CORROSION INHIBITOR

Abstract

The present invention relates to a corrosion inhibitor and a corrosion inhibiting coating provided for coating a metal, particularly but not exclusively steel. The inhibitors pigment will also protect aluminium and magnesium alloys. The corrosion inhibitor particularly protects a sacrificial coating such as zinc or zinc alloy on galvanised steel, which in turn therefore provides improved corrosion resistance to the underlying steel. The present invention comprises an organic cation in a cation exchange resin.

Claims

1. A corrosion inhibitor comprising: an organic cation in a cation exchange resin.

2. A corrosion inhibitor according to claim 1, wherein the organic cation in the cation exchange resin is particulate.

3. A corrosion inhibitor as recited in claim 1, wherein the organic cation is an azole or an oxime.

4. A corrosion inhibitor as recited in claim 1, wherein the organic cation is benzotriazole or a derivative thereof.

5. A corrosion inhibitor as recited in claim 1, wherein the cation exchange resin is an organic cation exchange resin matrix.

6. A corrosion inhibitor as recited in claim 5, wherein the cation exchange resin is a styrene and/or divinylbenzene copolymer with a negatively charged group.

7. A corrosion inhibitor as recited in claim 6, wherein the styrene and divinylbenzene copolymer has a negatively charged sulphonated functional group.

8. A corrosion inhibitor as recited in claim 2, wherein particulate size of the corrosion inhibitor is less than 5 microns.

9. A corrosion inhibitor as recited in claim 1, wherein a ratio of organic cation to cation exchange resin is approximately 10% by weight of organic cation to cation exchange resin.

10. An additive for addition to a coating for imparting corrosion resistance upon a substrate comprising: a first corrosion inhibitor comprising an organic cation in a cation exchange resin; and a second corrosion inhibitor comprising inorganic cation modified silica.

11. An additive as recited in claim 10, wherein the inorganic cation modified silica comprises calcium cation modified silica.

12. An additive as recited in claim 10, wherein the inorganic cation modified silica is particulate.

13. An additive as recited in claim 10, wherein the first corrosion inhibitor and the second corrosion inhibitor comprise a particulate mixture.

14. An additive for addition to a coating for imparting corrosion resistance upon a substrate comprising a first corrosion inhibitor comprising an organic cation in a cation exchange resin, a second corrosion inhibitor comprising an inorganic cation in a cation exchange resin.

15. A coating for a metal substrate comprising: a corrosion inhibitor or additive comprising an organic cation in a cation exchange resin provided in a polymer binder.

16. A coating according to claim 15, wherein the polymer binder is selected from one or more of an acrylic, polyurethane or polyvinyl butyral.

17. A coating as recited in claim 15, in a form of a paint.

18. A metal substrate coated with a coating comprising a corrosion inhibitor or additive comprising an organic cation in a cation exchange resin provided in a polymer binder.

19. A metal substrate according to claim 18, wherein the metal substrate is steel, or an alloy based on steel.

20. A method of manufacturing a corrosion inhibitor comprising: combining organic cations with a cation exchange resin.

21. A method as recited in claim 20, further comprising: providing the organic cations in solution; and combining the cation exchange resin with the solution to form beads.

22. A method comprising: providing a corrosion inhibitor or additive comprising an organic cation in a cation exchange resin provided in a polymer binder; wherein the organic cations are produced by dissolving an organic compound into solution, the organic compound being capable of dissociating into at least two ions, one of the ions being the organic cation, wherein the solution has a pH of less than 3.

23. A method of manufacturing a corrosion inhibitor, comprising: providing an organic compound in solution and combining with a cation exchange resin having a negatively charged functional group for dissociating the organic compound to organic cations and anions, wherein the organic cations and cation exchange resin together form the corrosion inhibitor.

24. A method as recited in claim 23 wherein, the corrosion inhibitor is formed into beads, and the beads are filtered from solution.

25. A method as recited in claim 24, comprising drying the beads.

26. A method as recited in claim 25, further comprising breaking up the beads into smaller particles.

27. A method as recited in claim 20, wherein the organic cations are an azole and preferably comprise benzotriazole.

28. A method as recited in claim 20, wherein the cation exchange resin is an organic cation exchange resin.

29. A method as recited in claim 28, further comprising combining a second corrosion inhibitor comprising an inorganic cation in a cation exchange resin.

30. A method as recited in claim 26, further comprising mixing the smaller particles with a second corrosion inhibitor comprising particulate inorganic cation modified silica.

31. A method of manufacturing a coating for a metal substrate comprising: combining a corrosion inhibitor comprising an organic cation exchange resin with a polymer binder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] An embodiment of the invention will now be described by way of example only with reference to and as illustrated in the following figures and examples in which;

[0043] FIG. 1 shows: a schematic exploded view of a typical metal substrate and coating layers;

[0044] FIG. 2 shows: the action of a corrosion inhibitor in the event of a breach of coating layers to reach the metal substrate;

[0045] FIG. 3 shows: the corrosion progress of a coated metallic substrate without an inhibitor included in the coating (i) is 240 minutes after initiation, then every line 60 mins up to (ii) 780 mins. Inset is the distance from defect vs time for an uninhibited system and a strontium chromate inhibited system;

[0046] FIG. 4 shows: the delamination of a coating on a hot-dip galvanised steel (HDG) surface with a loading of 0.1 PVF of benzotriazole in a cation exchange resin.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0047] The present invention has been developed to provide a smart-release corrosion inhibitor which has particular but not exclusive application in the protection of galvanised steel from corrosion. The inhibitor, which is usually applied as a primer to a metal surface in liquid form at room temperature and pressure contains an organic ion, preferably an azole, and even more preferably benzotriazolate (BTA). This is added to an ion exchange matrix. The ion exchange resin matrix in one embodiment is a divinylbenzene copolymer with a sulphonate functional group as shown below. The benzene ring with the three nitrogen atoms is benzatriazolate and is positively charged due to extra hydrogen cation. The ion exchange resin matrix is the remainder and is shown as being negatively charged.

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[0048] The corrosion inhibitor structure is formed of repeating units of the ion exchange resin with a sulphonated group having a negative charge to hold the corrosion inhibiting cation of protonated benzotriazole in place until positively charged corrosion electrolyte ions are present.

[0049] To make the corrosion inhibitor according to an exemplary embodiment, benzotriazole is dissolved in water at a molar concentration of 0.25 M. The solution can be heated to dissolve the benzotriazole or the pH is adjusted using an acid. A suitable amount through experimentation of benzatriazole is 29.78 g per litre of water. An amount of the solution is taken which may be at room temperature, or it can also be heated, for example to 40 degrees Celsius and divinylbenzene copolymer with a sulphonated functional group is added to the solution. As an example 10 g of the divinylbenzene copolymer is added to 100 ml of the solution containing the benzotriazole. The mixture is stirred, typically for an hour and left to settle so that beads are formed. Once the beads have settled the supernatant solution is decanted off and replaced with more 0.25 M benzotriazole solution in the ratio of 100 ml to 10 g of original weight of exchanger. This encourages more ion exchange. The topped up solution is stirred for another period, typically an hour and any supernatant left after a further period of settling is decanted and replaced with further solution. The topped up solution is stirred further, for example for a further four hours to ensure saturation of BTA within the matrix. The resultant beads are filtered off and washed with de-ionised water. This process ensures the exchange of the Cl anion of the divinylbenzene copolymer with the BTA is maximised.

[0050] Another processing route for the inhibitor, is to run a benzotriazole solution through an ion exchange column process. The resin beads are static in the column and a solution of corrosion inhibitor is run through the column, where the beads pick up the corrosion inhibitors from the solution. The beads can be removed from the column and processed using the methods below.

[0051] The beads contain a BTA in a divinyl benzene matrix. The beads are then dried for a period of time such as overnight at 40 degrees Celsius and then ball milled (typically for 1 hour) to achieve a powdered form that can be added to a coating such as a primer coating. The powdered material that is formed may be added to a primer at a range of 1-30% w/w.

[0052] An inorganic cation in a cation exchange resin to provide an optional second corrosion inhibitor within the polymer binder is beneficial which may act synergistically. The second corrosion inhibitor may be achieved by the following exemplary procedure. Cation exchange resin beads (e.g. Amberlite Dowex) were dispersed in 1 mol dm.sup.3 aqueous solutions of the relevant metal chloride salt and the resulting suspensions stirred for 2 hours. The suspensions were subsequently left to settle overnight and the supernatant decanted. The resin beads were exhaustively washed by repeated cycles of centrifugation and re-dispersion in fresh distilled water, until no chloride ions could be detected in the supernatant by silver nitrate aqueous solution testing. The inorganic cation solution can be used in ion exchange columns to add the cations to the cation exchange resin.

[0053] Finally the resin beads were dried in air at 40 C. and ground in a planetary mill to give a particle size of <5 microns diameter, or milled in a jet mill to give a d50 of 5 m. The second corrosion inhibitor may then be incorporated with the polymer binder and first corrosion inhibitor.

[0054] The corrosion inhibitor may be mixed with an inorganic cation modified silica, preferably calcium cation modified silica (an example of which is sold under the tradename Shieldex). The particulate organic cation in the cation exchange resin is preferably mixed with particulate inorganic cation modified silica, however it will be appreciated that mixing may occur before breaking down into particulate form.

[0055] The primer may be used in a multi-layer system on coated Hot Dip Galvanised (HDG) Steel, to protect from under-film corrosion. The benzotriazolate is released when it comes into contact with a corrosive electrolyte after which it sequesters the electrolyte ions. Typically the primer is used on a zinc or zinc alloy surface and forms a protective layer by adhering onto the zinc surface. If there is any corrosion, the organic exchange matrix will sequester ions that have been formed as a result of the corrosion and by having the active agent in a matrix, there is also slow release of benzotriazole.

[0056] A series of coatings was prepared by dispersing various volume fractions of the corrosion inhibitor formed of benzatriazole in an ion exchange resin matrix, which is then mixed in a polyvinyl butyral binder. This mixture was then applied to HDG steel and an in situ scanning Kelvin probe was used to evaluate the efficiency of the mixtures in inhibiting corrosion driven coating failures by cathodic delamination. The Na.sup.+ and other cations present are sequestered into the coating and benzotriazole released and deprotonated due to the local pH as it is released into the defect electrolyte. The deprotonated benzotriazole can now remain in a neutral form, or if the pH of the environment is above approximately pH6 then the neutral deprotonated benzotraizole can be deprotonated again to form a benzotrialozate anion which can react to form a precipitate with Zn.sup.2+ (Zn(BTA).sub.2). An insoluble precipitate is thus formed blocking interfacial electron transport. Another effect is that benzotriazole is hydrophobic in nature and binds to the metal surface in a mono layer, which then attracts other benzotriazole molecules, creating a barrier to the electrolyte and oxygen.

[0057] Under normal corrosion conditions as described above, the organic cation (benzotrialozate) exchanges off the cation exchange resin to either react with a metal cation to form a complex and reacts with free ions of the corrosion process at the surface of the metal. The effect of the inorganic cation modified silica is to provide a third possibility where reaction occurs with the inorganic cation modified silica (calcium) to form insoluble precipitates. The precipitate formed by this third route is highly insoluble and provides a strong barrier to further corrosion.

[0058] FIG. 3 shows the corrosion progress of a coated metallic substrate without an inhibitor included in the coating (i) is 240 minutes after initiation, then every line 60 mins up to (ii) 780 mins. This shows the progress of corrosion over time as the corrosion progresses under the coating, and shows that after 780 minutes there is 12 mm of corrosion for an unprotected coating. The upper lines represent the intact measures potential of the coating and the lower line represents the delaminated potential of the coating, with the joining lines representative of the corrosion front at 60 minute intervals. Inset is the distance from defect versus time for an uninhibited system and a strontium chromate inhibited system, showing the relative effectiveness of use of traditional strontium chromate as corrosion inhibitor.

[0059] FIG. 4 shows the delamination of a coating on a hot-dip galvanised steel (HDG) surface with a loading of 0.1 PVF of benzotriazole in a cation exchange resin. This representation can be compared directly to the graph of FIG. 4 where at a defect site there is initial progression of a defect to a distance of 1 mm, following which there is no subsequent progression of that defect. Thus, the presence of a corrosion inhibitor according to an exemplary embodiment of the present invention halts subsequent defect progression by providing a highly corrosion inhibitive system. This is further shown through the presence of multiple overlaid plots up to 1800 minutes showing no additional defect progression.

[0060] The present invention has been described by way of example only and it will be appreciated by the skilled addressee that modifications and variations may be made without departing form the scope of protection afforded by the appended claims.