WO2009059798A2

WO2009059798A2 - A method for producing a coating on a metal substrate and a coating produced thereby

Info

Publication number: WO2009059798A2
Application number: PCT/EP2008/009475
Authority: WO
Inventors: Henagame Liyanage Mallika Bohm; Sivasambu Bohm; Wu Li
Original assignee: Corus Uk Limited
Priority date: 2007-11-08
Filing date: 2008-11-10
Publication date: 2009-05-14
Also published as: WO2009059798A3

Abstract

The invention relates to a method for producing a cured coating on a metal substrate, comprising the steps of: - providing a coating mixture comprising at least one hydrolysable silane-based component and at least one further component wherein the further component comprises a reactive functional group curable with the hydrolysable silane-based component; applying the mixture to the metal substrate; - curing the mixture, so as to provide a dense network structure of the cured coating for temporary protection of the substrate against corrosion. The invention also relates to a coated substrate provided with a coating produced by the method according to the invention.

Description

A METHOD FOR PRODUCING A COATING ON A METAL SUBSTRATE AND A

COATING PRODUCED THEREBY

The invention relates to a method for producing a coating on a metal substrate and a coating produced thereby for providing temporary protection of the substrate against corrosion.

Corrosion is a well-known problem relating to metallic structures, for example steel sections. A number of approaches may be invoked in order to prevent or reduce corrosion. For example, the flow of electrons can be interrupted in that the chemical composition of the materials concerned is changed, or the material to be protected against corrosion is separated from the electrolyte (for example, salty water). In addition, there are methods based on the use of a protective coating on a medium, which prevents formation of rust. The protective coatings preventing rust formation tend to change the surface chemistry of the substrate in the form of an ultra-thin adhesive layer between the substrate and the protective coating.

Protective coatings for steel sections, for example, are typically based on paints, organic layers, ceramic and inorganic layers, plastic coatings, and platings with non- rusting metal. In contrast, protective coatings such as zinc, aluminium-based metal and magnesium-based metal are distinguished by the fact that they have a tendency to corrode more quickly than the surface to be protected (these layers are therefore also termed sacrificial layers).

Alternatively, surface passivation layers may be provided to afford permanent protection of a substrate from environmental degradation. Surface passivation may be introduced by the application or production of an oxide layer onto the surface of the substrate.

After manufacture, metallic structures such as as-rolled metal sections are transported, in pristine condition, from the Rolling Mill to the customer. Transportation of the sections from source to destination could take from a few to several months, during which short periods, by the time the steel sections reach the customer, surface red rust is formed on the surface of the section. Progressive rusting over a few weeks to a few months of the surface of the section impairs its aesthetic appeal, particularly the pristine mill-scale appearance of as-rolled mill-scale steel sections. While the mechanical properties of the steel are not affected as a result of rusting, the steel surface tends to develop a poor visual appearance by the time steel arrives at the customer, the latter preferring to see the steel surface in its original pristine surface as-manufactured.

To combat the issue of this short-term rusting on steel sections, films are introduced on the surface of the steel, which promote a water-repellent steel surface. Coating materials such as petroleum jelly, castor oil and paraffin oil are conventionally used to provide steel surfaces with temporary protection against atmospheric corrosion typically lasting several weeks, especially during transportation and storage. In this context, the term 'temporary¹ refers to a coating on the steel, which protects its surface for a few to several months, i.e. for the period between production of the substrate by the producer and the use of the substrate by the customer.

Many of the temporary protective films referred to above are greasy and, hence, the slippery surface of the steel section may cause accidents when handled by personnel unloading the sections. Furthermore, excessive cleaning of the sections may be necessary, prior to additional processing stages, to remove the greasy contaminants from the steel surface during surface preparation. If the greasy contaminants are not removed before further processing steps are applied to the section, there will be a serious impact on the durability of subsequently applied coatings.

The drawbacks of the above mentioned surface protection methods are that they do not provide sufficient protection of metal substrates like steel sections, the coating handling characteristics are not suitable, and the coating layers may not be applied cost-effectively onto the surface of the metal substrates.

Therefore, it is an object of the invention to provide a protective coating for a metal substrate, preferably a steel substrate, which affords adequate temporary protection for the surface of the substrate over a few to several months.

A further object is to provide a coating on the steel substrate that is transparent and dry in that the original pristine surface of the steel substrate without rust may be seen when the coated substrate arrives at the customer. Moreover, the surface of the substrate is not slippery so that it can be easily handled. Because steel sections are to be further processed in the factory after being transported, a further object of the invention is to provide a protective coating, which is easily removable during conventional surface preparation operations, without leaving residues. The removed coating should preferably be benign (particularly important with re-circulating blast cleaning operations). Moreover, if the coating remains on the steel surface, it will have to be compatible with any subsequent operations; for example, over-coating, cutting or welding and the coating should have no adverse effect on fabrication and down-stream processing.

According to the invention, one or more of these objects may be achieved by providing a method for producing a coating on a metal substrate, comprising the steps of: providing a coating mixture comprising at least one hydrolysable silane-based component and at least one further component wherein the further component comprises a reactive functional group curable with the at least one hydrolysable silane-based component; applying the mixture to the metal substrate; curing the mixture, so as to provide a dense network structure of the coating for temporary protection of the substrate against corrosion.

Said method applies a coating to a metal substrate, the green coating comprising a hydrolysable silane component and a further component, characterised in that the further component comprises a reactive functional group curable with the hydrolysable silane component so as to provide a dense network structure of the cured coating for temporary protection of the substrate against corrosion. This combination has the advantage that silane components readily react with further components to provide a three-dimensional network including at least a silane network, which provides temporary protection for the substrate. Moreover the cured coating is removable. The term green coating is used to identify the coating which is formed on the metal substrate after the coating mixture has been applied to the substrate and before curing has taken place. After curing, the green coating has become a cured coating.

This coating is not only an improvement of the coatings thus far used in the sense that it protects the substrate against corrosion better, longer and more consistently, but also in the sense that upon removal of this temporary coating prior to using the steel substrate in a construction the removed coating does not burden the environment. The dry film thickness of the cured coating is preferably between 0 and 20 μm. Depending on the requirements to the cured coating thinner coatings may be preferable. For (thick) plate, rail or sections a robust and hard coating may be desirable, and this coating preferably has a thickness of over 10 μm. For wire and strip, a thin and flexible coating is desirable and this coating preferably has a thickness of less than 5 μm. The minimum thickness of the coating is the thickness wherein the coating can fully cover the surface area. It was found that a minimum thickness of 1 μm is adequate to achieve this.

In an embodiment of the invention, the silane component comprises tetraethoxysilane (TEOS) and vinylthmethoxysilane (VTMS). The further component in the coating mixture may comprise an aqueous solution of an acid, preferably a nitric or acetic acid solution such that an improved network structure is provided on the substrate. The acid is used to adjust the pH of the coating mixture to catalyse the hydrolysis and condensation of silanes. This combination of components provides for a removable, highly corrosion resistant coating having a silane network that is transparent or glass- like and providing additional flexibility. By means of example hydrochloric acid, sulphuric acid or acetic acid can be used instead of nitric acid Other catalysts can be also used, such as KOH, amines, titaniumalkoxides, and vanadiumalkoxides and oxides. Advantageously the coating having a dry film thickness of 2 to 3 microns provides temporary protection of the substrate for up to 6 months. The protection afforded for this period is suitable for longer transit periods where the substrate has to be transported abroad.

In an embodiment of the invention, the coating mixture also comprises one or more of the following compounds:

- phenyltriethoxysilane (Ph-TES);

- propyltrimethoxysilane (PTMS);

- titanium isopropoxide;

- alcohol such as ethanol and/or butanol; - a dispersion of nano-scale colloidal particles in water, such as an aqueous colloidal silica sol product;

- coating additives such as leveling agents, defoamer;

- a corrosion inhibitor having catalytic properties such as cerium-acetate. In this embodiment of the invention, the further component comprises a dispersion of nano-scale colloidal particles in water and a corrosion inhibitor.

Advantageously, the dispersion of nano-scale silica particles is an aqueous colloidal silica sol product, and the corrosion inhibitor component is a cerium acetate (Ce(CH₃COO)₃). The cerium acetate also catalyses the curing mechanism. The addition of these further components provides a coating with superior properties such as additional flexibility.

In an embodiment of the invention, the coating mixture comprises 13 to 16 %wt TEOS, 65 to 55 % wt VTMS and 22 to 29 %wt nitric acid solution. The cured coating produced from this coating mixture having a dry film thickness of 2 to 3 microns typically provides temporary protection for the substrate for up to 6 months.

In an embodiment of the invention, the coating mixture comprises 10 to 15 %wt TEOS, 55 to 45 %wt VTMS, 20 to 25 %wt nitric acid solution, 5 to 10 %wt of the dispersion and 0.001 to 0.05 %wt cerium acetate. Favourably, the cured coating produced from this coating mixture having a dry film thickness of 2 to 3 microns typically provides temporary protection for the substrate for up to 9 months. This level of temporary protection for the substrate is suitable for long transit times abroad and in marine environments. The coatings based on VTMS and TEOS as the only silane based compounds are hard and durable which is important for plate, section or rail applications. In these cases the coating may also be chosen significantly thicker than 3 μm, e.g. between 10 and 15 μm.

In an embodiment of the invention the coating mixture comprises VTMS, TEOS and Ph-TES, wherein VTMS and Ph-TES are present in a ratio in wt% of between 2 and 20, preferably of between 6 and 12, more preferably of about 10. The inventors found that when adding a small amount of Ph-TES to the coating mixture in the said ratio that the corrosion performance of the cured coating is improved. This is believed to be caused by the fact that Ph-TES is a more hydrophobic silane due to the phenyl-group which helps to decrease the diffusion of water through the coating. This type proves to be particularly suitable for coatings which have to be able to withstand high temperatures such as reinforcement wires for embedding in a glass. It proved to be beneficial to add methylthmethoxysilane (MTMS) to the coating mixture comprising Ph-TES. MTMS is a backbone silane which can partly replace VTMS if so desired. In an embodiment of the invention, the coating mixture comprises VTMS, TEOS and PTMS, wherein VTMS and PTMS are present in a ratio in wt% of between 2 and 20, preferably of between 6 and 12, more preferably of about 10. The inventors found that when adding a small amount of PTMS to the coating mixture in the said ratio that the corrosion performance of the cured coating is improved and, more importantly, that the flexibility of the cured coating increases considerably. This means that a cured coating produced from this coating mixture is particularly suitable for application to thin substrates like sheet, strip and wire which bend easily and which are cured at low temperatures. It proved to be beneficial to add MTMS to the coating mixture comprising PMTS.

In an embodiment of the invention, the coating mixture comprises titanium isopropoxide and the silane-based components in a ratio in wt% of between 0.1 and 2%, preferably between 0.2 and 1 %, more preferably between 0.3 and 0.7%. This compound was found to be particularly effective in catalysing the curing reaction so that the curing temperature can be reduced. This is particularly relevant for the application of the method in accordance with the invention in processes where a high curing temperature is undesirable because it would constitute an additional process step. By adding titanium isopropoxide in the said amounts, a curing temperature (expressed in peak metal temperature) of below 150⁰C or even below 12O⁰C may be achieved. By tuning the composition of the coating mixture curing temperatures involving a peak metal temperature of below 100⁰C is achievable.

In another embodiment of the invention, the silane component is a Y- glycidoxypropylthmethoxysilane (GPS) and the further component a solution of Chitosan. Preferably, the reactive functional group is an amine group. This leads to the formation of a transparent, 3-dimensional Chitosan-silane network, which is flexible and removable by acid pickling.

In an embodiment of the invention, the Chitosan solution comprises a homogeneous mixture of de-ionised water, acetic acid and low molecular weight Chitosan. The solution form of Chitosan enables it to be readily combinable with the other components to provide a network structure.

In an embodiment of the invention, the coating mixture comprises 45 to 55 %wt GPS and 55 to 45 %wt Chitosan solution. This range of components provides a robust network on the substrate, a transparent network and removability of the coating. Favourably, the coating mixture comprises 50 %wt GPS and 50 %wt Chitosan solution.

Preferably, the coating mixture further comprises a vinyltrimethoxysilane (VTMS). This component provides for the solubility of the coating if it is desired to remove it prior to additional processing.

In an embodiment of the invention, the coating mixture comprises 42 to 56 %wt GPS, 50 to 40 %wt Chitosan solution and 8 to 4 %wt VTMS. This combination of components provides protection of the substrate during transit and permits the removal of the cured coating if desired after transportation. Preferably, the cured coating is removable from the substrate by shot blasting or pickling. This is important because manufacturers often apply their own coatings or pre-treatment to make the final product.

Advantageously, the coating produced using the GPS-Chitosan compounds having a dry film thickness of 2 to 3 microns provides the substrate with temporary protection for up to three months. This is sufficient protection for the substrate for short transit periods often within the same territory. The period of protection is therefore somewhat shorter, but the advantage of this coating is that it is picklable as a result of the presence of the Chitosan.

The curing step is preferably incorporated in an existing process step. The choice of the curing temperature depends on the requirements to the cured coating. For applications where no additional heat treatment is desirable, for example in strip production, the use of Ti-isopropoxide allows to reduce the peak metal temperature to below 120⁰C, e.g. about 110⁰C.

The curing treatment may be performed by using the heat still present in the steel after rolling, or for example by using an infrared heating device, induction device or any other suitable means. Also, by selecting the proper composition, the coating mixtures can be chosen such that curing can take place at ambient temperatures.

Preferably all silane-based compounds used in the method according to the invention are fluor-free to avoid pollution upon removal of the temporary coating by a blasting or pickling operation. Preferably the substrate is cleaned before applying the coating mixture. For example rolling oil or some adhering scale may cause insufficient adhesion. The cleaning may be performed using conventional cleaning and/or descaling means.

According to a second aspect, a metal substrate is provided with a coating produced in accordance with the method of the invention as described hereinabove having a dry film thickness of 0 to 20 microns. The coating is removable from the substrate by shot blasting or pickling. In an embodiment of the invention the coating has a dry film thickness of 2 to 3 microns which provides the substrate with temporary protection for up to three months, preferably for up to 6 months, more preferably for up to 9 months. Preferably, the substrate is made of steel, and the substrate may be a sheet, plate, section, rail, coiled hot or cold rolled sheet, galvanized sheet, bar, rod or wire. Steels are prone to show red rust, particularly when stored under hot and/or moist conditions. The aforementioned coatings are chemically compatible with steel substrates and provide the steel substrate with an excellent finish. Examples

The invention will now be further described by way of examples undertaken on steel substrate panels, with reference to the accompanying figure in which figure 1 shows the structures of Chitosan and γ-Glycidyloxypropyltrimethoxysilane (γ-GPS).

The primary silane-based-compounds used in the embodiments of the invention include γ-GPS, Tetraethoxysilane (TEOS) and Vinyltrimethoxysilane (VTMS). A further chemical component used in the invention includes the Chitosan structure, which is a linear polyamine (poly d-glucosamine) that is entirely soluble in acidic solutions.

In the most generic form, the method for producing the coatings in accordance with the invention comprises a Silica "Sol-gel" technology including the steps of: mixing at least one hydrolysable silane-based component and at least one further component; applying the mixture on the metal product; and curing the mixture, characterised in that the at least one further component comprises at least a functional group for cross-linking with the at least one hydrolysable silane component so as to provide a dense network structure of the coating for temporary protection the substrate against corrosion. Silica Sol-gel technology has been used to produce uniform, coatings and the advantage of the technology is that coatings with specific properties can be produced for different applications. Silicone alkoxides (silanes) are easily hydrolysed by water. The hydrolysis reaction scheme is shown below with the condensation between the silanol groups and the hydroxyl groups leading the strong Si-O-Si bonds.

Hydrolysis RSi-(OR')₃ + H₂O RSi-(OH)₃ + R¹OH

Condensation RSi-(OH)₃

Transparent glass-like films may be obtained for a fully cured coating. The non- hydrolysable ¹R¹ group is used to adjust the degree of the cross-linking density and subsequently the flexibility of the coating. The characteristics and properties of a particular sol-gel inorganic network are related to a number of factors that affect the rate of hydrolysis and condensation reactions, such as, pH, temperature and time of reaction, reagent concentrations, catalyst nature and concentration, H₂O/Si molar ratio (R), aging temperature and drying time.

Silanes react on steel surfaces to form highly stable covalent Fe-Si and Fe-C bonds, and silanes have the intrinsic characteristics to condense and form a network. The network structure will depend on the chemical nature of the silane and therefore, the network could be tailored for the desired properties.

Details of the chemical formulations of examples of the invention are shown in Table 1 below.

In a first set of examples of the invention, to prepare the coating mixture for applying on the metal substrate, γ-GPS was mixed with a Chitosan solution in the ratios described in the formulations shown in table 1. The Chitosan solution was prepared from 170 mL de-ionised water, 5 ml_ acetic acid and 3.45g Low Molecular Weight

(LMW) Chitosan. The components were added and stirred very slowly until the

Chitosan solution was homogeneous. The components were mixed for at least two hours at room temperature or above (or preferably at 50⁰C or above).

The components of the coating were mixed for 16 hours, and the coating mixture was applied by spraying onto mild steel test panels (Grade A -BS 7079: Part A1 ). The coating was applied using a conventional spray coating for applying thin films, for example, on strip substrates. Alternatively the coating may be applied using a dip process or a dip process with or without squeegee rolls. The coating mixture was cured at room temperature for 2 days; alternatively, it may be cured between 30 seconds and 30 minutes at elevated temperatures.

The condition of the substrate surface prior to application of the mixture is important; preferably, the surface of the substrate should be free of contaminants to ensure good wettability and adsorption of the mixture of the silane and Chitosan solution. Good wettability promotes the formation of a uniform coating layer. Hence, degreasing of the substrate steel panels was performed using acetone prior to the application of the coating mixture. As shown in table 1 , in an example, designated formulation Y, the ratio of GPS to Chitosan was 1 :1 ; in this example, formulation Y comprised 50%wt GPS and 50%wt Chitosan solution.

Referring to table 1 , in a second example of the coating mixture designated formulation X, the coating mixture comprised 47.1 %wt Chitosan solution and 5.8%wt VTMS, mixed together for at least ten minutes. This was followed by mixing with 47.1 %wt of GPS. Again the coating was applied using the same methods and conditions described above.

Coating Formulations: Chemicals in coating formulation (%wt.)

(CS:GPS weight ratio = 1 :1 ) -{47.1 %w. of chitosan solution + 5.8%w. VTMS} +47.1%w. GPS

(CS:GPS weight ratio = 1 :1 )

-50%w. GPS + 50%w. chitosan solution

Table 1 : Coating ratios of Chitosan to silane in formulation X and Y. The coatings were tested in respect of their ability to be removed from the substrate and to assess the period of time the coatings protected the steel panels against rusting.

Picklability of the coating was measured by immersing the coated panels in a bath of pickling solution. One half of the coated steel panel was dipped in 14% HCI and 17% HCI solution both inhibited with 0.3%wt. hexamethylenetetramine. After 25 minutes, the samples were removed from the bath and rinsed with tap water. The dry film thicknesses (DFT) of the coating on the steel panel was tested, the thickness of the coating before pickling being 2-4 microns. For a 2-4 micron DFT, all the coatings and scale were removed by both HCI 14% and HCI 17% after 25min. It was determined that formulation Y was generally easier to pickle by HCI 14-17% solution than formulation X.

Panels sprayed with formulation X and Formulation Y at 2-4 micron DFT were exposed in an outdoors environment. It was determined that after 2 months exposure, the reference samples revealed a rusted surface whilst the panels coated with the temporary coating were in a good condition.

With the 2 months exposure in an outdoor environment, formulation X and formulation Y with a 2-4 micron DFT were still protecting the panels from rusting. Based on the above results, it was decided to check the corrosion protection of the formulations X&Y applied to mild steel sections. Samples of sections were cut from a single bar. Half of each section was sprayed, while the other half was left free of protection as a reference.

The sections were exposed to a semi-marine environment. After 3 months, the part of the section unprotected showed severe surface rust. The part of the section that was protected with the temporary coating started to rust after 3 months. Formulation X and formulation Y significantly slowed down the rusting process over 3 months.

Two formulations, namely formulation X {CS+GPS+VTMS} and formulation Y {CS+GPS}, developed in accordance with the invention, have shown removability in acid HCI 14-17% and section protection in semi-marine conditions for up to 3 months. Clearly, the Chitosan structure having amino groups reacted very well with silanes such as γ-Glycidyloxypropyltrimethoxysilane (γ-GPS) thereby forming a 3D chitosan- silane network film.

In further examples of the invention, an additional two coating formulations were tested during trail experiments and the compositions of the formulations (formulation 1 and 2) of the invention are summarised in Table 2 below. In these examples the invention utilises the hydrolysis function of silanes to form part of the network structure of the protective coatings.

In accordance with the invention, formulation 1 (see table 2) comprised 14.8 %wt TEOS, 59.1 %wt VTMS and 26.1 %wt nitric acid solution.

Table 2: Composition of temporary protective coating formulations

In Table 2 above, TEOS is a silane compound with the chemical structure (CH₃CH₂O)₄Si. VTMS is another silane compound having the chemical structure (CH₃O)₃Si-CH=CH₂. The nitric acid solution (H₂O/HNO₃) provides for hydrolysis of the silane components. Levasil^® 200E -20% is a registered trade name of H. C. Starck and it is an aqueous colloidal dispersion of amorphous silica. These are silica nano-particles and are an aqueous colloidal silica sol product - essentially a stable dispersion of nano-scale glass particles in water. Levasil^® 200E -20% was incorporated into the coating formulation to reinforce the hardened silane network and improve its barrier properties. The applicability of the method is not restricted to the Levasil^® 200E -20% materials.

Cerium acetate is a salt which plays two roles in the temporary protective coating. Firstly, it catalyses gelling of the silane/silica sol network and secondly, it is a highly effective corrosion inhibitor. The rare earth salt solution was prepared by dissolving 1 g of cerium acetate in 4 ml of 3x10^"3 M nitric acid solution. The curing mechanism of this formulation at room temperature is provided by catalysis via the rare earth salt accelerator.

The components of formulation 1 were mixed together for a period of 15 hours, and the formulation was applied on the steel panel by conventional spraying (as described above) on a pre-heated steel test panel. The panel was exposed to a thermal pre- treatment at temperatures between 50 and 100 degrees, preferably 70-80 degrees. Alternatively, the coating may be applied using a dip process or a dip process with or without squeegee rolls. These application methods can be used for each of the coating mixtures in accordance with the invention.

The DFT of the coating on the steel panel was tested, the thickness of the coating being 2-3 microns. The corrosion protection was similarly experimented upon as described above for the first set of experiments relating to the silane-Chitosan coatings of the invention.

Experiments conducted on formulation 1 demonstrated up to six months corrosion protection and a high resistance to scratch; therefore high robustness was imparted by the coating. Moreover, the protective coating was removable by shot blasting without contamination thereby rendering it amenable to further processing steps.

In yet a further example of the invention, formulation 2 comprised 12.8% of TEOS, 51.1 %wt of VTMS, 23.4 %wt of nitric acid solution and further having a dispersion of nano-scale silica particles in water and a corrosion inhibitor component having catalytic properties, the corrosion inhibitor preferably being cerium acetate. The latter two components were present in the amounts 8.5 %wt of nano-silica particles and 0.005 wt % of cerium acetate.

The components of formulation 2 were mixed together for 15 hours, preferably between 10 to 20 hours, and the formulation was applied on the steel panel by spraying on a preheated surface. The pre-treatment temperatures were 50 to 100 degrees, preferably 70 to 80 degrees.

The dry film thicknesses (DFT) of the coating on the steel panel was tested, the thickness of the coating being 2 to 3 microns. Evaluation of the extent of corrosion protection was similarly experimented upon as described above for the first set of examples of the invention. Tests on formulation showed up to nine months corrosion protection and manifested favourable durability. Moreover, the protective coating prepared from formulation 2 was removable for the panels by shot blasting without contamination. The latter feature rendering the panel amenable to further processing steps such as hot dip galvanising. It is thought that, because the TEOS structure has no organic side group and VTMS has a small side group, the result is that a much denser silicate network was obtained. To produce a temporary coating with an even further improved corrosion resistance and a better flexibility the formulations in accordance with table 3 were produced. It will be obvious that for large quantities of the coating mixture the amounts given in table 3 must be suitably increased.

The addition of the titaniumisopropoxide (Ti-ip in table 3) allows a curing treatment with a peak metal temperature as low as 110⁰C. It was found that a level of about 0.5% in relation to the total silane content was adequate to achieve this. By varying the amount of titaniumisopropoxide between e.g. 0.3 and 0.7 the required PMT can be influenced. Both the PTMT-type as the Ph-TES-type showed good corrosion performance. The pH is adjusted to about 2, in this example by the use of nitric acid.

The inventors also found that aqueous colloidal dispersion of amorphous silica could be added to the coating mixtures presented in table 3. In the example the pH value of the colloidal silica Levasil® 200E 20% was first adjusted to 2 after which the MTMS and Ph-TES was added slowly while stirring, which then was diluted by iso-propanol at 1 :1 ratio. Iso-propanol is added to increase the stability of the sol, and thereby increase the pot-life of the solution. Instead of iso-propanol 2-butanol is used in some coating mixtures. Both have the same functionality, used as solvent and can be replaced by other organic solvents as well.

Table 3: Composition of temporary protective coating formulations comprising PTMS or Ph-TES.

Ph-TES type: MTMS and Ph-TES were hydrolysed in acidified water (pH=2) for 24 hours which was then diluted with iso-propanol at 1 :1 ratio. 0.5 mm thick wire was cleaned with acetone and then dipped into the sol-gel solution. The wet coatings were cured at 300^°C for 60 seconds. MTMS and Ph-TES are used together to form sol-gel coating having the right balance between flexibility and corrosion barrier property. Figure 2a and b show the results of the wire after 9 days of immersion in water at 4O⁰C (a=wax coated wire, b=Ph-TES type). Ph-TES type with colloidal silica: The pH value of colloidal silica Levasil 200E 20% was first adjusted to 2 by adding 20% nitric acid. Then MTMS and Ph-TES were added slowly into the colloidal silica solution under stirring. The mixture was then diluted by iso-propanol at a 1 :1 ratio. 0.5 mm thick wire was cleaned by acetone and then dipped into the hybrid sol-gel solution. The wet coatings were cured at 300⁰C for 60 seconds.

Figure 3a and b show the results of the wire after 6 days of cyclic humidity testing (BS3900, part F2, April 1973, cont 1989) (a=wax coated wire, b=Ph-TES type).

PTMS type: VTMS/TEOS + 10% PTMS with 0.5% Ti Figure 4 shows an uncoated (a.) and coated section (b.) with the above formulation, after 10 weeks exposure in industrial environment. The uncoated section shows large amounts of red rust, whereas the coated section still shows the as-rolled condition. This type is especially suitable for the coating of strips, particularly steel strip, because it is flexible, offers good corrosion resistance and cures at low temperatures.

Claims

1. A method for producing a cured coating on a metal substrate, comprising the steps of: providing a coating mixture comprising at least one hydrolysable silane-based component and at least one further component wherein the further component comprises a reactive functional group curable with the hydrolysable silane-based component; applying the mixture to the metal substrate; curing the mixture, so as to provide a dense network structure of the cured coating for temporary protection of the substrate against corrosion.

2. Method as claimed in claim 1 , wherein the hydrolysable silane component comprises tetraethoxysilane (TEOS) and vinyltrimethoxysilane (VTMS).

3. Method as claimed in claim 1 or 2, wherein the coating mixture comprises a solution of an acid, preferably a nitric acid solution.

4 Method as claimed in any one of claims 1 to 4 wherein the coating mixture also comprises one or more of the following compounds: phenyltriethoxysilane (Ph-TES); propyltrimethoxysilane (PTMS); titanium isopropoxide; alcohol such as ethanol, propanol and/or butanol; - a dispersion of nano-scale colloidal particles in water, such as an aqueous colloidal silica sol product; coating additives such as leveling agents, defoamer. and a corrosion inhibitor having catalytic properties such as cerium- acetate.

5. Method as claimed in any one of the preceding claims, wherein the coating mixture comprises 13 to 16 %wt TEOS, 65 to 55 % wt VTMS and 22 to 29 %wt nitric acid solution.

6. Method as claimed in any one of the preceding claims, wherein the coating mixture comprises 10 to 15 %wt TEOS, 55 to 45 %wt VTMS₁ 20 to 25 %wt nitric acid solution, 5 to 10 %wt of the dispersion of nano-scale colloidal silica particles in water and 0.001 to 0.05 %wt cerium acetate.

7. Method as claimed in any one of the preceding claims wherein the coating mixture comprises VTMS, TEOS and PTMS, and wherein VTMS and PTMS are present in a ratio in wt% of between 2 and 20, preferably of between 6 and 12, more preferably of about 10.

8. Method as claimed in any one of the preceding claims wherein the coating mixture comprises VTMS, TEOS and Ph-TES, and wherein VTMS and Ph-TES are present in a ratio in wt% of between 2 and 20, preferably of between 6 and 12, more preferably of about 10.

9. Method as claimed in any one of the preceding claims wherein the coating mixture comprises titanium isopropoxide and the silane-based components in a ratio in wt% of between 0.1 and 2%, preferably between 0.2 and 1 %, more preferably between 0.3 and 0.7%.

10. Method as claimed in any one of the preceding claims wherein the coating mixture also comprises methyltrimethoxysilane (MTMS).

11. Method as claimed in claim 1 , wherein the silane-based component comprises a Y-glycidoxypropyltrimethoxysilane (GPS) and the further component comprises a solution of Chitosan.

12. Method as claimed in claim 11 , wherein the coating mixture comprises 45 to 55 %wt GPS and 55 to 45 %wt Chitosan solution.

13. Method as claimed in any one of claim 11 to 12, wherein the coating mixture further comprises a vinyltrimethoxysilane (VTMS).

14. Method as claimed in claim 13, wherein the coating mixture comprises 42 to 56 %wt GPS, 50 to 40 %wt Chitosan solution and 8 to 4 %wt VTMS.

15. Metal substrate provided with a coating produced according to the method of any one of the preceding claims having a dry film thickness of 0 to 20 microns.

16. Coated substrate according to claim 15, wherein the coating is removable from the substrate by shot blasting or pickling.

17. Coated substrate as claimed in claim 15 or 16 wherein the coating having a dry film thickness of 2 to 3 microns provides the substrate with temporary protection against corrosion for up to three months, preferably for up to 6 months, more preferably for up to 9 months.

18. Coated substrate as claimed in any one of claim 15 to 17 wherein the substrate is made of steel.

19. Coated substrate as claimed in any one of claim 15 to 18 wherein the substrate is a sheet, plate, section, rail, coiled hot or cold rolled sheet, galvanized sheet, bar, rod or wire.