WO2013029090A1 - Corrosion resistant coatings for aluminium and aluminium alloys - Google Patents

Abstract

Disclosed is a coating composition comprising a bis-silane of Formula (I) (R⁷O)₃Si -(CR³R⁴)_m −X⁵-C(X³)-X¹-(CR¹R²)₁−X²−C(X⁴)−X⁶ −(CR⁵R⁶)_n−Si(OR⁸)₃ The coating composition can be used to form corrosion resistant coatings on metal articles, such as aluminium or aluminium alloy containing articles.

Description

CORROSION RESISTANT COATINGS FOR ALUMINIUM AND ALUMINIUM

ALLOYS

PRIORITY DOCUMENT

[001 ] The present application claims priority from Australian Provisional Patent Application No. 201 1903432 entitled "CORROSION RESISTANT COATINGS FOR ALUMINIUM AND

ALUMINRJM ALLOYS" and filed on 26 August 201 1 whose contents are hereby incorporated by reference in their entirety.

FIELD

[002] The present invention relates to coatings that improve the resistance of aluminium and aluminium alloy articles to corrosion. The present invention also relates to methods that can be used to improve the resistance of aluminium and aluminium alloy articles to corrosion.

BACKGROUND

[003] High strength-weight ratio metal alloys, such as aluminium (Al) alloys are used in numerous applications in aerospace, automotive, marine and construction industries due to their light weight and attractive mechanical properties. Unfortunately, in practice, the corrosion resistance of these alloys in general tends to be poor.

[004] Corrosion involves the reaction between a metal or alloy and its environment. Corrosion is affected by the properties of the metal or alloy as well as environmental variables such as pH, oxidation potential, temperature, fluid flow, and solution constituents. With aluminium and its alloys there are two main types of corrosion that occur; pitting corrosion and cathodic corrosion. Cathodic corrosion occurs in high pH environments where the outer layer of aluminium oxide dissociates and hydroxide ions react with, and dissolve, metallic aluminium. Pitting corrosion of aluminium occurs in aqueous media with a pH range of 4.5 to 9.0 and during the exposure of aluminium to halogen (i.e. CY) rich environments.

[005] Corrosion control is an ongoing problem with metal articles made from these alloys and it has been estimated that corrosion control in the USA costs up to $200 billion/yr alone (see Koch, G. http://www.corrosioncost.com/news/2002/corrosioncosts.htm). Consequently, mitigation of corrosion of alloys has been the focus of much research. [006] Articles can be protected from corrosion by coating the base material. Coatings may prevent corrosion by forming a barrier between the metal and its environment and/or by incorporating corrosion inhibiting substances in the coating. There are a number of possible technologies available for coating metals and their alloys and these include conversion coatings, anodizing, electrochemical plating, hydride coatings, vapour-phase processes, and organic coatings. Conversion coatings and anodizing are widely used commercial coating processes for aluminium and its alloys.

[007] Chromates (deposited as hard-chrome) have gained wide acceptance as corrosion inhibitors for a variety of metal substrates. However, they do not provide adequate corrosion and wear protection from harsh service conditions when used alone and they pose serious health and environmental risks due to the presence of leachable hexayalent chromium in the coating.

[008] Coatings produced by anodizing are porous ceramic-like coatings. These properties impart good paint-adhesion characteristics and excellent wear and abrasion resistance to the coating. However, without sufficiently sealing topcoats, they are not adequate for use in applications where corrosion resistance is of primary importance.

[009] There is a need for coatings that improve the corrosion resistance of metals and metal articles that overcome one or more of the problems with prior art coatings.

SUMMARY

[010] The present invention arises from research into siloxanes that can be used to form corrosion resistant coatings for articles containing Al and Al alloys. Specifically we have found that a group of siloxanes have properties that are particularly suitable for forming corrosion resistant coatings on these metals. ·

[01 1 ] In a first aspect, the present invention provides a coating composition comprising a bis-silane according to Formula (I)

(R⁷0)₃Si -(CR³R'')_ro -X⁵-C(X³)-X¹-(CR¹R²)₁-X²-C(X⁴)-X⁶-(CR⁵R⁶)n-Si(OR⁸)3

(I)

wherein:

R¹, R², R³, R⁴, R⁵, R⁶, R⁷, and R⁸ are each independently selected from the group consisting of: H, optionally substituted C_rC₆ alkyl, optionally substituted C_rC₆ cycloalkyl, optionally substituted C₂- C_fi alkenyl, optionally substituted C2-C₆ cycloalkenyl, optionally substituted C2-C6 alkynyl, optionally substituted C₂-C6 cycloalkynyl, and optionally substituted aryl; X¹, X², X³, X⁴, X⁵, and X⁶ are each independently selected from the group consisting of: O, S, and NR⁹;

each R⁹ is independently selected from the group consisting of: H, optionally substituted C_\-C alkyl, optionally substituted C C₆ cycloalkyl, optionally substituted C₂-C₆ alkenyl, optionally substituted C₂-C₆ cycloalkenyl, optionally substituted Ci-C^ alkynyl, optionally substituted C₂-C₆ cycloalkynyl, and optionally substituted aryl; and

1, m, and n are integers independently selected from the group consisting of: 1 , 2, 3 , 4, 5, and 6.

[012] In embodiments, X¹ and X² are S, X³ and X⁴ are O, and X⁵ and X⁶ are NH. This provides compounds of formula (la):

(R⁷0)₃Si -(CR³R⁴)_M -NH-C(0)-S-(CR¹R²)_rS-C(0)-NH-(CR⁵R⁶)„-Si(OR⁸)₃

(la)

wherein R¹ , R\ R\ ⁴, R⁵, R⁶, R⁷, and R⁸ are as defined previously.

[013] In a second aspect, the present invention provides a method of coating an aluminium- or aluminium alloy-containing article to improve the corrosion resistance thereof, the method comprising:

- providing an aluminium- or aluminium alloy-containing article to be coated;

- providing a coating composition according to the first aspect of the invention;

- if necessary, at least partially hydrolysing the bis-silane in the coating composition;

- contacting the coating composition and the article under conditions that result in at least part of the surface of the article being coated with the coating composition; and

- curing the coating composition to form a corrosion resistant coating on said at least part of the surface of the article.

[014] In a third aspect, the present invention provides a coated aluminium- or aluminium alloy- containing article produced by the method of the second aspect of the invention. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS/FIGURES

[015] Illustrative embodiments of the present invention will be discussed with reference to the accompanying drawings.

[016] Figure I shows the infrared absorption spectra of bis-(triethoxysilylpropyl)-ethane-dithiol- carbamate ("BTESPEDC") thin films on AA2024-T3 aluminium alloy. Hydrolysis (72 hrs) was carried out in acidic conditions (pH 3.5) and condensation was carried out in basic conditions (pH 7.5).

[017] Figure 2 shows SEM images of (a) bis-(trimethoxysilylpropyl)-amine ("BTMSPA") and (b) bis- (triethoxysilylpropyl)-tetrasulfide ("BTESPT") coated on AA2024-T3 aluminium alloy.

[018] Figure 3 shows SEM images of BTESPEDC coated on AA2024-T3 aluminium alloy after (a) 48 hr hydrolysis (b) 72 hr hydrolysis.

[019] Figure 4 shows EDAX results for BTESPEDC (5 %vol) coated of A A2024-T3 deposited after 72 hrs hydrolysis.

[020] Figure 5 shows a TM AFM image obtained for BTESPT film on AA2024-T3.

[021 ] Figure 6 shows the results of TGA of 10 %vol BTESPEDC cured at pH 3.5. Both the mass loss (TG) and first derivative of mass loss with temperature (DTG) curves are shown.

[022] Figure 7 shows the results of HR-TGA of 10 %vol BTESPEDC cured at pH 3.5. Both the mass loss (TG) and first derivative of mass loss with temperature (DTG) curves are shown.

[023] Figure 8 shows an electrical circuit model representing the diffusion of ions through a polymeric coating on a metal substrate.

[024] Figure 9 shows Bode magnitude plots of impedance against frequency for AA2024-T3 coated with (a) 2% and (b) 10% BTESPEDC.

[025] Figure 10 shows Nyquist plots of imaginary impedance versus the real impedance for AA2024- T3 coated with (a) 2% and (b) 10% BTESPEDC.

[026] Figure 1 1 shows photographs of (a) a coupon of uncoated AA2024-T3; (b) a coupon of AA2024-T3 coated with BTMSPA; (c) a coupon of AA2024-T3 coated with BTESPT; and (d) a coupon of AA2024-T3 coated with BTESPEDC after 24 hours neutral salt spray testing. [027] Figure 12 shows photographs of (a) a coupon of uncoated AA2024-T3; (b) a coupon of AA2024-T3 coated with BTMSPA; (c) a coupon of AA2024-T3 coated with BTESPT; and (d) a coupon of AA2024-T3 coated with BTESPEDC after 48 hours neutral salt spray testing.

[028] Figure 13 shows photographs of (a) a coupon of uncoated AA2024-T3; (b) a coupon of AA2024-T3 coated with BTMSPA; (c) a coupon of AA2024-T3 coated with BTESPT; and (d) a coupon of AA2024-T3 coated with BTESPEDC after 72 hours neutral salt spray testing.

[029] Figure 14 shows photographs of (a) a coupon of uncoated AA2024-T3; (b) a coupon of

A A2024-T3 coated with BTMSPA; (c) a coupon of AA2024-T3 coated with BTESPT; and (d) a coupon of AA2024-T3 coated with BTESPEDC after 96 hours neutral salt spray testing.

[030] Figure 1 5 shows photographs of (a) a coupon of uncoated AA2024-T3; (b) a coupon of AA2024-T3 coated with BTMSPA; (c) a coupon of AA2024-T3 coated with BTESPT; and (d) a coupon of A A2024-T3 coated with BTESPEDC after 168 hours neutral salt spray testing.

DETAILED DESCRIPTION

[031 ] In this specification a number of terms are used which are well known to a skilled addressee. Nevertheless for the purposes of clarity a number of terms will be defined.

[032] As used herein, the term "unsubstituted" means that there is no substituent or that the only substituents are hydrogen.

[033] The term "optionally substituted" as used throughout the specification denotes that the group may or may not be further substituted or fused (so as to form a condensed polycyclic system), with one or more non-hydrogen substituent groups. In certain embodiments the substituent groups are one or more groups independently selected from the group consisting of halogen, =0, =S, -CN, -NO2, -CF₃, - OCF₃, alkyl, alkenyl, alkynyl, haloalkyl, haloalkenyl, haloalkynyl, heteroalkyl, cycloalkyl, cycloalkenyl, heterocycloalkyl, heterocycloalkenyl, aryl, heteroaryl, cycloalkylalkyl, heterocycloalkylalkyl, heteroarylalkyl, arylalkyl, cycloalkylalkenyl, heterocycloalkylalkenyl, arylalkenyl, heteroary!alkenyl, cycloalkylheteroalkyl, heterocycloalkylheteroalkyl, arylheteroalkyl, heteroarylheteroalkyl, hydroxy, hydroxyalkyl, alkyloxy, alkyloxyalkyl, alkyloxycycloalkyl, alkyloxyheterocycloalkyl, alkyloxyaryl, alky loxy heteroaryl, alkyloxycarbonyl, alkylaminocarbonyl, alkenyloxy, alkynyloxy, cycloalkyloxy, cycloalkenyloxy, heterocycloalkyloxy, heterocycloalkenyloxy, aryloxy, phenoxy, benzyloxy, heteroaryloxy, arylalkyloxy, amino, alkylamino, acylamino, aminoalkyl, arylamino, sulfonylamino, sulfinylamino, sulfonyl, alkylsulfonyl, arylsulfonyl, aminosulfonyl, sulfinyl, alkylsulfinyl, arylsulfmyl, aminosulfinylaminoalkyl, -C(=0)OH, -C(=0)R^a, -C(=0)OR^a, C(=0)NR^aR^b, C(=NOH)R^a,

C(=NR^a)NR^bR^c, NR^aR^b, NR^aC(=0)R^b, NR^aC(=0)OR^b, NR^aC(=0)NR^bR°, NR^aC(=NR^b)NR^cR^d,

NR^aS0₂R^b,-SR^a, S0₂NR^aR^b, -OR^a, OC(=0)NR^aR^b, OC(=0)R^a and acyl,

wherein R^a, R^b, R^c and R^d are each independently selected from the group consisting of H, Ci-Cealkyl, Ci-Cshaloalkyl, C2-C₆alkenyl, C₂-C₆alkynyl, C₂-C₆heteroalkyl, C₃-C₆cycloalkyl, C₃-C₆cycloalkenyl, C₂- C₆heterocycloalkyl, C₂-C₆heterocycloalkenyl, Cg-Cisaryl, Ci-Cigheteroaryl, and acyl.

[034] In embodiments each optional substituent is independently selected from the group consisting of: halogen, =0, =S, -CN, -N0₂, -CF₃, -OCF₃, alky!, alkenyl, alkynyl, haloalkyl, haloalkenyl, haloalkynyl, heteroalkyl, cycloalkyl, cycloalkenyl, heterocycloalkyl, heterocycloalkenyl, aryl, heteroaryl, hydroxy, hydroxyalkyl, alkyloxy, alkyloxyalkyl, alkyloxyaryl, alkyloxyheteroaryl, alkenyloxy, alkynyloxy, cycloalkyloxy, cycloalkenyloxy, heterocycloalkyloxy, heterocycloalkenyloxy, aryloxy, heteroaryloxy, arylalkyl, heteroarylalkyl, arylalkyloxy, amino, alkylamino, acylamino, aminoalkyl, aryiamino, sulfonyl, alkylsulfonyl, arylsulfonyl, aminosulfonyl, aminoalkyl, -COOH, -SH, and acyl.

[035] Examples of particularly suitable optional substituents include F, CI, Br, I, CH₃, CH₂CH₃, OH, OCH₃, CF₃, OCF₃, N0₂, NH₂, and CN.

[036] In the definitions of a number of substituents below it is stated that "the group may be a terminal group or a bridging group". This is intended to signify that the use of the term is intended to encompass the situation where the group is a linker between two other portions of the molecule as well as where it is a terminal moiety. Using the term alkyl as an example, some publications would use the term

"alkylene" for a bridging group and hence in these other publications there is a distinction between the terms "alkyl" (terminal group) and "alkylene" (bridging group). In the present application no such distinction is made and most groups may be either a bridging group or a terminal group.

[037] "Acyl" means an R-C(=0)- group in which the R group may be an alkyl, cycloalkyl, heterocycloalkyl, aryl or heteroaryl group as defined herein. Examples of acyl include acetyl and benzoyl. The group may be a terminal group or a bridging group. If the group is a terminal group it is bonded to the remainder of the molecule through the carbonyl carbon.

[038] "Alkenyl" as a group or part of a group denotes an aliphatic hydrocarbon group containing at least one carbon-carbon double bond and which may be straight or branched preferably having 2-12 carbon atoms, more preferably 2-10 carbon atoms, most preferably 2-6 carbon atoms, in the normal chain. The group may contain a plurality of double bonds in the normal chain and the orientation about each is independently E or Z. Exemplary alkenyl groups include, but are not limited to, ethenyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl and nonenyl. The group may be a terminal group or a bridging group.

[039] "Alkyl" as a group or part of a group refers to a straight or branched aliphatic hydrocarbon group, preferably a C-_rCi2 alkyl, more preferably a Q-Cio alkyl, most preferably Ci-Ce unless otherwise noted. Examples of suitable straight and branched Ci-C₆ alkyl substituents include methyl, ethyl, n-propyl, 2-propyl, n-butyl, sec-butyl, t- butyl, hexyl, and the like. The group may be a terminal group or a bridging group.

[040] "Alkynyl" as a group or part of a group means an aliphatic hydrocarbon group containing a carbon-carbon triple bond and which may be straight or branched preferably having from 2-12 carbon atoms, more preferably 2- 10 carbon atoms, more preferably 2-6 carbon atoms in the normal chain. Exemplary structures include, but are not limited to, ethynyl and propynyl. The group may be a terminal group or a bridging group.

[041 ] " Aryl" as a group or part of a group denotes (i) an optionally substituted monocyclic, or fused polycyclic, aromatic carbocycle (ring structure having ring atoms that are all carbon) preferably having from 5 to 12 atoms per ring. Examples of aryl groups include phenyl, naphthyl, and the like; (ii) an optionally substituted partially saturated bicyclic aromatic carbocyclic moiety in which a phenyl and a C5.7 cycloalkyl or C5-7 cycloalkenyl group are fused together to form a cyclic structure, such as tetrahydronaphthyl, indenyl or indanyl. The group may be a terminal group or a bridging group.

Typically an aryl group is a C -Cip aryl group.

[042] A "bond" is a linkage between atoms in a compound or molecule. The bond may be a single bond, a double bond, or a triple bond.

[043] "Cycloalkyl" refers to a saturated monocyclic or fused or spiro polycyclic, carbocycle preferably containing from 3 to 9 carbons per ring, such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and the like, unless otherwise specified, it includes monocyclic systems such as cyclopropyl and cyclohexyl, bicyclic systems such as decalin, and polycyclic systems such as adamantane. A cycloalkyl group typically is a C3-C_]2 alkyl group. The group may be a terminal group or a bridging group.

[044] "Halogen" represents chlorine, fluorine, bromine or iodine. [045] Formula (I) is intended to cover, where applicable, solvated as well as unsolvated forms of the compounds. Thus, each formula includes compounds having the indicated structure, including the hydrated as well as the non-hydrated forms.

[046] As used herein, the term "corrosion", and similar terms, means the destructive attack of a metal or metal alloy through interaction with its environment.

[047] As used herein, the term "aluminium- or aluminium alloy-containing" in relation to an article means that at least the exposed surfaces of the article are formed from aluminium or an aluminium alloy.

[048] The person skilled in the art understands that the chemistry at the surfaces of the article that is coated is important in terms of the coating and, as such, the interior or non-exposed surfaces of the article may be made from any material or substance.

[049] The following abbreviations used throughout this specification have the following meanings:

BTESPEDC: bis-(triethoxysilylpropyl)-ethane-dithiol-carbamate;

BTMSPA: bis-(trimethoxysilylpropyl)-amine; and

BTESPT: bis-(triethoxysilylpropyl)-tetrasulfide.

[050] In a first aspect, the present invention provides a coating composition comprising a bis-silane accordin to Formula (I)

(I)

wherein:

R¹, R\ R³, R⁴, R⁵, R⁶, R⁷, and R⁸ are each independently selected from the group consisting of: H, optionally substituted C,-C₆ alkyl, optionally substituted C_rC₆ cycloalkyi, optionally substituted C₂- C alkenyl, optionally substituted C₂-C cycloalkenyl, optionally substituted C2-Q alkynyl, optionally substituted C₂-C6 cycloalkynyl, and optionally substituted aryl;

X', X², X³, X⁴, X^s, and X⁶ are each independently selected from the group consisting of: O, S, and NR⁹;

each R⁹ is independently selected from the group consisting of: H, optionally substituted CpCe alkyl, optionally substituted Ci-C₃ cycloalkyi, optionally substituted C - ₆ alkenyl, optionally substituted C₂-C₆ cycloalkenyl, optionally substituted C₂-Q alkynyl, optionally substituted C₂-C₆ cycloalkynyl, and optionally substituted aryl; and

I, m, and n are integers independently selected from the group consisting of: 1 , 2, 3, 4, 5, and 6.

[051 ] In embodiments, R¹, R², R³, R⁴, R⁵, R⁶ are H.

[052] In embodiments, I is 1 or 2. In specific embodiments, 1 is 2.

[053] In embodiments, m is 1 , 2 or 3. In specific embodiments, m is 3.

[054] In embodiments, n is 1 , 2 or 3. In specific embodiments, n is 3. .

[055] In embodiments, R⁷ and R⁸ are C C₃ alkyl. In specific embodiments, R⁷ and/or R⁸ are - CH₂CH₃. In specific embodiments, R⁷ and/or R⁸ are H. As described in more detail later, the bis- alkoxysiloxane is at least partially hydrolysed prior to coating a metal article and, therefore, the coating composition (in this partially hydrolysed state) may be a mixture of alkyl- and hydroxyl-bis-silane molecules.

[056] In embodiments, X¹ and X² are S or O. In specific embodiments, X¹ and X² are S. [057] In embodiments, X³ and X⁴ are O.

[058] In embodiments, X⁵ and X⁶ are NR⁹. In embodiments, R⁹ is H.

[059] In embodiments, X¹ and X² are S, X³ and X⁴ are O, and X⁵ and X⁶ are NH. This provides compounds of formula (la):

(R⁷0)₃Si -(CR³R⁴)_m -NH-CiOi-S-tCR'R^rS-CiOi-NH-iCR'R^n-SiiOR⁸),

(la)

wherein R¹ , R², R³, R⁴, R⁵, R⁶, R⁷, and R⁸ are as defined previously.

[060] In specific embodiments, the bis-silane is of Formula (lb):

(EtO),Si -(CH₂)₃ -NH-C(0)-S-(CH₂)2-S-C(0)-NH-(CH₂)₃-Si(OEt)₃

(lb) [061 ] In embodiments the coating composition contains a solvent. Suitable solvents include (but are not limited to): methanol, ethanol, n-propanol, i-propanol, n-butanol, sec-butanol, tert-butanol, toluene, and xylene. Typically, the solvent will also contain water to bring about hydrolysis of the bis-silane. The ratio (v/v/v) of bis-silane, water and solvent may be about 2/2/96 to about 10/1 0/80. In

embodiments, the solvent is ethanol and the ratio of silane, water and ethanol is about 5/5/90 (v/v/v).

[062] A composition containing bis-silane may be particularly suitable for forming a corrosion resistant coating on an aluminium alloy. In embodiments, the aluminium alloy is AA2024-T3 which has the following composition:

[063] In a second aspect, the present invention provides a method of coating an aluminium- or aluminium alloy-containing article to improve the corrosion resistance thereof, the method comprising: providing an aluminium- or aluminium alloy-containing article to be coated;

providing a coating composition according to the first aspect of the invention; if necessary, at least partially hydrolysing the bis-silane in the coating composition;

contacting the coating composition and the article under conditions that result in at least part of the surface of the article being coated with the coating composition; and

curing the coating composition to form a corrosion resistant coating on said at least part of the surface of the article.

[064] In embodiments, the metal article is an aluminium alloy-containing article.

[065] The condensation of the bis-silane on the surface of the metal article requires the bis-silane to be in an at least partially hydrolysed form (i.e. at least some of the bis-silane molecules in the composition must contain -Si(OH)₃ groups). The coating composition may be provided in a fully hydrolysed or a partially hydrolysed form. If that is the case, no additional hydrolysis step may be required prior to contacting the coating composition with the metal article. Alternatively, the coating composition may be provided in a non-hydro lysed form in which case it will be necessary to hydrolyse the bis-silane. The bis-silane may be hydrolysed by contacting it with water under suitable conditions. Suitable conditions may include adjusting the pH of a solution or mixture containing the bis-silane and water to below 7. The pH may be adjusted using a suitable mineral or organic acid. Suitable acids include (but are not limited to): formic acid, acetic acid, hydrochloric acid, and sulfuric acid. [066] Hydrolysis is a kinetically favourable reaction for silanes at low and high pH and hence occurs readily in water and protic solvents, such as ethanol. The extent of hydrolysis varies depending on silane and solvent concentration, pH, the type of acid catalyst and reaction temperature. The silane

concentration is critical to obtaining a 'workable' solution (solution with sufficient silanol groups to react with the metal surface). The concentration of silane in the hydrolysing solution needs to be relatively dilute to provide stability of the silanols through hydrogen bonding.

[067] The step of contacting the coating composition and the aluminium- or aluminium alloy- containing article under conditions that result in at least part of the surface of the article being coated with the coating composition may be carried out using any suitable coating technique known in the art. In embodiments, the article is coated with the coating composition by dip coating.

[068] The step of curing the coating composition may be carried out using any suitable curing technique known in the art for curing siloxanes. In embodiments of the invention in which the coating composition contains a solvent, the curing step may involve removing the solvent from the coating on the article. The solvent may be removed by heating the article and/or by air drying. The curing step may also involve a step of heating the article.

[069] The curing of the silane on the substrate involves the formation of covalent bonds between molecules and the metal substrate with the loss of water. The extent of crosslinking determines the morphology of the structure, which is important to provide the required anti-corrosion characteristics: To achieve a uniform coverage, there must be an even distribution of Si-O-Al covalent bonds. Hence, the functionality of the silane is important to increase the adhesion between the silane and the substrate.

[070] The structure of the silane molecule can be a major factor affecting the extent of surface adhesion. The characteristics, such as surface thickness and morphology, of the siloxane film vary depending on the functionality of the silane. Mono-silanes are not as efficient at producing a thick, homogenous crosslinked network, as bis-silanes. The higher density of the Al-O-Si bonds formed by bis-silanes on Al alloy surfaces result in an increased hydrophobic and desirable homogenous film. Also, the functionality in the alkyl chain can aid in the rate of condensation. For example, sulfur has the capability of bonding to copper-rich precipitates in the alloy and hence provides better film adhesion.

[071 ] The corrosion inhibiting mechanism for Al is thought to be due to the hydrophobic interfacial layer. The interface layer is enriched with Si-O-AI and Si-O-Si and has extensive crosslinking compared to the siloxane film. The Si-O-AI bonds form readily but are not hydrolytically stable (reaction reverses in the presence of water), therefore the hydrophobic barrier is important not only to prevent the transfer of corrosion promoting elements (i.e. CI ) through the film but also to prevent film delamtnation. This illustrates the significance of producing a film that incorporates full crosslinking by making the film denser and enhancing the films hydrophobic nature.

[072] The anti-corrosion ability of the silane films can be tested using various techniques. The most valuable technique is Electrochemical Impedance Spectroscopy (EIS). This technique investigates the water permeability and corrosion resistance of the film. Salt spray testing can also be conducted over extended periods of times using various salts.

[073] In a third aspect, the present invention provides a coated metal article produced by the method of the second aspect of the invention. Whilst coatings will typically be applied to articles having flat or rounded surfaces, it is also possible to coat articles having intricate shapes using dip coating methods.

[074] The present invention is hereinafter further described by way of the following, non-limiting example(s) and accompanying figure(s).

EXAMPLE(S)

[075] Solutions and Reagents

[076] The following reagents were analytical grade used as supplied from Aldrich: 3- isocyanatopropyltriethoxysilane, 1 , 2 ethane dithiol, triethyl amine, toluene, 100% ethanol, d- chloroform, acetone, ethanol, hexane, sodium hydroxide, dibutyltin dilaurate, bis-(triethoxysilylpropyl)- tetrasulfide (BTESPT), bis-(trimethoxysilylpropyl)-amine (BTMSPA), octadecyltrichloro silane.

TURCO 5948. DPM solution was supplied by the Defence Science and Technology Organisation's (Australian Department of Defence) Maritime Platforms Divisions.

[077] Example 1 - Synthesis of Bis-(triethoxysilylpropyl)-ethane-dithiol-carbamate (BTESPEDC) [078] A process for producing BTESPEDC is.shown in Scheme 1 .

Scheme 1

[079] a) 3-Isocyanatopropyltriethoxysilane (12 ml, 0.048 mol) was added to a 50 ml round bottom flask and a slight excess 1 , 2 ethanedithiol (2.1 ml, 0.025 mol) was added. The solution was mixed under nitrogen and dibutyltin dilaurate (0.05 ml, 8.71 x 10^"5 mol) was added. The clear solution was allowed to react whilst being stirred at 60 °C for 24 hours. The product was then heated under vacuum

(l mbar/room temperature) for 6 hours to remove the excess 1 , 2-ethanedithiol, yielding a white waxy solid (12.8 g, 90%). This product contained residual 1 , 2 ethanedithiol and it was thought that the use of a solvent may allow an increased product yield.

[080] b) 3-Isocyanatopropyltriethoxysilane (12 ml, 0.048 mol) was added to a 50 ml round bottom flask and stoichiometric amount of 1 , 2 ethanedithiol (2.0 ml, 0.024 mol) was added and mixed under nitrogen and in a fume hood. Toluene (2.0 ml) and triethylamine (0.05 ml) were added and the clear solution was allowed to react whilst being stirred at 60 °C for 24 hours. The product was purified under vacuum (1 mbar/RT) for 6 hours to remove the excess 1 , 2-ethanedithiol, yielding a white waxy solid (13.15g, 91 %).

[081 ] Ή (CDC1₃): δ 5.99 ppm (t, 2H, J=6.3 Hz, NH), δ 3.8 ppm (q, 12, J=6.9 Hz, OCH₂CH₃), δ 3.26 ppm (m, 4H, N-CH₂), 6 3.10 ppm, (s, 4H, S-(CH₂)₂-S), δ 1.62 ppm, (m, 4H, J=6.3, 8. t Hz,

CH₂CH₂CH₂), 61.20 ppm, (t, 18H, J=6.9 Hz, OCH₂CH₃) 60.61 ppm, (t, 4H, J=8.1 , CH₂CH₂CH₂)

[082] ^,3C (CDClj): δ 166.27 ppm, CO, δ 58.42 ppm, OCH₂CH₃, δ 43.63 ppm, NH-CH₂, δ 0.61 ppm, S-(CH₂)₂-S, δ 22.81 ppm, CH₂CH₂CH₂, δ 18.19 ppm, OCH₂CH₃, δ 7.65 ppm, SiCH₂

[083] FT1R: 3300 cm-¹ (s, N-H), 2900 cm-' (s, C-H), 1648 cm-' (s, C=0), 1527 cm-' (s,N-H), 1389 cm-¹ (w, Si-O-C), 1219 cm-¹ (w, Si-O-C), 1206 cm-¹ (w, -S-CH₂), 1 165 cm-¹ (CH₃CH₂-), 1072 cm-¹ (w, Si-O-C), 951 cm-¹ (w, Si-O-C), 775cm-¹ (w, Si-C)

[084] Example 2 - Preparation of AA2024-T3 Aluminium Alloy

[085] A A2024-T3 (also known as UNS A92024; ISO AlCu4Mgl ; NF A-U4G1 (France); DIN AlCuMg2; AA2024-T3, ASME SB21 1 ; CSA CG42 (Canada)) aluminium alloy is available commercially. In the present case, panels with dimensions of 1 cm x 2 cm x 0.2 cm were obtained from the Defence Science and Technology Organisation's Maritime Platforms Division. The panels were prepared by immersing and ultrasonicating (10 min) each Al panel in hexane, acetone and ethanol. This was followed by immersion of each panel for 5 min at 40-50 °C in 30 g L aqueous solution of TURCO 4215 (alkaline cleaner). Between each step the Al panels were rinsed with DI water. The last step involved a thorough rinse with deionised (DI) water for 5 min and blow drying of the panel making it ready for immersion into the silane hydrolysing solution. The Al surface needed to be water 'break free', indicating a high concentration of hydroxy! groups on the surface.

[086] Example 3 - Preparation of silane hydrolysing solutions

[087] The silanes BTESPT and BT SPA were purchased from Sigma-Aldrich and BTESPEDC was synthesised using the method described in Example 1. The silanes were used without further purification. The silanes were hydrolysed in a small amount of water and ethanol as the solvent.

[088] BTESPT hydrolysing solution was prepared by mixing 5 vol% of the BTESPT silane with 90 vol% ethanol and 5 vol% DI water. Acetic acid was added to the solution until a pH 4.5 was reached (BTESPT had a natural H 7) and the solution was aged for 24 hr to obtain a workable solution.

[089] BTMSPA was hydrolysed using an analogous procedure. A BTMSPA silane solution of 5 vol.% was prepared and added to a mixture of water and ethanol. The ratio of BTMSPA silane/ DI water/ ethanol was 5/5/90 (v/v/v). Acetic acid was added to the mixture until a pH of 5.5 was reached. The natural pH of this silane (pH 9) was too high, hence in practice the solution was unstable and gelled within 1 hr. With the addition of acetic acid the solution was able to be used for the next step immediately.

[090] BTESPEDC was hydrolysed by dissolving 5 vol% of the BTESPEDC in absolute ethanol (90%). The samples were ultrasonicated for 20 min to ensure dissolution. Addition of 5% DI water and acetic acid to decrease the pH from 4.5 to 3.5 initiated the hydrolysis reaction. The reaction was relatively slow and the solution was allowed to age for 3 days to ensure a workable solution was prepared.

[091 ] The 5/5/90 (v/v/v) ratio of silane, water and ethanol was adopted as that concentration of water and silane had the best stability for silanes BTESPT and BTMSPA. It should be noted that the hydrolysis reaction occurs in steps and using ethanol as a solvent prevents complete hydrolysis from occurring. Hence, when the film condenses and cures there are still unhydrolysed groups present.

[092] Kinetics Study on the Hydrolysis of Bis-Silanes

[093] The rate of hydrolysis of the silanes in water and ethanol solution was investigated. The ethanol and water makes it difficult to observe changes to the silane chemical structure due to the interference of water and ethanol absorptions in techniques such as infrared analysis. Method 1 , method 2 and method 3 set out below are based on the work of Pu and Ogasawara (Pu, Z. et al., Journal of Adhesion Science Technology 1996, 11, 1 8; Ogasawara, T. et al, J. Colloid Polymer Science 2000, 278, 14) where Fourier Transform Infrared Spectroscopy (FTIR) was used with different sampling techniques to determine the optimum conditions for silane hydrolysis.

[094] Method 1 : To obtain the rate of hydrolysis of silane in ethanol and water based solutions, a technique employing Attenuated Total Reflectance (ATR) FTIR in the mid infrared region was initially trialled. A hydrolysed solution of BTESPT was prepared and the pH was varied to study the effect of solution pH on the rate of hydrolysis. A 20 μΐ, sample was taken from the reaction vessel and placed on the diamond and a spectrum was recorded. The frequency at which spectra were recorded was every two minutes for three hours. This allowed time for the instrument to take 64 scans with a resolution of 4 (the average spectral collection time was 90 sec). The overlayed spectra were scrutinised to find a peak that illustrated that the hydrolysis reaction was occurring. A band at 1 1 10 cm^'1 corresponded'to the Si-O-C symmetric stretch, which would be expected to decrease over time. An ethanol band at 1050 cm^"1 was used as a reference; it had the same intensity for each spectrum. The difference in heights of these two peaks (absorption of ethanol ( 1050 cm^"') / absorption of Si-O-C (1 1 10 cm^'1)) was recorded and plotted as a function of time.

[095] Method 2: This method involved the use of a BaF₂ liquid cell, in mid-infrared transmission mode FTIR. The silane (BTESPT) and ethanol solution was prepared and water was added. Immediately after the water was added the silane solution was expelled into the liquid cell and data was collected every 2 min. The spectra were overlayed and the bands present were evaluated but no useful peaks were identified. The method was repeated using a ZnSe liquid cell but similar problems were prevalent.

[096] Method 3: This method used the NIR region to observe the hydroxy 1 peaks present in a hydrolysing solution. The FTIR instrument was set-up by changing the detector from TE-DLaTGS Mid IR to TE-InGaAs NIR and the grating was changed from Br to CaF₂. The FT-NIR was allowed to purge under nitrogen for 2-3 hrs and a series data collection method was set up. The ratio of peak height of an ethanol band and water band (ethanol absorption (4188 cm^"')/water absorption (5158 cm^'1)) was collected every 30 s. Bands of interest appeared at 5158 cm^'' (water) and 4811 cm^"' (ethanol) with baselines at 5350 to 5000 cm^"' and 5000 to 4650 cm^"1. The intensity of the ethanol band at 4188 cm^'1 remained unchanged, despite evolution/consumption of ethanol in hydrolysis equilibrium. The water concentration decreases as it is consumed by hydrolysis of the silane.

[097] The amounts of silane, ethanol and water used were 0.5 mg, 9.0 ml and 0.5 ml respectively. The pH was adjusted using acetic acid. The natural pH of BTESPEDC was 4.5 and the lowest pH 3.5 was achieved and the rate of reaction was observed by producing a graph illustrating the change in peak intensity against time. The slopes were calculated and used to reflect the rate of hydrolysis of the silane solution at pH 3.5 and pH 4.5. The rate constants (slopes) were determined using the LINEST function in Microsoft Excel, using the average of the triplicate spectra.

[098] Example 4 - Condensation of Bis-Silanes

[099] A A2024-T3 was first cleaned and the hydrolysing solution prepared as described in Examples 2 and 3, respectively. The BTESPT and BTMSPA silane solutions were then applied by immersion of the cleaned Al alloy panel into the workable silane solution for 30 s at room temperature. The coated panel was then cured in an oven at 100 °C for 1 hr.

[0100] Condensation of BTESPEDC was optimised to compliment the hydrolysis conditions. A summary of samples prepared for condensation of BTESPEDC are shown in Table 1. Samples 1 to 10 a, were prepared using acidic hydrolysis and condensation conditions (pH 3.5). Samples 1 to 10 b, were prepared in an acidic hydrolysing solution (pH 3.5), followed by condensation under basic conditions (pH 7.5). The silane solutions were then applied by immersion of the cleaned Al alloy panel into the solution for 30 s at room temperature The coated panels were cured in an oven at 100 °C for 1 hr

[0101 ] Many samples were prepared for the condensation of BTESPEDC for which the pH of condensation, hydrolysis aging time and concentration of BTESPEDC were varied (Table 1 ). This was to observe the effect of pH on crosslinking and to determine the optimum pH for condensation to compliment the optimised hydrolysis conditions. Samples of varying BTESPEDC concentration were analysed to observe the effect of BTESPEDC concentration on the physical properties of the films, such as film thickness. Each sample was subsequently analysed by PhotoAcoustic (PA-FTIR) to determine the relative film thickness, Scanning Electron Microscopy (SEM) to determine the uniformity and homogeneity and Electrical Impedance Spectroscopy (EIS) to determine the diffusion rate ions through the film (details provided later). [0102] Table 1: A summary of the samples prepared for condensation.

[0103] Characterisation Methods

[0104] Ή and C Nuclear Magnetic Resonance (NMR) Spectroscopy

[0105] NMR spectroscopy is a common technique used to determine structures of organic compounds. In the present study, NMR spectra were collected with a Varian 300 NMR spectrometer using CDCI3 as the solvent and an internal lock.

s

[0106] Fourier Transform Infrared (FTIR) Spectroscopy

[0107] FTIR spectroscopy is a common technique used to determine the chemical functionality present in organic and inorganic compounds. A Thermo-Nicolet Nexus 870 FT-IR spectrometer (Thermo Electron Corporation) fitted with either the Attenuated Total Reflectance (ATR) attachment, Photo- acoustic (PA) module or a liquid cell in transmission mode was used to generate FT-IR spectra and data was manipulated using OMNIC software. [0108] ATR uses the phenomenon of total internal reflection of an infrared laser beam in a diamond crystal. The diamond crystal (used in this work) has a high refractive index, hence allowing total internal reflectance to occur and can be used to observe hard as well as soft samples. The infrared beam is incident at the critical angle to the diamond and is reflected off the walls of the diamond. The evanescent wave produced by total internal reflectance passes through the sample, typically to a depth of a few micrometres. The sample will absorb the infrared beam with characteristic energies corresponding to its chemical functionality, thus altering the beam. The attenuated IR beam exits the crystal and hit's the detector in the spectrometer.

[0109] Photoacoustic (PA) Infrared Spectroscopy is a relatively new technique in which a sensitive microphone is used to detect pressure waves. A modulated infrared beam is incident on the sample, which absorbs some energy characteristic to the functionality of the molecule. The release of the energy results in temperature fluctuations at the samples surface, thus producing pressure waves which are detected by the microphone. The microphone and sample is surrounded by helium gas, which transmits the waves produced by the sample. The summary of spectral collection parameters is provided in 2 below for each type of analysis. This includes PA-FTIR, ATR-FTIR and NIR.

[01 10] Table 2: Summary of spectral collection parameters used in Attenuated Total Reflectance (ATR) photoacoustic (PA) and Near-Infrared (NIR) experiments.

FTIR Sampling Techniques

Parameter

Description (ATR) Description (PA) Description (NIR)

Spectral Range 4000 to 500 cm^"1 4000 to 500 cm^"1 5400 to 4500 cm^"1

Number of Scans 64 256 32

Resolution 4 cm^"1 4 cm^'1 4 cm^"1

Detector DTGS TEC PAC 300 TE-InGaAs beamsplitter Br KBr CaF₂

Mirror Velocity 0.6329 cm s^"1 0.9494 cm s^"1 0.6329 cm s^'1 [01 1 1 ] ThermoGravimetric Analysis (TGA)

[01 1 2] TGA measures a change in mass of a sample as a function of temperature and/or time. The sample is positioned on a tared pan suspended from a sensitive microbalance and is then fully enclosed in a furnace. The system is purged with a specified gas and parameters such as the gas flow rate and the heating profile supplied by the furnace are programmed by the user.

[01 13] Cured silane samples were analysed under continuous heating conditions between room temperature (25 ^*C) and 600 ^°C. The heating rate was kept constant at 1 0 °C min^'1 with a nitrogen gas flow rate of 50 ml min^'1. The instrument used for this work was a TA Instrument Hi-Res Modulated TGA 2950.

[01 14] High Resolution (HR) TGA uses the same principle as standard TGA, but the heating rate is altered in response to sample mass variation. To get the best resolution in standard TGA experiments, the slowest heating rate possible is required but this technique can take a long time for each analysis. HR-TGA decreases the heating rate when there is. a loss of sample mass, and increases the heating rate when there is no mass loss. The method is faster than using a constant low heating rate and has similar resolution. This technique was used with a sensitivity of 3 and a resolution of 6. The underlying heating rate was 10 ^*C/min, with a nitrogen gas flow rate of 50 ml/min.

[01 1 5] Scanning Electron Microscopy (SEM)

[01 16] SEM is used in many circumstances to determine the surface morphology and elemental composition of a material. SEM uses an incident beam of electrons produced from an electron gun. The highly focused beam of high energy electrons scans a specified area on the surface of the material and the electrons collide with atoms on the surface. Various emissions are produced, such as X-rays, Auger electrons (both provide compositional information), backscattered electrons (atomic number topography information), cathodoluminescence (electrical information) and secondary electrons (topography). The information provided by the secondary and primary backscattered electrons is of most interest as they provide the topography and compositional information. The various emissions are detected and the signals are converted to images on a cathode ray tube. The samples are mounted onto stubs and coated with a conducting material, such as platinum, carbon or gold (if the sample is an insulator) and put into the sample holder, which is put under a vacuum to minimise interactions between the electrons and air.⁵' The SEM used was the Phillips XL30 Field Emission SEM with an EDAX detector for X-ray analysis and the results were manipulated with microscope control software. [01 17] Atomic Force Microscopy (AFM)

[01 18] A typical AFM contains four major components: a tip that is mounted onto a cantilever, a piezoelectric stage, a sensitive detection system and an electrical feedback loop, which controls the height of the tip. Common materials for the tip can be silicon or silicon nitride and when the tip is brought into close proximity to the surface of the sample, forces between the sample and the tip cause deflection due to Hookes law, which is measured with a sensitive photodiode detector.

[01 19] Contact Mode (CM) and Tapping Mode (TM) were used for this work. CM AFM images the topography of the sample by keeping the force between the tip and the sample (cantilever deflection) constant. As the tip is brought into close proximity to the surface (a few angstroms), van der Waals force between the tip and the sample cause deflection of the cantilever. As the sample beneath the tip moves, the features on the sample cause changes in the force and hence the deflection of the cantilever. The deflection on the laser (reflected off the cantilever) is measured by the photodiode detector and is plotted to produce an image. In TM AFM, topography is imaged similar to CM AFM. The tip is oscillated at or near its resonant frequency, thus only touching at the bottom of its swing (lightly taps). The feedback loop maintains constant oscillating amplitude and a topographic image of the sample surface is produced.

[0120] The AFM technique allows high resolution imaging and can be used to determine how homogenous the surface is. VEECO silicon nitride (NP-10) tips were used for CM AFM and Silicon (FESP) tips were employed for TM-AFM. The instrument used was a Multimode Atomic Force Microscope, equipped with a Nanoscope TV scanning probe and the data was manipulated with Nanoscope 5.3 l rl software.

[0121] Example 5 - PA-FTIR of the silane films

[0122] Each silane was hydrolysed, condensed and cured onto the AA2024-T3 Al alloy substrate. The Al coated with silane was analysed using PA-FTIR to determine the extent of crosslinking within the film. Characteristic bands occurred for a typical siloxane film at 3300 cm^"1, 1 100 cm^"1 and 900 cm^'1 which represent the OH, Si-O-Si and Si-OH stretching vibrations respectively. [0123] Table 3: Assigned bands for a) BTESPT, b) BTMSPA and c) BTESPEDC using PA-FTIR, where v is the stretching mode and δ is the bending mode.

BTESPT

Band Position (cm^"') Assignments

3393 OH-Si (v)

2926 C-H (CH₂,v)

1408 Si-0-CH₂CH₃ (asymmetric v)

1300 Si-0-CH₂CH₃ (CH₂, δ)

1 121 Si-O-Si (S-O, asymmetric v)

886 Hydrogen bonded Si-OH

786 Si-C (v)

BTMSPA

Band Position (cm ¹) Assignments

3500-2500 OH-Si /N-H (v)

2934 C-H (v)

1559 N-H (δ)

1405 S1-O-CH₃ (asymmetric v)

1055 Si-O-Si (S-O, asymmetric v)

926 Hydrogen bonded Si-OH

764 Si-C (v) BTESPEDC

Band Position (cm^"') Assignments

3301 N-H /OH (v)

2934 G-H (CH₂, v)

1653 C=0 (v)

1 522 N-H (5)

1065 Si-O-Si (S-O, asymmetric v)

908 Hydrogen bonded Si-OH

784 Si-C (v)

[0124] The films produced on Al alloy with varied silane concentration were analysed using PA-FTIR (Figure 1 ). The thickness of the film was estimated and compared with thickness values obtained using other techniques. The spectra indicate that the silane film thickness has a proportional relationship to the corresponding BTESPEDC concentration. This is apparent for most silanes, as found by Zhu (Zhu, D.; van Ooij, W. J. Progress in Organic Coatings 2004, 49, 12) and Siindararajan et al. (Sundararajan, G. P., University of Cincinnati, 2000). When the substrate is immersed into the hydrolysing silane solution, silane molecules adhere to the surface through hydrogen bonds. The more silane molecules present in the solution, the more will absorb onto the surface of the Al substrate, thus resulting in a thicker film.

[0125] Example 6 - Morphology of silane coating using SEM

[0126] Morphology of silane coatings is extremely important in relation to their ability to inhibit corrosion. Any pores that may be present may initiate corrosion by enabling the transfer of ions and water across the film. The uniformity of the surface establishes the extent of crosslinking and the overall orientation of the silanes on the metal surface. For a film where the silane has many conformations, a less homogenous film would be obtained. Conversely, a film with mainly unidirectional crosslinked silanes would produce a homogenous coating. [0127] SEM enabled the microstructure of the silane films to be imaged at high resolution (μιτι). SEM allows resolution at the nanoscale but due to electrons charging the surface, a microscale image could only be obtained. Figure 2 shows SEM images obtained for AA2024-T3 coated with BTESPT and BTMSPA. Both show a uniform coating with only the BTMSPA showing signs of pores (Figure 2a). The morphology of the silane was homogenous with not much visual depth variation or coagulation. Part of the silane solution agglomerated on the film surface, appearing as white particles on the image.

[0128] BTESPEDC was hydrolysed for 48 hrs and 72 hrs to observe the effect of hydrolysis time on the film morphology. This is clearly illustrated in Figure 3, where (a) is a film produced after a 48 hrs hydrolysis and (b) is a film produced after a 72 hrs hydrolysis. Both condensation steps were carried out at pH 7.5. The coagulation evident in image (a) illustrates the importance of the hydrolysis step when producing a homogenous film. If there is not enough silanol groups present after hydrolysis (ie; if the solution is not workable), the degree of crosslinking is hindered. Furthermore, the network produced through crosslinking is reduced, resulting in pores and reduced stability. This has also been seen for mono-silanes, as they only have three hydrolysable groups and hence produce a more heterogeneous film , when compared to bis-silanes.

[0129] The samples were treated with the same solution and the results were found to be reproducible. The film produced in Figure 3 (b) is comparable and may even be more superior than that produced for the model silanes. No pores were identified and Electron Dispersive Spectroscopy (EDAX) was used to prove that the film was present (Figure 4). Peaks at 0.5, 1 .7 and 2.3 eV represented oxygen, silicon and sulfur respectively. This was concurrent with the observations of Ghandi et al.( Gandhhi, J. S.; Metroke, T. L.; Eastman, M. A.; Ooij, W. J. v.; Apblett, A. Corrsion Science Section 2006, 62, 1 1 ) where it was concluded that less silanol groups cause less interaction between the metal surface and the silane, thus having less anchorage and poorer corrosion inhibition.

[0130] Example 7 - Roughness Determination Using Atomic Force Microscopy (AFM) of BTESPT and BTMSPA

[0131 ] AFM was used for analysis of the morphology and surface roughness of films adhered onto the AA2024-T3 Al alloy substrate. The films were prepared as described previously and were analysed by both contact mode (CM) and tapping mode (TM).

[0132] The images obtained using both techniques (CM and TM AFM) were poorly resolved and the tip seemed to get stuck on the film (Figure 5). The artefacts appearing as streaks on Figure 5 were caused by contamination on the tip, either by dust on the surface or from the film itself. It was also difficult to get the tip close enough to the surface for imaging, especially for an extended period of time. The tip drifted away from the surface over time, thus preventing a full image to be obtained. It was thought that this was due to the sample being hydrophobic. To overcome this problem a technique could be used to make the tip hydrophobic by coating it with octadecyltrichlorosilane (OTS). This technique was described by Wei et al. (Wei, Z. Q.; Wang, C; Bai, C. L. Surface. Science 2000, 467, 5) where they first cleaned the silicon nitride tip using a plasma reactor and then coated it with a hydrolysed solution of OTS forming a Self Assembled Monolayer (SAM).

[01 33] Example 8 - Thermal stability of the siloxane films

[01 34] The thermal stability of a silane relates to the extent of crosslinking; the more crbsslinking, the more stable the network is when put under extreme conditions such as heat. The ability of a silane to withstand extreme environments is an important factor that needs to be considered for industrial use. The stability of a silane is related to many aspects, such as the degree of hydrolysis and condensation and the functional groups present in the silane molecule.

[0135] The number of silanol groups present after hydrolysis strongly determines the degree of condensation that occurs between molecules. The more crosslinking that occurs, the more stable and homogenous the film is. This is one reason why bis-silanes (with 6 Si-OR groups) were chosen for this work rather than mono-silanes (with 3 Si-OR groups). The type of functionality present in the silane can affect the density of the film and thus the films thermal stability. More hydrophobic films, such as those produced using BTESPT, are generally less dense as there are no strong secondary interactions (such as hydrogen bonding) within the film. A silane such as BTMSPA, which contains a hydrophilic functionality group, implements the strength of H-bonding as well as crosslinking, producing a film of high density and stability. TGA was used to observe the thermal stability of films produced using the two model silanes (BTESPT and BTMSPA) and compare them to films formed using the novel silane (BTESPEDC).

[0136] The thermal stabilities of BTESPT and BTMSPA were assessed by TGA and compared to the literature (van Ooij, W.; Zhu, D.; Palanivel, V.; Lamar, A.; Stacey, M. Silicon Chemistry 2006, 10, 25). The results are summarised in Table 4, where two events (mass loss) occurred for BTESPT and three for BTMSPA. BTESPT produces a highly hydrophobic film that only contains sulfur and alkyl functionality. The only interactions that can occur between the molecules to enhance the crosslinking effect are between S-bridges. BTMSPA is a water-based silane with amine group that can undergo hydrogen bonding between molecules, producing a more dense film.

[0137] The mass loss events that occurred during TGA analysis of BTESPT and BTMSPA are summarised in Table 4. The first event (T_max = 90 °C for BTESPT; 96 °C for BTMSPA) for each silane was loss of entrapped water from the films, consistent with the literature (van Ooij, W.; Zhu, D.;

Palanivel, V.; Lamar, A.; Stacey, M. Silicon Chemistry 2006, 10, 25). Event 2 for BTMSPA has been proposed to be the result of hydrogen bonding that is involved throughout the siloxane network. The hydrogen bonding also accounts for the high decomposition temperature of BTMSPA (event 3) when compared to BTESPT (event 2). The results obtained were analogous to the literature values and will be used to compare the results of the novel silane (BTESPEDC). The onset temperature (T_onset) further supports that BTMSPA (T_onse, = 464 °C) is more stable when compared to BTESPT (T_onset = 293 °C).

[0138] Table 4: The TGA analysis for cured BTESPT and BTMSPA

[0139] The characteristics of BTESPEDC are different compared to the two reference si lanes, BTESPT and BTMSPA. The model silanes are both oily liquids, whereas the novel silane has a white waxy crystal structure with a melting point of 50 °C. In the TGA results, the cured BTESPEDC product showed differences when compared to the model cured silanes. The TGA showed multiple mass loss steps that were involved in the decomposition. Some of these steps were not resolved, preventing the complete thermal decomposition process to be explored; hence High Resolution (HR) TGA was used to resolve the mass loss steps.

[0140] A comparison between Figure 6 and Figure 7 illustrates the better resolution that was obtained using HR-TGA. This allowed improved determination of the decomposition of cured BTESPEDC. The first three peaks in the DTG curve were not identified immediately, hence PA-FT1R was used in conjunction with TGA to identify each step and the cause of mass loss. The sample was heated until the temperature of each decomposition peak was reached, and was then held isothermally for 30 min. The sample was analysed using PA-FTIR after every run to observe any changes in the infrared spectrum.

[0141] Two BTESPEDC (10 %vol) samples were analysed, sample 6 a) condensed at pH 3.5 and sample 6 b) condensed at pH 7.5. This was to observe the extent of crosslinking during condensation and the effect that pH had on the thermal stability results summarised in Table 5. [0142] Table 5: The thermal stability of cured BTESPEDC using HR-TGA.

[0143] PA-FTIR was carried out for each decomposition step and the changes in absorption are summarised in Table 6. This allowed for the type of decomposition to be hypothesised. The first decomposition peak had little change in IR absorption (Table 6). The only difference occurred at the OH band at 3300 cm^'1. There was a slight decrease, which indicated that this mass loss was due to entrapped water within the film. As no other changes in the spectrum were observed, it was concluded that there was no decomposition of the siloxane film itself. In addition, the onset temperature of the BTESPEDC sample condensed at pH 7.5 was thirty degrees lower than the onset temperature for BTESPEDC condensed at pH 3.5. This provides evidence that there was more entrapped water in the basic condensed sample and the resulting contribution of the excess water caused a decrease in overall thermal performance of the sample. This was also shown in the first mass loss event (Table 5), where the basic condensed sample had a 17.0 % mass loss and the acidic condensed sample had a 1 5.1 % mass loss due to entrapped water.

[0144] For event 2, the IR absorption for N-H (3050 cm-1), Si-OH (908 cm-1) and OH (3301 cm-1 ) all decreased in intensity indicating part of the silane decomposed during this event. It was also observed that the sample went yellow after the heat treatment (140 and 180 ^°C). Work done by Dyer et al. (Dyer, E.; Wendell Osborne, D. Journal of Polymer Science 1960, 47, 10) proposed a mechanism where carbonyl sulfide (COS) was formed along with an amine. This would account for the changes observed by FTIR and the colour of the sample. The expected mass loss of COS was 10 % and the actual mass loss was 8.5 %, thus supporting the degradation mechanism. Event 3, continues with the decreased intensity of N-H (3050 cm^-1), Si-OH (908 an ¹), OH (3301 cm^"') and -S-CH- (1214 cm ') bands. The sample was a light brown colour after the heat treatment. Work done by Barth et al.( Barth, A.; Munch, E. Die Pharmazie 1 69, 24, 6) illustrated the decomposition of a thiolcarbamate herbicide. The resulting products included carbon dioxide, thiol and an amine. The removal of ethane dithiol would cause the S- CH2 band to be removed and the carbon dioxide emitted would account for the colour. The expected mass loss would be 12%, which corresponds to the actual mass loss (Table 6).

[0145] Upon the next event, a new band appeared at 201 5 cm^"1, which is not a common band. It was proposed that this could have been a cyanate or an NC stretching band due to the decomposition of the isocyanate or amine functionalities.

[0146] Table 6: PA-FTIR data of cured BTESPEDC taken at each decomposition event as determine by HR-TGA.

[0147] Corrosion Testing

[0148] Electrochemical Impedance Spectroscopy (EIS)

[0149] EIS is a technique that measures the total resistance (impedance) of a system to the flow of electrical current. In terms of the evaluation of a metal/coating system using EIS, an electrochemical cell is created and an AC potential at varying frequencies is applied. Current is measured to evaluate the impedance and the data obtained is fitted to a model electrical circuit to generate plots of information such as total impedance over frequency, interface impedance over frequency and double layer capacitance over time. Sources of impedance in EIS experiments are described as Ohmic, inductance and/or capacitance (Kelly, R.; Scully, J. R.; Shoesmith, D.; Buchheit, R. Electrochemrical Techniques in Corrosion Science and Engineering; Marcel Dekker, Inc.: New York, 2003; and Tait, W. An

Introduction to Electrochemical Corrosion Testing for Practicing Engineers and Scientists; Pair O Docs Publications: Rascin, 1994).

[01 50] EIS tests were conducted on BTESPEDC samples (2, 5 & 10 % vol) using a PAR 2273 Advanced Electrochemical System controlled using PAR Electrochemistry Powersuite. All the tests were conducted at room temperature (20-25 °C) after stabilization corrosion potential as monitored using Ecorr vs time tests. Platinum mesh was used as counter electrode and Saturated Calomel Electrode was used as Reference Electrode for the tests. The test area was about 90 sq. mm per sample and immersed in 0.5 M of NaCI solution.

[0151] Example 9 - Electrochemical Impedance Spectroscopy of BTESPEDC on AA2024-T3 Al Alloy

[01 52] Electrochemical Impedance Spectroscopy (EIS) is a common technique used to determine the anti-corrosion properties of films on metal substrates. The technique is similar to linear polarization but uses an Alternating Current (AC) instead of a Direct Current (DC). The low magnitude AC voltage is applied at varying frequencies and the resistance and impedance are recorded and used to determine properties, such as the diffusion of ions through the film to the metal. EIS testing was conducted in 0.5 M NaCI solution and obtained a Bode Magnitude plot, Bode Phase plot and a Nyquist plot for each of the nine samples (triplicate samples of 2 %, 5 % and 10 % BTESPEDC films on AA2024-T3, condensed at pH 7). From EIS it was concluded that the highest impedance values were given by samples containing 10% BTESPEDC (discussed later).

[01 53] A model circuit containing both resistors and capacitors (Figure 8) was used to represent the silane coating on the Al substrate. For example, a capacitor is defined as two plates with an equal amount of charge on each but of opposite sign. The insulator between the plates stores electric charge. When a voltage is applied, the capacitor takes a while to reach its full charge and alters the output potential by introducing an impedance of its own. This change is measured as a phase angle change and can be plotted against frequency to determine the capacitance of the coating on the surface.

[01 54] The interface structure of a coated metal surface in an electrical circuit is shown schematically in Figure 8. The Electric Double Layer (EDL) is referred to as the layer where the negatively charged ions on the metal surface form an electrolyte layer to counteract the charge on the metal surface, or to eventually become neutral via diffusion of the surrounding electrolytes in solution. This layer contains an 'in-built' capacitor (C_EDL) and resistor (R_c0n-) and the type of capacitance and resistance can be analysed using EIS. Similarly, the polymeric film (in this case siloxane) can be represented by a simple circuit containing capacitance and pore resistivity, and the effect on the corrosion rate can be determined for a coated metal substrate.

[0155] AC polarization causes the movement of ions back and forth with the changing polarity from peak cathodic to peak anodic amplitudes, thus has variable magnitude (unlike direct current which has constant polarity/direction and magnitude). When a coating or substrate is submerged in an electrolyte, the dielectric properties of the coating are altered and other factors need to be taken into account. Such factors include the pore resistance or restricted movement of ions and water through a coating (restricted by the coating morphology). The dielectric properties of the coating may also change over time as the concentration of ions and water in the film increases.

[0156] Example 10 - Bode Magnitude Plot of 2% and 10% BTESPEDC

[0157] Figure 9 shows a simple Bode plot, impedance (Z) versus the frequency. As the impedance represents the resistance of current flow through a circuit, a high impedance value is needed for good corrosion resistance. At low frequencies the impedance is the sum resistance of the solution, pore and corrosion. As the frequency increases, the resistance due to corrosion and the pore resistance of the coating becomes negligible, thus decreasing the impedance. The lowest impedance value (at high frequencies) represents the solution resistance (which is very small), and does not affect the coatings resistance/anti-corrosion values.

[01 58] Sample b (10 % BTESPEDC) from Figure 9 had extraordinary polarization resistance, up to 40 ΜΩ. When a coating system has a resistance above 10 ΜΩ, it can be assumed that the layer acts as protective coating. This value is comparable (or better) than other tested silanes, such as BTESPT on AA2024-T3, where van Ooij et al.( van Ooij, W.; Zhu, D.; Palanivel, V.; Lamar, A.; Stacey, M. Silicon Chemistry 2006, 10, 25) found a 20 ΜΩ impedance value in 0.5 M ₂S0₄. The BTESPEDC sample also had a higher resistance as the frequency was increased, compared to BTESPT. Another sample is shown as sample a, Figure 9 which illustrates what is expected for a low-resistance coating (poor anti-corrosion properties). [0159) Example 11 - Nyquist Plot of 2% and 10% BTESPEDC

[0160] The Nyquist plot (Figure 10) is a plot of imaginary impedance versus the real impedance. The imaginary impedance is the reactance or the opposition to the electric circuit and the real impedance is the resistance. For a highly resistant film, this plot would be expected to have a large diameter for the semi-circle, as the diameter is equivalent to the polarization resistance. In the plot shown in Figure 10 b), the semi-circle is almost 40 ΜΩ wide. When compared to a protective film, the semi-circle diameter becomes significantly less, as shown in Figure 10 a). This technique provides a good visual on the polarization resistance of a sample. Overall, the Nyquist plot supports finding that 10 % BTESPEDC has excellent anti-corrosion properties.

[0161 ] In addition, the immersion time prior to EIS testing can be extended to see if any degradation (decreased corrosion resistance over time) occurs. Work by van Ooij et al.( van Opij, W.; Zhu, D.; Stacey, A.; Seth, T.; Mugada, J.; Gandhi, P.; Puomi. Tsinghua Science and Technology 2005, 10, 25; van Ooij, W.; Zhu, D.; Palanivel, V.; Lamar, A.; Stacey, M. Silicon Chemistry 2006, 10, 25; van Ooij, W.; Zhu, D. Corrosion 2001 , 57, 5) and Zhu et al.( Zhu, D.; van Ooij, W. J. Corrosion Science 2003, 45, 12; Zhu, D.; van Ooij, W. J. corrosion science 2003, 45, 20) observed the effect of immersion time of silane coated AA2024-T3 on the corrosion inhibition properties of the silane coating. It was discovered that upon extending the time of immersion in the complex environment ( ₂SO,), the impedance increased, therefore enhanced corrosion resistance was observed. FTIR was used to explain this phenomenon, which concluded that there was further formation of silanols (from remaining ethoxy groups), resulting in further crosslinking to form more Si-O-Si bonds.

[0162] Example 12 - Neutal salt spray testing

[0163] Coupons of AA2024-T3 aluminium alloy (control), AA2024-T3 aluminium alloy coated with 5% BTMSPA, AA2024-T3 aluminium alloy coated with 5% BTESPT and AA2024-T3 aluminium alloy coated with 10 % BTESPEDC were exposed to salt spray for a period of 168 hours in accordance with Australian standard test AS 2331.3.1 -2001. The coupons were placed on an angled plastic specimen rack (15 degrees frm vertical) and subjected to 168 hours neutral salt spray testing. The coupons were inspected every 24 hours throughout the exposure, excluding the weekend period. Prior to o

each inspection the coupons were rinsed with warm water (to remove salt deposits) and blown dry with compressed air. Staining leading from the edges and corrosion attack within 5mm from the edge of the coupon was disregarded from the evaluation. The results are shown in Figures 1 1 to 15.

[0164] In the control sample (Figures 1 1(a) to 15(a)), a significant amount of surface

staining/discolouration and uniform corrosion pitting (white corrosion product) was noted throughout the entire coupon surface after 24 hours exposure. The coupon continued to corrode, but no significant visual change was noted after 168 hours exposure.

[0165] In the BTMSPA coated sample (Figures 1 1 (b) to 1 5(b)), a moderate amount of localised pitting (white corrosion product) was noted throughout the coupon after 24 hours exposure. A negligible change in corrosion pitting frequency was noted after 48, 72 and 96 hours exposure. A slight increase in the amount of corrosion pitting was noted after 168 hours exposure. A slight amount of surface staining was noted from the corrosion pits, this was excluded from evaluation.

[0166] In the BTESPT coated sample (Figures 1 1 (c) to 1 5(c)), no change was noted after 24 hours exposure. A slight amount of localised corrosion pitting (white corrosion product) was noted after 48 hours exposure. Negligible change was noted after 72 hours exposure. However, an increase in corrosion pitting frequency was apparent after 96 hours and 168 hours exposure.

[01 67] In the BTESPEDC coated sample (Figures 1 1 (d) to 1 5(d)), no visible change was noted after 168 hours exposure. A trace amount of localised corrosion pitting was apparent in close proximity to the edge of the coupon and this was disregarded from evaluation.

[0168] Thus, we have shown that the BTESPEDC coated aluminium alloy was more resistant to corrosion than uncoated aluminium alloy and aluminium alloy coated with BTMSPA or BTESPT.

[0169] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

[01 70] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[01 71 ] All publications mentioned in this specification are herein incorporated by reference. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application.

Claims

1 . A coating composition comprising a bis-silane according to Formula (I)

(R⁷0),Si -(CR³R⁴)_m -X'-CiX^-X'-iCR'R^ X^CCX'VX^iCR'R^n-SiiOR⁸).,

(I)

wherein:

R' , R², I*³, R⁴, R⁵, R⁶, R⁷, and R⁸ are each independently selected from the group consisting of: H, optionally substituted C_rC₆ alkyl, optionally substituted C|-Q cycloalkyi, optionally substituted C2- C(, alkenyl, optionally substituted C2-C6 cycloalkenyl, optionally substituted C₂-C₆ alkynyl, optionally substituted C₂-C₆ c cloaikynyl, and optionally substituted aryl;

X', X²,

X⁴, X⁵, and X⁶ are each independently selected from the group consisting of: O, S, and NR⁹;

each R⁹ is independently selected from the group consisting of: H, optionally substituted Ci-C₆ alkyl, optionally substituted Q-Ce cycloalkyi, optionally substituted C2-C6 alkenyl, optionally substituted C₂-C₆ cycloalkenyl, optionally substituted C2-C₆ alkynyl, optionally substituted C₂-C₆ cycloaikynyl, and optionally substituted aryl; and

1, m, and n are integers independently selected from the group consisting of: 1 , 2, 3, 4, 5, and 6.

2. The coating composition of claim 1 , wherein R¹, R², R³, R⁴, R⁵, R⁶ are H.

3. The coating composition of either claim 1 or claim 2, wherein 1 is 1 or 2.

4. The coating composition of claim 3, wherein I is 2.

5. The coating composition of any one of claims 1 to 4, wherein m is 1 , 2 or 3,

6. The coating composition of claim 5, wherein m is 3.

7. The coating composition of any one of claims 1 to 6, wherein n is 1 , 2 or 3.

8. The coating composition of claim 7, wherein n is 3.

9. The coating composition of any one of claims 1 to 8, wherein R⁷ and R⁸ are C1-C3 alkyl.

10. The coating composition of claim 9, wherein R⁷ and/or R⁸ are -CH2CH3.

1 1. The coating composition of claim 9, wherein R⁷ and/or R⁸ are H.

12. The coating composition of any one of claims I to 1 1 , wherein X¹ and X² are S or O.

13. The coating composition of claim 12, wherein X¹ and X² are S.

14. The coating composition of any one of claims 1 to 13, wherein X³ and X⁴ are O.

1 5. The coating composition of any one of claims 1 to 14, wherein X⁵ and X⁶ are NR⁹.

16. The coating composition of claim 12, wherein R⁹ is H.

1 7. The coating composition of any one of claims 1 to 14, wherein the bis-silane is a compound of Formula (la):

(R⁷0)₃Si -(CR³R⁴)_m -NH-C(0)-S-(CR¹R²),-S-C(0)-NH-(CR⁵R⁶)_n-Si(OR⁸)₃

(la)

wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, and R⁸ are each independently selected from the group consisting of:

H, optionally substituted Ci-Ce alkyl, optionally substituted C|-C₆ cycloalkyl, optionally substituted C₂- C alkenyl, optionally substituted C₂-Ce cycloalkenyl, optionally substituted C₂-C₆ alkynyl, optionally substituted C₂-Ce cycloalkynyl, and optionally substituted aryl; and

I, m, and n are integers independently selected from the group consisting of: 1 , 2, 3, 4, 5, and 6.

18. The coating composition of any one of claims 1 to 15, wherein the bis-silane is a compound of Formula (lb):

(EtO),Si -(CH₂)₃ -NH-C(0)-S-(CH₂)₂-S-C(0)-NH-(CH₂),-Si(OEt)j

(lb)

19. A method of coating a metal article to improve the corrosion resistance thereof, the method comprising: providing a metal article to be coated;

providing a coating composition according to any one of claims 1 to 18;

if necessary, at least partially hydrolysing the bis-silane in the coating composition;

contacting the coating composition and the metal article under conditions that result in at least part of the surface of the metal article being coated with the coating composition; and curing the coating composition to form a corrosion resistant coating on said at least part of the surface of the metal article.

20. The method of claim 19, wherein the metal article is an aluminium or aluminium alloy containing article.

21. The method of claim 20, wherein the metal article contains AA2024-T3 aluminium alloy.

22. The method of any one of claims 19 to 21 , wherein the bis-silane is is a compound of Formula (lb): (EtO)₃Si -(CH₂), -NH-C(0)-S-(CH₂)2-S-C(0)-NH-(CH₂)3-Si(OEt)₃

(lb)

23. The method of claim 22, comprising a step of hydrolysing the bis-silane.

24. The method of claim 23, wherein the bis-silane is hydrolysed at ambient temperatui pressure for a period of at least 72 hours.

25. A coated metal article produced by the method of any one of claims 19 to 24.