Corrosion-resistant coating composition and the method of preparation thereof

Abstract

ABSTRACT CORROSION-RESISTANT COATING COMPOSITION AND THE METHOD OF PREPARATION THEREOF The present invention relates to a corrosion-resistant coating composition and a method for preparing and applying the same for corrosion protection of metallic substrates under aggressive acidic and high-temperature environments. The boron-integrated phthalonitrile coating exhibits improved resistance to acid-induced degradation while retaining the inherent thermal stability and chemical robustness of phthalonitrile polymers. The invention provides a simple, catalyst-free, and scalable coating system suitable for corrosion protection of substrates exposed to harsh chemical and thermal conditions in industrial applications Fig. 1

Description

FIELD OF THE INVENTION

[0001] The present invention relates to corrosion-resistant polymeric coatingcompositions and, more particularly, the present invention relates to phthalonitrile-basedcorrosion-resistant coating compositions for protecting metallic substrates againstaggressive acidic environments. The present invention also relates to the method ofpreparation of the corrosion-resistant coating composition.BACKGROUND OF THE INVENTION

[0002] Metallic substrates such as aluminium and its alloys are widely used inaerospace, chemical processing, transportation, and structural applications due to theirfavourable strength-to-weight ratio, ease of fabrication, and mechanical properties.However, aluminium surfaces, despite their good corrosion resistance in neutralenvironments, are highly susceptible to corrosion when exposed to strong acids, includingnitric acid and mixed mineral acids, particularly at elevated temperatures. Prolongedexposure to such corrosive environments leads to surface deterioration, coatingdelamination, loss of mechanical integrity, and reduction in service life of the components.

[0003] Various protective coatings have been developed to mitigate corrosion ofmetallic substrates. Conventional organic polymer-based coatings, including epoxy resins,polyurethanes, fluoropolymers, and polyimides, are commonly employed. Epoxy-basedcoatings are widely used due to their good adhesion and ease of application; however, theysuffer from chemical degradation, loss of adhesion, and reduced protective performancewhen exposed to strong acidic environments and elevated temperatures. Polyurethanecoatings provide flexibility but exhibit limited resistance to aggressive acids.Fluoropolymer coatings demonstrate superior chemical resistance but are expensive andoften require specialized processing techniques, thereby limiting their industrialapplicability. Polyimide coatings offer improved thermal stability; however, theygenerally require high curing temperatures, involve complex synthesis routes, and exhibitlimited adhesion to certain metallic substrates, including aluminium.

[0004] Phthalonitrile (PN) resins are known in the art as high-performance polymerspossessing excellent thermal stability, flame resistance, mechanical strength, and chemicalinertness. Owing to these properties, PN resins have been primarily developed for high-temperature structural composites, adhesives, and aerospace applications. However,existing phthalonitrile-based systems, when used as surface coatings, exhibit gradualdegradation under prolonged exposure to strong acidic environments and theirperformance in such conditions remains insufficiently explored. For instance,phthalonitrile resins specifically as coatings. Tay et al., (ACS Omega, 2022, 7,32996-33003 DOI: 10.1021 / acsomega.2c02667) studied alumina nanoparticle-filledphthalonitrile nanocomposites primarily for enhanced polymerization kinetics and thermalproperties; Wang et al., (ACS Appl. Mater. Interfaces, 2024, 16, 68316-68327 DOI:10.1021 / acsami.4c14729) reported in-situ generated organic-inorganic hybridphthalonitrile systems focused on thermal stability via hydrogen bonding interactions; andDerradji et al., (Prog. Org. Coat., 2016, 90, 34-43 DOI: 10.1016 / j.porgcoat.2015.11.004)examined silane-modified titania nanoparticles in bisphenol-A phthalonitrile resin forthermal, mechanical, and saline corrosion properties. Consequently, the application of PNresins as effective anticorrosion coatings has remained limited and insufficiently explored.

[0005] Several prior art disclosures describe corrosion-resistant coatings based onepoxy systems, hybrid organic-inorganic formulations, and polyimide or fluoropolymercompositions. Certain disclosures also relate to phthalonitrile resin formulations; however,such disclosures are mainly directed towards structural or load-bearing applications anddo not address corrosion protection of metallic substrates under acidic conditions.

[0006] Additionally, existing coating technologies often rely on complex chemistries,catalysts, fillers, or multi-step processing methods, which increase manufacturingcomplexity and limit scalability.(Progress in Organic Coatings 167, 106861 (2022), . Ind.Eng. Chem. Res. 60, 2178-2186 (2021), and ACS Symposium Series (ed. Gupta, R. K.)vol. 1452 1-20 (2023).

[0007] Therefore, there exists a need for an improved coating composition thatovercomes the drawbacks of existing coating systems, is simple in composition, scalable,and does not require catalysts or complex processing steps, while maintaining the superiorthermal and mechanical properties of phthalonitrile resins.OBJECTIVE OF THE INVENTION

[0008] Some of the objects of the present disclosure, which at least one embodimentherein satisfy, are listed herein below.

[0009] The primary objective of the present disclosure is to provide a coatingcomposition that addresses the above-mentioned problems.

[0010] Another objective of the present disclosure is to provide a corrosion-resistantcoating composition comprising a phthalonitrile(PN) resin system; and a boron-containingadditive.

[0011] It is yet another objective of the present invention to provide corrosion-resistantcoating composition that exhibit improved resistance to acid-induced degradationcompared to unmodified PN coatings.

[0012] It is yet another objective of the present invention to provide corrosion-resistantcoating composition that improve the durability and barrier performance of PN coatingsagainst acidic media, such as 30% nitric acid, thereby reducing substrate degradation,coating failure, and mass loss.

[0013] It is yet another objective of the present invention to provide a process forpreparation of a corrosion-resistant coating composition.

[0014] It is yet another objective of the present invention to provide a process forpreparation of a corrosion-resistant coating composition that is simple, reproducible, andscalable.

[0015] Still another objective of present invention is to provide a dip-coatingmethodology, including multilayer application and controlled thermal curing cycles, forproducing uniform, adherent, and chemically stable PN-based coatings on metallicsubstrates.

[0016] Still another objective of present invention is to provide a cost-effective,catalyst-free, and industrially viable fabrication route suitable for large-scale adoption insectors such as aerospace, marine, and chemical processing industries.

[0017] These and other objectives and advantages of the present subject matter will beapparent to a person skilled in the art after consideration of the following detaileddescription taking into consideration accompanying drawings in which preferredembodiments of the present subject matter are illustrated.SUMMARY OF THE INVENTION

[0018] In an aspect, the present disclosure relates to a corrosion-resistant coatingcomposition comprising:a phthalonitrile resin system; anda boron-containing additive;wherein the boron-containing additive is in an amount ranging from 1 to 5 wt.% basedon the total weight of phthalonitrile resin system.

[0019] In another aspect the present invention relates to a process for preparing acorrosion-resistant coating composition, comprising the steps of:a) mechanically blending 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile to obtain a phthalonitrile resin system;b) adding a boron-containing additive to the phthalonitrile resin system andblending for 10-15 minutes to obtain the coating composition.

[0020] In another aspect the present invention relates to coated article comprising ametallic substrate coated with the corrosion-resistant coating composition.BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0021] The illustrated embodiments of the subject matter will be best understood byreference to the drawings. The following description is intended only by way of example,and simply illustrates certain selected embodiments of composite and processes that areconsistent with the subject matter as claimed herein, wherein:

[0022] Figure 1 is schematic illustration of the overall process flow, showingpreparation of the boron-integrated phthalonitrile resin, dip-coating onto a metallicsubstrate, multi-stage thermal curing, and subsequent corrosion testing in aggressiveacidic media.

[0023] Figure 2 illustrates (FT IR spectra of cured phthalonitrile resin pellets (PNB-0,PNB-1, PNB-5) and corresponding coatings on aluminium substrates (AIPNB-nx5, n=0,1, 5), demonstrating preservation of the PN backbone and the emergence of B-O relatedbands upon boron integration.

[0024] Figure 3 illustrates UV-DRS spectra of coated and cured phthalonitrile resincoatings on aluminium substrates (AIPNBx-5, x=0,1,5), demonstrating preservation of thePN backbone and the emergence of B-O-related bands upon boron integration.

[0025] Figure 4 illustrates Tauc plots (direct and / or indirect) derived from UV-DRSdata for cured PNB-0, PNB-1, PNB-5 and their corresponding aluminium coatings,illustrating bandgap modification and electronic structure changes induced by boronintegration.

[0026] Figure 5 illustrates schematic representation of the preparation of neat andboron-doped phthalonitrile resin formulations, including mechanical blending of boricacid with PN components prior to solution preparation.

[0027] Figure 6 illustrates schematic or photographic sequence depicting dip-coatingof aluminium substrates with the PNB formulations, drying at ambient conditions, andsubsequent multi-stage thermal curing to obtain adherent protective coatings.

[0028] Figure 7 illustrates Photographic images of uncoated, coated and curedaluminium substrates.

[0029] Figure 8 illustrates the nitric-acid immersion weight-loss data for barealuminium and aluminium coated with PNB-0, PNB-1 and PNB-5.DETAILED DESCRIPTION OF INVENTION

[0030] A detailed description of various exemplary embodiments of the disclosure isdescribed herein. It should be noted that the embodiments are described herein in suchdetail as to communicate the disclosure. However, the amount of details provided hereinis not intended to limit the anticipated variations of embodiments; on the contrary, theintention is to cover all modifications, equivalents, and alternatives falling within the spiritand scope of the present disclosure.

[0031] The terminology used herein is to describe particular embodiments only and isnot intended to be limiting to the invention. As used herein, the singular forms "a", "an"and "the" are intended to include the plural forms as well, unless the context indicatesotherwise. It will be further understood that the terms "comprises" and / or "comprising",or "includes" and / or "including" or "has" and / or "having" when used in this specificationspecify the presence of stated features, regions, integers, steps, operations, elements,and / or components, but do not preclude the presence or addition of one or more otherfeatures, regions, integers, steps, operations, elements, components, and / or groups thereof.

[0032] Unless otherwise defined, all terms (including technical and scientific terms)used herein have the same meaning as commonly understood by one of ordinary skill inthe art to which example embodiments belong. It will be further understood that terms,e.g., those defined in commonly used dictionaries, should be interpreted as having ameaning that is consistent with their meaning in the context of the relevant art and will notbe interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0033] The term "further" is used in the embodiments and claims of the presentapplication. The said term is a well-accepted term to narrow down any principal feature.Therefore, the person skilled in the art would clearly understand the scope of the said termin the context of the present disclosure.

[0034] The present disclosure relates to a corrosion-resistant coating compositioncomprising:a phthalonitrile resin system; anda boron-containing additive;wherein the boron-containing additive is in an amount ranging from 1 to 5 wt.% based onthe total weight of phthalonitrile(PN) resin system.

[0035] In an embodiment of the present invention, phthalonitrile resin systemcomprises 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile.

[0036] In an embodiment of the present invention, the boron-containing additive isboric acid.

[0037] In an embodiment of the present invention, 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile is 100:6 partsby weight.

[0038] In an embodiment of the present invention, Diphthalonitrile (100 parts):provides backbone rigidity and thermal stability. Aminophthalonitrile (6 parts): Primaryamines (-NH2) trigger crosslinking during thermal curing by attacking nitrile groups(C-N). Boric acid [B(OH)3]: White crystalline powder; becomes dispersed throughout PNmatrix; B-O species form during curing.

[0039] The present disclosure relates to a process for preparing a corrosion-resistantcoating composition, comprising the steps of:a) mechanically blending 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile to obtain a phthalonitrile resin system;b) adding a boron-containing additive to the phthalonitrile resin system andblending for 10-15 minutes to obtain the coating composition.

[0040] In an embodiment of the present invention, a molar ratio of 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile is in the rangeof 10:1 to 11:1 (equivalent to 100:6 to 95:5 parts by weight).

[0041] In an embodiment of the present invention, the boron-containing additive is inthe range of 1.5 - 10 mmol per 10-12g of the phthalonitrile resin system, preferably 1.7 to8.6 mmol corresponding to approximately 1-5 wt.% of total solids.

[0042] Around 1.5 - 2.5 g of PBN-n (n=0,1,5) is dissolved in 4-6 mL of a polar organicsolvent (acetone, DMF, or NMP) under gentle heating to obtain a saturated coatingsolution and then applied to the metallic substrate via dip-coating followed by multi-stagecuring at 160°C, 200°C, and 250°C.

[0043] In an embodiment of the present invention, the boron-containing additive isboric acid.

[0044] In an embodiment of the present invention, PN components (100:6 ratio byweight) and boric acid were added into an agate mortar and pestle to grind with firm,circular motions. The process was continued for 10-15 minutes, periodically scraping thewalls to center. Grounded until a uniform fine powder with no clumping and confirmedvisually for the homogenous powder mixture.

[0045] This grinding-based route is solvent-free at the blending stage, rapid (tens ofminutes rather than hours to days), and readily scalable from gram to kilogram quantities,making it attractive for both laboratory and pilot-scale production. Importantly, theprimary PN backbone chemistry is preserved while boric acid is dispersed throughout thematrix, where it can generate B-O species during curing.

[0046] In an embodiment of the present invention, Figure 1 summarizes the overallworkflow starting from the integration of boron into the phthalonitrile monomer system,represented schematically as a boron-doped PN resin, where the boron species is depictedas a central black feature within the PN molecular framework. The resulting boron-integrated resin is subsequently dissolved in a suitable organic solvent to obtain a saturatedcoating solution, which is then applied onto a pre-cleaned metallic substrate using a dip-coating technique. The coated substrate, depicted as a grey cube with a beige surface, issubjected to multi-stage thermal curing in an oven to induce polymerization and cross-linking of the PN network. Upon curing, the substrate exhibits a characteristic greencoloration, attributed to the formation of highly conjugated heterocyclic structures such astriazine, polyisoindoline, and predominantly phthalocyanine moieties, with the green colorindicating enhanced phthalocyanine formation. Finally, the cured specimens are exposedto aggressive acidic media through immersion testing to evaluate the corrosion resistanceof the coated metal as a function of exposure time. Overall, the schematic highlights thatthe invention is implemented through a simple, catalyst-free, and scalable sequence ofmechanical blending, solution preparation, dip-coating, and thermal curing, without theneed for complex synthesis routes or specialized processing techniques.

[0047] The present disclosure relates to a coated article comprising a metallic substratecoated with the corrosion-resistant coating composition.

[0048] In an embodiment of the present invention, the metallic substrate is selectedfrom the group consisting of aluminium, stainless steel, cooper, titanium, or alloys.ADVANTAGES:

[0049] The present invention offers the following advantages, including:- Enhanced corrosion resistance: Provides superior protection to metallicsubstrates against aggressive acidic environments, including concentratednitric acid.- High-temperature stability: Retains the inherent thermal robustness ofphthalonitrile resins under elevated temperature conditions.- Simple boron incorporation: Achieves effective boron integrationthrough a catalyst-free, post-synthesis mechanical blending method.- Improved barrier performance: Forms dense, low-porosity coatings thatsignificantly limit acid and ion penetration.- Strong substrate adhesion: Produces uniform, adherent multilayercoatings with reduced risk of delamination.- Cost-effective processing: Utilizes inexpensive additives and simplefabrication steps compatible with industrial scale-up.- Tunability of properties: Allows easy adjustment of corrosion resistanceby varying boron content and coating layers.- Wide industrial applicability: Suitable for aerospace, chemicalprocessing, energy, and marine applications requiring combined thermaland corrosion resistance.

[0050] The invention is further illustrated by the following example, which is providedto be exemplary of the invention and does not limit the scope of the invention. While thepresent invention has been described in terms of its specific embodiments, certainmodifications and equivalents will be apparent to those skilled in the art and are intendedto be included within the scope of the present invention.EXAMPLES

[0051] Example 1: Process for corrosion-resistant coating composition

[0052] Referring to figure 5 is provided a schematic illustration of the formulationprocess. 10.0 g (27.59 mmol) of 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 0.6 g(2.55 mmol) of 4-(4-aminophenoxy)phthalonitrile, corresponding to a 100:6 parts byweight (10:1 molar) ratio were mechanically blended to yield PNB-0 (10.6 g total). Threeidentical PNB-0 batches (10.6 g each) were prepared: one retained as PNB-0, one used forPNB-1, and one for PNB-5. For boron integration, one batch of PNB-0 was combined with0.1 g (1 wt.%, 1.71 mmol) and the other batch, 0.5 g (5 wt.%, 8.57 mmol) boric acid andground 10-15 min to produce PNB-1 and PNB-5, respectively. The schematic highlightsthat boron is introduced via a simple post-synthesis mechanical blending route rather thancovalent modification, enabling easy tuning of boron content while preserving the basePN chemistry and making the process suitable for scale-up.

[0053] Table 1: Different coating compositionExample 2: FT-IR spectra of resin and the coated and cured substrates.

[0054] Bruker Alpha II in ATR mode (Neat) under room temperature was used foranalysis. Figure 2 (left) is the Neat FT-IR spectra of the cured resin and Figure 2 (right)is the Neat FT-IR spectra of the coated and cured substrate. The spectra was measuredbetween 500 cm-1 and 4000 cm-1 wavenumber as X-axis and transmittance (%) as Y-axis. Neat PNB-0, PNB-1 and PNB-5 resins were cured in pellet form and substratescoated with PNB-0, PNB-1 and PNB-5 by 5 dip-cycles were similarly analyzed tomonitor the coating chemistry on metal.

[0055] In the cured resin, the disappearance or significant reduction of nitrilestretching bands and the emergence of characteristic absorption features associated withheterocyclic structures such as triazine, polyisoindoline, and phthalocyanine confirmsuccessful curing and network formation of the phthalonitrile system. For the coatedaluminium substrates, the spectra retain the key fingerprint features of the PN backbone,indicating that the coating chemistry is preserved upon deposition and curing on themetal surface. Additionally, an increase in absorption intensity in the fingerprint regionassociated with B-O related vibrations is observed for the boron-integratedformulations, suggesting the presence of boron-containing species within the coating.While FT-IR analysis of the fingerprint region provides only preliminary chemicalconfirmation and cannot be solely relied upon for definitive structural assignment, itoffers useful evidence for curing progression and compositional consistency.

[0056] FT-IR confirms that boric acid is successfully incorporated into thephthalonitrile network without disrupting the fundamental PN structure, and that multi-dip coatings build up uniform, chemically consistent layers on aluminium.Example 3: UV-DRS spectra of the coated and cured substrates.

[0057] Instrument used: SHIMADZU UV-3600 UV-VIS-NIR Spectrophotometerunder room temperature.

[0058] Figure 3 (left) is the absorbance spectra and Figure 3 (right) is the reflectancespectra of the uncoated substrate and coated substrate with PNB-0, PNB-1 and PNB-5(5 dip cycles, fully cured), measured in diffuse reflectance mode.

[0059] Bare aluminium exhibits very low absorbance across the UV-visible region,as expected for a smooth metallic surface with minimal interaction with incident light.In contrast, all coated samples show a clear increase in absorbance, confirming that thephthalonitrile layer introduces strong UV absorption associated with the aromatic PNnetwork. In this context, the apparent optical density is directly determined from theabsorbance data using the usual relationship OD = A = log10(10 / I), so higherabsorbance values translate directly into higher optical density. All coated samples showa clear increase in apparent optical density, confirming that the phthalonitrile layerintroduces strong UV absorption associated with the aromatic PN network. Within thecoated series, the boron-modified films (PNB-1 and PNB-5) absorb more strongly thanthe boron-free PNB-0, following a dose-dependent trend (PNB-0 < PNB-1 < PNB-5),with the optical density rising by roughly 1.3-fold for PNB-1 and 1.6-fold for PNB-5relative to PNB-0. This behaviour is consistent with additional light-absorbing centresand electronic modification arising from boron-oxygen coordination within the PNmatrix.

[0060] Reflectance spectra: In terms of reflectance, bare aluminium shows highvalues across the UV-visible region, with around 60% reflectance at ~500 nm,characteristic of a reflective metallic surface. All coated substrates display markedlyreduced reflectance, indicating effective coverage by the phthalonitrile-based films andreduced direct exposure of the metal. At ~500 nm, the reflectance drops to about 18%,12% and 8% for PNB-0, PNB-1 and PNB-5, respectively, corresponding to an overallreduction of up to ~87% at the highest boron loading. This strong decrease inreflectance, together with the higher optical density of the boron-containing coatings, isconsistent with denser, less porous layers that limit diffusion pathways for corrosivespecies, supporting the interpretation that boron-modified PN coatings provide a moreeffective protective barrier than the unmodified PN coating.

[0061] UV-DRS demonstrates that the boron-modified PN coatings form continuous,optically dense layers on aluminium, supporting the presence of a more effective barriercompared to the unmodified PN coating.Example 4: Tauc analysis

[0062] Figure 4 is the UV-DRS data for the coated and cured metal substrates (PNB-0,PNB-1 and PNB-5; for 5-dip) transformed using Kubelka-Munk functions to obtainF(R) vs. wavelength and converted to energy (eV) for Tauc analysis (bandgapestimation). The Tauc plot was plotted for the coated and cured substrate (PNB-0, PNB-1, PNB-5), 5 dips using direct (F(R)hv)22 and indirect transition models (F(R)hv)1 / 2.The linear regions of the plots were used to estimate the optical bandgaps.

[0063] For the indirect transition model, the extracted indirect bandgap values fall inthe range of approximately 1.4-1.5 eV. A clear shift of the absorption edge towardhigher photon energies is observed with increasing boron content, progressing fromPNB-0 to PNB-1 and further to PNB-5. This trend reflects a widening of the indirectbandgap resulting directly from the incorporation of boron into the phthalonitrilepolymer matrix.

[0064] The direct bandgap Tauc plots, which describe allowed direct electronictransitions typically associated with stronger optical absorption, exhibit a much steeperabsorption onset at higher photon energies. The corresponding direct bandgap valuesare significantly larger than the indirect bandgaps, occurring in the range ofapproximately 3.1-3.3 eV. Compared to the boron-free coating, the boron-containingcoatings show increased absorption intensity and a measurable shift in the directtransition edge, indicating modification of the high-energy electronic states within thecured, crosslinked polymer network.

[0065] The boron-modified coatings indicate a denser and more strongly interactingpolymer structure. This densification reduces the number of diffusion pathwaysavailable for corrosive species, limits electrolyte ingress to the metal-coating interface,and thereby contributes to the improved corrosion resistance of the boron-containingcoatings.

[0066] The Tauc analysis supports that boron integration alters the electronicenvironment of the PN matrix, consistent with a denser, more strongly interactingnetwork that contributes to improved barrier performance under aggressive conditions.Example 5: Dip-coating and curing sequence

[0067] Preparation of dip coating

[0068] 2 g of powdered coating composition was dissolved in 5 mL of acetone, gentlyheated to obtain a saturated solution, filtered to remove any undissolved residue, andused for dip-coating.

[0069] Leica S9 D stereozoom microscope equipped with a Leica MC170 HD cameraand KL 300 LED was used to examine the coating. Figure 6 represents a dip-coatingand curing sequence. A simple manual dip-coated substrate with a hot-air oven forstaged curing. Commercially available aluminum sheets from Merck TLC plates weredesilicated and utilized as metallic substrates. Metal substrates (~0.2 mm thickness)were mechanically polished, degreased, and cleaned prior to coating. The preppedsubstrates were immersed into the PNB coating solution and withdrawn at controlledspeed to form a thin liquid film, then dried at room temperature overnight to removesolvent; this cycle was repeated (e.g., 5 dips) to build up coating thickness. Dried coatedsubstrates were subjected to multi-stage thermal curing, for example initial hold at 160°C, followed by higher-temperature holds at 200 °C and 250 °C to complete crosslinkingof the PN resin. Cooling rate - Natural or 2-5°C / min- Controlled cooling reduces stressat Air atmosphere (allows oxidative stabilization). Firmly adhered. color progression(golden tan -> dark green) provides instant visual confirmation of successful processing,eliminating expensive testing.

[0070] The sequence shown in Figure 6 illustrates that a uniform, adherent, multi-layerboron-integrated PN coating can be produced on aluminium using standard dip-coatingand oven curing equipment, confirming the practicality of the disclosed fabricationroute.Example 6: Photographic images of uncoated and coated aluminium substrates

[0071] Figure 7 illustrates the photographic images of the coated and cured substratebefore and after acid immersion. Mobile camera was used to record the images of coatedand uncoated metal substrates before and after immersion in nitric acid. Photographicimages of uncoated and coated aluminium substrates (PNB 0, PNB 1, PNB 5; five dipcycles) before and after immersion in nitric acid, showing visibly reduced surface attackand better coating retention for boron integrated systems.

[0072] Uncoated and coated metal substrate with PNB-0, PNB-1 and PNB-5 (5 dipcycles, fully cured) were immersed in 30% nitric acid solution for up to 48hrs, thenremoved, rinsed and dried prior to imaging.Example 7: Acid immersion weight-loss data versus exposure time for uncoatedaluminium and aluminium coated

[0073] Figure 8 shows the nitric-acid immersion weight-loss data for bare aluminiumand aluminium coated with PNB-0, PNB-1 and PNB-5 (five dip cycles), with massesmeasured on an analytical balance and the data plotted in OriginPro 8.5. Over 48 h in30% HNO3 at room temperature, all coated specimens lose less mass than the uncoatedaluminium, confirming that the phthalonitrile layer provides a measurable barrieragainst acid attack. Among the coated samples, the boron-containing formulations(PNB-1 and PNB-5) show consistently lower total weight loss than the unmodifiedPNB-0, demonstrating that boron integration further enhances corrosion resistanceunder these aggressive conditions.

[0074] The time-dependent curves also reveal differences in degradation behaviour.The PNB-0 coating exhibits a two-stage profile in which the rate of mass loss increasesnoticeably in the later part of the test, consistent with partial barrier breakdown andsubsequent rapid substrate attack. In contrast, the boron-modified coatings display moregradual, controlled mass-loss trajectories, with no sharp late-stage acceleration,indicating that the barrier remains more effective throughout the 48-h exposure. Thedistinction between PNB-1 and PNB-5 suggests a boron-loading effect on performance,implying that the boron content can be tuned to balance coating robustness andresistance to extended acid immersion. Overall, Figure 8 provides quantitative supportfor the conclusion that boron-integrated PN coatings significantly improveacid-corrosion resistance compared with both bare aluminium and unmodified PNcoatings.

[0075] It should be noted that the description and figures merely illustrate theprinciples of the present subject matter. It should be appreciated by those skilled in theart that conception and specific embodiment disclosed may be readily utilized as a basisfor modifying or designing other structures for carrying out the same purposes of thepresent subject matter. It should also be appreciated by those skilled in the art bydevising various systems that, although not explicitly described or shown herein,embody the principles of the present subject matter and are included within its spirit andscope. Furthermore, all examples recited herein are principally intended expressly to befor pedagogical purposes to aid the reader in understanding the principles of the presentsubject matter and the concepts contributed by the inventor(s) to further the art and areto be construed as being without limitation to such specifically recited examples andconditions. The novel features which are believed to be characteristic of the presentsubject matter, both as to its organization and method of operation, together with furtherobjects and advantages will be better understood from the following description whenconsidered in connection with the accompanying figures.

[0076] It will be further appreciated that functions or structures of a plurality ofcomponents or steps may be combined into a single component or step, or the functionsor structures of one step or component may be split among plural steps or components.The present invention contemplates all of these combinations. Unless stated otherwise,dimensions and geometries of the various structures depicted herein are not intended tobe restrictive of the invention, and other dimensions or geometries are possible. Inaddition, while a feature of the present invention may have been described in the contextof only one of the illustrated embodiments, such feature may be combined with one ormore other features of other embodiments, for any given application. It will also beappreciated from the above that the fabrication of the unique structures herein and theoperation thereof also constitute methods in accordance with the present invention. Thepresent invention also encompasses intermediate and end products resulting from thepractice of the methods herein.

Claims

1. A corrosion-resistant coating composition comprising: a phthalonitrile resin system; and a boron-containing additive; wherein the boron-containing additive is in an amount ranging from 1 to 5 wt.% based on the total weight of phthalonitrile resin system.

2. The coating composition as claimed in claim 1, wherein phthalonitrile resin system comprises 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4- aminophenoxy)phthalonitrile.

3. The coating composition as claimed in claim 1 , wherein the boron-containing additive is boric acid.

4. The coating composition as claimed in claim 2, wherein 4,4'-(1,3- phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile are 100:6 parts by weight.

5. A process for preparing a corrosion-resistant coating composition as claimed in claim 1, comprising the steps of: a) mechanically blending 4,4'-(1,3-phenylenebis(oxy))diphthalonitrile and 4-(4- aminophenoxy)phthalonitrile to obtain a phthalonitrile resin system; b) adding a boron-containing additive to the phthalonitrile resin system and blending for 10-15 minutes to obtain the coating composition.

6. The process as claimed in claim 5, wherein a molar ratio of 4,4'-(1,3- phenylenebis(oxy))diphthalonitrile and 4-(4-aminophenoxy)phthalonitrile is 10:1.

7. The process as claimed in claim 5, wherein the boron-containing additive is in the range of 1.7 to 8.6 mmol.

8. The process as claimed in claim 5, wherein the boron-containing additive is boric acid.

9. A coated article comprising a metallic substrate coated with the corrosion-resistant coating composition as claimed in 1.

10. The coated article as claimed in claim 9, wherein the metallic substrate is selected from the group consisting of aluminium, stainless steel, copper, titanium, or alloys.