Steel substrates comprising a rare-earth doped silica-alumina nanocoating, coating compositions, and methods thereof

Abstract

The present disclosure provides a steel substrate comprising a rare-earth doped silica-alumina coating comprising: a) about 0.5-6.5 wt% of Al; b) about 0.5-6.5 wt% of Si; and c) about 0.1- 11 wt% of a rare earth element. The rare-earth doped silica-alumina coating is a nanocoating and provides a very high corrosion resistance to steel substrates and at the same time renders the steel substrates formable, weldable, paintable, and fuel resistant. The present disclosure also provides a coating composition for depositing the rare-earth doped silica-alumina coating on steel substrates, wherein the coating composition comprises a) an aqueous solution of an acid; b) a silane; c) aluminium iso-propoxide; d) a compound of a rare earth element; and e) orthophosphoric acid or a derivative thereof. The present disclosure also provides a method for preparing the coating composition and a method for preparing the steel substrate comprising the rare-earth doped silica-alumina coating.

Description

[0001] “STEEL SUBSTRATES COMPRISING A RARE-EARTH DOPED SILICA- ALUMINA NANOCOATING, COATING COMPOSITIONS, AND METHODS THEREOF” TECHNICAL FIELD The present disclosure relates to a steel substrate comprising a low-cost, water-based nanocoating comprising silica and alumina doped with a compound of a rare earth element. In particular, the disclosure provides coated steel substrates, methods of preparing them, and ϱ^ coating compositions for providing the coating on the steel substrate. BACKGROUND OF THE DISCLOSURE There is a significant impact of corrosion on environment and the economy of a country. The economic impact of corrosion on metal structures is an important issue. Steel is a versatile ϭϬ^ material used in many infrastructures due to its durability and affordability. However, steel is always prone to corrosion because of electrochemical reactions (a spontaneous process) in its service environment. There is a huge economic loss due to corrosion of metal if proper care has not been taken. There are different methods to protect steel structures from corrosion which are listed as follows: (i) passive barrier protection, (ii) active protection, (iii) sacrificial ϭϱ^ protection, (iv) electrophoretic deposition, (v) metallic coating, and (vi) organic coatings. Different specialized coatings play a major role to mitigate corrosion. However, the selection of the coating material depends on the substrate and its end application. In galvanized steel (GI), zinc is in direct contact with steel (iron) substrate which offers sacrificial protection where preferential oxidation happens on zinc metal. Zinc corrodes due to its lower electrochemical ϮϬ^ potential compared to steel. The corrosion rate for zinc is usually slower in normal environment. However, this corrosion can be accelerated in the presence of ions such as chloride ions in coastal regions. So, secondary coatings (organic / inorganic / hybrid) are required over the zinc surface to protect zinc from corrosion and increase the life of whole structure. Ϯϱ^ Galvanized steel is mainly used in the field of construction. However, the galvanized steel surface undergoes oxidation and darkening easily. The forming behavior of galvanized steel is also not good. To avoid the above issues, the galvanized steel surface is usually passivated by the hazardous hexavalent chromate or thin organic passivation. The hexavalent chromates are not good for the environment. ϯϬ^

[0002] 1 ^ Motoaki Hara et al. investigated the chemical conversion treatment of galvanized steel using colloidal silica coating as an alternative treatment to chromate conversion. In this work, white rust appeared more quickly on the colloidal silicate film than on chromate chemical conversion (CCC) films. However, the colloidal silicate coating provides more corrosion resistance in ϱ^ terms of red rust compared with CCC films. R. V. Lakshmi et al. have studied the effect of ceria nanoparticles / cerium nitrate doped silica-alumina hybrid sol-gel coating on corrosion resistance. They observed that ceria nanoparticles doped coating shows better performance compared to the cerium nitrate doped coating which is due to the more compact coating in case of cerium nanoparticles doped coating. ϭϬ^ Rust preventive oils (RP oils) are also used over steel substrates (e.g., cold rolled closed annealed (CRCA), galvanized (GI) and galvannealed (GA) substrates) to protect steel from corrosion during the transportation to different end customers and storage at their end. These RP oils need to be removed later by degreasing followed by seven tanks phosphating processes.ϭϱ^ The phosphating pretreatment helps for better post-paint adhesion. After the phosphate pre- treatment, these steel substrates undergo electrophoretic deposition (ED) of paint followed by primer and topcoat respectively. These seven tanks’ processes consume a lot of time and increase the process cost. However, the cost and process time can be reduced by the use of a single coating system (instead of phosphating, ED, primer and topcoat), which can provide all ϮϬ^ the required properties (corrosion resistance, formability, weldability, and post-paintability). Thus, there is a need in the art to provide a coating which exhibits a high corrosion resistance, formability, weldability, and post-paintability, but at the same time saves cost and time. The present disclosure attempts to address said need. Ϯϱ^ STATEMENT OF THE DISCLOSURE The present disclosure relates to a steel substrate comprising a rare-earth doped silica-alumina coating, wherein the coating comprises: a) about 0.5-6.5 wt% of Al; b) about 0.5-6.5 wt% of Si; and c) about 0.1-11 wt% of a rare earth element. ϯϬ^ The present disclosure also relates to a coating composition for preparing the coated steel substrate, wherein the coating composition comprises: a) an aqueous solution of an acid; b) a silane; c) aluminium iso-propoxide; d) a compound of a rare earth element; and e)

[0003] 2 ^ orthophosphoric acid or a derivative thereof. In some embodiments, the coating composition comprises two silanes, e.g., TEOS and APTES, an organic green inhibitor, and a rust preventer. The present disclosure also provides a method for preparing the coating composition, ϱ^ comprising: a. preparing a first solution, comprising: • adding a portion of the acid to demineralized water to obtain a first aqueous solution of the acid; • adding the silane to the first aqueous solution of the acid; and ϭϬ^ • adding the compound of the rare earth element to the first aqueous solution of the acid after addition of the silane; b. preparing a second solution, comprising: • adding the remaining portion of the acid to demineralized water to obtain a second aqueous solution of the acid; ϭϱ^ • adding aluminium iso-propoxide to the second aqueous solution of the acid; and c. mixing the first solution and the second solution followed by adding orthophosphoric acid to obtain the coating composition. ϮϬ^ In the embodiments where the coating composition comprises two silanes, an organic green inhibitor, and a rust preventer; the second silane is added to the mixture of the first solution and the second solution prior to adding orthophosphoric acid and the organic green inhibitor and the rust preventer are added to the second solution prior to the addition of the remaining portion of glycolic acid. Ϯϱ^ The present disclosure also provides a method for preparing a steel substrate comprising the present coating, wherein the method comprises: a) applying the coating composition to the substrate to obtain a coated substrate; and b) drying the coated substrate at about 40-200 °C for about 2 - 20 minutes. ϯϬ^ BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES Figure 1 shows the results of a Salt spray test (SST) of bare / un-coated GA and GI substrates. Figure 2 shows the results of the SST of coated La-doped GA substrate.

[0004] 3 ^ Figure 3 shows the results of the SST of coated La-doped GI substrate. Figure 4 shows the results of the SST of coated Ce-doped GI substrate. ϱ^ Figure 5 shows variation of impedance with frequency (Bode plot). Figure 6 shows the results of adhesion (crosshatch) test of La-doped coated GA and GI samples. ϭϬ^ Figure 7 shows the formability tests on La-doped coated GA and GI samples. Figure 8 shows the results of Spot-welded GA / GI samples and their corresponding weld-lobe plots. ϭϱ^ Figure 9 shows the (i) Impact and (ii) cut bend test of painted samples. Figure 10 shows the Scanning electron microscopy and cross-section images of coated (a, b) GA, and (c, d) GI samples. ϮϬ^ Figure 11 shows SEM-EDX images (coated GA and GI samples) with their corresponding elemental compositions. Figure 12 shows the self-healing behavior of coated GA (a) on the first day, and (b) on the Ϯϱ^ sixth day (120 h exposure in ambient environment). Figure 13 shows the self-healing behavior of coated GI (a) on first day, and (b) sixth day (120 h exposure in ambient environment). ϯϬ^ Figure 14 shows XRD patterns of (a) coating in powder form (after drying), (b) bare and coated GI, and (c) bare and coated GA. Figure 15 shows Dynamic light scattering (DLS) spectrum showing the particle size distribution of the Nanocoat sol.

[0005] 4 ^ Figure 16 shows an exemplary schematic of the process of preparing the coating composition of the present disclosure. ϱ^ Figure 17 shows the fuel / petrol resistance of coated GA sheets. Figure 18 shows the FTIR spectra of coated GI and GA samples. DETAILED DESCRIPTION OF THE DISCLOSURE ϭϬ^ With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may ϭϱ^ be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having”, or “including but not limited to” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group ϮϬ^ of elements, integers or steps. Reference throughout this specification to “some embodiments”, “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Ϯϱ^ Thus, the appearances of the phrases “in some embodiments”, “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, ϯϬ^ described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

[0006] 5 ^ The term “about” as used herein encompasses variations of + / -5% and more preferably + / - 2.5%, as such variations are appropriate for practicing the present invention. The term “coated steel substrate” as used herein refers to a steel substrate having a rare-earth ϱ^ doped silica-alumina coating. In some embodiments, the rare-earth doped silica-alumina coating is La-doped silica-alumina coating. The present disclosure provides a steel substrate comprising a rare-earth doped silica-alumina coating, wherein the coating comprises: ϭϬ^ a) about 0.5-6.5 wt% of Al; b) about 0.5-6.5 wt% of Si; and c) about 0.1-11 wt% of a rare earth element. In some embodiments, the rare earth element present in the coating is cerium (Ce), lanthanum ϭϱ^ (La), neodymium (Nd), praseodymium (Pr) or a combination thereof. In some embodiments, the rare earth element present in the coating is cerium (Ce). In some embodiments, the rare earth element present in the coating is Lanthanum (La). In some embodiments, the rare-earth doped coating on the steel substrate comprises: ϮϬ^ a) about 0.5-6.5 wt% of Al, including values and ranges thereof, such as, for example, about 0.5-6 wt%, about 0.5-5 wt%, about 0.5-4.5 wt%, about 0.5-4 wt%, about 0.5-3.5 wt%, about 0.5-3 wt%, about 0.5-2.5 wt%, about 0.5-2 wt%, about 1-6.5 wt%, about 1-6 wt%, about 1-5.5 wt%, about 1-5 wt%, about 1-4.5 wt%, about 1-3.5 wt%, about 1.5-6.5 wt%, about 1.5-5 wt%, about 1.5-4 wt%, about 2-6.5 wt%, about 2-6 wt%, about 2-5.5 wt%, about 2-4.5 wt%, Ϯϱ^ about 3-6.5 wt%, about 3-6 wt%, about 3-5 wt%, about 4-6.5 wt%, about 5-6.5 wt%, about 0.5 wt%, about 0.75 wt%, about 1 wt%, about 2 wt%, about 2.5 wt%, about 3 wt%, about 3.5 wt%, 4 wt%, about 4.5 wt%, about 5 wt%, about 5.5 wt%, about 5.75 wt%, about 6 wt%, about 6.25 wt%, or about 6.5 wt% of Al; b) about 0.5-6.5 wt% of Si, including values and ranges thereof, such as, for example, ϯϬ^ about 0.5-6 wt%, about 0.5-5 wt%, about 0.5-4.5 wt%, about 0.5-4 wt%, about 0.5-3.5 wt%, about 0.5-3 wt%, about 0.5-2.5 wt%, about 0.5-2 wt%, about 1-6.5 wt%, about 1-6 wt%, about 1-5.5 wt%, about 1-5 wt%, about 1-4.5 wt%, about 1-3.5 wt%, about 1.5-6.5 wt%, about 1.5-5 wt%, about 1.5-4 wt%, about 2-6.5 wt%, about 2-6 wt%, about 2-5.5 wt%, about 2-4.5 wt%, about 3-6.5 wt%, about 3-6 wt%, about 3-5 wt%, about 4-6.5 wt%, about 5-6.5 wt%, about 0.5

[0007] 6 ^ wt%, about 0.75 wt%, about 1 wt%, about 2 wt%, about 2.5 wt%, about 3 wt%, about 3.5 wt%, about 4 wt%, about 4.5 wt%, about 5 wt%, about 5.5 wt%, about 5.75 wt%, about 6 wt%, about 6.25 wt%, or about 6.5 wt% of Si; and c) about 0.1-11 wt% of a rare earth element, including values and ranges thereof, such ϱ^ as, for example, about 0.1-11 wt%, about 0.1-10.5 wt%, about 0.1-10 wt%, about 0.1-9.5 wt%, about 0.1-9 wt%, about 0.1-8.5 wt%, about 0.1-7.5 wt%, about 0.1-6.5 wt%, about 0.1-5.5 wt%, about 0.1-4 wt%, about 1-11 wt%, about 1-10.5 wt%, about 1-9.5 wt%, about 1-8.5 wt%, about 1-7.5 wt%, about 1-6 wt%, about 2-11 wt%, about 2-10.5 wt%, about 2-10 wt%, about 2-9.5 wt%, about wt%, about wt%, about 2-8 wt%, about 2-7.5 wt%, about 2-7 wt%, ϭϬ^ about 3.5-10.5 wt%, about 3.5-10 wt%, about 3.5-9.5 wt%, about 3.5-8.5 wt%, about 3.5-8 wt%, about 4-10.5 wt%, about 4-10 wt%, about 4-9.5 wt%, about 4-9 wt%, about 4-8.5 wt%, about 4-8 wt%, about 4-7 wt%, about 5-11 wt%, about 5-10.5 wt%, about 5-10 wt%, about 5- 9.5 wt%, about 5-9 wt%, about 5-8.5 wt%, about 5-8 wt%, about 6-10.5 wt%, about 6-10 wt%, about 6-9.5 wt%, about 6-9 wt%, about 6-8.5 wt%, about 7-10.5 wt%, about 7-10 wt%, about ϭϱ^ 7-9 wt%, about 7.5-10.5 wt%, about 7.5-10 wt%, about 7.5-9.5 wt%, about 8-10.5 wt%, about 8-10 wt%, about 0.5 wt%, about 1 wt%, about 2 wt%, about 3 wt%, about 3.5 wt%, about 4 wt%, about 4.5 wt%, about 5 wt%, about 5.5 wt%, about6 wt%, about 6.5 wt%, about 7 wt%, about 7.5 wt%, about 8 wt%, about 8.5 wt%, about 9 wt%, about 9.5 wt%, about 10 wt%, about 10.5 wt%, or about 11 wt% of a rare earth element. ϮϬ^ In some embodiments, the rare-earth doped coating on the steel substrate comprises: a) about 0.5-6.5 wt% of Al, including values and ranges thereof, as described above; b) about 0.5-6.5 wt% of Si, including values and ranges thereof, as described above; and c) about 0.1-11 wt% of Ce or La, including values and ranges thereof, as described above. Ϯϱ^ In some embodiments, the rare-earth doped coating on the steel substrate is a nanocoating. In some embodiments, the present coating comprises nanoparticles or nanorods. The nano morphology of the coating provides better coverage of the substrate, prevents penetration of water molecules and other aggressive ions such chloride ions through the coating, provides ϯϬ^ better corrosion resistance compared to conventional coatings, and provides properties such as paint excellent stain-resistance and easy-clean properties. In some embodiments, the nanocoating has a particle size of about 35-500 nm, including values and ranges thereof, such as, about 35-450 nm, about 35-400 nm, about 35-350 nm, about 35-

[0008] 7 ^ 300 nm, about 35-275 nm, about 35-265 nm, about 35-250 nm, about 35-225 nm, about 50- 500 nm, about 50-450 nm, about 50-400 nm, about 50-350 nm, about 50-300 nm, about 50- 275 nm, about 50-265 nm, about 75-500 nm, about 75-400 nm, about 75-350 nm, or about 75- 265 nm. In some embodiments, the nanocoating has a particle a size of about 35-265 nm. ϱ^ Adhesion of paint to steel substrates depends on the contact angle and surface energy. Lower contact angle and high surface energy indicate that the coated substrates are hydrophilic in nature which helps in better paint adhesion. In some embodiments, coated steel substrates have lesser contact angles and higher surface energy compared to the corresponding bare samples. ϭϬ^ In some embodiments, the coated steel substrate has a contact angle of about 15-55°, including values and ranges thereof, such as about 19-49°, about 20-45°, about 25-50°, about 30-50°, about 19°, about 20°, about 25°, about 30°, about 35°, about 40°, about 45°, about 49°, or about 55°. In some embodiments, the coated steel substrate has a contact angle of 19° or 49°. In some ϭϱ^ embodiments, the coated steel substrate is a galvannealed (GA) substrate comprising the present coating and having a contact angle of 19°. In some embodiments, the coated steel substrate is a galvanized (GI) substrate comprising the present coating and having a contact angle of 49°. ϮϬ^ In some embodiments, the coated steel substrate has a surface energy of about 50-75 mN / m, including values and ranges thereof, such as, about 53-71 mN / m, about 55-70 mN / m, about 60- 75 mN / m, about 65-75 mN / m, about 50 mN / m, about 53 mN / m, about 55 mN / m, about 60 mN / m, about 63 mN / m, about 71 mN / m, or about 75 mN / m. Ϯϱ^ In some embodiments, the coated steel substrate has a contact angle of 15-55°, including values and ranges described above, and a surface energy of about 50-75 mN / m, including values and ranges described above. In some embodiments, the coated steel substrate has a contact angle of 19° and a surface energy ϯϬ^ of 71 mN / m. In some embodiments, the coated steel substrate has a contact angle of 49° and a surface energy of 53 mN / m. In some embodiments, an X-ray diffraction pattern of the substrate before and after coating is similar indicating that the uniformity of the substrate is maintained after coating. The inventors

[0009] 8 ^ observed that steel substrates such as GA and GI substrates prior to applying the present coating are crystalline in nature. After applying the present coating, there is some reduction in crystallinity of the substrate; however, after coating, most of the X-ray diffraction peaks remain intact and only a few diffraction peaks are eliminated. ϱ^ The coated steel substrates of the present disclosure fulfill all the four requirements - corrosion resistance, formability, weldability, and paintability. Thus, the present coated steel substrates are suitable for making product for the automobile and appliances industries. ϭϬ^ The present coating provides a high corrosion resistance to the steel substrate. In some embodiments, the coated substrate exhibits no white rust at least up to 200 hours, 300 hours, 400 hours, 500 hours, 600 hours, 700 hours, 800 hours, 900 hours, 1000 hours, 1200 hours, 1500 hours, 1800 hours, 2000 hours, 2300 hours, 2500 hours, 2800 hours, 3200 hours, 3500 hours, 4000 hours, 4500 hours, or 5000 hours of Salt Spray Test. ϭϱ^ In some embodiments, the coated substrates exhibit no white rust at least up to 900 hours. In some embodiments, the coated substrates exhibit no white rust at least up to 4500 hours. In some embodiments, the coated substrates exhibit no white rust even after 900 hours. In some ϮϬ^ embodiments, the coated substrates exhibit no white rust even after 4500 hours. In some embodiments, the coated substrate exhibits an increase in impedance compared to a bare substrate in an electrochemical impedance spectroscopy (EIS) test which indicates that the present coating is providing barrier protection. Ϯϱ^ In some embodiments, the present coating is well adhered to the steel substrate and does not peel off from the substrate in a standard crosshatch test. The present coating provides formability to the steel substrates. In some embodiments, the ϯϬ^ substrate shows no delamination or powdering of the coating up to 25 mm when tested by a dome test indicating that the coating is formable. The present coating also provides spot-weldability to the stee substrate. In some embodiments, the coating shows a nugget diameter of more than 4.5√t, where “t” is the thickness of the

[0010] 9 ^ substrate in a spot-welding test at a current of 5.5 kA or more indicating that the weld quality is acceptable as per automobile industries requirement. The coated steel substrates of the present disclosure show a very high corrosion resistance, for ϱ^ example, in some embodiments, no white rust formation for more than 900 hours or in some embodiments, no white rust formation for more than 4500 hours, and at the same time, the substrate is formable, weldable, and paintable. The coated steel substrates of the present disclosure show fuel resistance. In some ϭϬ^ embodiments, resistance to fuel, such as petrol, is measured by exposing the coated substrate to fuel and observing the formation of rust for certain time period. For example, in some embodiments, the coated substrates of the present disclosure do not show rust formation upon exposure to fuel after 450 hours, 500 hours, 750 hours, 1000 hours, 1250 hours, 1500 hours, 1750 hours, 2000 hours, 2250 hours, 2400 hours, 2472 hours, or 2500 hours. In some ϭϱ^ embodiments, the coated substrates of the present disclosure show good fuel resistance even after 2472 hours of exposure to fuel. Accordingly, in some embodiments, the coated steel substrates of the present disclosure can be employed in fuel tank applications. In some embodiments, the steel substrate is a galvanized iron (GI), galvannealed (GA), galume, ϮϬ^ galfan, magizin, or a super galva substrate. In some embodiments, the steel substrate is a GI substrate or a GA substrate. The present disclosure also provides a coating composition for preparing the coated steel substrate of the present disclosure. In some embodiments, the coating composition comprises: Ϯϱ^ a) an aqueous solution of an acid; b) a silane; c) aluminium iso-propoxide; d) a compound of a rare earth element; and e) orthophosphoric acid or a derivative thereof. ϯϬ^ In some embodiments, the coating composition comprises: a) an aqueous solution of an acid; b) a silane (e.g., tetraethylorthosilicate); c) a compound of a rare earth element;

[0011] 10 ^ d) a tannin compound (e.g., tannic acid); e) an organic green inhibitor (e.g., anhydrous caffeine); f) aluminium iso-propoxide; g) a second silane (e.g., 3-Aminopropyl)triethoxysilane); and ϱ^ h) orthophosphoric acid or a derivative thereof. The coating composition is a water-based / aqueous coating composition. The coating composition comprises an aqueous solution of an acid. In some embodiments, the ϭϬ^ acid is a carboxylic acid. In some embodiments, the coating composition comprises an aqueous solution of a carboxylic acid selected from lactic acid, formic acid, acetic acid, propionic acid, glycolic acid, benzoic acid, glutaric acid, caproic acid, butyric acid, valeric acid, fumaric acid, or a combination thereof. In some embodiments, the acid is glycolic acid. ϭϱ^ In some embodiments, the acid is present in the coating composition at a concentration of about 2-40 wt %, including values and ranges thereof, such as, about 2-35 wt%, about 2-30 wt%, about 2-25 wt%, about 2-20 wt%, about 2-15 wt%, about 5-40 wt%, about 5-35 wt%, about 5- 30 wt%, about 5-25 wt%, about 5-20 wt%, about 5-15 wt%, about 10-40 wt%, about 10-30 wt%, about 10-20 wt%, about 15-40 wt%, about 15-35 wt%, about 15-30 wt%, about 20-40 ϮϬ^ wt%, about 20-30 wt%, or about 30-40 wt%. In some embodiments, the acid is present in the coating composition at a concentration of about 2 wt%, 4 wt%, 5 wt%, 10 wt%, 12.5 wt%, 15 wt%, 20 wt %, 25 wt%, 30 wt%, 35 wt%, or about 40 wt%. In some embodiments, the silane present in the coating composition is selected fromϮϱ^ tetraethylorthosilicate (TEOS), 3-aminopropyltryethoxysilane (APTES), (3- Aminopropyl)trimethoxysilane (APTMS), (3-Glycidyloxypropyl)trimethoxysilane (GPTMS), (3-Glycidyloxypropyl)triethoxysilane (GPTES), Methyltrimethoxysilane (MTMS), or a combination thereof. In some embodiments, the coating composition comprises TEOS as the silane. In some embodiments, the coating composition comprises APTES as the silane. In some ϯϬ^ embodiments, the coating composition comprises TEOS and APTES as the silanes. In some embodiments, the silane is present in the coating composition at a concentration of about 3-30 wt %, including values and ranges thereof, such as about 3-27 wt%, about 3-25 wt%, about 3-20 wt%, about 3-15 wt%, about 3-10 wt%, about 5-30 wt%, about 5-25 wt%,

[0012] 11 ^ about 5-20 wt%, about 5-15 wt%, about 5-10 wt %, about 10-30 wt%, about 10-25 wt%, about 10-20 wt%, about 15-30 wt%, about 15-25 wt%, about 20-30 wt%, about 20-25 wt%, about 25-30 wt%. In the embodiments, where more than one silane is present in the coating composition, each silane is present at the concentration of about 3-30 wt%, including values ϱ^ and ranges thereof, as described above. In some embodiments, aluminium iso-propoxide is present in the coating composition at a concentration of about 5-40 wt %, including values and ranges thereof, such as, about 5-35 wt%, about 5-30 wt%, about 5-25 wt%, about 5-20 wt%, about 5-15 wt%, about 10-40 wt%, ϭϬ^ about 10-30 wt%, about 10-20 wt%, about 15-40 wt%, about 15-35 wt%, about 15-30 wt%, about 20-40 wt%, about 20-30 wt%, about 30-40 wt%, 5 wt%, 10 wt%, 12.5 wt%, 15 wt%, 20 wt %, 25 wt%, 30 wt%, 35 wt%, or about 40 wt%. The coating composition comprises a compound of the rare earth element selected from cerium ϭϱ^ (Ce), lanthanum (La), neodymium (Nd), praseodymium (Pr) or a combination thereof. In some embodiments, the compound of the rare earth element is selected from a nitrates, sulphates, chlorides, phosphates, and hydroxides of the rare earth element. In some embodiment, the compound of the rare earth element is present in the coating ϮϬ^ composition at a concentration of about 0.05-10 wt %, including values and ranges thereof, such as about 0.05-8 wt%, about 0.05-6 wt%, about 0.05-5 wt%, about 1-10 wt%, about 1-8 wt%, about 1-5 wt%, about 2.5-10 wt%, about 2.5-8 wt%, about 2.5-5 wt%, about 5-10 wt%, about 0.05 wt%, about 1 wt%, about 1.5 wt%, about 2 wt%, about 2.5 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 5.5 wt%, about 6 wt%, about 7 wt %, about 7.5 wt%, about 8 wt%, Ϯϱ^ about 9 wt%, or about 10 wt%. In some embodiments, the compound of the rare earth element present in the coating composition is cerium nitrate or cerium nitrate hexahydrate or cerium nitrate hydrate or cerium chloride or cerium chloride heptahydrate or cerium hydroxide or cerium phosphate or cerium ϯϬ^ sulphate or cerium isopropoxide or lanthanum nitrate or lanthanum nitrate hydrate or lanthanum nitrate hexahydrate or lanthanum chloride or lanthanum chloride or lanthanum chloride heptahydrate or lanthanum isopropoxide in any of the concentration ranges described above. In some embodiments, the compound of the rare earth element present in the coating composition is cerium nitrate or lanthanum nitrate.

[0013] 12 ^ The coating composition comprises orthophosphoric acid or a derivative thereof. In some embodiments, orthophosphoric acid or the derivative is present in the coating composition at a concentration of about 5-50 wt %, including values and ranges thereof, such as, about 5-45 ϱ^ wt%, about 5-40 wt%, about 5-35 wt%, about 5-30 wt%, about 5-25 wt%, about 5-20 wt%, about 5-15 wt%, about 10-50 wt%, about 10-45 wt%, about 10-40 wt%, about 10-30 wt%, about 10-20 wt%, about 15-50 wt%, about 15-45 wt%, about 15-40 wt%, about 15-35 wt%, about 15-30 wt%, about 20-50 wt%, about, about 20-40 wt%, about 20-30 wt%, about 30-50 wt%, about wt%, about 35-50 wt%, about 40-50 wt%, about 5 wt%, about 10 wt%, about ϭϬ^ 12.5 wt%, about 15 wt%, about 20 wt %, about 25 wt%, about 30 wt%, about 35 wt%, about 40 wt%, about 45 wt%, or about 50 wt%. In some embodiments, the derivative of orthophosphoric acid is selected from phosphoric acid, phosphonic acid, pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, trimetaphosphoric acid, and phosphoric anhydride. ϭϱ^ In some embodiments, the coating composition comprises an organic green inhibitor, a rust converter, or both. In some embodiments, the organic green inhibitor is selected from cysteine, folic acid, glycine, ϮϬ^ leucine, caffeine, alanine, tryptophan, methionine, or a combination thereof. In some embodiments, the organic green inhibitor is present at a concentration of about 0.05-10 wt%, including values and ranges thereof, such as about 0.05-8 wt%, about 0.05-6 wt%, about 0.05- 5 wt%, about 1-10 wt%, about 1-8 wt%, about 1-5 wt%, about 2.5-10 wt%, about 2.5-8 wt%, about 2.5-5 wt%, about 5-10 wt%, about 0.05 wt%, about 1 wt%, about 1.5 wt%, about 2 wt%, Ϯϱ^ about 2.5 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 5.5 wt%, about 6 wt%, about 7 wt %, about 7.5 wt%, about 8 wt%, about 9 wt%, or about 10 wt%. In some embodiments, the coating composition comprises caffeine as the organic green inhibitor in any of the amounts described above. ϯϬ^ In some embodiments, the rust converter is selected from tannins. In some embodiments, the rust converter is present at a concentration of about 0.05-10 wt%, including values and ranges thereof, such as about 0.05-8 wt%, about 0.05-6 wt%, about 0.05-5 wt%, about 1-10 wt%, about 1-8 wt%, about 1-5 wt%, about 2.5-10 wt%, about 2.5-8 wt%, about 2.5-5 wt%, about 5-10 wt%, about 0.05 wt%, about 1 wt%, about 1.5 wt%, about 2 wt%, about 2.5 wt%, about

[0014] 13 ^ 3 wt%, about 4 wt%, about 5 wt%, about 5.5 wt%, about 6 wt%, about 7 wt %, about 7.5 wt%, about 8 wt%, about 9 wt%, or about 10 wt%. In some embodiments, the coating composition comprises tannic acid as the rust converted in any of the amounts described above. ϱ^ In some embodiments, the coating composition comprises: a) an aqueous solution of a glycolic acid; b) TEOS and APTES; c) aluminium iso-propoxide; d) cerium nitrate or lanthanum nitrate; and ϭϬ^ e) orthophosphoric acid. In some embodiments, the coating composition comprises: (a) an aqueous solution of a glycolic acid; (b) TEOS and APTES; ϭϱ^ (c) aluminium iso-propoxide; (d) cerium nitrate or lanthanum nitrate; (e) orthophosphoric acid; (f) caffeine; and (g) tannic acid. ϮϬ^ In some embodiments, the coating composition comprises: a) about 2-40 wt% of an aqueous solution of a glycolic acid; b) about 3-30 wt% of TEOS and APTES; c) about 5-40 wt% of aluminium iso-propoxide; Ϯϱ^ d) about 0.05-10 wt% of lanthanum nitrate or cerium nitrate; and e) about 5-50 wt% of orthophosphoric acid. In some embodiments, the coating composition comprises: a) about 2-40 wt% of an aqueous solution of a glycolic acid; ϯϬ^ b) about 3-30 wt% of TEOS and APTES; c) about 5-40 wt% of aluminium iso-propoxide; d) about 0.05-10 wt% of lanthanum nitrate or cerium nitrate; e) about 5-50 wt% of orthophosphoric acid; f) about 0.05-5 wt% of caffeine; and

[0015] 14 ^ g) about 0.05-10 wt% of tannic acid. The present disclosure also provides a method of preparing the coating composition. In some embodiments, the method of preparing the coating composition comprises: ϱ^ a. preparing a first solution, comprising: x adding a portion of the glycolic acid to demineralized water to obtain a first aqueous solution of the acid; x adding the silane (e.g., tetraethylorthosilicate) to the first aqueous solution of the acid; and ϭϬ^ x adding the compound of the rare earth element (e.g., rare earth salt / hydroxide) to the first aqueous solution of the acid after addition of silane; b. preparing a second solution, comprising: x adding the remaining portion of the glycolic acid to demineralized water to obtain the second aqueous solution of the acid; and ϭϱ^ x adding aluminium iso-propoxide to the second aqueous solution of the acid; and c. mixing of the second solution into the first solution followed by the addition of orthophosphoric acid to obtain the coating composition. In some embodiments, the coating composition comprises two silanes. In these embodiments, ϮϬ^ one of the silanes is added to the first aqueous acid solution and the other silane is added to a mixture of the first and the second solution prior to addition of orthophosphoric acid. The organic green inhibitor and the rust converter, if present, are added to demineralized water in the step of preparing the second solution prior to addition of the acid. Ϯϱ^ In some embodiments, a method for preparing the coating composition comprises: (a) preparing a first solution, comprising: • adding a portion of glycolic acid to demineralized water to obtain a first aqueous solution of the acid; ϯϬ^ • adding the silane (tetraethylorthosilicate) to the first aqueous solution of the acid; and • adding the compound of the rare earth element (rare earth salt / hydroxide) to the first aqueous solution of the acid after addition of silane;

[0016] 15 ^ (b) preparing a second solution, comprising: • adding tannic acid to demineralized water to obtain a second aqueous solution; • adding anhydrous caffeine to the second aqueous solution; • adding the remaining portion of the glycolic acid to the second aqueous solution ϱ^ to obtain the second aqueous solution of the acid; and • adding aluminium iso-propoxide to the second aqueous solution of the acid; and (c) mixing of the second solution into the first solution followed by the addition of second silane (APTES) and orthophosphoric acid respectively to obtain the coating composition. ϭϬ^ An exemplary schematic for the preparation of the coating composition is shown in Figure 16. In embodiments of the above-described method, the concentrations or wt% of the components are as described in the embodiments of the coating composition. For the sake of brevity and to ϭϱ^ avoid repetition, each of those concentrations or wt% are not being reiterated in the context of the method. However, each of those concentrations or wt% completely fall within the purview of the method of preparing the coating composition. The components of the coating composition are added to demineralized water or the aqueous ϮϬ^ acid solution at a temperature of about 25-60°C, including values and range thereof. After addition of each component, the component is mixed in the solution by stirring at a speed of about 100-1000 rpm, including values and range thereof. The present disclosure also provides a method for preparing the steel substrates comprising the Ϯϱ^ coating of the present disclosure. In some embodiments, the method for preparing the steel substrate comprises: a) applying the coating composition described herein to a steel substrate to obtain a coated substrate; and b) drying the coated substrate at about 40-200 °C for about 2 - 20 minutes. ϯϬ^ The coating composition can be applied to a steel substrate by a dip-coating method or a roller coating method. After applying the coating composition, the steel substrate is dried at a temperature of about 40-200 °C, including values and ranges thereof, for about 2 - 20 minutes, including values and ranges thereof. In some embodiments, after application of the coating composition, the steel substrate is dried at a temperature of about 40-175 °C, 40-150 °C, 40-

[0017] 16 ^ 120 °C, 40-100 °C, 50-200 °C,, 50-180 °C, 50-140 °C, 50-100 °C, 60-200 °C, 60-180 °C, 60- 120 °C, 60-100 °C, 75-200 °C, 75-180 °C, 75-150 °C, 75-120 °C, 100-200 °C, 100-180 °C, 100-150 °C, or 150-200 °C for about 2-20 minutes, 2-15 minutes, 2-10 minutes, 2-5 minutes, 5-20 minutes, 5-15 minutes, 5-10 minutes, 10-20 minutes, or about 15-20 minutes. ϱ^ The steel substrate coated with the present coating composition is dried at the temperature and time conditions described above in a hot air oven. The substrates that can be coated with the present coating compositions include, but are not ϭϬ^ limited to, galvanized, galvannealed, galume, galfan, magizin, or a super galva substrate. In exemplary embodiments, the substrates coated with the present coating compositions include galvanized or galvannealed steel substrates. The performance characteristics, such as corrosion resistance, paintability, weldability, ϭϱ^ formability, adhesion, fuel resistance, etc., of the coated steel substrates obtained by the present method are described above. The present disclosure provides a low cost, low peak metal temperature (PMT) of 40-150 °C, water-based, rare earth-doped silica-alumina nano-coating system for steel substrates. The ϮϬ^ coating compositions prepared for providing the present coatings are found to be stable for more than one year. The present coating system is a reactive coating system both for the GA and GI substrates. After the uniform deposition and curing / drying of the coating composition, performance studies were carried out on the coated substrates. It is observed that the coated substrates show superior corrosion resistance and other required ready-to-paint properties. For Ϯϱ^ example, in some embodiments, the coating shows >4500 h white rust stability on GA and >900 h on GI substrates. The present coating fulfills all the four requirements (corrosion resistance, formability, weldability, and paintability) and also shows fuel resistance. So, it can be a suitable product for the automobile and appliances industries. The developed coating system is based on silica-alumina system doped with rare earth compounds / salts. The doping ϯϬ^ improves the corrosion resistance and electrical conductivity of the overall coating system. The increase in conductivity of the coating also helps in the post-welding process. The developed coating is weldable due to the semiconducting nature of the coating provided through doping of rare earth compounds. The low PMT coatings consume less power compared to high PMT coatings.

[0018] 17 ^ It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many ϱ^ changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein. ϭϬ^ Descriptions of well-known / conventional methods / steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure certain aspects have been employed. The examples used ϭϱ^ herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein. ϮϬ^ EXAMPLES Example 1: Synthesis of Coating Composition and Application on Substrates The coating composition was prepared in an aqueous acidic medium containing one or more of the following carboxylic acids: lactic acid, formic acid, acetic acid, propionic acid, glycolic Ϯϱ^ acid, benzoic acid, glutaric acid, caproic acid, butyric acid, valeric acid, and fumaric acid. The concentration of these acids in the total composition varied between 2 wt % to 40 wt %. The acidic medium helps in controlled growth of the sol and to avoid precipitation. These acids can also form metal complexes with metals thereby increasing the stability of the coating solution. Two silanes [tetraethylorthosilicate (TEOS) and 3-aminopropyltryethoxysilane (APTES)] ϯϬ^ were used along with aluminium iso-propoxide. The concentration of the above silanes varied between 3 wt % to 30 wt % and the concentration of aluminium iso-propoxide varied between 5 wt % to 40 wt %. The coating composition was doped with a compound of rare-earth elements (Ce / La / Nd / Pr, 0.05 wt % to 10 wt %) to improve the corrosion resistance and conductivity of the coating. The doping helped to make conducting coating easily spot weldable. One or more

[0019] 18 ^ of the listed organic green inhibitors (Cysteine, folic acid, glycine, leucine, caffeine, alanine, tryptophan, and methionine) were added to the coating composition in the range of 0.05 wt% to 10 wt% and rust converters (tannins, 0.05 wt% - 10 wt%) were added to make the coating robust. Ortho phosphoric acid was added at a concentration of 5 wt % to 50 wt %. The coating ϱ^ composition was applied to GA and GI substrates by dip and roll coating methods. After coating, the samples were dried at 40 °C - 200 °C for 2 - 20 minutes in hot air oven. Drying / curing of samples was performed at different temperatures / time and it was found that the peak metal temperature (PMT) is 40 °C – 150 °C. ϭϬ^ Example 2: Performance Characterization of coated steel substrates Salt spray test (SST) was carried out on coated steel substrates (GA and GI) as per ASTM B117 to understand the corrosion resistance of the present coating under aggressive saline environment. To understand the effect of coating on corrosion resistance, SST was also performed on bare GA and GI samples. ϭϱ^ Figure 1 shows the results of the SST. It was observed that both bare substrates corroded (i.e., white rust was formed) after 6h of exposure in SST chamber. However, red rust started after 48h in case of GA sample. ϮϬ^ Figure 2 shows the SST images of coated GA samples after different time intervals. It was found that the present coating on GA substrate is very stable and there is no white rust even after 4500 h. The SST images of GI samples are shown in Figure 3 with different time intervals. In this case, Ϯϱ^ it was observed that there is little (<1 %, two dots) white rust appeared after 960 h. Thus, this coating is stable on GI for 900 h or more. Such high SST values are obatined for the first time for galvanized substrates. As per previous studies, GA or GI substrates usually give a maximum of 200 h of white rust stability in SST. ϯϬ^ The corrosion resistance of Ce-doped coated samples was also tested. There was more than 10 % white rust after 264 h of SST for coated GA and GI substrates (see Figure 4). The corrosion resistance properties of both bare and coated steel substrates (GA and GI) were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy

[0020] 19 ^ (EIS) test. The EIS study is carried out in the frequency range of 0.01 Hz to 100 kHz. A three- electrode system was used for this test where the samples act as the working electrode (1 cm2of exposed area), saturated calomel electrode (SCE) as reference, and graphite was used as a counter electrode. In all the measurements, 3.5% aqueous NaCl solution was used as an ϱ^ electrolyte. Figure 5 shows the semi-log plots of impedance with the frequency (Bode plot), both for GA and GI substrates respectively. It was observed that there is an increase in impedance of coated samples (GA / GI) compared to the bare substrates which indicates coating is giving barrier protection. ϭϬ^ The adhesion test of coated GA and GI samples were performed by standard crosshatch test (Figure 6). It was clearly observed that no coating was removed by the adhesive tape, which indicates the coating is well adhered to both GA and GI substrates. The coating formability was checked by impact, conical mandrel bend test, and dome test ϭϱ^ (Figure 7). In dome test, the coated samples are drawn up to 25 mm and till fracture. There was no coating delamination or powdering in the case of samples drawn up to 25 mm. The dome tested samples (GA and GI) with 25 mm height were exposed to SST and it was found that there was no white rust till 144 h exposure. The impact test was done using a weight of 1 kg and from a height of 100 cm (Impact energy = 9.6 Joule). There was no coating delamination ϮϬ^ or powdering observed in this case. These samples were kept inside the SST chamber and it was observed that these samples were stable inside the SST chamber for more than 500 h without any white rust. Similarly, there was no coating delamination or powdering in case of conical mandrel bend test. These samples were also kept inside the SST chamber which also showed 144 h SST life without any white rust. Ϯϱ^ The spot-welding test was done to check the weldability of the developed coating system both on GA and GI substrates (Figure 8a and 8b). The spot-welding test was done by supplying current for 250ms. The current was gradually increased from 0.5 kA and to 6.5 kA. It was observed that there was current flow even at low current level (0.5 or 1 kA). There was a nugget ϯϬ^ formation even at low current (1 and 1.5 kA) though the nugget size was less. After spot welding, the nugget diameter was measured. If the nugget diameter is more than 4.5√t, where “t” is the thickness of the steel sheet, then it is acceptable. The required nugget diameter was formed when the current amount was 5.5 kA or more. The spot weld lobe test was done both

[0021] 20 ^ for GA and GA samples and found accepted weld qualities as per automobile industries requirement (Figure 8c and 8d). The sol-gel nano-coated samples were further coated / painted by epoxy-polyester hybrid ϱ^ powder coating system. Then, the powder coated samples were checked by different test like (i) impact test (Figure 9a), (ii) cross-hatch test, (iii) conical mandrel bend test, (iv) cut-bend test (Figure 9b), and (v) SST of cross-cut samples. It was observed that all the above tests were passed without any delamination. The cross-cut samples also passed more than 1000 h in SST without any blistering. ϭϬ^ The surface morphology of coated samples was checked by scanning electron microscopy (SEM) and protruded kind of morphology was observed (Figure 10a and 10c). This kind of morphology helps in the formability and post-paint adhesion. The paint adhesion was good as observed from the paintability study where this protruded structure may help to increase the ϭϱ^ mechanical paint adhesion along with chemical interaction. The coating thickness was found to be 2-3 μm as observed from Figure 10b and 10d. The SEM-EDX data is given in the Figure 11. The self-healing behavior of coating (La-doped) was studied through SEM-EDX. A crosscut ϮϬ^ was done on both coated GA and GA samples. Figure 12a and Figure 13a (and the corresponding table) show the SEM-EDX image and corresponding values on the 1stday. Figure 12b and Figure 13b (with the table) show the SEM-EDX image and corresponding values on the 6thday. It was observed that there is an increase in the “Si” content after 120 h exposure in ambient environment which demonstrates the self-healing behavior of the coating. Ϯϱ^ The crystalline / amorphous nature of bare GA / GI, coated GA / GI, and powder form of coating (after drying) are determined by X-ray diffraction (XRD) using an X-ray diffractometer (PANlytical Xpert pro diffractometer) with a monochromatic Cr-Kα source (λ = 1.540598 A°) operated at 40 kV in the range of 10° < 2θ < 90 ° with a step size of 0.03. The diffraction peaks ϯϬ^ for bare GI are observed at 2θ = 36.19, 39.01, 43.17, 54.32, 70.03, 76.97, 82.07, and 88.52. Out of these peaks, the peaks observed at 36.19 and 82.07 are for both Zn1 and O1Zn1 (Wurtzite type), whereas other peaks correspond to Zn [JCPDS: 98-065-3502 (Zn) and 98-016- 1836 (ZnO)]. Similarly, the diffraction peaks for bare GA are observed at 2θ = 35.51 (Fe0.911O1, Fe0.902O1, Fe1O1, Fe1.86O4Zn1.14, Fe1Zn13) 40.89 (Fe0.911O1, Fe0.902O1,

[0022] 21 ^ Fe1Zn13), 42.24 (Fe0.902O1, Fe1O1, Fe1.86O4Zn1.14, Fe1Zn13), 43.17 (Fe1, Fe0.902O1, Fe1.86O4Zn1.14, Fe1Zn13), 64.92 (Fe1, Fe1.86O4Zn1.14, Fe1Zn13), 73.96 (Fe1, Fe1.86O4Zn1.14, Fe1Zn13), and 82.29 (Fe1, Fe1Zn13). See Table 1 below. ϱ^ Table 1: X-ray diffraction peaks Diffraction peaks for bare GI at 2θ Diffraction peaks for bare GA at 2θ 36.19 (Zn1 and O1Zn1) 35.51 (Fe0.911O1, Fe0.902O1, Fe1O1, Fe1.86O4Zn1.14, Fe1Zn13) 39.01 (Zn1 and O1Zn1) 40.89 (Fe0.911O1, Fe0.902O1, Fe1Zn13) 43.17 (Zn / ZnO) 42.24 (Fe0.902O1, Fe1O1, Fe1.86O4Zn1.14, Fe1Zn13) 54.32 (Zn / ZnO) 43.17 (Fe1, Fe0.902O1, Fe1.86O4Zn1.14, Fe1Zn13) 70.03 (Zn / ZnO) 64.92 (Fe1, Fe1.86O4Zn1.14, Fe1Zn13) 76.97 (Zn / ZnO) 73.96 (Fe1, Fe1.86O4Zn1.14, Fe1Zn13) 82.07 (Zn1 and O1Zn1) 82.29 (Fe1, Fe1Zn13) 88.52 (Zn / ZnO) After coating, most of the above peaks remained intact whereas a few peaks were not found. The data in Figure 14a shows that the developed coating (La-doped) is amorphous in nature. ϭϬ^ Bare GA and GI samples are crystalline in nature. However, there is some reduction in crystallinity after the coating (Figure 14a and 14b) due to the amorphous behavior of the coating. Overall, this data indicates that the coating does not affect the surface uniformity of the steel substrates. ϭϱ^ The contact angle and surface energy of bare / coated GI / GA samples was studied (Table 2). It was observed that coated steel sheets have lesser contact angles compared to the corresponding bare samples. Moreover, coated samples have higher surface energy compared to the bare substrates. In the case of post-painting, paint adhesion depends on the contact angle / surface energy. Low contact angle / high surface energy indicates that the coated samples are ϮϬ^ hydrophilic in nature which helps in better paint adhesion.

[0023] 22 ^ Table 2: Contact angle and surface energy of bare / coated GI / GA samples The particle size of the coating sol (Nanocoat sol) was studied using a particle size analyzer ϱ^ through dynamic light scattering (DLS) technique. It is observed that the particle size varies from 35 nm to 500 nm (hydrodynamic diameter) (Figure 15). Fuel / petrol resistance study: Four coated GA samples were immersed in four different fuel mixtures. Petrol was the primary ϭϬ^ fuel where 1-35 % of water or ethanol was added by volume. Other ingredients (formic acid, acetic acid, and chlorine) were in ppm level with respect to water. The fuel mixtures were prepared as per the combinations given in Table 3 below. The images of the tested coated sheets after 2472 hrs are shown in Figure 17. ϭϱ^ It was observed that Samples A and B, immersed in the petrol+water and petrol+ethanol mixtures respectively, were found to be in excellent conditions even after 2472 hours of exposure. Sample C and D were kept in very aggressive fuel mixtures where petrol was mixed with water, formic acid, acetic acid, and chlorine at different concentration as shown in Table 3 below. The samples - C and D - also exhibited fuel resistance. The coating was still found to ϮϬ^ be almost intact on the surface without much degradation after 2472 h of exposure. An edge corrosion (red rust) was observed at the bottom after 500 h of exposure in case of sample C (500 h data not shown here) and 48 h in case of sample D (48 h data not shown here), but the red rust did not extensively aggravate further due to the present coating. Since the coating exhibits fuel resistance, it can be used in fuel tank applications. Ϯϱ^

[0024] 23 ^ Table 3: Fuel mixtures for testing fuel / petrol resistance of coated GA samples. Sample Petrol Water Ethanol Formic Acetic Chlorine (Cl) (% by (% by (% by acid acid (ppm) volume) volume) volume) (ppm) (ppm) Fuel A 65-99 1-35 0 0 0 0 Fuel B 65-99 0 1-35 0 0 0 Fuel C 65-99 1-35 0 20-150 50-300 20-150 Fuel D 65-99 1-35 0 200-1500 500-3000 20-150 Fourier-transform infrared spectroscopy (FTIR) Fourier transform infrared (FTIR) spectrum was recorded for the coated GI and GA substrates ϱ^ (Figure 18). The FTIR spectra is used to illustrate the structural detail of the oxides of “Si”, “Al”, “La” and their interaction with Zn and Zn / Fe of GI and GA substrates respectively. The spectrum was measured in the wavelength range of 400 – 4000 cm-1. The FTIR peaks obtained for both coated GI and GA are given in Table 4 below. ϭϬ^ Table 4: FTIR peaks of coated GI and GA samples. The foregoing description of the specific embodiments reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and / or ϭϱ^ adapt for various applications such specific embodiments without departing from the generic

[0025] 24 ^ concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have ϱ^ been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Throughout this specification, the term ‘combinations thereof’ or ‘any combination thereof’ or ϭϬ^ ‘any combinations thereof’ are used interchangeably and are intended to have the same meaning, as regularly known in the field of patents disclosures. As regards the embodiments characterized in this specification, it is intended that each embodiment be read independently as well as in combination with another embodiment. For ϭϱ^ example, in case of an embodiment 1 reciting 3 alternatives A, B and C, an embodiment 2 reciting 3 alternatives D, E and F and an embodiment 3 reciting 3 alternatives G, H and I, it is to be understood that the specification unambiguously discloses embodiments corresponding to combinations A, D, G; A, D, H; A, D, I; A, E, G; A, E, H; A, E, I; A, F, G; A, F, H; A, F, I; B, D, G; B, D, H; B, D, I; B, E, G; B, E, H; B, E, I; B, F, G; B, F, H; B, F, I; C, D, G; C, D, H; ϮϬ^ C, D, I; C, E, G; C, E, H; C, E, I; C, F, G; C, F, H; C, F, I, unless specifically mentioned otherwise. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many Ϯϱ^ changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation. ϯϬ^

[0026] 25 ^

Claims

We claim:

1. A steel substrate comprising a rare-earth doped silica-alumina coating, wherein the coating comprises: a. about 0.5-6.5 wt% of Al; ϱ^ b. about 0.5-6.5 wt% of Si; and c. about 0.1-11 wt% of a rare earth element.

2. The steel substrate as claimed in claim 1, wherein the substrate has a contact angle of 15- 55°.

3. The steel substrate as claimed in claim 1 or 2, wherein the substrate exhibits a surface ϭϬ^ energy of about 50-75 mN / m.

4. The steel substrate as claimed in any one of claims 1-3, wherein the coating is a nano- coating and exhibits a particle size of about 35-500 nm.

5. The steel substrate as claimed in any one of claims 1-4, wherein an X-ray diffraction pattern of the substrate before and after coating is similar. ϭϱ^ 6. The steel substrate as claimed in any one of claims 1-5, wherein the rare earth element is selected from cerium (Ce), lanthanum (La), neodymium (Nd), praseodymium (Pr) or a combination thereof.

7. The steel substrate as claimed in any one of claims 1-6, wherein the substrate exhibits no white rust at least up to 200 h of Salt Spray Test. ϮϬ^ 8. The steel substrate as claimed in any one of claims 1-7, wherein the substrate exhibits an increase in impedance compared to a bare substrate in an electrochemical impedance spectroscopy (EIS) test.

9. The steel substrate as claimed in any one of claims 1-8, wherein the coating does not peel off from the substrate in a standard crosshatch test. Ϯϱ^ 10. The steel substrate as claimed in any one of claims 1-9, wherein the coating shows no delamination or powdering up to 25 mm when tested by a dome test.

11. The steel substrate as claimed in any one of claims 1-10, wherein the coating shows a nugget diameter of more than 4.5√t, where “t” is the thickness of the substrate in a spot- welding test at a current of 5.5 kA or more. ϯϬ^ 12. The steel substrate as claimed in any one of claims 1-11, wherein the coating does not show substantial rust formation upon exposure to fuel even after 2472 hours.

13. The steel substrate as claimed in any one of claims 1-12, wherein the substrate is formable, weldable, and paintable.26 ^14. The steel substrate as claimed in any one of claims 1-13, wherein the substrate is a galvanized, galvannealed, galume, galfan, magizin, or a super galva substrate.

15. The steel substrate as claimed in any one of claims 1-13, wherein the substrate is a galvanized iron (GI) substrate or a galvannealed (GA) steel substrate. ϱ^ 16. A coating composition for preparing the coated steel substrate as claimed in any one of claims 1-15, comprising: a. an aqueous solution of an acid; b. a silane; c. aluminium iso-propoxide; ϭϬ^ d. a compound of a rare earth element; and e. orthophosphoric acid or a derivative thereof.

17. The coating composition as claimed in claim 16, wherein the acid is a carboxylic acid.

18. The coating composition as claimed in claim 17, wherein the carboxylic acid is selected from lactic acid, formic acid, acetic acid, propionic acid, glycolic acid, benzoic acid, ϭϱ^ glutaric acid, caproic acid, butyric acid, valeric acid, fumaric acid, or a combination thereof.

19. The coating composition as claimed in any one of claims 16-18, wherein the acid is present at a concentration of about 2-40 wt %.

20. The coating composition as claimed in any one of claims 16-19, wherein the silane is selected from tetraethylorthosilicate (TEOS), 3-aminopropyltryethoxysilane (APTES), (3- ϮϬ^ Aminopropyl)trimethoxysilane (APTMS), (3-Glycidyloxypropyl)trimethoxysilane (GPTMS), (3-Glycidyloxypropyl)triethoxysilane (GPTES), Methyltrimethoxysilane (MTMS), or a combination thereof.

21. The coating composition as claimed in claim 20, wherein the composition comprises TEOS and APTES. Ϯϱ^ 22. The coating composition as claimed in any one of claims 16-21, wherein the silane is present at a concentration of about 3-30 wt %.

23. The coating composition as claimed in any one of claims 16-22, wherein aluminium iso- propoxide is present at a concentration of about 5-40 wt %.

24. The coating composition as claimed in any one of claims 16-23, wherein the rare earth ϯϬ^ element is selected from cerium (Ce), lanthanum (La), neodymium (Nd), praseodymium (Pr) or a combination thereof.

25. The coating composition as claimed in any one of claims 16-24, wherein the compound of the rare earth element is present at a concentration of about 0.05-10 wt %.27 ^26. The coating composition as claimed in any one of claims 16-25, wherein orthophosphoric acid is present at a concentration of about 5-50 wt %.

27. The coating composition as claimed in any one of claims 16-26, wherein the composition comprises an organic green inhibitor, a rust converter, or both. ϱ^ 28. The coating composition as claimed in claim 27, wherein the organic green inhibitor is selected from cysteine, folic acid, glycine, leucine, caffeine, alanine, tryptophan, methionine, or a combination thereof.

29. The coating composition as claimed in claim 27 or 28, wherein the organic green inhibitor is present at a concentration of about 0.05-10 wt%. ϭϬ^ 30. The coating composition as claimed in claim 27, wherein the rust converter is selected from tannins.

31. The coating composition as claimed in claim 27 or 30, wherein the rust converter is present at a concentration of about 0.05-10 wt%.

32. A method for preparing the coating composition as claimed in any one of claims 16-31, ϭϱ^ comprising: a. preparing a first solution, comprising: • adding a portion of the acid to demineralized water to obtain a first aqueous solution of the acid; • adding the silane to the first aqueous solution of the acid; and ϮϬ^ • adding the compound of the rare earth element to the first aqueous solution of the acid after addition of the silane to obtain the first solution; b. preparing a second solution, comprising: • adding the remaining portion of the acid to demineralized water to obtain a second aqueous solution of the acid; Ϯϱ^ • adding aluminium iso-propoxide to the second aqueous solution of the acid to obtain the second solution; and c. mixing the first solution and the second solution followed by adding orthophosphoric acid to obtain the coating composition.

33. The method as claimed in claim 32, wherein a second silane is added to a mixture of the ϯϬ^ first and the second solution prior to adding orthophosphoric acid.

34. The method as claimed in claim 32 or 33, wherein in the preparation of the second solution, the organic green inhibitor, the rust converter, or both are added to demineralized water prior to the addition of the acid.

35. A method for preparing the steel substrate as claimed in any one of claims 1-15, comprising:28 ^a. applying the coating composition as claimed in any one of claims 16-31 to the substrate to obtain a coated substrate; and b. drying the coated substrate at about 40-200 °C for about 2 - 20 minutes.

36. The method as claimed in claim 35, wherein the substrate is a galvanized, galvannealed, ϱ^ galume, galfan, magizin, or a super galva substrate.

37. The method as claimed in claim 35 or 36, wherein the substrate is a galvanized iron (GI) substrate or a galvannealed (GA) steel substrate.

38. The method as claimed in any one of claims 35-37, wherein said applying is performed by dipping the substrate in the coating composition or by roll coating. ϭϬ^ 39. The method as claimed in any one of claims 35-38, wherein said drying is performed in a hot air oven.

40. The method as claimed in any one of claims 35-39, wherein the coating provided by the method on a substrate exhibits no white rust at least up to 200 h of Salt Spray Test.

41. The method as claimed in any one of claims 35-40, wherein the coating provided by the ϭϱ^ method shows no delamination or powdering up to 25 mm when tested by a dome test.

42. The method as claimed in any one of claims 35-41, wherein the coating provided by the method shows a nugget diameter of more than 4.5√t, where “t” is the thickness of the substrate in a spot-welding test at a current of 5.5 kA or more.

43. The method as claimed in any one of claims 35-42, wherein the coating provided by the ϮϬ^ method does not show substantial rust formation upon exposure to fuel even after 2472 hours.

44. The method as claimed in any one of claims 35-43, wherein the coating provided by the method is formable, weldable, and paintable.29 ^

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