Spot-weldable polyimide-polyaniline emeraldine salt (PI-PANI-es) coating with high corrosion resistance

Abstract

The present disclosure provides a steel or iron substrate comprising a polyimide-polyaniline emeraldine salt (PI-PANI-ES) coating The incorporation of polyaniline emeraldine salt increases the corrosion resistance of the coated substrate as well as renders the substrate spot-weldable. The present disclosure also provides a method for preparing the coated steel or iron substrate, a coating composition, and a method for preparing the coating composition. The coating composition comprises polyamic acid (PAA) and PANI-ES. The coating composition is applied to a steel or iron substrate and is cured to provide a substrate comprising polyimide-PANI-ES coating. The coated steel or iron substrates of the present disclosure are formable, post-paintable, spot-weldable, and show high corrosion resistance.

Description

[0001]“Spot-weldable polyimide-polyaniline emeraldine salt (PI-PANI-ES) coating with high corrosion resistance” TECHNICAL FIELD The present disclosure relates to metal substrates comprising a polyimide (PI)-polyaniline emeraldine salt (PANI-ES) coating and methods thereof. In particular, the disclosure provides a steel or iron substrate comprising the PI-PANI-ES coating; a coating composition for providing the PI-PANI-ES coating on the substrate; methods for preparing the coated substrates; and methods for preparing the coating composition. BACKGROUND OF THE DISCLOSURE Iron substrates and mild / carbon steel substrates are one of the most widely used engineering materials. Mild steels are selected for construction because of their mechanical properties, which are widely used in industry for their low price. These steels have limited corrosion resistance though they possess relatively high strength. Similarly, galvanized iron (GI) and galvannealed (GA) steels are also prone to environmental corrosion. Therefore, to reduce corrosion in harsh environment, there should be some protective secondary coating on its surface. Different secondary protective coating systems are used to enhance corrosion resistance of the surfaces of iron and steel structures that are used in various industrial applications. There are different coating methods to protect iron and steel components from corrosion which are listed as follows: (i) active protection, (ii) passive barrier protection, (iii) electrophoretic deposition, (iv) sacrificial protection, (v) metallic coating, and (vi) organic coatings. Moreover, there are different types of organic coatings which play a major role in mitigating corrosion. Rust preventive oils (RP oils) are generally used over mild steel substrates to protect it from corrosion during the transportation and storage at their end. These RP oils later need to be removed by degreasing followed by seven or nine tanks phosphating processes. 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 / nine tanks’ processes consume a lot of time and increase the process cost. Another secondary coating studied for steel substrates is a polyimide-polyaniline composite coating. Liangcai et al. (“Preparation and EIS studies on polyimide / polyaniline blend film for corrosion protection,” Polymers for Advanced Technologies 12.11‐12 (2001): 720-723) has studied the corrosion resistance behavior of polyimide-polyaniline composite coating on steel using electrochemical impedance spectroscopy (EIS). In this study, the composite coating was prepared through ex-situ blending process. The concentration of polyaniline in polyimide matrix was 10-15 wt %. The polyimide system was synthesized by employing 4,4’-diaminodiphenyl ether, and 3,3’,4,4’-biphenyl dianhydride in N-methylpyrrolidine (NMP) solvent, and the solid content of the coating was 15 wt %. Ayesha Kausar (“Polyimide, polybenzimidazole-in situ-polyaniline nanoparticle and carbon nano- onion-based nanocomposite designed for corrosion protection,” International Journal of Polymer Analysis and Characterization 22.6 (2017): 557-567) developed a composite coating system of polyimide / polybenzimidazole-in situ-polyaniline nanoparticle / carbon nano-onion (PI / PBI-in- PANI / CNO) and studied its corrosion resistance on steel. In this study, polyaniline nanoparticles were prepared by in-situ process in polybenzimidazole (PBI-in-PANI) system and carbon nano- onion (CNO) was used as the nanofiller. However, steel or iron substrates comprising polyimide- polyaniline based coatings show poor weldability. Thus, there is a need to provide a single coating system that can reduce the cost and process time associated with coating systems that involve multi-tank coating processes (phosphating, electrodeposition (ED), primer, and topcoat, etc.), and overcome the drawbacks associated with current polyimide-polyaniline based coatings to provide substrates with high corrosion resistance, formability, post-paintability, as well as spot-weldability. The present disclosure attempts to address this need. STATEMENT OF THE DISCLOSURE The present disclosure relates to a steel or an iron substrate comprising a polyimide-polyaniline emeraldine salt (PI-PANI-ES) coating. The present disclosure also relates to a coating composition comprising polyaniline emeraldine salt (PANI-ES) and a polyamic acid (PAA). The present disclosure also provides a method for preparing the coated steel or iron substrate, comprising: a) applying the coating composition comprising polyaniline emeraldine salt (PANI- ES) and a polyamic acid (PAA) to a steel or iron substrate; and b) curing the coating composition applied on the substrate at about 150 °C – 350 °C for about 0.5-20 minutes to obtain the coated steel or iron substrate comprising the polyimide-PANI-ES coating. The present disclosure further provides a method for preparing the coating composition, comprising: a) adding polyaniline emeraldine salt (PANI-ES) into an organic solvent; b) adding a first and a second diamine to the organic solvent; c) adding a first and a second dianhydride to the organic solvent to provide a reaction mixture; and d) stirring the reaction mixture for about 5-40 hours in a nitrogen atmosphere to obtain the coating composition. BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES Figure 1 shows an exemplary schematic of synthesis of neat polyamic acid (PAA) or a coating composition comprising PAA and PANI-ES. Figure 2 shows the results of a Salt spray test (SST) of bare / un-coated CRCA, GA and GI substrates. Figure 3 shows the results of a Salt spray test (SST) of coated CRCA samples (Compositions F1 to F6). Figure 4 shows the results of a Salt spray test (SST) of coated GA samples (Compositions F1 to F6). Figure 5 shows the results of a Salt spray test (SST) of coated GI samples (Compositions F1 to F6). Figure 6 shows (A) Bode impedance plot of neat polyimide and polyimide-PANI ES coated steel (CRCA) samples, and (B) Electrochemical equivalent circuits (EECs) used to fit the Bode impedance plot. Figure 7 shows the results of an AC conductivity study of (a) PANI ES powder and (b) polyimide- PANI-ES composite coating. Figure 8 shows potentiodynamic measurement of neat polyimide and polyimide-PANI-ES coating. Figure 9 shows the results of a spot-welding test of coated CRCA (a, b), GA (c, d) and GI samples (e, f) [F1 (a, c, e) and F6 (b, d, f). Figure 10 shows the results of a weld lobe study showing variation of nugget diameter with the increase in supplied current. Figure 11 shows the results of (a) Conical mandrel bend test, (b) crosshatch, and (c) Impact test of post-painted samples on polyimide coated CRCA substrates. Figure 12 shows the results of (a) Conical mandrel bend test, (b) crosshatch, and (c) Impact test of post-painted samples on polyimide-PANI ES coated CRCA substrates. DETAILED DESCRIPTION OF THE DISCLOSURE 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 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 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. 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, described in the context of a single embodiment, may also be provided separately or in any suitable sub- combination. 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. It is desirable that steel and iron substrates coated with secondary coatings show properties such as corrosion resistance, formability, post-paintability, and spot-weldability for various downstream applications. Steel and iron substrates coated with polyimide (PI) based composite coatings have been studied. However, PI coatings are generally insulating in nature, i.e., their electrical conductivity is poor. Thus, previous steel and iron substrates comprising PI coatings were found to be not weldable. The present inventors found that steel and iron substrates comprising polyimide and an emeraldine salt of polyaniline (PI-PANI-ES) increased the conductivity of the coated substrate rendering the substrate spot-weldable and at the same time also exhibited superior corrosion resistance, formability, and post-paintability. The presence of PANI-ES increased the corrosion resistance of the coating as well as improved the conductivity of the coating thereby making it spot-weldable. The coated substrates are also formable and post-paintable. Accordingly, the present disclosure provides a steel or an iron substrate comprising a polyimide- polyaniline emeraldine salt (PI-PANI-ES) coating (also referred to herein as a coated substrate). In some embodiments, the PI-PANI-ES coating applied to the substrate has a thickness of about 3-20 µm, including values and ranges thereof, such as about 3-18 µm, 3-15 µm, 3-12 µm, 3-10 µm, 3-7 µm, 3-5 µm, 5-20 µm, 5-15 µm, 5-10 µm, 10-20 µm, 10-15 µm, 12-20 µm, 12-15 µm, or 15-20 µm. In some embodiments, the coated substrate of the present disclosure shows electrical conductivity of about 1x10-5to 8x10-5σac S / cm, including values and ranges thereof, such as about 2x10-5to 8x10-5σacS / cm, 2x10-5to 7x10-5σacS / cm, 2x10-5to 6x10-5σacS / cm, 2x10-5to 5x10-5σacS / cm, 2x10-5to 4x10-5σacS / cm, 3x10-5to 8x10-5σacS / cm, 3x10-5to 7x10-5σacS / cm, 3x10-5to 6x10-5σac S / cm, 3x10-5to 5x10-5σac S / cm, 4x10-5to 8x10-5σac S / cm, 4x10-5to 7x10-5σac S / cm, 4x10-5to 6x10-5σac S / cm, 5x10-5to 8x10-5σac S / cm, or 5x10-5to 7x10-5σac S / cm. In some embodiments, the coated substrate 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 kA or more. In some embodiments, the coated substrate shows a corrosion current (Icorr) of about 0.0001 µA / cm2to 0.01 µA / cm2, including values and ranges thereof, such as about 0.0001 µA / cm2to 0.0009 µA / cm2, 0.0001 µA / cm2to 0.0007 µA / cm2, 0.0001 µA / cm2to 0.0006 µA / cm2, 0.0001 µA / cm2to 0.0005 µA / cm2, 0.0001 µA / cm2to 0.001 µA / cm2, 0.0001 µA / cm2to 0.005 µA / cm2, 0.0001 µA / cm2to 0.004 µA / cm2, or 0.001 µA / cm2to 0.005 µA / cm2. In some embodiments, the coated substrate shows a corrosion rate of about 0.00003 to 0.003 mpy, including values and ranges thereof, such as about 0.00003 to 0.00009 mpy, 0.00003 to 0.0003 mpy, 0.00003 to 0.0001 mpy, 0.0001 to 0.0005 mpy, 0.0001 to 0.0003 mpy, 0.0001 to 0.002 mpy, or 0.0002 to 0.0005 mpy. In some embodiments, the coated substrate shows a corrosion resistance of more than 200 h, more than 300 h, more than 400 h, more than 500 h, or more than 600 h, in a Salt Spray Test, including value and ranges thereof, such as about 200 h to 800 h, 200 h to 700 h, 200 h to 600 h, 200 h to 500 h, 300 h to 800 h, 300 h to 700 h, 300 h to 600 h, 300 h to 500 h, 400 h to 800 h, 400 h to 700 h, 400 h to 600 h, 500 h to 800 h, 500 h to 700 h, or 600 h to 800 h. In some embodiments, the coated substrate shows a red rust stability (e.g., <10 % red rust) of more than 200 h, more than 300 h, more than 400 h, more than 500 h, or more than 600 h in a Salt Spray Test, including value and ranges thereof, such as about 200 h to 800 h, 200 h to 700 h, 200 h to 600 h, 200 h to 500 h, 300 h to 800 h, 300 h to 700 h, 300 h to 600 h, 300 h to 500 h, 400 h to 800 h, 400 h to 700 h, 400 h to 600 h, 500 h to 800 h, 500 h to 700 h, or 600 h to 800 h. In some embodiments, the coated substrate that shows the red rust stability as described above is a mild steel substrate, e.g., cold rolled cold annealed (CRCA) substrate; or galvannealed steel or galvanized iron substrate. In some embodiments, the coated substrate shows a white rust stability of more than 200 h, more than 300 h, more than 400 h, more than 500 h, or more than 600 h, in a Salt Spray Test, including value and ranges thereof, such as about 200 h to 800 h, 200 h to 700 h, 200 h to 600 h, 200 h to 500 h, 300 h to 800 h, 300 h to 700 h, 300 h to 600 h, 300 h to 500 h, 400 h to 800 h, 400 h to 700 h, 400 h to 600 h, 500 h to 800 h, 500 h to 700 h, or 600 h to 800 h. In some embodiments, the coated substrate that shows the white rust stability as described above is a galvannealed steel or galvanized iron substrate. In some embodiments, the coated substrate shows formability, paintability, and weldability including spot-weldability and high corrosion resistance as described above. In some embodiments, the coated substrate of the present disclosure is a mild steel substrate such as cold rolled cold annealed (CRCA) substrate or a galvanized substrate such as galvannealed (GA) steel substrate or galvanized iron (GI) substrate. The present disclosure also provides a coating composition for preparing the coated substrate. The coating composition comprises polyaniline emeraldine salt (PANI-ES) and a polyamic acid (PAA). In some embodiments, PANI-ES is present in the coating composition in an amount of about 0.001-10 wt%, including values and ranges thereof, such as, about 0.001-0.01 wt%, 0.001- 0.05 wt%, 0.001-0.1 wt%, 0.001-0.5 wt%, 0.001-1 wt%, 0.001-3 wt%, 0.001-5 wt%, 0.001-7 wt%, 0.005-0.01 wt%, 0.005-0.05 wt%, 0.005-0.1 wt%, 0.005-0.5 wt%, 0.005-1 wt%, 0.005-3 wt%, 0.005-5 wt%, 0.005-7 wt%, 0.005-10 wt%, 0.01-0.05 wt%, 0.01-0.1 wt%, 0.01-0.5 wt%, 0.01-1 wt%, 0.01-3 wt%, 0.01-5 wt%, 0.01-7 wt%, 0.01-10 wt%, 0.05-0.1 wt%, 0.05-0.5 wt%, 0.05-1 wt%, 0.05-3 wt%, 0.05-5 wt%, 0.05-7 wt%, 0.05-10 wt%, 0.1-0.5 wt%, 0.1-1 wt%, 0.1-3 wt%, 0.1-5 wt%, 0.1-7 wt%, 0.1-10 wt%, 0.5-1 wt%, 0.5-3 wt%, 0.5-5 wt%, 0.5-7 wt%, 0.5-10 wt%, 1- 3 wt%, 1-5 wt%, 1-7 wt%, 1-10%, 3-10 wt%, 3-7 wt%, 3-5 wt%, or 5-10 wt%. In some embodiments, the PAA present in the coating composition is a reaction product of dianhydrides and diamines. In some embodiments, the PAA is a reaction product of two dianhydrides and two diamines. In some embodiments, the dianhydrides are selected from pyromellitic dianhydride (PMDA), 3,3’,4,4’-Biphenyltetracarboxylic dianhydride (BPDA), 4,4′-(4,4′-Isopropylidenediphenoxy) bis (phthalic anhydride) (BPADA), benzophenone-3,3’,4,4’-tetracarboxylic dianhydride (BTDA), 4,4’-oxydiphthalic anhydride (ODPA); 4,4’-(hexafluoro-isopropylidene) diphthalic anhydride (FDA); and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (HFDA), or any combination thereof. In some embodiments, the dianhydrides employed to prepare the PAA are PMDA and BPDA. In some embodiments, the diamines employed to prepare the PAA are selected from hexamethylene diamine (HMDA); 4,4’ oxydianiline (ODA); ethylene diamine (EDA); O,O’- Bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-block polypropylene glycol; 4,7,10-trioxa-1,13-tridecanediamine; poly(propylene glycol)bis(2-aminopropyl ether; 1,2-bis(2- aminoethoxyethane); 4,9-dioxa-1,12-dodecanediamine; 1,11-Diamino-3,6,9-trioxaundecane; 2,2′- (ethylenedioxy)bis(ethylamine); 2,2-bis(aminoethoxy)propane; or 1,8-diamino-3,6-dioxaoctane; a phenylenediamine; 4,4’-(1,3-Phenylenedioxy) dianiline; 4,4′-(4,4′-isopropylidenediphenyl-1,1′- diyldioxy)dianiline; 4,4′-diaminodiphenylmethane; 4,4′-(1,1′-Biphenyl-4,4′-diyldioxy)dianiline; or 4,4′-methylene-bis(2-methylaniline), or any combination thereof. In some embodiments, the diamines are ODA and HMDA. In some embodiments, the polyamic acid is a reaction product of PMDA, BPDA, ODA, and HMDA. In some embodiments, each of the dianhydrides and diamines are employed in equivalent ratios of about 0.1-1.0, including values and ranges thereof, such as about 0.1-0.8, 0.1-0.5, 0.1-0.3, 0.3-0.8, 0.5-1, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1. The present disclosure also provides a method for preparing a steel or iron substrate comprising the polyimide-PANI-ES coating (the “coated substrate”), wherein the said method comprises: a) applying the coating composition comprising polyaniline emeraldine salt (PANI-ES) and the polyamic acid (PAA) to a steel or iron substrate; and b) curing the coating composition applied on the substrate at about 150 °C – 350 °C for about 0.5-20 minutes to obtain the steel or iron substrate comprising the polyimide-PANI-ES coating. The coating composition can be applied to a steel or iron substrate by dip or bar coating. The step of curing converts the polyamic acid into a polyimide matrix to provide the polyimide- PANI-ES coated substrate. In some embodiments, curing is carried out at about 150-350 °C, 150- 300 °C, 150-275 °C, 150-250 °C, 150-200 °C, 200-350 °C, 200-300 °C, 200-250 °C, 250-350 °C, 300-350 °C, 150 °C, 200 °C, 250 °C, 300 °C, or 350 °C for about 0.5-20 minutes, 0.5-15 minutes, 0.5-12 minutes, 0.5-10 minutes, 0.5-5 minutes, 1-20 minutes, 1-15 minutes, 1-10 minutes, 1-5 minutes, 5-20 minutes, 5-15 minutes, 5-10 minutes, 10-15 minutes, 10-20 minutes, or 15-20 minutes. The polyimide-PANI-ES coated substrate provided by the method exhibits electrical conductivity, spot-weldability, corrosion current, corrosion rate, and corrosion resistance as described above. The present disclosure also provides a method for preparing the coating composition. In some embodiments, the method comprises: a) adding polyaniline emeraldine salt (PANI-ES) into an organic solvent; b) adding a first and a second diamine to the organic solvent; c) adding a first and a second dianhydride to the organic solvent to provide a reaction mixture; and d) stirring the reaction mixture for about 5-40 hours in a nitrogen atmosphere to obtain the coating composition. The first and second diamines and the first and second dianhydrides employed in the method are selected from the diamines and dianhydrides described above. The amounts of PANI-ES, the first and second diamines and the first and second dianhydrides employed to prepare the coating composition are described above. In some embodiments, the organic solvent is selected from N-methylpyrrolidone (NMP), N,N’- dimethylacetamide (DMAc), dimethylformamide (DMF), chloroform, acetone or any combination thereof. In some embodiments, the organic solvent comprises NMP and DMAc. In some embodiments of the method for preparing the coating composition, PANI-ES is added to a first organic solvent, such as NMP, to which a second organic solvent, such as DMAc, is added followed by the addition of a first and second diamine and a first and second dianhydride to obtain a reaction mixture which is stirred for about 5-40 hours in a nitrogen atmosphere to obtain the coating composition. In some embodiments, the reaction mixture is stirred for 5-40 hours, including values and ranges thereof, such as about 5-35 hours, 5-30 hours, 5-25hours, 5-20 hours, 5-15 hours, 5-10 hours, 10- 40 hours, 10-35 hours, 10-30 hours, 10-25 hours, 10-20 hours, 10-15 hours, 15-40 hours, 15-30 hours, 15-25 hours, 15-20 hours, 20-40 hours, 20-30 hours, 25-35 hours, 25-30 hours, or 30-40 hours in a nitrogen atmosphere to obtain the coating composition. Figure 1 shows an exemplary process for the preparation of a polyimide and polyimide-PANI-ES coated substrates. 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 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. 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 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. EXAMPLES Example 1: Synthesis of Polyaniline Emeraldine Salt (PANI ES) Polyaniline emeraldine salt (PANI ES) was synthesized at -24 °C to 102 °C using ice-NaCl salt mixture or ethylene glycol bath.1-20 g of aniline was dissolved in 100 ml of 0.1-10 M HCl (diluted from HCl 37%) and kept in bottle-A at the desired temperature (0 °C – 10 °C) for 1-10 hours. Then, 1-80 g of ammonium persulfate (APS) was dissolved in 100 mL of demineralized (DM) water, stored in bottle-B and kept in the same desired temperature (0 °C – 10 °C) for 1-10 hours. The polymerization reaction was carried out in a round bottom flask where APS solution was added to aniline-HCl solution in a vigorous stirring condition at the desired polymerization temperature (-24 °C to 102 °C). The polymerization reaction was continued for 6 h to 72 hours to ensure that all aniline monomers were converted into PANI ES (green in color). The prepared PANI ES samples were collected through filtration using filter paper on Büchner funnel, followed by washing with 0.1M – 10M HCl solution, DM water, and acetone. The washing process was repeated several times to make PANI ES samples free of oligomers. Finally, PANI ES samples were dried in hot air oven at 35 °C to 120 °C. Example 2: Synthesis of PI-PANI-ES coating composition Different PI-PANI-ES coating systems were prepared by varying the combinations of different dianhydrides, diamines and PANI-ES. Specifically, 0.001-15 wt% of PANI-ES synthesized in Example 1 was added to 5-80 vol% of N-methylpyrrolidone (NMP) to obtain a to the PANI-ES solution. N,N’-dimethylacetamide (DMAc) was added to the PANI-ES solution followed by the addition of ODA, HMDA, PMDA, and BPDA to obtain a reaction mixture which was stirred for about 5-40 hours in a nitrogen atmosphere to obtain the coating composition. A higher viscosity was observed in the case of PI-PANI-ES coating system compared to the pure polyimide coating system. Different formulations were prepared as shown in Table 1. Table 1: Polyimide (PI) and polyimide-polyaniline-ES (PI-PANI-ES) coatings and their performance on different steel substrates (CRCA, GA, and GI). Samples Monomers Quantity **SST Impact Cross Spot (*In (h) Test hatch Welding equivalent / wt Test %) F1 PMDA 0.1-1.0 120 Pass Pass Fail BPDA 0.1-1.0 (CR), (CR), (CR), (CR, ODA 0.1-1.0 96 (GA (GA GA, and HMDA 0.1-1.0 (GA), and GI and GI GI) 72 (GI) - †) - †) F2 PMDA 0.1-1.0 192 Pass Pass † BPDA 0.1-1.0 (CR), (CR), (CR), ODA 0.1-1.0 144 (GA (GA HMDA 0.1-1.0 (GA), and GI and GI PANI-ES 0.001-3.0 wt% 168 - †) - †) (GI) F3 PMDA 0.1-1.0 360 Pass Pass † BPDA 0.1-1.0 (CR), (CR), (CR), ODA 0.1-1.0 288 (GA (GA HMDA 0.1-1.0 (GA), and GI and GI PANI-ES 0.001-5.0 wt% 216 - †) - †) (GI) F4 PMDA 0.1-1.0 >500 Pass Pass † BPDA 0.1-1.0 (CR), (CR), (CR), ODA 0.1-1.0 456 (GA (GA HMDA 0.1-1.0 (GA), and GI and GI PANI-ES 0.001-7.0 wt% 96 (GI) - †) - †) F5 PMDA 0.1-1.0 288 Pass Pass † BPDA 0.1-1.0 (CR), (CR), (CR), ODA 0.1-1.0 456 (GA (GA HMDA 0.1-1.0 (GA), and GI and GI PANI-ES 0.001-10.0 120 - †) - †) wt% (GI) F6 PMDA 0.1-1.0 312 Pass Pass Pass BPDA 0.1-1.0 (CR), (CR), (CR), (CR, GA ODA 0.1-1.0 528 (GA (GA and GI) HMDA 0.1-1.0 (GA), and GI and GI PANI-ES 0.001-15.0 216 - †) - †) wt% (GI) PMDA = Pyromellitic dianhydride, BPDA = 3,3,4,4-Biphenyltetracarboxylic dianhydride, HMDA = Hexamethylenediamine, ODA = 4,4-Oxydianiline, PANI = Polyaniline, F1 = Formulation 1 (similarly other formulations). * PMDA, BPDA, ODA, and HMDA are given as per equivalent ratio whereas PANI-ES is given in wt % with respect to total weight. **Dimethylacetamide (DMAc) is used as solvent in case of F1 whereas mixture of DMAc and NMP (N- Methylpyrrolidone) is used in other formulations (F2 to F6). ***SST = Salt spray test [Test hour shows red rust (CR) and white rust (GA and GI) < 10% area of total exposed area]. † Test not done on these samples. Example 3: Performance of PI-PANI-ES coated substrates Non-chromated CRCA / GA / GI steels were collected from CRM (Tata Steel Limited), Jamshedpur, India. The above steel sheets were cut into 100 mm x 40 mm and A4 size for coating application. These sheets were degreased in 0.25 – 10.0 wt. % aqueous alkali solution at 30 °C to 120 °C for 10 - 100 seconds of dipping to remove all the oil and dirt particles. Then, these samples were rinsed with distilled water and dried. Finally, these samples (100 mm x 40 mm size samples by dip coating, and A4 sheets by bar coating) were used for coating deposition. The salt spray test (SST) was carried out on coated steel substrates (CRCA, GA, and GI) as per ASTM B117 to understand the corrosion resistance of PI-PANI ES composite coating under aggressive saline environment. To understand the effect of coating on corrosion resistance, SST of bare CRCA, GA and GI samples was also performed. The results for bare samples are shown in Figure 2. It is observed that bare samples were corroded (red rust in case of CRCA and white rust in case of GA / GI) after 6h of exposure in SST chamber. However, red rust was started after 48h in the case of GA sample. Figure 3 shows the SST results of coated CRCA samples with different formulations of polyimide-PANI ES composite coatings where the polyaniline concentration was increased from 0.005 to 10 wt % (Formulations 2 to 6). Formulation 1 is based on the pure polyimide system (without PANI ES). The coating based on PI-PANI-ES system shows >500 h (<10 % red rust) red rust stability on mild steel substrates (F4 shows the best performance for CRCA) whereas coating based on neat polyimide system (F1) shows 120 h red rust stability (Figure 3). Similarly, PI-PANI-ES coated substrates (F2 to F6) show >500 h (F4 and F6 showing the best results for GA) and >216 h (F3 and F6 showing the best results for GI) white rust stability on GA and GI substrates respectively whereas the pure polyimide coating (F1) shows 96 h and 72 h white rust stability on GA and GI substrates respectively (Figures 4 and 5). The corrosion resistance properties of coated mild steel (CRCA) substrates were also investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test. The EIS study was 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 electrolyte. Figures 6A and 6B show the log-log plots of impedance with the frequency (Bode plot) and the equivalent circuit respectively. The fitted data is given in Table 2. There was a decrease in impedance of coated CRCA samples with the increase in PANI-ES concentration. The AC conductivity (electrical conductivity) of both PANI-ES powder and PI- PANI-ES coating (in film form) was checked and PANI-ES was found to exhibit electrical conductivity. It was observed that there is an increase in the conductivity of the coated substrate with an increase in the PANI-ES concentration. The conductivity of PI-PANI-ES coated substrate falls in the semiconducting region (Figure 7). Similar changes in the electrical property was also observed in Bode plot (variation of impedance with frequency) (Figure 6). Figure 8 shows the potentiodynamic plot (TAFEL plot) of pure PI and PI-PANI-ES coating systems. The corrosion potential (Ecorr), corrosion current densities (icorr), anodic and cathodic Tafel slopes (Beta A and Beta C) for neat polyimide and PI-PANI-ES composite coatings at 25 °C are listed in Table 3. It was observed that there is a decrease in icorr value from 0.0332 µA / cm2(F1 - neat polyimide) to 0.000584 µA / cm2(F4 – Polyimide-PANI ES) with the increase in PANI ES concentration. This decrease can be due to the passivating effect of PANI ES which helps in delaying the corrosion. Table 2 Electrochemical parameters obtained by fitting the Bode impedance plot with the EECs. RsRctRfCPEctCPEfGoodness of Rt = Rct+ RfFormulations (Ohm) (Ohm) (Ohm) (S*s^n) n (S*s^m) m Fit (Ohm) F1 8.77E-02 1.20E+07 1.53E+07 2.86E-07 4.74E-01 4.80E-10 9.84E-01 9.94E-04 2.74E+07 F2 8.40E-03 5.54E+06 2.36E+04 4.14E-07 5.89E-01 4.88E-08 6.63E-01 1.50E-02 5.56E+06 F3 8.5 8.03E+07 1.29E+03 1.00E-05 4.05E-01 2.18E-06 4.23E-01 4.19E-03 8.03E+07 F4 7.68E-05 1.23E+04 1.38E+04 6.46E-05 4.70E-01 2.98E-09 8.98E-01 3.68E-04 2.61E+04 F5 1.28E-03 1.67E+07 5.11E+04 1.70E-08 8.20E-01 3.06E-09 8.42E-01 2.24E-03 1.67E+07 F6 2.10E-04 7.64E+03 6.73E+03 3.93E-05 6.23E-01 6.34E-09 8.16E-01 3.24E-03 1.44E+04 Table 3: Electrochemical data for different samples in 3.5% NaCl solution by DC polarization fittings. Corrosion Beta A Beta C IcorrEcorrRate Chi Samples (V / decade) (V / decade) (µA / cm2) (mV) (mpy) Squared F1 166.9e-3 347.5e-3 0.0332 -793.0 15.16e-3 9.178 2.022e-3 F2 341.0e-3 186.0e-3 0.00443 -748.0 (0.002) 4.221 49.42e-6 F3 133.2e-3 402.4e-3 0.000108 -592.0 (0.000049) 3.93E-02 266.8e-6 F4 797.3e-3 1.060 0.000584 -523.0 (0.00026) 4.95E-04 254.6e-6 F5 630.6e-3 295.0e-3 0.000558 -723.0 (0.00025) 1.04E-02 F6 130.9e-3 324.2e-3 0.0655 -659.0 29.89e-3 16.95 The spot-welding test was done to check the weldability of the developed coating system [F1 (without PANI-ES) and F6 (with PANI ES)] on coated CRCA, GA, and GI substrates (Figure 9). Panels (a), (c), and (e) of Figure 9 show the spot-welding test done on CRCA, GA, and GI substrates coated with pure PI (F1). Panels (b), (d), and (f) of Figure 9 show the spot-welding test done on CRCA, GA, and GI substrates coated with PI-PANI-ES (F4 and F6). The spot- welding test was done by supplying current for 250ms. The current was gradually increased from 1.0 kA and to 5.0 kA. It was observed that there was current flow even at low current level (1 kA) in case of F6. There was nugget formation even at low current (3 kA). 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 weld lobe study was done on only polyimide-PANI-ES coated CRCA substrates. As per the above formula, the required nugget diameter should be 3.6 mm (minimum) as per the supplied steel (CRCA) thickness of 0.65 mm. For the weld lobe study (Figure 10), the input current was varied from 4 kA to 7.5 kA. The required nugget diameter was formed when the current amount was 5 kA or more. However, there was an expulsion above 7.0 kA. Thus, the current of 5 kA to 7.0 kA provided the required nugget formation. Both polyimide (Figure 11) and polyimide-PANI ES (Figure 12) coated CRCA substrates were powder painted with polyester powder paint. Its performance was checked by salt spray test (SST), impact test, conical mandrel bend test, and crosshatch test. There was no rust or blister formation after 500 h in SST chamber. The powder painted samples also passed all the other required tests (Impact test, Conical mandrel bend test, and Crosshatch test) which are presented in Figures 11 and 12. Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well- known / conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein. 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 adapt for various applications such specific embodiments without departing from the generic 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 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 ‘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 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; 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 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.

Claims

We claim:

1. A steel or an iron substrate comprising a polyimide-polyaniline emeraldine salt (PI-PANI- ES) coating.

2. The substrate as claimed in claim 1, wherein the coating has a thickness of about 3-20 µm.

3. The substrate as claimed in claim 1 or 2, wherein the substrate shows electrical conductivity of about 1x10-5to about 8x10-5σacS / cm.

4. The substrate as claimed in any one of claims 1-3, wherein the substrate 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 kA or more.

5. The substrate as claimed in any one of claims 1-4, wherein the substrate shows a corrosion current (Icorr) of about 0.0001 µA / cm2to 0.01 µA / cm2.

6. The substrate as claimed in any one of claims 1-5, wherein the substrate shows a corrosion rate of 0.00003 to 0.003 mpy.

7. The substrate as claimed in any one of claims 1-6, wherein the substrate shows a corrosion resistance of more than 200 h in a Salt Spray Test.

8. The substrate as claimed in any one of claims 1-7, wherein the substrate shows a corrosion resistance of more than 500 h in a Salt Spray Test.

9. The substrate as claimed in any one of claims 1-8, wherein the substrate is formable, post- paintable, and spot-weldable.

10. The substrate as claimed in any one of claims 1-9, wherein the substrate is selected from mild steel substrate and galvanized substrate.

11. The substrate as claimed in claim 10, wherein the mild steel substrate is cold rolled cold annealed (CRCA) substrate and the galvanized substrate is galvannealed (GA) steel substrate or galvanized iron (GI) substrate.

12. A coating composition for preparing the substrate as claimed in any one of claims 1-11, comprising: polyaniline emeraldine salt (PANI-ES) and a polyamic acid (PAA).

13. The coating composition as claimed in claim 12, wherein PANI-ES is present in an amount of about 0.001-10 wt%.

14. The coating composition as claimed in claim 12 or 13, wherein the PAA is a reaction product of dianhydrides and diamines.

15. The coating composition as claimed in claim 14, wherein the dianhydride is selected from pyromellitic dianhydride (PMDA), 3,3’,4,4’-Biphenyltetracarboxylic dianhydride (BPDA), 4,4′-(4,4′-Isopropylidenediphenoxy) bis (phthalic anhydride) (BPADA), benzophenone-3,3’,4,4’-tetracarboxylic dianhydride (BTDA), 4,4’-oxydiphthalicanhydride (ODPA); 4,4’-(hexafluoro-isopropylidene) diphthalic anhydride (FDA); and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (HFDA), or any combination thereof.

16. The coating composition as claimed in claim 14, wherein the diamine is selected from hexamethylene diamine (HMDA); 4,4’ oxydianiline (ODA); ethylene diamine (EDA); O,O’-Bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-block polypropylene glycol; 4,7,10-trioxa-1,13-tridecanediamine; poly(propylene glycol)bis(2- aminopropyl ether; 1,2-bis(2-aminoethoxyethane); 4,9-dioxa-1,12-dodecanediamine; 1,11- Diamino-3,6,9-trioxaundecane; 2,2′-(ethylenedioxy)bis(ethylamine); 2,2- bis(aminoethoxy)propane; or 1,8-diamino-3,6-dioxaoctane; a phenylenediamine; 4,4’-(1,3- Phenylenedioxy) dianiline; 4,4′-(4,4′-isopropylidenediphenyl-1,1′-diyldioxy)dianiline; 4,4′-diaminodiphenylmethane; 4,4′-(1,1′-Biphenyl-4,4′-diyldioxy)dianiline; or 4,4′- methylene-bis(2-methylaniline), or any combination thereof.

17. The coating composition as claimed in any one of claims 14-16, wherein each of the dianhydrides and diamines are employed in equivalent ratios of 0.1-1.

0.

18. A method for preparing the steel or iron substrate as claimed in any one of claims 1-11, comprising: a. applying a coating composition comprising polyaniline emeraldine salt (PANI- ES) and a polyamic acid (PAA) to a steel or iron substrate; b. curing the coating composition applied on the substrate at about 150 °C – 350°C for about 0.5-20 minutes to obtain the steel or iron substrate comprising the polyimide-PANI-ES coating.

19. The method as claimed in claim 18, wherein the coating composition is applied by dip or bar coating.

20. The method as claimed in claim 18 or 19, wherein the coating composition is prepared by a method comprising: a. adding polyaniline emeraldine salt (PANI-ES) into an organic solvent; b. adding a first and a second diamine to the organic solvent; c. adding a first and a second dianhydride to the organic solvent to provide a reaction mixture; and d. stirring the reaction mixture for about 5-40 hours in a nitrogen atmosphere to obtain the coating composition.

21. The method as claimed in claim 20, wherein the organic solvent is selected from N- methylpyrrolidone (NMP), N,N’-dimethylacetamide (DMAc), dimethylformamide (DMF), chloroform, acetone or any combination thereof .

22. The method as claimed in claim 20 or 21, wherein the organic solvent comprises NMP and DMAc.

23. The method as claimed in any one of claims 20-22, wherein the dianhydride is selected from pyromellitic dianhydride (PMDA), 3,3’,4,4’-Biphenyltetracarboxylic dianhydride (BPDA), 4,4′-(4,4′-Isopropylidenediphenoxy) bis (phthalic anhydride) (BPADA), benzophenone-3,3’,4,4’-tetracarboxylic dianhydride (BTDA), 4,4’-oxydiphthalic anhydride (ODPA); 4,4’-(hexafluoro-isopropylidene) diphthalic anhydride (FDA); and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (HFDA), or any combination thereof.

24. The method as claimed in any one of claims 20-23, wherein the diamine is selected from hexamethylene diamine (HMDA); 4,4’ oxydianiline (ODA); ethylene diamine (EDA); O,O’-Bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-block polypropylene glycol; 4,7,10-trioxa-1,13-tridecanediamine; poly(propylene glycol)bis(2- aminopropyl ether; 1,2-bis(2-aminoethoxyethane); 4,9-dioxa-1,12-dodecanediamine; 1,11- Diamino-3,6,9-trioxaundecane; 2,2′-(ethylenedioxy)bis(ethylamine); 2,2- bis(aminoethoxy)propane; or 1,8-diamino-3,6-dioxaoctane; a phenylenediamine; 4,4’-(1,3- Phenylenedioxy) dianiline; 4,4′-(4,4′-isopropylidenediphenyl-1,1′-diyldioxy)dianiline; 4,4′-diaminodiphenylmethane; 4,4′-(1,1′-Biphenyl-4,4′-diyldioxy)dianiline; or 4,4′- methylene-bis(2-methylaniline), or any combination thereof.

25. The method as claimed in any one of claims 18-24, wherein PANI-ES is present in an amount of about 0.001-10 wt% of the coating composition.

26. The method as claimed in any one of claims 20-25, wherein the dianhydrides and diamines are present in equivalent ratios of 0.1-1.

0.

27. The method as claimed in any one of claims 18-26, wherein the steel or iron substrate is selected from mild steel substrate and galvanized substrate.

28. The method as claimed in claim 27, wherein the mild steel substrate is a cold rolled cold annealed (CRCA) substrate and the galvanized substrate is galvannealed (GA) steel substrate or galvanized iron (GI) substrate.

29. The method as claimed in any one of claims 18-28, wherein the substrate provided by the method has a thickness of about 3-20 µm.

30. The method as claimed in any one of claims 18-29, wherein the substrate provided by the method shows electrical conductivity of about 1x10-5to about 8x10-5σacS / cm.

31. The method as claimed in any one of claims 18-30, wherein the substrate 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 kA or more.

32. The method as claimed in any one of claims 18-31, wherein the substrate provided by the method shows a corrosion current (Icorr) of about 0.0001 µA / cm2to 0.01 µA / cm2.

33. The method as claimed in any one of claims 18-32, wherein the substrate provided by the method shows a corrosion rate of 0.00003 to 0.003 mpy.

34. The method as claimed in any one of claims 18-33, wherein the substrate provided by the method shows a corrosion resistance of more than 200 h in a Salt Spray Test.

35. The method as claimed in any one of claims 18-34, wherein the substrate provided by the method shows a corrosion resistance of more than 500 h in a Salt Spray Test.

36. The method as claimed in any one of claims 18-35, wherein the substrate provided by the method is formable, post-paintable, and spot-weldable.

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