GB2206358A

GB2206358A - Corrosion-resistant aluminium-bearing iron base alloy coating

Info

Publication number: GB2206358A
Application number: GB08814918A
Authority: GB
Inventors: Dr Michael John Bennett; Dr Keith Taylor Scott; Leonard Lewis Wassell; Lesley Ray Forbes
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1987-07-03
Filing date: 1988-06-23
Publication date: 1989-01-05
Anticipated expiration: 2008-06-23
Also published as: GB8814918D0; GB8715713D0; GB2206358B

Abstract

Metals such as mild steels are coated to improve their properties such as their resistance to corrosion with an aluminium-bearing iron base alloy such as a ferritic Fe-Cr-Al-Y steel in the form of first and second layers, the first layer being in contact with the substrate and carrying the second layer, and the concentration of oxide at the interface between the first layer and the substrate being less than that in the second layer, thereby giving both good bond strength and corrosion resistance. The layers may be applied by thermal spraying (e.g. flame spraying) where the spraying conditions for producing the first layer differ from those for producing the second layer thereby to give rise to the above-mentioned difference in oxide concentration.

Description

Metal Substrate Coatings This invention relates to coated metal substrates and their production.

There is natural interest in improving the efficiency of industrial processes and one way of so doing is to operature the known process at a higher temperature than hitherto. Doing this can, however, cause process plant component materials to corrode or oxidise more readily.

Existing materials may then have to be replaced by different, higher quality materials, which has the disadvantage of being expensive. Alternatively, existing materials may be coated to improve their resistance to corrosion and/or oxidation, for example, as described by V E Carter in 'Metallic Coatings for Corrosion Control', Newnes-Butterworth, Chapter 1 passim, Chapter 2 passim, Chapter 3 passim.

The present invention addresses the problem of improving the properties of materials by coating and is concerned with coating metals, for example steels such as mild steels, for this purpose. In one aspect the invention provides a coated metal substrate wherein the substrate carries a coating comprising a first layer of an aluminium-bearing iron base alloy in contact with the substrate and a second layer of an aluminium-bearing iron base alloy carried by the first layer, the concentration of oxide at the interface between the first layer and the substrate being lower than the concentration of oxide in the second layer.

The bond strength of the first layer to the substrate is found to be surprisingly high and the second layer forms a coherent barrier layer for resisting corrosion.

A particular example of substrate that may be used is a mild steel substrate since mild steel is a conventional constructional material having excellent mechanical properties, being easily fabricated and cheap, but having poor resistance to corrosion in most environments when its rusting causes the structure of a component of which it is made to deteriorate progressively. The invention therefore enables this deficiency of mild steel to be overcome.

Other examples of substrates that may be used are alloy steels such as Cr:Mo ferritic steels (e.g. 2} Cr:lMo), stainless steel (e.g. Type 321), and nickel containing alloys such as INCOLOY 800H (Trade Mark). In a test of-a coated INCOLOY alloy substrate of the invention, the coating was found to adhere satisfactorily to the substrate for 10,000 hours at temperatures between 4000C and 10000C and not to interact chemically therewith during that time.

The substrate may be in the form of a component, for example in or suitable for use in a fossil fuel power generating plant or in a waste recovery system.

The coating materials used in the present invention aluminium-bearing iron base alloys - are known to resist corrosion. For example, aluminium-bearing ferritic steels are known to ox id ise to form a substantially alumina protective oxide layer, the integrity of which is maintained by supply of aluminium from within the alloy to sustain oxide growth and to repair flaws. Examples of such steels are those of the following composition by weight: 10 - 30% Cr, 1 - 10% Al, 0 - 0.5% C and the balance Fe.

Such steels may be available in the UK under the UK Registered Trade Mark 'FECRALLOY' where the steel composition includes a small proportion of Y; examples are steels in the composition range by weight: 15 - 22% Cr, 4 - 5.2% Al, 0.05 - 0.4% Y and the balance Fe. The art contains many references to aluminium-bearing ferritic steels. See, for example, J E Antill, 'Fecralloy Steels', Stainless Steel Ind. 7 (36) March 1979 and P.T. Moseley et al, 'The Microstructure of the Scale Formed During the High Temperature Oxidation of Fecralloy Steel', Corrosion Science 24 No. 6 (1984) p. 547.

The coating may, for example, have a thickness in the range of 0.2 mm to 1 mm though it may be possible and/or desirable in some circumstances to produce coatings of a thickness outside this range. The layers of the coating may consist of metal splats surrounded by fine oxide layers which are rich in alumina and, if an Y containing alloy is used, in yttria. Also, it has been generally found that the proportion of oxide in the layers is directly related to the Al content of the alloy used.

The coated metal substrates of the invention may be made by thermal spraying. Thus, in another aspect, the invention provides a method of providing a metal substrate with an aluminium-bearing iron base alloy coating comprising the steps of (i) thermally spraying the alloy onto the substrate to give a first layer, and (ii) thermally spraying the alloy onto the first layer to give a second layer; step (i) being carried out under different conditions from step (ii) so that the oxide concentration at the interface of the first layer with the substrate is lower than the oxide concentration in the second layer.

Thermal spraying is a known technique that can be used readily and economically in a range of different circumstances. The method of the invention therefore provides an economical way of improving the properties of metals, for example low alloy steels such as mild steels mentioned above.

A coating formed by thermally spraying an aluminium-bearing iron base alloy in air normally contains a significant proportion of oxide. However, in this invention, the alloy is thermally sprayed in step (i) under conditions where less oxide than normal is formed. It is then found that the sprayed layer, ie the first layer, is bonded very strongly to the substrate. The second layer, containing more oxide, is to provide greater corrosion resistance than would be provided by the first layer alone.

The resulting coating comprising the first and second layers adheres well to the substrates and forms a continuous barrier that isolates the substrate from the environment.

'Thermal spraying' is a generic term for a known method of spraying a solid material, e.g. in the form of wire or powder, by melting it and projecting the molten material with high kinetic energy onto a target which is, in this invention, the substrate. The term includes plasma spraying and flame spraying though the latter is preferred in the practice of this invention.

A commercially available spray gun such as exemplified hereinafter may be used to flame spray the alloy wherein, for example, wire of the alloy is fed into the gun and melted by an oxy-fuel gas flame. The wire issues from the gun nozzle into the centre of the heating flame where its tip is continuously melted and then atomised by a stream of compressed air and propelled from the nozzle towards the substrate in the form of droplets of the alloy. The droplets flatten on striking the substrate and adhere to form a coating.

The spray gun may have three gas feed systems: compressed air to activate feeding of the wire and project the alloy to the substrate, oxygen to increase flame temperature and give the flame stability, and fuel gas as the source of heat. When such a gun is used in the practice of the invention, it has been found that a higher ratio of oxygen: fuel gas and a lower volume of gas used per unit time (termed the 'power' of the flame) together minimise formation of oxide in the layer produced. Thus, when the above spray gun is used in the practice of this invention, a low power, high oxygen potential flame is preferred for step (i) and a high power, low oxygen potential flame for step (ii), where proportionately more oxide is formed.

Flame spraying is the preferred method of thermal spraying in this invention because it can be done using readily available, portable equipment such as the spray gun mentioned above and using cheap, readily available raw materials such as alloy wire and powder. It can therefore be used in situ for coating components in large scale plant, for example to repair such a component.

When performing the method of the invention, the substrate must be in a suitable condition for receiving the coating; for example it must be free of surface contaminants or superficial corrosion. It may therefore be necessary to treat the substrate before carrying out the invention; examples of such treatment are methods known in the art such as degreasing to remove surface contaminants and grit blasting to remove superficial corrosion.

The following examples illustrate the invention wherein the aluminium-bearing ferritic steel has the following composition by weight: 19.13% Cr, 4.75% Al, 0.45% Y, 0.34% Si, 0.12% Ni, 0.12% C, 0.07% Mn, 0.001% S, 0.007% P and the balance Fe.

EXAMPLE 1 A hollow mild steel substrate (internal diameter 7.6 mm, outer diameter 12.7 mm and length 12.7 mm) was vapour degreased, then grit blasted with alumina grit (average diameter 350 m) at a pressure of 551 KPa for 5 seconds and at an angle of 900 to the substrate, and vapour degreased again.

Immediately after treatment, the substrate was held in a headstock and rotated at 680 rpm and flame sprayed at a distance of 17.8 cm using a Metco-IOE metallising gun traversed at 0.3 m/s wherein an aluminium bearing ferritic steel wire (diameter 3.175 mm) was fed automatically therethrough by an air turbine mechanism. The wire was melted by an oxygen-acetylene flame and atomised into a fine spray by a blast of compressed air. The metal particles in the spray were accelerated to the substrate surface where they flattened and froze to form a first layer. The conditions of the spraying were as follows: acetylene flowmeter setting (arbitrary units) 30 oxygen flowmeter setting (arbitrary units) 40 atomising gas flowmeter setting 50 (arbitrary units) The oxygen:acetylene setting ratio was therefore 1.33 and the power (in arbitrary units) 120.

The substrate, coated with the first layer, was flame sprayed again to give a second layer, the conditions being the same as above with the following exceptions: acetylene flowmeter setting (arbitrary units) 40 oxygen flowmeter setting (arbitrary units) 44 The oxygen:acetylene setting ratio was therefore 1.10 and the power (in arbitrary units) 134.

The overall coating consisting of the first and second layers was found to have a thickness of about 0.22 mm and to be satisfactorily bonded to the substrate. Scanning electron microscopy (SEM) examination showed that there was metal:metal contact between the substrate and the first layer, ie that the oxide concentration at the interface between the substrate and the first layer was depleted.

This is possibly due to a thinner oxide film, which is formed around the molten droplets during spraying, being broken when the droplets strike the substrate surface bence enabling a metal:metal bond to be formed; the oxide remains on the surface of the splat remote from the bond.

EXAMPLE 2 The procedure of Example 1 was repeated using a mild steel bar (19 mm diameter, 15 mm length) as substrate. The coating produced was about 1 mm thick, was very well bonded to the substrate and had only a fine layer of oxide between the substrate and the first layer.

Claims

1. A coated metal substrate wherein the substrate carries a coating comprising a first layer of an aluminium-bearing iron base alloy in contact with the substrate and a second layer of an aluminium-bearing iron base alloy carried by the first layer, the concentration of oxide at the interface between the first layer and the substrate being lower than the concentration of oxide in the second layer.

2. A coated metal substrate according to claim 1 wherein the metal substrate is a mild steel substrate.

3. A coated metal substrate according to claim 1 wherein the metal substrate is a Cr:Mo ferritic steel substrate.

4. A coated metal substrate according to claim 1 wherein the metal substrate is a nickel containing alloy substrate.

5. A coated metal substrate according to any of the preceding claims in the form of a component for an industrial plant.

6. A coated metal substrate according to any of the prceding claims wherein the aluminium-bearing iron base alloy is an aluminium-bearing ferritic steel.

7. A coated metal substrate according to claim 6 wherein the steel has a composition including a small proportion of yttrium.

8. A coated metal substrate according to any of the preceding claims wherein the coating has a thickness in the range of 0.2 mm to 1 mm.

9. A coated metal substrate according to claim 1 substantially as described herein with reference to either of the examples.

10. A method of providing a metal substrate with an aluminium-bearing iron base alloy coating comprising the steps of (i) thermally spraying the alloy onto the substrate to give a first layer, and (ii) thermally spraying the alloy onto the first layer to give a second layer; step (i) being carried out under different conditions from step (ii) so that the oxide concentration at the interface of the first layer with the substrate is lower than the oxide concentration in the second layer.

11. A method according to claim 10 wherein the thermal spraying in each of steps (i) and (ii) is carried out by flame spraying.

12. A method according to claim 11 wherein, in step (i), flame spraying is carried out using a lower power and higher oxygen potential flame than in step (ii).

13. A method of providing a metal substrate with an aluminium-bearing iron base alloy coating according to claim 1 substantially as described herein with reference to either of the examples.

14. A coated metal substrate made by a method according to any of claims 10 to 13.