FIELD OF THE INVENTION The present invention generally relates to a method for protecting an article from sulfate corrosion and an article having an improved resistance to sulfate corrosion, and more specifically, to a method for protecting an article from sulfate corrosion resulting from exposure to a sulfate containing material at an elevated temperature and an article having an improved resistance to such sulfate corrosion. BACKGROUND OF THE INVENTION Hot corrosion is a typical problem for metallic components exposed to fuels or materials which contain corrosive contaminants in aviation and power industries. It is accelerated corrosion that occurs in the presence of environmental salts and sulfates containing elements such as sodium, magnesium, potassium, calcium, vanadium, and various halides. The corrosion may damage a protective oxide surface or oxide coating of a metallic component. At a relatively higher temperature, such as higher than about 850°C, the hot corrosion occurs above the melting point of most of the sulfates and simple salts. The sulfates and salts may form a liquid deposit on the component surface, and the liquid deposit may attack the component surface through a fluxing mechanism. As such, dissolution (fluxing) may occur to the protective oxide surface of the component. At a relatively lower temperature, for example of about 650-800°C, the sulfates may attack the component surface through a pitting mechanism. Sulfidation and oxidation reactions may initiate on discontinuities on the surface and propagate on a localized basis, generating pitting. The pits may occur at an unpredictably rapid rate and initiate cracks that propagate into the base alloy of the component, leading to catastrophic failure. Consequently, the load-carrying ability of the component is reduced, leading eventually to its catastrophic failure. Efforts have been made to study characteristics and mechanism of the hot corrosion and develop different approaches to mitigate the hot corrosion. But there is still no mature technology to address such hot corrosion. Especially, as 2 most of the study so far are focusing on the hot corrosion caused by molten salts with high conductivities, there is no approach to effectively mitigate the hot corrosion caused by pitting at a relatively lower temperature such as 650-800°C, which may be common under an operation condition in the aviation and power industries. Accordingly, it is desirable to develop new methods and materials for preventing such sulfate corrosion. BRIEF DESCRIPTION In one aspect, a method for protecting a surface of an article from sulfate corrosion resulting from exposure to a sulfate containing material at an elevated temperature includes coating the surface with a nickel based material to form an anti-corrosion coating. The nickel based material includes NiO, a spinel of formulation AB2O4, or a combination thereof, wherein A includes nickel, and B includes iron or a combination of manganese and a B site dopant. In another aspect, an article having an improved resistance to sulfate corrosion resulting from exposure to a sulfate containing material at an elevated temperature includes a metallic substrate and an anti-corrosion coating deposited on the metallic substrate. The anti-corrosion coating includes NiO, a spinel of formulation AB2O4, or a combination thereof, wherein A includes nickel, and B includes iron or a combination of manganese and a B site dopant. BRIEF DESCRIPTION OF THE DRAWINGS The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the subsequent detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a graph showing SO2 intensity signals for evaluating catalytic activity of different tested spinels. 3 FIG. 2A is a graph showing a scanning electron microscopy (SEM) image of a cross section of Sample 1, and FIG. 2B is a diagram showing compositions of areas labeled in FIG. 2A. FIG. 3A is a graph showing a SEM image of a cross section of Sample 2, and FIG. 3B is a diagram showing compositions of areas labeled in FIG. 3 A. FIG. 4A is a graph showing a SEM image of a cross section of Sample 3, and FIG. 4B is a diagram showing compositions of areas labeled in FIG. 4A. FIG. 5A is a graph showing a SEM image of a cross section of Sample 4, and FIG. 5B is a diagram showing compositions of areas labeled in FIG. 5 A. FIG. 6A is a graph showing a SEM image of a cross section of Sample 5, and FIG. 6B is a diagram showing compositions of areas labeled in FIG. 6A. FIG. 7A is a graph showing a SEM image of a cross section of Sample 6, and FIG. 7B is a diagram showing compositions of areas labeled in FIG. 7A. FIG. 8A is a graph showing a SEM image of a cross section of Sample 7, and FIG. 8B is a diagram showing compositions of areas labeled in FIG. 8A. FIG. 9A is a graph showing a SEM image of a cross section of Sample 8, and FIG. 9B is a diagram showing compositions of areas labeled in FIG. 9A. FIG. 10A is a graph showing a SEM image of a cross section of Sample 9, and FIG. 10B is a diagram showing compositions of areas labeled in FIG. 10A. FIG. 11A is a graph showing a SEM image of a cross section of Sample 10, and FIG. 1 IB is a diagram showing compositions of areas labeled in FIG. 11 A. FIG. 12A is a graph showing a SEM image of a cross section of Sample 11, and FIG. 12B is a diagram showing compositions of areas labeled in FIG. 12A. FIG. 13A is a graph showing a SEM image of a cross section of Sample 12, and FIG. 13B is a diagram showing compositions of areas labeled in FIG. 13 A. 4 FIG. 14A is a graph showing a SEM image of a cross section of Sample 13, and FIG. 14B is a diagram showing compositions of areas labeled in FIG. 14A. FIG. 15A is a graph showing a SEM image of a cross section of Sample 14, and FIG. 15B is a diagram showing compositions of areas labeled in FIG. 15 A. FIG. 16A is a graph showing a SEM image of a cross section of Sample 15, and FIG. 16B is a diagram showing compositions of areas labeled in FIG. 16A. FIG. 17A is a graph showing a SEM image of a cross section of Sample 16, and FIG. 17B is a diagram showing compositions of areas labeled in FIG. 17A. FIG. 18A is a graph showing a SEM image of a cross section of Sample 17, and FIG. 18B is a diagram showing compositions of areas labeled in FIG. 18 A. DETAILED DESCRIPTION One or more embodiments of the present disclosure will be described below. Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms "a" and "an" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about" and "substantially", are not to be limited to the precise value specified. Additionally, when using an expression of "about a first value - a second value," the about is intended to modify both values. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here, and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 5 Any numerical values recited herein include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. As an example, if it is stated that the amount of a component or a value of a process variable such as, for example, temperature, pressure, time and the like is, for example, from 1 to 90, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner. Embodiments of the present disclosure relate to a type of nickel based coating materials that can be used in power generation, aviation, and other applications involving hot and corrosive environment, to protect metallic articles such as gas turbine or engine components from sulfate corrosion and thereby significantly improve the service life of the articles. This type of nickel based coating materials are stable while exposed to a sulfate containing material (corrodent) at an elevated temperature, and can be used to provide a multifunctional coating for anti-corrosion applications. The unique anti-corrosion property of the nickel based coating material may be related to its high chemical stability and high catalytic activity for sulfate decomposition, which may change interfacial interaction between the corrodent and the coating. In some embodiments, the nickel based coating material and a coating of this material (also referred to as "nickel based coating", "nickel based anti-corrosion coating", or "anti-corrosion coating" hereinafter) can not only make sulfate decompose, for example, at about 750°C, earlier than the sulfate decomposes itself, but also prevent SCVsulfate formation by converting sulfur trioxide (SO3) to sulfur dioxide (SO2). The sulfate may decompose to produce the corresponding metal oxide, SO2 and oxygen: 2MS04^2MO+2S02+02, 6 wherein M represents a metal. SCVsulfate formation may be prevented by converting SO3 to SO2 and oxygen: 2S03^2S02+02 The nickel based coating may have a composition substantially the same with that of the nickel based coating material, and therefore they may be described together hereinafter. The nickel based coating material or the coating may include nickel 9+ ^+ 9 oxide (NiO), a nickel based spinel of general formulation AB2O4 (A B 20 "4), or a combination thereof. Although the charges of A and B in a prototypical spinel structure are +2 and +3, respectively, combinations incorporating univalent, divalent, trivalent, or tetravalent cations, such as potassium, magnesium, aluminum, chromium, and silicon, are also possible. It is found that the spinel AB2O4 has the catalytic activity for sulfate decomposition when A includes nickel (Ni) and B includes one or more transition metals such as chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co). Besides Ni, A may further include an A site dopant. The A site dopant may be any suitable element(s) that can be doped to the A sites of the spinel. Similarly, B may further include a B site dopant. The B site dopant may be any suitable element(s) that can be doped to the B sites of the spinel. In some embodiments, the A site dopant or the B site dopant may include aluminum (Al), gallium (Ga), indium (In), silicon (Si), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), sodium (Na), potassium (K), magnesium (Mg), a rare earth element, or a combination thereof. The rare earth element may include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), ebium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), yttrium (Y), scandium (Sc), or a combination thereof. The dopant(s) may increase the stability of the spinel AB2O4. For example, NiFe204 is stable whereas NiC^C^, MMJ^CM and MC02O4 are not stable while 7 exposed to the sulfate containing corrodent, but a B-site dopant can increase the stability of NiMn204. The B-site doped NiMn204 has both catalytic activity for sulfate decomposition and high chemical stability. In some embodiments, the nickel based material or the coating includes NiO, a spinel of formulation AB2O4, or a combination thereof, wherein A includes Ni, and B includes Fe, or a combination of Mn and a B site dopant as described above. In some particular embodiments, the B site dopant includes Cr, Co, Al, or a combination thereof. Some examples of suitable spinels include NiFe204, Ni(Fe2-xCox)04, Ni(Fe2-xAlx)04 and Ni(Mn2-xAlx)04, wherein 04 and NiO show anti-sulfur corrosion capability and stability similar to these of NiFe2C>4 under the sulfate corrosion condition. NiMnA104 also shows anti-sulfur corrosion capability but there is cation diffusion to corrodent. The combination of NiMn204 and NiO are unstable under the sulfate corrosion condition. Similar to Example 1, SEM images of the cross-sections of the Samples 6-10 are shown in FIGS. 7 A, 8A, ..., and 11 A, respectively. Mass percent compositions of labeled areas in each of the SEM images of FIGS. 7A, 8A, ..., and 11A are illustrated in a corresponding diagram in FIGS. 7B, 8B, ..., and 1 IB, respectively. Each of FIGS. 7A, 8 A and 9A shows a clean pellet cross section image. FIGS. 7B and 8B indicate that in Samples 6 and 7 there is neither S migration to the pellet nor cation leaching from the pellet into the corrodent, but only a very small amount of Si diffused from the corrodent to the pellet, which may not affect the anti-corrosion performance of the tested material very much. FIG. 9B indicates that there is no inter diffusion between the corrodent and pellet across the pellet surface of Sample 8. It can thus be seen that the pellet made from doped NiFe204, or a combination of NiFe204 and NiO is stable through the test. No sulfur diffusion is observed in the pellet of sample 9 in FIG. 10A, which indicates that NiMnA104 has anti-sulfur capability. However, FIG. 10B indicates that Mn and Ni diffused from the pellet side are detected in spectrum 93 which is located in the corrodent side. As for Sample 10 made from a combination of NiMn204 and NiO, FIG. 11B indicates that there is S migration to the pellet, and S diffused from the corrodent side is detected in spectrum 104 which is located in the pellet side and adjacent to the pellet surface. Example 3: In this example, pellets respectively made from NiO, NiFe204, NiFeCo04, NiFeA104, a combination of NiFe204 and NiO, NiMn204 and NiMnA104 (Samples 11-17) were subjected to a simulated corrosion test as described herein above at a temperature of about 704°C for about 500 hours (much longer than the testing duration in Examples 1 and 2). As for each sample, 15 a depth of S penetration into the pellet and cation leaching observed in the sulfate corrodent are illustrated in the following Table 3. Table 3 Samples Materials Corrosion Test Depth of S penetration Cation leaching 11 NiO 704°C, 500 hours 0 - 12 NiFe204 704°C, 500 hours 0 - 13 NiFeCo04 704°C, 500 hours 0 Co 14 NiFeA104 704°C, 500 hours 0 - 15 80wt%NiFe2O4+20%NiO 704°C, 500 hours 0 - 16 NiMn204 704°C, 500 hours R Mn, Ni 17 NiMnA104 704°C, 500 hours R Mn, Ni Notes: R means a reaction layer is formed in the pellet. It can be seen from Table 3 that, in the corrosion test of a longer duration, NiO, NiFe2C>4, a combination of NiO and NiFe204, and NiFeA104 remain stable, but NiFeCo04 shows phase segregation and leaching of Co into the sulfate corrodent. MMJI2O4 and its Al-doped derivative MM11AIO4 show severe S penetration and element leaching. Similar to Example 1, SEM images of the cross-sections of the Samples 11-17 are shown in FIGS. 12A, 13A, ..., and 18A, respectively. Mass percent compositions of labeled areas in each of the SEM images of FIGS. 12A, 13A, ..., and 18A are illustrated in a corresponding diagram in FIGS. 12B, 13B, ..., and 18B, respectively. Each of FIGS. 12A, 13A, 15A and 16A shows a clean pellet cross section image. The measured results in FIGS. 12B, 13B and 16B also prove that there is no diffusion across the pellet surfaces of Samples 11, 12 and 15. FIG. 15B indicates that in Sample 14 there is neither S migration to the pellet nor cation leaching from the pellet into the corrodent, but only a very small amount of Mg diffused 16 from the corrodent to the pellet, which may not affect the anti-corrosion performance very much. It can thus be seen that the pellet made from NiO, NiFe2C>4, a combination of NiFe204 and NiO, or NiFeA104 is stable through the test of a long duration. As shown in FIGS. 14A and 14B, NiFeCo04 can also block the penetration of sulfur in the pellet, suggesting the good anti-sulfur corrosion capability. However, FIG. 14B indicates that Co diffused from the pellet side is detected in spectrum 134 which is located in the corrodent side. This may affect the life time of the material. As for Sample 16 made from NiMn204, a reaction zone is observed near the interface between the tested material and the corrodent in FIG. 17A. FIG. 17B indicates that Na, Mg, Al, Si, S, K and Ca diffused from the corrodent side are detected in spectrum 164 which is located in the pellet side. As for Sample 17 made from NiMnA104, Mn and Ni diffused from the pellet side is detected in spectrums 173-175 which are located in the corrodent side, as indicated in FIG. 18B. However, the reaction layer in FIG. 18A is much thinner than the reaction layer in FIG.17A, confirming that doped NiMn204 (NiMnAlC^) has improved anti-sulfur corrosion capability and is a potential anti-sulfur corrosion material. Although in the above examples, only AB2O4 spinels with a B site dopant were tested, it should be noted that AB2O4 spinels with an A site dopant are also applicable. The A site doping strategy may be the same as the B site doping strategy based on the common knowledge in this art. This written description uses examples to describe the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 17 WE CLAFM: 1. A method for protecting a surface of an article from sulfate corrosion resulting from exposure to a sulfate containing material at an elevated temperature, comprising coating the surface with a nickel based material to form an anti-corrosion coating, the nickel based material comprising NiO, a spinel of formulation AB2O4, or a combination thereof, wherein A comprises nickel, and B comprises iron or a combination of manganese and a B site dopant. 2. The method as claimed in claim 1, wherein A further comprises an A site dopant. 3. The method as claimed in claim 2, wherein the A site dopant comprises aluminum, gallium, indium, silicon, titanium, vanadium, chromium, iron, cobalt, copper, zinc, sodium, potassium, magnesium, a rare earth element, or a combination thereof. 4. The method as claimed in claim 1, wherein B comprises iron. 5. The method as claimed in claim 1, wherein B comprises a combination of manganese and the B site dopant. 6. The method as claimed in claim 1, wherein the B site dopant comprises aluminum, gallium, indium, silicon, titanium, vanadium, chromium, iron, cobalt, copper, zinc, sodium, potassium, magnesium, a rare earth element, or a combination thereof. 7. The method as claimed in claim 1, wherein the nickel based material comprises NiO, NiFe204, Ni(Fe2-xC0x)04, Ni(Fe2-xAlx)04, Ni(Mn2-xAlx)04, or a combination thereof, wherein 0*tt AMIT SINGH Agent for the Applicant [IN/PA-1554] 20