GB2534433A

GB2534433A - Plating a metal

Info

Publication number: GB2534433A
Application number: GB1513407.5A
Authority: GB
Inventors: Sakhawat Hussain Mohammad
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-07-31
Filing date: 2015-07-30
Publication date: 2016-07-27
Also published as: GB201413633D0; GB201513407D0; GB2528873A

Abstract

A method of depositing nickel onto an object including; using a heat source 7 to heat a nickel containing electrolyte solution 3, using a pump 12 to allow the electrolyte 3 to continuously flow between the anode 4 and cathode 5, and applying direct current to the anode 4 and cathode 5 to allow nickel to deposit on the cathode 5, where the cathode 5 is the object. The object may contain one or more of aluminium, titanium, stainless steel or molybdenum, possibly covered in an oxide layer. The electrolyte 3 may be heated first before being allowed to continuously flow or flow continuously prior to heating. The electrolyte 3 may be heated to a predetermined temperature between 45 and 75 °C, before applying a current. The electrolyte 3 may also continuously flow until a thermal equilibrium is reached with the electrolyte 3, anode and cathode, prior to applying current to the anode 4 and cathode 5. The electrolyte 3 may be circulated in a turbulent flow. The anode 4 and cathode 5 or object may be spaced between 1 and 4 mm apart. A further aspect relates to a method of thickening an aluminium oxide layer on an aluminium object, using a heat source 7 to heat a nickel containing electrolyte solution 3 to a predetermined temperature and using a pump 12 to allow the electrolyte 3 to continuously flow between surfaces of an anode 4 and a cathode 5; where the cathode is an object and no current is supplied.

Description

PLATING A METAL

The present invention relates to a method of depositing nickel or a nickel alloy onto a metal that is known to be hard to coat, in particular a metal such as aluminium, but also metals such as titanium, molybdenum or stainless steel.

Aluminium is the second most widely used metal after iron. Aluminium is used in the manufacture of many household appliances, aerospace and automotive products and is lightweight, soft, durable and ductile.

It is common to want to plate other metals onto the surface of aluminium as a decoration or to enhance its physical properties.

Nickel is often coated on to the surface of aluminium to improve corrosion resistance. It may also improve wear resistance and/or increase its luster.

Nickel and other metals are often plated onto the surface of aluminium. The electroplating process uses an electrolyte and an electric current. The electric current is applied between the aluminium, the substrate (cathode), and an electrode (anode). The process is a chemical redox reaction. The process deposits a layer of nickel or another metal onto the substrate.

It is known however that as soon as aluminium is exposed to an atmosphere including oxygen, a thin oxide layer forms on the outer surface of the aluminium. This thin oxide layer affects the adhesion between the aluminium and the nickel or other metal that is to be deposited onto the surface of the aluminium. In order to achieve effective adhesion, the oxide layer is removed and prevented from reforming. Before electroplating, the aluminium is therefore usually pre-treated and/or pre-plated with an intermediate metallic layer. The aluminium may be cleaned to remove contaminants including the oxide layer that may interfere with the flow of ions between the anode and the aluminium.

It is known to pre-plate the aluminium by immersing it in a solution of zinc or tin. The function of pre-plating and/or multiple layers of nickel is to improve the adhesion of the nickel or other metal to be applied to the aluminium or titanium.

It is also known to first clean the surface of the aluminium with a solution of sodium or potassium hydroxide. The solution typically has a pH of about. 12 or above. The solution may also comprise nickel or cobalt. A non-cyanide complexing agent is used to keep the nickel or cobalt in solution at a pH of about 12 or above.

Direct plating on aluminium has long been an engineering pursuit. By "direct plating" is meant without the need for cleaning, pre-treatment or pre-plating. The direct plating is plating that is straight onto the aluminium, or other metal, or any oxide coating that is formed on it i.e. without removing naturally occurring surface layer.

Plating of metals on aluminium is a complex and highly developed art. The prior art discloses different methods to deposit nickel onto aluminium. Since 1927 almost all of the successful coating techniques reported in the literature incorporate some method of removing the natural oxide layer and subsequently depositing a metal either directly onto the aluminium or onto the previously formed immersion film. However, there is no simple method that enables nickel to be plated directly onto aluminium, titanium, molybdenum and stainless steel, which are loosely termed as "difficult to plate metals" in the electroplating industry.

These metals have never been plated directly with nickel without elaborate pretreatment, which can be fairly lengthy and are also expensive. No true direct plating of these metals has been reported. Currently in the plating industry prior to depositing nickel on aluminium, this oxide fihn is removed and stopped from reforming by a pre-treatment process, followed by the deposition of a metal e.g. zinc on aluminium surface. Even after all these treatments the level of adhesion is not always acceptable. Plating of nickel on aluminium has therefore emerged as a complex and highly-developed art since it was first reported in 1913, more than a century ago.

The historical approach has required one or more extra steps, including removing the oxide layer: in 1927, the "zincate" process was disclosed, which results in removal of this aluminium oxide layer and the creation of a zinc layer on the new aluminium surface, which can then be classically electroplated with nickel. The 1939, the "double zincate" process addressed the nickel-aluminium substrate adhesion aspect. Not all types and grades of aluminium can be treated by these processes. In US Patent No: 4197722 Schardein et al. have reviewed many other approaches to plating aluminium. The prior art recognises the desirability of being able to apply a nickel coating directly to an aluminium substrate. Several patents claim to disclose a "direct" process for plating nickel on aluminium. Upon closer examination, however, these disclosures either include an additional pre-plating immersion process step (which may involve cyanide solutions or involve up to 13 separate steps, including a fusion step above 500°C).

Aluminium is the only metal that has been commercially prepared for the reception of adherent electrodeposits by anodizing. Although anodizing has been used as a pre-treatment for plating on aluminium for more than 70 years, the process is not as common as the zincate and stamtatc process. Anodizing is a process used to thicken the oxide layer on aluminium, used industrially since 1923. When anodising, there is an electric field, acidity/alkalinity, oxygen and water. With these variables, anodized coatings of aluminium oxide, ranging from a few microns to more than 100fim, can be created. The porosity of these anodised coatings is due to the equilibrium between the acid dissolution of the oxide and the electrolytic growth of the oxide layer.

Three understandings have emerged of "high-speed plating". These are as follows: first, this term is applied to fast substrate movement, such as in roll-to-roll plating with a high linear substrate speed, or in a cathode rotating at high speed; a second use of high-speed plating is the fast electrodeposition of the metal; a third meaning is used here and relates to the volumetric flow rate of the electrolyte.

The invention relates to a method of depositing nickel onto an object, comprising the step of heating a nickel containing electrolyte solution to a predetermined temperature and causing the electrolyte to continuously flow between surfaces of an anode and a cathode each at least partially submerged in the electrolyte, and applying current to the anode and cathode to deposit nickel on to the cathode, wherein the cathode is the object.

The invention provides a method of depositing nickel onto an object, the method comprising: providing a source of direct current having a posit.ve and a negative terminal and a nickel containing electrolyte, connecting the object to the negative terminal; connecting the anode to the positive terminal; positioning the object in the electrolyte; providing a heat source and heating the electrolyte solution to a predetermined temperature; providing a pump and causing the electrolyte to continuously flow between surfaces of the anode and object; and applying direct current to the anode and object from the direct current source and performing electroplating through the continuously flowing nickel containing electrolyte solution in between surfaces of the anode and the object.

The method may include positioning the anode in the electrolyte.

The object may comprise one or more of aluminium, titanium, stainless steel and molybdenum.

The object or surface of the object may be coated in an oxide layer. The object or surface may be shrouded in an oxide layer in the as received condition.

The method may comprise first heating the electrolyte and then causing it to continuously flow.

The method may comprise first causing the electrolyte to continuously flow and then heating it.

The method may further comprise maintaining the temperature of the electrolyte during all or some of the process.

The method may further comprise causing the electrolyte to continuously flow and/or heating the electrolyte to a predetermined temperature before applying current to the anode and cathode.

The method may further comprise causing the electrolyte to continuously flow until there is thermal equilibrium with the electrolyte, anode and cathode before applying current to the anode and cathode.

The electrolyte may be heated and caused to continuously flow for up to 10 minutes; preferably for between 2 and 8 minutes or between 4 and 6 minutes, for example between 3 and 5 minutes before the electroplating begins.

The present invention preferably uses flow rates of up to 5m/s. The electrolyte may be circulated at an average velocity of up to 5 m/s, but is preferably circulated at an average velocity of I to 5, or 2 to 5 or 3 to 5 m/s.

For aqueous electrolytes, this gives a Reynold's Number around 4700, which defines purely turbulent or non-laminar flow.

The electrolyte may be circulated in a turbulent flow.

The pre-determined temperature may be between 45 and 75'C; for example between 50 and 70'C preferably the pre-determined temperature is between 55 and 65 le, for example 65 C. The pH of the nickel containing electrolyte may be between 2 and 6, or 3 and 5. Preferably the pH of the nickel containing electrolyte is 4.

The object may comprise, consist or consist essentially of or be any suitable metal for example aluminium, titanium, molybdenum and stainless steel or an alloy or combination thereof. Preferably the object is aluminium. In one embodiment the object has, or the surface of the object has, an oxide layer.

The method of depositing nickel on the object or surface of the object may include the step of thickening the oxide layer before the nickel in the electrolyte comprising nickel is deposited on the object or surface of the object.

it may be an advantage of the present invention that the oxide layer is thickened and/or the surface of the object becomes susceptible to the nickel in the solution comprising nickel being deposited on the surface of the object. Thickening of the oxide layer may be part of the step of depositing the nickel on the surface of the object.

This may be particularly suited to the object when it comprises aluminium, the object coated in an aluminium oxide layer.

in one embodiment, the anode and the cathode or object are spaced I to 4mm apart for example 2 to 3mm apart in another embodiment the anode and the cathode or object are spaced 2mm apart.

The anode and cathode or object may be in contact with the electrolyte when the pump is not in use.

In one embodiment, the heat source is activated before the pump. In another embodiment the pump is activated before the heat source. In a further embodiment, the heat source and pump are activated at the same time.

The electrolyte may be any suitable nickel solution such as a Watts-type nickel plating solution, for example nickel sulphamate, nickel fluoborate, nickel sulphate, nickel chloride or any mixture thereof. The electrolyte may be boric acid or a mixture of boric acid and another suitable component.

The anode may comprise, consist of, consist essentially of or be nickel or an inert material.

The invention also relates to a method of thickening an aluminium oxide layer on an aluminium object or surface of an aluminium object, comprising the step of heating a nickel containing electrolyte solution to a predetermined temperature and causing the electrolyte to continuously flow between surfaces of an anode and a cathode, wherein the cathode is the object and no current is being supplied to the anode or the cathode.

The invention also provides a method of thickening an aluminium oxide layer on an aluminium object, the method comprising: positioning an anode and an object in a nickel containing electrolyte; providing a heat source and heating the electrolyte to a predetermined temperature; providing a pump causing the electrolyte to continuously flow between surfaces of the anode and object; wherein there is no current applied to the anode or object.

The method may further comprise providing a source of direct current having a positive and a negative terminal, connecting the object to the negative terminal of a source of direct current and connecting the anode to a positive terminal of a source of direct current.

The method may further comprise the step of applying direct current to the anode and object from the direct current source and performing electroplating through the continuously flowing electrolyte solution in between surfaces of the anode and the object.

The applicant has observed a morphology in the process of the invention that seems to correspond with the growth in the film thickness, and the development of the surface morphology. In this new process of the direct nickel plating of aluminium, even before the electric field is applied, there is a change in the chemical environment of the metal there is an increased temperature, acidity, oxygen and water. The electrolyte is circulated until it, and the apparatus, attain the pre-set temperature (e.g. 65°C). During this initial, start-up phase, of duration only a few minutes, the high speed of the acidic electrolyte, flowing turbulently over the top surface of the oxide, creates an irregular or electrically conductive morphology, similar to that observed in anodising ("horizontal" morphology development).

The applicant has managed to deposit nickel directly onto the oxidized layer surface of aluminium by firstly using a turbulent flow of electrolyte without using a power supply and then using a power supply which has not previously been done.

A number of embodiments of the invention will now be described with reference to the drawings. The description and drawings are not intended to be limiting on the scope of protection and are purely example embodiments. In the figures: Figure 1 shows schematically an apparatus for the high-speed direct plating of nickel according to the present invention.

Figures 2a and 2b show through schematic sections the development of the film surface morphology during the early stages of electrolyte flow process without any current flow according to the present invention.

Figure 3 shows a schematic diagram showing the thickening of the existing oxide layer between the electrodeposited Nickel and the aluminium substrate prior to electrodeposition.

Figure 4 is a HR-TEM image showing the existence of an oxide layer (40 to 47nm) between the aluminium substrate and the electrodeposited Ni.

Figure 1 shows an apparatus for the high-speed direct plating of nickel according to the present invention. The apparatus includes a circulating flow system 1 which comprises chemical and heat resistant plastic pipes 1I and includes a container 2 (e.g. a two litre glass beaker) acting as an electrolyte reservoir to supply the flow system 1. The container 2 contains a nickel electrolyte 3. A conventional nickel electrolyte may be used for example this embodiment uses nickel sulphate. In this embodiment, the nickel solution has a pH around 4.

The circulating flow system 1 receives an anode 4 and a cathode 5, which are each attached to a power supply 6 by wires 6a and 6b. In this embodiment the anode 4 and cathode 5 are spaced 2mm apart. The circulating flow system pushes electrolyte between the anode and cathode. This embodiment also utilises a heat source 7 positioned beneath the container 2 and a pump 12.

in order to electroplate the substrate according to the present invention, the heat source 7 and the pump 12 are activated before the power supply 6 is activated. In the present embodiment, the heat source 7 is first activated and the nickel electrolyte 3 within the container 2 is heated to 65°C. The pump 12 is then activated which causes the nickel electrolyte 3 to flow around the circulating flow system I at an average velocity of 3 m/s. The power source 6 is activated when the plating is to begin, ideally when the electrolyte is flowing round the circulating flow system at the required flow rate of 3 to 5 metres per second and is being maintained at the desired temperature of 50 to 65°C.

In other embodiments the pump 12 may be activated before the heat source 7 or at the same time as the heat source 7.

The nickel solution 3 flow is turbulent.

Turbulent solution flow in the narrow, preferably 2mm wide, channel between conforming cathode 5 surfaces and soluble or insoluble anodes 4 is utilised to deposit dense, nano-crystalline nickel directly on the cathode 5 (the substrate) without any pre-treatment. In this embodiment, the substrate is aluminium.

Turbulent flow is an important aspect of this new industrial process for plating nickel on aluminium. Without wishing to be bound by theory, it is believed that the turbulent flow of the acidic electrolyte 3 contributes to oxide layer thickening, and to the preparation of the amphoteric oxide layer for the nickel electrodeposition. An aluminium surface, cathode 5 (the substrate) adsorbs oxygen within milliseconds of exposure to air. It then forms an A1,03 oxide layer 13 that continues to grow because there is a potential between the metal substrate 5 and the adsorbed oxygen. This enables aluminium ions to move through the film. The limiting thickness of this stable film at ambient conditions is typically 2nm for aluminum. This is showing in Figures 2a and 3a.

This limit is reached when aluminium ions can no longer cross the layer 13 by diffusion without the aid of an electric field). If the environment of the surface is changed, this layer can grow further. Factors that can cause this film to thicken include increasing the temperature, applying an electrical potential, the presence of water and oxygen, and modifying the pH (acidic or basic, since aluminium oxide is arnphoteric).

in this new process of the direct nickel plating of aluminium, there is a change in the chemical environment of the aluminium even before the electric field is applied -there is increased temperature, acidity, oxygen level and water level.

The circulation of electrolyte, at for example 3 to 5 metres per second, causes a change in the chemical environment of the metal being plated (the cathode 5) -there is an increased temperature as the electrolyte is being heated/is heated to 50 to 65°C, acidity (the electrolyte has a pH of 2 to 6), oxygen and water. During this initial start-up phase, the high speed movement of the acidic electrolyte (the nickel solution 3), flowing turbulently over the top surface of the aluminium outer oxide layer (of cathode 5), creates an irregular and electrically conductive; a "horizontal" morphology development -similar to that observed in anodizing. The change in chemical environment allows further A13 ions 14 to join the oxide layer 13 causing it to grow.

This is shown in Figure 2b where the oxide layer is 45nm.

A morphology has been observed, as a result of the process of the present invention that seems to correspond with growth in the film thickness, and the development of the surface morphology, that were observed in the initial and mid-stages of anodizing, schematically shown in Figures 2a and 2b and in Figures 3a and 3b Using High Resolution -Transmission Electron Microscope (HR-TEM) as shown in figure 4 we observe that nickel 42 is deposited on the pre-existing metal-oxide (MO) layer of the cathode, for example, A1203 layer 43. The figure shows a considerable thickening 41 of the naturally existing metal-oxide layer (MO) 43 after electrodeposition. During high speed movement of the electrolyte, the existing oxide (A1203) thin film on the surface of aluminium or the cathode enabled positive metal ions to diffuse through the oxide. At the surface between the oxide and air, they are able to combine with oxygen to form new layers of oxides assuming electrons too are able to get through the layer from the metal to the surface. It is proposed that during the start-up phase -the few minutes prior to plating -film thickening occurs, and conditions are created for effective nickel plating: the flow of the hot electrolyte increases thermal diffusion of (A13 /02-) ions; turbulent flow places fresh electrolyte at the surface, increasing the chemical potential that becomes a further driver for the ionic diffusion. HR-TEM combined with elemental analysis shows that the aluminium oxide layer has increased in thickness to about 45nm. This film growth is similar to anodising, but has not been reported before.

As stated above, the electrolyte is circulated until it, and the apparatus, attain the pre-set temperature (e.g. 65°C). During this initial start-up phase with a duration of only a few minutes, two dimensions of oxide development occur: (i) There is an environment conducive to the thickening of the oxide layer ('vertical" growth), from the diffusion of aluminium ions through the oxide film, and oxygen availability from the surface.

(ii) The high speed of the acidic electrolyte, flowing turbulently over the top surface of the oxide, creates an irregular and electrically conductive morphology, similar to that observed in anodising ("horizontal' morphology development).

The change in the surface layer thickness and morphology is illustrated schematically in Figures 2 and 3. The existence and importance of this start-up (pre-electric-field) phase, in the process of the plating of aluminium, has not previously been identified and reported. It is known experimentally that humidity is one parameter that can profoundly influence the oxidation rate. It is therefore expected that the oxide growth rates will generally be increased by crystalline imperfections.

In this embodiment the nickel electrolyte 3 at 65°C is allowed to flow for five minutes to allow the circulating flow system 1 to reach thermal equilibrium (suitable current densities include 0.1, 0.3, 0.5, 1.0 and 1.5 A/cm2). The power supply 6 is activated then, which powers the electrodes 4, 5.

When the electric field is applied for nickel plating, the surface of this thicker oxide layer is already in a very activated state because of the ionic movement. This provides a surface on which nickel is readily nucleated under the applied electric field. Further nucleation and growth is the mechanism by which an adherent nickel layer is formed on this aluminium oxide.

A critical aspect of this process is the proximity of the conforming electrodes 4,5. The preferred 2mm spacing provides a high potential gradient. A rectifier provides the current required for the electroplating. At about 10V the potential gradient is typically 5kV/m. A preliminary study of the process variables of direct plating of nickel on aluminium has established that the electrodeposition is increased by current density (up to 0.6 A/cm2), electrolyte temperature (up to 65°C) and electrolyte flow rate (up to 5ms1).

Claims

CLAIMS1. A method of depositing nickel onto an object, comprising the step of heating a nickel containing electrolyte solution to a predetermined temperature and causing the electrolyte to continuously flow between surfaces of an anode and a cathode each at least partially submerged in the electrolyte, and applying current to the anode and cathode to deposit nickel on to the cathode, wherein the cathode is the object.
2. A method of depositing nickel onto an object, the method comprising: providing a source of direct current having a positive and a negative terminal and a nickel containing electrolyte, connecting the object to the negative terminal; connecting the anode to the positive terminal; positioning the object in the electrolyte; providing a heat source and heating the electrolyte solution to a predetermined temperature; providing a pump and causing the electrolyte to continuously flow between surfaces of the anode and object; and applying direct current to the anode and object from the direct current source and performing electroplating through the continuously flowing nickel containing electrolyte solution in between surfaces of the anode and the object.
3. The method of claim 1 or claim 2 wherein the method includes pos t on ng the anode in the electrolyte.
4. The method of any one of the preceding claims wherein the object comprises one or more of aluminium, titanium, stainless steel and molybdenum.
5. The method of any one of the preceding claims wherein the object or surface of the object is coated in an oxide layer.
6. The method of any one of the preceding claims wherein the method comprises first heating the electrolyte and then causing it to continuously flow.
7. The method of any one of claims I to 5 wherein the method comprises first causing the electrolyte to continuously flow and then heating it.
8. The method of any one of the preceding claims further comprising causing the electrolyte to continuously flow and/or heating the electrolyte to a predetermined temperature before applying current to the anode and cathode.
9. The method of any one of the preceding claims further comprising causing the electrolyte to continuously flow until there is thermal equilibrium with the electrolyte, anode and cathode before applying current to the anode and cathode.
10. The method of any one of the preceding claims wherein the electrolyte is circulated in a turbulent flow.
11. The method of any one of the preceding claims wherein the pre-determined temperature is between 45 and 75 C; for example between 50 and 70 C, or between 55 and 65 C, for example 65 C.
12. The method of any one of the preceding claims wherein the method of depositing nickel on the object or surface of the object includes the step of thickening the oxide layer before the nickel in the electrolyte comprising nickel is deposited on the object or surface of the object.
13. The method of any one of the preceding claims wherein the anode and the cathode or object are spaced 1 to 4mm apart, for example 2 to 3mm apart, for example 2mm apart.
14. A method of thickening an aluminium oxide layer on an aluminium object or surface of an aluminium object, comprising the step of heating a nickel containing electrolyte solution to a predetermined temperature and causing the electrolyte to continuously flow between surfaces of an anode and a cathode, wherein the cathode is the object and no current is being supplied to the anode or the cathode.
15. A method of thickening an aluminium oxide layer on an aluminium object, the method comprising: positioning an anode and an object in a nickel containing electrolyte; providing a heat source and heating the electrolyte to a predetermined temperature; providing a pump causing the electrolyte to continuously flow between surfaces of the anode and object; wherein there is no current applied to the anode or object.
16. The method of claim 14 or claim 15 further comprising providing a source of direct current having a positive and a negative terminal, connecting the object to the negative terminal of a source of direct current and connecting the anode to a positive terminal of a source of direct current.
17. The method of claim 16 further comprising the step of applying direct current to the anode and object from the direct current source and performing electroplating through the continuously flowing electrolyte solution in between surfaces of the anode and the object.
18. A method of depositing nickel onto an object substantially as defined herein and with reference to the drawings.
19. A method of thickening an aluminium oxide layer on an aluminium object substantially as defined herein and with reference to the drawings.