CZ-46 aluminum alloy disks were treated using steps (1) - (4) of the procedure of Example I. A group of disks was randomly selected and immersed in a conventional HNO 3 solution (50% by volume) for 1 minute at room temperature. The other group was immersed in a 50 vol.% HNO 3 solution containing 0.5 g / l iron III ions (supplied as iron III chloride) for the same time length and temperature. The disks were then immersed in a second zincate bath (as in step (6) of Example I) for either 10, 20, 30, 40, 50 or 60 seconds at room temperature. The galvanized discs were then stripped in 50% by volume HNO 3 and the amount of zinc deposited on the disc was determined by Atomic Absorption Spectroscopy.

Fig. 4 shows that less zinc was deposited on the discs when the method of the invention was used compared to the conventional double zincate method. This is important because less surface distortion and consequently a thinner but denser zinc coating is obtained. The iron probably acts as an inhibitor by thus slowing down and thereby controlling the solution of aluminum by zinc. In addition, the thinner settlements of zinc will not contaminate the subsequent electroless nickel bath as quickly.

EXAMPLE V

Aluminum 5586 alloy disks were ostensibly plated using the procedures of Example I to a thickness of about 0.40 mils. All tests were performed under the same plating conditions of 84 ° C, pH 4.6, a working load of 0.31 ft2 per gallon, continuous filtration and a plating time of 2 hours. The nickel, pH and sodium hyprofosphite were continuously replenished during the 2 hour plating time using an automatic controller.

Fig. 5A represents the nickel surface area resulting from the use of the above conventional galvanizing and planting procedure.

Fig. 5B depicts the nickel surface resulting from the use of the above procedure with the exception that 0.5 g / l iron III ions (as iron III chloride) were added to the nitric acid (step (5).

Fig. 5C represents the nickel surface resulting from the above procedure except that 0.75 mg / l cadmium was added to the nickel plating bath before plating and was not replenished for 2 hours of plating time.

Fig. 5D shows the nickel surface resulting from the use of the above procedure with the exception that 0.5 g / l iron III (as iron III chloride) was added to nitric acid (step (5) and 0.75 mg / l cadmium was added added to nickel plating bath and not replenished for 2 hours of plating time.

As can be clearly seen from the figures, the conventional process produces a rough surface with many nodules. Fig. 5B and 5C show the beneficial effects of using iron III ions and cadmium, respectively. and Fig. 5D shows the extremely smooth surface obtained using the preferred method of the invention.

It will be understood that many changes and modifications of the various characteristics described herein can be made without departing from the spirit and scope of the invention. It is therefore understood that the foregoing description has been made by way of illustration of the invention rather than as limiting the invention.

Claims (12)

A method for depositing a smooth electroless metal coating on an aluminum substrate comprising: (a) applying a zincate coating to the aluminum using a double zincate procedure in which the nitric acid bath used to treat the first zincate coated substrate has an effective amount of a group VIII ion; and (b) plating the galvanized aluminum substrate using an electroless metal plating bath containing an effective amount of cadmium ions. The method of claim 1 wherein the group VIII ion is the iron III ion. The method of claim 2 wherein the iron III ion is present in an amount of about 0.1-2 g / l. The method of claim 3 wherein the cadmium is present in an amount of about 0.1-1 mg / L. 5. Double galvanizing process for the preparation of aluminum and aluminum alloys for metal plating in which the aluminum is galvanized after pretreatment by immersion in a galvanizing bath, then the galvanized aluminum is dipped in a nitric acid bath to remove the zincate coating, followed by immersion in a zincate bath around the aluminum and coating the metal plating of the galvanized aluminum, using an effective amount of a Group VIII ion in the nitric acid bath. The method of claim 5 wherein the Group VIII ion is selected from the group consisting of iron, cobalt and nickel. The method of claim 6, wherein the group VIII ion is the iron III ion. The method of claim 6, wherein the iron III ion is present in an amount of about 0.1-1 g / l. A method of depositing a smooth electroless metal coating on a galvanized aluminum substrate comprising coating a thin first layer of the metal on the substrate using an electroless metal plating bath containing cadmium, followed by using another electroless metal plating bath to plating substrate with a second metal coating to the desired thickness, the thickness of the second metal coating being thicker than the first metal coating. A method of depositing a smooth electroless metal coating on a galvanized aluminum substrate in which the galvanized substrate is immersed in an electroless metal plating bath to plate a coating of a desired thickness and in which the bath contains metal ions and a reducing agent which components are controlled within desired machining limits by periodic replenishment where an effective amount of cadmium is used in the bath at the start of the plating operation and the cadmium is not replenished to the desired machining level until the plating operation is completed and / or new unplated galvanized aluminum substrates are immersed in the bath for plating. The method of claim 10 wherein cadmium is present at a level of about 0.1-1 mg / l. A method according to claim 11, wherein the galvanized aluminum substrate is prepared according to the method of claim 5.