The present invention relates to a method for providing corrosion protection to a metal substrate and to a metal substrate provided with a corrosion protective coating.

Unprotected metal substrates can suffer severe corrosion when exposed to the environment. This can have a significant impact on the mechanical properties of substrates being corroded, as well as the products produced therefrom. In order to suppress corrosion propagation and extend service life, metal substrates are often provided with corrosion protective organic coatings, e.g. polyurethane or polyester coatings. A drawback of such organic coatings is that they do not adhere strongly to the metal substrate and therefore it is often required to provide the metal substrate with a pre-treatment layer to promote adhesion between the organic coating and the metal substrate. This leads to a more complex and expensive manufacturing process, which is undesirable.

Many organic coatings also have the disadvantage that they are limited to certain applications in view of their mechanical properties. For example, commercially available polyester and polyurethane coatings often exhibit good flexibility and corrosion resistance but inadequate abrasion resistance. Thus, such coatings are unsuitable for post-treatment operations such as bending and forming and for applications where the coated substrate may be subjected to mechanical damage during service.

In order to suppress corrosion propagation still further, organic coatings may be provided with corrosion inhibitors, e.g. corrosion inhibitors based on chromium. However, such corrosion inhibitors are known to be carcinogenic and are therefore harmful to human health.

The service life of steel may be increased by providing the steel with a galvanised coating. The corrosion of steel is inhibited because zinc in the coating corrodes in preference to iron in the steel, thereby reducing the onset of rust and prolonging the mechanical integrity of the steel substrate. The service life of such galvanised steel substrates typically depends on the thickness of the galvanised coating. Thus, for applications that require an extended service life, the thickness of the galvanised coating may be increased to meet the demands of that particular application. However, the thickness of the galvanised coating cannot be increased indefinitely, otherwise the costs associated with manufacturing galvanised steel products become too expensive due to the high cost of zinc. In addition, at higher coating thicknesses, failure of the galvanised coating may occur prematurely, since above a certain coating weight, galvanised coatings are known to delaminate due to insufficient adhesion, and in certain instances, a lack of coating integrity. Another factor that limits galvanised coating thickness is the thermal mass of the article being coated. For instance, high thermal mass articles such as steel strips or sheets may be provided with thicker galvanised coatings, whereas the galvanised coating thickness for low thermal mass articles such as steel wires is often limited.

In view of the above drawbacks, it is an object of the invention to increase the corrosion resistance and abrasion resistance of a corrosion protective coating for metal and hot-dip coated metal substrates, while maintaining good coating flexibility.

Another object of the invention is to provide a corrosion protective coating for metal and hot-dip coated metal substrates that exhibits good adhesion to the metal or hot-dip coated metal substrate, and which does not require a pre-treatment layer.

A further object of the invention is to provide a cost effective method for manufacturing, at high speed, coated metal and hot-dip coated metal substrates that exhibit good flexibility and good corrosion and abrasion resistance.

According to a first aspect of the invention one or more of the above objects is reached by a method for providing corrosion protection to a metal substrate, which comprises the steps of applying a sol-gel on the metal substrate and thereafter heating the sol-gel to form a corrosion protective coating, wherein the sol-gel comprises water, a first organofunctional siloxane oligomer and a second organofunctional siloxane oligomer, and wherein the sol-gel is prepared by hydrolysing the first organofunctional siloxane oligomer in a first solution, hydrolysing the second organofunctional siloxane oligomer in a second solution and mixing the first solution and the second solution to form the sol-gel, wherein the pH of the first solution and the second solution is adjusted to an acidic pH . The corrosion protective coating formed from heating the sol-gel shall hereinafter be referred to as "siloxane coating or siloxane corrosion protective coating".

US2010/0209719 Al discloses a composition for coating metallic surfaces, comprising water-soluble and fully hydrolyzed polysiloxanes wherein the pH of the composition is greater than 8. The inventors found that when the pH of the composition is greater than 7 (i.e. basic) that excessive cross-linking occurred leading to inflexible and brittle siloxane coatings. Also, since the reaction rate is so high, this also leads to a short pot life of the composition. The inventors found the method according to the invention very suitable for providing corrosion protection to metal and hot-dip coated metal substrates, having the benefit that the service life of the metal substrate can be extended. The coating flexibility allows the substrate to bend without cracking of the coating, and the hardness of the coating provides the durability. Metal substrates provided with the siloxane coating exhibited improved corrosion resistance compared to bare metal substrates, and comparable corrosion resistance when compared to metal substrates provided with a pre-treatment layer and a further organic coating, e.g. polyurethane. It was also found that the siloxane coating exhibited excellent abrasion and scratch resistance and that the abrasion and scratch resistance was better than that of metal substrates provided with polyurethane or polyester protective coatings. An increase in coating adhesion was also observed when metal substrates provided with the siloxane coating were compared with organic coated metal substrates. Thus, it is not necessary to provide a metal substrate with pre-treatment layer prior to providing the siloxane coating. The siloxane coating additionally exhibited good flexibility making it particularly suitable for use in applications where the metal substrate is formed.

It is important to note the essential difference between silane and siloxane. Silane is an inorganic compound with chemical formula Si - R ₄ where R can be H, alkyl or alkoxy group, whereas siloxane is an organosilicon compound with a Si-O-Si linkage. For the purpose of the invention silanes are useless, because the films formed with silane chains are glassy, lack flexibility and may produce significant amounts of VOC during hydrolyzing and curing (e.g. alcohols as alkoxy groups break down during hydrolysing).

The sol-gel is prepared by hydrolysing the first organofunctional siloxane oligomer in a first solution, hydrolysing the second organofunctional siloxane oligomer in a second solution and mixing the first solution and the second solution to form the sol-gel. By hydrolysing the first and second organofunctional siloxane oligomer in separate solutions, the hydrolysation in the solutions can be carefully controlled to balance the reactivity of the two components in the solutions with each other and the reactivity of components with the substrate during curing. This is important in order to strike the right balance between the flexibility of the coating, reflected in the 0T bending test, and the hardness of the coating. If both the first and second organofunctional siloxane oligomers would be hydrolysed in the same solution, then precipitation of the reaction product would occur straightaway, before being applied to the metal substrate. The pH of the first solution and the second solution is adjusted to an acidic pH. By acidifying the solutions, the rate of siloxane hydrolysis and the rate of reaction between the first and second hydrolysed organofunctional siloxane oligomers is controlled. This prevents against excessive cross-linking and the formation of less flexible and brittle siloxane coatings. It also prevents premature precipitation of the reaction product, before being applied to the metal substrate.

In a preferred embodiment the sol-gel comprises a cathodic corrosion inhibitor. When the siloxane coating comprised cathodic corrosion inhibitors further improvements in corrosion resistance were obtained. The cathodic corrosion inhibitors react with corrosion products at the metal surface to form an insoluble precipitate. This insoluble precipitate then acts as a physical barrier to corrosive agents such as water and oxygen, which inhibits the further corrosion of the metal substrate.

In a preferred embodiment the cathodic corrosion inhibitor comprises salts of zinc, magnesium, titanium, zirconium, yttrium, lanthanum, vanadium or cerium. Very good corrosion protection was afforded to the metal substrate when the salts comprised one or more of the above cations. Preferably the salts comprise acetate, nitrate or sulphate anions. Cerium acetate is a particularly preferred salt. Preferably the cathodic corrosion inhibitor does not comprise chromium salts. Improved corrosion resistance was obtained when the sol-gel comprised up to 200 ppm of the cathodic corrosion inhibitors. From an economic perspective, it is preferred not to exceed 200 ppm.

In a preferred embodiment the first siloxane oligomer comprises one or more organofunctional groups selected from monoamines, diamines, amino-alkyls and alkyls. Such siloxanes are water soluble and may be provided in a water based or aqueous solution. This has the advantage that the risks associated with the handling and disposal of solutions containing organic solvents is avoided or at least reduced. Siloxane coatings exhibiting good corrosion resistance, abrasion resistance and flexibility can be obtained when using such organofunctional siloxane oligomers. In addition, improved adhesion between the siloxane coating and subsequently applied organic coatings, e.g. a top coat provided to improve the aesthetics of the corrosion protected metal substrate, is also possible.

In a preferred embodiment the organofunctional groups of the second organofunctional siloxane oligomer are capable of reacting with the first organofunctional siloxane oligomer. By providing a second organofunctional siloxane oligomer that is capable of reacting with the first organofunctional siloxane oligomer, a cross-linked siloxane coating network is formed at the metal surface. This siloxane coating exhibits improved barrier properties, flexibility and abrasion resistance relative to other organic coatings.

In a preferred embodiment the second siloxane oligomer comprises one or more organofunctional groups selected from monoamines, diamines, amino-alkyls, alkyls, epoxies and hydroxyls. Preferably siloxane oligomers that comprise epoxy functional groups additionally comprise hydroxyl groups to improve the solubility of the siloxane oligomer in water. Siloxane oligomers functionalised with one or more of the above functional groups are particularly suitable for reacting with the first organofunctional siloxane oligomer to form a dense cross-linked siloxane network that exhibits very good corrosion resistance, abrasion resistance and flexibility. The silanol (Si-OH) groups that are formed are capable of forming strong chemical bonds with the metal surface, thereby increasing adhesion between the siloxane coating and the metal surface.

In a preferred embodiment the sol-gel comprises organosilane functionalised silica particles. The presence of the functionalised silica particles in the sol-gel improves the surface coverage of the siloxane coating, the barrier properties and the hydrophobic properties, all of which contribute to improving the corrosion resistance of the coating.

In a preferred embodiment the sol-gel is prepared by hydrolysing the first organofunctional siloxane oligomer in a first solution, hydrolysing the second organofunctional siloxane oligomer in a second solution and mixing the first solution and the second solution to form the sol-gel. The pH of the first solution and the second solution is adjusted to an acidic pH, preferably a pH of 2-3. By acidifying the solutions, the rate of siloxane hydrolysis and the rate of reaction between the first and second hydrolysed organofunctional siloxane oligomers is controlled. This prevents against excessive cross-linking and the formation of less flexible and brittle siloxane coatings.

In the context of the present invention and unless explicitly stated, a hydrolysed organofunctional siloxane oligomer refers to organofunctional siloxane oligomers that are fully hydrolysed as well as to organofunctional siloxane oligomers that are partially hydrolysed.

In a preferred embodiment the first solution and the second solution are aqueous solutions. Although the first and second solutions are aqueous, the sol-gel will contain a small amount of organic solvent, e.g. ethanol, which is produced as a by-product when the first and second organofunctional siloxane oligomers are hydrolysed. Nevertheless, this has the advantage that the amount of organic solvent present in the sol-gel is minimised relative to sol-gels that are prepared from solutions that comprise organic solvents. Although less preferred, the first and second solutions could also be solvent based solutions, meaning that a greater variety of organofunctional siloxane oligomers can be used, including those having a low sol utibi lity in water. Such solutions will also contain water, which will be added to hydrolyse the organofunctional siloxane oligomers.

In a preferred embodiment the first solution containing the hydrolysed first organofunctional siloxane oligomer and the second solution containing the second organofunctional siloxane oligomer are mixed in a weight ratio of 1 : 1 to 6 : 1. It was found that a mixing ratio lower than 1 : 1 increased the rate of reaction between the first and second organofunctional siloxane oligomers to an extent that siloxane coatings exhibiting reduced flexibility were obtained. Moreover, the pot-life of the mixed solution is reduced if a mixing ratio lower than 1 : 1 is used.

In a preferred embodiment the mixing ratio is between 2 : 1 to 6 : 1, and more preferably between 3 : 1 and 6 : 1. Although the lower the ratio the higher the solute content and thereby the higher the corrosion protection, the pot life reduces as well, which for industrial applications is undesirable. So preferably the mixing ratio is at least 2.5 : 1 and more preferably at least 3 : 1. The corrosion protection increases when reducing the mixing ratio from 6 : 1 to 3 : 1, and the pot life decreases when reducing the mixing ratio from 6 : 1 to 3 : 1.

It was found that between 2.5 : 1 and 4: 1 a good combination of life of pot-life and optimum corrosion protection was obtained.

In a preferred embodiment the pH of the mixed solution comprising the first and second hydrolysed organofunctional siloxane oligomers is adjusted to between pH 4 and pH 6. This results in a controlled rate of reaction between the first and second hydrolysed organofunctional siloxane oligomers to produce a siloxane coating that exhibits good flexibility, abrasion resistance and corrosion resistance. If the pH of the mixed solution is adjusted to a neutral or alkaline pH, cross-linking between the siloxane oligomers may occur too quickly, leading a dense and brittle siloxane coating with reduced flexibility. Adjusting the pH to between pH 4 and pH 6 has proven to be beneficial when the mixed solution has a solids content above 5%.

In a preferred embodiment the coating is formed by heating the coated metal substrate with near infrared radiation (NIR) or by induction heating.

NIR and induction heating were both very suitable means for heating the coated metal substrate and for curing the sol-gel to form the siloxane coating. An advantage of using NIR or induction heating is that the sol-gel could be cured quickly, and much faster than when the sol-gel was cured in a convection oven. The use of NIR heating also has the advantage that both the coating and the metal substrate are heated and therefore the sol-gel can be cured at a lower substrate temperature. This helps prevent against the formation of brittle siloxane coatings. A particular advantage of using induction heating as a means for curing the sol-gel is that heat is generated within the metal substrate itself. This avoids the wasteful and unnecessary heating of the background space and apparatus. The use of NIR or induction heating also enables lower temperatures to be used compared to those that are necessary when curing the sol-gel in a convection oven. By reducing the curing time and the curing temperature, siloxane coatings having improved flexibility and coating integrity can be obtained. A further advantage of the NIR or induction heating is that it can be used to cure the coating at high speed in a high speed continuous line. Preferably, the coated metal substrate is heated by NIR or induction heating for a period of 10 seconds or less, preferably between 2 and 5 seconds.

When curing the sol-gel using NIR or induction heating, it is preferred to heat the coated substrate to a temperature of 200°C or less.

According to a second aspect, the invention relates to a metal substrate provided with a corrosion protective coating, wherein the corrosion protective coating is formed from the reaction between a first organofunctional siloxane oligomer and a second organofunctional siloxane oligomer. The siloxane coating exhibited very good flexibility making the siloxane coated metal substrate particularly suitable for subsequent forming or coiling operations. The siloxane coating also exhibited excellent scratch and abrasion resistance and therefore the metal substrate is afforded additional corrosion protection during storage, and when the coated metal substrate is handled or transported. By virtue of the siloxane coating having a cross- linked network, superior corrosion protection was also afforded to the metal substrate.

Preferably the coated metal substrate according to the second aspect of the invention is produced according to the method of the first aspect of the invention.

In a preferred embodiment the metal substrate comprises aluminium, zinc, copper, brass or steel. The siloxane coating is chemically compatible with such substrates and affords such substrates excellent corrosion and abrasion resistance. It was also found that the siloxane coating was highly adhesive towards such metal substrates and therefore it was not necessary to provide a pre-treatment layer between the metal substrate and the siloxane coating.

In a preferred embodiment the metal substrate is a hot-dip coated steel substrate, preferably the hot-dip coating comprises zinc, aluminium or alloys thereof. By providing the steel substrate with a hot-dip coating and the siloxane coating, the steel substrate is afforded additional corrosion protection. For instance, when the hot- dip coating comprises zinc or a zinc alloy, sacrificial corrosion protection is afforded to the steel substrate because zinc is corroded in preference to iron in the steel. Additional corrosion protection is afforded to the steel substrate because the siloxane coating exhibits very good corrosion resistance and abrasion resistance, which protects the zinc or zinc alloy coating from corrosion and substantially reduces the zinc or zinc alloy coating from being cut or scratched, both of which are known to accelerate the zinc corrosion reaction. In addition, the siloxane coating retains its shiny appearance for a longer period of time relative to uncoated hot-dip galvanized substrates, making the siloxane coated substrate aesthetically more appealing.

In a preferred embodiment the metal substrate is a wire, preferably a zinc or zinc alloy coated steel wire. The siloxane coating provides additional corrosion protection to the zinc or zinc alloy wire. This is important because the thermal mass of the wire places a limitation on the maximum weight of the zinc or zinc alloy coating, and ultimately the service life of the zinc or zinc alloy coated wire. The siloxane coating is also flexible enough to be used in downstream processes such as bending and twisting.

In an embodiment a pigment is added to the mixture of the first solution and the second solution, preferably wherein the pigment is a phthalocyanine, more preferably wherein the pigment is copper phthalocyanine (blue). The addition of a pigment to the mixture, and thus to the sol-gel coating allows the coating to be distinguished from the substrate. If the substrate is galvanised, an unpigmented coating will not be visible as such, whereas the unpigmented coating is visible. It is important that the pigment is UV resistant for colourfastness and for this reason the phthalocyanine or one of its derivatives is a good choice. A copper phthalocyanine pigment proved to be very useful in this regard. To disperse the pigment in the mixture it is beneficial to use a dispersing agent (aka surfactant) such as Disperbyk 190, Antiterra 250 or Disperbyk 2012. By using the dispersing agent in combination with stirring the particle size of the dispersed pigment drops considerably from about 20 - 30 μιτι without dispersing agent to 100 nm with dispersing agent.

The invention will now be elucidated by way of example. These examples are intended to enable those skilled in the art to practice the invention and do not in anyway limit the scope of the invention as defined by the claims.

A first siloxane solution was prepared by providing (30ml_) of a Dynasylan ^® HYDROSIL 2627 solution and adjusting the pH to pH 3 using acidified water. Dynasylan ^® HYDROSIL 2627 comprises an amino alkyl-functional oligomeric siloxane in water and contains no volatile organic solvents. The concentration of the amino alkyl-functional oligomeric siloxane was 20 wt%. A second siloxane solution was prepared by providing (5ml_) of a Dynasylan ^® HYDROSIL 2909 solution and adjusting the pH to pH 3 using acidified water. Dynasylan ^® HYDROSIL 2909 comprises an amino alkyl-functional oligomeric siloxane in water and contains no volatile organic solvents The concentration of this amino alkyl-functional oligomeric siloxane was 37 wt%. These solutions were mixed in a weight ratio of 6 : 1 respectively and subsequently stirred for at least 5 hours under ambient conditions to form the sol- gel. Hydrosil 2909 contains 37% solid content and Hydrosil 2627 contains 20% solid content. Silanes are difficult to keep in aqueous base because they tend to crosslink between themselves in aqueous solutions as a result of hydrolysation. Siloxanes are made from silanes by carefully controlling the solid content in solution combined with pH adjustments so that controlled cross linking occurs till only oligomers are formed. Final formulation contain 6% (w/w) of first siloxane and 1.85% (w/w) of second siloxane making the final solid content in the formulation 7.85%.

To prepare a sol-gel that comprised cathodic corrosion inhibitors, 100 ppm of cerium acetate was provided in the mixed siloxane solution that was prepared according to the method above.

A hot-dip galvanised steel wire having a cross section of 2.65mm and a zinc coating weight of 60 g/sm (= g/m ² ) was provided, subsequently cleaned with iso- propanol to remove residual lubricant and then dried. The galvanised wire was then immersed in the sol-gel for 1 second. The sol-gel coated hot-dip galvanised steel wire was then cured in a conventional oven for 10 seconds to form the siloxane coating. The peak metal temperature during curing was 220°C.

The siloxane coatings were tested to investigate their flexibility (T-bend test ASTM D145), abrasion resistance (pencil hardness test ASTM D3363) and corrosion resistance (salt spray test (SST) according to ASTM B117. The test results are shown in Table 1 together with the test results of comparative examples C1-C7.

Example 1 relates to a hot-dip galvanised steel wire provided with the siloxane coating produced according to the method above. Comparative examples C1-C7 relate to hot-dip galvanised steel wires with and without additional organic coatings. Specifically, CI is a hot-dip galvanised steel wire provided with a polyurethane coating. C2 is the same as CI except that a pre-treatment layer is provided between the galvanised coating and the polyurethane coating. C3 relates to a hot-dip galvanised steel wire provided with a polyester coating. C4 is analogous to C3 except that a pre-treatment layer is provided between the galvanised coating and the polyester coating. C5 and C6 relate to uncoated hot-dip galvanised steel wires having a zinc coating weight of 60 g/sm and 300 g/sm. C7 is an uncoated hot-dip galvanised steel wire with a Zn/AI coating and a coating weight of 230 g/sm. Finally, C8 is a commercially available polyester coated galvanised steel wire.

Table 1

C8 500 pass 2H

The results show that the corrosion resistance of the siloxane coated hot-dip galvanised steel wire (El) is comparable to the corrosion resistance of organic coated hot-dip galvanised steel wires (C2) and (C4) that additionally comprise a pre- treatment layer, and the commercially available organic coated steel (C8). The results also show that the corrosion resistance of the siloxane coated hot-dip galvanised steel is better than the corrosion resistance of samples CI and C3, which relate to organic coated hot-dip galvanised steels without a pre-treatment layer, and the uncoated galvanised steel wires (C5-C7).

Regarding flexibility, the flexibility of the siloxane coated galvanised steel was comparable to that of comparative examples C1-C4 and C8. However, as can be seen from the pencil hardness test, the abrasion resistance of the siloxane coated steel (El) was significantly better than the abrasion resistance of comparative examples C1-C8.

In a further example copper phthalocyanine (supplied as Hostafine Blue B2G, with a particle size of 17 to 28 μιτι as measured by a light scattering technique) was mixed with a dispersing agent (Disperbyk 190) and water in a weight ratio of 5 : 0.35 :45 to produce 50 g of copper phthalocyanine containing solution and mixing it in a dispersing apparatus at 2000 RPM resulted in an average particle size of 100 nm. This copper phthalocyanine containing solution was then added to a mixture consisting of a first solution of Hydrosil 2627 and a second solution of Hydrosil 2909 and water in a weight ratio of 30: 5 : 65, wherein the final solution has pH of 5 and to which 1% by weight of the copper phthalocyanine containing solution was added after completion of the mixing process of the first solution and second solution. The curing conditions are the same for a pigmented or an unpigmented coating. Figure 2 shows a photo of the final product.

The durability performance in an electrochemical test (A, ISO 16773) and a salt spray test (B, ASTM B117) of the sol-gel coating (Coated) resulting from this mixture after curing is solution is compared with regular galvanised steel (GI) in Figure 1. In this it shows that the corrosion resistance under these conditions is at least three times larger than the corrosion resistance offered by galvanising alone. It should be noted that the sol-gel coating without the copper phthalocyanine pigment has the same durability performance as the pigmented sol-gel coating.

In figure 3 a corrosion protection index (on the Y-axis) is shown of GI (corrosion protection index = 1) against a GI wire coated with two different coatings as measured by measuring polarisation resistance by Electrochemical Impedance (EIS). This technique does not provide absolute values, but it does allow a comparison between various coatings. Figure 3 shows that compared to bare GI a coating with a mixing ratio of 6 : 1 provides 3.4 times the corrosion protection of bare GI and 3 : 1 provides 4.4 times the corrosion protection of bare GI.

By means of dynamic polarisation techniques the cathodic and anodic corrosion current can be measured, and from these two values the average value was determined. The polarisation resistance R _p is then calculated by dividing the corrosion potential by the corrosion current. The corrosion current can also be converted to a corrosion rate (CR) using (with M _Zn being the molar mass of Zn, n the number of electrons participating in the corrosion reaction and d the density of zinc, 3.28 is a constant) :

CR (mm/yr) = I _corr (mA/cm ² ) ^■ 3.28 ^■ M _Zn /(n-d)

From this CR (corrosion rate which is under accelerated conditions in the dynamic polarisation techniques) a comparison (Index) can again be made (see table 2 (note the Index for 6 : 1 = 3.3, which is nearly identical to the value in Figure 1, demonstrating the consistency of the results).

Table 2

Industrial production was performed using an industrial solution which was prepared by

- Prepared 305 L of acidified water in tank 1 adjusting pH to 3 with acetic acid;

- Decant 150 L of this water into tank 2 and 150 I into tank 3;

- Add 25 L of Hydrosil 2909 to tank 2 while stirring;

- Add 150 L of Hydrosil 2627 to tank 3 while stirring;

- Decant the content of tank 3 to tank 4 and slowly add the content of tank 2 to tank 3 while stirring;

- Leave the mixture stirring for 5 hrs before optionally adding 20 L of the colour dispersion prepared as previously described while stirring.

Now the solution is ready for coating application and provided into the in-line coating apparatus where the wire is coated. After exiting the coating apparatus the excess liquid is removed, e.g. by two point wiping to prevent spillage and carry over before entering the induction oven.

It should be noted that the density of Hydrosil 2909 and Hydrosil 2627 is about the same as water.

It should be noted that the examples given hereinabove are not intended to be limiting the shape of the metal substrate. The method according to the invention can be used on any substrate shape such as strip, beam, rail, etc.