A coating composition comprising an epoxy resin and a corrosion inhibitor and a coated metal substrate

Abstract

A coating composition comprising a resin component comprising an epoxy resin, a corrosion inhibitor comprising one or more thiol groups, and an alkyl ester of phosphoric acid, wherein the corrosion inhibitor comprising one or more thiol groups is or comprises a thiazole, an azole thiol, a trithiocyanuric acid, or a salt thereof, and wherein the resin component is substantially free of water. Use of an alkyl ester of phosphoric acid as stabilizer for such resin component, and a metal substrate at least partly coated with a coating layer deposited from such coating composition, wherein the corrosion inhibitor comprising one or more thiol groups comprises 2-mercaptobenzothiazole, 2,5-dimercapto-1,3,4-thiadiazole, a salt of 2,5-dimercapto-1,3,4-thiadiazole, or a combination of two or more thereof.

Description

A COATING COMPOSITION COMPRISING AN EPOXY RESIN AND A CORROSION INHIBITOR AND A COATED METAL SUBSTRATEField of the InventionThe present invention relates to a coating composition comprising a resin component comprising an epoxy resin and a corrosion inhibitor comprising one or more thiol groups, the use of a stabilizer for such resin component, and to a metal substrate at least partly coated with a coating layer deposited from such coating composition.Background of the InventionProtective coatings are widely used to protect substrates, particularly metal substrates from corrosion. Hexavalent chromium compounds have long been in use as corrosion inhibitors in protective coatings and conversion coatings for metal surfaces, in particular for metal surfaces of vehicles such as trains and aircrafts. However, hexavalent chromium is toxic and is therefore due to be phased out for environmental, worker safety, and regulatory reasons. Several chromate- free inhibitors have been proposed in recent years.In WO 2010 / 112605 is disclosed a low temperature curable chromate-free anticorrosive coating composition comprising a film-forming resin, a curing agent, and a lithium salt as corrosion inhibitor.Thiol compounds such as thiazoles, azole thiols, and derivates thereof have been proposed as corrosion inhibitors. In US 7,662,241 is disclosed the use of 2,5-dimercapto-1 ,3,4-thiadiazole (DMTD), 2,4-dimercapto-s-triazolo-[4,3-b]-1 ,3,4-thiadiazole, trithiocyanuric acid (TMT), its derivatives and salts as corrosion inhibitor for application to a metal substrate.In WO 2023 / 135327 is disclosed a corrosion resistant coating composition for metal substrates comprising an organic film-forming resin, a lithium salt, and a zinc salt of 2, 5-dimercapto-1 ,3,4- thiadiazole.In coating compositions comprising an epoxy resin, the presence of compounds with a thiol group can lead to instability and limited pot life, since thiol groups are known to be reactive towards epoxy resins.In WO 2023 / 135328 is disclosed an anticorrosive coating composition comprising an epoxy resin as film-forming resin, a lithium salt, and a zinc salt of 2,5-dimercapto-1 ,3,4-thiadiazole (DMTD),wherein the zinc salt of DMTD is present as encapsulated solid particles to improve pot life of the coating composition. Encapsulation of corrosion inhibiting pigments is, however, expensive.Therefore, there is a need for stabilization of anticorrosive coating compositions comprising, in a single component, an epoxy resin and a corrosion inhibitor comprising thiol groups.Summary of the inventionThe inventors have been found that if, in a coating composition comprising an epoxy resin and a corrosion inhibitor with a thiol group in one component, a small amount of an alkyl ester of phosphoric acid is present in such component, the component is stable, and the pot life is improved.Accordingly, in a first aspect, the invention provides a coating composition comprising a resin component comprising: a) an epoxy resin; b) a corrosion inhibitor comprising one or more thiol groups; and c) an alkyl ester of phosphoric acid wherein the corrosion inhibitor is or comprises an azole, an azole thiol, a trithiocyanuric acid, or a salt thereof, and wherein the resin component is substantially free of water.An advantage of the coating composition according to the invention is that no encapsulation of the corrosion inhibitor with a thiol group is needed in order to protect the epoxy resin from reacting with the thiol groups of the corrosion inhibitor.In a second aspect, the invention provides use of an alkyl ester of phosphoric acid as stabilizer for a resin component that is substantially free of water and comprises an epoxy resin and a corrosion inhibitor comprising one or more thiol groups, wherein the corrosion inhibitor comprising one or more thiol groups is or comprises a thiazole, an azole thiol, a trithiocyanuric acid, or a salt thereof.In a final aspect, the invention provides a metal substrate at least partly coated with a coating layer deposited from a coating composition according to the first aspect of the invention, wherein the corrosion inhibitor comprising one or more thiol groups comprises 2-mercaptobenzothiazole,2,5-dimercapto-1 ,3,4-thiadiazole, a salt of 2,5-dimercapto-1 ,3,4-thiadiazole, or a combination of two or more thereof.Detailed Description of the InventionThe coating composition according to the invention comprises a resin component comprising an epoxy resin, a corrosion inhibitor comprising one or more thiol groups, and an alkyl ester of phosphoric acid.The coating composition may be a one-component composition ora two-component composition. If the composition is a one-component composition, the resin component is the only component, and the coating composition consists of the resin component. In a one-component coating composition, the resin component preferably comprises a curing catalyst, so that the epoxy resin can cure even in the absence of a curing agent. Typically, a one-component composition can be cured by heating the resin component comprising the epoxy resin and a curing catalyst (catalytic homopolymerization).Preferably, the coating composition is a two-component composition comprising a first component comprising the epoxy resin (the resin component) and a second component comprising a curing agent reactive with the epoxy resin in the first component. Before application of the coating composition on a substrate, the two components are mixed, and the mixed composition is then applied on a substate to form a coating upon curing. The two-component coating composition may comprise more than two components. Such composition with more than two components comprises the first component and the second component as described and comprises a further component such as for example a catalyst component or a thinner component.The curing agent reactive with the epoxy resin in the first component may be any suitable curing agent. Such curing agents are well known in the art and include polyamines, polyamidoamines, polyamides, anhydrides, polyphenols, polyisocyanates, and thiols. Preferably the curing agent is a polyamine, a polyamidoamine, a polyamide, a combination of two or more thereof, or a polyisocyanate. Non-cyclic aliphatic polyamines, cycloaliphatic polyamines, polyamidoamines, and polyamides are particularly suitable curing agents.The resin component is substantially free of water.Reference herein to ‘substantially free of water’ is to the absence of intentionally added water. Trace amounts of water that may be present in organic solvents or other ingredients of thecomponent may be present. Preferably, the resin component comprises at most 1 wt.% of water, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%.The component comprising the curing agent (second component or curing component) may be water borne, solvent borne, or solvent-free. Preferably, the curing component is solvent borne or solvent-free and is substantially free of water. Preferably, the curing component comprises at most 1 wt.% water, more preferably at most 0.5 wt.%, even more preferably at most 0.1 wt.%.The resin component is a liquid component. The resin component may be a solvent borne component. Reference herein to a solvent borne component is to a component comprising a liquid carrier for its solid components that essentially consists of one or more organic solvents. Alternatively, the resin component is a liquid solvent-free component. A solvent-free resin component will typically comprise a solvent-free epoxy resin of low viscosity.If the resin component is a solvent borne component it will comprise an organic solvent. The resin component may comprise organic solvent in any suitable amount. It will be appreciated that the amount of organic solvent will mainly be determined by the desired viscosity of the resin component and the viscosity of the epoxy resin. Typically, the resin component will comprise up to 60 wt.% organic solvent, preferably up to 50 wt.%. Any suitable organic solvent may be used. Organic solvents suitable as liquid carrier for epoxy resins are known in the art and include ketones (e.g., methyl ethyl ketone, cyclohexanone, acetyl acetone), alcohols (e.g., benzyl alcohol, isopropyl alcohol, butanol), esters (e.g., butyl acetate), ethers, and hydrocarbons (e.g., xylene, aliphatic hydrocarbons).The epoxy resin may be any resin with one or more epoxide functional groups that is able to form a film upon curing. Curing may be achieved by heating the epoxy resin (homopolymerization) or by crosslinking the epoxy resin with a curing agent reactive with the epoxy resin. Suitable epoxy resins are well-known in the art and include polyglycidyl ethers of aromatic or aliphatic cyclic and non-cyclic polyols. The epoxy resin may be a mixture of more than one epoxy resins. Low viscosity epoxy resins (often referred to as reactive diluents) may be used, preferably in combination with a further epoxy resin.The epoxy resin may have any suitable epoxy equivalent weight (EEW). The EEW is the amount of solid epoxy resin (in grams) per epoxy equivalent. Reference herein to EEW is to EEW as determined according to ASTM D1652-11. The invention is particularly advantageous for a coating composition with a resin component comprising an epoxy resin with a relatively low EEW,i.e., a relatively high density of epoxy functional groups, since such epoxy resin is more likely to react with a thiol group of the corrosion inhibitor, leading to instability, and thus benefits most from the presence of a stabilizer.Preferably the epoxy resin has an epoxy equivalent weight in the range of from 100 to 1 ,000 grams / epoxy equivalent, more preferably of from 100 to 600 grams / epoxy equivalent. In case the resin component comprises more than one epoxy resin, the EEW of the epoxy resin is the weight average EEW, calculated as the sum of the products of EEW and mass fraction for all epoxy resins, wherein the mass fraction is based on the total mass of solid epoxy resin. As an example, for two epoxy resins (epoxy resin 1 and epoxy resin 2):EEWaverage = Rlfl EEWi + mf2EEW2wherein EEWaverage is the weight average EEW; mfi is the mass fraction of epoxy resin 1 ; EEWi is the EEW of epoxy resin 1 ; mf2is the mass fraction of epoxy resin 2; and EEW2is the EEW of epoxy resin 2.The resin component may comprise any suitable amount of epoxy resin. It will be appreciated that the pot life of a component comprising epoxy resin in the presence of a thiol-functional corrosion inhibitor will depend inter alia on the amount and epoxy functionality (EEW) of the epoxy resin and on the amount and thiol-functionality of the thiol-containing corrosion inhibitor.Typically, the resin component has an epoxy resin solids content in the range of from 10 to 70 wt.%, preferably of from 15 to 60 wt.%, based on the total weight of the resin component.The resin component comprises a corrosion inhibitor comprising one or more thiol groups. The corrosion inhibitor comprising one or more thiol groups is or comprises a thiazole, an azole thiol, a trithiocyanuric acid, or a salt thereof. Suitable corrosion inhibitors are for example corrosion inhibitors comprising an anion formed by ionization of any one of the thiol-comprising organic compounds disclosed in paragraph

[0047] of WO 2008 / 143735 and any one of the thiol- comprising organic compounds disclosed in paragraph

[0048] of WO 2008 / 143735. Preferably, the corrosion inhibitor comprising one or more thiol groups is or comprises a substituted thiadiazole, or a salt thereof. Still more preferably the corrosion inhibitor comprising one or more thiol groups is or comprises 2-mercaptobenzothiazole, 2,5-dimercapto-1 ,3,4-thiadiazole, a salt of 2,5-dimercapto-1 ,3,4-thiadiazole, or a combination thereof. Metal salts of 2, 5-dimercapto-1 ,3,4- thiadiazole are particularly preferred, more in particular zinc salt of 2,5-dimercapto-1 ,3,4-thiadiazole. Zinc salt of 2,5-dimercapto-1 ,3,4-thiadiazole is commercially available, for example as Inhibicor 1000 (ex. WPC Technologies).The corrosion inhibitor comprising one or more thiol groups may be present in the form of a hybrid with other compounds. Hybricor™ 204 (ex. WPC Technologies) for example is a commercially available hybrid pigment that is particularly suitable as the corrosion inhibitor comprising one or more thiol groups. It comprises a hybrid of 2,5-dimercapto-1 ,3,4-thiadiazole and other compounds such as zinc phosphate and zinc oxide.The resin component may comprise the corrosion inhibitor comprising one or more thiol groups in any suitable amount. The resin component may for example comprise in the range of from 5 to 60 wt.% thiol-comprising corrosion inhibitor, preferably of from 10 to 40 wt.%, based on the weight of epoxy resin solids in the resin component.The coating composition according to the invention is preferably a chromate-free coating composition.The coating composition according to the invention may comprise a further corrosion inhibitor. Preferably it comprises one or more further chromate-free corrosion inhibitors. If the coating composition is a two-component composition, the further corrosion inhibitor(s) may be present in any component, i.e., in the resin component, the curing component, or any further component.The further corrosion inhibitor may be any corrosion inhibitor known in the art, preferably a chromate-free corrosion inhibitor known in the art. A suitable further corrosion inhibitor may be selected from the group consisting of a lithium salt, a magnesium compound, a phosphate, an oxide or salt of zinc, an oxide or salt of a rare earth metal such as cerium, praseodymium, or lanthanide. Examples of suitable magnesium compounds are magnesium oxide and magnesium hydroxide. Examples of suitable phosphates are zinc phosphate, zinc-aluminium polyphosphate, strontium aluminium polyphosphates, cerium diphenyl phosphate, calcium aluminium polyphosphate silicate, and any hydrates thereof. Examples of suitable lithium salts include lithium carbonate, lithium phosphate, lithium oxalate, and lithium borate.More preferably, the coating composition comprises a further corrosion inhibitor selected from a lithium salt, magnesium oxide, magnesium hydroxide, zinc phosphate, zinc oxide, cerium oxide, and combinations of two or more thereof. Lithium salts, magnesium oxide, and cerium oxide are particularly preferred further corrosion inhibitors. Particularly preferred lithium salts are lithium carbonate and lithium phosphate.The resin component comprises an alkyl ester of phosphoric acid. The alkyl ester of phosphoric acid may be a mixture of two or more alkyl esters of phosphoric acid. It may comprise mono-, di-, or tri-alkyl ester of phosphoric acid, or a combination thereof. The one, two, or three alkyl group(s) in the alkyl ester of phosphoric acid preferably have, independently, in the range of from 1 to 20 carbon atoms, more preferably of from 4 to 16 carbon atoms, even more preferably of from 6 to 12 carbon atoms. The alkyl group may be linear or branched. A particularly preferred alkyl ester of phosphoric acid is a mixture of octyl esters of phosphoric acid and decyl esters of phosphoric acid. In a particularly preferred embodiment, the alkyl ester of phosphoric acid is a mixture of octyl esters of phosphoric acid and decyl esters of phosphoric acid and the coating composition is free of any alkyl ester of phosphoric acid other than octyl esters of phosphoric acid and decyl esters of phosphoric acid.Preferably, the resin component comprises the alkyl ester of phosphoric acid in an amount in the range of from 0.05 to 5.0 wt.% based on the weight of epoxy resin solids, more preferably in the range of from 0.1 to 3.0 wt.%, even more preferably of from 0.2 to 2.0 wt.%.The coating composition may comprise further compounds such as fillers, pigments, and additives. Additives may include any additive typically used in epoxy-based coating compositions, for example wetting agents, dispersing agents, rheology modifying agents, levelling agents, adhesion promoters.In a second aspect, the invention provides use of an alkyl ester of phosphoric acid as stabilizer for a resin component that is substantially free of water and comprises an epoxy resin and a corrosion inhibitor comprising one or more thiol groups, wherein the corrosion inhibitor comprising one or more thiol groups is or comprises a thiazole, an azole thiol, a trithiocyanuric acid, or a salt thereof. The alkyl ester of phosphoric acid has the features and preferred features as hereinbefore described with respect to the coating composition according to the invention. The alkyl ester of phosphoric acid is preferably mixed with the epoxy resin, the corrosion inhibitor comprising one or more thiol groups, and any other ingredients of the resin component (e.g., solvent, pigments, additives) in an amount in the range of from 0.05 to 5.0 wt.% based on the weight of epoxy resin solids, more preferably in the range of from 0.1 to 3.0 wt.%, even more preferably of from 0.2 to 2.0 wt.%. The alkyl ester of phosphoric acid may be provided as a solution in organic solvent. Such solutions are commercially available, for example as BLISTER Free 3 (ex. Bernd Schwegmann GmbH & Co. KG).The coating composition according to the invention is a corrosion resistant coating composition and is particularly suitable to be used on a metal substrate. The coating composition may be used as a primer coating composition. Such primer coating composition is directly applied on an optionally pretreated metal surface. Alternatively, or additionally, the coating composition may be used to provide a further coating on top of a primer coating. In a multi-layer coating system comprising a primer coating and one or more further coatings, the coating composition may be used to provide any of these coatings, i.e., primer coating or any one of the further coatings, preferably the primer coating.In a final aspect, the invention therefore provides a metal substrate at least partly coated with a coating layer deposited from a coating composition according to the first aspect of the invention, wherein the corrosion inhibitor comprising one or more thiol groups comprises 2- mercaptobenzothiazole, 2,5-dimercapto-1 ,3,4-thiadiazole, a salt of 2,5-dimercapto-1 ,3,4- thiadiazole, or a combination of two or more thereof. The metal substrate may be any metal substrate that needs protection from corrosion. Preferably, the substrate is a steel substrate, an aluminum substrate, or an aluminium alloy substrate. An example of a suitable substrate is a metal surface of a vehicle, in particular a metal surface of an aircraft or part thereof.The substrate may be a bare substrate or a pretreated substrate, preferably a substrate pretreated with a common pretreatment for metal substrates for vehicles or aircrafts such as pretreatment with a chemical conversion coating, an anodization coating, a wash coat, galvanizing, or electrocoating.The bare or pretreated substrate may be coated with a single coating layer, wherein the coating layer is deposited from the coating composition according to the first aspect of the invention. Preferably, the bare or pretreated substrate is coated with one or more further coating layers. In such multi-layer coated substrate, the coating layer deposited from the coating composition according to the first aspect of the invention may be any one of the coating layers. Preferably, the coating layer deposited from the coating composition according to the first aspect of the invention is a primer coating layer. Such primer coating layer is obtained by directly applying the coating composition on the, optionally pretreated, metal substrate.The present invention will further be illustrated by the following non-limiting examples.The following epoxy resins were used:EPIKOTE™ Resin 828 (ex. Westlake Epoxy): Diluent-free, medium viscosity, bisphenol-A based liquid epoxy resin produced from bisphenol A and epichlorohydrin. 100% solids; EEW: 184-190 g / epoxy equivalent.Dynasylan® GLYMO (ex. Evonik): 3-Glycidyloxypropyltimethoxysilane. EEW: 236 g / epoxy equivalent.Dynasylan® GLYEO (ex. Evonik): 3-Glycidyloxypropyltriethoxysilane. EEW: 278 g / epoxy equivalent.D.E.N. ® 431 (ex. Olin): Epoxy Novolac Resin. EEW: 172-179 g / epoxy equivalent.EPIKOTE™ Resin 1001-X-75 (ex. Westlake Epoxy): Bisphenol-A based epoxy resin in solution (75 wt.% solids in xylene). EEW 450-500 g / epoxy equivalent (based on nonvolatiles).DLVE® 52 (ex. Olin): Ultra-low viscosity epoxy resin modified with a cycloaliphatic polyglycidyl ether (free of any organic solvent). EEW 165-180 g / epoxy equivalent.DLVNE® 59 (ex. Olin): Low viscosity novolac resin modified with a cycloaliphatic polyglycidyl ether (free of any organic solvent). EEW 160-175 g / epoxy equivalent.Other ingredients used:Hybricor™ 204 (ex. WPG Technologies): a hybrid corrosion inhibitor pigment comprising 37 wt.% zinc salt of 2,5-dimercapto-1 ,3,4-thiadiazole.BLISTER Free 3 (ex. Bernd Schwegmann GmbH & Co. KG): a solution of a mixture of octyl- and decyl esters of phosphoric acid in organic solvent (3-10 wt.% octyl- and decyl esters of phosphoric acid).Example 1 - Stability of liquid mixtures of epoxy resin and thiol-containing corrosion inhibitorThe storage stability of mixtures comprising an epoxy resin and a thiol-containing corrosion inhibitor was determined in a model experiment.Epoxy resin and Hybricor™ 204 were mixed in the amounts as given in Table 1 (Experiments 1a to 1g) to yield a homogenous dispersion. The dispersion was stored at 23 °C for three weeks.After three weeks, precipitation of solids was observed in all dispersions. It was not possible to re-disperse the solids. Analysis of the precipitate by IR spectroscopy showed that a chemical reaction between the epoxy groups of the epoxy resin and the thiol groups of the corrosion inhibitor had occurred.Example 2 - Stability of liquid mixture of epoxy resin and thiol-containing corrosion inhibitor comprising alkyl esters of phosphoric acidThe storage stability of mixtures comprising an epoxy resin, a thiol-containing corrosion inhibitor, and alkyl ester of phosphoric acid was determined in a model experiment.Epoxy resins, Hybricor™ 204, and an additive comprising octyl and decyl esters of phosphoric acid (Blister Free 3) were mixed in the amounts as given in Table 1 (Experiments 2a and 2b) to yield a homogenous dispersion. The dispersion was stored at 23 °C and at 40 °C.The dispersion of Experiment 2a was stored at 23 °C and at 40 °C. After 8 months storage at 23 °C, the dispersion showed no solid precipitate and no viscosity increase. IR spectroscopy of the dispersion showed that no chemical reaction between epoxy and thiol groups had occurred.After 4 months storage at 40 °C, the dispersion showed no solid precipitate and no viscosity increase.The dispersion of Experiment 2b was stored at 23 °C and at 40 °C. After 2 months storage, no solid precipitate and no viscosity increase was observed for both storage temperatures.Table 1 - Type and amounts (in wt.%) of ingredients used in Examples 1 and 2Example 3 - Exemplary resin componentIn Table 2 an exemplary resin component for the coating composition according to the invention is given. All weight percentages are based on the total weight of the resin component. The weight average EEW of the epoxy resins was 440 g / epoxy equivalent. Table 2 - Resin component

Claims

CLAIMS1 . A coating composition comprising a resin component comprising: a) an epoxy resin; b) a corrosion inhibitor comprising one or more thiol groups; and c) an alkyl ester of phosphoric acid wherein the corrosion inhibitor comprising one or more thiol groups is or comprises a thiazole, an azole thiol, a trithiocyanuric acid, or a salt thereof, and wherein the resin component is substantially free of water.

2. A coating composition according to claim 1 , wherein the coating composition is a two- component coating composition comprising a first component and a second component, wherein the first component is the resin component and comprises the epoxy resin and the second component comprises a curing agent reactive with the epoxy resin.

3. A coating composition according to claim 2, wherein the curing agent is a polyamine, a polyamidoamine, a polyamide, a combination of two or more thereof, or a polyisocyanate.

4. A coating composition according to any one of the preceding claims, wherein the epoxy resin has an epoxy equivalent weight (EEW) in the range of from 100 to 1 ,000 grams / epoxy equivalent, preferably in the range of from 100 to 600 grams / epoxy equivalent.

5. A coating composition according to any one of the preceding claims comprising a further chromate-free corrosion inhibitor.

6. A coating composition according to claim 5, wherein the further corrosion inhibitor is selected from the group consisting of a lithium salt, magnesium oxide, magnesium hydroxide, zinc phosphate, zinc oxide, cerium oxide, and combinations of two or more thereof, preferably selected from the group consisting of a lithium salt, magnesium oxide, and combinations thereof.

7. A coating composition according to any one of the preceding claims, wherein the alkyl ester of phosphoric acid has alkyl groups having, independently, in the range of from 1to 20 carbon atoms, preferably of from 4 to 16 carbon atoms, more preferably of from 6 to 12 carbon atoms.

8. A coating composition according to claim 7, wherein the alkyl ester of phosphoric acid is a mixture of octyl esters of phosphoric acid and decyl esters of phosphoric acid.

9. A coating composition according to any one of the preceding claims, wherein the resin component comprises the alkyl ester of phosphoric acid in an amount in the range of from 0.05 to 5.0 wt.% based on the weight of epoxy resin solids, preferably in the range of from 0.1 to 3.0 wt.%, even more preferably of from 0.2 to 2.0 wt.%.

10. A coating composition according to any one of the preceding claims, wherein the corrosion inhibitor comprising one or more thiol groups comprises 2- mercaptobenzothiazole, 2,5-dimercapto-1 ,3,4-thiadiazole, a salt of 2,5-dimercapto-1 ,3,4- thiadiazole, or a combination of two or more thereof.

11. A coating composition according to claim 10, wherein the corrosion inhibitor comprising one or more thiol groups comprises a zinc salt of 2,5-dimercapto-1 ,3,4-thiadiazole.

12. Use of an alkyl ester of phosphoric acid, preferably an alkyl ester of phosphoric acid as specified in claim 7 or 8, as stabilizer for a resin component that is substantially free of water and comprises an epoxy resin and a corrosion inhibitor comprising one or more thiol groups as specified in any one of claims 1 , 10, and 11 .

13. Use according to claim 12, wherein the alkyl ester of phosphoric acid is used in an amount in the range of from 0.05 to 5.0 wt.% based on the weight of epoxy resin solids in the resin component, preferably in the range of from 0.1 to 3.0 wt.%, even more preferably of from 0.2 to 2.0 wt.%.

14. A metal substrate at least partly coated with a coating layer deposited from a coating composition according to claim 10 or 11 , preferably a steel substrate, an aluminium substrate, or an aluminium alloy substrate.

15. A metal substrate according to claim 14, wherein the coating layer deposited from a coating composition according to claim 10 or 11 is a primer coating layer.