A coating composition

Abstract

Disclosed is a coating composition for forming a free radical scavenging coating on a substrate. The coating composition can comprise a matrix-forming polymer and an organic free radical scavenger that is reactable with hydrogen radicals. The coating composition can form a polymeric organic radical scavenging coating when applied to a metallic substrate surface to at least partly prevent free radical contact with the substrate surface.

Description

[0001] A COATING COMPOSITION

[0002] Field of Invention

[0003] The present invention relates to a coating composition for forming a free radical scavenging coating on a substrate.

[0004] The present invention relates particularly, although by no means exclusively, to a coating composition for providing an organic free radical scavenging coating on a substrate surface to reduce hydrogen embrittlement of the substrate.

[0005] The present invention also relates to a method of forming a coated substrate that is capable of scavenging free radicals contacting the coated substrate surface.

[0006] The present invention relates particularly, although by no means exclusively, to a method of forming a coated substrate that is capable of scavenging free radicals contacting the coated substrate surface to reduce hydrogen embrittlement of the substrate.

[0007] Embrittlement caused by contact with hydrogen radicals is a generally recognised problem faced by infrastructure constructed from steel that are exposed to a hydrogen-rich environment. Hydrogen embrittlement compromises the structural strength of the steel structure, making it susceptible to cracking and fracturing under stress, thereby limiting its practical utility for transportation and storage of hydrogen and blends of hydrogen and natural gas.

[0008] This problem becomes more significant as society transitions to a hydrogen economy due to most natural gas infrastructures being constructed of steel. As a consequence, one method of managing embrittlement is to carefully control the amount of hydrogen in the fluid that would be delivered or contact the steel structures, which are typically in the form of storage vessels and pipes.

[0009] Hydrogen embrittlement in steel is believed to initiate at surface imperfections and fractures of the steel substrate. It is also understood that hydrogen molecules dissociate into hydrogen radicals at these sites before permeating the substrate. These atomic hydrogen species then dissolve into the steel, diffusing from areas of high pressure to low pressure. As hydrogen accumulates within the steel, it detrimentally affects its mechanical properties, including ductility, fracture toughness, and resistance to fatigue. Addressing the challenge of hydrogen embrittlement is vital for ensuring the safe and efficient utilization of steel infrastructure such as pipelines for hydrogen transport and storage.

[0010] To mitigate this issue, various methods have been explored, including the formation of a metallic coating on the steel substrate to shield the steel substrate from the hydrogen. Such a coating can be formed using electroplating. Another approach is to construct the structures using a different type of steel. However, these methods are not suitable to treat existing steel infrastructure, particularly underground steel pipelines, which are often immobile.

[0011] Effectively addressing the challenge of hydrogen embrittlement is crucial for ensuring the safe and reliable operation of steel pipelines and other metallic infrastructure in the emerging hydrogen economy. Innovative coating solutions that mitigate hydrogen embrittlement of steel surfaces are also expected to facilitate the widespread adoption of hydrogen as a clean energy carrier.

[0012] Summary of Invention

[0013] In response to the previously mentioned challenges, the Applicant has investigated alternative approaches to mitigate hydrogen embrittlement. One promising avenue is the development of functionalised coatings for the internal surface of metallic structures that can scavenge hydrogen atoms to reduce radical diffusion or propagation through the steel. These coatings aim to convert hydrogen radicals contacting the coating surface into a non-destructive form to impede diffusion into the steel substrate. One expected benefit of this approach over traditional gaseous barrier coatings is to minimise the effect of physical damage to the performance of the coating. A further benefit of the present invention is that the coating can be manufactured from a thin polymer layer, which would be easier and cheaper to apply to existing infrastructure than a metallic barrier coating. In comparison, gaseous barrier coatings often require a very thick polymer layer to be effective e.g. greater than 1 mm in thickness.

[0014] The present invention may be complementary to existing gaseous barrier coatings and can augment protection afforded by these barrier coatings against hydrogen embrittlement. The effectiveness of functionalised barrier coatings in reducing hydrogen embrittlement depends on various factors, including the type of coating material, its application method, and the environmental conditions to which the coated steel is exposed. The Applicant is continuing research in this area to explore novel coating formulations and application techniques to optimize the performance of such coatings to enhance the durability of steel infrastructure in a hydrogen-rich environment. One such coating formulation is disclosed in International application PCT / AU2023 / 051220, which is wholly incorporated herein by cross-reference. The Applicant’s research is also directed towards a solution that can be applied to existing metallic infrastructure.

[0015] In this respect, the present invention provides a coating composition for forming an organic radical scavenging coating on any suitable substrate surface that would typically experience embrittlement when exposed to hydrogen radicals.

[0016] Suitably, the hydrogen radical scavenging coating is applied to a metallic substrate surface to decrease the amount of hydrogen radicals accumulating in the underlying metallic substrate. The hydrogen radical scavenging coating thereby advantageously provides protection from the effects of hydrogen radical degradation of the underlying substrate on which the coating is applied. The hydrogen radical scavenging coating is particularly adapted to reducing hydrogen embrittlement of steel, including high strength steel and high manganese content steel. However, it is equally effective for reducing embrittlement of other metallic surfaces including aluminium, magnesium, nickel, titanium, cobalt, and their alloys.

[0017] A first aspect of the present invention provides a coating composition for forming a polymeric organic radical scavenging coating when applied to a metallic substrate surface, wherein the composition comprises an organic free radical scavenger that is reactable with hydrogen radicals.

[0018] In this specification, the term “polymeric organic radical scavenging coating” refers to a coating that comprises at least one polymeric component, and an organic scavenger that is reactable with free radicals, wherein the organic scavenger may be the polymeric component.

[0019] In this specification, the term “organic free radical scavenger” refers to an organic scavenger that is reactable with free radicals, the free radicals including hydrogen radicals. It is believed that damage to steel induced by hydrogen radicals contributes to hydrogen embrittlement of steel infrastructure. Therefore, a free radical scavenging coating in accordance with the present invention when applied to steel, desirably provides protection to the steel from hydrogen radical degradation by scavenging the hydrogen radicals.

[0020] The organic free radical scavenger is a compound that participates in a single-electron reaction with free radicals including hydrogen free radicals. The organic free radical scavenger advantageously converts free radicals into less reactive chemical species.

[0021] The coating prevents or reduces hydrogen embrittlement of the substrate, particularly due to exposure of the substrate to hydrogen radicals in a hydrogen gas-containing environment.

[0022] It is believed that damage to steel induced by hydrogen radicals contributes to hydrogen embrittlement of steel infrastructure. Therefore, a free radical scavenging coating in accordance with the present invention when applied to steel, desirably provides protection to the steel from hydrogen radical degradation by scavenging the hydrogen radicals.

[0023] The organic free radical scavenger may be radical bearing. In this embodiment, the organic free radical scavenger itself may comprises a free radical (i.e. it is “radical bearing”), which can scavenge hydrogen radicals by radical -radical coupling. Suitable examples of such scavengers are melanin / eumelanin materials including PDA. Such radical-bearing scavengers react with a hydrogen radical without resulting in the generation of a further product radical. This is opposed to a non-radical-bearing scavenger that react with a hydrogen radical to generate a less reactive product radical (e.g. BHT, or triarylphosphites).

[0024] The organic free radical scavenger may react with other free radicals such as those formed from oxygen and nitrogen.

[0025] Suitable organic free radical scavengers include butylated hydroxytoluene (BHT), polydopamine (PDA), (eu)melanin, phenols, catechols, polyphenols such as tannic acid and flavonoids, triaiylphosphit.es, sterically hindered phenols and 2, 5-di-terA-butyl hydroquinone. The organic free radical scavenger may be a polymer. A suitable polymeric organic free radical scavenger is PDA.

[0026] PDA is a synthetic melanin comprising a heterogenous mix of catechol and hydroquinone functional groups. PDA advantageously displays excellent radical scavenging ability toward oxygen-, carbon- and nitrogen-centered radical species. The reaction between PDA and free radicals is believed to occur through electron transfer between PDA and the free radicals, hydrogen abstraction from PDA by the free radicals, and / or through radical-radical coupling between PDA and the free radicals. Those skilled in the art would recognize that a radical being centered on a particular element, such as oxygen or carbon, means that the electron density of the unpaired electron constituting the radical is located within that element’s electron shell. Free radicals centered on different elements, comprise different free energy levels, and thereby different reactivity. Therefore, the reactivity of a free radical differs whether it is centered on carbon, oxygen, or otherwise. For this reason, free radical scavengers that are reactive with free radicals centred on a particular element, are not necessarily reactive with free radicals centred on a different element. As a corollary, antioxidants and other known free-radical scavengers in the art may be suitable as scavengers of oxygen-centred radicals, but their reactivity with hydrogen-centred radicals has been mostly unexplored.

[0027] Organic free radical scavengers, such as phenolic compounds, are known scavengers of oxygen and carbon-centre radicals in the art. In one example, PDA is known to have broad application in biomedical areas as an antioxidant due to its capability to rapidly scavenge various free radicals such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) present in cells and tissues. However, the inventors have surprisingly and advantageously discovered that PDA is particularly adapted to scavenge hydrogen-centered radicals.

[0028] The organic free radical scavenger may be in a solubilised, polymeric, or particulate form, or a mixture thereof. When the organic free radical scavenger is PDA, the PDA may be in a polymeric or particulate form, or a mixture thereof.

[0029] Whist the present invention can function with the organic free radical scavenger, suitably PDA, in either polymeric or particulate form, there are benefits of having the organic free radical scavenger in particulate form. A coating composition containing organic free radical scavenger-functionalised particles, for example PDA particles, can be formed more readily than a coating composition formed solely from non-particulate PDA which can take several days to form within an aqueous alkaline solution. The use of PDA particles also allows a range of polymer matrices to be used that are already in commercial use as coatings and linings of steel for corrosion protection or to reduce frictional loss, such as epoxy and polyurethane coatings.

[0030] The organic free radical scavenger-containing particles may have an average diameter of less than 1 cm. Preferably, the organic free radical scavenger-containing particles have an average diameter ranging from 100 nm to 1cm. More preferably, the organic free radical scavengercontaining particles have an average diameter ranging from 100 to 1000 nm.

[0031] The composition may comprise an organic free radical scavenger having a minimum concentration of IxlO'6mol PDA or 0.0148 g PDA per mol of hydrogen gas(H2) occupying any section of a hydrogen gas delivery conduit such as a pipeline or hydrogen gas storage vessel at a given time. The composition may comprise up to and including 100% PDA.

[0032] The organic free radical scavenger may be in a polymeric form. The organic free radical scavenger in a polymeric form may itself provide a coating or film over a substrate. In some embodiments, a coating of polymeric PDA may be applied over a substrate surface as a film.

[0033] The composition may comprise a matrix-forming polymer that is capable of forming a matrix when applied to a substrate surface. The organic free radical scavenger may be physically or chemically dispersed through the matrix-forming polymer, or a mixture of both.

[0034] The matrix-forming polymer may be a linear, branched, or crosslinked polymer.

[0035] When the matrix-forming polymer is a linear polymer, the matrix-forming polymer may have a number average molecular weight (Mn) less than 500,000 g / mol, or less than 1,000,000 g / mol. Suitably, the matrix-forming polymer has a number average molecular weight (Mn) ranging from 30,000 g / mol to about 500,000 g / mol. When the polymer is a branched or crosslinked polymer, the Mn may be greater than 500,000 g / mol. The matrix forming polymer may be water-soluble or soluble in an organic solvent. Suitable examples of matrix-forming polymer include polyimides, poly(vinyl chloride), poly(vinyl alcohol), polyurethane and epoxy-group containing polymers.

[0036] The matrix forming polymer may be applied to the substrate surface without being solubilised or dispersed in a carrier (e.g. solvent). For example, polyurethane and epoxycontaining polymer coatings are known in the art to be typically applied to a surface without a solvent. The matrix-forming polymer may otherwise be solubilised, dispersed or diluted in a carrier prior to application to a surface. A carrier may provide improved mechanical properties to the matrix-forming polymer to facilitate application as a coating.

[0037] The matrix-forming polymer may form a hydrogen-barrier coating when applied to a substrate surface. A suitable polymer for imparting hydrogen-barrier properties to the coating is poly(vinyl alcohol). Beneficially, a coating having hydrogen-barrier and hydrogen scavenging properties provides a two-pronged system for reducing hydrogen radical ingress into the substrate.

[0038] The organic free radical scavenger may be incorporated into the matrix-forming polymer. In one embodiment, the matrix-forming polymer is functionalised with the organic free radical scavenger. In another embodiment, the matrix-forming polymer is reactable with the organic free radical scavenger to form a chemical bond between the two entities.

[0039] The organic free radical scavenger may be blended with the matrix-forming polymer to form a homogenous or a heterogenous mixture. Forming a homogenous mixture may provide a coating wherein the organic free radical scavenger is evenly distributed throughout the coating. Forming a heterogenous mixture may provide a coating wherein the organic free radical scavenger is concentrated in certain regions of the coating, for example, being concentrated in proximity to the interface of coating with the substrate. By being concentrated at the substrate interface, the interaction of radicals in the substrate with the organic radical scavenger may be improved.

[0040] The composition may include a curing agent. The curing agent may be a substance that reacts with a matrix-forming polymer to solidify the polymer composition to form the organic radical scavenging coating. The curing agent may comprise a cross-linker. The cross-linker may be used to cross-link the matrix-forming polymer to form the organic radical scavenging coating.

[0041] The matrix-forming polymer may provide a medium to facilitate delivery of particles including the organic free radical scavenger to a substrate surface. The matrix-forming polymer may also be capable of forming a matrix containing a dispersion of the particles when applied to a substrate surface. In some embodiments, the matrix-forming polymer is capable of being cured after application to a substrate surface. This may form a sealed organic radical scavenging coating.

[0042] The composition may comprise a monomer that is polymerizable to form the matrix-forming polymer.

[0043] The composition may include an initiator to initiate polymerisation of the matrix-forming polymer. A skilled person would understand that any suitable initiators may be used such as UV-activated initiators and heat-activated initiators. In some embodiments, the coating is formed without polymerisation after application of the composition onto a substrate surface.

[0044] In some other embodiments, one or more coatings may be applied onto the organic radical scavenging coating, preferably in situ. In these embodiments, the organic radical scavenging coating serves as a primer layer for the subsequent coatings.

[0045] The coating composition can further include a carrier. A carrier may desirably improve application of the coating to a substrate surface. Suitable carriers include any fluids such as solvents capable of facilitating delivery of the organic free radical scavenger and any other components of the coating composition such as the matrix-forming polymer to a substrate surface. Suitably, the carrier is capable of reducing the viscosity of the coating composition. More suitably, the carrier is capable of solubilising the matrix-forming polymer. Suitable examples of substances that can be used as a carrier include acetone, ethyl acetate, methyl ether ketone, tetrahydrofuran, dichloromethane, water, and ethanol, or mixtures thereof. The coating composition can further include an anti-corrosive additive. This additive imparts a further layer of protection to the underlying substrate from chemical and or physical degradation.

[0046] For large scale infrastructure, and particularly for piping that is buried underground, specialised polymeric coatings and methods for their application onto the infrastructure are required. For such infrastructure, it is advantageous if the coating composition can be readily applied to the infrastructure without requiring relocating or repositioning the infrastructure to be coated. Preferably, the application process involves use of near-ambient temperature solutions with very limited heating. Advantageously, it is desirable that embodiments of the coating composition are formulated for being applied to the substrate surface using conventional means, for example, by brushing or spraying.

[0047] The substrate surface may be any surface that may be exposed to hydrogen. Suitable substrates include pipes and storage vessels. For example, the substrate surface can be part of an internal surface of a pipe through which hydrogen or a hydrogen-containing gas is to be transported or the internal surface of a storage vessel for holding hydrogen or a hydrogencontaining gas.

[0048] A second aspect of the present invention provides a coated substrate comprising an organic radical scavenging coating formed from the coating composition according to the first aspect of the present invention.

[0049] The organic radical scavenging coating may be applied directly to a substrate surface. This maximises its interactions with radicals formed at the metallic substrate surface.

[0050] The organic radical scavenging coating may have a thickness of less than 1 mm. Preferably, the organic radical scavenging coating has a thickness ranging from 10 and 500 pm, ranging from 10 and 200 pm, or ranging from 10 and 100 pm. More preferably the organic radical scavenging coating has a thickness of ranging from 20 and 50 pm, or ranging from 20 and 30 pm. When the organic radical scavenging coating comprises PDA without a further matrix forming polymer, the organic radical scavenging coating may be thinner, and may have a thickness ranging from 10 and 100 nm, or ranging from 100 and 1000 nm. The coated substrate may comprise multiple layers of the organic radical scavenging coating.

[0051] The coated substrate may comprise a primer layer comprising the organic radical scavenging coating. Preferably, the coated substrate includes a hydrogen-barrier coating located on the hydrogen radical scavenging coating.

[0052] A third aspect of the present invention provides a method of forming a coated substrate, the coated substrate comprising a polymeric coating including an organic free radical scavenger that is reactable with hydrogen radicals.

[0053] The coating composition may be in accordance with the first aspect of the present invention.

[0054] The coated substrate may be in accordance with the second aspect of the invention.

[0055] The method may include applying a coating composition, comprising an organic free radical scavenger that is reactable with a hydrogen radical, to a metallic substrate surface; and forming a polymeric organic radical scavenging coating from the coating composition on the metallic substrate surface, wherein the polymeric organic radical scavenging coating prevents or reduces diffusion of hydrogen radicals into the metallic substrate when the substrate is exposed to a hydrogen gas-containing environment.

[0056] The method may include applying a coating composition comprising an organic free radical scavenger in solubilised, polymeric or particulate form, or a mixture thereof to the substrate surface.

[0057] The method may include applying a coating composition comprising a matrix-forming polymer including the organic free radical scavenger to the substrate surface.

[0058] The method may include applying a coating composition comprising a matrix-forming polymer and a polymeric organic free radical scavenger to the substrate surface.

[0059] The method may include applying a coating composition comprising a matrix-forming polymer and particles including an organic free radical scavenger that is reactable with a hydrogen radical to the substrate surface. Preferably, the method includes forming an organic radical scavenging coating from the coating composition on the substrate surface, wherein the organic radical scavenging coating comprises a dispersion of the particles in a matrix formed from the matrix-forming polymer.

[0060] The method may include applying a coating composition comprising an organic free radical scavenger having a minimum concentration of IxlO'6mol PDA or 0.0148 g PDA per mol of hydrogen gas(H2) occupying any section of a hydrogen gas delivery conduit such as a pipeline or hydrogen gas storage vessel at a given time to the substrate surface.

[0061] The method may include calculating the minimum concentration of organic free radical scavenger required for the coating composition. Suitably, the method includes calculating the minimum concentration of organic free radical scavenger based on a given partial pressure of hydrogen contacting a metallic surface, combined with the following assumptions:

[0062] 1. It is known that 100 ppm of oxygen gas impurities blended in the hydrogen gas supply is sufficient to prevent hydrogen radicals causing embrittlement (Komoda, R., et al., Inhibitory effect of oxygen on hydrogen-induced fracture of A333 pipe steel. Fatigue & Fracture of Engineering Materials & Structures, 2019. 42(6): p. 1387-1401). One part of oxygen blocks one part of hydrogen dissociation sites on the metallic surface (e.g. steel). All oxygen impurities added would dissociate on the metallic surface to occupy dissociation sites. In the absence of oxygen, the same amount of hydrogen would dissociate on the metallic surface. Assuming an effective oxygen impurity concentration of 100 ppm, this means 100 ppm of hydrogen will dissociate if no oxygen is present, translating to a hydrogen dissociation percentage of x = 100 ppm = 0.01%.

[0063] 2. Calculation of the hydrogen radical scavenging stoichiometry of the scavenger. For example, BHT is understood to scavenger 1 mole of hydrogen radicals for each mole of BHT, therefore the stoichiometry is 1 : 1.

[0064] Where the organic free radical scavenger is PDA, the PDA is assumed to be a linear polymer with 100 repeating unit as illustrated below (Liebscher, J., et al., Structure of polydopamine: a never-ending story? Langmuir, 2013. 29(33): p. 10539-48):

[0065] For one PDA chain with 100 repeating unit, p=31, o=23, n=23 and m=23 and the molecular weight of PDA is MWPDA= 14,776 g / mol.

[0066] Hydrogen radicals can react with both catechol and quinone groups in PDA. Therefore, 1 mole of PDA with 100 repeating units can react with 200 moles of hydrogen radicals, equivalent to 100 moles of hydrogen molecules.

[0067] Hence, the amount of radical scavenger needed is equivalent to IxlO'6mol PDA or 0.0148 g PDA per mol of hydrogen gas (H2) in any section of pipeline.

[0068] As an illustrative example, if the pipeline operates at a pressure of 100 bar and a temperature of 25 °C and has a diameter of 1 meter, the amount of PDA required for coating one meter of the pipeline should be at least 47 g.

[0069] The method may include applying the coating composition directly onto the substrate surface.

[0070] The method may include applying the coating composition by brushing or spraying.

[0071] The method may include performing the application step at ambient temperature. Preferably, the application step is performed at a temperature ranging from 10-40 °C. More preferably, the application step is performed at a temperature ranging from 20-40 °C.

[0072] The method may include polymerising a monomer to form the matrix-forming polymer. Suitably, the method includes adding an initiator to initiate polymerisation of the matrixforming polymer.

[0073] The method may include curing the organic radical scavenging composition. Curing may allow the coating forming composition to be applied as a liquid, then solidified in situ to form the coating. Curing may be suitably achieved by techniques known in art, for example drying, or crosslinking. Suitably, the method includes curing the matrix-forming polymer. More suitably, the method includes adding a cross-linker to the coating composition to cross-link the matrix-forming polymer.

[0074] Depending on the form of the organic radical scavenger, the curing step may form an organic radical scavenging coating comprising particles including an organic free radical scavenger dispersed in a matrix or an organic radical scavenging coating including a cross-linked polymeric organic free radical scavenger.

[0075] The curing step may involve polymerising or crosslinking the matrix-forming polymer. Preferably, the curing step is performed after the coating composition is applied to the substrate surface.

[0076] The method may include forming one or more organic radical scavenging coatings on the substrate surface. Preferably, the method includes drying each organic radical scavenging coating before the next coating is formed. This enables a multi-layered coating with a desired thickness to be more easily achieved. It may also allow control over the distribution of the particles containing the organic radical scavenger in the multi-layered coating.

[0077] The method may include forming the organic radical scavenging coating as a primer layer to enable the deposition of further coatings on the primer layer. The further coatings may be functionalised to impart corrosion or hydrogen embrittlement resistance to the substrate.

[0078] The method may include applying one or more coatings onto the organic radical scavenging coating. Suitably, at least one of the coatings is a hydrogen-barrier coating.

[0079] The method may include adding a carrier to the coating composition. Suitably, the method includes adding a carrier to the coating composition such that the viscosity of the coating composition is sufficient to apply the coating composition on the substrate or the matrixforming polymer is solubilised.

[0080] The method may include adding an anti-corrosive additive to the coating composition.

[0081] An embodiment of the invention is hereinafter described by way of example only with reference to the accompanying figures, wherein:

[0082] Figure 1 provides high-resolution XPS spectra of C Is core levels of PDA (a. without plasma treatment; b. with 15 min plasma treatment).

[0083] Figure 2 provides percentage of C=O and C-OH / C-N in C Is core levels of PDA under different treatment times (0, 15, 30, 45, 60 min).

[0084] Figure 3 provides high-resolution XPS spectra of O Is core levels of PDA (a. without plasma exposure; b. with 15 min plasma exposure; c. with 45 min plasma).

[0085] Figure 4 provides percentage of C=O in O Is core levels of PDA under different treatment times (0, 15, 30, 45, 60 min).

[0086] Figure 5 is a cross-sectional view of a coated substrate according to one form of the present invention.

[0087] Figure 6 is a cross-sectional view of a coated substrate according to another form of the present invention.

[0088] Figure 7 provides high-resolution XPS spectra of C Is core levels of PDA particles (i. before plasma treatment; ii. after 60 min plasma treatment).

[0089] Detailed

[0090] In one example a homogeneous PDA-containing coating is formed as a matrix by reaction of the monomer dopamine hydrochloride in an alkaline solution and applied to a substrate (see Figure 5). This avoids the need to use a matrix-forming polymer in the coating composition to form the free radical scavenging coating.

[0091] As shown in Figure 5, the PDA containing coating 12 is located directly on a metallic substrate surface such as the internal surface 10 of a steel pipe. In this embodiment, the PDA- containing coating 12 serves as a primer layer to receive a hydrogen-barrier coating 16 which provides an additional layer of protection against hydrogen radical ingress into the steel pipe. It is preferred that the hydrogen-barrier coating 16 is thicker than the PDA-containing coating 12. Suitably, the hydrogen-barrier coating 16 is at least twice the thickness of the PDA - containing coating 12.

[0092] To demonstrate the ability of PDA in reacting with the hydrogen radicals, the Applicant used microwave assisted plasma chemical vapor deposition (MPCVD) to generate hydrogen radicals in the gas phase and to expose PDA films to this environment for different time periods. XPS analysis of the PDA films was then performed to determine the change in elemental composition of the PDA before and after plasma exposure for 60 min (see Table 1 below).

[0093] Table 1 presents the elemental composition of PDA films before and after plasma exposure for 15 min and 45 min. The percentages of carbon (C) and nitrogen (N) atoms increased after the treatment, while the oxygen (O) atom percentage decreased. The carbon in the PDA structure forms the polymer backbone, thus the number of C atoms should remain the same before and after the treatment. The atom percentage ratios of N / C also confirmed that the number of N atoms remained unchanged after treatment, while the number of O atoms halved. This indicates that oxygen is being evolved from the PDA, possibly as water molecules.

[0094] Table 1 Atom percentage of C, O, N in PDA films and O / C, N / C ratios before and after 15 min, 45 min plasma treatment, measured by XPS

[0095] Exposure _ Atom percentage (%) _ Percentage ratio _ time (min) C 0 N O / C N / C

[0096] 0 73.70±0.70 20.35±0.65 5.58±0.15 0.28 0.079

[0097] 15 81.15±0.95 12.35±1.15 6.50±0.20 0.15 0.080

[0098] 45 81.98±0.90 11.60±0.45 6.42±0.46 0.14 0.078

[0099] The high resolution XPS (see Figure 1) shows that compared with the C-C / CHXbonds, the intensity of C=O bonds and C-OH / C-N bonds decreased after hydrogen plasma exposure. The percentage of each bond were calculated based on the peak XPS areas and this data is illustrated in Figure 2 as a function of time. The C=O percentage decreased from 9% to 5% while the C- OH / C-N percentage decreased from 35% to 20% after 15 min of plasma treatment. These results indicate that the hydrogen radicals react with both the quinone groups and catechol groups in the PDA structure.

[0100] The O ls spectrum without plasma treatment (Figure 3a) exhibited two major contributions at 530.7 eV and 532.2 eV, which represent the oxygen bonds of C=O and C-OH respectively. The intensity ratio of C=O and C-OH bonds significantly decreased after exposure to hydrogen plasma for 15 min and showed a further decrease after exposure for 45 min (Figure 3b&c). The fraction of C=O bonds under different exposure times was also calculated from these intensity results (Figure 4).

[0101] Based on these XPS results, it can be concluded that PDA reacts with hydrogen radicals, and the scavenging reactions appear to occur with both the quinone and catechol groups in the PDA structure. The reaction between hydrogen radicals and catechol groups can generate water molecules, leading to a decrease in the O atom percentage after the reaction. The reaction between PDA and hydrogen radicals primarily occurs within the first 15 min under the conditions provided in the plasma reactor.

[0102] XPS analysis of PDA revealed obvious changes in the composition and concentration of functional groups after exposure to hydrogen plasma. The elemental composition revealed a clear shift in the atomic percentages, with a decrease in oxygen after plasma exposure, suggesting that the reaction primarily occurred within the quinone and catechol groups in the PDA structure. High-resolution XPS spectra further supported these findings, illustrating a decrease in C=O and C-OH / C-N bonds after plasma treatment, consistent with the observed changes in atomic percentages. The experimental data shows that hydrogen radicals interact with quinone and catechol groups.

[0103] Overall, the experimental data confirmed that PDA undergoes reactions with hydrogen radicals, and the radical scavenging ability is attributed to both quinone and catechol groups in the PDA structure.

[0104] To further expand on this work, the Applicant performed research and development on the use of particulate PDA as the organic radical scavenger. In this second example, a coating composition according to the present invention is prepared by synthesising PDA particles and mixing them with a matrix-forming polymer in the form of Matrimid® 5218 (Huntsman Advanced Materials America). A carrier in the form of dichloromethane may be added to the mixture to facilitate delivery of the coating composition to a substrate surface.

[0105] The suspension is then applied to a metallic substrate surface such as to the internal surface 10 of a steel pipe, by spraying. Having the suspension in a form that can be applied using conventional means enables the existing steel infrastructure to be treated without requiring them to be moved. This forms a coating comprising a dispersion of PDA particles 14 on the substrate surface.

[0106] The coating composition may be cured to form the final PDA particle-containing coating 12 having a thickness ranging from about 20-30 pm. The curing process forms a matrix that fixes the PDA particles in the coating and can involve polymerisation of the matrix-forming polymer. The coating composition may include a catalyst or a UV-activated initiator to trigger the polymerisation process. Other suitable curing methods include heating or drying. In some embodiments, the curing process triggers movement of the PDA particles towards the metallic substrate. An illustration of this embodiment wherein the coating is formed directly on the steel substrate surface is shown in Figure 6.

[0107] In this embodiment, the PDA particle-containing coating 12 serves as a primer layer to receive a hydrogen-barrier coating 16 which provides an additional layer of protection against hydrogen radical ingress into the steel pipe. It is preferred that the hydrogen-barrier coating 16 is thicker than the PDA particle-containing coating 12. Suitably, the hydrogen-barrier coating 16 is at least twice the thickness of the PDA particle-containing coating 12.

[0108] Once coated, the steel pipe can be used to transport hydrogen or hydrogen-containing gas. Hydrogen radicals formed within the pipe that contact the internal surface of the steel pipe react with the PDA particles. This quenches the radicals and prevents ingress of the hydrogen radicals into the substrate. The Applicant again used microwave assisted plasma chemical vapor deposition (MPCVD) to generate hydrogen radicals in the gas phase and to expose the PDA particle-containing coating to this environment for different time periods.

[0109] It was observed that there was an increase in the percentages of carbon (C) and nitrogen (N) atoms, accompanied by a decrease in the percentage of oxygen (O) atoms after the plasma treatment. The number of C atoms should remain the same before and after the treatment. The atom percentage ratios of N / C and O / C were calculated to investigate the change in N and O (Table 2). The number of N atoms remained unchanged after treatment, while the number of O atoms halved. This indicates that PDA particles, when integrated into an organic radical scavenging coating, are capable of reacting with hydrogen radicals.

[0110] Table 2: Atom percentage of C, O, N in PDA particles and O / C, N / C ratios before and after 60min plasma treatment, measured by XPS.

[0111] The C is core-level spectra of PDA particles before and after plasma treatment for 60min are presented in Figure 7. The spectral fitting yielded three distinct functional groups associated with carbon atoms: C-C / CHx (284.4 eV), C-OH / C-N (285.7 eV), and C=O (287.4 eV). It is assumed that the amount of C-C / CHx bonds remain consistent before and after the plasma treatment. The change in the amount of C-OH / C-N and C=O can be compared using their ratio with the C-C / CHx bond. The ratios were calculated based on the peak areas and presented in Table 3 below. Table 3: Bond ratio before and after 60 min plasma treatment as calculated from XPS peak areas.

[0112] Compared with the C-C / CHx bonds, the amount of C=O bonds and C-OH / C-N bonds decreased after hydrogen plasma exposure. This suggests that the hydrogen radical reaction occurs with the quinone and catechol groups in the PDA particles. The result also indicates that PDA particles incorporated into the organic radical scavenging coating can react with hydrogen radicals. This confirms that an approach of blending pre-formed PDA particles with a coating can be used to scavenge hydrogen radicals.

[0113] The Applicant’s work shows that PDA has the potential to serve as a pipeline coating to prevent hydrogen embrittlement by scavenging hydrogen atoms in the gas phase. The experimental results show that the hydrogen radicals react with both the quinone and catechol groups in the PDA.

[0114] It can be appreciated that the coating composition may include particulate and non-particulate PDA. In the first example, PDA forms the polymer matrix and in the second example, PDA particles are dispersed in the matrix. The formed coating may further include a hydrogenbarrier coating to enhance the substrate’s hydrogen resistance.

[0115] Materials and Methods

[0116] Materials

[0117] Dopamine hydrocholoride and Trizma base (> 99.9% titration) were purchased from Sigma- Aldrich (St. Louis, USA). Hydrochloric acid (HC1) solution (32 %) was purchased from UNIVAR (Victoria, Australia). Hydrogen gas (99.99 % purity) was supplied by Coregas Pty Ltd. All the chemicals were used without further purification.

[0118] Polydopamine film fabrication PDA film was fabricated onto a cellulose acetate (CA) supporting film by selfpolymerisation. Dopamine hydrochloride solution (2 g / L) was prepared by dissolving in 10 mM Tris-HCl buffer (pH 8.5) solution. The CA support was fixed on the bottom of a petri dish and immersed in the dopamine solution for 3 days at room temperature. The film was then left at ambient conditions to dry overnight before testing. The thickness of the dry PDA layer is 20 to 30 pm.

[0119] Synthesis of PDA particles

[0120] Dopamine hydrochloride (1 g) was introduced into 10 mM Tris-HCl buffer (pH 8.5, 500 mL) to form a 2 g / L solution. The solution was stirred for 24 h at room temperature in darkness. Subsequently, the solution was centrifuged for 20 min at 10,000 rpm to gather PDA particles. The PDA particles were then dispersed again into deionised (DI) water to remove the buffer solution and unreacted dopamine hydrochloride, followed by centrifugation to collect the particles. The rinsing process was repeated three times. After that, the PDA particles were dried under vacuum at room temperature. The yield of PDA particles was about 20% on the basis of the dopamine hydrochloride added.

[0121] Fabrication of PDA incorporated polymer films

[0122] Matrimid® 5218 (Huntsman Advanced Materials America) was dissolved in di chloromethane and PDA particles were added into the solution as 50 wt% of the total polymer (see Eq. 1).

[0123] Where wMatrimidand wPDAparticlearethe weight of Matrimid and PDA particles, respectively.

[0124] The mixture was stirred to achieve a uniform particle dispersion before films were fabricated using a solvent casting method. The final thickness of the film was about 20-30 pm.

[0125] MPCVD experiment

[0126] Hydrogen radicals can be generated in the gas phase using plasma generation. The microwave assisted plasma chemical vapor deposition (MPCVD) system utilizes a combination of microwave energy and plasma discharge to dissociate precursor gases such as hydrogen into gas phase radicals. The MPCVD system is generally used in polymer modification and diamond deposition. A Seki-ASTeX Model AX5200S microwave plasma chemical vapor deposition (MPCVD) reactor was utilized to generate hydrogen radicals within the instrument chamber. A 2.45 GHz ASTeX AX2100 microwave generator generated microwaves that were propagated through a rectangular waveguide, a TE-TM mode converter, and was then directed into the MPCVD quartz microwave cavity plasma chamber. A low-pressure hydrogen environment is maintained inside the plasma chamber with a variable vacuum value and a constant hydrogen gas flow of 100 SCCM (standard cubic centimeter per minute). The sample stage at the bottom of the chamber is capable of vertical movement, allowing for adjustments of the sample's exposure height. A hydrogen plasma was stabilized near the top of the plasma chamber, and the sample can be raised up to vary the flux of different hydrogen species.

[0127] The PDA-coated CA film or the PDA incorporated Matrimid film was positioned inside the instrument chamber on the graphitic susceptor and exposed to plasma conditions featuring 800 W microwave power, a hydrogen pressure of 20 Torr, and a stage height set at 20 mm (~80 cm away from the plasma center). This height was chosen to minimize plasma induced heating of the polymer films. The total exposure time was 60 min and the temperature inside the instrument was below 40°C.

[0128] X-ray photoelectron spectroscopy (XPS)

[0129] XPS spectra were obtained using a K-Alpha equipment from Thermo Fisher Scientific (Waltham, USA) with a monochromated aluminium Ka X-ray source (photon energy of 1486.7 eV). High-resolution scans were acquired for C Is, O Is and N Is regions. Peak fitting considered the position of each peak, its width at half of the maximum intensity, and the intensity using CasaXPS software.

[0130] The PDA coated CA film was directly measured after the MPCVD experiment. The PDA particle incorporated Matrimid film was dissolved back into dichloromethane after the MPCVD experiment. The resulting particle dispersion underwent centrifugation at 10,000 rpm for 20 min to collect the PDA particles. These PDA particles were then re-dispersed into dichloromethane and centrifuged again to remove any residual Matrimid. The rinsing process was repeated three times. The PDA particles were dried under vacuum at room temperature. The XPS spectra were obtained for PDA particles both before and after the hydrogen plasma exposure.

Claims

Claims1. A coating composition for forming a polymeric organic radical scavenging coating when applied to a metallic substrate surface, wherein the composition comprises an organic free radical scavenger that is reactable with hydrogen radicals.

2. The coating composition of claim 1 wherein the organic free radical scavenger is one or more compounds selected from the group comprising butylated hydroxytoluene, polydopamine, (eu)melanin, phenols, catechols, polyphenols, flavonoids, triarylphosphites, sterically hindered phenols and 2,5-di-tert-butylhydroquinone.

3. The coating composition of claim 1 or 2, wherein the organic free radical scavenger is in a polymeric form.

4. The coating composition of any one of claims 1-3, wherein the organic free radical scavenger is in a particulate form.

5. The coating composition of any one of claims 1-4, wherein the organic free radical scavenger comprises polydopamine.

6. The coating composition of any one of claims 1-5, wherein the organic free radical scavenger is dispersed in a matrix-forming polymer.

7. The coating composition of claim 6, wherein the composition comprises one or more matrix-forming polymers selected from the group consisting of polyimides, poly(vinyl chloride)s, poly(vinyl alcohol)s, polyurethanes, polydopamines, and epoxy-group containing polymers.

8. The coating composition of claim 6, wherein the composition comprises one or more monomers that are polymerizable to form a matrix-forming polymer selected from the group consisting of polyimides, poly(vinyl chloride)s, poly(vinyl alcohol)s, polyurethanes, polydopamines, and epoxy-group containing polymers.

9. The coating composition of any one of claims 6 to 8, wherein the one or more matrixforming polymers forms a hydrogen barrier coating when applied to a metallic substrate surface.

10. The coating composition of any one of claims 1-9, wherein the composition comprises a curing agent, crosslinker, solvent, and / or polymerisation initiator.

11. A coated substrate comprising an organic radical scavenging coating formed from the coating composition according to any one of claims 1-10.

12. The coated substrate of claim 11, wherein the organic radical scavenging coating has a thickness of less than 1 mm.

13. The coated substrate of claim 11 or 12, comprising a primer layer comprising the organic radical scavenging coating.

14. A method of forming a coated substrate, the coated substrate comprising a polymeric coating including an organic free radical scavenger that is reactable with hydrogen radicals, the method comprising: applying a coating composition, comprising an organic free radical scavenger that is reactable with a hydrogen radical, to a metallic substrate surface; and forming a polymeric organic radical scavenging coating from the coating composition on the metallic substrate surface, wherein the polymeric organic radical scavenging coating prevents or reduces diffusion of hydrogen radicals into the metallic substrate when the substrate is exposed to a hydrogen gas-containing environment.

15. The method of claim 14, comprising applying a coating composition comprising a matrix-forming polymer including an organic free radical scavenger to the substrate surface.

16. The method of claim 14 or 15, comprising applying a coating composition comprising a matrix-forming polymer and particles including an organic free radical scavenger that is reactable with a hydrogen radical to the substrate surface.

17. The method of any one of claims 14-16, comprising calculating the minimum concentration of organic free radical scavenger required for the coating composition.

18. The method of any one of claims 14-17, comprising forming the organic radical scavenging coating as a primer layer.

19. The method of any one of claim 14-18, comprising curing the organic radical composition to form the polymeric organic radical scavenging coating.

20. The method of claims 14-19, including performing the application step at ambient temperature.

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