KR20070095331A - Method of electroplating and pre-treating aluminium workpieces - Google Patents

Abstract

The invention relates to a method of applying a metal layer onto at least one surface of an aluminium or aluminium alloy workpiece, including the steps of pre-treating the surface by cathodic activation in a pre-treatment bath containing sulphuric acid and metal-ions selected from the group consisting of nickel, iron and cobalt, and applying a metal layer by electroplating the pretreated workpiece, and wherein the metal layer is selected from the group consisting of nickel, iron, cobalt, and alloys thereof.

Description

METHOD OF ELECTROPLATING AND PRE-TREATING ALUMINIUM WORKPIECES}

The present invention relates to a method of coating a metal layer on one or more surfaces of an aluminum or aluminum alloy processing member or article, comprising a simple pretreatment step of cleaning and activating the surface, and subsequently achieving good adhesion of the coated metal layer. It is about a method of including. The invention also relates to an aluminum alloy product in which at least one surface is plated with a metal layer. In particular, the present invention relates to a method of coating a metal layer of brazing-promoting metal on a clad layer of an aluminum alloy brazing sheet product used in a flux-free brazing process.

Except as otherwise indicated, all alloy designations and temper designations, as detailed below, are in accordance with the aluminum standards published by the Aluminum Association and the Aluminum Association designations of data and registration records.

Nickel plating of aluminum products has been widely used for a long time because nickel provides a bright and glossy appearance, and is durable and conductive. A particular use of nickel plating is also in the manufacture of brazing sheet products. The aluminum alloy brazing sheet includes an aluminum alloy core and a cladding layer of filler alloy on one or both sides of the core. Aluminum brazing sheets are widely used in the manufacture of heat exchangers.

However, the use of aluminum-silicon alloys as filler material has the problem of breaking the aluminum oxide layer during brazing. This can be done by applying chemical flux on the workpiece before brazing. Flux for use in aluminum alloy brazing typically consists of a mixture of alkali and alkaline earth metal chlorides and fluorides or cryolites. The flux operates at brazing temperatures that destroy, spread and dissolve the oxide film. However, applying chemical flux on the workpiece is quite cumbersome and therefore expensive.

In the past, flux-free brazing techniques have been developed and employed as described in US-2003 / 0098338-A1, which is hereby incorporated by reference in its entirety. One such technique is coated on the part where the brazing-promoting metal of cobalt, iron or more preferably nickel is brazed. During brazing, nickel exothermicly reacts with the bottom aluminum alloy, thereby breaking the aluminum oxide layer and flowing and bonding with the bottom molten aluminum clad metal. Since this method does not require fluoride flux, it is also suitable for use with magnesium-rich aluminum alloys which are advantageously used as heat exchanger components.

In addition to nickel, iron or cobalt coatings, wetting agents may also be added to improve the wettability of the clad alloy during the brazing treatment. However, nickel plating requires extensive pretreatment of metal surfaces such as cleaning, etching, desmutting, and the like. This is also due to the presence of a strong oxide layer. If the aluminum alloy surface was not properly pretreated, the nickel coating would have low adhesion or would be contaminated, thereby impairing the brazing of the product. Thus, nickel plating with all the necessary pretreatment steps is a costly and environmentally unfriendly process.

It is an object of the present invention to provide a method of electroplating a metal layer on an aluminum alloy product, which requires as few steps as possible and does not require the use of fluoride containing components.

It is also an object of the present invention to obtain a metal coated aluminum alloy product, wherein the coated metal coating adheres well and acts to destroy the oxide layer during the subsequent brazing process.

The present invention solves one or more of these objects through a method of coating a metal coating according to claim 1 and an aluminum alloy product according to claim 14.

A method of coating a metal layer on at least one surface of an aluminum or aluminum alloy processing member, the method comprising pretreating the surface by cathodic activation in a pretreatment bath containing sulfuric acid and metal ions selected from the group consisting of nickel, iron and cobalt. And coating the metal layer by electroplating the pretreated workpiece, wherein the metal layer is selected from the group consisting of nickel, iron, cobalt and alloys thereof. Those skilled in the art will appreciate that if the coated metal layer contains nickel or nickel-alloy, the pretreatment bath must contain nickel-ion, and if the pretreatment bath contains iron or cobalt or alloys thereof, the pretreatment bath is each iron-ion. And will immediately contain cobalt-ions.

1 shows the angle on a coupon for a brazing test.

The present invention enables metal, for example nickel, to be plated on an aluminum alloy product after cathodic activation in a simple sulfuric acid solution containing only nickel ions in the form of nickel ions, such as nickel sulphate. It is based on the fact that In this activation treatment, no fluoride component is necessary. The activation bath contains the same ingredients as the Watts bath, which is preferably used as the nickel plating bath, so cross contamination is excluded. It is also expected that there will be no problems in wastewater treatment.

During cathodic activation in a sulfuric acid solution containing nickel ions, nickel nuclei are produced on the surface through a thin aluminum oxide film, thereby forming anchor spots for the nickel layer covered in subsequent plating steps. Thus, the cathode activation step has the same effect as the creation of a thin bonding layer between the aluminum surface and the nickel coating. The same applies to situations where iron or cobalt is used.

Preferably, the pre-treatment bath is about 15 to 200 g / l, preferably 80 to 150 g / l NiSO ₄ H ₂ 0 and about 50 to 350 g / l, preferably about It contains 150 to 250 g / l H ₂ SO ₄ . In a preferred embodiment, the pretreatment bath also contains boric acid as a buffer, such as boric acid in the range of 1 to 50 g / l, preferably 20 to 40 g / l.

Preferred baths for pure nickel plating are Watts baths containing nickel sulfate, nickel chloride and boric acid.

Preferred baths for nickel-bismuth plating are citrate-gluconate baths containing nickel sulfate, nickel chloride, (NH ₄ ) ₂ SO ₄ , bismuth concentrate, sodium citrate and sodium gluconate )to be. Preferred concentration ranges for these materials are from 1 to 10 ml / l containing 100 to 180 g / l NiSO ₄ .6H ₂ 0, 10 to 50 g / l NiCl ₂ .6H ₂ 0, 100 g / l Bi. Bismuth concentrate, 10-50 g / l of (NH ₄ ) ₂ SO ₄ , 100-180 g / l of sodium citrate.2H ₂ 0, and 10-50 g / l of sodium gluconate. This bath can also be used for pure nickel plating, in which case bismuth concentrate is excluded.

It has been found that the pretreatment is sufficiently effective at high temperatures below 95 ° C, preferably in the range of 55 ° C to 80 ° C. This has the great advantage of facilitating the introduction into the strip plating line since working at lower temperatures can limit evaporation losses. In addition, aluminum dissolution is very low at temperatures below 70 ° C., thereby increasing the life of the activation bath. Thus, the pretreatment bath is preferably maintained at a temperature of 55 ° C. to 80 ° C., most preferably about 60 ° C. to 70 ° C.

The activation current is cathodic. As shown by way of example, current density is not critical to the quality of the final product. The activation time of the product in the pretreatment bath is also the same. The activation current of the cathode activation is preferably in the range of -200 to -2000 A / m 2, more preferably -500 to -1400 A / m 2. The time spent by the product in the pretreatment bath is typically in the range of 1 to 50 seconds, preferably 5 to 15 seconds.

The average thickness of the coated metal layer of Ni, Co, Fe or alloys of each of these metals is in the range of less than 2 μm, more preferably less than 1.0 μm, most preferably 0.2 to 1.0 μm.

Preferably, the method is carried out as a continuous plating process that allows for continuous processing of an endless strip of metal.

In an optional further step, an additional metal layer may be coated on top of the layer of Ni, Fe, Co or alloys thereof, for example to improve the corrosion resistance of the final product. For example, a thin tin layer can be coated on the nickel layer of the brazing sheet product, thereby achieving a sufficient improvement in corrosion resistance after brazing.

The method according to the invention may comprise an additional step of degreasing the surface prior to the cathode activation and / or electroplating step to clean the surface.

In order to avoid the work hardening of the soft annealing coil during processing in the (vertical) plating line, it is desirable to plate a full hard material. Moreover, fully hard materials break more easily than soft anneal materials. Therefore, it is desirable to plate a wide coil in a completely hard state and then slit it into a plurality of coils of a desired width, thereby reducing the switching cost. The coil may later be soft annealed.

In one embodiment of the method according to the invention, the aluminum working member is a brazing sheet product, the brazing sheet product comprising a brazing alloy comprising aluminum and 2 to 18 wt% silicon, preferably 7 to 14 wt% silicon. A cladding layer and a core layer formed of AA4343 and 4045 alloy, and the like, and a metal layer is coated on the cladding layer. The metal layer of nickel, iron, cobalt or alloys of each of these metals acts as a brazing-promoting element during brazing.

In a preferred embodiment of the brazing sheet product, the cladding layer further comprises a humectant as the alloying element in the range of up to 1 wt% to improve the wettability of the clad alloy during the brazing treatment. Preferably, the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, thallium and mixtures thereof.

In another embodiment of the method according to the invention, the aluminum working member is an aluminum conductor and is preferably made of an alloy selected from the group consisting of AA1370, AA1110 and AA6101. The aluminum conductor may be in the form of an aluminum strip or an aluminum wire or an aluminum tube. In an embodiment, the coated metal layer is preferably composed of nickel to improve electrical contact properties. Aluminum conductors can be used for the transfer of electrical and / or thermal energy. These conductors are typically in the form of bars, wires or cables when used as electrical conductors, and in the form of strips, bars or tubes when used as thermal conductors.

In another aspect of the present invention, an aluminum alloy product, preferably electroplated with a metal layer selected from the group consisting of nickel, iron, cobalt and alloys thereof, produced by the process of the invention as disclosed in the specification and claims of the invention To provide a brazed sheet product. Such brazing sheet products can be successfully applied to controlled atmosphere brazing ("CAB") processing without brazing flux.

As can be seen by the examples below, the aluminum alloy product according to the invention has a nickel or nickel-bismuth coating that adheres well. In a particularly preferred embodiment, the article is an aluminum alloy brazing sheet comprising a core, a cladding layer and a nickel-containing layer plated on top of the cladding layer, which has good brazing properties and low manufacturing costs. Wetting agents such as Bi may be contained in the clad alloy or in the nickel-containing layer.

The invention is illustrated by the following non-limiting examples.

Example

Two different types of aluminum brazing sheet products of 0.4 mm thickness were used for plating with nickel or nickel-alloy layers having an average thickness of 0.5 μm. An aluminum brazing sheet composed of an AA3003-series aluminum core alloy clad with AlSi brazing alloy in a conventional manner was used, while cladding layer A contained 10 wt% Si, 1.5 wt% Mg and 0.08 wt% Bi, while the cladding Layer B contains 12 wt% Si and no Mg or Bi.

In making nickel plated brazing sheet products, the following procedure was used:

Washing after washing at 50 ° C. for 180 seconds using 35 g / l Chemtech 30014 (commercially available bath);

Cleaning after activation with a current density of -1000 A / m 2;

Cleaning after Ni or Ni-Bi plating using a current density of -1000 A / m 2.

The cathodic activation bath according to the invention was prepared based on sulfuric acid (see Table 1). Nickel sulfate was chosen to feed the nickel-ion to the solution, preferably boric acid was added as a buffer. For comparison, a fluoride based activation bath consisting of anodic activation at a current density of +1000 A / m 2 disclosed in US Pat. No. 6,780,303-B2, incorporated herein by reference, was used (see Table 2). Cathodic activation was carried out at various temperatures. Two samples 10 and 11 were conducted using the same activation bath but the current was switched to produce bipolar activation.

After activation, the nickel layer was plated from the Watts bath (see Table 3) or the nickel-bismuth alloy layer was plated from the citric acid-gluconic acid bath (see Table 4).

The quality of the plated substrate obtained was measured using the adhesion test and the brazing test. The adhesion test consisted of an Ericsson dome test (5 mm high cup) and peeled off once after applying the adhesive tape (Scotch tape 3M No. 610) to the deformed area. Adhesiveness was quantified by analyzing the amount of nickel on the tape. Overall adhesion ratings ranged from 1 (poor) to 10 (excellent), and level 6 was considered suitable because it is comparable to commercially available brazing sheets with Ni-Pb layers.

For laboratory scale testing, the brazing test was conducted in a small quartz furnace. Nickel-bismuth-plated sheets were cut into small cut specimens of 25 mm x 25 mm. A small strip of bare AA3003 alloy of 30 mm x 7 mm x 1 mm was bent at a 45 ° angle from the center and placed on the cut specimen (see Figure 1). Samples with angles on the cut specimens were heated to 580 ° C. at room temperature under nitrogen flow, lasted for 1 minute at 580 ° C., and cooled to room temperature at 580 ° C. The brazing treatment was determined by the possibility of formation of wrinkles, capillary depression and fillet formation. Overall evaluation was given as (-) = brazing poorness, (±) = brazing titration and (+) = good brazing good.

Experimental results of adhesion and brazing performance for various samples are summarized in Table 5.

Comparisons of Samples 3, 4 and 5 show that both adhesion and brazing are poor when no activation is used. In contrast, hydrofluoric acid baths obtain good adhesion and good brazing, comparable to sulfuric acid baths. However, hydrofluoric acid baths contain fluorides and are therefore not environmentally desirable methods.

From Examples 10 and 11, anode activation provides good adhesion, but there is a possibility that the brazing property is seriously impaired by the formation of an oxide film. According to the present invention, it has been found that both good adhesion and brazing are obtained by cathodic activation. However, it may be an effective pretreatment method if brazing is not required for its application, such as aluminum conductors.

The temperature of the negative electrode activation bath shows a strong influence on the adhesion of the plated Ni layer. Sample 7 shows good adhesion and brazing even at a temperature of about 60 ° C. However, lower temperatures, below 50 ° C. (sample 6), result in inadequate adhesion levels. From Samples 1, 2, 8 and 9, it can be seen that even when the temperature is lowered from 93 ° C to 70 ° C, neither adhesion nor brazing is affected. This makes it very easy to introduce into a continuous strip plating line since the evaporation losses will be limited. In addition, aluminum dissolution is much less below 70 ° C., thereby increasing the life of the activation bath.

The addition of humectants, such as Bi, is beneficial for the brazing performance of the resulting brazing sheet product. It can be seen from Samples 1 and 8 that wetting agents can be added to the Ni layer or brazing clad layer without affecting adhesion or brazing properties. Adding a humectant to both the clad layer and the nickel layer does not adversely affect the brazing properties.

Thus, it has been demonstrated that nickel plating directly on brazing sheet products is possible after cathodic activation in simple sulfuric acid solution with only nickel sulfate added. Fluoride is not necessary for this activation treatment. Since the activation bath contains the same ingredients as the Watts bath, cross-contamination is excluded. It is also expected that there will be no problems in wastewater treatment. Satisfactory results are obtained at bath temperatures of about 60 ° C. The process can be carried out in a reliable manner over a wide range of current densities and processing times.

Similar results can be obtained when iron or cobalt instead of nickel is used as the brazing-promoting metal on the brazing sheet product.

The present invention described above may be variously modified and changed by those skilled in the art without departing from the technical spirit of the present invention.

Claims (14)

In a method of coating a metal layer on at least one surface of an aluminum or aluminum alloy processing member, Pretreating the surface by cathodic activation in a pretreatment bath containing metal ions and sulfuric acid selected from the group consisting of nickel, iron and cobalt and Coating a metal layer by electroplating the pretreated processing member, And said metal layer is selected from the group consisting of nickel, iron, cobalt and alloys thereof. The method of claim 1, The pretreatment bath comprises 15 to 200 g / l of NiSO ₄ H ₂ 0 and 50 to 350 g / l of H ₂ SO ₄ . The method according to claim 1 or 2, The pretreatment bath further comprises 1 to 50 g / l of boric acid (H ₃ BO ₃ ) method of coating a metal layer. The method according to any one of claims 1 to 3, And the plating bath is a Watts bath containing nickel sulfate (NiSO ₄ ), nickel chloride (NiCl ₂ ) and boric acid (H ₃ BO ₃ ). The method according to any one of claims 1 to 4, And the plating bath is a citrate-gluconate bath containing NiSO ₄ and (NH ₄ ) ₂ SO ₄ and sodium citrate and sodium gluconate. The method according to any one of claims 1 to 5, And said pretreatment bath is free of fluoride containing components. The method according to any one of claims 1 to 6, The temperature of the pretreatment bath is maintained at a high temperature in the range of up to 95 ℃, preferably 55 ℃ to 80 ℃ metal layer coating method. The method according to any one of claims 1 to 7, The coated metal layer has a mean thickness in the range of less than 2 μm, preferably less than 1.0 μm, more preferably in the range of 0.2 to 1.0 μm. The method according to any one of claims 1 to 8, And the method is performed by a continuous plating process. The method according to any one of claims 1 to 9, The aluminum processing member is a brazing sheet product, The brazing sheet product comprises a cladding layer and a core layer formed of a brazing alloy comprising aluminum and 2 to 18 wt% silicon, preferably 7 to 14 wt% silicon, wherein the metal layer is coated on the clad layer. Metal layer coating method. The method of claim 10, The cladding layer further comprises a wetting agent in the range of up to 1 wt% as an alloying element. The method of claim 11, And the wetting agent is selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, thallium and mixtures thereof. The method according to any one of claims 1 to 9, The aluminum processing member is an aluminum conductor, and is preferably made of an alloy selected from the group consisting of AA1370, AA1110 and AA6101. A brazing sheet product electroplated with a metal layer selected from the group consisting of nickel, iron, cobalt and alloys thereof by using the method according to any one of claims 1 to 13.

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