CH684275A5 - The method of depositing an electroless metal coating on a smooth aluminum substrate. - Google Patents

Description

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Description

This invention relates to the metallic coating of galvanized aluminum, and more particularly to the provision of better adhesion and a smooth coating by employing an improved double zinc coating process and preferably in combination with a single autocatalytic metallic coating process using a special formula autocatalytic metal coating bath.

The metallic coating of aluminum is of considerable commercial interest and one application is the preparation of memory disks used in a variety of electronic applications such as computers and data processing systems. Aluminum is the preferred substrate for the disc; however, other metals are also suitable. In general, a relatively thin layer of non-magnetic autocatalytic nickel is applied to the aluminum, followed by a thin layer of a magnetic material such as cobalt. A signal is stored on the disc by magnetizing the cobalt layer to represent the signal at a selected time.

The alloys typically used for memory disks are numbers 5086 and 5586 of the Aluminum Association. These discs contain an amount of about 4% by weight of magnesium. Generally, aluminum discs 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, the rest consisting of aluminum and unavoidable impurities.

The finished metal coated disc must be extremely smooth and uniform in order to prevent the magnetic head of the device flying over the surface of the disc in the immediate vicinity (usually 5-8 microinches / 125-200 x 10_9m) from spitting. While the starting aluminum substrate must itself be extremely smooth and flat, as described in US Pat. No. 4,825,680, the metallic coating of the disc must likewise be smooth and uniform, so that the final disc produced has the exact specifications required for products of this type.

Unfortunately, however, the metallic coating of a substrate and even a coating of an autocatalytic metal does not necessarily produce a smooth coating. Holes in the coating, inclusions, bridge formations and the like are only a few of the coating problems that can make a disc surface rough and unacceptable.

Aluminum and its alloys also present additional problems during coating, because of the speed with which an oxidized film forms when exposed to air. As a result, special treatments must be employed when aluminum is to be coated. These treatments include mechanical treatments; chemical attacks, especially attacks with an acid containing iron, nickel and manganese salts; alkaline displacement solutions, especially those depositing zinc, brass and copper; anodizing, especially in phosphoric, sulfuric or chromic acids; and a low current density electrolytic coating for a few seconds with zinc. Among these treatments, alkaline displacement solutions have the most commercial success.

Although many metals can be deposited on aluminum by displacement, zinc is the most common. In this case, the process is known as the zinc plating process.

Over the years, many improvements have been made to the conventional zinc plating formula and the zinc plating process, most of which relate to an acceleration of film formation and the degree of adhesion and uniformity of the coating. of zinc obtained. A detailed summary of the zinc plating processes can be found in US Patent 4,346,128 to Loch and US 3,216,835 to Saubestre, these patents being incorporated herein as references.

In the conventional zinc plating process, aluminum is prepared by alkaline cleaning to remove surface, organic and inorganic contamination, such as oil and grease, followed by rinsing with cold water. The cleaned aluminum is then attacked superficially in order to remove solid impurities and to combine the components which can create cavities causing bridges in the materials deposited later. After rinsing with water, the aluminum is scoured to remove metal residues and the aluminum oxides still remaining on the surface. Thorough rinsing is required, and then the zinc coating is applied using a zinc immersion bath to prevent re-oxidation of the cleaned surface.

The zinc coating is obtained by immersing the aluminum part in an alkaline solution containing zinc ions. The amount of zinc deposited is currently very low and depends on the duration and the type of immersion bath used, the aluminum alloy, the temperature of the solution and the pre-treatment process. The zinc coating bath also functions as an attack solution and all the oxides which are reformed during the transfer operations are dissolved by alkaline zinc plating during the deposition of the zinc on the aluminum.

The process currently followed by the industry is to double the zinc plating, the first film of zinc being removed with nitric acid, followed by application of a second deposition of zinc by immersion. Double zinc plating is a preferred method for coating aluminum and is specially used for certain aluminum alloys which are difficult to coat, in order to ensure better adhesion of the final layer of deposited metal.

Despite the feasibility and efficiency of the double zincing process, the need still exists for a better process allowing better adhesion of the coating as well as a smoother metallic coating on the zinc-coated aluminum substrate. Without being limited to theory, it is believed that the properties of the metal plate are directly related to thickness, uniformity and continuity

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zinc coating, thinner coatings generally providing a smoother and more adherent metal coating.

The object of the present invention is to provide a process for the preparation of articles as an aluminum substrate having extremely smooth metallic coatings.

Another object of the present invention is to provide an improved double zinc coating process for metallic coating of aluminum, which improved process provides a thinner, more uniform and continuous zinc coating and provides better adhesive coating deposits and smoother metal coating.

Another object of the invention is to provide an improved autocatalytic metallic coating composition and a coating method for the re-coating of zinc-coated aluminum substrates with extremely smooth coatings.

Other objects and advantages appear in the detailed description which follows.

It has been found that aluminum substrates coated with metal, extremely smooth, for example memory disks, can be obtained preferably using a special double zinc plating process in conjunction with an autocatalytic metal coating bath containing an effective amount of cadmium. The double zincing process for preparing aluminum and aluminum alloys for metal coating is improved by using a specially formulated HNO3 bath to remove the first zinc coated film from aluminum. Following conventional procedures, bare aluminum is then rinsed with water and covered with a second zinc coating film. The metal covers this second zinc coating film. Generally said, the HNO3 bath used to remove the zinc coating comprises group VIII ions and, preferably, ferric ions, in an effective amount, for example, from about 0.1 g / l to 2 g / l and HNO3 in an amount of about 250 or 350 to 600 g / l or more.

Following the zinc plating process, the autocatalytic metal, for example nickel, covering the aluminum is supplied by using an autocatalytic metal covering bath containing an effective amount of cadmium to obtain extremely smooth metallic coatings. Generally said, the autocatalytic metal bath contains a source of metal ions, a reducing agent such as a hypophosphite or a borane amine, an acid or a hydroxide pH adjuster to provide the desired pH, a complexing agent for metal ions , sufficient to prevent their precipitation in solution and an effective amount of cadmium ions to provide the extremely smooth coating according to the invention. In general, the concentration of cadmium is approximately 0.1 to 1 mg / l with a preferred concentration of approximately 0.4 to 0.7 mg / l.

It has been found that the consumption of cadmium occurs quickly in the first stages of recovery, that is to say for 10 min and that thereafter the consumption is very slow and that the presence of cadmium is not important for the next coating. A preferred procedure is to start coating the zinc-coated aluminum substrate in an autocatalytic bath containing an effective amount of cadmium of about 0.1 to 1 mg / l, and not to supply cadmium until a new zinc-plated substrate to be covered is introduced into the bath. The autocatalytic metal coating bath contains metal ions, reducing agents, chelators, etc., and these components are supplied in a conventional manner by measuring their concentration and adding the necessary component in order to maintain the concentration between the desired operating limits. . New recovery methods employ automatic controls that continuously measure and replenish the bath into components. Other methods, such as manual measurement and re-supply at certain intervals, for example hourly, etc., may also be used.

According to a highly preferred embodiment, multiple coating baths are used in which a thin coating of nickel is placed on the zinc-plated surface by an autocatalytic bath containing cadmium ions followed by a thicker final coating by a second final bath conventional autocatalytic coating. This preferred method is similar to the method described in US Pat. No. 4,567,066 to P. B. Schultz and E. F. Yarkosky, this patent being incorporated here for reference.

The invention is more particularly understandable from the attached drawing, with the figures where:

fig. 1A and 2A are 500 X scale photomicrographs of aluminum substrates coated with autocatalytic nickel which have been prepared for coating by a conventional process of double zinc plating,

fig. 1B and 2B are 500 X scale photomicrographs of aluminum substrates coated with autocatalytic nickel which have been prepared for coating by the double zinc plating method according to the present invention,

fig. 3 is a graph showing that the nitric acid stripping solution of the invention (containing Fe +++ ions) removes more zinc from a zinc-coated aluminum substrate than a conventional zinc nitric acid solution,

fig. 4 is a graph showing that the aluminum substrates prepared according to the invention have less zinc on the surfaces to be coated with metal (a thinner coating) than the substrates prepared by the conventional double zincing process, and FIGS. 5A, 5B, 5C and 5D are 500 X scale photomicrographs of aluminum substrates coated with autocatalytic nickel obtained by various zinc-coating and coating processes.

The double zincing method for the preparation of an aluminum part for a metallic coating is well known to the skilled person as indicated above. In general, each type of aluminum or aluminum alloy can be treated by the method according to the invention although the

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examples given relate to alloys of types 5086, 5586 and CZ-46. The aluminum part can be machined or foundry.

While the specific method of double zinc plating employed may vary depending on the alloys treated and the results required, all of the methods use a bath of HNO 3 acid to remove the first zinc plating film, this being the step which the present invention relates to. A typical procedure used in the industry is shown below, and it should be understood that water rinses are generally provided after each step of the process.

The first step is usually to clean the aluminum surface of grease and oil, and an alkaline, non-etching cleaner, such as EN-BOND (R) NS-35 from Enthone, Incorporated West Häven, Connecticut , can be favorably used. ENBOND NS-35 is a mildly alkaline, non-silicate cleaner, used in a temperature range of approximately 49 ° to 66 ° C for 1 to 5 minutes.

The attack on cleaned aluminum can then take place using attackers such as ACTANE (R) E-10, ENBOND E-14 or ENBOND E-24, each of them from Enthone. These products are either acidic or alkaline. The acid striker is generally preferred, particularly when the dimensions of the surface, tolerances and integrity are important. The attackers are generally employed at high temperatures of the order of 49 ° to 66 ° for 1 to 3 minutes.

The cleaning of the alloy can be obtained with an HNO3 solution (for example at 50% by volume) or mixtures of HNO3 and H2SO4 alone or combined with ACTANE 70 from Enthone. ACTANE 70 is an acid fluoride salt containing ammonium bifluoride. A typical cleaning solution contains 25% by volume of H2SO4, 50% by volume of HNO3 and 1 lb / gallon (0.12 kg / dm3) of ACTANE 70 in water.

It is at this time that a zinc coating is applied to the aluminum by immersion in a zinc bath as described in US Pat. No. 3,216,835 to Saubestre, supra. A preferred bath, thanks to its proven effectiveness, is the ALUMON (R) EN from Enthone. ALUMON EN contains an alkali metal hydroxide, a zinc salt (such as zinc oxide or zinc sulphate, etc.), a chelating agent, optional anionic wetting agents and metal additives. An article by DS Lashmore whose title is “Immersion Coatings on Aluminum”, Plating and Surface Finishing, 67, pages 36-42 (1980) shows the use of iron (for example in the form of ferric chloride) in the zinc coating solution to deposit iron with zinc and produce a more adhesive zinc coating which is very resistant and comparatively insoluble in HNO3. ALUMON (R) EN and other commercial zinc solutions contain iron.

Generally, the double zincing process comprises immersing the aluminum substrate in a dilute zinc bath such as ALUMON (R) EN for a period of 20-30 seconds followed by a complete rinsing with cold water, an operation for removing zinc in nitric acid, a subsequent rinse with cold water and a second zinc-plated immersion and a subsequent rinse. As indicated in US Pat. No. 4,346,128 to Loch, supra, the improved Loch process dips the galvanized part in nitric acid for one to three minutes instead of the usual 20-30 seconds, to remove the zinc coating. This process claims to produce a uniform and thin oxide coating on the substrate which then serves to reduce the amount of zinc deposited and thus allows better adhesion for the second (final) coating.

In contrast to the Loch process, the use of Vili group ions, for example ferric ions, in the nitric acid bath, allows a similar result to reduce the amount of zinc deposited, while producing a zinc coating. which is very adhesive, uniform and continuous, and to which an extremely smooth metallic coating can be applied.

Returning to the improvement according to the invention, the nitric acid solution used to remove the first coating of zinc is generally a 50% by volume solution with a concentration range generally ranging from 350 to 600 g / l, and preferably about 450 to 550 g / l. The improved solution also contains group VIII ions, preferably ferric ions, in an amount of about 0.1 g / l to 1 or 2 g / l, preferably 0.3 g / l to 0.8 g / l and more preferably from 0.4 g / l to 0.6 g / l. For concentrations below about 0.1 g / l, minimal effects are obtained, while at concentrations above about 2 g / l, the surface topography can be seriously damaged.

The nitric acid solution can be used at a suitable temperature, generally about 20 to 25 ° C or higher, and preferably 21 to 23 ° C. The immersion time can vary from about 30 to 90 seconds and preferably from about 40 to 60 seconds.

Exemplary group VIII ions that can be used include iron, nickel and cobalt. Ferric ions are particularly preferred.

It is understood by those skilled in the art that the concentration, the temperature of the solution and the duration of immersion are in mutual relation and that, in general, by increasing the temperature and the concentration, the duration of immersion required is reduced to obtain the desired surface effect, with the invention residing in the use of group VIII ions in the bath to increase adhesion and make the metal layer smoother.

Although other metals can now coat prepared aluminum, coated with zinc, the following description makes special mention of nickel, given its commercial importance.

Autocatalytic nickel coating compositions for applying nickel coatings are well known in the art, and coating methods and compositions are described in numerous publications. For example, compositions for the deposit of autocatalytic nickel are described in US Patents 2,690,401, US 2,690,402, US 2,762,723,

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US 2,935,425, US 2,929,742, and US 3,338,726. Other common compositions for depositing nickel and its alloys are discussed in the 35th annual edition of Metal Finish Guidebook for 1967, Metal and plastics publications Ine ,. Westwood, N.J. pages 483-486. Each of the previous publications is included here for reference.

In general, the autocatalytic nickel deposition solutions comprise at least four ingredients dissolved in a solvent, typically water. They are a source of nickel ions, a reducing agent such as a hypophosphite or a borane amine, an acid or hydroxide pH adjuster to provide the desired pH and a complexing agent to have enough metal ions to prevent their precipitation in solution. A large number of possible complexing agents for autocatalytic nickel solutions are described in the publications mentioned above. It is known to those skilled in the art that nickel, or other metal to be applied, is generally in the form of an alloy with the other materials present in the bath. Thus, if hypophosphite is used as a reducing agent, the deposit will contain nickel and phosphorus. Similarly, if a borane amine is used, the deposit will contain nickel and borane. Thus, the use of the term nickel also signifies the other elements normally deposited with it.

The nickel ion can be provided by each soluble salt, such as a nickel sulphate, a nickel chloride, a nickel acetate and their mixtures. The concentration of nickel in solution can vary widely and is from about 0.1 to 100 g / l, and preferably about 2 to 50 g / l, for example from 2 to 10 g / l.

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

The baths can be acidic, neutral or alkaline and an acidic or alkaline pH adjuster can be selected from a wide range of materials, such as ammonium hydroxide, sodium hydroxide, hydrochloric acid and others. The pH of the bath can be in the range of 5 to 12, with acid baths being preferred. A pH range from 4 to 5, for example from 4.3 to 4.6 is preferable for the cadmium-containing bath used to deposit the coating on the zinc-plated layer. A range of 4 to 5, for example 4.3 to 4.6 is also preferred for the bath used to deposit the final layer of nickel when the cadmium-containing bath is used to provide only a thin contact coating.

The complexing agent can be chosen from a wide variety of materials such as lactic acid, malic acid and those containing anions, such as acetate, citrate, glycollate, pyrophosphate and the like, mixtures of them being usable. The concentrations for the complexing agent based on the anions can vary widely, for example, about 1 to 300 g / l, preferably about 5 to 50 g / l.

The autocatalytic nickel coating baths may also contain other ingredients known in the art, such as damping agents, bath stabilizers, activators, bleaches, etc. Stabilizers such as lead, antimony, mercury, tin and mixtures of oxides such as iodate can be used.

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

The zinc coated aluminum part can be coated with the autocatalytic nickel-cadmium bath to the desired final thickness. Preferably, the part is immersed in the bath to apply a thin coating of nickel intended to provide a usable base for thick, extremely smooth deposits, of the final layer of nickel, by a different autocatalytic nickel bath. Thicknesses can reach about 0.1 mil (2.5 µm) or more,

thicknesses of 0.005 to 0.08 mils (0.125 to 2 µm), for example 0.01 to 0.05 mils (0.25 to 1.25 µm) being preferred. A soak time of 15 seconds to 15 minutes generally provides the desired coating, depending on the parameters of the bath. A temperature range of about 25 ° C at boiling, for example 100 ° C, can be employed, a range of 30 ° to 95 ° C being preferred.

The next step in the preferred process is to finish the nickel coating according to the desired thickness and to give it the physical characteristics by immersing the nickel-coated part in another autocatalytic nickel bath which is maintained in a temperature range of 30 ° at 100 ° C, ie boiling, and preferably between 80 ° and 95 ° C. A thickness greater than 5 mils (125 µm) or more can be used, with a range of about 0.1 to 2 mils (2.5 to 50 µm) used in most applications. When this contact bath method is used, it is preferable not to rinse the substrate coated with the contact layer before immersing it in the next (final) coating bath.

Cadmium ions can be supplied using any source of soluble cadmium, such as cadmium sulfate. It is important to control the concentration of cadmium in order to obtain the extremely smooth coating of the invention, and an effective concentration of approximately 0.1 to 1 mg / l, preferably 0.3 to 0.8, and to more preferably from 0.5 to 0.7 mg / l, can be used. Amounts as high as 2 or 3 mg / l or more can be used for certain applications that do not require as smooth a surface as for memory discs.

The use of cadmium in autocatalytic nickel baths is described in US Patent 2,929,742.

Cadmium is described as acting on the

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hydrogen potential and is beneficial for improving the shine of the coating. Amounts of more than 100 mg / l of cadmium chloride are described.

It is known to those skilled in the art that the amount of coating can be influenced by various factors including the pH of the coating solution, the concentration of reducing agent, the temperature of the bath, the concentration of soluble nickel, the ratio between the volume of the bath and the surface coated, the presence of soluble fluoride salts (activators) and the presence of wetting agent and / or stirring, and that the above parameters are only indicated to give a general line to apply the invention, the invention residing in the use of an autocatalytic coating bath containing cadmium, as described above, to provide an improved smooth coating on a zinc-coated aluminum substrate.

The composition and process according to the present invention are now more fully explained by following specific examples which are illustrative and not limiting, and in which all the quantities and percentages are by weight and the temperatures in centigrade degrees if not indicated differently.

In the first example, aluminum alloy discs GZ-46 have a double zinc coating and are coated with autocatalytic nickel by the following process (rinsing with cold water according to each of the steps):

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 HNO3 at 50% by volume for 1 minute at room temperature;

4. immersion in ALUMON EN for 35 seconds at room temperature;

5. immersion in HNO3 at 50% by volume for 1 minute at room temperature;

6. immersion in ALUMON EN for 16 seconds at room temperature;

7. immersion in ENPLATE ADP-300 for 1 hour at 84-87 ° C (pH 4.5 + 0.1).

The ENPLATE ADP-300 is an acidic autocatalytic nickel base bath (pH 4.6) containing in g / l nickel sulphate hexahydrate (26), sodium hydrophosphite (20), sodium lactate (60%) (71), malic acid (11.8), sodium hydroxide (4.6), potassium iodate (0.015), lead nitrate (0.0003) and an anionic subframe (0.02).

Fig. 1A shows the surface area of nickel resulting from the use of the conventional double galvanizing process above. When the same process was used, except that ferric ions (such as ferric chloride) were added to HNO3 in step 5, in an amount of 0.5 g / l Fe +++, a remarkably smoother nickel surface was obtained , as shown in fig. 1 B.

For the second example, the process of the first example 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 HNO3 at 50% by volume for 1 minute at room temperature;

4. immersion in ALUMON EN for 45 seconds

at room temperature;

5. immersion in HNO3 at 50% by volume 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 conventional double-zinc coating process above. When the same procedure was used, except that ferric ions were added to HNO3 in step 5, in an amount of 0.5 g / l, a remarkably smoother nickel surface was obtained as shown in fig. 2B.

For the third example, the method of the first example (steps 1 to 4) was used to prepare several zinc-plated CZ-46 aluminum alloy discs.

The disks were chosen randomly and a total area of 40 ft2 (3.7 m2) was stripped at room temperature for each HNO3 bath tested. The reference HNO3 was 50% by volume and was compared with the HNO3 of the invention which was 50% by volume and contained 0.50 g / l of ferric ions (supplied by ferric chloride).

Fig. 3 shows the amount of zinc coating removed per ft2 (0.0929 m2) from the bare disc and the results clearly show that HNO3 containing ferric ions removes more zinc coating than a conventional HNO3 solution. This is important since less zinc is introduced into the coating solution.

The fourth example demonstrates that less zinc is deposited on the substrate to be coated with metal when the double zinc plating method of the invention is used, compared with the conventional double zinc plating method.

Aluminum alloy discs CZ-46 were treated using steps 1 to 4 of the method according to the first example. A group of discs was chosen randomly and was immersed in a conventional HNO3 solution (at 50% by volume) for 1 minute at room temperature. The other group was immersed in a 50% by volume solution of HNO3 containing 0.5 g / l of ferric ions (supplied by ferric chloride) for the same duration and at the same temperature. The discs were then immersed in a second zinc plating bath (as in step 6 of the first example) for either 10, 20, 30 40, 50 or 60 seconds at room temperature. The galvanized discs were then stripped in HNO3 at 50% by volume and the quantity of zinc deposited on the disc determined by atomic absorption spectrophotometry.

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Fig. 4 shows that less zinc was deposited on the discs when the method according to the invention was used compared to the conventional double zincing method. This is important since there are fewer surface defects and therefore a thinner but denser coating of zinc is obtained. Iron apparently acts as an inhibitor, thus delaying and controlling the dissolution of aluminum by zinc. In addition, the thinner zinc deposits do not contaminate the autocatalytic nickel baths as quickly.

For the fifth example, 5586 aluminum alloy discs were comparatively coated using the method of the first example, to a thickness of about 0.40 mils (10 µm). All tests were conducted with the same coating conditions of 84 ° C, pH 4.6 a workload of 0.31 ft2 / gal. (0.076 cm "1), continuous filtering, a coating time of 2 The nickel, pH and sodium hypophosphite were continuously renewed during the 2 hours of coating time by an automatic control.

Fig. 5A represents the nickel surface resulting from the use of the above conventional process of zinc plating and covering.

Fig. 5B represents the area of nickel resulting from the use of the above method, except that 0.5 g / l of ferric ions (such as ferric chloride) were added to the nitric acid in step 5 .

Fig. 5C represents the area of nickel resulting from the use of the above process, with the exception that 0.75 mg / l of cadmium was added to the nickel bath before coating without being renewed for the 2 hours of duration of coating.

Fig. 5D represents the surface area of nickel resulting from the use of the above process, except that 0.5 g / l of ferric ions (such as ferric chloride) were added to the nitric acid in step 5 and that 0.75 mg / l of cadmium was added to the nickel bath without being renewed during the 2 hours of coating time.

As can be clearly seen in the figures, the conventional procedure produces a rough surface with many nodules. Figs. 5B and 5C show the beneficial effects of the use of ferric ions and cadmium respectively, and FIG. 5D shows the extremely smooth surface obtained by using the preferential method according to the invention.

It is obvious that several changes and modifications to several of the features described above can be made without departing from the spirit and scope of the invention. It is also obvious that the foregoing description is more an illustration than a limitation of the invention.

Claims (7)

Claims 1. A method of depositing at least one smooth autocatalytic metallic coating on an aluminum or aluminum alloy substrate comprising a first step of double zincing, before the final metallic deposition, in which the substrate, after a pretreatment, is zinc plated by immersion in a first zinc bath, the galvanized substrate is then dipped in a nitric acid bath to remove the zinc coating, followed by immersion in a second zinc bath to coat the substrate, characterized in that the bath nitric acid employed comprises an effective quantity of group VIII ions. 2. Method according to claim 1, characterized in that the ions of group VIII are chosen from the group comprising iron, cobalt and nickel. 3. Method according to claim 2, characterized in that the group VIII ions are ferric ions. 4. Method according to claim 3, characterized in that the ferric ions are in an amount of about 0.1 to 1 g / l. 5. Method according to claim 1, characterized in that the metallic coating is obtained by an autocatalytic bath containing an effective quantity of cadmium ions. 6. Method according to claim 1, characterized in that two different autocatalytic baths of metal coating are used to cover the zinc-plated substrate, the first bath containing cadmium, followed by the second bath to coat the substrate with a second metal coating , the second metallic coating being thicker than the first metallic coating. 7. Method according to claim 1, characterized in that an effective quantity of cadmium is available in a metal coating bath at the start of the coating operation, the quantity of cadmium not being restored to the desired operational level until until the coating operation is completed and / or new uncoated zinc coated aluminum substrates are immersed in the coating bath. 5 10 15 20 25 30 35 40 45 50 55 60 65 7