FERROUS STRUCTURAL COMPONENT FOR USE IN FOULING AND CORROSIVE ENVIRONMENTS, AND METHOD OF MAKING AND USING A FERROUS STRUCTURAL COMPONENT

Abstract

A method of using a ferrous structural component is described. The method comprises integrating a ferrous structural component into process equipment, where the ferrous structural component comprises an iron alloy body with a modified surface including an aluminized surface layer that comprises one or more iron aluminides. The modified surface of the iron alloy body is exposed to an oxidative environment, thereby forming, as part of the modified surface, a passivating layer comprising aluminum oxide on the aluminized surface layer. The modified surface is also exposed to a process fluid. The exposure to the oxidative environment occurs prior to and/or upon exposure of the modified surface to the process fluid. Due to protection afforded by the passivating layer, the modified surface resists fouling and corrosion while exposed to the process fluid, as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body.

Description

TECHNICAL FIELD

The present disclosure is related generally to ferrous structural components used in fouling and/or corrosive environments, and more particularly to surface modification technology to improve the fouling- and corrosion-resistance of ferrous alloys.

BACKGROUND

Degradation of structural surfaces via a variety of corrosion or fouling mechanisms in industrial and environmental process streams is a pressing challenge. Such degradation may lead to reduced efficiency or component failure. However, the complexity of the process conditions (temperature, pressure, chemistry, flow) has encumbered a comprehensive description of possible degradation mechanisms, thereby limiting the effectiveness of existing mitigation strategies.

Asphaltene deposition is a ubiquitous and undesirable phenomenon in the petroleum production chain. Asphaltenes are the heaviest and most surface active component within the aromatic composition group of crude oil. In addition to carbon and hydrogen, they can include heteroatoms such as sulfur, nitrogen, and oxygen. At low temperatures, asphaltenes may precipitate from process fluids and foul critical processing and refining equipment. At high temperatures, heteroatoms such as sulfur may contribute to the formation of deleterious corrosive deposits. Asphaltene precipitation on sidewalls of a pipeline can limit process fluid flow and reduce productivity. In severe cases, flow from petroleum wells may completely cease within a matter of days because of arterial clogging from asphaltene deposition.

BRIEF SUMMARY

A ferrous structural component for use in fouling and/or corrosive environments is described. The ferrous structural component comprises an iron alloy body having a modified surface. The modified surface includes an aluminized surface layer comprising one or more iron aluminides and a passivating layer comprising aluminum oxide on the aluminized surface layer. When in direct contact with a process fluid, the modified surface resists corrosion and fouling as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body.

Also described is a method of using a ferrous structural component with enhanced fouling- and corrosion-resistance. The method comprises integrating a ferrous structural component into process equipment, where the ferrous structural component comprises an iron alloy body with a modified surface. The modified surface includes an aluminized surface layer comprising one or more iron aluminides. The modified surface of the iron alloy body is exposed to an oxidative environment, thereby forming, as part of the modified surface, a passivating layer comprising aluminum oxide on the aluminized surface layer. The modified surface of the iron alloy is also exposed to a process fluid. The exposure to the oxidative environment occurs prior to and/or upon exposure of the modified surface to the process fluid. Due to protection afforded by the passivating layer, the modified surface resists fouling and corrosion while exposed to the process fluid, as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body.

Finally, a method of imparting fouling- and corrosion-resistance to a ferrous structural component is described. The method comprises introducing aluminum into a surface of an iron alloy body at an elevated temperature to form a modified surface of the iron alloy body, where the modified surface includes an aluminized surface layer comprising one or more iron aluminides. The iron alloy body comprising the modified surface is exposed to an oxidizing environment, thereby forming, as part of the modified surface, a passivating layer comprising aluminum oxide on the aluminized surface layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B (inset image) show an exemplary ferrous structural component comprising a modified surface.

FIG. 2A is a scanning electron microscope (SEM) image of a cross-section of an iron alloy body having a modified surface including an aluminized surface layer and an interdiffusion layer; as indicated, the iron alloy body comprises X65 steel. Indentations from nanoindentation (hardness) tests are visible in the image.

FIG. 2B shows a concentration profile of atomic percent (at. %) Al and Fe as a function of depth (distance from the surface) in microns as determined from an energy dispersive spectroscopy (EDS) linescan.

FIG. 2C identifies phases present in the aluminized surface layer as determined by grazing incidence x-ray diffraction (GIXRD).

FIG. 3 is a schematic of an exemplary configuration for low-temperature pack aluminization with nested crucibles.

FIG. 4 is a SEM image of a cross-section of an X65 steel sample having a modified surface as in FIG. 2A, but at a different magnification; the aluminized surface layer and the interdiffusion layer are both discernible on the X65 steel.

FIG. 5A is a typical load versus depth plot obtained from nanoindentation experiments on cross-sections such as those shown in FIG. 2A or FIG. 4.

FIG. 5B shows hardness as a function of depth as determined by nanoindentation.

FIG. 5C shows a bar plot revealing the average hardness of the aluminized surface layer and base X65 steel as determined by nanoindentation.

FIG. 6 is a plot showing normalized mass gain for aluminized and bare (untreated) X65 steel samples.

FIG. 7 shows a cross-sectional scanning transmission electron microscope (STEM) image taken after fouling experiments of an X65 steel sample having a modified surface, where a passivating layer comprising aluminum oxide is visible.

FIGS. 8A-8C show cross-sectional STEM images of a pack aluminized X65 steel sample after scratching the passivating layer and then annealing, demonstrating the reformation of a thick and dense aluminum oxide layer.

FIG. 9 shows results from corrosion tests of aluminized and untreated X65 steel samples.

DETAILED DESCRIPTION

Described in this disclosure is an approach to modifying surfaces of ferrous structural components, such as oil and gas pipelines, to improve their resistance to fouling and corrosion in use. Also described are ferrous structural components having such modified surfaces, and methods of using the ferrous structural components to exploit their fouling- and corrosion-resistance in industrial and environmental processes.

FIG. 1A is a schematic of an exemplary ferrous structural component which has undergone surface modification processing as described herein for improved performance in corrosive and/or fouling environments. The ferrous structural component 102 comprises an iron alloy body 104 having a modified surface 104a, which is shown in the inset (FIG. 1B) to include an aluminized surface layer 106 comprising one or more iron aluminides and a passivating layer 108 on the aluminized surface layer 106. The passivating layer 108 comprises aluminum oxide. The modified surface 104a may also include, beneath the aluminized surface layer 106, an interdiffusion layer 110 comprising a decreasing amount of aluminum and an increasing amount of iron in a depth direction 112 of the iron alloy body 104. The depth direction 112 is shown by the arrow in FIG. 1B, and generally may be described as a direction normal to the modified surface 104a and into the depth of the structural component 102.

When in direct contact with a process fluid, which may comprise hydrocarbon and/or oxygenate components and/or water, the modified surface 104a resists fouling and corrosion—as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body 104. The term “carbonaceous deposits” may refer to asphaltene deposits and/or other types of deposits (e.g., thiolate and sulfide deposits), the latter of which may be formed as a consequence of asphaltene decomposition at elevated temperatures (e.g., above 300° C.). The phrase “substantial absence of carbonaceous deposits” can be understood to mean that the iron alloy body 104 exhibits an area normalized mass gain of no greater than about 500 mg/m² after exposure to the process fluid for 1 hour at a temperature of 350° C. This is typically an upper bound for rough surfaces (e.g., average roughness of a few microns), where smoother surfaces may show a significantly lower mass gain. Accordingly, the phrase “substantial absence of carbonaceous deposits” may mean that the iron alloy body 104 exhibits an area normalized mass gain of no greater than about 100 mg/m², or preferably no greater than about 50 mg/m², after exposure to the process fluid for 1 hour at a temperature of 350° C. This enhanced resistance to fouling and corrosion can be attributed to the presence of the aluminum oxide-containing passivating layer 108, which acts as a protective barrier, mitigating asphaltene deposition and reducing sulfidic corrosion of the ferrous structural component 102. Advantageously, aluminum oxide has a high thermal stability and low diffusivity that can inhibit the uncontrolled formation of thiolate and sulfide deposits that commonly foul unpassivated steels.

The modified surface 104a of the ferrous structural component 104 may be formed in a pack bed aluminization process followed by exposure to an oxidative environment. As described below, the pack-bed aluminization process may be employed to modify the surface of the iron alloy body 104 (e.g., to create the aluminized surface layer 106 and the interdiffusion layer 110), while subsequent exposure to the oxidative environment may produce the passivating layer 108 comprising aluminum oxide. Since the pack bed aluminization process entails inert or reducing conditions, the passivating layer 108 may not be formed as a consequence of that process. The exposure to the oxidative environment may take place during an oxidizing heat treatment following pack bed aluminization and/or during use of the ferrous structural component 102 in an industrial or environmental process. Since the aluminized surface layer 106 is formed by diffusion of aluminum into the iron alloy body 104 and does not constitute a separate surface coating prone to delamination, the aluminized surface layer 106 may serve as a microscale aluminum “reservoir” for forming and reforming the passivating layer 108 as needed (e.g., upon damage or removal in use).

In the schematic of FIG. 1A, the ferrous structural component 102 takes the form of tubing (a tube) or a pipeline; however, the size and shape of the structural component 102 are not limited by this particular example. In other examples, the ferrous structural component 102 may comprise an expansion fitting, an orifice plate, a blind, a valve, a flange, a connector, a baffle, an agglomerator, a demister, a static mixer, a thermowell, a pitot tube, a sparger, a nozzle, a distillation or fractionating column, a component used in a distillation or fractionating column (e.g., a distillation tray or a downcomer), a heat exchanger, a component used in a heat exchanger, and/or a vessel. The term “iron alloy body” 104 is used without intending any limitation to the geometry or size of the ferrous structural component 102 and may be understood to refer to the mass of material that constitutes some or all of the component 102 and which includes the modified surface 104a.

The iron alloy body may comprise a ferrous alloy, such as cast iron or steel. The ferrous alloy may include Fe, C, and one or more other alloying elements, such as Cu, Mn, Mo, Ni, V and/or Cr. Exemplary steels include low alloy steels, such as low-carbon, medium-carbon, and high-carbon steels, and high alloy steels, such as tool steel (which may also be considered to be a high-carbon steel) and stainless steel, which includes at least about 11 wt. % Cr. Low-carbon steels typically include less than about 0.25 wt. % C; medium-carbon steels typically include between about 0.25 wt. % C and 0.60 wt. % C, and high-carbon steels typically include between about 0.60 wt. % C and about 1.4 wt. % C. Exemplary cast irons include gray iron, ductile iron, white iron, and malleable iron; typically, cast irons include above about 2.1 wt. % C and may include between about 3.0 wt. % C and about 4.5 wt. % C. A commercially-available low-carbon steel that is investigated in this disclosure is X65 steel, which may include, in wt. %: 0.16 C, 0.45 Si, 1.65 Mn, 0.020 P, 0.010 S, 0.09 V, 0.05 Nb, and 0.06 Ti. The iron alloy body may comprise any of the aforementioned ferrous alloys, or others known in the art, which may benefit from enhanced corrosion- and/or fouling-resistance.

The modified surface 104a of the iron alloy body 104 is now described. As indicated above, the modified surface 104a includes an aluminized surface layer 106 comprising one or more iron aluminides, such as one or more of the following: Fe₂Al₅, FeAl, Fe₃Al, Fe₅Al₈, FeAl₂, FeAl₃, and Fe₄Al₁₃. More typically, the one or more iron aluminides comprise Fe₂Al₅, FeAl, Fe₃Al, FeAl₂, and/or FeAl₃. The aluminized surface layer 106 may also include an elemental iron phase, an elemental aluminum phase, and/or a solid solution phase. The depth to which the aluminized surface layer 106 extends may range from a few microns to tens of microns, in contrast to the passivating layer 108, which may have a nanoscale thickness, as described below. For example, depending on the process conditions, the aluminized surface layer 106 may extend to a depth as large as about 10 microns, as large as about 20 microns, or as large as about 30 microns. Typically, the aluminized surface layer 106 extends to a depth of at least about 3 microns, at least about 5 microns, or at least about 7 microns. The aluminized surface layer 106 may include a decreasing amount of aluminum as a function of depth. In other words, the aluminized surface layer 106 may include a decreasing amount of aluminum at increasing depths into the modified surface 104a. Also or alternatively, the aluminized surface layer 106 may include a substantially constant amount of aluminum as a function of depth. In other words, the aluminized surface layer 106 may include a substantially constant amount of aluminum at increasing depths into the modified surface. (“Substantially constant amount” may be understood to mean an amount that does not vary by more than about +/−15%, or by more than about +/−10%, from an average value.) Similarly, the aluminized surface layer 106 may include an increasing amount of iron as a function of depth, and/or a substantially constant amount of iron as a function of depth.

The interdiffusion layer 110 typically has a thickness (in a depth direction) in a range from about 1 micron to about 5 microns. As described above, the interdiffusion layer 110 includes a decreasing amount of aluminum and an increasing amount of iron as a function of depth. A significant rise or fall in the amount of the respective element (Fe or Al) typically occurs in the interdiffusion layer 110. For example, a concentration change of ±50-90 wt. % may occur over a depth of just a few microns (e.g., 1-5 microns). This may be visualized with a concentration profile, as described below.

FIG. 2A shows a scanning electron microscope (SEM) image (cross-sectional view) of an exemplary iron alloy body 104 having a modified surface 104a. The iron alloy body 104 of this example is a 5 mm×5 mm×0.5 mm pipe steel sample comprising an X65 alloy that underwent pack aluminization as described below. Referring to the SEM image, an interdiffusion layer 110 of about 2 microns in thickness is visible between the aluminized surface layer 106, which extends to a depth of about 7 microns, and the iron alloy body 104. FIG. 2B provides a concentration profile and FIG. 2C shows an x-ray diffraction pattern obtained from the modified surface 104b. Referring to FIG. 2B, the aluminized surface layer 106 of this example comprises a substantially constant amount of aluminum (about 70 wt. %) as a function of depth. Similarly, the aluminized surface layer 106 includes a substantially constant amount of iron as a function of depth (about 30 wt. %). Depending on the process conditions, the aluminized surface layer 106 may contain an amount of aluminum in a range from about 50-90 wt. % and an amount of iron in a range from about 10-50 wt. %. The x-ray diffraction pattern of FIG. 2C reveals the presence of Fe₂Al₅ and Fe in the aluminized surface layer 106.

To be effective in inhibiting corrosion and fouling, the passivating layer 108 comprising aluminum oxide typically has a thickness greater than 5 nm and is continuous over an entirety of the aluminized surface layer 106. As indicated above, the aluminum contained in the aluminized surface layer 106 may serve as a “reservoir” to regenerate the passivating layer 108 if damaged or otherwise removed, ensuring that the continuity of the passivating layer 108 can be maintained. This regeneration, if needed, may occur either in use or in a separate oxidizing heat treatment, as discussed below. The thickness of the passivating layer 108 may lie in the range from greater than 5 nm to about 40 nm, and is typically in the range from about 10 nm to about 30 nm. Ideally the passivating layer 108 exhibits a uniform thickness, with a variation in average thickness no greater than about ±10% over the layer 108. The passivating layer 108 may act as a protective barrier against a corrosive environment and may mitigate the build-up of foulant from reactive species, such as sulfur and carbon; the passivating layer 108 may also prevent outward diffusion of metal species from the iron alloy body 104. In addition, there is evidence that an aluminum sulfate layer may, in some cases, form on the passivating layer 108 in use.

The passivating layer 108 may consist essentially of aluminum oxide, i.e., aluminum oxide and any incidental impurities only. Alternatively, the passivating layer 108 may comprise some amount of other metallic elements, such as chromium, depending on the composition of the iron alloy body 104. However, the presence of iron is not advantageous and it is preferred that the passivating layer 108 be substantially devoid of iron. In other words, iron may not be present in the passivating layer 108 beyond parts-per-million (ppm) or incidental impurity levels. It is believed that the presence of iron aluminide phase(s) such as Fe₂Al₅ in the aluminized surface layer 106 biases the native oxide chemistry to favor the formation of aluminum oxide (Al₂O₃) instead of iron oxide (e.g., Fe₂O₃). It is believed that the aluminum oxide present in the passivating layer comprises amorphous alumina. Gamma (γ) alumina is not known to be an effective diffusion barrier, and alpha (α) alumina may not be formed at temperatures below 1000° C., which can damage the mechanical properties of steels. Accordingly, this suggests there is a maximum temperature to which the modified surface 104a may be exposed during processing or use (about 550° C.) to prevent the amorphous alumina, which is an effective diffusion barrier, from transforming to γ alumina.

Also described in this disclosure is a method of using the ferrous structural component 102 in industrial or environmental applications which may benefit from resistance to fouling and corrosion. The method comprises integrating a ferrous structural component 102 into process equipment exposed to fouling and/or corrosive process streams in use. The ferrous structural component 102 comprises an iron alloy body 104 with a modified surface 104a including, as described above, an aluminized surface layer 106 comprising one or more iron aluminides. An interdiffusion layer 110 comprising a decreasing amount of aluminum and an increasing amount of iron in a depth direction of the iron alloy body 104 may lie beneath the aluminized surface layer 106. A passivating layer 108 forms on the aluminized surface layer 106 upon exposure to an oxidative environment, which may occur prior to use of the ferrous structural component 102 and/or in use. For example, the passivating layer 108 may be formed upon exposure of the aluminized surface layer 106 to an oxidizing heat treatment prior to use, or upon exposure of the modified surface to a process fluid, which may entail oxidative conditions. While a thin oxide layer may form on the aluminized surface layer 106 upon air exposure without heating, an oxidizing heat treatment is believed to be advantageous if not necessary to promote formation of a passivating layer 108 that is sufficiently thick and continuous to serve a protective function.

The process fluid to which the modified surface 104a of the iron alloy body 104 is exposed in use may include hydrocarbon and/or oxygenate components and/or water. For example, the process fluid may comprise petroleum, natural gas, oil, one or more petrochemicals, a biofuel, and/or water (e.g., seawater, fresh water, and/or cooling water). As explained above, the modified surface 104a includes a passivating layer 108 comprising aluminum oxide on the aluminized surface layer 106. Due to protection afforded by the passivating layer 108, the modified surface 104a resists fouling and corrosion while exposed to the process fluid, as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body 104. As described above, the term “carbonaceous deposits” may refer to asphaltene deposits and/or other types of deposits (e.g., thiolate and sulfide deposits), the latter of which may be formed as a consequence of asphaltene decomposition at elevated temperatures. The phrase “substantial absence of carbonaceous deposits” can be understood to mean that the iron alloy body 104 exhibits an area normalized mass gain of no greater than about 500 mg/m² (or no greater than about 200 mg/m², or preferably no greater than about 50 mg/m²) after exposure to the process fluid for 1 hour at a temperature of 350° C.

It is known that rough surfaces can provide physical entrapment sites for foulants in hydrodynamic environments. In fact, improving surface finish to reduce foulant deposition is a strategy employed in the oil and gas industry. Thus, it is notable that the protection afforded by the passivating layer 108 reduces the negative impact of surface roughness on foulant deposition. For example, even modified surfaces 104a exhibiting a surface roughness up to about 2.5 microns (e.g., 2.0-2.5 microns) can exhibit an area normalized mass gain of less than about 500 mg/m² after exposure to a process fluid (as described in the examples below) at 350° C. for 1 hour.

The oxidative environment referred to above may be understood to be an environment in which the aluminized surface layer 106 undergoes surface oxidation to form the passivating layer 108 comprising aluminum oxide. In other words, the oxidative environment may comprise conditions and/or characteristics (e.g., elevated temperature, oxygenate component(s)) conducive to surface oxidation of the aluminized surface layer 106, such that the passivating layer 108 is formed. An oxidizing heat treatment carried out in air or in a controlled environment (such as a furnace) at a suitable elevated temperature (e.g., from about 150° C. to about 350° C.) may provide the oxidative environment. Also or alternatively, exposure of the aluminized surface layer 106 to the process fluid in use in an industrial or environmental application may provide the oxidative environment. For example, exposure to the process fluid may occur at an elevated temperature, such as in the range from about 150-350° C. In such a situation, the passivating layer 108 may form essentially instantaneously on the aluminized surface layer 106 upon exposure to the process fluid. Dissolved oxygen in the process fluid may act as a source of oxygen and the elevated temperature may accelerate oxide growth rate. It is understood that the passivating layer 108 being formed is preferably greater than 5 nm in thickness and continuous over an entirety of the aluminized surface layer 106 to be effective in mitigating fouling and corrosion.

The ferrous structural component 102 may be integrated into process equipment utilized in oil or gas production, oil refining, petrochemicals processing, and/or biofuels processing. The integration may comprise assembly and/or connection with one or more other structural components that may or may not have undergone the pack bed aluminization process. As described above, the ferrous structural component 102 may comprise tubing (a tube), a pipeline, an expansion fitting, an orifice plate, a blind, a valve, a flange, a connector, a baffle, an agglomerator, a demister, a static mixer, a thermowell, a pitot tube, a sparger, a nozzle, a distillation or fractionation column, a component used in a distillation or fractionating column (e.g., a distillation tray or a downcomer), a heat exchanger, a component used in a heat exchanger, and/or a vessel, for example.

A method of imparting fouling- and corrosion-resistance to a ferrous structural component is now described. The method includes introducing aluminum into a surface of an iron alloy body at an elevated temperature to form a modified surface of the iron alloy body. The introduction of aluminum into the surface may be referred to as aluminization. The modified surface includes, as described above, an aluminized surface layer comprising one or more iron aluminides. An interdiffusion layer including a decreasing amount of aluminum and an increasing amount of iron in a depth direction of the iron alloy body may also be formed beneath the aluminized surface layer. The aluminum is introduced into the surface under inert or reducing conditions, and thus a passivating layer may not be formed during aluminization. After aluminization, the iron alloy body is exposed to an oxidative environment, thereby forming, as part of the modified surface, a passivating layer comprising aluminum oxide on the aluminized surface layer.

The exposure to the oxidative environment may occur while the ferrous structural component is in use, such as while the iron alloy body is in contact with a process fluid. Alternatively, the exposure to the oxidative environment may occur prior to use, such as during exposure to an oxidizing heat treatment following the formation of the aluminized surface layer.

It is also contemplated that the method may entail introducing chromium instead of or in addition to aluminum into the surface, in which case the aluminized surface layer 106 may comprise iron and chromium instead of or in addition to aluminum. The aluminum and/or chromium may be introduced into the surface of the iron alloy body 104 via pack aluminization (or chromization) using a pack comprising an aluminum source (a chromium source), a halide salt, and aluminum oxide. Pack aluminization (or chromization) may be described as a halide salt activated chemical vapor deposition process in which a structural component of any size or geometry is embedded in a powder-based pack and surface-treated with aluminum (or chromium) through thermal activation. Two processes may occur simultaneously during thermal activation: (1) the formation of aluminum (chromium) halide vapors and their infusion into the surface of the iron alloy body; and (2) their reduction at the surface to form a Fe—Al (Fe—Cr) diffusion couple and the consequent interdiffusion process. Pack aluminization (chromization) may create a chemical gradient as a function of depth into the surface, with decreasing Al (or Cr) concentration in a depth direction of the iron alloy body (i.e., in a direction away from the surface). The chemical profile may be modified by altering the aluminization (chromization) conditions or by post-aluminization (chromization) annealing or heat treatment steps in inert or oxidizing conditions. Aluminization and chromization can be done simultaneously or sequentially to produce the aluminized surface layer 106 described above.

Referring now to FIG. 3, the pack 330 and the ferrous structural component 340 to be aluminized (chromized) may be placed in a controlled environment 350, such as in a crucible with a gas-tight seal or in a semi-open crucible placed in a furnace under vacuum conditions or inert gas (e.g., He, Ar, or N₂) flow. Alternatively, a reducing environment, such as a forming gas flow (e.g., N₂ or Ar+H₂) may be employed. For example, the controlled environment may comprise an environmental control furnace under gettered argon (≤10⁻¹² partial pressure of O₂). The argon may be maintained at a pressure of 1 atm and continuously purged at a flow rate in a range of 0.5-2 L/min. Advantageously, the controlled environment is substantially devoid of oxygen.

To promote diffusion into the surface, the introduction of the aluminum (chromium) may be carried out at an elevated temperature in a range from 500° C. to 1000° C., and preferably below 900° C. to avoid having a detrimental impact on the mechanical properties of the ferrous structural component 102. Preferably, the elevated temperature may lie in the range from about 550° C. to about 900° C. For example, pack aluminization of alloy steels including 13 wt. % Cr (“13 Cr alloys”) may be carried out at an elevated temperature of about 900° C. or less, and pack aluminization of X65 steels may be carried out at an elevated temperature of 600° C. or less. The elevated temperature at which aluminization (chromization) takes place may be understood to be the temperature attained by the ferrous metal component during processing. The elevated temperature may be maintained for 12 hours or less, for 6 hours or less, or for 3 hours or less, and typically for at least one hour to achieve aluminization (chromization).

The aluminum source may comprise an aluminum alloy including from 50% to 99% aluminum or pure aluminum including only incidental impurities. When chromization takes place, the chromium source may comprise chromium (e.g., 99.5% purity). The halide salt may comprise NH₄Cl or AlCl₃, for example. The halide salt may serve as an activator and the aluminum oxide may act as an inert diluent. All three ingredients may take the form of a powder that can be tumbled together to form a mixture that serves as the pack. For example, the powder may be ball milled for 24-48 hours to ensure a homogeneous mixture. The pack may comprise the aluminum (or chromium) source at a concentration from about 5 wt. % to about 40 wt. %; the halide salt at a concentration from about 1 wt. % to about 5 wt. %; and the aluminum oxide at a concentration from about 70 wt. % to about 90 wt. %. The relative amounts of the pack ingredients are selected so that the pack has a sufficiently high aluminum (chromium) activity to be effective at the lower temperatures desired to preserve the mechanical properties of the iron alloy body and minimize grain growth.

Examples

In experiments described below, low-temperature pack aluminization is applied to samples comprising API 5L X65, a high strength pipe steel that is low in alloying elements. High-temperature and high-pressure (HPHT) autoclave fouling with a model sulfur-containing asphaltene, specifically, 1,6-bis(pyren-1-ylthio)hexane, referred to as BPH, is conducted on untreated and aluminized sample surfaces. The results suggest that using low-temperature pack aluminization can be a viable route to improve the fouling resistance of ferrous structural components, with beneficial effects on other metrics, such as mechanical properties and corrosion resistance.

Metallographic Sample Preparation

As indicated above, the ferrous alloy utilized for this study is API 5L X65. After being machined down to 5 mm×5 mm×0.5 mm size by electrical discharge machining (EDM), the coupons are brought to a level but stochastic finish (˜20 nm RMS roughness) by grinding with silicon carbide papers up to 1200 grit followed by polishing with a 1 μm neutral alumina solution. While the aluminization process increases the original surface roughness of the polished X65 specimens (˜180 nm RMS roughness), the benefits of reduced asphaltene fouling are realized (see below) due to the presence of the passivating layer.

Low-Temperature Pack Aluminization

The pack used for the aluminization process includes three ingredients: 82 wt. % inert Al₂O₃ powder [Baikowski/US Research Nanomaterials, 99.9% pure], 3 wt. % NH₄Cl activator [Alfa Aesar, 99.999% pure], and a 15 wt. % Raney-nickel aluminum source [Ni-50 wt. % Al, Acros, 99.99% pure]. The relative amounts of the ingredients are selected to ensure that the pack has a sufficiently high aluminum activity to work at the lower temperatures desired to minimize grain growth and preserve the base mechanical properties of the steel. After the ingredients are combined, they are mixed with Al₂O₃ ball milling media for 48 hours to ensure homogeneous mixing. The X65 samples 340 are embedded in the pack 330, as shown in FIG. 3, and placed in an environmental control furnace 350 under 1 atmosphere of gettered argon (10⁻¹² partial pressure of O₂) that was continuously purged at a flow rate of 1 L/min. In order to produce Al chloride vapors that reduce at the surface of the metal to leave behind Al metal, the embedded samples are heated to 600° C. at a ramp rate of 20° C./min and held at this temperature for 6 hrs. Several 5 mm×5 mm×0.5 mm X65 pipe steel coupons 340 are aluminized, as illustrated in FIG. 3. The samples 340 are then cooled under argon for an hour, followed by a nitrogen purge until they reach room temperature.

High-Temperature Fouling Test

The model asphaltene used in this study is 1,6-bis(pyren-1-ylthio)hexane, referred to as BPH. The BPH is synthesized through a nucleophilic aromatic substitution reaction. For the HPHT fouling test, an autoclave [60 ml EZE Seal Pressure Vessel, Parker Hannifin] made from 316 stainless steel is used. The steel coupons are fully submerged in 5 mL of the model asphaltene solution before being air-sealed and then heated from room temperature at a rate of 10° C./min. Once the furnace reaches its final temperature of 350° C., the temperature is held for 1 hour before the furnace is naturally cooled to room temperature. After the autoclave reaches room temperature, the coupons are removed and rinsed with 1 mL of petroleum ether to remove loosely adhered deposits. The mass gain of the coupons is measured using a microbalance [XPE26, Mettler-Toledo] after the fouling test.

Mechanical Characterization

The hardness of the aluminized coupons is measured along the depth of the cross-sectional samples using a nanoindentation system [Hysitron TI 950 Tribolndenter, Bruker Corporation] with a diamond Berkovich tip. All indentation tests are made to a maximum depth of 100 nm with a constant rate of 10 nm/s. The obtained load-displacement data are analyzed using the Oliver-Pharr method to calculate the hardness.

Hydrogen Chloride Corrosion Test

Corrosion tests are performed by immersing X65 samples in a fixed pH (˜3) HCl solution at room temperature. The HCl solution is composed of 1.0 M HCl and distilled water in the appropriate proportions. The solution is monitored daily to keep the pH value relatively constant throughout the test. The mass of the samples is measured every three days using a microbalance. The total area normalized mass loss of specimens is calculated and compared to the original mass before immersion.

Results

Cross-sectional SEM images of X65 samples 104 after aluminization are shown at different magnifications in FIGS. 2A and 4. The modified surface 104a includes an aluminized surface layer 106 overlying a smaller interdiffusion layer 110. Beneath the interdiffusion layer 110 is the base X65 alloy, which is not altered by the aluminization.

Results from nanoindentation tests carried out on the cross-sections of aluminized X65 steel samples as described above are shown in FIGS. 5A-5C, where indentations from the tests are visible in FIG. 2A. The hardness of the aluminized top layer and the X65 substrate beneath is evaluated from the indentation load-depth curves. FIG. 5A shows the typical load-depth curves for the indents made in the aluminized surface layer and in the base X65 alloy. The aluminized surface lay has a much higher maximum indentation load, thus a higher hardness compared to the base X65. The variation of hardness as a function of depth is shown in FIG. 5B; the aluminized surface layer is significantly harder than both the interdiffusion layer and the base X65 alloy. The average hardness of the aluminized surface is 8.77±1.66 GPa, and it drops to 3.22±0.15 GPa in the base X65 region, as shown in FIG. 5C. The hardness data as a function of depth correlate well with the concentration profile data, discussed above and shown in FIG. 2B. Combined, these data confirm the formation of an aluminized surface layer including one or more iron aluminides. The high hardness in the aluminized surface layer suggests that Fe₂Al₅, the hardest intermetallic phase formed among Fe—Al compounds, may be present.

Results from fouling experiments conducted on aluminized and untreated (bare) X65 coupons at 350° C. as described above are presented in FIG. 6 as area normalized mass gain. The difference in mass gain between the aluminized and bare X65 samples is significant. The average amount of deposition measured for the bare X65 samples is nearly 3000 mg/m² (2914 mg/m²), while negligible mass gain is observed for the aluminized X65 sample. Optical images reveal large and numerous carbonaceous deposits on the bare X65 samples, while there is no significant foulant build-up on the aluminized X65 samples.

A detailed cross-sectional scanning transmission electron microscope (STEM) image of an aluminized X65 steel surface after fouling is shown in FIG. 7. The STEM data reveal a dense and continuous film of Al₂O₃ with a thickness of ˜10 nm. In agreement with XPS data, the results show that during the fouling experiment, the existing thin Al₂O₃ layer grows thicker. Thus, in contrast to the bare X65 substrate, the alumina layer prevents the formation of thiolate species and iron sulfide at the surface. The passivating layer also mitigates the diffusion of other metallic species from the X65 substrate. Therefore, the aluminized surface shows no significant mass gain from the fouling deposits, in contrast to the bare X65 coupons.

In the aluminization process, the aluminum diffuses into the X65 steel and establishes an aluminum reservoir for the formation and healing of the passivating layer, as described above. In order to examine the reformation of the passivating layer in aluminized samples, the aluminized surface layer undergoes polishing to remove the existing passivating layer. A nanoindenter scratch test is then performed along a 100 μm line with a continuously increasing load from zero to the maximal load of 150 mN in order to determine the desired depth of penetration. A few scratches are made at a constant load of 50 mN and a depth of ˜500 nm (0.5 micron). After introducing the scratches, the sample is annealed in air at 350° C. for 1 hour (similar to the fouling experiment) for further analysis. An STEM cross-sectional image of a representative scratch is shown in FIG. 8A. The high magnification images confirm the reformation of a ˜10 nm thick and uniform oxide layer both at the side and tip of the scratch (FIGS. 8B and 8C). The results show that if the passivating oxide layer on the iron alloy body is damaged within the aluminized region, it can be reformed continuously from the Al reservoir.

A summary of the results of the corrosion tests is shown in FIG. 9, where it can be observed that the average mass loss after 30 days is higher for the bare X65 sample than for the aluminized X65 sample. This indicates increased corrosion resistance for the aluminized X65 sample. As explained above, an aluminum oxide layer may form immediately upon air exposure; this oxide layer may be very thin and discontinuous. However, exposure to higher temperatures can rapidly help it grow thick and continuous to form the passivating layer 108, which provides protection against corrosion and deposition.

Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.

Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.

Claims

1. A ferrous structural component for use in fouling and corrosive environments, the ferrous structural component comprising: an iron alloy body having a modified surface comprising: an aluminized surface layer comprising one or more iron aluminides; anda passivating layer on the aluminized surface layer, the passivating layer comprising aluminum oxide,wherein, when in direct contact with a process fluid, the modified surface resists fouling and corrosion as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body.

2. The ferrous structural component of claim 1, wherein the modified surface further comprises an interdiffusion layer beneath the aluminized surface layer, the interdiffusion layer comprising a decreasing amount of aluminum and an increasing amount of iron in a depth direction of the iron alloy body.

3. The ferrous structural component of claim 2, wherein the interdiffusion layer comprises a thickness in a range from about 1 micron to about 3 microns, wherein the aluminized surface layer has a thickness in a range from about 3 microns to about 30 microns, andwherein the passivating layer has a thickness in a range from greater than 5 nm to about 40 nm.

4. The ferrous structural component of claim 1, wherein the one or more iron aluminides are selected from the group consisting of Fe2Al5, FeAl, Fe3Al, Fe5Al8, FeAl2, FeAl3, and Fe4Al13.

5. The ferrous structural component of claim 1, wherein the aluminized surface layer comprises a substantially constant amount of aluminum as a function of depth and/or a decreasing amount of aluminum as a function of depth.

6. The ferrous structural component of claim 1, wherein the iron alloy body comprises cast iron or steel.

7. The ferrous structural component of claim 1 being a tube, pipeline, expansion fitting, an orifice plate, a blind, a valve, a flange, a connector, a baffle, an agglomerator, a demister, a static mixer, a thermowell, a pitot tube, a sparger, a nozzle, a fractionating column, a component used in a fractionating column, a distillation tray, a downcomer, a heat exchanger, a component used in a heat exchanger, and/or a vessel.

8. A method of using a ferrous structural component, the method comprising: integrating a ferrous structural component into process equipment, the ferrous structural component comprising an iron alloy body with a modified surface including an aluminized surface layer comprising one or more iron aluminides;exposing the modified surface of the iron alloy body to an oxidative environment, thereby forming, as part of the modified surface, a passivating layer comprising aluminum oxide on the aluminized surface layer; andexposing the modified surface to a process fluid,wherein the exposure to the oxidative environment occurs prior to and/or upon exposure of the modified surface to the process fluid, andwherein, due to protection afforded by the passivating layer, the modified surface resists fouling and corrosion while exposed to the process fluid, as exhibited by a substantial absence of carbonaceous deposits on the iron alloy body.

9. The method of claim 8, wherein the modified surface further comprises an interdiffusion layer beneath the aluminized surface layer, the interdiffusion layer comprising a decreasing amount of aluminum and an increasing amount of iron in a depth direction of the iron alloy body.

10. The method of claim 8, wherein the exposure to the process fluid occurs at an elevated temperature.

11. The method of claim 8, wherein the exposure to the oxidative environment occurs prior to the exposure to the process fluid during an oxidizing heat treatment.

12. The method of claim 8, wherein the process fluid comprises hydrocarbon and/or oxygenate components and/or water.

13. The method of claim 12, wherein the process fluid comprises petroleum, natural gas, oil, one or more petrochemicals, a biofuel, and/or water.

14. The method of claim 8, wherein the ferrous structural component comprises a tube, a pipeline, an expansion fitting, an orifice plate, a blind, a valve, a flange, a connector, a baffle, an agglomerator, a demister, a static mixer, a thermowell, a pitot tube, a sparger, a nozzle, a fractionating column, a component used in a fractionating column, a distillation tray, a downcomer, a heat exchanger, a component used in a heat exchanger, and/or a vessel.

15. The method of claim 8, wherein the passivating layer is reformable if damaged or removed.

16. A method of imparting fouling- and corrosion-resistance to a ferrous structural component, the method comprising: introducing aluminum into a surface of an iron alloy body at an elevated temperature to form a modified surface of the iron alloy body, the modified surface including an aluminized surface layer comprising one or more iron aluminides;exposing the iron alloy body comprising the modified surface to an oxidizing environment, thereby forming, as part of the modified surface, a passivating layer comprising aluminum oxide on the aluminized surface.

17. The method of claim 16, wherein the modified surface further comprises an interdiffusion layer beneath the aluminized surface layer, the interdiffusion layer comprising a decreasing amount of aluminum and an increasing amount of iron in a depth direction of the iron alloy body.

18. The method of claim 16, wherein the exposure to the oxidizing environment occurs upon exposure of the modified surface to a process fluid while the ferrous structural component is in use.

19. The method of claim 16, wherein the exposure to the oxidizing environment occurs during an oxidizing heat treatment prior to use of the ferrous structural component.

20. The method of claim 16, wherein the aluminum is introduced into the surface of the iron alloy body via pack aluminization using a pack comprising an aluminum source, a halide salt, and aluminum oxide.