GB2216899A - Control of electroplating of a batch of articles - Google Patents

Abstract

The apparatus comprises an individual plating cell for each article, each cell having its own power supply and electrolyte supply from a common reservoir and the current being continuously monitored by computer means and continuously integrated until a total desired level of charge has been passed to each cell. Once the desired charge has been passed to a cell that cell is then switched off. An example is given of plating turbine components with platinum. The plated articles may be subsequently treated by an aluminising operation. <IMAGE>

Description

Coatings The present invention relates to a process and apparatus for the electrodeposition of metaLlic coatings.

Components such as blades and nozzle guide vanes, for example, in gas turbine engines have to operate under highly agressive conditions. Such components require resistance to corrosion at high temperatures; specifically oxidation and sulphidation corrosion resistance and additionally, in the case of engines operating in a marine environment, resistance to salt corrosion.

Gas turbine engines operating in an exclusively aero environment such as in civil airlines frequently have aluminide coatings formed on the components. Such coatings have very high resistance to oxidation due to the highly adherent oxide scales produced on the component surface during operation.

Aluminide coatings, however, do not have adequate resistance to the corrosion mechanisms occurring in engines operating in industrial or marine environments.

One type of coating which has been successfully used for components under such conditions is pLatinum aluminide where the surface of the component is enriched with pLatinum.

The means of enrichment may vary and has included, for example, molten salt-bath diffusion techniques, sputtering and electrodeposition of a Layer of pLatinum. A thin coating of platinum is usually applied and the coated component is subjected to an aluminising operation and heat treatment which by diffusion forms the required pLatinum aluminide coating.

Electroplating of the platinum layer has a certain advantage. It is relatively economic to set up when compared to some other deposition techniques such as molten salt-bath, for example. The technology is reasonably well documented and the technique is economic for smalL batch sizes of components.

The conventional method of electroplating a batch of components in a single bath does however, have disadvantages. Because of the volume of the bath the cost of electrolyte may be very high especially with platinum containing solutions. Contamination of the bath may render the whole volume of electrolyte useless.

In an electroplating process, the thickness of coating at any point is primarily a function of the total charge passed per unit area at that point. Therefore, for a given sized component, the average thickness over its total plated area is proportional to the total charge passed through it. There are thus two options for controlling mean thickness. These are either passing a controlled current for a defined period of time or passing a non-constant current, at a constant voltage LeveL, for example, and terminating the cycle after the passage of a pre-defined charge.

In either case, with multi-component processing in a single bath only overall process control may be exercised.

The positioning of components to be plated within the bath may cause appreciable variation in the quality and quantity of coating deposited on each component due to each component receiving different quantities of electrical charge.

This latter variable may also cause inhomogenity in the electrolyte within the bath.

Furthermore, the variation with time of the current/voltage relationship for a given component is a good indication of the "state of health" of the -p lati ng process with regard to that component. With common bath processing any such variation in a single, or indeed in several, components may be masked by the ambient values for the bath as a whole.

In any case, even if a fault were detected, the only course of action available would be to stop the complete process thus prejudicing all those components which are being plated satisfactorily.

According to a first aspect of the present invention a method for the electrolytic deposition of a metallic coating onto a plurality of articles comprises the steps of immersing that part of each article to be coated in its own individual electroplating cell, changing the electrolyte of each cell by suppLying electrolyte from an electrolyte reservoir, monitoring the quantity of charge passed to each article and terminating the electricity supply when a predetermined quantity of charge has been passed.

The coating material may be selected from the platinum group of metals.

Controlling the processing of components individually overcomes all of the above disadvantages and at the same time allows the simultaneous plating of different types of component.

The individual cells may have constant level overflow weirs and be supplied with electrolyte by a pump such that the electrolyte is being changed continuously.

The electrolyte is supplied to each cell from a reservoir where it may be monitored for composition and temperature.

According to a second aspect of the present invention a method for the deposition of a precious metal aluminide coating on an article comprises the steps of depositing a coating according to the first aspect of the invention and then subjecting the coated article to an aluminising step.

The aluminising step may be carried out by any suitable technique known in the art, examples of which are the so-called pack cementation method or a vapour phase technique such as that described by Restall et al in GB 1549845.

According to a third aspect ov the present invention apparatus for the electrolytic deposition of a metallic coating onto a plurality of articles comprises an individual plating cell for each article, each cell having electrolyte level control means and electrolyte supply means, an electrolyte reservoir having associated temperature control for the electrolyte, pump means for supplying electrolyte from the reservoir to the cells and electrical supply and control means for supply and control of electrical charge to each cell.

In a preferred embodiment of the present apparatus the voltage to each cell is controlled and the current is monitored.

The individual cells may be arranged in groups, the apparatus having one or more groups of cells. In one embodiment of the apparatus the cells are arranged in groups of eight, there being twelve groups giving a total of ninety six cells in the apparatus. The use of eight cells in a group stems from this being a convenient number in computing terms. Much electronic hardware is available in packages incorporating logical functions grouped in multiples of eight. The use of twelve groups is not mandatory and any reasonable number of groups may be employed.

A common electrical power supply with a voltage output of, for example, 4 volts may be used. This may supply each cell through its own controller, to establish a programmed voltage at each cell. The controller may be programmed to ramp up the voltage from 0 to the desired supply voltage over a predetermined period, the desired supply voltage may then be held constant for the duration of the plating process. The voltage may, for example, be ramped up from 0 to a range of from 0.8 to 1.2 V over a period of typically 2 minutes.

The controller may continuously monitor the current to each cell and integrate this to establish the total charge passed. When a predetermined quantity of electricty has been passed to the component in a cell, that cell is switched off. Thus each individual component coated by the apparatus is controlled separately and the continuous monitoring and integration of the charge supplied to a cell is used as a control parameter to terminate the plating process at a predetermined point.

In one embodiment of the apparatus the controller monitors and samples the current in all ninety six cells every second. Upper and lower current limits may be defined; operation outside of which may yield substandard plating. The system may be programmed to switch off any individual cell which operates outside the set limits.

Furthermore, all the data relating to the electroplating of each individual component may be recorded for future analysis purposes.

The practical effect of the method and apparatus is that the plating parameters for each component are controlled and may be recorded individually. For components which are used in critical applications such as in gas turbine engines, it is possible to provide a full production history for each individual component rather than merely for a batch of plated components.

In order that the present invention may be more fully understood, an example will now be described by way of ill-ustration only with reference to the accompanying drawings of which: Figure 1, shows a section in elevation through an individual electroplating cell; Figure 2, shows a plan view of the cell of Figure 1; Figure 3, shows a general perspective view of the "plumbing" arrangements of part of the apparatus; Figure 4, shows a plan view of part of two rows of cells; Figure 5, shows a view in elevation of part of one row of cells; Figure 6, shows an end view in elevation of two rows of cells; and Figure 7, which shows a block diagram for the plating controller applied to one cell.

Referring now to the drawings and where the same features are denoted by common reference numerals. Not all features are shown in all figures.

The apparatus may comprise any reasonable number, twelve in this instance, of rows or groups 10, each row or group having up to eight individual electroplating cells 12.

Each cell comprises a base cup 14 having therein a tubular extension 16 sealed to the cup 14 by an "0" ring seal 18.

The base cup 14 has a tubular spigot 20 in the bottom thereof and a level controlling overflow weir 22 fitted in the spigot 20. A cap 24 is fitted to the top of each extension 16. The cap has a clip arrangement 26 fixed thereto for retaining in a predetermined orientation a component 28 to be plated. Electrical connectors 30 are provided to make the component 28 cathodic. In this case the component is a turbine blade. Contained within the cell is an anode 32 which in this case is a platinum plated titanium mesh basket. A tag 34 is provided-for an electrical connection. Each cell 12 is connected to a drain conduit 36 by means of the spigot 20 which fits into one branch of a tee-piece 38. A number of tee-pieces 38 are joined by pipe portions 40 to form the drain coduit 36 for each row 10.Each drain conduit 36 is joined to a manifold 42 by tee-pieces 44, the manifold 42 returning electrolyte to a central reservoir 46 via a conduit 48.

Electrolyte is supplied to each row of cells 10 via a supply manifold 50 by a pump 52 from the reservoir 46.

Each row 10 is connected to the supply manifold 50 via a tee-piece 54 and shut-off valve 56. Electrolyte is supplied to each cell 12 via flexible tubes 58 which are themselves connected to a four way distributor 60 and supply conduit 62. A flow monitor (not shown) is included in each cell supply and if flow falls below a predetermined level that cell is switched off. Shut- off clamps 64 are provided should any individual cells 12 of a row 10 not be in use. Should all the shut-off valves 56 be closed a supply by-pass valve 66 and return conduit 68 to the reservoir 46 are provided. An immersion heater 70 and thermostat 72 appropriately connected are provided in the reservoir 46 for electrolyte heating. The end of the supply conduit 62 remote from the shut-off valve 56 is supported by a leg 74 which also blanks off the end of the conduit 62.Similarly the drain conduits 36 are supported and blanked off by legs 76. The rows of cells 10 are arrayed over a drip tray 78.

The control layout is shown in Figure 7. A main power supply unit 80 supplies power to a control circuit 82 which is dedicated to a single plating cell 12, there are, therefore, as many control circuits 82 as there are plating cells. The voltage from the power supply 80 is controlled to a desired level by an amplifier 84. The current consumed by the cell 12 is read by a second amplifier 86. A computer 88 instructs the amplifier 84 at what Level to set the voltage via a latching, multi-channel D/A converter 90. The current consumed is read by the computer via a multiplexed, multi-channel A/D converter 92. Appropriate software in the computer 88 contains all the parameters necessary for the controlled plating of components in each individual cell.Data relating to the electrolyte such as flow rate and temperature are received by the computer via the input port 94 and appropriate control signals are issued by the computer via the output port 96.

The control system scans all ninety six cells in the apparatus approximately every second to ensure that the current flow to each cell is being maintained between predetermined upper and lower limits set in the computer software. The actual amount of electricity passed to each cell is continuously integrated by the computer 88 and when the total predetermined amount of electricity has been passed to each cell that cell is switched off.

Any cells having a current flow outside the desired upper and lower limit is switched off. This ensures that only components having electroplating parameters within the desired optimum range are electroplated. The computer also records the entire electroplating history of each component in a memory. The memory content may be printed out by suitable -means to provide the necessary information in a convenient form.

Not all cells in the apparatus or all cells within any one group must be occupied with a component being plated.

By substitution of the tubular extension 16 different length components may be plated. The minimum height extension consistent with the component length being coated may generally be used as this minimises electrolyte volume and hence cost.

It is also a facility of the apparatus to be able to remove one or more entire rows of eight cells and replace it with one or more larger cells in order to coat larger area components such as a turbine disc, for example. The controlling factor is the current required for larger components. It may be necessary, for example, to utilise the total "normal" current requirement for eight small components for one large component.

Gas turbine engine components have been electroplated with platinum using the method and apparatus described. An electrolyte based on NaPtOH6 was employed. Temperature was controlled to + 1 0C within a range from 650C to 800C. Current density was typically 0.7 A/dm2 which for a typical component yielded a current of about 100 mA at a voltage of 0.8 to 1.2. A plating time of 1 to 2 hours produced a coating thickness of 0.005 to 0.01mm.

Platinum plated components as described above were then given an aluminising treatment which consisted of packing the components in an alumina and aluminium powder mixture in a retort, heating the retort to a temperature of 9000C for 3 hours under an atmosphere of Argon gas. The aluminised components were found to have a uniform coating comprising platinum, aluminium and elements from the nickel alloy substrate from the component.

Claims (21)

1. A method for the electrolytic deposition of a metallic coating onto a plurality of articles, the method comprising the steps of immersing that part of each article to be coated in its own individual electroplating cell, changing the electrolyte of each cell by supplying electrolyte from an electrolyte reservoir, monitoring the quantity of charge passed to each article and terminating the electricity supply when a predetermined quantity of charge has been passed.

2. A method according to Claim 1 wherein the electrolyte is changed continuously.

3. A method according to either Claim 1 or Claim 2 wherein the voltage to each electroplating cell is maintained at a substantially constant value.

4. A method according to any one preceeding claim wherein the current to each cell is frequently monitored.

5. A method according to any one preceeding claim wherein the total electrical charge to each cell is continuously integrated by computer means and compared to an amount of charge which it is desired to achieve.

6. A method according to any one preceeding claim and where instantaneous current Level for each cell is continuously compared to desired minimum and maximum current levels in computer memory means and any cell not lying within the set range is switched off.

7. A method according to any one preceeding claim wherein electrolyte flow to each cell is monitored and any cell receiving insufficient flow is switched off.

8. A method according to any one preceeding claim wherein the condition of the electrolyte composition in the reservoir is monitored.

9. A method according to any one preceeding claim wherein the electrolyte temperature is continuously monitored and controlled.

10. A method according to any one preceeding claim wherein computer control means are programmed to ramp up the voltage to a desired level over a predetermined time period.

11. A method according to any one p re ceedi ng claim wherein the plating history for each individual article plated is recorded by computer means.

12. A method according to any one preceeding claim wherein the metallic coating is selected from the platinum group of metals.

13. A method according to any one preceeding claim further including the step of subjecting the electro plated article to an aluminising treatment.

14. Apparatus for the electrolytic deposition of a metallic coating onto a plurality of articles, the apparatus comprising an individual plating cell for each article, each cell having electrolyte level control means and electrolyte supply means, an electrolyte reservoir having associated temperature control for the electrolyte, pump means for supplying electrolyte from the reservoir to the cells and electrical supply and control means for supply and control of electrical charge to each cell.

15. Apparatus according to Claim 14 wherein the electrolyte level control means comprises overflow weirs in each cell.

16. Apparatus according to either Claim 14 or Claim 15 wherein the individual cells are arranged in one or more groups.

17. Apparatus according to Claim 16 wherein each group comprises eight cells.

18. Apparatus according to Claim 17 wherein one or more groups of eight cells may be replaced by a group having a lesser number of cells.

19. Apparatus according to any one preceeding claim from 14 to 18 wherein the electrolyte flow to each cell is monitored.

20. A method substantially as hereinbefore described with reference to the accompanying specification and drawings.

21. Apparatus substantially as hereinbefore described with reference to the accompanying specification and drawings.