IE20170181A1

IE20170181A1 - Coating

Info

Publication number: IE20170181A1
Application number: IE20170181A
Authority: IE
Inventors: Guest Peter
Original assignee: Spencer Coatings Ltd
Priority date: 2016-09-15
Filing date: 2017-09-13
Publication date: 2018-05-30
Also published as: IE87051B1; GB201615701D0; GB2553813B; GB2553813A

Abstract

A primer for application to a metal substrate is provided. the primer comprises a primer formulation including an amount of volatile corrosion inhibitor (VCI) additive, wherein the VCI additive content of the primer formulation is between 0.1% and 1% by weight. <Figure 1>

Description

FIELD The present disclosure relates to coatings, a method of preparing such coatings and metal substrates to which such coatings have been applied.

BACKGROUND It is known that metal structures are susceptible to corrosion, particularly those structures in locations having high levels of humidity. Such structures may for example be electricity pylons, girders, bridges, pipes, off-shore structures or any other metal structure.

Painting such a metal structure with a coating is a commonly used method of preventing or reducing corrosion. However, where a metal surface is already heavily corroded, the surface of the metal is often very uneven. In these conditions it is difficult to evenly apply a primer or a similar coating to such a surface, resulting in regions of the metal surface which the paint does not contact. This reduces the anti-corrosive effect of the paint or coating.

One option to address this problem is to prepare the corroded metal surface prior to application of a paint, e.g. by sanding, to create a more even surface for painting. However this is typically time intensive and therefore costly. It also may be difficult to carry out such preparation, for example where the metal surface is difficult to access.

The present disclosure aims to reduce or overcome such problems.

SUMMARY In a first aspect a primer is provided for application to a metal substrate, wherein the primer comprises a primer formulation including an amount of volatile corrosion inhibitor (VCI) additive, wherein the VCI additive content of the primer formulation is between 0.1% and 1% by weight.

In some embodiments, the VCI additive content of the primer is between, 0.1 to 0.9%, for example between 0.2 to 0.8%, for example between 0.3% and 0.7%, for example between 0.4 to 0.6% of the primer formulation by weight, optionally 0.5%.

In some embodiments the primer is a moisture-cure primer which is formulated to cure with water.

In some embodiments the moisture-cure primer is a moisture-cure polyurethane primer and the primer formulation comprises a resin containing an isocyanate prepolymer formulated to polymerise with water to form a polyurea coating.

In some embodiments the resin content of the primer formulation is between 10% and 80% of the primer formulation by weight, for example between 20 and 70%, for example between 30 and 60%, for example between 40 and 50% by weight, optionally between 45% and 50%.

In some embodiments the VCI additive is added to the primer formulation in powder form.

In some embodiments the VCI additive comprises an amine based compound. In some embodiments the VCI additive comprises cyclohexylamine benzoate.

In some embodiments the primer formulation comprises a drying additive formulated to remove moisture from the primer formulation. In some embodiments the drying additive is a monofunctional isocyanate drying additive.

In some embodiments the drying additive content of the primer formulation is up to 5% of the primer formulation by weight, for example between 0.5 and 3%, for example between 1 and 1.5% by weight, optionally up to 1.5%.

In some embodiments the primer formulation comprises a cure accelerator to accelerate curing of the primer. In some embodiments the cure accelerator comprises a metal-based cure accelerator, for example a tin-based cure accelerator.

In some embodiments the cure accelerator content of the primer formulation is up to 0.5% of the primer formulation by weight, for example up to 0.25%, for example up to 0.1% by weight, optionally less than 0.1%.

In some embodiments the primer is formulated to cure with ambient moisture in the atmosphere.

In a second aspect, a metal substrate is provided to which the primer described herein has been applied.

In some embodiments the metal substrate is a steel structure.

In some embodiments the metal substrate comprises at least a portion of a pylon, a girder, or a pipe.

In a third aspect a pylon is provided for supporting a power line, the pylon comprising one or more structural elements, wherein at least a portion of at least one of the structural elements is coated with the primer described herein.

In a fourth aspect a primer is provided, wherein the primer is a moisture-cure polyurethane primer which is formulated to cure with moisture, and wherein the primer comprises a primer formulation including: a volatile corrosion inhibitor (VCI) additive wherein the VCI content of the primer formulation is between 0.1% and 1% by weight; an isocyanate containing resin formulated to polymerise with water to form a polyurea coating, wherein the amount of resin present is between 10% and 80% of the primer formulation by weight; a monofunctional isocyanate drying additive formulated to remove moisture from the primer formulation, wherein the amount of drying additive present is up to 5% of the primer formulation by weight; aluminium powder; and a tin based cure accelerator to accelerate curing of the primer, wherein the amount of cure accelerator present is up to 0.5% of the primer formulation by weight.

In a fifth aspect a method of preparing a primer for application to a metal substrate is provided, wherein the method comprises preparing a primer formulation and, during preparation of the primer formulation, adding an amount of volatile corrosion inhibitor (VCI) additive to the formulation, wherein the amount of volatile corrosion inhibitor added is between 0.1% and 1% of the resulting primer formulation by weight.

In some embodiments the amount of VCI additive added during preparation of the primer formulation is between 0.1 to 0.9%, for example between 0.2 to 0.8%, for example between 0.3% and 0.7%, for example between 0.4 to 0.6% of the resulting primer formulation by weight, optionally 0.5%.

In some embodiments preparing the primer formulation includes the step of providing a resin containing an isocyanate prepolymer.

In some embodiments the amount of resin provided is between 10% and 80% by weight of the resulting primer formulation, for example between 20 and 70%, for example between 30 and 60%, for example between 40 and 50% ofthe resulting formulation, optionally between 45% and 50%.

In some embodiments the VCI additive is added as a powder.

In some embodiments preparing the primer formulation includes the step of adding a drying additive to the formulation, wherein the drying additive is formulated to remove moisture from the primer formulation.

In some embodiments the drying additive is a mono-functional isocyanate drying additive.

In some embodiments the amount of drying additive added to the formulation is up to 5% of the resulting primer formulation by weight, for example between 0.5 and 3%, for example between 1 and 1.5% by weight, optionally up to 1.5%.

In some embodiments preparing the primer formulation includes the step of adding a cure accelerator to the formulation to accelerate curing ofthe primer.

In some embodiments the cure accelerator comprises a metal-based cure accelerator, for example a tin-based cure accelerator.

In some embodiments the amount of cure accelerator added is up to 0.5% of the resulting primer formulation by weight, for example up to 0.25%, for example up to 0.1% by weight, optionally less than 0.1%.

In a sixth aspect a paint is provided for application to a metal substrate, wherein the paint comprises a paint formulation including an amount of volatile corrosion inhibitor (VCI) additive, wherein the VCI additive content of the paint formulation is between 0.1% and 1% by weight.

In a seventh aspect, a metal substrate is provided to which the paint described herein has been applied.

In an eighth aspect a pylon is provided for supporting a power line, the pylon comprising one or more structural elements, wherein at least a portion of at least one of the structural elements is coated with the paint described herein.

In a ninth aspect a paint is provided, wherein the paint is a moisture-cure polyurethane paint which is formulated to cure with moisture, and wherein the paint comprises a paint formulation including: a volatile corrosion inhibitor (VCI) additive wherein the VCI content of the paint formulation is between 0.1% and 1% by weight; an isocyanate containing resin formulated to polymerise with water to form a polyurea coating, wherein the amount of resin present is between 10% and 80% of the paint formulation by weight; a monofunctional isocyanate drying additive formulated to remove moisture from the paint formulation, wherein the amount of drying additive present is up to 5% of the paint formulation by weight; aluminium powder; and a tin based cure accelerator to accelerate curing of the paint, wherein the amount of cure accelerator present is up to 0.5% of the paint formulation by weight.

In a tenth aspect a method of preparing a paint for application to a metal substrate is provided, wherein the method comprises preparing a paint formulation and, during preparation of the paint formulation, adding an amount of volatile corrosion inhibitor (VCI) additive to the formulation, wherein the amount of volatile corrosion inhibitor added is between 0.1% and 1% of the resulting paint formulation by weight.

In some embodiments, the paint comprises a primer.

It will be appreciated that features described herein in relation to primers are also applicable to paints. All combinations contemplated will not be explicitly recited here for the sake of brevity.

BRIEF DESCRIPTION OF FIGURES Embodiments will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating a primer in accordance with this disclosure when applied to a metal substrate; and Figure 2 shows a flow chart illustrating a method of making the primer shown in Figure 1.

DETAILED DESCRIPTION Primers are disclosed herein for application to metal substrates. In some embodiments the primer is a moisture-cure primer, commonly referred to as a moisture-cure polyurethane primer or moisturecure urethane primer. As will be discussed below, when the moisture-cure primer is applied to a substrate, the moisture-cure primer cures with ambient water to form a hard polyurea film on the substrate. This film inhibits moisture accessing the substrate and so reduces further corrosion occurring.

The primers disclosed herein include an amount of volatile corrosion inhibitor (VCI) additive. The VCI additive content in the primer formulation is between 0.1% and 1% by weight. This has the advantage of increasing the anti-corrosion properties of the primer, as will be discussed in more detail below. It has also been found that this amount of VCI additive is sufficiently low that it does not impair the hardness of the cured primer. Therefore a robust coating with enhanced anticorrosion properties is provided.

In some embodiments, primers disclosed herein have a formulation including a resin containing an isocyanate prepolymer. The isocyanate prepolymer is formulated to cure with moisture in the air, in other words to polymerise with water in the atmosphere, to form a polyurea film according to the following reaction mechanism: H2O + R-N=C=O (isocyanate) A R-NH2 + CO2 (1) R-NH2 + R'-N=C=O -> RN(H)C(O)N(K)R' (urea linkage) As is understood in the art, although referred to as a moisture-cure polyurethane or urethane primer, the primer containing isocyanate cures with water to form a polyurea coating.

The isocyanate prepoiymer may comprise more than one isocyanate functional group. For example, the isocyanate prepolymer may be a di-isocyanate.

The term primer as used herein will be understood as meaning a coating which can be applied directly to a substrate, for example prior to painting with a top coat. Whilst it will be understood that a top coat can be applied to a primer coating, this is not necessarily always the case and the primer can be applied to the substrate without subsequent application of a top coat.

In some embodiments the resin is dissolved in a solvent which contributes to the desired thickness of the primer by acting as a thinner. The solvent may be any suitable hydrocarbon solvent. In this way the thickness of the primer can be adjusted such that it is suitable for a desired application.

Since the primer is formulated to cure with moisture, it can be advantageous to remove water from the primer formulation, or reduce the water content, to prevent curing of the primer before it is desired. To achieve this, in some embodiments the primer formulation also includes a moisture scavenger component or drying additive. Any suitable moisture scavenger liquid or solid may be used, for example a monomeric or monofunctional isocyanate drying additive. if desired, a colour pigment can be added to the primer formulation to give the primer a desired colour. Any colour pigment may be used as desired. Alternatively, no colour pigment need be added. Such colour pigments may also contain water and so the moisture scavenger in the primer formulation also acts to remove water from these pigments.

In some embodiments, the primer formulation includes a metallic element component in powder form e.g. aluminium. This may be any grade of aluminium and/or any metallic element or combination of metallic elements in powder form. This component acts as a corrosion inhibitor as is commonly known in the art. The primer formulation may also contain a thickener, for example a mineral thickener in solvent, which acts to keep the metallic powder dispersed throughout the formulation, for example by altering the rheology of the mixture.

A volatile corrosion inhibitor (VCI) additive is included in the primer formulation. This may be any suitable VCI additive, for example, in powder form. The use of a VCI additive in powder form is advantageous since the powder is relatively dry and so introduces minimal, if any, water into the composition. In the case of moisture-cure primers, the resin is formulated to cure with water, as discussed above, therefore reducing the amount of water which is introduced into the formulation via the VCI additive is advantageous in preventing curing of the primer before it is desired.

Volatile Corrosion Inhibitors are known to act as a corrosion inhibitor. Volatile Corrosion Inhibitors vaporise within an enclosed volume of air and sublime onto a metal surface to form a protective film on the metal surface. Typically VCIs are applied to a substrate by wrapping the substrate in a paper or film carrying VCI molecules. When wrapped around the substrate, the paper of film forms an enclosed space around the substrate. The VCI molecules vaporise into the atmosphere around the substrate, within the enclosed space, and when the vapour concentration reaches a certain level, sublimes onto the surface of the substrate to form a thin molecular layer on the surface of the substrate.

In some alternative known methods, VCIs are applied by blowing a powder into an enclosed space. The powder vaporises and then sublimes onto the surface of the substrate, as described above, to form a film on the surface of the substrate.

When a VCI additive is added to a moisture-cure primer in an amount of 0.1% to 1% by weight, it would be expected that the VCI molecules would be bound in the moisture-cure primer structure such that they are not free to vaporise and therefore would have no effect on the anti-corrosion properties of the primer. However, it has been found that the addition of a VCI additive in a moisture-cure primer in this amount has the effect of increasing corrosion resistance. It is believed that at least some of the VCI additive in the primer is free to vaporise out of the primer such that it forms a film on the surface of the substrate in a similar manner as described above. As will be described in more detail below, this acts to enhance the anti-corrosive properties of the primer.

This is described in more detail below in relation to Figure 1. Further, it has been found that adding a VCI in this amount does not impair the hardness of the cured primer. Therefore a robust coating with enhanced anti-corrosion properties is provided.

In some embodiments an amine based VCI is used. In embodiments where the VCI additive used is an amine based VCI and the substrate is a metal substrate, the lone pair of electrons on the nitrogen atom of the amine is attracted to the polar metal surface. The remainder of the VCI molecule is hydrophobic and so, when the VCI film is formed on the metal surface, the VCI molecules act to repel water from the metal surface, thereby reducing corrosion.

In order to adjust and/or tailor the time taken for the primer to cure, a cure accelerator may be included in the formulation. The cure time will therefore depend, at least in part, on the amount of cure accelerator present. Any cure accelerator which acts to catalyse the above reaction (1) may be used, for example a metal based cure accelerator. In some embodiments a tin based cure accelerator is used. In some embodiments a bismuth based cure accelerator is used.

Exemplary primer formulations are provided below. The examples herein are provided to facilitate an understanding of the invention. The examples are not intended to limit the scope of the claims.

Example i A moisture-cure primer for application to a metal substrate has the following composition: Component Percentage by weight Isocyanate containing resin For example a liquid isocyanate prepolymer resin in solvent, e.g. the product sold as Desmodur® E21 by Covestro. In some embodiments the primer formulation comprises 10 to 80% isocyanate resin by weight, for example 20-70%, for example 30 to 60%, for example 40 to 50% by weight. In some embodiments the content of isocyanate resin is between 45 and 50% by weight. Solvent/thinner For example a hydrocarbon solvent, e.g. the product sold as Solvesso™ 100 by ExxonMobil Chemical Hydrocarbons. In some embodiments the primer formulation comprises 0 to 50% solvent/thinner by weight, for example 10% to 40%, for example 20% to 30%, for example 20% to 25% by weight. Moisture scavenger liquid or solid For example a monomeric or monofunctional isocyanate drying additive, e.g. the product sold as Additive Tl by OMG Borchers GmbH, or the product sold as PTSI by Luna Chemicals Co., Ltd. In some embodiments a zeolite based drying agent is used. Any drying agent which is compatible with an isocyanate resin may be used. In some embodiments the primer formulation comprises 0 to 5% moisture scavenger by weight, for example 0.5% to 3%, for example 1% to 1.5% by weight, in some embodiments the amount of moisture scavenger added is sufficient to remove all or substantially all water present in the composition.

Thickener For example a mineral thickener, e.g. a clay based thickener. In some embodiments a bentonite clay thickener is used. In some embodiments a silica based thickener is used e.g. Aerosil® by Evonik Industries. In some embodiments the primer formulation comprises 0 to 40% thickener by weight, for example 0 to 30%, for example 0 to 20%, for example 0 to 10%, for example 0 to 5% by weight. Metallic element in powder form For example aluminium powder, e.g. fine aluminium powder in a solvent paste. For example non-leafing aluminium paste. In some embodiments the primer formulation comprises 0 to 40% metallic element by weight, for example 10% to 30%, for example 20% to 25% by weight. Volatile corrosion inhibitor (VCI) For example a volatile corrosion inhibitor powder. In some embodiments an amine based VCI is used. In some embodiments cyclohexylamine benzoate is used. In some embodiments the primer formulation comprises 0.1 to 1% VCI additive by weight, for example, 0.1 to 0.9%, 0.2 to 0.8%, 0.3 to 0.7%, 0.4 to 0.6%, for example approximately 0.5% by weight. Cure accelerator For example a tin based cure accelerator liquid, e.g. dibutyltin dilaurate. In some embodiments the primer formulation comprises 0 to 0.5% cure accelerator by weight, for example 0% to 0.25%, for example 0% to 0.1% by weight. Colour pigment For example a colour pigment dispersion in solvent, e.g. red oxide pigment, or other desired pigment. In some embodiments the primer formulation comprises 0 to 20% of colour pigment by weight, for example up to 15%, for example up to 10%, for example up to 5% by weight. In some embodiments, any amount of pigment sufficient to achieve the desired colour is used.

It will be appreciated that the above proportions of ingredients may be adjusted to provide a primer with the properties desired.

Example 2 A moisture-cure primer for application to a metal substrate was made using the following composition: Component Percentage by weight Desmodur® E21 47.061 Solvesso™ 100 22.122 Additive Tl 1.444 A bentonite clay thickener 3.757 Non-leafing aluminium paste 20.198 Cyclohexylamine benzoate (VCI) 0.500 Dibutyitin dilaurate 0.071 Red oxide pigment 4.847 In use, a moisture-cure primer disclosed herein, for example as defined in either example 1 or 2 above, is applied to the surface of a metal substrate 102 (see Figure 1), for example a metal pylon. The primer is suitable for application directly onto corroded metal surfaces, including those which are heavily corroded, those comprising unstable lamellar rust, and/or those comprising crumbly and/or loosely adherent rust layers. Pre-preparation of the corroded metal surface e.g. by sanding, prior to application of the primer is not required.

The primer is applied onto the corroded metal surface in any suitable manner, for example painting or spraying. Advantageously, special application of the primer is not required, standard application methods are appropriate. When the primer is applied to the metal surface it is exposed to atmospheric conditions. As described above, the isocyanate prepolymers present in the resin cure with moisture in the air, in other words, the isocyanate prepolymers polymerise with water in the atmosphere. The speed of this reaction will depend, at least in part, on the amount of cure accelerator present in the primer formulation.

When the primer cures it hardens to form a tough, impervious polyurea film 104 on the surface of the metal. This polyurea layer acts to reduce further corrosion of the metal surface by preventing water accessing the metal substrate or substantially reducing the amount of water accessing the metal substrate. The moisture-cure primer has a low viscosity and hence soaks into the rust on the surface of the metal, penetrating some of the unstable lamellar rust and crumbly loosely adherent rust. However, in particular where the metal substrate is heavily corroded, there may be regions of the rusted surface that the moisture-cure primer cannot penetrate and so voids 106 form between the polyurea film 104 and the surface of the metal 108. In this way, enclosed volumes between the polyurea film 104 formed by the primer and the metal surface 108 are formed.

As described above, it is believed that an amount of the VCI additive in the primer is free to vaporise out of the primer, the vapour 110 occupying the voids 106. The VCI vapour 110 is believed to then sublime onto the surface of the metal 108 to form a thin, uniform molecular layer 112. The VCI layer 112 forms a barrier to water, thereby further reducing or preventing corrosion of the metal substrate 102. This is an unexpected effect since it would be expected, in particular in view of the relatively small amount of VCI present in the primer formulation, that all the VCI molecules would be bound in the moisture-cure primer structure and so not free to vaporise. However, it has been found that at least some of the VCI present in the primer is free to vaporise. In this way, it is believed that the VCI penetrates the corroded metal structure beyond that achievable by the moisture-cure primer alone, and deposits a layer 112 of corrosion inhibitor onto the surface of the metal. This slows or stops further corrosion beneath the primer.

Consequently, the inclusion of between 0.1 to 1% by weight in the primer formulation results in the surprising effect of enhanced anti-corrosion properties of the primer by virtue of the VCI film formed on the metal surface. In addition, the selected amount of VCI additive has been found not to impair the hardness of the cured primer coating, therefore the integrity of the coating and the anticorrosion effect of the cured primer layer is maintained. This dual effect results in an improved anticorrosion coating.

As previously described, where the VCI is an amine based VCI additive, the lone pair of electrons on the nitrogen atom of the amine is attracted to the polar metal surface. The remainder of the VCI molecule is hydrophobic and so, when the VCI film is formed on the metal surface, the VCI molecules act to repel water from the metal surface, thereby reducing corrosion.

In view of the above described hydrophobic properties of amine based VCI additives, removal or reduction in the amount of water present in the primer formulation is also believed to be advantageous to the action of the VCI additive. It is thought that presence of water in the primer formulation may inhibit vaporisation of the VCI molecules out of the primer formulation due to the hydrophobic properties of the VCI molecules, thereby reducing the amount of VCI that is free to vaporise out of the primer and resulting in a reduction the anti-corrosive effect obtained. By including amine based VCI molecules in a moisture-cure primer that is kept as dry as possible to avoid premature curing of the primer, vaporisation of the VCI molecules out of the primer, and therefore the anti-corrosion effect which can be achieved by the VCI additive, is optimised.

Tests were carried out on the anti-corrosive effect ofthe moisture-cure primer detailed in Example 2 above. The anti-corrosive properties of a piece of steel coated with the primer of Example 2 above (i.e. containing VCI additive) was compared to that of a piece of steel coated with a primer the same as that in Example 2 but containing no VCI additive. Both pieces of steel were subjected to 500 hours of accelerated Hot Salt Spray exposure using the standard test known as ASTMB117. These tests showed that there was a 25% reduction in corrosion creep on steel beneath the moisture-cure primer of Example 2 above, as compared to the same moisture-cure primer formulation containing no VCI additive.

The use of a moisture-cure primer is particularly advantageous when applied to metal structures located in areas of high humidity, for example up to 90% humidity. In such high humidity environments, metal structures are more prone to corrosion and so the addition of a VCI component to moisture-cure primers further improves the corrosion resistance achievable in these conditions.

The process for making the primer described herein will now be described with reference to Figure 2.

Firstly the isocyanate resin, about half of the solvent/thinner, the thickener and the metallic element in powder form are mixed 202 together slowly, for example at about lOOrpm, in a clean and dry mixing vessel for approximately 5 minutes. The moisture scavenger is then added 204 slowly, for example at about lOOrpm, and in 10% additions ofthe total amount of moisture scavenger until the gas release/bubbling of the mixture stops.

The colour pigment, e.g. in paste form, is separately mixed 206 with the moisture scavenger as a pre-mix. The moisture scavenger is added to the colour pigment in 10% additions ofthe total amount of moisture scavenger until the gas release/bubbling ofthe mixture stops. The colour pigment pre-mix is then added 208 to the resin blend.

The VCI additive is then added 210 to the mixture under slow stirring, for example at about lOOrpm. The viscosity of the mixture is then adjusted 212 with solvent/thinner.

The cure accelerator is added 214 to the mixture to achieve the desired cure time. The viscosity is checked again, adjusting 216 if necessary with the solvent/thinner to achieve the desired viscosity.

The resulting mixture is then immediately transferred 218 to nitrogen purged cans and the lids sealed. Rapid filling and sealing of the cans is preferable to reduce exposure of the primer formulation to the atmosphere and any moisture therein.

Typically the primer is manufactured using a high speed disperser.

Although the invention has been described above with reference to one or more embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. For example, whilst the examples above are illustrative of a specific moisture-cure primer, it will be understood that any suitable primer or paint may be used, for example, any primer or paint that is formulated to cure to form a hard, impervious layer.

Claims

1. A primer for application to a metal substrate, wherein the primer comprises a primer formulation including an amount of volatile corrosion inhibitor (VCI) additive, wherein the VCI additive content of the primer formulation is between 0.1% and 1% by weight.

2. A primer according to claim 1, wherein the VCI additive content of the primer is between 0.3% and 0.7% of the primer formulation by weight, optionally 0.5%.

3. A primer according to any preceding claim, wherein the primer is a moisture-cure primer which is formulated to cure with water.

4. A primer according to claim 3, wherein the moisture-cure primer is a moisture-cure polyurethane primer and the primer formulation comprises a resin containing an isocyanate prepolymer formulated to polymerise with water to form a polyurea coating.

5. A primer according to claim 4, wherein the resin content of the primer formulation is between 10% and 80%, optionally between 45% and 50%.

6. A primer according to any preceding claim, wherein the VCI additive is added to the primer formulation in powder form.

7. A primer according to any preceding claim, wherein the VCI additive comprises an amine based compound.

8. A primer according to claim 7, wherein the VCI additive comprises cyclohexylamine benzoate.

9. A primer according to any preceding claim, wherein the primer formulation comprises a drying additive formulated to remove moisture from the primer formulation.

10. A primer according to claim 9, wherein the drying additive is a mono-functional isocyanate drying additive.

11. A primer according to claim 9 or 10, wherein the drying additive content of the primer formulation is up to 5% of the primer formulation by weight, optionally up to 1.5%.

12. A primer according to any preceding claim, wherein the primer formulation comprises a cure accelerator to accelerate curing of the primer.

13. A primer according to claim 12, wherein the cure accelerator comprises a metal-based cure accelerator, for example a tin-based cure accelerator.

14. A primer according to claim 12 or 13, wherein the cure accelerator content of the primer formulation is up to 0.5% of the primer formulation by weight, optionally less than 0.1%. 15. A primer according to any preceding claim, wherein the primer is formulated to cure with ambient moisture in the atmosphere. 16. A metal substrate to which the primer according to any of claims 1 to 15 has been applied. 17. A metal substrate according to claim 16, wherein the metal substrate is a steel structure. 18. A metal substrate according to claim 16 or 17, wherein the metal substrate comprises at least a portion of a pylon, a girder, or a pipe. 19. A pylon for supporting a power line, the pylon comprising one or more structural elements, wherein at least a portion of at least one of the structural elements is coated with the primer according to any of claims 1 to 15. 20. A primer, wherein the primer is a moisture-cure polyurethane primer which is formulated to cure with moisture, and wherein the primer comprises a primer formulation including: a volatile corrosion inhibitor (VCI) additive wherein the VCI content of the primer formulation is between 0.1% and 1% by weight; an isocyanate containing resin formulated to polymerise with water to form a polyurea coating, wherein the amount of resin present is between 10% and 80% of the primer formulation by weight; a monofunctional isocyanate drying additive formulated to remove moisture from the primer formulation, wherein the amount of drying additive present is up to 5% of the primer formulation by weight; aluminium powder; and a tin based cure accelerator to accelerate curing of the primer, wherein the amount of cure accelerator present is up to 0.5% of the primer formulation by weight. 21. A method of preparing a primer for application to a metal substrate, wherein the method comprises preparing a primer formulation and, during preparation of the primer formulation, adding an amount of volatile corrosion inhibitor (VCI) additive to the formulation, wherein the amount of volatile corrosion inhibitor added is between 0.1% and 1% of the resulting primer formulation by weight. 22. A method according to claim 21, wherein the amount of VCI additive added during preparation of the primer formulation is between 0.3% and 0.7% of the resulting primer formulation by weight, optionally 0.5%. 23. A method according to claim 21 or 22, wherein preparing the primer formulation includes the step of providing a resin containing an isocyanate prepolymer. 24. A method according to claim 23, wherein the amount of resin provided is between 10% and 80% by weight of the resulting primer formulation, optionally between 45% and 50%. 25. A method according to any of claims 21 to 24, wherein the VCI additive is added as a powder. 26. A method according to any of claims 21 to 25, wherein the VCI additive comprises an amine based compound. 27. A method according to claim 26, wherein the VCI additive comprises cyclohexylamine benzoate. 28. A method according to any of claims 21 to 27, wherein preparing the primer formulation includes the step of adding a drying additive to the formulation, wherein the drying additive is formulated to remove moisture from the primer formulation. 29. A method according to claim 28, wherein the drying additive is a mono-functional isocyanate drying additive. 30. A method according to claim 28 or 29, wherein the amount of drying additive added to the formulation is up to 5% of the resulting primer formulation by weight, optionally up to 1.5%. 31. A method according to any of claims 21 to 30, wherein preparing the primer formulation includes the step of adding a cure accelerator to the formulation to accelerate curing of the primer. 32. A method according to claim 31, wherein the cure accelerator comprises a metal-based cure accelerator, for example a tin-based cure accelerator.

15. 33. A method according to claim 31 or 32, wherein the amount of cure accelerator added is up to 0.5% of the resulting primer formulation by weight, optionally less than 0.1%.