Reactive coating processes

Abstract

Reactive coating processes are provided that can include providing a coating material, reacting the coating material to form a shell about the coating material, contacting the shelled coating material with a substrate to be coated, depositing the coating material from within the shelled coating material on the substrate, and removing the shells from the substrate. Coating materials may be deposited upon a substrate to be coated and reacted to form a shell about the coating material. The coating materials can be particles and a shell can be formed about each of the individual particles.

Claims

1. A reactive coating process comprising: reacting a coating material to form shelled coating material comprising a solid shell material about liquid coating material, wherein the coating material comprises Al; depositing the shelled coating material upon a substrate to be coated; adhering the liquid coating material from within the solid shell material to the substrate, wherein the coating material is in the liquid form and the shell material is in the solid form during the adhering; and removing the solid shell material from the substrate.

2. The reactive coating process of claim 1 wherein the reacting comprises reduction, oxidation, corrosion, radiation, and/or vibration.

3. The reactive coating process of claim 1 wherein the reacting is performed at a temperature of from about 660 C. to about 1200 C.

4. The reactive coating process of claim 1 wherein the substrate comprises Fe.

5. The reactive coating process of claim 4 wherein the substrate further comprises Cr.

6. The reactive coating process of claim 4 wherein the substrate further comprises Ni.

7. The reactive coating process of claim 1 wherein the solid shell material is removed with a brush.

8. The reactive coating process of claim 1 wherein the substrate comprises Ti.

9. The reactive coating process of claim 8 wherein the substrate further comprises Al.

10. The reactive coating process of claim 1 wherein the shell comprises oxidized Al.

11. The reactive coating process of claim 10 further comprising diffusing the Al into the substrate.

Description

DESCRIPTION OF DRAWINGS

(1) Embodiments of the disclosure are described below with reference to the following accompanying drawings.

(2) FIG. 1 is an illustration depicting the aluminization of a metal substrate surface according to one embodiment of the present disclosure.

(3) FIG. 2 is a plot showing button cell performance curves.

(4) FIG. 3 is a process flow for a reactive process according to an embodiment of the disclosure.

(5) FIG. 4 is a depiction of various reactive process intermediates according to an embodiment of the disclosure.

DETAILED DESCRIPTION

(6) The following description includes the best mode of one embodiment of the present disclosure. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments, but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the disclosure is susceptible of various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.

(7) FIGS. 1 and 2 show a variety of aspects of the present disclosure. Referring first to FIG. 1, the illustration depicts the aluminization of a metal substrate (e.g., Cr-containing iron alloy) according to one embodiment of the present disclosure. While the present invention is not limited to Al coatings on Cr-containing alloys, many aspects of the invention are demonstrated well by this material system because of the particular relevance of Cr-containing alloys to high-temperature applications and because of the processing difficulties of Cr-containing alloys. For example, the list of candidate high-temperature alloys considered applicable in SOFCs generally is limited to those that form a stable, protective chromium oxide scale in-situ during use. However chromia is susceptible to volatilization at high temperature (>600 C.), particularly in water vapor bearing environments, due to the formation of gaseous CrO.sub.3 and CrO.sub.2(OH).sub.2. Furthermore, it has been established that these species will preferentially adsorb onto oxygen catalysis sites at the cathode/electrolyte interfaces within the cells, which causes continual degradation in the overall power output of the fuel cell system. While the use of complex conductive oxide coatings (such as chromium manganite spinel) is a useful means of mitigating chromia volatility in interconnect components, they are not necessarily applicable to other in-stack and balance-of-plant components, all of which can have a potential role in the formation of deleterious Cr-gas species. While the use of electrically conductive coatings (such as cobalt manganite spinel) is a useful means of mitigating chromia volatility in interconnect components, they have reactions with other components (such as sealing materials) which can lead to mechanical failure and electrical short circuits. In at least some of these embodiments, aluminum-based coatings may afford greater functionality (i.e. a combination of low-cost application, long-term oxidation resistance, minimal chromia volatility, and improved seal durability).

(8) According to the instant embodiment, aluminization comprises applying a relatively low viscosity Al powder/polymer binder slurry 102 onto the surfaces of the metal substrate 101, drying the coated component in air at 80 C. to drive off the slurry solvent 105, and heating in air at temperatures above 660 C. to initiate diffusion 104 of the aluminum into the underlying substrate and begin forming a protective alumina scale 103 in a combined step.

(9) In a particular example, a Cr-containing Fe alloy coupon was employed (e.g., 73.7% Fe, 24% Cr, 0.8% Mn, 0.5% Si, 0.5% Cu, and 0.5% Al). Coupons measuring 10 mm25.4 mm were machined from these plates, then ultrasonically cleaned in isopropanol and rinsed with acetone prior to coating. As listed in Table 1, three different nominal sizes of aluminum powder were evaluated (all high purity aluminumthe actual purities are listed). In addition, two soak temperatures and four binder systems were examined. Because the nominal particle size of the powder can define the minimum applied slurry coating thickness, this factor was investigated as a function of powder particle size.

(10) TABLE-US-00001 TABLE 1 Variables tested and compared in the aluminzation of Cr-containing Fe alloys Soak Temperature/Time Binder Type Al Powder Particle Size 700 C., 4 h None 0.1 m (purity: 97.5%) 1000 C., 1 h Isopropanol 3 m (purity: 97.5%) Heraeus V-006 44 m (purity: 99.5%) ESL 450

(11) The aluminum slurry pastes were prepared by mixing aluminum powder with a given binder in a ratio of 70:30 by weight, first by hand and then by using a three-roll mill to achieve uniform dispersion. Substrate coupons were coated using a stencil printing technique and dried in a convection oven at 80 C. for a minimum of 2 h. The coupons were then heated in an air muffle furnace at 3 C./min to the final soak conditions and cooled at 3 C./min. Loose debris on the surface of the coated materials was removed with a nylon brush. We have since employed a light fluidized sand attrition process that yields similar results with minimal labor.

(12) A 700 C., 4 h heat treatment in air was conducted on coupons having a 200 m thick layer of a slurry comprising 44 m Al powder and various binders (e.g., isopropanol, organic binders HERAEUS V-006 and ESL 450). This low-temperature treatment resulted in acceptable coatings, but ones that demonstrated relatively poor adhesion and that would likely exhibit poor chromia retention behavior due to only partial coverage of the underlying chromia-scale forming substrate. Based on these findings, subsequent heat treatments were conducted at a more preferred temperature, 1000 C. for 1 h.

(13) With respect to appropriate binder selection, three separate slurries were prepared, each containing 44 m Al powder suspended in either: isopropanol, HERAEUS V-006, or ESL 450, which are organic binders. Results from coating experiments conducted with these slurries were compared with a baseline case in which no binder was used, but the coupons were coated with loose aluminum powder (44 m size). In all cases, the coated coupons were heat treated in air at 1000 C. for 1 h. It was found that only slurries prepared with the Heraeus binder resulted in well-adhered coatings displaying uniform surface coverage. Both the loose power and isopropanol slurry coatings yielded virtually no aluminization phenomenon. The ESL-based slurries resulted in coatings with better surface coverage than either loose powder or isopropanol slurries, but still a relatively patchy appearance; one in which chromia formation and subsequent volatilization could still foreseeably take place. The success of the HERAEUS binder is due to organic constituents in the binder that carburize and/or volatilize at temperatures near or above the melting point of aluminum, thereby delaying excessive oxidation in the powder particles and allowing greater melting and diffusion of aluminum into the substrate to take place.

(14) The average size of the aluminum powder can play a significant role in the structure of the final coating, including surface roughness and morphology and the extent of aluminum diffusion into the substrate (i.e. depth of the Al diffusion zone). These factors in turn lead to changes in the thermal expansion properties of the substrate, which affect the likelihood of oxide scale spallation and general coating durability under thermal cycling conditions. Fundamentally, powder particle size can determine the balance between competing oxidation and diffusion mechanisms that occur during the air heat treatment step. Surface oxidation consumes aluminum within each particle, forming an Al.sub.2O.sub.3 scale layer. In general, the larger the powder particle size, the lower the surface area available for particle oxidation to take place and therefore the greater the amount of aluminum available to diffuse into the substrate. Conversely at very small powder particle sizes, a larger fraction of the aluminum will be consumed via oxidation during heat treatment in air, leaving less molten aluminum to react with and/or diffuse into the substrate alloy. That is: rate.sub.oxid 4r.sup.2 and rate.sub.diff 1.333r.sup.3, assuming the powder is approximately spherical with an average radius of r. While uniform Al.sub.2O.sub.3 coatings can be formed from aluminum powders with a wide range of sizes, the relatively larger particle sizes afford a measurable Al diffusion zone. This diffusion zone represents a store of aluminum that can be tapped during stack operation so that a protective alumina scale inherently reforms if it becomes damaged, for example during thermal cycling.

(15) The Cr-containing Fe alloy of the above example was aluminized using 44 m, 3 m, and 0.1 m size aluminum powder. Referring to Table 2, the extent of aluminum diffusion decreases as the powder size decreases. Below the 0.1 m powder size there essentially was no diffusion zone, just a thin alumina scale layer. It appears that a minimum particle size exists at which aluminum diffusion does not take place because a sufficiently strong oxide shell forms on each particle, which encases the molten aluminum within. Above a threshold particle size, the volumetric ratio of oxide and metal is such that the molten aluminum will break through this shell due to high stresses associated with volumetric expansion during melting. Accordingly, in some embodiments, the average powder granule diameter is between 10 nm and 1000 m. In preferred embodiments, the diameter is between 0.1 m and 45 m.

(16) TABLE-US-00002 TABLE 2 Surface roughness and scale and diffusion zone thickness data Average Al Particle Powder Size Measurement 44 m 3 m 0.1 m Target coating thickness 160* 160 160 Average surface roughness 50 6 <1 Scale thickness 0.5 0.5 >100 Diffusion zone thickness 120 105 ~0 *All values are reported in microns. **Much of the scale is lost due to spallation.

(17) Referring to Table 2, powder particle size can also affect the morphology of the final coated surface. The larger particle size tends to lead to higher values of surface roughness, with little roughness being observed for the 0.1 m powder coatings. However, it should be noted that regardless of the degree of surface roughness, the top scale appears to be nearly pure aluminum oxide.

(18) Referring next to FIG. 3, a portion of a reactive coating process is shown wherein coating material is provided. This coating material can be an aluminum material, for example, and the coating material is converted to a shelled coating material. Referring to example implementations, the coating material can be reacted to form a shell about the coating material. The coating material can also include nickel, for example, and the shell of this coating material can be formed by oxidizing the coating material, for example, forming an oxidized aluminum outside an aluminum coating material. The process of forming this oxidized or shell component can include exposing the coating material to an oxidizing atmosphere, for example. Other reactions that can form the coating material can include reduction, corrosion, radiation, and/or vibration, for example. Temperatures can be utilized to form the shelled portion of the coating material, and example temperatures are those disclosed herein, as well as temperatures from about 660 C. to about 1200 C. in the case of pure aluminum particles.

(19) Referring next to FIG. 4, an example reactive coating process is depicted wherein a particle has formed. Coating material 401 is reacted to form a shell 402 about the coating material 401. This particle can have a size of less than 100 It can also have a size from about 10 nm to about 100 m in specific embodiments. In other embodiments, the particle size can range from 0.1 m to about 45 m. It is to be noted that the smaller the particle size, the more the particle takes on the composition of the shell component, and the larger the particle size, the less shell component is available and less internal component is available for coating.

(20) Referring again to FIG. 4, shelled coating materials are provided to substrate 406. Substrate 406 can include an iron chromium substrate and/or an iron nickel substrate, for example. In alternative embodiments, the substrate can comprise titanium and also titanium and aluminum and/or titanium and nickel components. In accordance with example implementations, the process can include depositing the coated material 401 on substrate 406 as is shown wherein internal portions 401 break away from shelled portions 402 and adhere to substrate 406. In accordance with example implementations, a layer 401 is deposited upon substrate 406 and shell portions 402 are above the layer portion 401. The shell portions 402 can be removed through brushing as they form a fragmented layer that can be removed from coated substrate 406. In accordance with example implementations, material 401 can penetrate into substrate 406, causing an infused layer 407 that includes the coating material into substrate 406.

(21) Energy-dispersive X-ray (EDX) data from these surfaces as well as from a region 10 m deep (i.e., the diffusion zone) into an aluminized sample and a bare coupon heated in air are provided in Table 3. The scale formed on the uncoated Cr-containing Fe alloy coupon consists of a mixture of iron and chromium oxide, whereas that of the aluminized Cr-containing Fe alloy coupon contains very little iron or chromium. It should be noted that the error associated with these numbers is significant (as high as 10 wt %). Unless conducted using a glancing angle technique, EDX tends to collect compositional information not only from the thin surface scale but also from a region 1-2 m deep into the sample. That is, it is likely that scale formed on the aluminized material contains virtually no Fe or Cr and that formed on the uncoated sample is closer in composition to pure Cr.sub.2O.sub.3.

(22) TABLE-US-00003 TABLE 3 Results from direct EDX analysis of the materials in FIGS. 3 (a) and (b). O Fe Cr Al Crofer Aluminized surface 15.02* 4.26 1.53 79.19 Region ~10 m deep 0.67 43.62 12.79 42.92 into the sample aluminized sample Uncoated surface 17.10 20.16 62.74 0.00 Alloy composition 1.26 75.86 22.52 0.35 *All values are reported in wt %.

(23) Results from a Cr poisoning test can provide another indication of scale composition and coating functionality. Shown in FIG. 2 is a set of button cell performance curves in which a potential source of Cr (either uncoated Cr-containing Fe alloy or Cr-containing Fe alloy aluminized on all exposed surfaces using the 44 m Al powder) was placed in the humid air environment to which the cathode was exposed. Two baseline trials were made using no potential Cr source. As seen by the slopes of the curves, cells operated with aluminized Cr-containing Fe alloy coupons exhibit the same degree of intrinsic power degradation (due to micro structural changes in the cathode, anode, and electrolyte materials) as cells operated with components that contain no source of Cr or Cr.sub.2O.sub.3. By comparison, the incorporation of uncoated Cr-containing Fe alloy coupons leads to exaggerated power degradation (a steeper time-dependent power slope) due to the volatility of chromia. In summary, the aluminization coating appears to block the formation of volatile chromium species.

(24) While a number of embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the disclosure in its broader aspects. The appended claims, therefore, are intended to cover all such changes and modifications as they fall within the true spirit and scope of the invention.