NL194398C - Double galvanizing method for treating an aluminum substrate for metal plating. - Google Patents

Description

1 194398

Double galvanizing method for treating an aluminum substrate for metal plate ring

The invention relates to a double galvanizing method for treating an aluminum or aluminum alloy substrate for metal plating, wherein the substrate is pre-treated after immersion by immersion in a galvanizing bath, the galvanized aluminum is then dipped in a nitric acid bath to remove the zinc coating, followed by immersion in a galvanizing bath to coat the aluminum and the metal plating of the galvanized aluminum.

Such a method is known from United States Patent Specification 4,346,128. This publication describes a plating process of an aluminum substrate, in which the substrate is galvanized twice after pre-treatment to clean the substrate. Between the two galvanizing steps, zinc is removed with an acid, for example nitric acid, and a uniform oxide layer is applied to the substrate to delay the zinc deposition during the subsequent galvanizing step. After the second galvanizing step, the substrate is provided with a non-porous layer in a heated metal plating solution, which may be electroless, and finally a conductive metal is applied as an outer plating in a plating bath

Double galvanizing processes in which the first zinc film is removed with nitric acid, followed by applying a second zinc layer, is now generally followed by the industry. Although such an aluminum plating method is particularly useful on certain aluminum alloys that are difficult to plate to ensure better adhesion to the final metal layer deposition, there is still a need for an improved process that improves adhesion and smoothness of the metals. plating provided on the galvanized aluminum substrate. Without being limited to a theory, it may be assumed that the properties of the metal coating are directly related to the thickness, uniformity and continuity of the zinc coating, with thinner coatings generally giving a smoother and more adhesive metal plating.

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

It has surprisingly been found that this goal can be achieved with a double · galvanizing method, i

Wherein an aluminum substrate is treated with an I between a first and a second galvanizing step

nitric acid solution containing an effective amount of certain metal ions. 1

Therefore, the present invention is characterized in that the nitric acid bath initially contains an effective amount of ions from one or more of the Group VIII Iron Group metals from the Periodic Table of Elements.

Such a method has been found to be very suitable for obtaining aluminum substrates with a very smooth metal plating, as for example illustrated in examples I up to and including V. The method according to the invention is for example very suitable for the treatment of memory disks.

With the method according to the invention, a thinner, more uniform and continuous zincate coating can be applied. The method according to the invention also provides for improved adhesion of a metal plating precipitate and an improved metal plating smooth hero.

Preferably, the plating in a galvanizing method according to the invention is carried out in a method further characterized in that the galvanized aluminum substrate is plated in an electroless metal plating bath containing an effective amount of cadmium ions.

It has been found that the presence of cadmium ions in the metal plating bath has an additional beneficial influence on the smoothness of the surface of the substrate.

Such a method is characterized in that the galvanized aluminum substrate is plated in an electroless metal plating bath containing an effective amount of cadmium ions.

A substrate that has been treated according to such a method has an extremely smooth coating, as illustrated in Example V.

Metal plating of aluminum is of considerable commercial interest and one application is in the manufacture of memory disks 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 may be used. Generally, a relatively thin layer of non-magnetic nickel is applied to the aluminum 55 without any current, 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 selected time.

Typical aluminum alloys used for memory disks have the 194398 2 classification numbers 5086 and 5586. These disks contain about 4% magnesium by weight. The aluminum discs are generally about 1.25 to 5 mm thick and contain about 4 to 4.90% by weight of magnesium, 0.01 to 0.40% by weight of copper, 0.01 to 0.40% of zinc , chromium, nickel, iron, silicon and, for the rest, aluminum and unavoidable impurities.

The finished metal-plated disk must be extremely smooth and uniform to prevent crashing against the magnetizing head of the device which floats extremely dense (generally 127-203 nm) over the disk surface. Because the starting aluminum substrate itself must be extremely smooth and flat as described in U.S. Pat. No. 4,825,680, the metal plating of a disc must be equally smooth and uniform so that the disc as a finished 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 provide a smooth coating. Plating activities, inclusions, couplings and the like are just some of the plating problems that can cause a rough disk surface, which is not acceptable. is.

Aluminum and its alloys also exhibit additional plating problems due to the speed at which they form an oxide coating when exposed to air. As a result, special treatments must be used when plating on aluminum must take place. These treatments include mechanical treatments; treatments with chemical etchants, in particular with acidic etchants containing iron, nickel and manganese salts; basic displacement solutions, in particular those which precipitate zinc, bronze and copper; anodizing, in particular in phosphoric, sulfuric 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 method is known as the zincate process. Over the years, a number of improvements have been made to the conventional zincate formulation and the zincate process, most of which aimed to accelerate the degree of film formation, and the degree of adhaesion and uniformity of the zinc coating that was produced. A detailed summary of the zincate process can be found in U.S. Patent Nos. 4,346,128 and 3,216,835. In the conventional zincate process, the aluminum is pre-treated 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 sufficiently etched to remove solid impurities and alloying components that potentially create activities that result in bridging of successive deposits. After rinsing with water, the aluminum is burned clean to remove metallic residues and aluminum oxides that have remained on the surface.

Thorough rinsing is required and then the zinc coating is applied using an immersion zinc bath to prevent re-oxidation of the cleaned surface.

The zinc coating is obtained by immersing the aluminum part in a basic solution containing zinc ions. The amount of precipitated zinc is actually very small and depends on the time and 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 etching solution and all the oxides that have been re-formed during the transfer operations are dissolved by the basic zincate, depositing zinc on the aluminum.

45 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 zinc film from the aluminum. When conventional procedures are followed, the stripped aluminum is then rinsed with water and coated with a second zinc film. The metal is plated on this second zinc film. Broadly stated, the HNO3 bath used to strip the zinc coating comprises 50 ions from Group VIII iron group and, preferably, iron (III) ions, in an effective amount, for example, from 0.1 to 2, 0 g / l and HNO3 in an amount of about 250 or 350 to 600 g / l or higher.

Following the zincate procedure, the electroless plating of metal, e.g., nickel, on the aluminum is improved by using an electroless metal plating bath containing an effective amount of 55 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 more complex - 3,998,198 metal ions 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 0.1 to 1.0 mg / l, with a preferred level of about 0.4 to 0.7 mg / l.

It has been found that the consumption of cadmium occurs quickly in the first steps of plating, namely 10 minutes, and thereafter this consumption is very slow and the presence of the cadmium is not important during further plating. A preferred form of the operation is to start plating the galvanized aluminum substrate in an electroless bath containing an effective amount of cadmium from 0.1 to 1.0 mg / l, and not to replenish the cadmium until new galvanized substrates must be plated in the bath. The electroless metal plating bath contains metal ions, reducing agents, chelators, etc. and these components are conventionally supplemented by determining 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 continuously measure and supplement the bath components. Other methods such as measuring by hand and topping up at certain intervals, for example every hour can also be used.

In a very preferred embodiment, multiple plating baths are used, wherein a thin nickel coating is provided on the galvanized surface of an electroless ion-free bath followed by plating a thicker end 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.

Figures 1A and 2A are 500-fold enlargements of micrographs of electroless nickel-plated aluminum substrates made by plating with a conventional double zinc coating process.

Figures 1B and 2B are 500-fold enlargements of micrographs of electroless nickel-plated aluminum substrates made by plating with the double galvanizing method of the present invention.

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

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

Figures 5A, 5B, 5C and 5D are 500-fold magnifications of micrographs of electroless nickel-plated aluminum substrates using different galvanizing and plating procedures. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

The double galvanizing method for treating aluminum for metal plating is well known in the art as discussed above. In general, any 3 aluminum alloy can be treated by the method of the invention and preferred alloys are 5086, 5586 and CZ-46. The aluminum can be forged or cast.

5

While the specific double galvanizing method used may vary with the alloys to be treated and the required results, all of these methods use an HNO3 immersion bath to remove the first 7 zinc coating; this is the step to which the present invention is directed. A typical 8 procedure used in the industry is as follows. It will be clear that after every process step, 9 water washes are generally used; 10

The first step is generally cleaning the aluminum surface by removing grease and oil, which includes a basic non-etching cleaning agent such as ENBOND (R) NS-35, sold by 12

Enthone, Incorporated, West Haven, Connecticut, can be used appropriately. ENBOND NS-35 is a 13 non-silicate weak basic cleaning agent that can be used for 1 to 5 minutes in a temperature range of approximately 49 to 66 ° C.

15

The etched aluminum can then be etched using 16 etchants such as ACTANE (R) E-10, ENBOND E-14 or ENBOND E-24, all of which are sold by 17

Enthone. These materials are either acidic or basic. An acid etchant is generally preferred 18, especially when surface dimensions, tolerances, and integrity are important. The etchants are generally used for 1 to 3 minutes at elevated temperatures of about 49 to 66 ° C.

21

Further cleaning of the alloy can be carried out using an HNO3 solution (for example 50% by volume) or mixtures of HNO3 and H2SO4 alone or in combination with ACTANE 70, sold by Ethone. ACTANE 70 is an acid fluoride salt product that contains ammonium bifluoride. A typical 194398 cleaning solution contains 25 volume% H 2 SO 4, 50 volume% HNO 3 and 119.84 g / l ACTANE 70 in water.

At present, a zinc coating is applied to the aluminum by immersion in a zincate bath as described in the aforementioned U.S. Patent No. 3,216,835. A bath that is preferred because of its demonstrated effectiveness is ALUMON (R) EN, sold by Ethone. ALUMON EN contains an alkali metal hydroxide, a zinc salt (for example, zinc sulphate) or zinc oxide, a chalcing agent, possibly 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 (e.g., iron (III) chloride) in the zankate solution to iron down with zinc. and to produce a more adhesive zinc coating, which is highly resistant and relatively insoluble in HNO 3, ALUMON (R) AND other commercial zincate solutions contain iron.

In general, the double zincate process involves immersing the aluminum substrate in a diluted zincate bath such as ALUMON (R) AND for 20 to 50 seconds followed by a thorough rinse with cold water and zinc strip processing in nitric acid, another rinse with cold water, and a second zincate immersion and then rinsing. According to the aforementioned U.S. Patent No. 4,346,128, the galvanized workpiece is immersed in nitric acid for 1 to 3 minutes instead of the usual 20 to 30 seconds used to strip the zincate coating. This procedure would apply a thin uniform oxide coating to the substrate 20, which serves for a further reduction of the deposition rate of zinc and thereby provides for better zinc adhesion for the final (second) zincate coating.

In contrast to the procedure according to U.S. Pat. No. 4,346,128, the use of ions of one or more of the Group VIII iron group metals, for example iron (III) ions, in the nitric acid bath brings about the same result of reducing the deposition rates of the zinc while obtaining a zinc coating which is very adhesive, uniform and continuous and on which an extremely smooth metal plating can be applied.

According to the invention, the nitric acid solution used to strip the first zinc coating is generally a 50% by volume solution with a concentration range of generally about 350 to 600 g / l and preferably about 450 to 550 g / l. The improved solution also contains 30 ions of one or more metals from the group VIII iron group, preferably iron (III) ions, in an amount of about 0.1 to 1 or 2 g / l, preferably 0.3 to 0.8 g / l and most preferably 0.4 to 0.6 g / l. 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 at about 20 to 25 ° C or higher and preferably 21 to 23 ° C. Immersion times can vary from about 30 to 90 seconds and preferably about 40 to 60 seconds.

The ions of the Group VIII iron group metals that can be used are those of iron, nickel, and cobalt. Iron (III) ions are preferred.

It is clear that the concentration, solution temperature and immersion time are interdependent and that it generally holds that the higher the temperature and concentration, the shorter the immersion time necessary to achieve the desired surface effect will be, the invention relies on the use of ions of one or more metals from the Group VIII iron group in the bath to achieve the improved adhesion and smoothness of the metal plating.

The method of the present invention will now be illustrated by the following examples. 45 In these examples, all parts and percentages are by weight and temperatures in degrees Celsius are indicated, unless otherwise indicated.

Example / CZ-46 aluminum alloy discs were 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.% HNOa for 1 minute at room temperature; 4. Immersion in ALUMON AND for 35 seconds at room temperature; 55 5. Immersion in 50 vol.% HNO3 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).

194398 ENPLATE ADP-300 is an electroless acid-free nickel bath (pH 4.6) containing in g / l, nickel sulfate 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 the use of the above-mentioned conventional double galvanizing procedure. When the same procedure was used except that iron (III) ions (such as iron (III) chloride) were added to the HNO3 in step (5) in an amount of 0.5 g / l iron (III), a considerably smoother nickel surface became obtained than shown in Figure 1B.

EXAMPLE II

The procedure of Example I was repeated essentially as follows on 5586 aluminum alloy discs: 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 vol.% HNOg for 1 minute at room temperature; 4. Immersion in ALUMON AND for 45 seconds at room temperature; 5. Immersion in 50 volume% HNO3 for 30 seconds at room temperature; 6. Immersion in ALUMON AND for 15 seconds at room temperature; 7. Immersion in ENPLATE ADP-300 for 2 hours at 84-87 ° C (pH 4.5 ± 0.1).

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

EXAMPLE III

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

The disks were randomly selected and a total of 3.72 m2 was stripped at room temperature by each HNO3 bath tested. The control HNO3 was 50% by volume and was compared to the HNO3 of the invention which was 50% by volume and contained 0.5 g / l of iron (III) ions (supplied as iron (III) chloride).

Figure 3 shows the amount of zinc coating removed per m2 of the stripped disk. The results clearly show that the HNOs containing iron (III) ions removes more of the zinc coating than a conventional HNO3 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 the steps (1) - (4) of the procedure of Example I. A group of disks was randomly selected and immersed in a 40 conventional HNO 3 solution (50% by volume) for 1 minute at room temperature. The other group was immersed for the same time length and temperature in a 50% by volume HNO3 solution containing 0.5 g / l iron (III) ions (supplied as iron (III) chloride). The disks were then immersed in a second zincate bath (as in step (6) of Example 1) at room temperature for either 10.20, 30, 40, 50 or 60 seconds. The galvanized disks were then stripped in 50 vol.% HNO 3 and the amount of 45 deposited zinc on the disk was determined by Atomic Absorption Spectroscopy.

Figure 4 shows that less zinc was deposited on the disks when the method according to the invention was used compared to the conventional double zincate method. This is important because less surface disruption and, as a consequence, a thinner but denser coating of zinc is obtained. The iron probably acts as an inhibitor by thus delaying and thereby controlling the solution of aluminum by zinc. In addition, the thinner deposits of zinc will not so quickly contaminate the subsequent electroless nickel bath.

Example V

Aluminum 5586 alloy discs were plated using the procedures of Example I to a 55 thickness of about 10.16 µm. All tests were performed under the same plating conditions of 84 ° C, pH 4.6, a working load of 76.1 cm 2 / l, continuous filtration and a plating time of 2 hours. The nickel, pH and sodium hypophosphite were continuously used during the 2 hour plating time

Claims (6)

194398 6 an automatic controller supplemented. Figure 5A shows the nickel surface resulting from the use of the above-mentioned conventional galvanizing and plating procedure. Figure 5B shows the nickel surface resulting from the use of the above-mentioned procedure with the exception that 0.5 g / l iron (III) ions (as iron (III) chloride) were added to the nitric acid (step (5) ). Figure 5C shows the nickel surface resulting from the above procedure with the exception that 0.75 mg / l cadmium was added to the nickel plating bath for plating and that was not replenished during the 2 hour plating time. Figure 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 to the nickel plating bath and was not replenished during the 2 hour plating time. As can be clearly seen from the figures, the conventional process produces a rough surface 15 with many modules. Figures 5B and 5C show the beneficial effects of using iron (III) ions, cadmium, respectively, and Figure 5D shows the extremely smooth surface obtained using the preferred method of the invention. Conclusions 1. Double galvanizing method for treating an aluminum or aluminum alloy substrate for metal plating, wherein the substrate is pre-treated by immersion in a galvanizing bath, the galvanized aluminum is then dipped in a nitric acid bath to remove the zinc coating, followed by immersion in a galvanizing bath to coat the aluminum and the metal plating of the galvanized aluminum, characterized in that the nitric acid bath initially contains an effective amount of ions from one or more of the Group VIII iron group metals from the Periodic Table of Elements . A galvanizing method according to claim 1, characterized in that said ions are initially present in an amount of 0.1 to 2.0 g / l. The galvanizing method according to claim 1 or 2, characterized in that the ion is group VIII of the metal from the iron group of the iron (III) ion. The galvanizing method according to claim 3, characterized in that the iron (III) ion is present in an amount of 0.1 to 1 g / l. The galvanizing method according to any of claims 1 to 4, characterized in that the galvanized aluminum substrate is plated in an electroless metal plating bath containing an effective amount of cadmium ions. The galvanizing method according to claim 5, characterized in that the cadmium ions are initially present in an amount of 0.1 to 1.0 mg / l. Hereby 6 sheets of drawing