Abstract:
A coating system and process for protecting component surfaces exposed to sulfur-containing environments at elevated temperatures. The coating system includes a sulfidation-resistant overlay coating that is predominantly niobium or molybdenum.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to protective coatings for components exposed to high temperatures. More particularly, this invention is directed to a coating system that provides sulfidation protection at elevated temperatures within a sulfur-containing environment, such as the hostile thermal environment of a gasification system used in gas turbine power generation plants. 
     Power generation plants exist that burn coal to produce coal gas or synthesis gas (SNG), which is then combusted in a gas turbine. The output of the gas turbine may be used directly to power an electric generator. In combined cycle gas turbine plants, the hot exhaust gases of the turbine are used to generate steam for powering a steam turbine. The production of coal gas occurs in what is commonly termed a gasifier. 
     The components of a coal gasifier, including injectors and nozzles, are subjected to a thermally and chemically hostile environment. For this reason, coal gasifier components may be coated with protective coatings. Common examples include oxidation-resistant coatings and thermal barrier coatings (TBCs). A particular example is a thermal barrier coating system comprising a ceramic coating bonded to the component surface with an oxidation-resistant metallic bond coat. Various ceramic materials have been used and proposed as TBCs, the most widely used being zirconia (ZrO 2 ) partially or fully stabilized by yttria (Y 2 O 3 ), magnesia (MgO), or ceria (CeO 2 ). TBCs can be deposited by a variety of processes, including plasma spraying, flame spraying, and physical vapor deposition (PVD) techniques. The bond coat serves to promote adhesion of the TBC to the component. Common examples of bond coat materials include aluminum-rich compositions, for example, diffusion coatings such as diffusion aluminides and diffusion platinum aluminides, and overlay coatings of an MCrAlX alloy where M is typically iron, cobalt and/or nickel, and X is yttrium, rare earth elements, and/or reactive elements. These bond coat materials develop an aluminum oxide (alumina) scale as a result of oxidation, such as during deposition of the TBC on the bond coat as well as at high temperatures in an oxidizing environment. The alumina scale chemically bonds the TBC to the bond coat and provides environmental protection to the bond coat and underlying substrate. 
     While effective for providing oxidation resistance, traditional bond coat materials are not effective for protecting components from sulfidation in the high-temperature sulfur-rich environment of a coal gasifier. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a coating system suitable for components exposed to high temperatures, and particularly a sulfidation-resistant overlay coating capable of protecting a component surface when exposed to a sulfur-containing environment at elevated temperatures. Notable nonlimiting examples include the components of gasification systems used in gas turbine power generation plants. 
     According to a first aspect of the invention, the coating system lies on a surface region of a component subjected to sulfur and/or sulfur compounds at an elevated temperature, and the coating system comprises a sulfidation-resistant overlay coating that is predominantly niobium or molybdenum. 
     According to a second aspect of the invention, a process is provided by which a coating system is deposited on a surface region of a component adapted to be subjected to sulfur and/or sulfur compounds at an elevated temperature. The deposition step comprises depositing on the surface region a sulfidation-resistant overlay coating that is predominantly niobium or molybdenum. 
     The overlay coating of the coating system preferably contains a sufficient amount of niobium or molybdenum to grow an adherent niobium sulfide or molybdenum sulfide layer capable of inhibiting further sulfidation of the overlay coating and the underlying surface region of the component. In this manner, the overlay coating is useful for protecting gasifier components, as well as other components exposed to a sulfur-containing environment at elevated temperatures. The sulfide layer is also preferably slow growing and capable of promoting the spallation resistance and life of an optional coating deposited on the overlay coating. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically represents a cross-sectional view of a gasification component and a thermal barrier coating system on the component in accordance with an embodiment of the invention. 
         FIG. 2  is a scanned image of a cross-section of a substrate protected by a molybdenum coating following an extended high-temperature exposure to a syngas. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention is applicable to a variety of components, and especially those exposed to sulfur and sulfur compounds at high temperatures, the invention will be discussed in particular reference to components of coal gasification systems used in gas turbine power generation plants. A surface region  12  of one such component  10  is schematically represented in  FIG. 1 . The surface region  12  defines a portion of the component  10  contacted by gases containing sulfur or its compounds at elevated temperatures and which is therefore subjected to sulfidation. Depending on the particular application, the surface region  12  may also be subject to attack by oxidation and corrosion. The component  10  and its surface region  12  can be formed of a variety of materials, nonlimiting examples of which include nickel and cobalt-based superalloys. The invention is believed to be especially advantageous for use on nickel-based superalloys, which are particularly prone to sulfidation attack. An example of such an alloy is INCONELL™ alloy X-750, having a nominal composition of, by weight, about 15.5% Cr, 7.0% Fe, 2.5% Ti, 1.0% Nb, 0.7% Al, the balance Ni. 
     The surface region  12  represented in  FIG. 1  is protected by a coating system  14  in accordance with an embodiment of the present invention. As shown, the coating system  14  includes a bond coat  16  overlying and directly contacting the surface region  12 . The bond coat  16  is shown as adhering a thermal-insulating ceramic layer  18 , or TBC, to the surface region  12 . The ceramic layer  18  may have a dense vertically cracked (DVC) microstructure produced by plasma spraying or high velocity oxy-fuel (HVOF) spraying a liquid precursor or by an air plasma spraying process, for example, as disclosed in U.S. Pat. Nos. 5,830,586, 5,897,921, 5,989,343 and 6,047,539. Alternatively, the ceramic layer  18  may be produced to have a noncolumnar structure, as is commonly produced by conventional plasma spray techniques. The ceramic layer  18  could also be formed to have a strain-tolerant structure with columnar grains produced by depositing the ceramic layer  18  using a physical vapor deposition technique known in the art, for example, electron beam-physical vapor deposition (EBPVD). A particularly suitable material for the ceramic layer  18  is believed to be an yttria-stabilized zirconia (YSZ), a preferred composition being about 6 to about 8 weight percent yttria (6-8% YSZ), optionally with additional oxides to reduce thermal conductivity. Other ceramic materials could be used for the ceramic layer  18 , such as yttria, nonstabilized zirconia, or zirconia stabilized by magnesia, ceria, scandia, and/or other oxides. The ceramic layer  18  can be deposited to a thickness that is sufficient to provide a desirable level of thermal protection for the underlying surface region  12  and component  10 , typically on the order of about 75 to about 300 micrometers, though lesser and greater thicknesses are also possible. 
     While shown and described in reference to a coating system  14  that includes a ceramic layer  18 , the present invention is also applicable to coating systems that do not include a ceramic coating or any other overlying coating layer. In the absence of an overlying coating layer, the bond coat  16  serves as an environmental coating that defines the outermost layer of the coating system  14  and the outermost surface of the component  10 . 
     The bond coat  16  is represented in  FIG. 1  as an overlay coating, as opposed to a diffusion coating. Various deposition processes can be used to deposit the overlay bond coat  16 , including cold spraying (kinetic metallization), HVAF (high velocity air-fuel), HVOF (high velocity oxy-fuel), plasma spraying (air, vacuum, etc.), cathodic arc deposition (also called ion plasma deposition (IPD)), EB-PVD, and cored-wire arc spray. As an overlay coating, the bond coat  16  forms a limited diffusion zone in the surface of the surface region  12 , though over time at elevated temperatures some level of interdiffusion will occur between the bond coat  16  and the surface region  12  as a result of diffusional gradients and changes in elemental solubility in the local region of the surface region  12 . 
     According to a particular aspect of the invention, the bond coat  16  has a niobium-based or molybdenum-based metallic composition. The metallic composition may be entirely niobium or molybdenum with typical impurities, or may be alloyed, blended, or clad with, for example, chromium, aluminum, cobalt, yttrium, silicon, boron, hafnium, iron, etc. Niobium or molybdenum is the preferred predominant constituent of the bond coat  16  (the composition contains more niobium or molybdenum than any other individual constituent) so as to be capable of forming a passivating sulfide layer  20  (niobium disulfide (NbS 2 ) or molybdenum disulfide (MOS 2 )) on the surface of the bond coat  16  that inhibits further sulfidation of the bond coat  16  and protects the underlying surface region  12  of the component  10  from sulfidation. Advantageously, the sulfide layer  20  is also adherent and slow growing, and therefore is capable of promoting the spallation resistance and life of the ceramic layer  18  or another overlying coating layer that might be present on the bond coat  16 . 
     For niobium-based metallic compositions for the bond coat  16 , a minimum niobium content is believed to be about 25 weight percent to ensure sufficient sulfide formation, while the maximum niobium content can be 100 weight percent. A suitable niobium content is in a range of about 50 to about 100 weight percent of the bond coat composition, and a particularly preferred niobium content is believed to be about 50 to about 100 weight percent. 
     For molybdenum-based metallic compositions for the bond coat  16 , a minimum molybdenum content is believed to be about 20 weight percent to ensure sufficient sulfide formation, while the maximum molybdenum content can be 100 weight percent. A suitable molybdenum content is in a range of about 35 to about 100 weight percent of the bond coat composition, and a particularly preferred molybdenum content is believed to be about 40 to about 100 weight percent. 
     Suitable constituents that can be alloyed, blended, or clad with niobium or molybdenum to form the niobium- or molybdenum-based bond coat  16  can vary depending on the particular application, including the composition of the surface region  12 , the composition of the ceramic layer  18  (if present), and the environmental conditions to which the bond coat  16  is exposed. With the exception of nickel, suitable constituents are believed to include typical MCrAlX constituents, for example, cobalt, iron, chromium, aluminum and yttrium. Other potential constituents that may be used with or instead of MCrAlX constituents include silicon, boron, hafnium, and chromium carbides (CrC and/or Cr 3 C 2 ). The presence of oxide formers such as chromium, aluminum, yttrium, silicon, hafnium, etc., can be advantageous if the bond coat  16  will be exposed to an oxidizing environment. Alloying with aluminum, silicon, boron, etc., can be desirable to promote the ductility of the bond coat  16 . The bond coat  16  is preferably free of nickel in view of the susceptibility of nickel to sulfidation, though the bond coat  16  may contain nickel as an impurity, preferably accounting for less than one weight percent of the bond coat composition. 
     It is foreseeable that the bond coat  16  may be used in combination with a diffusion barrier between the surface region  12  and bond coat  16  to inhibit interdiffusion. Alternatively or in addition, interdiffusion between the bond coat  16  and surface region  12  can be inhibited by forming the bond coat  16  to be a blend of niobium or molybdenum with one of the above-noted MCrAlX bond coat compositions, for example, CoCrAlY or FeCrAlY. 
     The bond coat  16  is believed to be particularly effective when deposited to a thickness of at least about 100 micrometers. A maximum thickness is believed to be about 500 micrometers, though it is foreseeable that a bond coat  16  of greater could be used. A particularly suitable thickness range is believed to be on the order of about 150 to about 400 micrometers. Following deposition, the bond coat  16  may undergo a heat treatment, for example, at a temperature of about 1800° F. to about 2000° F. (about 980° C. to about 1090° C.) and for a duration of about two to about four hours, to relieve any stresses induced by the coating process. 
     In an investigation leading to this invention, molybdenum coatings were deposited as environmental coatings (in other words, without a thermal barrier coating) on coupons formed of a cobalt-based alloy containing, by weight, about 30% chromium, about 20% iron, and the balance essentially cobalt. The molybdenum coatings were deposited by ion plasma deposition (IPD) to have thicknesses of about 0.002 to about 0.005 inch (about 50 to about 125 micrometers, and subjected to a syngas at a temperature of about 1400° F. (about 760° C.) at a pressure of about 35 psi (about 2.4 bar). The composition of the syngas was, by volume, about 2% hydrogen sulfide (H 2 S), about 10% carbon dioxide (CO 2 ), about 40% carbon monoxide (CO), and the balance hydrogen (H 2 ).  FIG. 2  shows the appearance of one of the molybdenum coatings following an exposure duration of about 1800 hours, and evidences the growth of an adherent scale that was found to be almost entirely sulfur, oxygen, carbon, and molybdenum. The molybdenum and sulfur contents were indicative of molybdenum sulfide present in the scale. A thin interaction zone visible beneath the molybdenum coating was found to contain chromium, iron and cobalt from the substrate, molybdenum from the coating, a very low amount of oxygen, and no detectable amounts of sulfur, evidencing that the molybdenum coating successfully protected the substrate from sulfidation as well as provided a significant barrier to oxidation. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.