Composition and method for galvanising metal

Abstract

The invention concerns a conductive anticorrosion coating composition for application onto a metallic substrate, and methods for preparing such coated metallic substrates The invention also extends to the use of such compositions for coating a metal substrate to provide a conductive anticorrosion layer, and to articles comprising a metal component and a conductive anticorrosion coating layer bound thereto.

Description

[0001] Composition and Method for Galvanising Metal

[0002] Field of the Invention

[0003] The invention concerns a conductive anticorrosion coating composition for application onto a metallic substrate, and methods for preparing such coated metallic substrates. The invention also extends to the use of such compositions for coating a metal substrate to provide a conductive anticorrosion layer, and to articles comprising a metal component and a conductive anticorrosion coating layer bound thereto.

[0004] Background of the Invention

[0005] The corrosion of engineering metals is a ubiquitous problem that requires constant attention, thought and counter measures. It affects almost all metals and results in the failure of products, infrastructure and services. It is defined as the degradation of a material’s properties by chemical and / or electrochemical reaction with the environment1. In 2013, the National Association of Corrosion Engineers (NACE) estimated the global cost of corrosion to be US$2.5 trillion, equivalent to 3.4% of the global GDP at the time2. Consequently, a great deal of research and effort is directed to the prevention, or delaying of corrosion in the environment, primarily through the application of coatings.

[0006] For the corrosion protection of metals such as iron or steel, there are two main coating types. Organic coatings such as paints and oils; and metallic coatings such as hot dip galvanised (HDG) and electroplated. There are also less common options such as powder coatings, PVD I CVD coatings and thermal spray used in specialist applications. The advantage of organic coatings such as paints is they can be applied at room temperature to flat and not flat surfaces either as part of the manufacturing process of the final product or, in the field either as part of a build or as a repair. Paints protect the metal substrates by acting as a barrier between the surface of the metal and the environment. The barrier prevents ionic conduction through to the metal surface providing an Ohmic resistance barrier to electrochemical corrosion reactions. Therefore, if the barrier is damaged, the coating is no longer effective in that damaged area.

[0007] HDG coatings are one of the most popular metallic coatings and have been employed for over 200 years. They are applied either continuously onto successive coils of sheet metals such as iron or steel, or through a batch process, where formed components are chemically cleaned and dipped into large baths of liquid zinc. Metallic coatings also provide a barrier between the substrate and the environment, and are indeed often overcoated with an organic coating for further protection. However, in addition to such barrier protection, zinc based HDG coatings also provide sacrificial, galvanic protection to the coated metal substrate. If the coating is damaged, exposing the metal substrate, the zinc will preferentially corrode instead of the metal substrate. Moreover, the zinc-based corrosion products will precipitate onto the exposed metal, providing some barrier protection in a “self-healing” type mechanism. The corrosion reactions occurring on a galvanised steel substrate are shown below. Equation 1 is the oxidation reaction where metallic zinc is converted to Zn2+. Equation 2 is the cathodic reaction that occurs on the steel surface under aerated and pH neutral conditions. Here, oxygen is reduced to produce OH’ ions but no metal loss occurs.

[0008] Anodic: 2Zn — 2Zn2++ 4e_Equation (1 )

[0009] Cathodic: 2H2O + O2 + 4e_— 4OH’ Equation (2)

[0010] The main disadvantage of HDG coatings is they are applied at temperatures in excess of 450°C, which means that repair of damaged coatings is not possible using conventional HDG coatings without transporting the whole component to a dedicated plant where the zinc is stripped and re-applied. This is an issue particularly when parts need welding or repairing post coating. In these instances, the zinc is often removed locally with a grinder, to prepare the substrate material for welding. After the repair or weld process, the area has no coating so becomes susceptible to corrosion. Further, many speciality steels, in particular high strength steels cannot be galvanised using traditional HDG methods due to the required isothermal treatment in excess of 450 °C that is imposed to keep the Zn molten because such metals require fast cooling rates to temperatures below 300°C in order to form non-equilibrium, hard phases within the microstructure that are responsible for the overall strength of the material. Therefore, exposing such metals to the elevated temperatures required for HDG coating would degrade the mechanical properties of the final product. As a consequence of this incompatibility with HDG coatings, market use of such speciality steels has typically been limited to applications where they are not exposed to the environment.

[0011] To address the limitations of HDG coatings, corrosion resistant coatings may be applied to metal substrates using conventional paints, which do not offer galvanic protection, or to use Zinc Rich organic Coatings (ZRC). ZRCs contain >80 wt% zinc in the form of zinc powder and are bound in an organic matrix, typically epoxy3. For the ZRC to provide galvanic protection, the zinc particles must be in electrical contact with the steel substrate, hence the need for high zinc loading. The organic matrix is not conductive, so a critical percolation between the particles must be met or galvanic protection is lost. This can break down when a zinc particle corrodes and forms a layer of insulating corrosion product. This electrically disconnects the particle from the percolation network meaning it can no longer provide galvanic protection. Furthermore, the organic matrix can fail by cathodic under cutting and by UV degradation.

[0012] Here we demonstrate an improved galvanic coating that, compared to conventional HDCs, can be applied at significantly lower temperatures (i.e. less than 350 °C, and preferably less than 250 °C) and so not only enables the possibility of in situ repair of damaged coatings, but also allows for the galvanisation of speciality metals such as certain high strength steels, which must be rapidly cooled to temperatures below 300 °C to form non-equilibrium hard phases within the microstructure that are responsible for the overall strength of the material. In addition, compared to conventional ZRCs, such coatings are metallic in nature and so may comprise significantly lower zinc levels whilst retaining continuous electrical conductivity between the coating and the substrate (and so galvanic protection to the substrate). Further, the coatings comprise an alloy as opposed to an organic matrix, which is not degraded by UV, susceptible to cathodic undermining or permeable to ionic solutions and so ensures continuous electrical conductivity is maintained between the coating and the substrate for the lifetime of the coating.

[0013] Statements of Invention

[0014] The present invention, in its various aspects, is as set out in the accompanying claims.

[0015] According to a first aspect of the invention there is provided a conductive anticorrosion coating composition for application onto a metallic substrate, the composition comprising, based on the total weight of the composition:

[0016] (i) from about 95 to 50 wt.% of a metallic binder comprising fusible alloy particles; and

[0017] (ii) from about 5 to 50 wt.% zinc particles.

[0018] Reference herein to a fusible alloy refers, in the context of the present invention, to a metal alloy having a melting point below about 200 °C. Further, as will be apparent to the skilled person, the fusible alloy preferably has a melting point above 100 °C so that, in normal use, the coating composition of the invention remains in the solid phase after application to a substrate.

[0019] The specific choice of fusible alloy is not particularly limited provided that the alloy is compatible with the metal substrate to be coated, and can establish electrical contact between said substrate and the zinc particles within the coating. However, in preferred embodiments, the fusible alloy comprises two or metals selected from bismuth (Bi), Lead (Pb) and Tin (Sn), and more preferably comprises Bi and Sn. In particularly preferred embodiments, the fusible alloy is a eutectic mixture, i.e. , a homogenous mixture that has a melting point lower than those of the individual constituents. Exemplary fusible alloys suitable for use in the coating compositions of the invention are set out in Table 1.

[0020] The metallic binder preferably comprises, in addition to fusible alloy particles, one or more flux. In such embodiments, the metallic binder preferably comprises from about 75 to 95 wt.% fusible alloy particles and from about 5 to 25 wt.% flux. In exemplary embodiments, the metallic binder comprises about 88 wt.% fusible alloy particles and about 12 wt.% flux.

[0021] As used herein, and as the skilled person readily understands, the term flux refers to a chemical additive that is added to fusible alloys to facilitate soldering, brazing and / or welding by removing oxidation from metals to be joined. In the context of the present invention, the coating composition preferably comprises a flux to prevent oxidation during heating and to promote liquid metal flow.

[0022] The specific choice of flux is not particularly limited, and an appropriate flux can be selected by the skilled person without difficulty. Organic flux such as rosin or tallow, and inorganic flux such as ammonium chloride, zinc chloride or zinc phosphate are suitable. However, in preferred embodiments, the flux comprises an inorganic flux, most preferably zinc phosphate. Such inorganic fluxes may impart additional corrosion resistance to the coated metal substrate.

[0023] The coating composition comprises from about 5 to 50 wt.% zinc particles, which in use provide sacrificial, galvanic protection to a coated metal substrate. In preferred embodiments, the coating composition comprises from about 10 to 30 wt.%, and most preferably about 20 wt.% zinc particles. It follows, therefore, that coating composition preferably comprises from about 90 to 70 wt.%, and most preferably about 80 wt.% metallic binder.

[0024] Additionally or alternatively, the zinc particles preferably have an average particle size, measured by laser diffraction or scanning electron microscopy, of from about 2 to 50 pm. More preferably, the zinc particles have a particles size of from about 5 to 20 pm and, most preferably, about 10 pm. The compositions as herein described provide resilient and long-lasting galvanic protection when applied as a coating to a metal substrate. Further, due to the use of a fusible alloy binder, said coatings can be applied at significantly lower temperatures (i.e. less than 350 °C, and preferably less than 250 °C) in comparison to conventional HDG coatings, and so can be applied to metal substrates in situ to repair damaged coatings, and / or for the galvanisation of speciality steels such as high strength steels that cannot tolerate prolonged exposure to elevated temperature associated with hot dip galvanization without degradation of mechanical properties.

[0025] In addition, in contrast to ZRCs, due to the use of a metallic fusible alloy binder, the compositions require significantly lower zinc levels whilst retaining continuous electrical conductivity between the coating and the substrate and, so galvanic protection to the substrate. Further, once applied to a substrate, a coating comprising discrete zinc islands embedded in a continuous fusible alloy matrix is formed. In contrast to the use of an organic binder matrix in ZRCs, the alloy matrix formed in the coatings of the invention is not susceptible to UV degradation and / or cathodic undermining, nor is it permeable to ionic solutions. Therefore, continuous electrical conductivity is maintained between the coating and the substrate for the lifetime of the coating.

[0026] Therefore, according to a second aspect, the invention extends to the use of the composition according to the first aspect for coating a metal substrate with a conductive anticorrosion layer.

[0027] As the skilled reader will readily appreciate, any metal that is more noble than zinc can be galvanically protected by coating with the composition of the present invention. Therefore, as the standard electrode potential of zinc is - 0.76 volts, the coated metal substate may comprise any metal having a standard electrode potential of -0.7 volts or higher. However, in preferred embodiments, the metal substrate is selected from: iron and steel, including high strength steel. In particularly preferred embodiments, the metal substrate is steel. As used herein, and as the skilled person readily understands, the term high strength steel refers to a steel alloy, such as a high-strength low-alloy (HSLA) steel or dual phase steel, that provides better mechanical properties than plain carbon steel. In particular, high strength steels have a yield strength of at least about 400 MPa, preferably at least about 500 MPa and most preferably at least about 600 MPa. Such high strength steels typically have a carbon content between about 0.05 and 0.25%, and their enhanced strength is typically attributed to a martensitic or bainitic microstructure. However, as hard martensitic phases can soften when heated to temperatures above 300°C, some high strength steels are not suitable for HDG coating.

[0028] According to a third aspect of the invention, there is provided a process for coating a metal substrate with a conductive anticorrosion layer, the process comprising:

[0029] (i) applying a coating composition according to the first aspect of the invention to a metal substrate surface; and

[0030] (ii) heating the coated metal substrate to a temperature of from about 100 to 300°C to melt the fusible alloy binder, encapsulate the solid zinc particles and bond the coating to the metal substrate surface.

[0031] Preferred features relating to the coating composition and metal substrate are as set out in relation to the first and second aspects, respectively.

[0032] In preferred embodiments, the coating is applied at a thickness of from about 15 to 60 pm, and more preferably about 30 pm. The coating can be applied according to any conventional method such as draw down, spray or brushing. However, in some embodiments, the coating is applied by printing, preferably a continuous printing process.

[0033] The skilled reader will readily appreciate that the coated metal substrate must be heated to a temperature sufficient to melt the fusible alloy binder component. For example, when the coating composition comprises Bi 57, Sn 43 binder, the coated substrate must be heated to a temperature above 139 ° In preferred embodiments, the coated metal substrate is heated in step (ii) to a temperature of from about 175 to 300 °C, and more preferably from about 200 to 250 °C, ideally for a period of from about 15 to 60 minutes. The specific method by which heat is applied to the coated metal substrate is not particularly limited and can be achieved simply by the use of a hot air emitting device such as a heat gun, or simple convection ovens such as those conventionally used in organic coating applications. However, in some embodiments, the requisite heat is applied to the coated metal substrate by Near Infrared (NIR) irradiation as such a process, particularly when coupled with printing process for applying the coating composition, is scalable to a continuous process for providing a conductive anti-corrosion layer.

[0034] Upon completion of the heating process of step (ii), a coated metal substrate is formed, wherein the coating comprises discrete zinc particle regions or islands dispersed throughout and embedded in a continuous fusible alloy matrix, and provides both barrier and sacrificial, galvanic protection to the coated metal substrate.

[0035] Therefore, according to a fourth aspect of the invention, an article is provided, said article comprising a metal component and a conductive anticorrosion coating layer bound thereto, wherein said coating layer comprises, based on the total weight of the composition:

[0036] (i) from about 95 to 50 wt.% of a metallic binder comprising fusible alloy particles; and

[0037] (ii) from about 5 to 50 wt.% zinc particles, and wherein said zinc particles are dispersed within a continuous matrix of said fusible alloy in the coating layer.

[0038] In preferred embodiments, the metal substrate is coated by the process according to the third aspect of the invention.

[0039] As will be readily apparent to the skilled person, the coating will provide galvanic protection to any coated metal component provided that said component is more noble (i.e. has a less negative standard electrode potential) than the sacrificial zinc particles in the coating. Therefore, as the standard electrode potential of zinc is -0.76 volts, the coated metal component may comprise any metal having a standard electrode potential of -0.7 volts or higher. However, in preferred embodiments, the metal component is selected from: iron; and steel, including high strength steel . In particularly preferred embodiments, the metal component is steel.

[0040] The choice of fusible alloy is also not particularly limited, provided that the alloy is compatible with the metal component to be coated, and can establish electrical contact between said metallic component and the zinc particles within the coating. However, in preferred embodiments, the fusible alloy comprises two or metals selected from bismuth (Bi), Lead (Pb) and Tin (Sn), and more preferably comprises Bi and Sn. In particularly preferred embodiments, the fusible alloy is a eutectic mixture, i.e. a homogenous mixture that has a melting point lower than those of the individual constituents.

[0041] Exemplary fusible alloys suitable for use in the coating composition are set out in Table 1.

[0042] In preferred embodiments, the metallic binder also comprises, in addition to fusible alloy particles, one or more flux. In such embodiments, the metallic binder preferably comprises, at the point of application onto the metal component, from about 75 to 95 wt.% fusible alloy particles and from about 5 to 25 wt.% flux. However, it will be apparent to the skilled person that the flux may be lost after application of the coating, particular where organic flux is used, due to the application of heat during the coating process.

[0043] In exemplary embodiments, the metallic binder comprises, at the point of application onto the metal component, from about 88 wt.% fusible alloy particles and about 12 wt.% flux. Organic flux such as rosin or tallow, and inorganic flux such as ammonium chloride, zinc chloride or zinc phosphate are suitable. However, in preferred embodiments, the flux comprises an inorganic flux, most preferably zinc phosphate. The presence of such inorganic fluxes may impart additional corrosion resistance to the coated metal substrate.

[0044] The coating comprises from about 5 to 50 wt.% zinc particles, which in use provide sacrificial, galvanic protection to a coated metal component. In preferred embodiments, the coating comprises from about 10 to 30 wt.%, and most preferably about 20 wt.% zinc particles. It follows, therefore, that coating composition preferably comprises from about 90 to 70 wt.%, and most preferably about 80 wt.% metallic binder.

[0045] Additionally or alternatively, the zinc particles preferably have a particle size, laser diffraction or scanning electron microscopy, of from about 2 to 50 pm. More preferably, the zinc particles have a particles size of from about 5 to 20 pm and, most preferably, about 10 pm.

[0046] In preferred embodiments, the coating layer has a thickness of from about 15 to 60 pm, and more preferably about 30 pm.

[0047] Preferred features of each aspect of the invention may be as described in connection with any of the other aspects.

[0048] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to” and do not exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Throughout the description and claims of the specification, the word “about” allows for a variation of ± 10%, preferably ± 5%, and most preferably ± 1 %, of the recited numerical value unless the context otherwise requires.

[0049] All references, including any patent or patent application, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. Further, no admission is made that any of the prior art constitutes part of the common general knowledge in the art.

[0050] Other features of the present invention will become apparent from the following examples. Generally speaking, the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including the accompanying claims and drawings). Thus, features, integers, characteristics, compounds or chemical moieties described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith.

[0051] Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.

[0052] The Invention will now be described by way of example only with reference to the Examples below and to the following Figures and Tables wherein:

[0053] Figure 1. Draw down tape method of applying the coating;

[0054] Figure 2. schematic representation of a galvanic coated steel substrate of the invention;

[0055] Figure 3. NIR heat profile using a 1 m / min speed at 40 % power;

[0056] Figure 4. Open Circuit Potential experimental set up;

[0057] Figure 5. Schematic illustrating ZRA experimental set up;

[0058] Figure 6. SEM and EDS inset of Sn-Bi microstructure;

[0059] Figure 7. Optical image of the coating heat-treated at 240 for 45 mins showing Zn surrounded by continuous Bi-Sn matrix;

[0060] Figure 8. Micrograph Image of heat-treated (NIR oven , 40% power, 1 m / min speed) 20 wt.% Zn Bi-Sn coating;

[0061] Figure 9. Micrograph cross-sectional Image of heat-treated (NIR oven , 40% power, 1 m / min speed) 20 wt.% Zn Bi-Sn coating;

[0062] Figure 10. Image of 0T Bend tested sample with 20 wt.% Zn. Coupon is 5cm in length;

[0063] Figure 11. SVET scans showing the anodic (red) activity focused on the coating; Figure 12. OCP results for coatings and constitutes over 24 hour test duration;

[0064] Figure 13. OCP results for inventive coating (20% Zn Bi-Sn) over 48 hours demonstrating that the does not increase towards the OCP of steel;

[0065] Figure 14. ZRA data for zinc connected to steel (representing standard galvanised coatings);

[0066] Figure 15. ZRA for inventive coating (Bi-Sn, 20wt% Zn) connected to steel; and

[0067] Table 1. Exemplary known fusible alloys.

[0068] Example 1 : Galvanic Coating of Steel

[0069] Zinc powder (Sigma Aldrich 209988) with particle size of <10 pm was mixed with a fusible Bismuth-Tin alloy (88 wt.% Sn-Bi powder, 12 wt.% flux)) solder paste purchased from Somerset Solders (P090) to create a slurry. The resultant solder paste consisted of Bi-Sn powder (<10 pm) immersed in an organic flux. The solder and zinc slurry was then subjected to shear mixing at 250 rpm for 15 minutes. Ethanol was used to reduce the viscosity of the slurry to allow easy coating application.

[0070] The mixture was then bar coated onto 0.5 mm gauge Interstitial Free (IF) grade steel from Tata Steel cut into 5 x 5 cm coupons. The steel substrate was first abraded with 1200 grit paper to remove surface debris and washed with water and ethanol before being dried. The dried steel was then dipped in an inorganic flux of zinc phosphate dissolved in water (50 wt.%) to provide a clean surface for the coating. A strip of electrical tape was placed each side of the coupon, as shown in Figure 1 , to give a coating thickness of 50 pm.

[0071] After bar coating, the samples were placed in a Nabertherm oven heated to 250°C in air for 30 minutes, thereby melting the solder material around the zinc particles and to form a bonded coating to the steel substrate. In contrast to the fusible Bismuth-Tin alloy solder, which melts at a temperature significantly below 250°C, zinc melts at 419.5°C so remained solid during this coating curing stage. The resultant coating produced was therefore composed of discrete zinc particle regions or islands dispersed throughout and embedded in a continuous Bismuth-Tin fusible alloy matrix as shown schematically in Figure 2.

[0072] Results

[0073] Macroscale comparison of the resultant coatings with standard hot dip galvanised (HDG) steel and the leading Zinc Rich Coated steel demonstrated that the new coating is rougher compared to the two convention coatings (data not shown), which is a result of agglomerated zinc particles protruding through the continuous Bi-Sn solder layer. The coating thickness of the HDG is 20 pm and the novel coating under investigation here was 30 pm. However, this surface roughness can be minimised through greater mixing and optimising of the fluxing component of the coating.

[0074] Example 2: Near Infrared (NIR) Heat Treatment

[0075] As a modification of the galvanic process of Example 1 , NIR heating was investigated as method of reducing heat treatment times. An AdPhos Coil Lab V2 was used in these trials and has been described elsewhere4. 5 x 5cm samples were coated as per the method of Example 1. A thermocouple was then spot welded to the reverse side of the steel to record the temperature as the samples passed through the NIR. The time and speed of travel was systematically altered to identify the optimum settings that would melt the Bismuth-Tin alloy but not the zinc particles. Results

[0076] The optimised heat profile of an NIR heated sample running at 1 m / min at 40 % power output is shown in Figure 3. Here a maximum temperature of 300°C was achieved, thus melting the Bi-Sn alloy but not the zinc particles.

[0077] Example 3: Metallographic Assessment

[0078] Samples were assessed with optical and electron microscopy. To examine the coating I substrate interface and the through coating microstructure, coated samples were cut into 2 cm2coupons and mounted in metallographic resin. They were then ground with progressively finer grit before being finished with a 1 pm diamond polish.

[0079] A Keyence VHX7000 3D optical microscope was used to image surface and cross sections of the coatings. Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray Spectroscopy (EDS) analysis was performed on a Hitachi TM 4000 microscope.

[0080] Results

[0081] The Bi-Sn solder component of the composition was provided at the eutectic composition and consisted of a coarse lamella microstructure as shown in Figure 6 below. The eutectic composition has the lowest melting point and under equilibrium conditions and during cooling freezes to a solid as soon as that temperature is reached. This composition was selected to minimise the post coating heat treatment temperature.

[0082] This lamella microstructure gives rise to a continuous tin phase surrounded by bismuth. Bismuth and tin are both electrochemically more noble than steel.

[0083] A range of Zn loadings from 0 to 40 wt.% were assessed for coating uniformity, adhesion and corrosion resistance, following the coating procedure of Example 1 . 20 wt.% Zn was identified as the optimum composition to give the best mix of mechanical and electrochemical properties. Figure 7 shows the 20 wt.%Zn coating microstructure with islands of zinc embedded in a Bi-Sn eutectic matrix. Good wetting between the Bi-Sn and the Zn can be observed.

[0084] Micrographs of 20wt% Zn Bi-Sn coatings, cured via the NIR heat treatment process of Example 2, are also shown in Figure 8 and Figure 9. These images clearly show that NIR can be used as a rapid heating method to prepare the galvanic coatings of the present invention.

[0085] Adhesion of the same 20wt% Zn Bi-Sn coatings was assessed using the simple, industry standard OT bend test, wherein the coated stell sample was bent in half and the bend was then examined for any loss of adhesion at the maximum radius. As shown in Figure 10, the coating remained largely intact after the full 180° bend, a very harsh test of coating adhesion.

[0086] Example 4: Corrosion assessment (SVET)

[0087] The Scanning Vibrating Electrode Technique (SVET) was used to assess the cut edge corrosion resistance of the coating formed according to the process of Example 1. The SVET has been successfully employed to study cut edge corrosion and has been described elsewhere5’6 7’8. Coated steel samples were cut into 2 x 2 cm coupons and mounted in non-conductive metallographic mount so that the cross section was exposed. They were polished to a 1 pm metallographic finish. A 1 cm2area was masked off using PTFE tape so only the area under investigation was exposed to the electrolyte. Samples were immersed in 1 wt.% NaCI solution (0.17 M) and scanned at a height of 100 pm with a vibrating amplitude of 30 pm and frequency of 140 Hz. The scan area was 3 mm x 10 mm with a total of 1500 points per scan. Scans were made once an hour for 24 hours. The SVET data was calibrated and plotted using the topographical software package, Surfer to reveal maps of current density. Anodic currents were plotted in red and cathodic currents in blue. If the steel is galvanically protected by the coating, the steel area will be the site of cathodic oxygen reduction and the coating will be the site of anodic oxidation. Results

[0088] The primary aim of this coating is to provide galvanic protection to the steel substrate. The SVET maps provided in Figure 11 show a cross section of the coated steel, with the coating on the left-hand side of the steel. This figure demonstrates that the anodic activity (coloured red) is focused on the coating and the cathodic (shown in blue), oxygen reduction reaction, is focused on the steel. Representative scans at 1 , 6, 12, 18 and 24 hours are shown. Clearly, the steel remained protected throughout the duration of the experiment. The scale shows the magnitude of the corrosion current density.

[0089] Example 5: Corrosion assessment (OCP)

[0090] Open Circuit Potential (OCP) was also used to measure the change in potential of the model coatings and compare them to steel, zinc and zinc free Bi-Sn solder with respect to time. All samples were polished to a metallographic finish to remove any scales or oxides. A Gamry Potentiostat was used alongside a personal computer installed with the Gamry Framework and Gamry Echem Analyst software. For all experiments, a saturated calomel (SCE) electrode was used as the reference electrode. The sample to be tested was placed into an electrochemical sample holder that connects it to a wire. 0.76 cm2of the sample was exposed to the electrolyte. This experimental setup is shown in Figure 4. The Electrolyte used was once again 1 wt.% NaCI in DI water. No current is applied during open circuit potential measurements so that the sample is undriven. The potential of the working electrode (sample) was measured against the SCE reference. All OCP experiments were conducted at 20°C for 24 hours.

[0091] Results

[0092] OCP tests were performed on steel, zinc, Bi-Sn and 20 wt.% Zn Bi-Sn electrodes formed according to the method of Example 1. This provides information on the thermodynamic driving force for corrosion and also the extent and time of galvanic protection. More noble materials have a relatively more positive OCP and are likely to be galvanically protected by metals with more negative OCP values. Figure 12 shows OCP results over 24 hours for all samples. Further, the 20% Zn Bi-Sn coating test was extended to 48 hours (Figure 13) to monitor OCP over the longer period.

[0093] It can be seen from these data that the Bi-Sn alloy has the highest OCP of - 400 mV followed by steel (-700 mV), Zn Bi-Sn (-900 mV), Zn, and Zn rich coating at -1V. As expected, each of the zinc-based materials occupy an OCP position below the steel substrate and therefore would be expected to provide galvanic protection to steel based on the thermodynamics. Further, the OCP of the novel Zn Bi-Sn coating remains below the steel for the duration of the 48-hour test, showing no sign of moving towards the OCP of steel. Galvanic protection can be considered exhausted once the OCP of the coatings moves to a value equal to that of the steel.

[0094] Example 6: Corrosion Resistance (ZRA)

[0095] Further, Zero Resistance Ammetry (ZRA) was used to measure the extent of galvanic coupling between the electrodes. Electrodes of zinc, zinc free Bi-Sn solder, steel and the model coatings composed of 20% Zn remainder solder as prepared in Example 1 were tested in 1 wt.% NaCl for24 hours. The working electrodes had a wire soldered to back of the sample to allow connection to the potentiostat. The sample was mounted in non-conductive resin and polished to a 1 pm finish with 0.87cm exposed to the electrolyte.

[0096] A measurement was taken every second for 24 hours. The ZRA provides information on the direction and magnitude of corrosion current flowing between two electrodes. If the coating provides galvanic protection to steel, electrons will flow from the coating to steel. However, when the sacrificial protection is exhausted, the electron flow will reduce and even reverse. A schematic is shown in Figure 5. Results

[0097] ZRA monitors the direction and magnitude of current flow between two working electrodes corroding in solution. Figure 14 shows the response of steel connected to zinc, as is in the case for conventional galvanic coating. A positive current flows from the zinc to the steel in a redox reaction of oxidation at the anode (zinc) and reduction at the cathode (steel). This can be compared with the ZRA data over 24 hours for the Bi-Sn 20 wt% Zn coating connected to the steel. As with Figure 15, electrons flow from the coating material to the steel electrode, demonstrating galvanic protection of the steel by the coating. Over the 24 hour tests, the current increases, which indicates activation control.

[0098] Summary

[0099] Galvanised coatings are ubiquitous due to their combined barrier and galvanic protection. They are conventionally applied by dipping a steel substrate into a bath of molten zinc, typically above 450°C. When galvanising is not possible, such as in the repair of components or in the case of high strength steels which cannot tolerate the 465°C isothermal without a deleterious effect on the mechanical properties, alternative coatings are required

[0100] Zinc Rich Coatings are commonly used to repair galvanised components or to paint formed components on site. They consist of zinc powder contained within an organic matrix. To achieve galvanic protection, there must be through coating electrical conductivity between the zinc particles and the substrate. This requires high zinc loading to meet the critical percolation condition. These coatings can fail when zinc particles corrode and form insulating corrosion products that prevent electrical conductivity. They can also fail by cathodic disbanding and organic matrix degradation under UV light.

[0101] A novel coating is presented here which uses a low temperature, fusible alloy as a matrix to hold zinc particles. This allows a lower zinc loading as the entire coating is electrically conductive so can provide galvanic protection. The coating can be applied as a slurry by draw down or with a brush. Other methods can also be employed.

[0102] Heat treatment of below 240°C can be used to melt the fusible alloy to envelop the zinc. This can be done in a conventional oven or with a local heat source. In this report, NIR was demonstrated as a rapid heating option.

[0103] The corrosion investigation shows the coating provides galvanic protection to the steel substrate and suffers no obvious self-corrosion despite the nobility of fusible alloy components.

[0104] Table 1. Exemplary known fusible alloys

[0105] References

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[0107] 2NACE, Koch G, Varney J, Thompson N, Moghissi O, Gould M, et al. International Measures of Prevention, Application, and Economics of Corrosion Technologies Study. 2013.3A. K. Hussain, N. Seetharamaiah, M. Pichumani, and C. S. Chakra, “Research progress in organic zinc rich primer coatings for cathodic protection of metals - A comprehensive review,” Prog. Org. Coatings, vol. 153, p. 106040, 2021.

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Claims

Claims1 ) A conductive anticorrosion coating composition for application onto a metallic substrate, the composition comprising, based on the total weight of the composition:(i) from about 95 to 50 wt.% of a metallic binder comprising fusible alloy particles; and(ii) from about 5 to 50 wt.% zinc particles.2) The composition according to claim 1 , wherein said fusible alloy has a melting point of from about 100 to 200 °C.3) The composition according to claim 1 or claim 2, wherein said fusible alloy comprises two or metals selected from bismuth (Bi), Lead (Pb) and Tin (Sn), and / or wherein said fusible alloy is a eutectic mixture.4) The composition according to any of the preceding claims, wherein said metallic binder comprises from about 75 to 95 wt.% fusible alloy particles and from about 5 to 25 wt.% flux.5) The composition according to claim 4, wherein said flux comprises zinc phosphate.6) The composition according to any of the preceding claims, comprising from about 90 to 70 wt.% metallic binder and from about 10 to 30 wt.% zinc particles.7) The composition according to any of the preceding claims, wherein said zinc particles have an average particle size of from about 2 to 50 pm.8) Use of the composition according to any of the preceding claims for coating a metal substrate with a conductive anticorrosion layer.9) The use according to claim 8, wherein said metal substate is selected from: iron, steel, and high strength steel.10) A process for coating a metal substrate with a conductive anticorrosion layer, the process comprising:(i) applying a coating composition according to any of claims 1 to 7 to a metal substrate surface; and(ii) heating the coated metal substrate to a temperature of from about 100 to 300°C to melt the fusible alloy binder, encapsulate the solid zinc particles and bond the coating to the metal substrate surface.11 ) The process according to claim 10, wherein said coating is applied at a thickness of from about 15 to 60 pm.12) The process according to claim 10 or claim 11 , wherein said coating is applied by a continuous printing process.13) The process according to any of claims 10 to 12, wherein the coated metal substrate is heated in step (ii) to a temperature of from about 175 to 300 °C, optionally for a period of from about 15 to 60 minutes.14) The process according to any of claims 10 to 13, wherein heat is applied to the coated metal substrate by Near Infrared (NIR) irradiation.15) An article comprising a metal component and a conductive anticorrosion coating layer bound thereto, wherein said coating layer comprises, based on the total weight of the composition:(i) from about 95 to 50 wt.% of a metallic binder comprising fusible alloy particles; and(ii) from about 5 to 50 wt.% zinc particles, and wherein said zinc particles are dispersed within a continuous matrix of said fusible alloy in the coating layer.16) The article according to claim 15, wherein said metal component is selected from: iron, steel, and high strength steel.17)The article according to claim 15 or claim 16, wherein said fusible alloy:has a melting point of from about 100 to 200 °C; comprises two or metals selected from bismuth (Bi), Lead (Pb) and Tin (Sn); and / or is a eutectic mixture.18) The article according to any of claims 15 to 17, wherein said metallic binder comprises, at the point of application onto the metal component, from about 75 to 95 wt.% fusible alloy particles and from about 5 to 25 wt.% flux.19) The article according to claim 18, wherein said flux comprises zinc phosphate.20) The article according to any of claims 15 to 19, wherein said coating comprises from about 90 to 70 wt.% metallic binder and from about 10 to 30 wt.% zinc particles.21 ) The article according to any of claims 15 to 20, wherein said zinc particles have an average particle size of from about 2 to 50 pm.22) The article according to any of claims 15 to 21 , wherein said coating layer has a thickness of from about 15 to 60 pm.