WO2021048411A1 - Method for reducing metal-dusting corrosion - Google Patents

Abstract

The present invention relates to methods for improving the resistance of alloys to corrosion, and in particular metal-dusting corrosion. The present invention also relates to alloys obtainable by such methods, and products formed from such alloys.

Description

METHOD FOR REDUCING METAL-DUSTING CORROSION

Field of the invention

The present invention relates to methods for improving the resistance of alloys to corrosion, and in particular metal-dusting corrosion. The present invention also relates to alloys obtainable by such methods, and products formed from such alloys.

Background to the invention

Metal-dusting corrosion is a high-temperature degradation phenomenon affecting alloys when exposed to strongly carburizing gases (with a carbon activity a_c > 1) at elevated temperatures (400 ^°C to 900^°C) ^{[1 4]}. Metal-dusting corrosion causes the alloy to disintegrate into a powdery mixture of metallic, carbidic and carbonaceous dusts.

This degradation of bulk metallic materials via metal-dusting corrosion is a critical issue in a range of chemical industries where alloys are in contact with strongly carburizing gas atmospheres at elevated temperatures ^{[1 4]}. Under these conditions, carbon can transfer to the alloy surface by one or more of: CO reduction, the Boudouard reaction and alkane thermal cracking reactions.

Fe and Ni are two of the most important components of high temperature alloys. However, both metals, and their alloys, are susceptible to carbon formation and metal- dusting corrosion. Thus, the development and improvement of methods for reducing or preventing metal-dusting corrosion of these metals is of critical importance.

For Fe-based alloys ^{[1 3]}, metal-dusting corrosion begins with the formation of Fe3C or FesCi carbides. The volume expansion resulting from the formation of these carbides creates defects on the alloy surface layer ^{[4, 5]}. Carbon atoms can diffuse through the carbides and precipitate as carbon at defects. Accumulation of carbonaceous deposits then separates the carbide particles from the metallic matrix that can be transported away from the surface of the alloy to leave a pit on the surface. For Ni-based alloys, metal-dusting corrosion proceeds without the formation of nickel carbides. Nickel carbides are unstable, so do not form under metal-dusting corrosion conditions ^{1 6}’ ^26]. During exposure of Ni- based alloys to strongly carburizing conditions, metal-dusting corrosion occurs by direct graphite penetration into the metal phase.

Oxide scale covering on the surface of metallic materials is known and provides an effective and relatively inexpensive way to protect alloys against high temperature degradation for sustainable use in a wide range of applications ^{[8 9]}. The properties of the oxide scale affects the oxidation and corrosion resistance of the alloy ^{| l0 |}.

In many industrial processes at elevated temperatures, Fe- and Ni-based alloys are used with Cr as a constituent. Cr is preferentially oxidised over Fe or Ni to form slow- growing, dense chromium oxide (CriCb) layers which are strongly adhered to the alloy surface.

(¾0₃ is generally stable in carbon atmospheres and provides a protective layer in preventing alloys from metal corrosion. It is known in the art that diffusion of carbon atoms through the (¾0₃ lattice is not possible ^{[10 12]}. However, the (¾0₃ oxide produced on the alloy surface does not generally provide a protective layer, because defects on the oxide can allow ingress of carbon ^[13].

Manganese is frequently used as a component in high temperature alloys. The high mobility of Mn in (¾0₃ enables it to readily form a Mh(¾q₄ spinel structure, which is also known in the art as a protective scale. Common oxidation processes usually form oxide films composed of two layers: an outer layer enriched in Fe oxide and an inner layer enriched in Cr oxides ^{[18 21}( The outer layer oxides can be reduced under working conditions to form Fe particles, which are then available to act as catalysts for carbon formation. Carbon is then able to be transported through spinels containing high concentrations of Fe ^[22], resulting in metal-dusting corrosion and breakdown of the bulk metal structure. Thus, the formation of an outer layer of Cr₂C>₃ scale is desirable to protect the alloy from metal-dusting corrosion.

Surface pre-treatment of alloys prior to exposure to carburizing conditions is known in the art. Pre-treatment can form microstructures (including dislocations and grain refinement) in the near-surface region to help Cr to diffuse to the surface ^{[11, 2 -241}. This diffusion of Cr to the surface of the alloy promotes the formation of a Cr-rich spinel oxide film. Such treatments are known to improve metal-dusting corrosion resistance, but do not eliminate it. Continued presence of Fe and/or Ni on the surface results in a the formation of an imperfect layer of Cr oxide. Carbon can diffuse through these imperfections into the bulk structure of the metal, resulting in metal-dusting corrosion. Several coatings have been applied to different alloys to protect them to prevent metal-dusting corrosion^{16, 7]}, however, such coatings may have issues with cost, practicality, adherence and thermal expansion.

An alternative approach for combatting metal-dusting corrosion is to design novel alloys that are corrosion-resistant. Although some promising results have been achieved, the costs are often prohibitively high and the novel alloys may have less desirable mechanical or thermal properties compared to presently-used alloys ^[8].

In view of the above, there remains a need to develop improved methods of surface- treatment of alloys intended to be exposed to carburizing conditions, to improve the resistance of such alloys to metal-dusting corrosion and improve the useful lifetime of components made from such alloys. There is also a need to develop methods that could be used on-site to improve the resistance of alloy-based equipment and critical surfaces to metal-dusting corrosion. Summary of the invention

The present invention arises from the surprising finding that it is possible to improve the corrosion resistance of alloys by subjecting the surface of the alloy to one or more mechanical treatment methods and then exposing the surface of the alloy to oxidising conditions at elevated temperatures. In particular, the present invention relates to a method for improving the resistance of an alloy to metal-dusting corrosion, the method comprising; a. mechanical treatment of a surface of the alloy to produce a mechanically-treated surface; b. oxidation of the mechanically-treated surface at a temperature from about 100 °C to about 1000 °C to produce a surface scale comprising CnCb and/or a Mn-Cr-0 spinel on the mechanically-treated surface; wherein the alloy comprises a) Cr and b) one or more of Mn, Fe, Ni, Co, Mo, Al,

Ti, Si, Cu Sn, Zn and Pb.

The present invention also relates to an alloy obtainable by this method. The present invention also relates to a product comprising the alloy.

Brief description of the drawings

Figure l is a phase equilibrium diagram of the equilibrium composition in the solid phase solution of the Cr-Fe-0₂ system. The equilibrium analysis shows O2 molar fraction in the Cr-Fe-0₂ system at 0.01, 0.1 and 0.3, representing deficient (i.e. sub-stoichiometric) O2, close to stoichiometric O2 (as per the above oxidation reaction), and sufficient O2 respectively for the reaction 4M +3O2 ® 2M2O3 (M = Fe /Cr).

Figure 2 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon at various stages of treatment as set out in example 1. Figure 3 consists of a Raman line scan across the NS-SPD area and non-NS-SPD area of an Incoloy 800 alloy coupon following treatment as set out in example 1.

Figure 4 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon following treatment as set out in example 2. Figure 5 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon following treatment as set out in example 3.

Figure 6 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon following treatment as set out in example 4.

Figure 7 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon following treatment as set out in example 5.

Figure 8 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon following treatment as set out in example 6.

Figure 9 consists of scanning electron microscopy (SEM) images of an Incoloy 800 alloy coupon following treatment as set out in example 7. Figure 10 consists of scanning electron microscopy (SEM) images of an Incoloy

800 alloy coupon following treatment as set out in comparative example 1.

Figure 11 consists of stitched bright field transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) maps of an Incoloy 800 alloy coupon following treatment as set out in example 1. Figure 12 consists of further stitched bright field transmission electron microscopy

(TEM) images and energy-dispersive X-ray spectroscopy (EDS) maps of an Incoloy 800 alloy coupon following treatment as set out in example 1.

Figure 13 consists of further stitched bright field transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) maps of an Incoloy 800 alloy coupon following treatment as set out in example 1.

Detailed description of the invention

The present invention is concerned with methods for improving the corrosion resistance of alloys, and with alloys obtainable by such methods. In particular, the invention is concerned with improving the resistance of alloys to metal-dusting corrosion.

The resistance of an alloy to metal-dusting corrosion can be determined by any techniques known to those of skill in the art. As set out in the examples, various methods can be used to inspect the surface of an alloy for evidence of metal-dusting corrosion after exposure to strongly carburizing conditions. Visual inspection of the alloy’s surface can be used, and can be conducted using any method known in the art, for example by scanning electron microscopy (SEM). Other analytical techniques such as energy-dispersive X-ray spectroscopy (EDS) may also be used.

The method of the present invention includes mechanically treating the surface of an alloy comprising chromium to produce a mechanically-treated surface, followed by oxidation of the mechanically-treated surface at a temperature above ambient temperature to produce a surface scale comprising Cr, preferably comprising (¾0₃ and/or a Mn-Cr-0 spinel, for example Mh(¾q4_, Mn1_.5Cr1_.5O4, MmCrCE etc.

Alloys suitable for use in the present method include any alloy susceptible to metal- dusting corrosion that contains chromium. Typically, the alloys comprise Cr and one or more of Mn, Fe, Ni, Co, Mo, Al, Ti, Si, Cu, Sn, Zn and Pb. Preferably, the alloy comprises Cr and one or more of Fe, Ni and Mn. Preferably, the alloy comprises Cr and Fe. Preferably, the alloy comprises greater than about 1% by mass Cr, preferably greater than about 3% by mass Cr, preferably greater than about 5% by mass Cr, preferably greater than about 10% by mass Cr. Preferably, the alloy comprises greater than 0.05% by mass Mn, preferably greater than about 0.5% by mass Mn, preferably greater than about 1% by mass Mn. Preferably, the alloy comprises 30.0-35.0% by mass Ni, 19.0-23.0% by mass Cr, greater than 39.5% by mass Fe, less than 0.01% by mass C and 0.30-1.20% by mass of a combination of Al and Ti, or the alloy comprises 58.0-63.0% by mass Ni, 21.0-25.0% by mass Cr, 1.0-1.7% by mass Al, less than 0.10% by mass C, less than 1.0% by mass Mn, less than 0.015% by mass S, less than 0.50% by mass Si, less than 1.0% by mass Cu and the remainder iron.

The mechanical treatment of the alloy is preferably by one or more of near-surface severe plastic deformation (NS-SPD) and severe plastic deformation (SPD). As used herein, the term SPD relates to metalworking techniques affecting the entire structure of a metal. SPD techniques involve large strains typically involving a complex stress state or high shear, resulting in a high defect density and a decrease in the average grain size of the alloy. As used herein, NS-SPD relates to metalworking techniques which only affect the near-surface region of the alloy i.e. the bulk metal is not affected by the mechanical treatment. NS-SPD techniques result in a high defect density and a decrease of the average grain size of the alloy. When the mechanical treatment is by NS-SPD, the NS-SPD is preferably conducted by one or more of scratching, sanding, grinding or sand-blasting. When the mechanical treatment is by SPD, the SPD is preferably conducted by one or more of rolling, bending, folding, equal channel angular pressing (ECAP), multistep isothermal forging (MIF) or accumulative roll bonding (ARB). Preferably, the mechanical treatment is by NS-SPD. Preferably, the NS-SPD is conducted by scratching or sanding. Most preferably, the NS-SPD is conducted by scratching the surface of the alloy. Scratching can be conducted with any implement, including a metal knife, tweezers, a wire brush or a hard diamond tip.

It is desirable for the entire surface of the alloy to be mechanically treated. When mechanical treatment of the alloy is by scratching, it is preferred that the scratches cover the entire surface. Preferably, the distance between adjacent scratches is about 20 pm or less, preferably about 15 pm or less, preferably about 10 pm or less, most preferably about 5 pm or less.

The grain size of the alloy before being subjected to mechanical treatment can be measured by visual inspection from a SEM image. The grain size of the alloy before being subjected to mechanical treatment is from about 5 pm to about 300 pm, preferably from about 10 pm to about 200 pm, more preferably from about 15 pm to about 150 pm and most preferably about 30 pm to about 70 pm. Following the mechanical treatment of the alloys surface, the grain size is preferably reduced to from about 10 nm to about 10 pm, preferably from about 0.1 pm to about 8 pm, more preferably from about 0.1 pm to about 6pm, most preferably from about 0.1 pm to about 3 pm.

The mechanical treatment of the alloy’s surface produces an ultrafme-grained structure with a higher fraction of grain boundaries together with twin boundaries and dislocations near the surface. Without wishing to be bound by theory, these microstructures are shown to increase the effective diffusion coefficient for Cr in the alloy by introducing a higher density of rapid diffusion paths, promoting formation of a thin, protective Cr-rich oxide scale in the alloy surface region following the oxidation step set out below.

After the surface of the alloy has been mechanically treated, it is subjected to oxidation. There are typically no intervening steps between mechanical treatment and oxidation. For example, the mechanically-treated surface is generally not subjected to a reduction step (e.g. in the presence of hydrogen) prior to the oxidation step. During oxidation, the mechanically-treated surface is exposed to an oxidative atmosphere comprising one or more gases selected from O2, CO, CO2 and H2O.

Preferably, the oxidative atmosphere comprises CO and/or H2O. More preferably the atmosphere comprises one of CO or H2O. The atmosphere may additionally comprise Fb and/or one or more gaseous hydrocarbons C_xH_y, preferably methane, ethane and/or ethene. The atmosphere may additionally comprise an inert gas, preferably an inert gas selected from N2, Ar and He. The atmosphere may be a flowing or a static atmosphere. The pressure of the atmosphere is typically 0.5 to 25 bar, preferably 1 to 20 bar.

One or more of the non-oxidative gases above may be included to reduce the partial pressure of the oxidative gas during the oxidation of the mechanically-treated surface.

This enables the degree of oxidation to be controlled. Chromium and/or manganese and/or aluminium and/or titanium and/or silicon in the metal alloy are oxidised, allowing the formation of a protective scale of chromium and/or manganese oxides. Other metal atoms, for example one or more of Fe, Ni, Co, Mo, Sn, Zn and Pb, are not oxidised during the oxidation of the mechanically-treated surface.

In support of this, thermal chemical calculations using commercial thermochemical software were performed to analyse the equilibrium composition in the solid phase solution of the Cr-Fe-Ch system. It is well-known that Fe and Cr have tendency to be oxidised under oxidative conditions. The oxidation reaction can be represented by the following: 4M +3O2 ® 2M2O3 (M = Fe /Cr)

Figure 1 shows the phase equilibrium diagram with O2 molar fraction in the Cr-Fe- O2 system at 0.01, 0.1 and 0.3, representing deficient (i.e. sub-stoichiometric) O2, close to stoichiometric O2 (as per the above oxidation reaction), and sufficient O2 respectively.

The equilibrium analysis in Figure 1 shows that Cr is thermodynamically prone to be oxidised in preference to Fe. Under the same oxidative conditions, Cr₂0₃ is formed prior to the oxidation reaction of O2 with Fe. In the present invention, the Cr oxide is formed primarily on the surface of the alloy. The thin and compact layer of Cr₂0, on the surface of the alloy prevents the migration of oxygen from the surface to the bulk body of the alloy and prevents the iron in the alloy from being exposed to oxygen. Thus, formation of Cr₂0, prevents metal-dusting corrosion of the alloy.

Oxidation of the mechanically-treated surface is conducted a temperature above ambient temperature. Preferably, the oxidation is conducted at a temperature of from about 100 ^°C to about 1000 ^°C, preferably from about 200 ^°C to about 900 ^°C, preferably from about 300 ^°C to about 800 ^°C, preferably from about 400 ^°C to about 780 ^°C, preferably from about 500 ^°C to about 760 ^°C, and most preferably from about 540 ^°C to about 750 ^°C.

Typically, the oxidation of the mechanically-treated surface has a duration of from about 1 hour to about 80 hours, preferably from about 5 hours to about 70 hours, preferably from about 10 hours to about 60 hours, preferably from about 15 hours to about 50 hours, preferably from about 20 hours to about 40 hours, preferably from about 20 hours to about 30 hours, and most preferably about 26 hours.

In a preferred aspect, the oxidation occurs at a temperature of from about 540 ^°C to about 750 ^°C under an atmosphere comprising CO and/or H2O and has a duration of from about 20 hours to about 30 hours.

Oxidation of the mechanically-treated surface may occur in one stage, two stages, three stages or more. Each stage may be conducted at a different temperature, and under a different atmosphere. Preferably, oxidation of the mechanically-treated surface occurs in one or two stages, more preferably two stages. When oxidation of the mechanically-treated surface occurs in two stages, the first of the two stages is conducted at a temperature of between about 100 ^°C and about 650 ^°C, preferably between about 200 ^°C and about 600 ^°C, preferably between about 300 ^°C and about 600 ^°C, preferably between about 400 ^°C and about 550 ^°C, preferably between about 500 ^°C and about 550 ^°C, and most preferably at about 540 ^°C. Stage one preferably has a duration of from about 1 hour to about 100 hours, preferably from about 1 hour to about 40 hours, preferably from about 1 hour to about 30 hours, preferably about 2 hours to about 20 hours, preferably about 4 hours to about 10 hours, and most preferably about 6 hours. Preferably, stage one is conducted under an atmosphere comprising H2O. Preferably the atmosphere comprises about 10% H2O by volume, with the remainder one or more inert gases. Preferably, the inert gas is argon. Preferably, the gas mixture is at a pressure of 0.5 to 1.5 bar, for example about 1 bar. In a preferred aspect, stage one occurs at a temperature between about 400 ^°C and about 550 ^°C, has a duration of from about 4 hours to about 10 hours, and occurs in an atmosphere of (by volume) about 10% H2O and about 90% Ar at a pressure of about 1 bar. When oxidation of the mechanically-treated surface occurs in two stages, the second of the two stages is conducted at a temperature of between about 500 ^°C and about 1000 ^°C, preferably between about 600 ^°C and about 900 ^°C, preferably between about 700 ^°C and about 800 ^°C and most preferably at about 750 ^°C. Stage two preferably has a duration of from about 0.5 hours to about 100 hours, preferably from about 0.5 hours to about 40 hours, preferably about 5 hours to about 35 hours, preferably about 10 hours to about 30 hours, preferably about 15 hours to about 25 hours and most preferably about 20 hours. Preferably, stage two is conducted under an atmosphere comprising CO.

Preferably, the atmosphere also comprises one or more of Eh, CO2, H2O and Ar. Most preferably, the atmosphere consists of (by volume) about 25% Eh, about 20% CO, about 15% CO2, about 10 % H2O and about 30% Ar. Preferably, the atmosphere is at a pressure of 15 to 25 bar, for example about 20 bar. In a preferred aspect, stage two occurs at a temperature between about 700 ^°C and about 800 ^°C, has a duration of from about 15 hours to about 25 hours, and occurs in an atmosphere comprising (by volume) about 20% CO at a pressure of about 20 bar.

In a preferred aspect, the oxidation of the mechanically-treated surface occurs in two stages, with the first stage conducted at a temperature between about 400 ^°C and about 550 ^°C, for a duration of about 4 hours to about 10 hours, under an atmosphere of (by volume) about 10% H2O and 90% Ar at a pressure of about 1 bar, and the second stage is conducted at a temperature between about 700 ^°C and about 800 ^°C, for a duration of about 15 hours to about 25 hours, under an atmosphere of (by volume) about 25% Fh, about 20% CO, about 15% CO2, about 10 % H2O and about 30% Ar at a pressure of about 20 bar.

Preferably, the alloy comprising the mechanically-treated surface is heated up from ambient temperature to the desired temperature(s) with a heating ramp rate of from about 0.1 ^°C/min to about 20 ^“C/min. Preferably, the heating ramp rate is from about 2 ^“C/min to about 18 ^“C/min, preferably from about 4 ^“C/min to about 16 ^“C/min, preferably from about 6 ^“C/min to about 14 ^“C/min, preferably from about 8 ^“C/min to about 12 ^“C/min, preferably about 10 ^“C/min.

The surface of the alloy can be subjected to pre-treatment before the mechanical treatment, to clean the surface of the alloy i.e. to remove stains, fats, impurities and so on. The pre-treatment step can comprise pre-treatment of the surface by polishing and/or chemical pre-treatment. Preferably, the pre-treatment step includes both polishing and chemical pre-treatment. Preferably, polishing includes first grinding the alloy with abrasive grinding paper, preferably SiC grinding paper, followed by polishing with diamond dust. Preferably, chemical pre-treatment includes ultrasonically cleaning the alloy in hexane.

The method of the present invention results in the formation of a surface scale on the surface of the alloy. A surface scale is a metal oxide forming a layer around the bulk metal. In the present invention, the surface scale comprises Cr, preferably C^Cb and/or a Mn-Cr-0 spinel. When the alloy comprises Mn, the scale preferably comprises a Mn-Cr-0 spinel, preferably MnC^Cb. Figure 11 shows the formation of a scale comprising Cr on the surface of an Fe-based alloy following the method of the present invention.

Preferably, the surface scale has a thickness of from 30 nm to 5 mm, preferably from 30 nm to 10 pm, more preferably from 30 nm to 1 pm, more preferably from 50nm to 750 nm. Preferably, the surface scale is resistant to corrosive, oxidative and erosive conditions. Most preferably, the surface scale is resistant to metal-dusting corrosion. Preferably, the surface scale is impervious to diffusion of carbon. Thus, even when exposed to strongly carburizing gases (with a carbon activity a_c > 1) at elevated temperatures (200 ^°C to 900 ^°C) carbon cannot diffuse through the surface scale and into the bulk metal. Thus, the surface scale protects the bulk metal from corrosion.

The method of the present invention may be applied directly on the alloy of a product, such as plate, sheet, vessel, pipe, tubes, joints, pipeline, heat exchanger, containers, reactors, gaskets and so on. The product may be used or new. The method can be applied to a product either on-site or off-site. The method can be applied to a product which is either mounted or dismounted from a system/unit. The operation of the method can be conducted by human, robot or both. The method of the present invention is preferably applied to alloys and products which are intended to be used in strongly carburizing conditions i.e. those alloys and products which are at high risk of undergoing metal-dusting corrosion.

The present invention is also concerned with alloys formed by applying methods as outlined above i.e. alloys resistant to corrosion. Preferably, alloys formed by the methods of the invention are resistant to metal-dusting corrosion. Alloys formed by the methods of the invention are useful in a variety of products, in particular as plates, sheets, vessels, pipes, tubes, joints, pipelines, heat exchangers, containers, reactors, gaskets and so on. The examples show that the metal-dusting resistance performance of alloys in industrial applications can be significantly improved by the methods of the present invention.

Examples The following are examples that illustrate the present invention. However, these examples are in no way intended to limit the scope of the invention.

Preparation of alloy coupons

In each of the examples below, metal coupons were prepared as follows. A 15 mm x 8 mm x 0.5 mm coupon of commercially-available Incoloy 800 alloy was ground using P2400 (800 grit) SiC paper. The coupon was then polished using 1 pm diamond dust to give a mirror finish. The coupon was then ultrasonically cleaned in hexane. Example 1

A coupon of Incoloy 800 alloy was prepared as above. One section of the coupon was subjected to NS-SPD by manually scratching using tweezers at ambient temperature. The mechanically-treated coupon was then cleaned with ethanol and acetone. The metal coupon was then exposed to an oxidative gas atmosphere of (by volume) 10% ThO and 90% Ar at 1 bar and 540 ^°C for 6 hours, followed by (by volume) 20% CO, 25% ¾, 15% CO2, 10% H2O and 30% Ar at 20 bar and 750 ^°C for 20 hours. Finally, the coupon was exposed to a strongly carburizing atmosphere of (by volume) 10% CO and 90% Ar at 550 ^°C for 20 hours. The coupon was then cooled in Ar to ambient temperature. The coupon was imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected. Raman spectra were collected by focusing a Horiba Jobin Yvon LabRAM HR800 spectrometer, using the emission line at 633 nm from a He-Ne laser, on the sample with a 50 ^c LWD objective with a motorized x-y stage. The output power of the laser was 8mW with a spot diameter of approximately 1.5 pm. The spectra were recorded with continuous scans in the range 300-3000 cm ¹. Spectra were taken at three different locations on a given sample to verify that the observed surface characteristics were uniform and representative. Finally, cross-section transmission electron microscopy (TEM) lamellas were prepared by a FEI Helios G4 UX focused ion beam (FIB). Carbon or platinum protection layers (the first part of the layer made by e- beam assisted deposition to avoid ion-beam induced surface damage) were deposited on the selected regions prior to cutting out the TEM lamella. Coarse thinning was performed at 30 kV acceleration voltage. The last part of the thinning was performed at 5 kV and finally 2 kV to minimize ion-beam induced surface damage on either side of the TEM lamellas. TEM analysis was done on a double C_s aberration corrected cold FEG JEOL ARM 200F, operated at 200 kV and equipped with a large solid angle Centurio SDD

(0.98 sr) for X-ray energy dispersive spectroscopy (EDS) and a Quantum ER GIF for dual electron energy loss spectroscopy (EELS).

Figure 2a) is an SEM image of the coupon after NS-SPD treatment but before the oxidation. The right side of the figure shows the section of the coupon treated by NS-SPD. A thin protection layer of Pt was deposited on the alloy surface before collecting the SEM image, to prevent ion-beam induced surface damage during ion milling when preparing TEM lamellas. This can be observed in the centre of the image.

Figure 2b) is an SEM image of the coupon after the first (6 hour) oxidation treatment. The right side of figure 2b) shows the section of the coupon treated by NS-SPD. A protection layer of C was deposited on the alloy surface before collecting the SEM image, to prevent ion-beam induced surface damage during ion milling when preparing TEM lamellas. This can be observed in the centre of the image.

Figure 2c) is an SEM image of the coupon after the second oxidation treatment. The right side of figure 2c) shows the section of the coupon treated by NS-SPD. A protection layer of Pt was deposited on the alloy surface before collecting the SEM image, to prevent ion-beam induced surface damage during ion milling when preparing TEM lamellas. This can be observed in the square drawn in centre of the image.

Figure 2e) is a cross-section TEM lamella of the section highlighted in figure 2c), showing the NS-SPD and the region of the coupon not treated by NS-SPD following two- step oxidation treatment.

Figure 2d) is an SEM image of the coupon after exposure to the carburizing atmosphere. The right side of figure 2d) shows the section of the coupon treated by NS- SPD. A protective layer of C was deposited on the alloy surface after collecting the SEM image, to prevent ion-beam induced surface damage during ion milling when preparing TEM lamellas. This can be observed in the Figure 2f).

Figure 2f) is a cross-section TEM lamella of the section highlighted in figure 2d), showing the NS-SPD and the region of the coupon not treated by NS-SPD following exposure to the carburizing atmosphere. Figure 2 shows that the region of the coupon that underwent NS-SPD region did not undergo metal-dusting corrosion. No carbon filaments or powdery mixture of metallic, carbidic and carbonaceous dusts were present in this region. The region of the coupon not treated by NS-SPD shows substantial buildup of carbon filaments and metallic, carbidic and carbonaceous dusts, indicating significant metal-dusting corrosion. Figure 3 shows a Raman line scan across the NS-SPD area and the region of the coupon not treated by NS-SPD, following ultrasonic cleaning in acetone for 30 min to remove loosely adhered metal-dusting product. The Raman spectra show the D-, G- and 2D- bands of carbon outside the NS-SPD. There are no peaks from carbon within the NS- SPD region, showing that no metal-dusting corrosion occurred in the NS-SPD region. In addition, a small peak is present at 555 cm ¹ within the NS-SPD region, indicating this region is rich in (¾0_3.

Figure 11 shows stitched bright field transmission electron microscopy (TEM) images of the coupon following the metal-dusting corrosion test. Three areas were further analysed by energy-dispersive X-ray spectroscopy (EDS) - namely a section of the non- NS-SPD region, a transition area between the non-NS-SPD and NS-SPD region, and the NS-SPD region. The images of the polished region show severe metal-dusting corrosion, shown by the degradation of the surface of the alloy and fragmentation of iron and nickel from the surface of the alloy. The transition region shows significantly less metal-dusting corrosion, and a build-up of a thin surface scale containing chromium. The NS-SPD region shows the formation of a somewhat thicker layer of surface scale containing chromium. This region shows no evidence of metal-dusting corrosion.

Therefore, mechanical treatment by NS-SPD followed by two-step oxidation at elevated temperature is shown to provide excellent protection from metal-dusting corrosion.

Example 2

A coupon of Incoloy 800 alloy was prepared as above. One section of the coupon was subjected to NS-SPD by manually scratching using tweezers at ambient temperature. The mechanically-treated coupon was then cleaned with ethanol and acetone. The metal coupon was then exposed to an oxidative gas atmosphere of (by volume) 20% CO, 25% ¾, 15% CO2, 10% H2O and 30% Ar at 20 bar and 750 ^°C for 26 hours. Finally, the coupon was exposed to a strongly carburizing atmosphere of (by volume) 10% CO and 90% Ar at 550 ^°C for 20 hours. The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected.

Figure 4a) shows both the non-NS-SPD and NS-SPD region at a magnification of lOOOx.

Figure 4b) shows the NS-SPD region at a magnification of 3000x. Figure 4c) shows the NS-SPD region at a magnification of lOOOOx.

Figure 4 shows that the non-NS-SPD region of the metal coupon had undergone substantially more metal-dusting corrosion than the region that underwent NS-SPD. This is shown by the increase in the quantity of carbon filaments and powdery mixture of metallic, carbidic and carbonaceous dusts in the non-NS-SPD region. NS-SPD followed by high-temperature oxidation is shown to inhibit the metal dusting corrosion of Fe-based alloy. Example 3

The method of example 2 was repeated with a second coupon of Incoloy 800 alloy. The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected.

Figure 5a) shows the interface of the polished region and NS-SPD region at a magnification of 2160x.

Figure 5b) shows the interface of the polished region and NS-SPD region at a magnification of 8000x. Figure 5c) shows the interface of the polished region and NS-SPD region at a magnification of 2120x.

As shown in the SEM images of figure 5, no carbon filaments were formed in the region of coupon that underwent NS-SPD. A thick layer of carbon filaments and metallic, carbidic and carbonaceous dust formed in the region of the coupon not treated by NS-SPD. The result of example 2 is therefore verified.

Example 4

A coupon of Incoloy 800 alloy was prepared as above. One section of the coupon was subjected to scratching by a hard diamond tip in both x- and y-directions, with a region of overlap of the scratches in the centre of the coupon. The mechanically-treated coupon was then cleaned with ethanol and acetone. The metal coupon was then exposed to an oxidative gas atmosphere of (by volume) 10 % FhO and 90% Ar at 1 bar and 540 ^°C for 6 hours, followed by (by volume) 20% CO, 25% Eh, 15% CO2, 10% FhO and 30% Ar at 20 bar and 750 ^°C for 20 hours. Finally, the coupon was exposed to a strongly carburizing atmosphere of (by volume) 10% CO and 90% Ar at 550 ^°C for 20 hours.

The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected.

Figure 6a) shows the interface of the polished region and mechanically-treated region at a magnification of lOOx. Figure 6b) shows the mechanically-treated region where scratches in the x and y- directions meet at a magnification of 500x.

Figure 6c) shows one section of the coupon that underwent mechanical treatment in both the x- and y-directions at a magnification of 5000x. Figure 6 shows that few carbon filaments formed in the region of the coupon which underwent mechanical treatment by diamond tip. Where the scratches in the x- and y- directions meet, very few carbon filaments formed. Thus, it is shown that more severe deformation of the metal surface results in increased protection of the metal to metal- dusting corrosion.

Example 5

The method of example 4 was repeated with a second coupon of Incoloy 800 alloy, except that the coupon was only scratched in the x-direction. The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected.

Figure 7a) shows the interface of the non-NS-SPD region and mechanically-treated region at a magnification of lOOx.

Figure 7b) shows the mechanically-treated region with scratches in only the x- direction at a magnification of 500x.

Figure 7b) shows one section of the coupon that underwent mechanical treatment in only the x-direction at a magnification of 5000x.

Figure 7 shows that few carbon filaments formed in the region of the coupon which underwent mechanical treatment, compared to the region of the coupon not treated by NS- SPD. A comparison of figure 7c) and figure 6c) shows that scratching in only one direction provides less protection than scratching in both x- and y-directions, showing that more severe deformation results in increased protection of the alloy to metal-dusting corrosion. Example 6

A coupon of Incoloy 800 alloy was prepared as above. One section of the coupon was subjected to NS-SPD by manually scratching with tweezers at ambient temperature. The mechanically-treated coupon was then cleaned with ethanol and acetone. The metal coupon was then exposed to an oxidative gas atmosphere of (by volume) 10% FhO and 90% Ar at 1 bar and 540 ^°C for 26 hours. Finally, the coupon was exposed to a strongly carburizing atmosphere of (by volume) 10% CO and 90% Ar at 550 ^°C for 20 hours.

The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected. Figure 8a) shows the interface of the NS-SPD region and the non-NS-SPD region at a magnification of 2000x.

Figure 8b) shows the interface of the NS-SPD region and the non-NS-SPD region at a magnification of lOOOOx. Figure 8c) shows the interface of the NS-SPD region and the non-NS-SPD region at a magnification of 20000x.

As shown in figure 8, the region of the metal coupon not treated by NS-SPD had undergone more metal-dusting corrosion than the NS-SPD region, as shown by the increase in carbon filaments and powdery mixture of metallic, carbidic and carbonaceous dusts in the polished region.

However, the difference between the two regions was significantly less than that of examples 1-3. Thus, treatment of the surface following NS-SPD with (by volume) 10% H2O and 90% Ar at 540 ^°C at 1 bar for 26 hours is shown to be less effective at protecting the alloy from metal-dusting corrosion than exposure to either (by volume) 20% CO, 25% H2, 15% CO2, 10% H2O and 30% Ar at 20 bar and 750 ^°C for 20 hours alone, or to (by volume) 10% H2O and 90% Ar at 1 bar and 540 ^°C for 6 hours, followed by (by volume) 20% CO, 25% H₂, 15% C0₂, 10% H₂0 and 30% Ar at 20 bar and 750 ^°C for 20 hours

Example 7 A coupon of Incoloy 800 alloy was prepared as above. One section of the coupon was subjected to mechanical treatment by scratching with tweezers in one region (the NS- SPD region), and to scratching with a hard diamond tip in a second region. The mechanically-treated coupon was then cleaned with ethanol and acetone. The metal coupon was then exposed to an oxidative gas atmosphere of (by volume) 10 % H2O and 90% Ar at 1 bar for 6 hours, followed by (by volume) 20% CO, 25% ¾, 15% CO2, 10%

H2O and 30% Ar at 20 bar and 750 ^°C for 20 hours. Finally, the coupon was exposed to a strongly carburizing atmosphere of (by volume) 10% CO and 90% Ar at 550 ^°C for 20 hours.

The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected.

Figure 9a) shows both mechanical treatment regions, and the polished region, at a magnification of 30x.

Figure 9b) shows the interface of the region of the coupon that had been scratched with tweezers (the NS-SPD region) and the polished region at a magnification of 1200x. Figure 9c) shows one section of the coupon that underwent mechanical treatment by the hard diamond tip in one direction at a magnification of 2000x.

As shown in figure 9, the NS-SPD region did not undergo metal-dusting corrosion. No carbon filaments or powdery mixture of metallic, carbidic and carbonaceous dusts were present in this region. The region that underwent scratching with a hard-diamond tip shows some evidence of metal-dusting corrosion. The polished region shows substantial buildup of carbon filaments and metallic, carbidic and carbonaceous dusts, indicating significant metal-dusting corrosion.

Therefore, mechanical treatment by NS-SPD is shown to provide excellent protection from metal-dusting corrosion. Scratching with a hard diamond tip is shown to be less effective at protecting the alloy from metal -dusting corrosion.

Comparative Example 1

A coupon of Incoloy 800 alloy was treated as for example 2, except that the oxidative gas atmosphere was replaced with an inert atmosphere of argon at 1 bar and 750 ^°C. The coupon was then imaged using scanning electron microscopy (SEM) to allow the surface of the alloy to be visually inspected.

Figure 10a) shows the interface of the polished region and NS-SPD region at a magnification of lOOOx.

Figure 10b) shows the interface of the polished region and NS-SPD region at a magnification of 2000x.

Figure 10c) shows a section of the NS-SPD region at a magnification of lOOOOx. Figure 10 shows that there is very little difference in the rate of metal -dusting corrosion between the polished region and the NS-SPD region. Thus, it is shown that exposing the mechanically-treated surface of an alloy to an inert gas alone is insufficient to protect the alloy from the effects of metal-dusting corrosion.

List of references

[1] H. Grabke, R. Krajak, and E. Miiller-Lorenz, “Metal dusting of high temperature alloys ” Materials and Corrosion, vol. 44, no. 3, pp. 89-97, 1993.

[2] H. Grabke, R. Krajak, and J. N. Paz, “On the mechanism of catastrophic carburization: ‘metal dusting’,” Corrosion Science, vol. 35, no. 5-8, pp. 1141-1150, 1993.

[3] H. J. Grabke, “Metal dusting,” Materials and Corrosion, vol. 54, no. 10, pp. 736- 746, 2003.

[4] B. Schmid, N. Aas, 0. Grong, and R. Odegard, “In situ environmental scanning electron microscope observations of catalytic processes encountered in metal dusting corrosion on iron and nickel,” Applied Catalysis A: General, vol. 215, no.

1, pp. 257-270, 2001.

[5] D. J. Young, J. Zhang, C. Geers, and M. Schiitze, “Recent advances in understanding metal dusting: A review,” Materials and Corrosion, vol. 62, no. 1, pp. 7-28, 2011.

[6] H. Grabke, “Thermodynamics, mechanisms and kinetics of metal dusting,” Materials and Corrosion, vol. 49, no. 5, pp. 303-308, 1998.

[7] H. Yin, J. Zhang, and D. J. Young, “Effect of gas composition on coking and metal dusting of 2.25Cr-lMo steel compared with iron,” Corrosion Science, vol. 51, no. 12, pp. 2983-2993, 2009.

[8] M. P. Ryan, D. E. Williams, R. J. Chater, B. M. Hutton, and D. S. McPhail, “Why stainless steel corrodes,” Nature, vol. 415, no. 6873, pp. 770-774, 2002.

[9] P. Marcus, Corrosion mechanisms in theory and practice. Crc Press, 2011.

[10] H. Grabke and E. Miiller-Lorenz, “Protection of high alloy steels against metal dusting by oxide scales,” Materials and corrosion, vol. 49, no. 5, pp. 317-320,

1998.

[11] K. Voisey, Z. Liu, and F. Stott, “Inhibition of metal dusting of Alloy 800H by laser surface melting,” Applied Surface Science, vol. 252, no. 10, pp. 3658-3666, 2006.

[12] S. J. M. P. Kurihara, “Resistance of Ni-Base Alloy UNS N 06696 to Metal Dusting,” vol. 50, no. 8, pp. 66-70, 2011.

[13] S. Madloch, A. Dorcheh, and M. Galetz, “Effect of Pressure on Metal Dusting Initiation on Alloy 800H and Alloy 600 in CO-rich Syngas,” Oxidation of Metals, vol. 89, no. 3-4, pp. 483-498, 2018.

[14] R. Lobnig, H. Schmidt, K. Hennesen, and H. Grabke, “Diffusion of cations in chromia layers grown on iron-base alloys,” Oxidation of Metals, vol. 37, no. 1-2, pp. 81-93, 1992.

[15] I. Wolf and H. Grabke, “A study on the solubility and distribution of carbon in oxides,” Solid state communications, vol. 54, no. 1, pp. 5-10, 1985.

[16] I. Wolf, H. Grabke, and P. Schmidt, “Carbon transport through oxide scales on Fe- Cr alloys,” Oxidation of Metals, vol. 29, no. 3-4, pp. 289-306, 1988.

[17] J. C. Walmsley, J. Z. Albertsen, J. Friis, and R. H. Mathiesen, “The evolution and oxidation of carbides in an Alloy 601 exposed to long term high temperature corrosion conditions,” Corrosion Science, vol. 52, no. 12, pp. 4001-4010, 2010.

[18] M. Fulger, D. Ohai, M. Mihalache, M. Pantiru, and V. Malinovschi, “Oxidation behavior of Incoloy 800 under simulated supercritical water conditions,” Journal of Nuclear Materials, vol. 385, no. 2, pp. 288-293, 2009.

[19] M. Yamawaki, M. Mito, and M. Kanno, “Oxidation of Heat-Resistant Fe-Base Incoloy 800 Alloy,” Transactions of the Japan Institute of Metals, vol. 18, no. 8, pp. 567-573, 1977.

[20] M. W. Edwards and N. S. McIntyre, “Gas Phase Initial Oxidation of Incoloy 800 Surfaces,” Oxidation of metals, vol. 79, no. 1-2, pp. 179-200, 2013.

[21] Z. Zeng, K. Natesan, Z. Cai, D. Gosztola, R. Cook, and J. Hiller, “Effect of element diffusion through metallic networks during oxidation of Type 321 stainless steel,” Journal of materials engineering and performance, vol. 23, no. 4, pp. 1247-1262, 2014.

[22] Z. Zeng and K. Natesan, “Initiation of Metal -Dusting Pits and a Method to Mitigate Metal-Dusting Corrosion,” Oxidation of Metals, vol. 66, no. 1-2, pp. 1-20, 2006.

[23] C. Chun and T. Ramanarayanan, “Metal dusting corrosion of austenitic 304 stainless steel,” Journal of the Electrochemical Society, vol. 152, no. 5, pp. B169- B177, 2005. [24] H. Grabke, E. Muller-Lorenz, S. Strauss, E. Pippel, and J. Woltersdorf, “Effects of grain size, cold working, and surface finish on the metal-dusting resistance of steels,” Oxidation of Metals, vol. 50, no. 3, pp. 241-254, 1998.

[25] Z. Zeng, K. Natesan, Z. Cai, and S. B. Darling, “The role of metal nanoparticles and nanonetworks in alloy degradation,” Nat Mater, vol. 7, no. 8, pp. 641-6, Aug

2008.

[26] H. Grabke, R. Krajak, E. Muller-Lorenz, and S. StrauB, "Metal dusting of nickel - base alloys," Mater. Corros., vol. 47, no. 9, pp. 495-504, 1996.

Claims

1. A method for improving the resistance of an alloy to metal-dusting corrosion, the method comprising; a. mechanical treatment of a surface of the alloy to produce a mechanically- treated surface; b. oxidation of the mechanically-treated surface at a temperature from about 100 ^°C to about 1000 ^°C to produce a surface scale comprising (¾0₃ and/or a Mn-Cr-0 spinel on the mechanically-treated surface; wherein the alloy comprises a) Cr and b) one or more of Mn, Fe, Ni, Co, Mo, Al, Ti, Si, Cu Sn, Zn and Pb.

2. The method of claim 1, wherein the alloy comprises a) Cr and b) one or more of Fe, Ni and Mn.

3. The method of claim 1 or 2 wherein the alloy comprises greater than about 1% by mass Cr.

4. The method of any one of claims 1-3, wherein the alloy comprises greater than about 5% by mass Cr.

5. The method of any one of claims 1-4, wherein the alloy comprises greater than 0.05% by mass Mn.

6. The method of any one of claims 1-5, wherein the mechanical treatment comprises one or more of near-surface severe plastic deformation (NS-SPD) and severe plastic deformation (SPD).

7. The method of any one of claims 1-6, wherein the mechanical treatment comprises NS-SPD, the NS-SDP being conducted by one or more of scratching, sanding, grinding or sand-blasting.

8. The method of claim 7, wherein the scratching is conducted with one or more of a metal knife, tweezers, a wire brush or a hard diamond tip.

9. The method of any one of claims 1-6, wherein the mechanical treatment comprises SPD, the SPD being conducted by one or more of rolling, bending, folding, equal channel angular pressing (ECAP), multistep isothermal forging (MIF) or accumulative roll bonding (ARB).

10. The method of any one of claims 1-9, wherein the method further comprises a pre- treatment step before the mechanical treatment of the surface, the pre-treatment step comprising polishing and/or chemical pre-treatment of the surface.

11. The method of any one of claims 1-10, wherein oxidation of the mechanically- treated surface occurs in an atmosphere comprising one or more gases selected from O2, CO, CO2 and H2O.

12. The method of any one of claims 1-11 , wherein the atmosphere further comprises one or more of a) an inert gas and b) Eh and/or one or more gaseous hydrocarbons (C_xH_y).

13. The method of any one of claims 1-12, wherein oxidation of the mechanically- treated surface has a duration of from about 1 hour to about 80 hours.

14. The method of any of claims 1-13, wherein oxidation of the mechanically-treated surface comprises a first stage and a second stage, wherein the temperature of the first stage is from about 100 ^°C to about 650 ^°C and the temperature of the second stage is from about 500 ^°C to about 1000 ^°C.

15. The method of any one of claim 1-14, wherein the mechanically-treated alloy is heated up from ambient temperature to the desired temperature with a heating ramp rate of from about 0.1 ^°C/min to about 20 ^“C/min.

16. The method of claim 14, wherein the first stage has a duration of from about 1 hour to about 40 hours, and the second stage has a duration of from about 0.5 hours to about 40 hours

17. The method of any one of claims 1-15, wherein oxidation of the mechanically- treated surface is carried out under a flowing atmosphere.

18. The method of any one of claims 1-15, wherein said oxidation of the mechanically- treated surface is carried out under a static atmosphere.

19. The method of any one of claims 1-18, wherein said surface scale has a thickness of from about 30 nm to about 10 pm.

20. The method of any one of claims 1-19, wherein the alloy is part of a product.

21. The method of claim 20, wherein the product is a plate, sheet, vessel, pipe, tube, joint, pipeline, heat exchanger, container, reactor or gasket.

22. An alloy obtainable by the method according to any one of claims 1-21.

23. A product comprising the alloy of claim 22.

24. The product of claim 23 which is selected from a plate, sheet, vessel, pipe, tube, j oint, pipeline, heat exchanger, container, reactor and gasket.