EP4713162A1

EP4713162A1 - Zinc-containing particles, method of manufacturing such particles by a thermal evaporation-condensation process, and coating composition comprising such particles

Info

Publication number: EP4713162A1
Application number: EP24724292.8A
Authority: EP
Inventors: Chunping QI; Hao Wu; Claus Erik Weinell; Kim Dam-Johansen
Original assignee: Danmarks Tekniske Universitet
Current assignee: Danmarks Tekniske Universitet
Priority date: 2023-05-15
Filing date: 2024-05-13
Publication date: 2026-03-25
Also published as: WO2024235906A1

Abstract

The invention relates to zinc-containing particles and a method of manufacturing such zinc-containing particles by a thermal evaporation-condensation process performed in a heated chamber (2). A zinc-containing source material (9) is supplied to the heated chamber. The heated chamber comprises an evaporation section where the zinc-containing source material is fully or partly vaporized to the gas phase, and a condensation section where zinc vapour is nucleated and/or condensed heterogeneously, and may be collected on a surface, such as a particle collector (11), in a cyclone or a filter. The zinc-containing particles have special shapes and properties compared to prior art zinc-containing particles. The invention further relates to a coating composition comprising such zinc-containing particles.

Description

ZINC-CONTAINING PARTICLES, METHOD OF MANUFACTURING SUCH PARTICLES BY A THERMAL EVAPORATION-CONDENSATION PROCESS, AND COATING COMPOSITION COMPRISING SUCH PARTICLES

FIELD OF THE INVENTION

The present invention relates to zinc-containing particles and the manufacturing thereof by a thermal evaporation-condensation process. In particular, it relates to such a method by which the obtained particles provide improved anticorrosion properties of a coating composition comprising such particles as compared to known compositions comprising spherical zinc particles. What is referred to as "coating composition" could also be called "anticorrosion composition" or "anticorrosive coating", since that is the intended functioning of the coating composition when being applied as a coating to a structure to be protected against corrosion.

BACKGROUND OF THE INVENTION

Zinc-rich coatings (ZRCs) have been one of the most effective and widely used anticorrosive coatings for the heavy-duty corrosion protection of steel structures such as ships, offshore platforms, and bridges.

A ZRC provides corrosion protection based on an initial cathodic protection from sacrificial active zinc particles and a subsequent barrier protection enhanced by the formed corrosion products filling up micro-pores within the coating and/or precipitating on the exposed steel surface in case of a damage to the coating system. A large amount (typically >80 wt% in dry film) of zinc particles contacting well with each other and with the steel substrate is required to ensure effective cathodic protection.

However, studies leading to the present invention have shown that only a fraction of the added zinc in a standard epoxy ZRC contributes to the cathodic protection. In other words, most of the added zinc is not effectively used in a known ZRC, which is a waste of the zinc resources. In addition, such a high loading of zinc particles could cause other problems such as a low cohesive and adhesive strength, poor mechanical properties such as low flexibility, and in-can sedimentation of zinc during storage before use of the ZRC.

OBJECT OF THE INVENTION

It is an object of the present invention to provide zinc-containing particles having a non-spherical shape and/or a more reactive crystal structure than prior art zinc particles used for coating compositions.

It is another object of the present invention to provide zinc-containing particles, which particles are suitable for improving the anticorrosion properties of a coating composition when compared to prior solutions comprising zinc particles.

It is another object of the present invention to provide a method of manufacturing said zinc-containing particles.

It is another object of at least some embodiments of the present invention to provide a method of manufacturing said zinc-containing particles in a one-step process.

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide zinc- containing particles and a method of manufacturing thereof that solves at least some of the above mentioned problems of the prior art.

SUMMARY OF THE INVENTION

The above described objects are intended to be obtained in a first aspect of the invention by providing zinc-containing particles whereof at least 80%, such as at least 90%, such as at least 95% have a crystal structure which is:

- a polygonal layered structure with a high percentage of exposed (100) facet, wherein the relative peak intensity I(100)/I(002) > 1, or - a dendrite with a high percentage of exposed (100) facet, wherein the relative peak intensity I(100)/I(002) > 1 when determined from X-ray diffraction (XRD) patterns.

This way of characterizing a crystal structure will be well known to a skilled person within this technical field. For further details and explanation, please refer to the description of figure 6. The relative peak intensity is the ratio of the peak intensity to the intensity of a reference peak within the same XRD-spectrum.

In some embodiments of the invention, the zinc-containing particles are characterized by at least 80%, such as at least 90%, such as at least 95% having a crystal structure which is:

- a polygonal layered structure with a high percentage of exposed (100) facet, wherein the relative peak intensity I(101)>I(100)>I(002), or

- a dendrite with a high percentage of exposed (100) facet, wherein the relative peak intensity I(101)>I(100)>I(002).

Zinc-containing particles according to the first aspect of the invention may be obtained by a method according to the second aspect of the invention as will be described below.

In preferred embodiments of the invention, the "zinc-containing particles" are zinc-rich particles which may be either pure zinc or zinc-rich composites.

The particles may be used in different fields, e.g. batteries, catalysts, biodegradable metallic implants, anticorrosive coatings, etc. Alternatively, the particles may be oxidized to synthesize zinc oxide, which may have different properties and which may also have different applications, such as catalysts, coatings, pharmaceuticals, etc. Of particular interest are the zinc-containing particles with the crystal structure as described above. The coating compositions comprising such particles have good performance in providing cathodic protection to a steel substrate.

In a second aspect, the invention relates to a method of manufacturing zinc- containing particles according to the first aspect of the invention by a thermal evaporation-condensation process, the method comprising: - providing a heated chamber comprising:

- an evaporation section where zinc-containing source material, when placed therein, can be fully or partly vaporized to the gas phase, and

- a condensation section where zinc vapour can be nucleated and/or condensed heterogeneously and can be collected on a surface, such as a particle collector, in a cyclone and/or a filter, and

- supplying a zinc-containing source material to the heated chamber.

What is referred to as "zinc vapour" could also be referred to as "vaporized zinc- containing particles".

In some embodiments of the invention, the heated chamber is heated by fuels, directly / indirectly electricity, a hot gas, and/or combinations thereof.

In the condensation section, there may be a temperature gradient. However, in principle the condensation and thereby the formation of the zinc-containing particles could alternatively be obtained by an increase in the pressure. In the condensation section, zinc vapour changes from gas phase directly / indirectly to solid phase thereby forming the zinc-containing particles. By "directly / indirectly" is meant that the formation mechanic may be vapour-solid or vapour-liquid-solid.

In some embodiments of the invention, the method further comprises the following steps:

- providing a furnace comprising:

- the heated chamber in which at least one of the temperature and the pressure is controllable as a function of time, and

- a cooling section forming at least a part of the condensation section,

- controlling the atmosphere and the operating conditions in the heated chamber in accordance with a pre-determined parameter profile, and

- collecting the zinc-containing particles on a particle collector, such as a particle collecting surface.

In some embodiments of the invention, the atmosphere and the operating conditions in the heated chamber are controlled so that during the thermal evaporation-condensation process: - the temperature in an evaporation section is 400 to 2000 degrees Celsius, such as 400 to 1200 degrees Celsius, such as 500 to 1000 degrees Celsius, such as 700 to 900 degrees Celsius,

- the partial pressure of zinc vapour produced by evaporation from the source material at the evaporation section is 500 to 1200 Pa, such as 700 to 1100 Pa,

- the temperature in a condensation section where the particles are collected is 150 to 400 degrees Celsius, such as 170 to 350 degrees Celsius, such as 170 to 250 degrees Celsius or 250 to 350 degrees Celsius.

In some embodiments comprising the experiments to be described below, the residence time of zinc vapour in the condensation section is 0.5 to 20 seconds, such as 0.5 to 10 seconds, such as 0.5 to 5 seconds, such as 0.5 to 3 seconds, such as 1 to 2.5 seconds. However, for other sizes of equipment used and dependent on how much surface is available per time for particle collecting, these numbers may be different.

In some embodiments of the invention:

- the heated chamber or furnace further comprises:

- a gas inlet for supplying at least one gas into the heated chamber, and

- a gas outlet through which the at least one gas can flow out of the heated chamber after having been cooled by flowing through at least a part of the cooling section,

- the method further comprises supplying the at least one gas into the heated chamber via the gas inlet.

In some embodiments comprising the experiments to be described below, the velocity of the flow of the at least one gas in the condensation section is 0.01 to 0.50 m/s, such as 0.01 to 0.15 m/s, such as 0.03 to 0.10 m/s, such as 0.04 to 0.08 m/s. However, for other sizes of equipment used and dependent on how much surface is available per time for particle collecting, these numbers may be different.

In some embodiments of the invention, the heated chamber has at least one gas inlet, one gas outlet, and a cooling section where zinc is condensed. The potential gas inlet is a system of supplying gas with controlled flow rate and composition into the heated chamber. When there is more than one gas, they can be supplied via the same inlet or via different inlets; it will just be referred to as "gas inlet" here and in the following. The choice of gas will depend on the type of particles to be manufactured, the zinc-source materials, cost, availability, and the system used. It may e.g. be inert gases, such as N2, He, or Ar, or reducing gases; e.g. gases from gasification or similar processes, such as CO or H2.

The gas in the heated chamber should preferably be inert or reducing. Instead of, or in combination with, a gas inlet, there could be a liquid inlet, or the zinc could be supplied as a slurry. In such embodiments without a gas supply, the gas can be created by evaporation of liquid.

When the gas is inert, the gas leaving the outlet will be the same as the one entering via the inlet. However, the zinc particles can also be produced under a slightly reducing gas atmosphere. In this case, a reducing gas, such as H2, would react with ZnO if the source material contains ZnO. By selecting the gas for a given application, the atmosphere of the heated chamber can be controlled, e.g. to an inert or slightly reducing atmosphere so that during the thermal evaporation and condensation processes, oxidation of zinc is avoided.

In some embodiments of the invention, the pressure in the heated chamber is around atmospheric pressure.

The zinc-containing particles that can be manufactured by a method according to the invention may be pure zinc or mixtures containing zinc, such as zinc alloyed particles or zinc coated particles.

What is referred to as "heated chamber" could also be referred to as "furnace chamber" or "inner chamber". The heated chamber may comprise more zones which can be heated to have different temperature profiles in one or more zones. The temperature may be controllable in all zones, or it may be controllable in only one or some of the zones independent of the temperature in other zones.

Furthermore, the temperature of the surface for collecting the particles may be controlled. The partial pressure of zinc vapour can be controlled by the gas flow and the evaporation rate of zinc. What is referred to as "the evaporation section" is the part of the heated chamber where the source material is placed and where the evaporation takes place.

The zinc-containing source material may be in solid form. Alternatively, the source material could be in liquid form, or a slurry of liquid and solid. The choice would e.g. depend on availability. The source material could also be referred to as "feed material".

In experiments leading to the present invention, the source materials were kept in the evaporation section for 60 minutes. Zinc is gradually vaporized until depleted, and the vapour is carried by the gas, so the real residence time in the condensation section can be estimated based on the distance between the location of the source material and the particle collecting surface and on the gas velocity.

The particle collecting surface could also be referred to as condensation surface or substrate. The particles may be created in the gas and fall down onto the surface, or they may grow/develop thereon. For some processes, a part of the inner surface of the heated chamber may also act as a particle collecting surface, or injected particle surface may act as particle collecting surface. Such injected particles could e.g. be hollow glass spheres and other lightweight materials, metallic particles such as aluminium, iron, or even zinc particles. The use of injected particles may be used for the synthesis of e.g. core-shell zinc composites in the form of particles coated with a shell of zinc.

In some embodiments of the invention, the particle collecting surface is movable along at least a part of the heated chamber, so that condensed zinc-containing particles can be withdrawn from the collecting surface and make the process in a continuous manner. This may be a preferred embodiment for large scale production facilities.

The source material may be supplied to the heated chamber in a container or on a plate, the source material being in a solid form, such as in the form of pure zinc, zinc compounds, or waste materials containing zinc. Such waste materials could e.g. be batteries, paint waste, cables, scrapped galvanized steel, mining waste. Many different sizes and shapes will be useful for the source material. Hereby waste material could find a valuable use instead of being disposed.

Alternatively, the source material may be supplied to the heated chamber by being sprayed into the heated chamber. It is also possible to supply some of the source material in a container or on a plate and supply some of the source material by spraying. The unwanted forms (e.g. shapes) of collected zinc particles may also be recycled as the source material to achieve a higher yield of the preferred form.

In some embodiments of the invention, the particle collector is arranged stationary during the process. This may be a batch process in which a certain amount of zinc-containing particles is produced before being removed from the heated chamber. When the particles have been removed from the particle collector, the particle collector may be re-used in a later batch. A particle collector for use in such a process may e.g. be a substrate made from aluminium oxide.

The inner surface of the heated chamber may be made from a material which is configured to function as the particle collector at least for some types of zinc- containing particles being manufactured and for some processing conditions. If the particles are formed on the inner surface of the heated chamber, it may be necessary to discontinue the process for the removal of the particles; this is also considered as a batch process.

A furnace suitable for use in a thermal evaporation-condensation process may be provided with a filter at the gas outlet. In such embodiments of the invention, at least a part of the filter may be configured to function as a particle collector. Studies on which the present invention is based have shown that the zinc- containing particles being collected at the filter may differ from those being collected on a particle collector in the form of a substrate or on other particles, such as injected particles, arranged at other locations. The heated chamber may be horizontally arranged. However, the scope of protection also covers other orientations, such as a vertically arranged heated chamber. The heated chamber may be a fixed bed system, an entrained flow system, a fluidized bed system, or other high-temperature heated systems depending on the form of source material.

In a third aspect, the invention relates to a coating composition comprising: more than 30wt%, preferably 60-95wt% in dry film of zinc-containing particles being obtained by a method according to the second aspect of the invention by weight of the total coating composition on a solvent free basis, and a binder system.

Thus, in the third aspect, the invention relates to a coating composition which provides improved cathodic protection performance and/or improved utilization efficiency of zinc as compared to the coatings comprising conventionally used zinc dust pigment. As used herein, the term 'improved cathodic protection performance' refers to an improvement in a coating's ability to provide cathodic or sacrificial protection to the underlying substrate via the oxidation of the more electrochemically active metallic zinc particles instead of iron present in the underlying substrate.

In some embodiments of the coating composition, the wt% in dry film of the zinc- containing particles is within one of the following ranges:

- 65 to 77 percent,

- 77 to 85 percent, and

- above 85 percent.

These ranges are those referred to in the SSPC-Paint 20 standard relating to zinc- rich primers and coatings. An alternative standard used for coating compositions is the ISO 12944 standard stating at least 80 wt% in the dry film as a zinc rich primer. Thus, this number could also be used to define specific embodiments of the invention. By ensuring that the wt% in dry film of the particles is within one of the ranges specified in a standard, it may be easier to have a coating composition approved for a given application. However, studies on zinc-containing particles made in relation to the present invention have shown that particles manufactured with a method according to the second aspect of the invention have improved anticorrosive properties. It may therefore be possible to obtain the same galvanic protection as with the traditionally used spherical zinc particles even if a lower wt% in dry film of the particles made with the present invention is used.

In addition to the total amount of the particles, a given coating composition may be further defined by specifying the size and/or the size distribution of the particles that makes it suitable for a given application. Particles should generally be smaller than the thickness of the dry coating film in which they are comprised. In some cases, sieving or classifying is necessary to get rid of coarse particles (e.g. particles coarser than 100 pm) as they may protrude from the coating film, causing defects and deteriorating the barrier effect and anticorrosion properties.

The binder may be selected from: silicate-based binder systems, epoxy-based binder systems, polyurethane-based binder systems, phenoxy resin-based binder systems, or cyclic rubber-based binder systems. However, the scope of protection in the broader sense covers any type of binder which is suitable for use in a coating composition.

Coating composition

The coating compositions obtained by methods according to the present invention have excellent anticorrosive properties. The synthesized zinc-containing particles may be used in combination with conventional binder systems, organic as well as inorganic, in a similar manner as zinc dust pigment is used in e.g. conventional zinc-rich, anti-corrosive coating compositions.

Preferably, the binder system of the present invention is selected from an epoxybased binder system or a silicate-based binder system. For example, a preferred epoxy-based binder system can be a combination of the one or more epoxy resins selected from e.g. bisphenol A, bisphenol F, novolac epoxies, one or more curing agents such as polyamines or polyamides, any reactive epoxy diluents and any reactive acrylic modifiers. Alternatively, new types of binders, e.g. epoxy resins or hardeners produced from lignin or other bio-based materials may also be used.

The blend of synthesized zinc-containing particles and zinc dust pigment can be used in the coating compositions of the present invention alone or together with other conventional or bio-based pigments, extenders, fillers, and corrosion inhibitors. For example, zinc particles can be employed together with conventional pigments such as titanium dioxide and iron oxide pigments, various fillers and extenders such as silica, talc, mica, or clays. In addition, corrosion inhibitors such as zinc phosphate or tannin derivatives may also be employed in the coating compositions.

The coating compositions of the present invention may also include other ingredients that are conventionally employed in anti-corrosive coating compositions of the prior art. For example, additives may be employed to improve the film properties. A rheology controlling agent, such as bentonite, may also be employed in the coating composition. Other ingredients that may be employed include anti-settling agents, wetting / dispersing agents, catalysts, deformers, plasticizers, adhesion promoters, and other additives.

The solvent possibly used in the coating compositions of the present invention may be any type of solvents and solvent mixtures conventionally employed in solvent-based coatings. Examples of solvents are selected from the group consisting of alcohols, esters, ketones, aromatic hydrocarbons, and mixtures thereof. The total weight of solvents should meet local regulations relating to volatile organic compounds (VOC). The coating compositions may also be formulated with water or without any solvents.

The zinc-containing particles are crucial components of the coating compositions of the present invention. The expression 'zinc-containing' is preferably intended to mean high-purity zinc so that at least 94% by weight of the particles is metallic zinc, the main unavoidable impurity typically being oxygen, which forms zinc oxide at the surface of the particles. Alternatively, it may also be the composites containing zinc, such as particles, e.g. hollow glass spheres and other lightweight materials, coated with zinc as a shell, or mixtures with other conductive materials or metals, e.g. graphite, char, aluminium and even iron, which may activate the zinc further. The composites and mixtures may have a zinc content lower than 94%. The particles can be in different shapes and sizes, but the size may be smaller than the designed coating film thickness. The coating compositions of the invention have outstanding utility as primers for use in corrosion-protection of e.g. bridges, off-shore oil drilling platforms, offshore wind turbine structures, guard rails, and structural metallic frames.

Full coating system

The coating compositions of the present invention may be used in a full coating system consisting of one or more layers. For example, the coating comprising silicate-based binder systems can be used as a stand-alone coating layer. The coating compositions comprising epoxy-based binder systems may be used as a primer and top-coated with e.g. epoxy coatings, polyurethane coatings, acrylic coatings, or other coatings specified in ISO 12944 part 5 and other standards.

Preparation of the coating composition

Coating compositions in accordance with the invention can be prepared either as single component or two component types, depending upon the resin which is used as a binder. It can be prepared by any applicable method that is commonly used within the coating industry or that is well known in the prior art. For example, the coating compositions can be prepared by mixing binder together with zinc particles and other ingredients using a high-speed dissolver, an ultrasonic homogenizer, a ball mill, a pearl mill, in-line disperser, etc.

Applying

The term 'applying' is used in its normal meaning within the coating industry. It is conducted by any conventional means, e.g. by dipping, by brush, by roller, by airspray, by air-less spraying, etc. The coating may be applied in a dry film thickness of 40-300 pm, preferably 60-100 pm.

The first, second and third aspects of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The method of manufacturing zinc-containing particles, characteristics of the manufactured particles, as well as a coating composition comprising such particles according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 schematically shows the experimental set-up of the manufacturing equipment which has been used during the development of the present invention.

Figures 2.a-2.d show SEM micrographs of four types of zinc particles which were studied during the work leading to the present invention.

Figure 3 shows results of accelerated salt spray tests made on four coating compositions.

Figure 4 shows results of open circuit potential measurements made with the same four coating compositions as in figure 3.

Figure 5 shows the particle size distributions as determined by a light scattering apparatus with a wet dispersion method.

Figure 6 shows XRD patterns of different zinc particles with the corresponding reference pattern for hexagonal phase zinc.

Figure 7 shows the relative texture coefficients determined for four reflection planes.

Figure 8 shows SEM micrographs of surface and cross sections of the same four coating compositions as in figure 3 after 30 days salt spray test.

Figure 9 shows variations of low-frequency impedance values ( | Z |f=0.01 Hz) for epoxy ZRCs formulated with different zinc particles over immersion time in 3.5 wt% NaCI solution. The results were obtained with the same four coating compositions as in figure 3.

Figure 10 shows results of salt spray tests with different combination ratios (Zn3 or Zn4:Znl - 1: 1, 1:4) with commercial Znl.

DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 schematically shows the experimental set-up of the manufacturing equipment which has been used during the development of the present invention. The method used is a thermal evaporation-condensation process with the features as described above. The zinc particles were synthesized in a furnace 1 with a horizontally arranged heated chamber 2 in which the temperature and the pressure are controllable. However, in the experiments to be described in the following, the pressure was kept constant at approximately atmospheric pressure. The heated chamber 2 is an alumina ceramic tube 3 surrounded by electrical heating elements 4 along a central region. In the illustrated embodiment, the actual temperature inside the heated chamber 2 is measured with a thermocouple 5. The furnace 1 comprises a gas inlet 6 for supplying N2 gas into the heated chamber 2 and a gas outlet 7 through which the N2 gas can flow out of the heated chamber 2. The N2 gas flows towards the gas outlet 7 via a cooling section 8 which may be water-cooled. A filter (not shown) is arranged at the gas outlet 7. In the studies leading to the invention, the source material was zinc in the form of powder 9 which was supplied to the heated chamber 2 in a container 10. During the synthesis process, the N2 gas was supplied into the heated chamber 2 via the gas inlet 6, and the conditions in the heated chamber 2 were controlled in accordance with a pre-determined parameter profile. The synthesized zinc particles were collected on a particle collector 11 in the form of a substrate made from aluminium oxide. The particle collector 11 was arranged stationary during the process.

In order to study the properties of coating compositions comprising zinc particles made by a method as described above and in relation to figure 1, a number of experiments were performed and will be described in the following. The experiments leading to the manufacturing of the zinc-particles were performed in a heated chamber 2 having an inner diameter of 51 mm and length of 100 cm. They were made under a continuous N2 gas flow of 3.0-4.0 NL/min at atmospheric pressure. Such a flow rate corresponds to 0.033 m/s in normal conditions (1 bar, 0 degrees Celsius), and varies with the temperature. E.g. the flow rate will be 0.075 m/s at the condensation section with a temperature of 350 degrees Celsius and 0.053 m/s at 170 degrees Celsius. The zinc vapour pressure was about 700- 1000 Pa in the evaporation section. It is controlled by the evaporation rate of zinc and the gas flow rate.

The source material was arranged so that zinc was evaporated at the middle of the evaporation section at a temperature of around 800 degrees Celsius and then deposited on a substrate placed downstream in the heated chamber at the condensation section having a temperature of approximately 170-350 degrees Celsius. These particles are referred to as Zn3 and Zn4 in the following. The temperature at the condensation section influences which type of particles that is obtained. E.g. Zn3 is found in a temperature region of 250-350 degrees Celsius and Zn4 is found in the region of 170-250 degrees Celsius. Thus, the parameter profiles that can be affected by temperature are different for the regions, such as gas velocity and zinc vapour pressure. The experiments showed that another type of zinc-particles could be collected on the filter equipped at the end of the heated chamber; these particles are referred to as Zn2 in the following. Thus, this filter can also be considered as a particle collector as explained above.

In order to study the anticorrosive properties of the newly developed zinc- particles, they were compared with commercially available zinc dust used in known anticorrosion coatings; these zinc dust particles are referred to as Znl in the following.

Figures 2.a-2.d show SEM micrographs of the four types of particles mentioned above. The large images are all obtained with a magnification of 2000, and the small images are all obtained with a magnification of 20,000. Figure 2. a is a commercial spherical zinc dust Znl, figure 2.b is spherical (oblate) zinc Zn2 collected on the filter, figure 2.c is irregular-shaped polygonal zinc Zn3 collected on the substrate, and figure 2.d is leave-like zinc Zn4 collected on the substrate. The inserted images in figures 2. a and 2.b are corresponding high magnification SEM images to show the differences more clearly between these two types of particles. As will be described in more detail in the following, the Zn3 and Zn4 particles have the properties according to claim 1.

The particles in figures 2.b-2.d were obtained in the following way.

Zn2:

The evaporation and condensation of zinc was carried out in a horizontally arranged chamber heated by electrical resistance as shown in Figure 1. Under nitrogen, zinc was vaporized from zinc powder at 800 degrees Celsius in the evaporation section. Zinc particles with a spherical (oblate) structure were collected on a glass fibre filter.

Zn3:

The evaporation and condensation of zinc was carried out in a horizontally arranged chamber heated by electrical resistance as shown in Figure 1. Under nitrogen, zinc was vaporized from zinc powder at 800 degrees Celsius in the evaporation section. Zinc particles with a polygonal layered structure and a high percentage of exposed (100) facet were condensed and collected on the surface of an alumina substrate where the temperature was approximately 250-350 degrees Celsius.

Zn4:

The evaporation and condensation of zinc was carried out in a horizontally arranged chamber heated by electrical resistance as shown in Figure 1. Under nitrogen, zinc was vaporized from zinc powder at 800 degrees Celsius in the evaporation section. Zinc particles with a dendritic structure and a high percentage of exposed (100) facet were condensed and collected on the surface of an alumina substrate where the temperature was approximately 170-250 degrees Celsius.

The four types of zinc particles were applied as sacrificial pigments at a zinc content of 78 wt% in dry film on a solvent free basis in an epoxy ZRC. A two- component coating was prepared by mixing the constituents listed below in base and curing part, and the application was initiated by mixing two parts. The quantities were in percent by volume based on the total volume: Part 1: Base

Epoxy resins 17.01

Zinc-containing particles 21.16

Rheological agents 2.07

Solvents (n-butanol, xylene) 32.43

Wetting/dispersing agents 0.61

Deformer 0.61

Reactive diluent 1.82

Part 2: Curing agent

Polyamide adduct 17.86

Accelerator 1.58

Solvents (n-butanol, xylene) 4.86

Their anticorrosion performance was evaluated in an accelerated salt spray test by following the standard ISO 9227 and the results are shown in figure 3. The four rows of images correspond to figures 2.a-2.d. The columns show images taken after the number of days marked at the top of each column in figure 3. The results of the accelerated salt spray test show that Zn3 and Zn4 exhibited superior anticorrosion performance compared to Znl. Less and later formation of red rust (caused by steel corrosion) was observed on the coatings containing Zn3 and Zn4; especially Zn3 where no red rust was observed after 30 days of exposure. This indicates the sufficient sacrificial corrosion of active zinc particles around the scribe of both of these coatings. Zn2 showed surprisingly worse anticorrosion performance than Znl. It had a higher localized galvanic activity near the scribe areas (more white rust observed after one day exposure), where active zinc particles were consumed faster, and red rust was observed within five days of exposure.

The open circuit potential of the zinc-rich coating is an indication of the efficiency of electrical connectivity between zinc particles and between the zinc particles and the underlying steel substrate. The open circuit potential (OCP) and electrochemical impedance spectroscopy tests were conducted using an electrochemical set-up (Gamry Potentiostat Reference 600+, Gamry). A glass cylinder was affixed to the coated steel surface to be tested. The counter electrode (graphite rod) and reference electrode (saturated calomel electrode) were then inserted into the electrolyte solution (3.5 wt. % NaCI solution). The working electrode was connected to the sample (coated steel substrate) with an exposing area of 10 cm². The OCP was measured until the system was stable and at equilibrium. The electrochemical impedance spectroscopy test was conducted over a frequency range of IO^-2 - 10⁵ Hz, with a signal amplitude of 20 mV at the stable OCP. Please refer to standard ASTM C876 for further details about this type of measurements.

Figure 4 shows results of OCP measurement made with the same four coating compositions as were described in relation to figure 3 during immersion in 3.5 wt% NaCI solution. OCP measurements are widely used to characterize the cathodic protection ability and duration of ZRCs. An OCP less than -0.86 V/SCE (i.e. the dotted line) represents the thermodynamic protection limit that has been commonly accepted as the criterion for cathodic protection of iron. Figure 4 shows that the film electrical conductivity was significantly improved for the coating compositions containing Zn3 and Zn4 when compared to Znl, since both Zn3 and Zn4 exhibited very negative OCP values located in the cathodic protection region (< -0.86 V/SCE) during the entire test period. Such negative values indicate a high active zinc-to-steel area ratio, i.e. good electrical conductivity and thus high cathodic protection intensity. These results therefore show that the Zn3 and Zn4 particles made with a method according to the present invention contributed to a better and longer period of cathodic protection than ZRC based on Znl. The coating compositions with Zn2 showed a dramatic increase of potential due to a burst corrosion of zinc particles, providing cathodic protection for a short time.

In order to study different possible explanations of the improved performance of Zn3 and Zn4, the following characterisations of the different types of zinc particles were performed.

Figure 5 shows the particle size distributions as determined by a light scattering apparatus with wet dispersion method. As seen, the particle size distribution of Zn3 is significantly wider than the other distributions. It is expected that this contributed to better electrical contacts between both the zinc particles themselves and between the zinc particles and the steel substrate to be protected. Figure 6 shows X-ray diffraction (XRD) patterns of different zinc particles. The XRD patterns were obtained using a Huber G670 powder diffractometer with Cu- Ko radiation at the scanning range of 3-100 ° for 30 min.

The obtained XRD patterns exhibit diffraction peaks consistent with zinc, as indexed by the JCPDS (card no. 03-065-5973) data. The sharp and narrow peaks imply good crystallinity of the structures. The following table shows the relative intensity of the (100) peak, the (110) peak, and the (101) peak to the (002) peak in the XRD patterns:

Table 1

The values referred to as "Zn standard" in the table is one type of hep zinc crystal with the card no. of 03-065-5973. Zn3 and Zn4 particles show relatively higher intensity peak attributed to the (100), (101) and (110) planes in comparison to other particles, suggesting a higher prevalence of these planes in Zn3 and Zn4.

The crystal orientation of zinc particles was further analyzed by determining the relative texture coefficient RTC(hkl) which is an important index for the degree of orientation. It is calculated using the following formula: 100 where is the intensity of each plane (hkl) from the zinc particles, and is the intensity from the aforementioned Zn standard. Figure 7 shows the relative texture coefficients determined for the four crystal planes of interest. Zn3 and Zn4 particles exhibit the highest RTC value for the (100) plane, while the lowest RTC value for the (002) plane is observed for Zn3 particles, highlighting a higher exposure of (100) plane.

It is at present assumed that the difference in crystalline structure of the zinc particles, e.g., different exposure levels of particularly (100) and (002) planes (Figure 6, Table 1 and Figure 7) could influence the electrochemical corrosion of zinc particles and thus the electrochemical characteristics of the coating composition.

As mentioned above, zinc has a hexagonal close-packed (hep) structure with two exposed crystal planes, (100) and (002) planes. The surface atomic structure varies greatly for each crystal plane and may have main influence on the electrochemical characteristics. A surface atom coordinates with nine other atoms on the (002) plane surface, while there are only six atoms adjacent to each atom on the (100) plane surface, which has low binding forces between the surface atoms, and it is more easy to break the bonds and dissolve the atoms compared to other planes in the hep structure. The (100) plane with a higher surface energy is less stable and more electrochemically active. In the work described above, Zn3 and Zn4 particles with a higher percentage of reactive (100) planes exposed compared to (002) planes are obtained, whereas in the prior art, it is more common that zinc dust is naturally grown to a structure in which more (002) planes are exposed. Therefore, Zn3 and Zn4 particles may have a higher galvanic activity.

As described above, the source material, the operating conditions, and the atmosphere in the heated chamber are selected so that the thermal evaporationcondensation process results in synthesized zinc particles in which the crystal structure is:

- a polygonal layered structure with a high percentage of exposed (100) facet, wherein the relative peak intensity I(100)/I(002) > 1, or

- a dendrite with a high percentage of exposed (100) facet, wherein the relative peak intensity I(100)/I(002) > 1.

As seen from table 1, Zn3 and Zn4 fulfil this criteria.

In order to find a possible explanation for the slightly worse performance of Zn4 compared to Zn3 as seen in figure 3, SEM micrographs of surfaces and cross sections of the same four coating compositions as in figure 3 after 30 days salt spray test were made as shown in figure 8. They show the epoxy ZRCs with different zinc particles after 30 days salt spray tests. Within the coating film, the black areas are the binder system, the bright areas are un-corroded zinc, and the grey areas are zinc corrosion products. Surface of coatings containing Znl and Zn2 was covered by a layer of spongy corrosion products, primarily composed of zinc and oxygen according to EDS analysis (Table), indicating the formation of zinc oxides/hydroxides. These corrosion products could block the ionic pathways in the coating and delay the diffusion of corrosive media, resulting in less severe attack of zinc particles in the lower layer. In contrast, the 78%Zn3 and 78%Zn4 coatings featured a thick and compact corrosion product layer ((g) and (h)), contributing to enhanced post-cathodic protection. In addition to those spongy corrosion products, quite a few hexagonal crystals, rich in zinc, oxygen and chloride, were observed on the surface ((c) and (d)), indicating the formation of plate-like simonkellite (Zns(OH)8Cl2-H2O) crystals known for superior barrier protection compared to zinc oxides/hydroxides. The variance in corrosion product formation may be attributed to differences in crystalline structures of zinc particles. A few micro-cracks were observed in the corrosion products layer of 78%Zn4 coating, which resulted in corrosion in the intact area during the late stage of exposure, as evidenced by the salt spray test. This difference between Zn3 and Zn4 has been studied by use of |Z|f=0.01 Hz values obtained from impedance Bode plots to qualitatively evaluate the coating barrier property; see figure 9 showing results obtained with the same four coating compositions as in figure 3. The initial decrease in | Z|f=o.oiHz values resulted from water uptake and zinc activation, followed by a gradual increase due to the formation of insoluble zinc corrosion products. The low and steadily decreasing | Z |f=o.oi HZ values, along with negative OCP values, suggested enduring effectiveness of the coatings containing Zn3 and Zn4 in providing cathodic protection over an extended period.

Table 2. Elemental composition of corrosion products formed on coating surface obtained from EDS analysis*.

Sample Zn C 0 Si Cl Fe

78%Znl 63.9 7.4 26.8 0.4 1.4 0.1

78%Zn2 61.7 9.9 25.6 0.6 2.0 0.3

78%Zn3 50.0 3.8 31.5 14.7

78%Zn4 67.4 2.3 15.2 0.2 15.0

* It is an average elemental composition of the entire coating surface for the inserted high magnification images in Figures 8 (a) and (b), and the area marked by a red box in Figures 8 (c) and (d). Figure 10 shows results of salt spray tests with different combination ratios (Zn3 or Zn4:Znl - 1: 1, 1:4) with commercial Znl. A two-component coating was prepared by mixing the constituents listed below in base and curing part, and the application was initiated by mixing two parts. The coating contains approximately 78% zinc (half commercial zinc dust pigment and half synthesized zinc-containing particles) by weight of the total coating composition on a solvent free basis. The quantities are in percent by volume based on the total volume:

Part 1: Base

Epoxy resins 17.01

Zinc dust pigment 10.58

Zinc-containing particles 10.58

Rheological agents 2.07

Solvents (n-butanol, xylene) 32.43

Wetting/dispersing agents 0.61

Deformer 0.61

Reactive diluent 1.82

Part 2: Curing agent

Polyamide adduct 17.86

Accelerator 1.58

Solvents (n-butanol, xylene) 4.86

The results of the accelerated salt spray test show that the coatings exhibited superior anticorrosion performance when only part of commercial zinc (Znl) was replaced with Zn3 and Zn4. This indicates that sufficient sacrificial protection to steel can also be achieved with a combination of commercial zinc dust and synthesized zinc particles.

Thus, in summary, the following parameters are considered to influence the anticorrosion performance of ZRCs made with zinc-particles produced by a method according to the present invention: the particle size and size distribution, the particle shape, the oxidation activity, and the crystal structure. The selection of these properties is based on experiments, involving typically coating formulation and testing. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. Zinc-containing particles whereof at least 80%, such as at least 90%, such as at least 95% have a crystal structure which is:

- a dendrite with a high percentage of exposed (100) facet, wherein the relative peak intensity I(100)/I(002) > 1, when determined from X-ray diffraction (XRD) patterns.

2. Zinc-containing particles according to claim 1, whereof at least 80%, such as at least 90%, such as at least 95% have a crystal structure which is:

3. Method of manufacturing zinc-containing particles according to claim 1 or 2 by a thermal evaporation-condensation process, the method comprising:

- providing a heated chamber (2) comprising:

- an evaporation section where zinc-containing source material (9), when placed therein, can be fully or partly vaporized to the gas phase, and

- a condensation section where zinc vapour can be nucleated and/or condensed heterogeneously and can be collected on a surface, such as a particle collector (11), in a cyclone and/or a filter, and

- supplying a zinc-containing source material (9) to the heated chamber (2).

4. Method according to claim 3, wherein the heated chamber (2) is heated by fuels, directly / indirectly electricity, a hot gas, and/or combinations thereof.

5. Method according to claim 3 or 4, the method further comprising the following steps:

- providing a furnace (1) comprising:

- the heated chamber (2) in which at least one of the temperature and the pressure is controllable as a function of time, and - a cooling section (8) forming at least a part of the condensation section,

- controlling the atmosphere and the operating conditions in the heated chamber

(2) in accordance with a pre-determined parameter profile, and

- collecting the zinc-containing particles on a particle collector (11), such as a particle collecting surface.

6. Method according to claim 5, wherein the atmosphere and the operating conditions in the heated chamber (2) are controlled so that during the thermal evaporation-condensation process:

- the temperature in an evaporation section is 400 to 2000 degrees Celsius, such as 400 to 1200 degrees Celsius, such as 500 to 1000 degrees Celsius, such as 700 to 900 degrees Celsius,

- the partial pressure of zinc vapour produced by evaporation from the source material (9) at the evaporation section is 500 to 1200 Pa, such as 700 to 1100 Pa, and

7. Method according to any of claims 3 to 6, wherein:

- the heated chamber (2) or furnace (1) further comprises:

- a gas inlet (6) for supplying at least one gas into the heated chamber (2), and

- a gas outlet (7) through which the at least one gas can flow out of the heated chamber (2) after having been cooled by flowing through at least a part of the cooling section (8),

- the method further comprises supplying the at least one gas into the heated chamber via the gas inlet (6).

8. Method according to any of claims 3 to 7, wherein the pressure in the heated chamber (2) is around atmospheric pressure.

9. Method according to any of claims 3 to 8, wherein the source material (9) is supplied to the heated chamber (2) in a container (10) or on a plate, the source material (9) being in a solid form, such as in the form of pure zinc, zinc compounds, or waste materials containing zinc.

10. Method according to any of claims 3 to 9, wherein the source material (9) is supplied to the heated chamber (2) by being sprayed into the heated chamber (2).

11. Method according to any of claims 3 to 10, wherein the particle collector (11) is:

- arranged stationary during the process, or

- moving along at least a part of the heated chamber (2) and out thereof via an exit so that the process can run continuously.

12. Method according to any of claims 3 to 11, wherein an inner surface of the heated chamber (2) is made from a material which is configured to function as the particle collector (11).

13. Method according to any of claims 3 to 12, wherein a filter is arranged at the gas outlet (7), and wherein at least a part of the filter is configured to function as the particle collector (11).

14. Coating composition comprising:

- more than 30wt%, preferably 60-95wt% in dry film of zinc-containing particles being obtained by a method according to any of claims 3 to 13, by weight of the total coating composition on a solvent free basis, and

- a binder system.

15. Coating composition according to claim 14, wherein the wt% in dry film of the zinc-containing particles is within one of the following ranges:

- 65 to 77 percent,

- 77 to 85 percent, and

- above 85 percent.

16. Coating composition according to claim 14 or 15, wherein the binder is selected from: silicate-based binder systems, epoxy-based binder systems, polyurethane-based binder systems, phenoxy resin-based binder systems or cyclic rubber-based binder systems.