WO2004085551A1

WO2004085551A1 - Corrosion inhibiting coating

Info

Publication number: WO2004085551A1
Application number: PCT/AU2004/000314
Authority: WO
Inventors: Glen Deacon; Craig Forsyth; Maria Forsyth; Daniel Ho
Original assignee: Monash University
Priority date: 2003-03-13
Filing date: 2004-03-12
Publication date: 2004-10-07
Also published as: AU2003901149A0

Abstract

The present invention provides a corrosion inhibiting coating comprising a rare earth-based organic compound and/or a combination of a rare earth metal and an organic compound, a method of inhibiting corrosion involving the coating and a method of preparing said coating.

Description

Corrosion Inhibiting Coating

Field of the Invention

This invention relates to a corrosion inhibiting coating and a method of inhibiting corrosion of a metal substrate involving the corrosion inhibiting coating. In particular, it relates to a corrosion inhibiting coating which is an epoxy primer for the corrosion protection of metals .

Background of the invention

Metallic corrosion is a natural process driven by thermodynamics, in which elements in their metallic form obtain a lower energy state by reacting with the surrounding environment to form stable oxide ores .

Most forms of corrosion are of the electrochemical type, involving the establishment of corrosion cells (galvanic cells) comprising of anodes, cathodes and an electrolyte. Metal dissolution occurs at the anodes where the metal is oxidized (M -^ Mⁿ + ne^") , generating free electrons and metallic ions. The free electrons travel to the cathodic sites and participate n the reduction reactions (0₂ + 2H₂0 + 4e^~ -ϊ> 40fT) (2H⁺ + 2e^~ -ϊ> H₂) . The circuit is completed by the flow of ionic charge through the electrolyte, resulting in the formation of hydroxide layers (Mⁿ⁺ + nOHT ^■*^■ M(OH)_n). Pitting corrosion occurs if the anodes and cathodes are clearly distinguishable. General corrosion occurs if numerous anodes and cathodes are very closely spaced thus indistinguishable and change place at short intervals of time ^1X .

Corrosion inhibitors are substances that reduce the rate of corrosion when added to a corrosive environment in suitable concentrations . This is achieved without altering the concentration of corrosive species present in the environment. Most inhibitors interact with the anodic or cathodic reactions and increase the resistance to the flow of corrosion current. There are two main types of inhibitors, inorganic and organic, belonging to three categories of anodic, cathodic, and mix inhibitors.

Inorganic inhibitors typically bond ionically to the metal surface. They deposit a protective barrier film to hinder corrosion. Many inorganic anions are anodic inhibitors . These include phosphate and molybdate ions that require the presence of oxygen, and chromate and nitrate which do not require the presence of oxygen. Chromate and nitrate protect by inducing the formation of a stable passive film with their redox properties that readily oxidises the metal surface.

Organic inhibitors function by absorption into the metal/metal oxide surface. Hence steric factors, molecular weight, and electron density are important factors that govern their performances . Cathodic organic inhibitors improve corrosion protection by reinforcement of the passive film. Anodic organic inhibitors typically form a hydrophobic network and repel water and aggressive ions away from the surf ce . Rare earth based compounds have shown corrosion inhibition properties ¹' ²' ³' ' ⁵' ^δ. The corrosion inhibition properties of lanthanum nitrate on mild steel has been reported⁷. Also, corrosion inhibition properties of CeCl on Al-Zn alloy with further studies on steels and zinc ⁸' ⁹. Solutions of rare earth metals and known organic carboxylate inhibitors (eg salicylate) has also been described¹⁸ In this study the corrosion inhibition performance of rare earth metal-organo phosphate compounds on aluminium alloy (AA 20204-T3) in NaCl solutions were investigated. However most work has concentrated on the formation of insoluble rare earth hydroxides such as Ce(OH) on the surface of the metal as a protective layer.

Organics have also been investigated as potential corrosion inhibitors. Research into organic inhibitors has been conducted in parallel with inorganic inhibitors¹⁰. Most organic inhibitors function by deposition of an insoluble biofilm on the surface of the metal to be protected. Oxygen diffusion to the metal surface was found to be hindered by the surface film deposited by the inhibitors benzotriazole and sodium 2- mercaptobenzothiazole ^u. Sodium sebacate (NaOOC (CH₂) ₈- COONa) , potassium hydrogen phthalate (C₈H₅O₄K) and sodium molybdate (Na₂Mo0₄.H₂0) were investigated by Jeffocoate ¹², with sodium sebacate found to function as an inhibitor to pit nucleation and pit growth with chloride concentrations below 0.3M. Raspini¹³ investigated the effectiveness of sodium acetate (NaC₂H₃0₂) and thioglycolic acid (HSCH₂COOH) and found them to be effective in preventing pit initiation but not pit growth in NaCl solutions. Salicyclic acid and various cinnamate compounds have also been shown to exhibit corrosion inhibition properties ^{14 15}. Some organics such as phosphates are already in use in water systems to protect pipelines^16,17. However, they typically require high concentrations in order to provide adequate inhibition.

Currently, metals such as aluminium alloys and steels are protected by paint-primer systems containing corrosion inhibitors . The paint systems serve as a barrier protection layer, whereas the corrosion inhibitors protect exposed metal should the barrier layer become damaged . Traditionally, chromate inhibitors, such as strontium chromate have been used in these paint systems. Fine inhibitor powders (micron/sub-micron) of 20wt% or more are mechanically mixed into the paints prior to application.

Leaching of chromate into the surface water and absorbed moisture is greatly enhanced by the fine inhibitor particle size. Protection of exposed metal occurs by passivation of the substrate by the chromate dissolved in the electrolyte, hence reducing the rate of corrosion.

Nitrate based corrosion inhibitors have also been very effective. Unfortunately, both the hexavalent chromium ion and nitrate compounds are highly toxic and carcinogenic, thus posing an undesirable threat to the environment and personnel .

An effective replacement of chromate and nitrate based corrosion inhibitors is highly desirable. There is also a need for coatings such as chromate paint-primer systems to be replaced by corrosion inhibiting coatings which do not contain chromate or nitrate based corrosion inhibitors .

Summary of the Invention

According to a first aspect, the present invention provides a corrosion inhibiting coating comprising a rare earth-based organic compound and/or a combination of a rare earth metal and an organic compound. According to another aspect the present invention provides a method of inhibiting corrosion of a metal substrate comprising applying the corrosion inhibiting coating defined above to the substrate.

According to a further aspect, the present invention provides a method of preparing the corrosion inhibiting coating defined above comprising the step of incorporating a rare earth-based organic compound and/or a combination of a rare earth metal and an organic compound in a coating.

Detailed Description of the Invention

In the claims of this application and in the description of the invention, except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as

"comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. As used in the specification the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise. Thus, for example, reference to "a rare earth metal" includes mixtures of rare earth metals, reference to "an organic compound" includes mixtures organic compounds, and the like.

The term "coating" is used in its broadest sense to describe decorative topcoats, undercoats, intermediate coatings, primers, sealers, lacquers, coatings which are pigmented or clear, coatings designed specifically for corrosion protection, coatings which are high gloss, matte, textured, or smooth in finish, or coatings containing specialty additives such as metal flakes. Preferably the coating is a paint system which includes solvent based systems such as epoxy and polyurethane based systems; water based paint systems such as acrylic based systems; or other forms of paint systems such as gel based systems. More preferably, the coating is an epoxy based paint system, such as Epikote 1001 (with the hardening agent Epicure 3115x70) or Eprirez 123®.

The coating of the present invention generally inhibits the corrosion of metal substrates . The term "metal" also includes metal alloys and materials containing metals which may be susceptible to metallic corrosion. Suitable examples are aluminium, aluminium alloys such as AA2024T3, steel etc. AA2024 T3 is the basic structural aluminium alloy used in the aerospace industry. It is an aluminium-copper alloy with 93.5wt% aluminium and 3.8-4.9wt% of copper. There are 2 forms of AA2024-T3 alloy used in the aerospace industry. The "bare" alloy is used in structural components such as fittings, gears, shafts and bolts. The "clad" (AA2024+zinc alloy laminate) alloy sheet is used as the aircraft skin. Both the ba^"re and sandwiched forms are susceptible to pitting and intergranular corrosion.

The term "rare earth metals" (REM) refers to elements in the Lanthanide series of the periodic table . This group comprises elements from ⁵⁸Ce to ⁷¹Lu, but commonly takes into account ²¹Sc, ³⁹Y and ⁵⁷La. The general understanding is that rare earth metals have low toxicity and as such are particularly advantageous as a replacement for the known toxic chromate paint-primer systems. Preferred rare earth metals include yttrium (Y) , lanthanum (La) , cerium (Ce) , praseodymium (Pr) , neodymium (Nd) , promethium (Pm) , samarium (S ) , europium (Eu) , gadolinium (Gd) , terbium (Tb) , dysprosium (Dy) , holmium (Ho) , erbium (Er) , thulium Tm; ytterbium (Yb) and/or misch metal. It will be understood that the misch metal comprises a combination of rare earth metals, and the composition of the combination depends on the batch of ore from which the misch metal is obtained. The geographical source will determine the composition of the misch metal. Further preferred rare earth metals include Ce, Sm, Tb, Y, La, Yb and/or misch metal, more preferably Ce and/or La.

It will be appreciated that the rare earth metals are present in the coating in the form of a salt. Suitable salts include halides and/or nitrates.

Suitable organic compounds include carboxylates, organo molybdates, organo tungstate, organo vanadate, organo phosphates, organo phosphites, phosphinates, phosphonates, sulfonates and/or derivatives thereof. By "derivative" is meant any analogue residue or other compound which is not undesirable and induces the desired corrosion inhibition when combined with the rare earth metal .

Examples of preferred organic compounds include phosphate derivatives such as phosphate esters and organo phosphites . Preferred phosphate esters comprise mono- and di- phosphate esters of the general formulae (RO)₂P(0)0^~, (RO) (R^O) P (O) 0^" and (RO) P (0) 0^" ₂. Preferred organo phosphites are of the general formulae RP(OR)0-, RPO^~ and/or P(OR) 0. R and R¹ of the phosphate esters and organo phosphites are independently selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted heterocyclyl and/or optionally substituted aryl . The term "alkyl" denotes straight chain, branched or mono- or poly-cyclic alkyl, preferably Cι-Cχ alkyl or cycloalkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, amyl, isoamyl, sec-amyl, 1, 2-dimethylpropyl, 1, 1-dimethylpropyl, pentyl, hexyl, 4-methylpentyl, 1- ethylpentyl , 2-methylpentyl , 3-methylpentyl , 1,1- dimethylbutyl , 2,2-dimethylbutyl , 3,3 -dimethylbutyl , 1,2- dimethylbutyl , 1,3 -dimethylbutyl , 1,2,2-dimethylbutyl , 3 , 3 -dimethylbutyl , 1,2-dimethylbutyl , 1,3 -dimethylbutyl , 1, 2 , 2-trimethylpropyl, 1, 1, 2-trimethylpropyl, heptyl, 5- methylhexyl , 1-methylhexyl , 2 , 2-dimethylpentyl , 3,3- dimethylpentyl , 4 , 4-dimethylpentyl , 1,2-dimethylpentyl , 1 , 3 -di ethylpentyl , 1, 4-dimethylpentyl, 1,2,3- trimethylbutyl , 1,1, 2-trimethylbutyl , 1,1,3- trimethylbutyl , octyl, 6-methylheptyl, 1-methylheptyl,

1, 1, 3 , 3-tetramethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-methyloctyl, 1-, 2-, 3-, 4- or 5-ethylheptyl, 1-, 2- or 3-propylhexyl , decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8- methylnonyl, 1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7- ethylnonyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9- methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4- or 5- propyloctyl, 1-, 2- or 3- butylheptyl, 1-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10- methylundecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8-ethyldecyl, 1-, 2-, 3-, 4-, 5- or 6-propylnonyl, 1-, 2-, 3- or 4- butyloctyl, 1 , 2-pentylheptyl and the like. Examples of cyclic alkyl include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl and the like.

The term "alkenyl" denotes groups formed from straight chain, branched or mono- or poly-cyclic alkenes including ethylenically mono- or poly-unsaturated alkyl or cycloalkyl groups as defined above, preferably C₂-ι₂ alkenyl. Examples of alkenyl include vinyl, allyl, 1- ethylvinyl, butenyl, iso-butenyl, 3-methyl-2-butenyl, 1- pentenyl, cyclopentenyl, 1-methyl-cyclopentenyl, 1- hexenyl, 3-hexenyl, cyclohexenyl, 1-heptenyl, 3-heptenyl, 1-octenyl, cyclooctenyl, 1-nonenyl, 2-nonenyl, 3-nonenyl, 1-decenyl, 3-decenyl, 1, 3-butadienyl, 1, 4-pentadienyl, 1, 3-cyclopentadienyl, 1, 3-hexadienyl, 1, 4-hexadienyl, 1,3- cyc1ohexadieny1 , 1,4-eye1ohexadieny1, 1,3- cycloheptadienyl, 1, 3 , 5-cycloheptatrienyl, 1,3,5,7- cycloocta-tetrenyl and the like.

The term "alkynyl" denotes groups formed from straight chain, branched, or mono- or poly-cyclic alkynes . Examples of alkynyl include ethynyl, 1-propynyl, 1- and 2- butynyl, 2-methyl-2-propynyl, 2-pentynyl, 3-pentynyl, 4- pentynyl, 2-hexynyl, 3-hexenyl, 4-hexenyl, 5-hexenyl, 10- undecynyl, 4-ethyl-l-octynyl-3-yl, 7-dodecynyl, 9- dodecynyl, 10-dodecynyl, 3-methyl-l-dodecyn-3-yl, 2- tridecynyl, 11-tridecynyl, 3-tetradecynyl, 7-hexadecynyl , 3-octadecynyl and the like.

The term "aryl" denotes single, polynuclear, conjugated and fused residues of aromatic hydrocarbons. Examples of aryl include phenyl, biphenyl, terphenyl, quaterphenyl , phenoxyphenyl , naphthyl , tetrahydronaphthyl , anthracenyl, dihydranthracenyl, benzanthracenyl, dibenzanthracenyl, phenanthrenyl and the like.

The term "heterocyclyl" denotes mono- or poly-cyclic heterocyclyl groups containing at least one heteroatom selected from nitrogen, sulphur and oxygen. Suitable heterocyclic groups include N-containing heterocyclic groups, such as, unsaturated 3 to 6 membered heteromonocyclic groups containing 1 to 4 nitrogen atoms, for example, pyrrolyl, pyrrolinyl, imidazolyl, pyrazolyl pyridyl, pyrimidinyl, pyrazinyl, pyridazinyl, triazolyl or tetrazolyl; saturated 3 to 6-membered heteromonocyclic groups containing 1 to 4 nitrogen atoms, such as pyrrolidinyl, imidazolidinyl, piperidino or piperazinyl; unsaturated condensed heterocyclic groups containing 1 to 5 nitrogen atoms, such as, indolyl, isoindolyl, indolizinyl, benzimidazolyl, quinolyl, isoquinolyl, indazolyl, benzotriazolyl or tetrazolopyridazinyl; unsaturated 3 to 6-membered heteromonocyclic group containing an oxygen atom, such as pyranyl or furyl; unsaturated 3 to 6-membered heteromonocyclic group containing 1 to 2 sulphur atoms, such as, thienyl; unsaturated 3 to 6-membered heteromonocyclic group containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, such as, oxazolyl, isoxazolyl or oxadiazolyl; saturated 3 to 6-membered heteromonocyclic group containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, such as, morpholinyl; unsaturated condensed heterocyclic group containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, such as, benzoxazolyl or benzoxadiazolyl; unsaturated 3 to 6-membered heteromonocyclic group containing 1 to 2 sulphur atoms and 1 to 3 nitrogen atoms, such as thiazolyl or thiadiazolyl; saturated 3 to 6-membered heteromonocyclic group containing 1 to 2 sulphur atoms and 1 to 3 nitrogen atoms, such as, thiadiazolyl; and unsaturated condensed heterocyclic group containing 1 to 2 sulphur atoms and 1 to 3 nitrogen atoms , such as benzothiazolyl or benzothiadiazolyl . In this specification, "optionally substituted" means that a group may or may not be further substituted with one or more groups selected from oxygen, nitrogen, sulphur, alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, alkoxy, alkenyloxy, alkynyloxy, aryloxy, carboxy, benzyloxy, haloalkoxy, haloalkenyloxy, haloalkynyloxy, haloaryloxy, nitro, nitroalkyl, nitroalkenyl, nitroalkynyl, nitroaryl, nitroheterocyclyl, azido, amino, alkylamino, alkenylamino, alkynylamino, arylamino, benzylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acylamino, acyloxy, aldehydo, alkylsulphonyl, arylsulphonyl, alkylsulphonylamino, arylsulphonylamino, alkylsulphonyloxy, arylsulphonyloxy, heterocyclyl, heterocycloxy, heterocyclylamino, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy, mercapto, alkylt io, arylthio, acylthio and the like. The rare earth element and organic compound may be physically or chemically combined and then incorporated into the coating. The physical combination can be achieved by physically mixing the rare earth element and the organic compound. Alternatively, the rare earth element and the organic compound can be chemically combined by reacting the components together to form a rare earth based organic compound.

A synergistic effect between the rare earth metal and organic compound is observed in the rare earth-based organic compound over the individual components. It is understood that the same synergistic effect is expressed in the physical combination of the two components in the coating.

Preferred rare earth-based organic compounds are cerium diphenyl phosphate Ce(dpp)₃, cerium dibutyl phosphate Ce(dbp)₃ and /or Ce (salicylate) ₃. It will be understood that the cerium compounds can include Ce(III) and related Ce(IV) derivatives.

When the inhibitor is a combination, the rare earth metal and organic compound are preferably present in a molar ratio of at least about 1 : about 3, preferably a molar ratio of at least about 1 : about 10, more preferably a molar ratio of at least about 1 : about 20 of rare earth metal to organic compound. When the inhibitor is a compound, it is preferably present in an amount of about 0.001 to about 40wt%, preferably about 0.001 to about 30wt%, more preferably about 0.001 to about 10wt%, most preferably about 0.1 to about 10wt% of the total weight of the coating.

It will be appreciated that known optional additives may be included in the coating of the present invention. Examples include fillers, such as Ti0₂; rheology modifiers such as hydroxypropyl methyl cellulose, modified urea and polyhydroxycarboxylic acid amides; film formers such as esters of dicarboxylic acid and glycol ethers; wetting agents such as fluorochemical surfactants and polyether modified poly-dimethyl-siloxane; surfactants such as fatty acid derivatives and quaternary ammonium salts; dispersants such as non-ionic surfactants based on primary alcohols and alkylphenol-formaldehyde-bisulfide or bisulfite condensates; substrate cling agents; anti foaming agents; stabilizers such as banzimidazole derivatives; levelling agents such as fluorocarbon- modified polymers; pigments such as fluorescents, and organic and inorganic dyes such as fluoroscein.

The corrosion inhibitor compound and/or combination may be incorporated into the coating by one or more of the following methods:

o Direct mechanical incorporation:

The inhibitor compound and/or combination is refined to the order of sub-micron or nano particle, by techniques such as ball milling or cryo-milling/grinding .

The refined inhibitor compound and/or combination is mechanically mixed and evenly distributed into the coating.

• Mechanical incorporation, combined with filler:

A suitable filler, such as Ti0₂ or Si0₂, is optionally added. The corrosion inhibitor compound and/or combination is mixed together with Ti0 preferably by milling, thus coating the filler with the corrosion inhibition compound and/or combination. The coated filler is then mechanically evenly distributed into the coating.

• Chemical incorporation via a solvent :

The inhibitor compound and/or combination is dissolved in a solvent compatible with the inhibitor and coating being used. Alternatively, the inhibitor compound and/or combination is dissolved directly into the coating if there is solvent already present. In neither case, the inhibitor should then re-precipitate into the coating, for example, the resin matrix, during evaporation of the solvent .

Any solvent which is compatible with the epoxy coating and inhibitor compound. Solvents include organic solvents, protic and aprotic solvents such as toluene, acetone, substituted benzenes such as xylene, nitrobenzene, linseed oil, alcohols, N'methyl pyrrolidinone, turpene or mixtures thereof, water, dimethylsulfoxide . Solvents may also include room temperature ionic liquids (liquid salts) . The resultant dissolved inhibitor is then mixed with the coating in proportions adequate to achieve the desired level of inhibition protection.

• Chemically - in solution

The inhibitor compound and/or combination is dissolved in a solution compatible with a water based/gel based coating or dissolved directly into the coating. Suitable solutions include the solvents or combinations of these mentioned above.

The resultant dissolved inhibitor is then mixed with the coating, in amounts adequate in achieving desired level of inhibition protection, such as ... please insert. The inhibitor should then re-precipitate into fine particles during drying of the coating.

Preferably, the inhibitor combination or compound are evenly distributed throughout the coating in the above methods .

The inhibitor coating may be applied to the metal substrate using any suitable known technique, such as, for example, spray, brush, dip, knife, blade, hose, roller, wipe, pipette or combinations thereof. Application by spray is preferred.

Brief Description of the Drawings

In the examples, reference will be made to the accompanying drawings in which: Fig. 1 shows a coated coupon indicating the location at which the thickness is measured.

Fig. 2 shows the results of cerium di-butyl phosphate inhibitor in Epikote 1001 on AA2024-T3 substrates.

Fig. 3 shows triplicates of coated steel substrates, immersed in 0.01M NaCl for 168 hrs .

Fig. 4 confirms that the inhibitor has leached formed a cerium containing protective layer.

Examples The invention will now be illustrated by the following non-limiting examples.

1. Manufacture of Inhibitor compounds: 1.1 Rare earth (III) organo phosphate: Rare earth complexes of composition

Ln(organophosphate) 3 can be prepared by precipitation from the reaction of the relevant rare earth chloride with the sodium salt of the organophosphate . Specifically, compounds of Ln(dbp)3 (Ln=Ce, Sm, Tb, Y, Yb) were obtained by precipitation from reaction of ethanolic solutions of selected rare earth chlorides with Na(dbp) . The latter was prepared in highly crystalline form from the stoichiometric addition of H(dbp) and NaOH in ethanol followed by evaporation under vacuum. Products were obtained in reasonable yield (50-80%) with high crystallinity.

1.2 Cerium-phosphate salts

1.2.1 Cerium-di-butyl-phosphate (Ce(dbp)₃)

Ce(dbp)₃ was prepared by combining cerium chloride and hydrogen-di-butyl-phosphate (H(dbp)) in an ethanol solution. Cerium chloride is dissolved in a solution of ethanol. Addition of H(dbp) results in the saturation of the solution and subsequent precipitation of Ce(dbp)₃ compound. The initial yield is about 50 wt%, which is increased to above 90 wt% if followed by reduction/evaporation of the remaining ethanol solution. The inhibitor compound is filtered out of solution and dried before verification of purity using ATR.

1.2.2 Cerium-di-phenol-phosphate (Ce (dpp) ₃) Ce(dpp)₃ is produced in an identical manner to

Ce(dbp)_3/ with addition of H(dpp) into a cerium chloride- ethanol solution causing precipitation of Ce(dpp)₃ compound. The inhibitor compound is then dried and verified for purity using ATR. The yield is above 90 wt% if the solution reduction step was used (appendix A.3.1).

2. Comparative example.

Manufacture of sodium Salts.

2.1 Sodium-di-butyl-phosphate (Na-dbp) Combining NaCl with H(dbp) in an ethanol solution results in the Na(dbp) rapidly precipitating out after saturation is achieved producing Na(dbp) . Again, it is dried then verified by ATR.

2.2 Sodium-di-phenyl-phosphate (Na-dpp) As with Na-dbp, H(dpp) is combined with NaCl in an ethanol solution. The precipitate of Na(dpp) was dried and verified using ATR.

3. Incorporation of inhibitor combination into coating. 3.1 Cerium dibutyl phosphate

(Ce(dbp)₃) was incorporated into an epoxy primer coating via several different methods :

Two epoxy-amide primer systems were prepared: 1. Epoxy primer with strontium-chromate "Epoxy primer 37002" and "Hardener 92057" by AKZO NOBEL as per MIL-PRF-23377 Type 2 class C specification . 2. Epikote 1001 with the hardening agent Epicure 3115 X70. This is a basic epoxy system with minimal amount of fillers / additives and no corrosion inhibitors . Two separate batches of this epoxy system were made with incorporation of the corrosion inhibitor compound cerium-di- butyl-phosphate, of approximate, 30wt% and

160ppm respectively.

3.1.1 Direct mechanical incorporations

The Ce(dbp)₃ inhibitor compound was dry ball milled using ceramic balls of 8mm in diameter. Wet milling could not be used as the compound was found to recrystallise into larger crystals during drying after wet milling. A low shear standard mixer head was used to incorporate approximately 30wt% inhibitor powder into the epoxy. A mechanical paint shaker was used to ensure adequate mixing of the components . The surf ces of the aluminium coupons were polished using 800 grit silicon carbide paper to assist adhesion between the substrate and the coating. A doctor blade was used to spread the epoxy film to 400μm over one face of the 10 cm x 10 cm x 2 mm AA2024 T3 coupons. All other faces and sides were protected against corrosion by a thick layer of non-inhibited epoxy primer. After drying for 24hrs, the coupons were submerged in 0.01M and 0.001M NaCl solution for 7 days. If no sign of corrosion was observed, they were then subjected to wet- dry cycles at 70°C in 5M NaCl solution.

3.1.2 Chemical incorporation via solvent

Ce(dbp)₃ was dissolved in toluene and then mixed with the epoxy to reach a level of 160ppm inhibitor in the final composition. The hardener was then mixed in prior to spraying the coating using an air-propelled spray gun onto A12024-T3 substrates.

3.1.3 Direct mechanical incorporation of Ce(sal)₃

Ce(sal)₃ was mechanically mixed into epoxy primer at concentration of lOwt. The hardener was then added and coatings were applied to cleaned steel substrates via doctor blading.

3.1.4 Mechanical incorporation combined with filler. A 50/50 mixture of Ce(sal)₃ was ballmilled with nanosized (~20nm) Si0₂ particles to thoroughly combine these . The mixture was incorporated into the epoxy primer to achieve a 10wt% loading of each component via mechanical mixing. The hardener was then added and coatings were applied to cleaned steel substrates. Coated samples were also prepared with just 10wt% Si0 and without any additive to evaluate the effect of the inhibitor.

3.2 Application of coating. All metal specimens that were tested were mechanically abraded by polishing to 1200 grit followed by a rinse using de-ionised water. Samples were then chemically cleaned as follows (as per MIL-C-5541) a. 5 minutes immersion in an agitated alkaline cleaner to remove oil and grease, followed by washing with tap water. b. 5 minutes immersion in agitated Deoxidizer 6^® solution to remove the surface oxide, followed by washing with tap water.

The coatings were applied using an air-propelled spray gun. The thickness of the coating was then measured using an ultrasound thickness-measuring instrument at 4 points along the length of the coupons .

The epoxy primers were thinned with MEK to appropriate workability during the spray painting operation. After application, the coatings were allowed to air dry for 1 hr followed by post-curing at 80°C for 24 hours .

The following epoxy primer-surface treatment coupons were made (see fig. 1 for location at which the thickness were measured)

Table 1 - surface treatment of AA2024 T-3 coupons prior to coating

3.3 Corrosion inhibition performance

Constant immersion tests and alternate immersion tests are designed to examine the corrosion inhibition effectiveness of inhibited coatings. In corrosion testing, the coatings are typically cross-scribed to simulate localised coating damage.

Constant immersion test, as suggested, has the test coupons immersed in the test solution for the entire duration of the test. Alternate immersion, on the other hand, only has the test coupon immersed for l/6^th of the test duration, resulting in a wet/dry cycle, Both tests are used for coating evaluation in industry.

3.3.1 Cross-scribed constant immersion test. An X was scribed through the coating to the substrate using a sharp razor blade on each of the coupons to be tested. The coupons were then submerged in low salt solution of 0.001M NaCl opened to atmosphere. Coupons were suspended from the base of the containers and the solution was regularly stirred and traces of loose corrosion products on the coupon surfaces were dislodged. The solutions were replenished daily with de-ionised water. Coupons were removed from the solution at the first signs of corrosion after 4 days, then at 8 days, 5 weeks and 3 months. Corrosion damage was assessed visually using an Olympus BH2-UMA microscope. Selected coupons were then analysed using Energy Dispersive X-ray Spectroscopy (EDXS) .

The basic epoxy-amide coating system (Epikote) with addition of ~30wt% Ce(dbp)₃ exhibited the best corrosion inhibition in the constant immersion test. No corrosion was observed for the duration of the 3 month test.

Table 2 - Coating performance after 4 days immersion in 0.001 NaCl solution

3.3.2 Corrosion testing on AA2024-T3 substrates.

An X was scribed through the coating to substrate using a sharp razor blade on each of the coupons to be tested. The coupons were then submerged in a salt solution of 0.001M NaCl opened to atmosphere. Coupons were suspended from the base of the containers and the solution was regularly stirred and traces of loose corrosion products on the coupon surfaces were dislodged. The solutions were replenished daily with de-ionised water. Coupons were removed from the solution at the first signs of corrosion after 4 days, then at 8 days, 5 weeks and 3 months. Corrosion damage was assessed visually using an Olympus BH2-UMA microscope. Fig. 2 shows the results of the cerium di-butyl phosphate inhibitor in Epikote 1001 on AA2024-T3 substrates.

3.3.3 Corrosion testing on steel substrates

An X was scribed through the coating to substrate using a sharp razor blade on each coupon. The coupons were then immersed horizontally in a salt solution bath (0.01M NaCl) opened to atmosphere. Coupons were removed after 4, 24, 48,168 and 508 hours for comparison.

3.3.4 Results of corrosion testing

Even after 3 months immersion the AA2024-T3 substrates coated with epoxy containing either 30wt% or 160ppm Ce(dbp) showed no signs of visible corrosion in contrast to the control sample (see 8 day test in Fig. 2) . Steel substrates coated with Ce(Sal)₃ containing epoxy showed less corrosion than the control after 168 hours whilst the Ce (Sal) ₃/Si0 containing epoxy reproducibly showed the least corrosion on scribed coupons . The photos shown in Fig. 3 are triplicates of coated specimens immersed in 0.01M NaCl for 168hrs. The inhibited coatings consistently show less corrosion at the scribe. The coating on coupon 3a is 10wt% Ce(sal)₃ + 10wt% Si0₂ in epoxy on steel (168 hours immersion in 0.01M NaCl solution) .

The coating on coupon 3b is 10wt% Ce(sal)₃ in epoxy with no filler, after 168 hours immersion in 0.01M NaCl.

The coating on coupon 3c is a control sample (no inhibition) after 168 hours immersion in 0.01 M NaCl solution.

3.4 Adhesion of coating

Similar to the hexavalent chromate primer of MIL-PRF- 23377, the inhibitor compound Ce(dbp)₃ was first mechanically milled to a refined particle size prior to incorporated into the Epikote 1001 primer. However, the waxy nature of the inhibitor compound led to difficulties in refining the particle size to sub micron levels required for a smooth surface finish. This resulted in the rough uneven finish observed in the high loading (30wt%) Epikote - Ce(dbp)₃ coated specimens. In the case of the low loading (160ppm) Ce(dbp)₃ coating specimens, the problem of particle size was able to be overcome by formulating a Ce(dbp)₃ gel for incorporation with the Epikote resin, due to the lesser amount of inhibitor concerned. Mixing of the inhibitor with a fine inorganic filler such as Ti0₂ or Si0 can also overcome the difficulties of surface finish.

Coating performance. This 'suggests that the Cedbp inhibitor has some additional benefit in the adhesion of the epoxy to bare metal surface.

3.5 Coating performance

The EDXS/SEM of Fig. 4 confirms that the inhibitor has leached to the scribed area and formed a Cerium containing protective layer. Cerium oxide was found within the scribes made on the coupons coated with Epikote / Epicure with Ce(dbp)₃, whereas the oxides found within the scribes on the chromium based primer (MIL-PRF-23377 spec) contained only aluminium and oxygen, as shown in Figure 4. The formation of cerium oxide, together with the drastically reduced corrosion observed in the immersion specimen, indicated that the Ce(dbp)₃ incorporated into the coating was able to leach into the surrounding moisture and actively inhibit corrosion on the surface of the substrate. It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention.

References:

(1) Bethencourt, M.; Botana,^" F.J. Calvino, J.J.; Marcos, M. ,« Rodriguez-Chacon, M,A. , " anthanide compounds as environmentally-friendly corrosion inhibitors of aluminum alloys: A Review", Corrosion Science, v 40, n 11, Nov, 1998, pl803-1819.

(2) p. Lu, W.J- van Ooij, "Protection of A12024 aluminum alloys by rare earth compound inhibitors", Cailiao

Gongscheng/Journal of Materials Engineering, n 7, 1998, p9-ll.

(3) Forsyt M., Wilson, K. , Behrsing, T- , Forsyth, C, Deacon, G.B., Phanasgoan ar, A., "Effectiveness of rare-earth metal compounds as corrosio inhibitors for steel*. Corrosion, ¥58, nil, Sϊo 2002, p 953-960,

(4)- H. Bruce, ^Corrosion inhibition with rare, earth metal sal s", Journal o£ Allo s and Compounds, ¥130,, n 1-2, 1SS2, pl5-25.

(5) Powell, s., "Evaluation of alternative corrosion inhibitors to chromate for use in organic coatings using scanning refe ence -electrode technique^e , Surface Engineering, -v 16 n2, 2000 , pi69-175.

(6) Ramanathan, L. ., "Corrosion control with rare earths", Corrosion . Prevention fi control,- V 4-5, n 3, Jun, 1998^", pB7-92. „

(7 Goldie B. P. F. , cCarroll J. J-, 'Method of inhibiting corrosion in aqueous systems', Australia Patent Office, 1984.

(8) Hinton B. R, W- , A nott D. R. , Ryan N. E., Mater. Forum, 1986, v 9, n 3, p. 162. [9> Uinton B._. R. . , *Corrosion inhibitor with rare earth metal salts', J. Alloys Compd., V 180, n 1-2, 1992, p. 15-25.

(10) Twite R.L., Bierwagen G.P., 'Review of alternatives to chromate for corrosion protection of aluminium aerospace alloys', Progress in Organic Coatings, n 33, 1998, p. 91 -r 100.

(11) Isobe Y., TanaJa s., Hine F-. , 'Effects of organic inhibitors on pitting corrosion of aluminium alloys', corrosion Engineering (Boshάku Gijutsu), v 39, n 4, 1990, p. 185 - 191

(12) Jafferocoate C.S., i?hD Thesis, university of Manchester, Institute¹ of Science and Technology, Manchester, 1994.

(13). R&spini Σ.&., Corrosion, v 49, n 10,. 1S93,^' . 831.

(14) Bl±-B.3?., Le j S. ,_ Wilson K._f Deacon «£._P Junk P., Forsyh M. , EVAUATION OF THE CORROSION MITIGATION POTENTIAL OF SOME NEW RARE EARTH METAL BASED COMPOUNDS ON STEEL, Corrosion & Prevention-02 ,

Australasian Corrosion association Inc, Adel i e, Hov, 2UQ2 .-

(15) Wilson K., Forsyth M., "New Green Inhibitors incorporating a Rare Earth arid an Organic Compound for Corrosion Mitigation", PhD transfer report, Department of Materials engineering, Monash

University, Clayton, Melbourne, Australia (2000).

(16) Edwards M. , Hidmi L., Gladwell D., "Phosphate inhibition of soluble, copper corrosion by-product release*. Corrosion Science, Vol 44, 2002, pl057- 1071. (17) spon R., ^Phosphate corrosion inhibitor chemistry made simple*, Journal of the New England Water Works Association, V110 n3 Setp 1996, p218-220.

(18) Ho, D, -Forsyth M. , Apateanu ., Brack. N, Deacon, G.B., "ASSESSMENT OF THE SYNERGY EFFECT OF CERIUM - PHOSPHATE COMPOUNDS IN INHIBITING CORROSION ON AA 02 T-3 ALLOY IN A CHORIDE ENVIRONMENT", ACA Conference, Adelaide, Australia, 2002.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1- A corrosion inhibiting coating comprising a rare earth-based organic compound and/or a combination of a rare earth metal and an organic compound.

2. A coating according to claim 1, wherein the rare earth metal comprises elements in the Lanthanide series of the periodic table, from ^5βCe to ⁷¹Lu, ²¹Sc, ³⁹Y and/or ^SLa.

3. A coating according to claim 1 or claim 2, wherein said rare earth metal comprises yttrium (Y) , lanthanum

(La) , cerium (Ce) , praseodymium (Pr) , neodyraium (Nd) , promethium (P ) , samarium (Sm) , europium (Eu), gadolinium (Gd) , terbium (Tb) , dysprosium (Dy) , holmium (Ho) , erbium (Er) , thulium Tm ytterbium (Yb) and/or misch metal.

4. A coating according to claim 3, wherein the rare earth metal . comprises Ce, Sm, Tb, Y, La, Yb and/or misch m tal.

5. A coating according to claim 3 or claim 4, wherein the rare earth metal i® Ce and/or La.

6. A coating according to any one of the preceding claims, wherein the rare earth metal is present in the coating in the form of a salt.

7. A coating according to claim 5, wherein the salt is a halide and/or nitrate.

8. A coating according to any one of the preceding claims wherein the organic compound comprises carboκylates, organo molybdates, organo tungstate, organo vanadate, organo phosphates, organo phosphites, phosphiήates, phosphonates, sulfonates and/or derivatives thereof .

9. A coating according to claim 8 , wherein the organic compound comprises organic phosphates of the general formulae (RO) _aP (0) θ^", (RO) (RO) ? (O)O^' and (RO) P (0)O^".₂.

5

10. A coating according to claim 8, wherein the organo compound comprises organo phosphites of the general formulae RP(0R)0-, R₂P0- and/or P(OR)₂0.

±0 11. A coating according to claim 9. or claim 10, wherein R and R¹ comprise optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted heterocyclyl and/or

15 optionally substituted aryl .

12. A coating according to any one of the preceding claims wherein the coating comprises a rare earth-based or anic compoun .

20

13. Α coating according to claim 12 , wherein the rare earth-based organic compound comprises cerium diphenyl phosphate (Ce (dpp} ₃) , cerium dibutyl phosphate (Ce (dbp) ₃) and/or -C®(salicylate) a .

25

14. A coating according to any one of the preceding, claims, wherei the coating comprises' rare earth-base organic compound in an amount of about Q-Q01 to about 0wt% of the total weight of the coating-

30

15. A coating according to claim 14, wherein the rare earth-based' organic compound s present in an amount of about 0.001.to about 30wt% of the total weight of the coating.

35

16. A coating according to claim 14 or claim 15, wherein the rare earth-based organic compoud is present in an amount of about 0.001 to about 10wt% of the total weight of the coating.

17. A coating according to any one of claims 14 to 16, wherein the rare earth-based organic compound is present in an amount of about 0.1 to about 10wt% of the total weight of the coating.

18. A coating according to any one of claims 1 to 9 , wherein the coating comprises a combination of a rare earth metal and an organic compound.

19. A coating according to claim 18, wherein the rare earth metal and organic compound are present in a molar ratio of at least about 1 : about 3.

20. A coating according to claim 19, wherein the. rare earth metal and organic compound are present in a molar ratio of at least about 1 : about 10.

21. A coating according to claim 19 or claim 20, wherein the rare earth metal and organic compound are present in a molar ratio of at least about 1 : about 20.

22. A coating according to any one of the preceding claims, further comprising additive(s).

23. A coating according to claim 22, wherein the optional additive comprises at least one of fillers, rheology modifiers, film formers, wetting agents, surfactants, dispersants, substrate cling agents, anti foaming agents, stabilizers, levelling agents, pigments and organic and inorganic dyes .

24. A coating according to claim 22 or claim 23, wherein the optional additive is a filler.

25. A coating according to claim 24, wherein the filler is Si0₂.

26. A method of inhibiting corrosion of a metal substrate comprising applying the corrosion inhibiting coating as defined in any one of the preceding claims, to the substrate.

27. A method according to claim 26, wherein the metal substrate comprises aluminum, aluminium alloys and steel.

28. A method according to claim 26 or claim 27, wherein the application of the coating to the substrate comprises spray, brush, dip, knife, blade, nose, roller, wipe, pipette or combinations thereof.

29. A method according to any one of claims 26 to 28, wherein the application of the coating to the substrate comprises spraying.

30. A method of preparing the corrosion inhibiting coating as defined in any one of claims 1 to 25, comprising the step of incorporating a rare earth-based organic compound and/or a combination of a rare earth metal and an organic compound in a coating.

31. A method according to claim 30, wherein the step of incorporation comprises direct mechanical incorporation, mechanical corporation combined with a filler, chemical incorporation with a solvent and chemical incorporation in solution.

32. A method according to claim 30 or claim 31, wherein the step of incorporation comprises mechanical incorporation with a filler and wherein the filler comprises Ti0₂ or Si0₂.

33. A method according to claim 31 or claim 32, wherein the filer is Si0 .

34. A method according to any one of claims 30 to 33, wherein the step of incorporation comprises incorporation with a solvent.

35. A method according to claim 34, wherein the solvent comprises organic, protic and aprotic solvents.

36. A method according to claim 34 or claim 35, wherein the solvent comprises acetone, toluene, substituted benzenes, linseed oil, alcohols, N-methyl pyrrolidinone, turpene, water, dimethyl sulfoxide and/or room temperature ionic liquids (liquid salts) .

37. A method according to claim 36, wherein, the substituted benzenes are xylene and/or nitrobenzene.

38. A method according to any one of claims 34 to 37, wherein the solvent is toluene .