GB2429465A

GB2429465A - Aluminide coating for a substrate and a method for providing same.

Info

Publication number: GB2429465A
Application number: GB0517259A
Authority: GB
Inventors: Zhidong Xiang; Prasanta Kumar Datta
Original assignee: Northumbria University
Current assignee: Northumbria University
Priority date: 2005-08-24
Filing date: 2005-08-24
Publication date: 2007-02-28
Also published as: GB0517259D0

Abstract

A coating provided on a substrate (1) comprising a first metal or an alloy of said first metal. In one embodiment the substrate comprises iron or steel. The coating comprises two layers: a first layer (2) comprising the second metal which is different to the first metal of the substrate and a second layer (3) comprising an aluminide of said second metal. The second metal or precursor may be nickel or chromium. The precursor layer comprising the second metal is applied on top of the substrate by electrolytic plating or electroless plating or thermal spraying. The second coating layer is obtained by pack aluminisation processes or by thermal spraying. The thickness of the second layer is smaller than the thickness of the precursor layer. The coating provides improved long term oxidation and corrosion resistance, particularly in high temperature working environments, compared to conventional single layer coatings.

Description

ALUMINIDE COATING FOR A METAL OR METAL ALLOY

The present invention relates to a coating provided on a metal or metal alloy substrate, and a method for providing such a coating. The coating comprises a metal alurninide layer, particularly, hut not exclusively, a nickel or chromium alurninide coating which is eminently suitable for use on a steel substrate at high temperatures.

There is currently a need to increase the efficiency of power generation processes and to reduce the amount of carbon-gas emissions dunng such processes. For steam turbine power generation, this demand could be met by increasing the steam operating temperature above the current temperature of 500 C to 550 C.

Many components of conventional steam turbines are manufactured from metals and/or metal alloys, such as alloy steels. Although high strength alloy steels containing typically 9-12 wt. % chromium and 1 wt. % molybdenum or tungsten have recently been developed which possess acceptable strength and exhibit satisfactory creep resistance at temperatures above 550 C, an increase in steam temperature to these levels is still problematic because it can cause enhanced oxidation and corrosion to the alloy steels and thereby reduce the lifetime of turbine components. Coatings are therefore needed to protect steels and other metals/metal alloys against degradation in supercritical steam.

For many metals and/or alloys, an important problem that will have to be addressed in providing high temperature oxidation and corrosion resistant coatings is how to deposit a suitable coating at temperatures that do not exceed the level at which the rnicrostructure of the substrate may become fatally disrupted. For example, for the components made of alloy steels, oxidation and corrosion resistant coatings will have to be deposited at temperatures below 700 C in order to preserve the microstructure of the alloy steel and hence the mechanical strength and creep resistance.

An object of the present invention is to obviate or mitigate one or more of the aforementioned problems.

A first aspect of the present invention provides a coating provided on a substrate comprising a first metal or an alloy of said first metal, wherein the coating comprises a first layer comprising a second metal which is different to said first metal, and a second layer comprising an aluminide of said second metal.

A substrate, such as a component of a steani turbine, having a coating in accordance with the first aspect of the invention, should exhibit improved oxidation and corrosion resistance and thus enjoy a longer operational lifetime than a substrate protected with a conventional single layer coating. It is envisaged that the second aluminide layer of the coating provides high temperature oxidationlcorrosion resistance. Accordingly, the second layer is preferably an oxidation barrier which impedes oxidation of the first layer.

A second related aspect of the present invention provides a method for providing a coating on a substrate comprising a first metal or an alloy of said first metal, wherein the method comprises providing a first layer comprising a second metal which is different to said first metal, and providing a second layer comprising an aluniinide of said second metal.

Interdiffusion between aluminide coatings and metal substrates can take place, particularly when operating at relatively high temperatures, during which aluminium atoms in the coating may diffuse inwardly into the substrate and nîetal atoms outwardly into the coating. This continuous interdiffusion process degrades the usefulness of the coating. Aluminium diffusion into metals is particularly fast at high temperatures, which can degrade the mechanical properties of the metal substrate. Accordingly, it is preferred that the first layer is interposed between the substrate and the second layer. In this way, the first layer of the coating can act as a barrier layer to prevent or to slow down aluminium diffusion into the substrate. Thus, it is preferable that the first layer is a diffusion barrier which restricts diffusion of aluminium atoms from the second layer into the substrate so that the coating can provide long-ten-n protection for the metal substrate. The second aluminide layer can also serve as an effective barrier preventing or slowing down the outward diffusion of metal atoms from the substrate.

In a preferred embodiment of the present invention the first metal is iron and it is particularly preferred that the substrate comprises a commercial steel, such as carbon steel, stainless steel or alloy steel containing 1-12 wt. % chromium and I wt. % molybdenum or tungsten or other refractory or transition metals.

Preferably the second metal is nickel or chromium and it is preferred that the first layer is substantially pure nickel or chromium metal. The second layer may comprise nickel aluminide or chromium aluminide. Where the second layer comprises nickel aluminide, the nickel aluminide may have a formula selected from the group consisting of Ni2AI3, NiAI and Ni3AI. When the second layer comprises chromium aluminide, the chromium aluniinide preferably has a formula selected from the group consisting of Cr2Al, Cr5AI8, Cr4Al9, Cr2A111 and CrAl7.

A particularly preferred embodiment of the present invention provides an alloy steel substrate with a coating comprising a first layer of substantially pure nickel metal deposited on to a surface of the substrate and a second outer layer of nickel aluniinide (for instance Ni2A13) provided on the first substantially pure nickel layer.

An alternative preferred embodiment provides an alloy steel substrate with a coating comprising a first layer of substantially pure chromium metal deposited on to a surface of the substrate and a second outer layer of chromium aluminide (for instance Cr2A111) provided on the first substantially pure chromium layer.

Regarding the coating, while any appropriate ratio of the thickness of the second layer to the thickness of the first layer may be used to suit a specific application, it is preferred that said ratio (second layer thickness first layer thickness) is less than or equal to approximately 2: 1, more preferably less than or equal to I: 1. Thus, the ratio (second layer thickness: first layer thickness) may be less than or equal to approximately 0.7: 1, more preferably in the range (0.2 - 0.5) 1 and yet more preferably in the range (0.3 - 0.4) : 1. Most preferably the ratio of the thickness of the second layer to the thickness of the first layer is approximately 0.35: I. For certain applications it may be desirable to provide a coating in which the ratio of the thickness of the second layer to the thickness of the first layer is less than or equal to approximately 3: 1, more preferably less than or equal to approximately 2: 1.

The first layer may possess any suitable thickness although it is preferred that the first layer has a thickness of less than approximately 100 m, more preferably less than approximately 50 jim, still more preferably less than approximately 20 m, and most preferably approximately 10 pm.

As with the first layer, the thickness of the second layer may take any suitable thickness for a given application. Appropriately, the second layer may have a thickness of less than approximately 50 tim, more preferably less than approximately 20 tm and most preferably a thickness of approximately 10 jim.

To provide additional oxidation resistance it is preferred that the coating comprises a third layer comprised of aluminium oxide provided on said second aluniinide layer. The layer of aluminium oxide may be provided by any convenient means, including exposing a surface of the second aluminide layer to ambient air or operating environment so that an aluminium oxide scale forms in situ on the exposed surface.

As described above, the second aspect of the present invention sets out a method for providing the coating on the substrate. In a preferred embodiment of this aspect of the present invention a precursor layer comprising the second metal is deposited on the substrate and said precursor layer is aluminised to a predetermined depth which is less than the total depth of the precursor layer such that the aluminised region of the precursor layer forms the second aluminide layer and the unaluminised region of the precursor layer forms the first layer.

Preferably the precursor layer is provided by electrolytic plating or electroless plating.

Alternatively, the precursor layer may be provided by thermal spraying in which case it is preferred that the thermal spraying process employs the second metal (e.g. nickel or chromium) in the form of a powder.

As described above the second layer may be substantially pure nickel aluminide having the formula Ni2Al3, NiAI or Ni3AI, or chromium aluminide having the formula Cr2A1, CrsAl8, Cr4AI9, Cr2AI1 i or CrAI7. The composition of the second aluminide layer can be controlled by the alurninising conditions, i.e. by adjusting aluminising pack composition or temperature, or by thermal spraying conditions via controlling the aluminium to nickel ratio or aluminium to chromium ratio in the metal powders.

When the second layer is provided by aluminising a region of the precursor layer, it is preferred that the alun-iinisation process is effected at a temperature in the range 500 C to 1150 C, more preferably 550 C to 950 C, still more preferably 550 C to 700 C. It has been determined that particularly preferred temperatures for the aluminisation process lie in the range 600 C to 650 C.

The aluminisation reaction time may be used to control the amount of aluminium deposited and therefore the thickness of the second aluminide layer. Preferably the aluminisation process is effected over a reaction time period of up to 24 hours, niore preferably a time period in the range 2 to 10 hours and most preferably 4 to 9 hours. In accordance with the Examples set out below, it has been determined that a particularly preferred reaction time period over which the aluminisation process is effected lies in the range 6 to 8 hours.

As described above, the second layer of the coating is preferably provided by aluminising the precursor layer comprising the second metal (e.g. nickel or chromium).

In this case, it is preferred that, prior to aluminisation, the precursor layer has a thickness of less than approximately 200 tm, more preferably less than approximately m, still more preferably less than approximately 75 urn, and most preferably less than approximately 40 m.

The second layer is preferably provided by aluminising the precursor layer to a predetermined depth which is less than the total depth of the precursor layer prior to aluminisatjon. Conveniently, the ratio of said predetermined depth to the thickness of the precursor layer (predetermined depth: precursor layer thickness) is less than around 0.9: 1, more preferably in the range (0.1 - 0.8) : I. In further preferred embodiments of the invention the ratio of said predetermined depth to the thickness of the precursor layer is in the range (0.2 - 0.7) : I or may lie in the range (0.4 - 0.5) 1.

Although any appropriate aluminisation process may be used, it is preferred that the second layer is provided by a pack aluminisatjon process, which may be an in-pack or out-of-pack alurninisation process. In the case where an in-pack aluniinisation process is used it is preferred that the process employs a powder mixture comprising powdered aluminium metal. The powder mixture may comprise up to 20 wt. % powdered aluminium metal. More preferably the powder mixture comprises 1 to 15 wt. 0/) 2 to 10 WI. % and most preferably 2 to 4 wt. % powdered aluminium metal. Preferably the powder mixture further comprises powdered aluminium oxide. The powdered aluminium oxide is preferably present in the powder mixture in an amount of up to 98 wt. %, more preferably 76 to 98 wt. % and most preferably 85 to 95 wt. % The powder mixture may further comprise a powdered chloride or fluoride salt, preferably one of the types including AiX3, NaX, NH4X (where X = Ci or F). The powder mixture may comprise I to 4 wt. % of the chloride or fluoride salt. A chloride salt such as AICI1, NH4C1 or NaCI can be used, and is particularly preferable when it is desired to activate the alurninisation process at temperatures below about 700 C. A fluoride salt, such as AIF3, NH4F and NaF may be used to activate the aluminisation process and is preferred when the aluminisation process is effected at temperatures higher than approximately 700 C.

In a further preferred embodiment of the second aspect of the present invention the first layer comprising the second metal is deposited on the substrate and the second aluminide layer is deposited on said first layer. The first layer may be deposited by electrolytic plating, electroless plating or thermal spraying. Said thermal spraying of the first layer preferably employs the second metal in the form of a powder.

As an alternative to providing the second layer by aluminisation, the second layer may be provided by thermal spraying. In this case it is not necessary to deposit a precursor layer of greater thickness than the intended thickness of the first layer. The first layer can be deposited to the appropriate thickness by any appropriate method (e.g. electrolytic plating, electroless plating or thermal spraying) and then the second layer deposited on the first layer by thermal spraying. In this way the first layer may be interposed between the second layer and the substrate. Said thermal spraying of the second layer preferably employs an aluminide of said second metal in the form of a powder and/or at least one of the second metal in the form of a powder and aluminium metal in the form of a powder. The particle sizes of the powders used for thermal spray are normally below 10 micron.

In accordance with a preferred embodiment of the second aspect of the present invention the coating may be provided on other metal substrates such as refractory metals niobium, molybdenum and tungsten, or their alloys, by a two-step process comprising electrolytic or electroless plating to form a substantially pure nickel precursor layer on a surface of the metal substrate followed by aluminisation to form an outer nickel aluminide layer using a pack aluminisation process at a temperature in the range of 500 C to 1150 C depending on the substrate material and the initial nickel precursor layer thickness. A higher temperature will effect a faster growth rate of the aluminised layer. It will be evident to the person skilled in the art that the precursor layer should be of sufficient thickness to enable it to be aluminised under a set of suitable aluminising conditioiîs including temperature, time and pack composition to an appropriate depth to provide the second aluniinide layer of the desirable thickness whilst leaving an unaluminised layer of the second metal of the desired thickness interposed between the second layer and the substrate. For example, as descnbed below in Example 1, a precursor layer of nickel is first deposited on to a steel substrate. The precursor layer is then aluminised under suitable conditions to a depth of around 36 im to provide an outer nickel aluminide layer having a thickness of around 36 m and an unalurninised layer of substantially pure nickel metal on the substrate having a thickness of around 94 jim.

A further preferred embodiment of the second aspect of the present invention provides a two-step thermal spraying process comprised of spraydepositing an initial substantially pure nickel precursor layer using nickel powder on to a surface of the metal substrate and then spraydepositing a nickel aluminide layer using N1AI powder, Ni2AI3 powder, or a mixture of nickel and aluminium powders in appropriate proportions.

A further preferred embodiment of the second aspect of the present invention provides a two-step process comprised of spray-depositing an initial substantially pure nickel precursor layer using nickel powder on to a surface of the metal substrate and then partially aluminising the precursor layer using a pack aluminising process at a temperature in the range of 500 C to 1150 C to provide the second outer nickel aluminide layer.

It will be appreciated that further preferred embodiments of the second aspect of the present invention are as set out above in the preceding two paragraphs save for replacing nickel/nickel aluminide with chromium/chromium aluminide. It is envisaged that metals other than nickel and chromium could be employed in the method of the present invention to provide coatings on metal or metal alloy substrates. For example, cobalt/cobalt aluminide may be used in the place of nickel/nickel aluminide or chromium/chromium aluminide on metals or metal alloys.

The present invention will now be further described with reference to the following non-limiting examples and figures in which: Figure 1(a) is a cross sectional SEM micrograph showing the microstructure of a Ni/Ni2AI3 coating provided on a steel substrate; Figure 1(b) is a graphic representation of the element concentration depth profile for the coating shown in Figure 1(a); Figure 2 is an X-ray diffraction pattern of the coating of Figure 1(a); Figure 3(a) is a cross sectional SEM micrograph showing the microstructure of a Cr/Cr- aluminide coating provided on a steel substrate; and Figure 3(b) is a graphic representation of the element concentration depth profile for the coating shown in Figure 3(a).

EXAMPLES

The following Examples demonstrate methods by which coatings can he formed on substrates in accordance with the present invention.

EXAMPLE 1

Production Of A Nickel/Nickel Aluminide Coating On A Steel Substrate A substrate of a commercial type of an alloy steel P92 containing 9 wI. % chromium, I wt. % molybdenum and trace amount of other elements was first nickel-plated to form a substantially pure nickel precursor layer having a thickness of approximately 130 tm using a conventional electrolytic nickel-plating process of a Watts type using NiSO4, NiCl2 and Boric acid as the plating solution, pure Ni as the anode and the steel substrate as the cathode.

A pack aluminisation process was used to partially aluminise the substantially pure nickel layer as follows. The substrate was buried in a mixture of powders filled in an alumina container. The powder mixture consisted of 4 wt. % Al, 2 wt. % AICI3 and 94 wt. % AI2O3. The container was then covered with an alumina lid and sealed with cement. The sealed pack was heated to 650 C in a furnace under an argon atmosphere and maintained at that temperature for 8 hours. The power supply to the container was then switchedoff to allow the container to cool before removing it from the furnace. The container was opened and the substrate removed and cleaned in water with light brushing.

The final coating obtained consisted of an outer Ni2AI3 layer having a thickness of approximately 36 trn and an inner substantially pure nickel layer of approximately 94 jim thickness. The ratio of the outer layer thickness to the inner layer thickness was therefore approximately 0.38. ii

EXAMPLE 2

Production Of A Nickel/Nickel Aluminjde Coating On A Steel Substrate A steel substrate was first nickel-plated to form a substantially pure nickel precursor layer having a thickness of approximately 146 l.tm using an electrolytic nickel-plating process as in Example 1.

The precursor layer was then aluminised using the pack aluminisation process as described above in Example I. The final coating obtained had an outer nickel aluminide layer with a thickness of approximately 36 tni and an inner substantially pure nickel layer with a thickness of approximately 110 tm. The ratio of the outer layer thickness to the inner layer thickness was therefore approximately 0.33.

A cross sectional SEM micrograph of the microstructure of the NI/NaA1b coating provided on the steel substrate is shown in Figure 1(a). The substrate 1, nickel 2 and nickel aluminide 3 layers can be clearly identified with reference to Figure 1(b) which provides a graphic represeiltation of the element concentration depth profile determined using the Energy Dispersive Spectroscopy (EDS) technique for the coating shown in Figure 1(a).

The concentration depth profile shows that the outer nickel aluniinide layer 3 contained approximately 60 at. % aluminium and approximately 40 at. % nickel across the whole depth of the layer. The aluminium: nickel atomic ratio was thus around 1.5 1, which is consistent with the Al: Ni atomic ratio in Ni2A13. This conclusion is further supported by the X-ray diffraction pattern of the coating of Figure 1(a) shown in Figure 2. The concentration depth profile also shows that the inner layer 2 is pure nickel across substantially its whole depth.

EXAMPLE 3

Production Of A Nickel/Nickel Alumjnide Coating On A Steel Substrate A steel substrate of the same type as in Example I was first nickel-plated to form a substantially pure nickel precursor layer of a thickness of approximately 30 m using an electrolytic nickel-plating process as in Example 1.

The precursor layer was then aluminised using the pack aluminisation process as described above in Example 1 except that the powder mixture used in the present example contained 2 wt. % Al, 2 wt. % AICI3 and 96 wt. % A1203.

The final coating obtained had an outer Ni2AI3 layer of approximately 20 m thickness and an inner substantially pure nickel layer of approximately 11 tm thickness. The ratio of the outer layer thickness to the inner layer thickness was therefore approximately 1.8.

EXAMPLE 4

Production Of A Nickel/Nickel Aluminide Coating On A Steel Substrate A steel substrate as in Example I was first mckel-plated to form a nickel precursor layer having a thickness of approximately 24 tm using an electroless nickel-plating process.

The nickel layer so formed preferably did not contain phosphorus and the boron content of the nickel layer did not exceed 1 wt. %. To achieve this, dimethylamineborane was used as a reducing agent with the temperature maintained in the range of 65 C to 75 C and p1-I in the range of 5 - 7. Plating solutions are prepared from nickel chloride and dimethylamineborane with sodium acetate, sodium or amnionium citrate and ammonium chloride used to adjust the pH value of the solutions.

The plated substrate was then aluminised using the pack aluminisation process as descnbed above in Example 2 except that the process was carried out at 600 C for 6 hours.

The final coating had an outer Ni2Al3 layer of approximately 12 jtrn thickness and an inner Ni layer of approximately l2un thickness. The ratio of the outer layer thickness to the inner layer thickness was therefore approximately 1.

EXAMPLE 5

Production Of A Chromium/Chromium Aluminjde Coating On A Steel Substrate A steel substrate as in Example 1 was first chromium-plated to form a substantially pure chromium layer having a thickness of approximately 30 jim using a conventional electrolytic chromium-plating process using a solution containing typically 200 - 300 g/l chromic acid with a chromic:sulphuric acid ratio of about 100:1 with pure Cr being the anode and the steel substrate the cathode.

The chromium-plated substrate was then aluminised using the pack aluminisation process described above in Example 1.

The final coating had an outer chromium aluminide layer of approximately 20 jim thickness and an inner substantially pure Cr layer of approximately 10 jim thickness.

The ratio of the outer layer thickness to the inner layer thickness was therefore approximately 2.

A cross sectional SEM micrograph of the microstructure of the Cr/CrAl coating provided on the steel substrate is shown in Figure 3(a). The substrate 4, chromium 5 and chromium aluminide 6 layers can be clearly identified with reference to Figure 3(b) which provides a graphic representation of the element concentration depth profile determined using the Energy Dispersive Spectroscopy technique for the coating shown in Figure 3(a).

The concentration depth profile shows that the outer chromium aluminide layer 6 contained approximately 75 at. % aluminium and approximately 25 at. % chromium across the whole depth of the layer. The aluminium: chromium atomic ratio was thus around 3: 1. The concentration depth profile also shows that the inner layer 5 was substantially pure chromium.

Claims

I. A coating provided on a substrate comprising a first metal or an alloy of said first metal, wherein the coating comprises a first layer comprising a second metal which is different to said first metal, and a second layer comprising an aluminide of said second metal.
2. A coating according to claim 1, wherein the first layer is interposed between the substrate and the second layer.
3. A coating according to claim I or 2, wherein the first layer is a diffusion barrier which restricts diffusion of aluminium atoms from the second layer into the substrate.
4. A coating according to claim 1, 2 or 3, wherein the second layer is an oxidation barrier which impedes oxidation of the first layer.
5. A coating according to any one of claims 1 to 4, whereiii the first metal is iron.
6. A coating according to any one of claims 1 to 4, wherein the substrate comprises steel.
7. A coating according to any one of claims I to 6, wherein the second metal is nickel.
8. A coating according to claim 7, wherein the first layer is substantially pure nickel metal.
9. A coating according to claim 7 or 8, wherein the second layer comprises nickel aluminide.
10. A coating according to claim 9, wherein the nickel aluminide has a formula selected from the group consisting of Ni2AI3, NiAI and Ni3Al.
11. A coating according to any one of claims I to 6, wherein the second metal is chromium.
12. A coating according to claim 11, wherein the first layer is substantially pure chromium metal.
13. A coating according to claim 11 or 12, wherein the second layer comprises chromium aluminide.
14. A coating according to claim 13, wherein the chromium aluminide has a formula selected from the group consisting of Cr2AI, Cr5A13, Cr4Al9, Cr2A111 and CrAl7.
15. A coating according to any preceding claim, wherein the first layer has a thickness of less than approximately 100 tim.
16. A coating according to any one of claims I to 14, wherein the first layer has a thickness of less than approximately 20 m.
17. A coating according to any one of claims I to 14, wherein the first layer has a thickness of approximately 10 ini.
18. A coating according to any preceding claim, wherein the second layer has a thickness of less than approximately 50 m.
19. A coating according to any one of claims I to 17, wherein the second layer has a thickness of less than approximately 20 pm.
20. A coating according to any one of claims I to 17, wherein the second layer has a thickness of approximately 10 tim.
21. A coating according to any one of claims 1 to 17, wherein the ratio of the thickness of the second layer to the thickness of the first layer is less than or equal to approximately 2: I.
22. A coating according to any one of claims I to 1 7, wherein the ratio of the thickness of the second layer to the thickness of the first layer is in the range (0.2 -0.5): 1.
23. A coating according to any one of claims I to 17, wherein the ratio of the thickness of the second layer to the thickness of the first layer is approximately 0.35: 1.
24. A coating according to any preceding claim, wherein a third layer of aluminium oxide is provided on said second layer.
25. A method for providing a coating on a substrate comprising a first metal or an alloy of said first metal, wherein the method comprises providing a first layer comprising a second metal which is different to said first metal, and providing a second layer comprising an aluminide of said second metal.
26. A method according to claim 25, wherein a precursor layer comprising the second metal is deposited on the substrate and said precursor layer is alurninised to a predetermined depth which is less than the total depth of the precursor layer such that the aluminised region of the precursor layer forms the second aluminide layer and the unaluminised region of the precursor layer forms the first layer.
27. A method according to claim 26, wherein the precursor layer is provided by electrolytic plating or electroless plating.
28. A method according to claim 26, wherein the precursor layer is provided by thermal spraying.
29. A method according to claim 26, 27 or 28, wherein the aluminisation process is effected at a temperature in the range 500 C to II 50 C.
30. A method according to claim 26, 27 or 28, wherein the aluminisation process is effected at a temperature in the range 550 C to 700 C.
3 1. A method according to any oiie of claims 26 to 30, wherein the aluminisation process is effected over a reaction time period of up to 24 hours.
32. A method according to any one of claims 26 to 30, wherein the aluminisation process is effected over a reaction time period in the range 4 to 9 hours.
33. A method according to any one of claims 26 to 30, wherein the alurninisation process is effected over a reaction time period in the range 6 to 8 hours.
34. A method according to any one of claims 26 to 33, wherein the precursor layer has a thickness of less than approximately 200 im prior to aluminisation.
35. A method according to any one of claims 26 to 33, wherein the precursor layer has a thickness of less than approximately 75 im prior to aluminisation.
36. A method according to any one of claims 26 to 33, wherein the precursor layer has a thickness of less than approximately 40 prn prior to aluminisation.
37. A method according to any one of claims 26 to 36, wherein the ratio of said predetermined depth to the thickness of the precursor layer is in the range (0. 1 - 0.8): 1.
38. A method according to any one of claims 26 to 36, wherein the ratio of said predetemiined depth to the thickness of the precursor layer is in the range (0.4 - 0.5): 1.
39. A method according to any one of claims 26 to 38, wherein the aluniinisation process is a pack aluminisation process.
40. A method according to any one of claims 26 to 38, wherein the aluminisation process is an in-pack or out-of-pack aluminisation process.
41. A method according to any one of claims 26 to 38, wherein the aluminisation process is an in-pack aluminisation process employing a powder mixture comprising powdered aluminium metal.
42. A method according to claim 41, wherein the powder mixture comprises up to wt. % powdered aluminium metal.
43. A method according to claim 41 or 42, wherein the powder mixture further comprises powdered aluminium oxide.
44. A method according to claim 41, 42 or 43, wherein the powder niixture comprises up to 98 wt. % powdered aluminium oxide.
45. A method according to any one of claims 41 to 44, wherein the powder mixture further comprises a powdered chloride or fluoride salt.
46. A method according to claim 45, wherein the powder mixture comprises up to 4 wt. % of the powdered chloride or fluoride salt.
47. A method according to claim 25, wherein the first layer comprising the second metal is deposited on the substrate and the second aluminide layer is deposited on said first layer.
48. A method according to claim 47, wherein the first layer is deposited by electrolytic plating or electroless plating.
49. A method according to claim 47, wherein the first layer is deposited by thermal spraying.
50. A method according to claim 49, wherein said thermal spraying employs the second metal in the form of a powder.
51. A niethod according to any one of claims 47 to 50, wherein the second layer is deposited by thermal spraying.
52. A method according to claim 51, wherein said thermal spraying employs an alurninide of said second metal in the form of a powder.
53. A method according to claim 51 or 52, wherein said thermal spraying employs at least one of the second metal in the form of a powder and aluminium metal in the form of a powder.