Patent Application
20150010751
The present invention relates to method and apparatus for protecting a metal substrate from corrosion via a sol-gel derived coating including polyaniline.
Magnesium and aluminium are exceptionally lightweight metals, with densities of 1.74 g/cm3 and 2.7 g/cm3, being 4.5 and 2.9 times less dense than steel respectively. Accordingly, these metals and their alloys are promising materials for a wide range of applications where weight reduction is important, such as the automotive and aerospace sectors, in order to lower fuel consumption and reduced CO2 emissions for example. Alloys of both materials have good mechanical properties, especially aluminium alloys containing copper and magnesium and magnesium alloys containing manganese, aluminium, zinc, zirconium and rare earths. However, the application of these metals and their alloys has been somewhat limited for many engineering applications due largely to their poor corrosion resistance, especially in chloride-containing aqueous electrolytes.
Historically passive metals such as magnesium, aluminium and zinc have relied upon the use of passivation treatments or primer coatings containing chromium compounds to impart corrosion resistance. Unfortunately, these treatments are toxic and carcinogenic and are to be banned in a large number of countries. Alternative chromium-free surface treatments or coatings such as, laser cladding, thermal spraying, physical vapour deposition, anodizing, conversion coating, and organic coatings have been explored in order to improve the corrosion resistance with limited universal success.
Sol-gel technology is fast becoming a recognised approach for producing an alternative environmentally-acceptable anticorrosion or functional coating, where the major advantage of hybrid sol-gel systems over conventional ceramic-based sol-gel coatings is that crack-free, thick coatings (above 10 μm), with controlled composition and morphology, can be fanned at low cure temperatures, in particular down to room temperature (H. M. Wang, R. Akid, Corr. Sci., 49 (2007) 449). In addition, sol-gel technology also provides the advantages of low cost, simple formulation chemistry and flexibility of application method for coating complex geometries, e.g., dip, spray, etc. To this end several attempts to apply sol-gel films on Aluminium and Magnesium-based alloys have been recently reported, particularly for the highly reactive structural alloys ‘AA2024’ ‘AZ31, in particular but not exclusively; Hamdy, Mat. Lettrs 60(21-22) (2006), 2633, Yue, J. Rare Earths, 25 (2), (2007), 193, Rajath Varma, Surf. Coat Technology, 204(3) (2009), 277, Voevodin, Surf. Coat Technology, 140(1), (2001), 29, Rosero-Navarro, Surf. Coat Technology, 203(13), (2009), 1897; U.S. Pat. No. 6,777,094; U.S. Pat. No. 7,011,719; R. Supplit, T. Koch, U. Schubert, Corr. Sci. 49 (2007) 3015; F. Zucchi, V. Grassi, A. Frignani, C. Monticelli, G. Trabanelli, Surf. Coat. Technol. 200 (2006) 4136; A. N. Khramov, V. N. Balbyshev, L. S. Kasten, R. A. Mantz, Thin Solid Films 514 (2006) 17; A. L. K. Tan, A. M. Soutar, I. F. Annergren, Y. N. Liu, Surf. Coat. Technol. 198 (2005) 478; M. F. Montemor, M. G. S. Ferreira, Electrochim. Acta 52 (2007) 7486; S. V. Lamaka et al. Electrochim. Acta 53 (2008) 4773. However, sol-gel coatings themselves do not exhibit the required anticorrosion properties for the passive metals and other lightweight metals susceptible to corrosion.
There is therefore a need for an effective anticorrosion treatment for lightweight metals that are particularly susceptible to corrosion that does not comprise the integrity of the metal, have adverse side affects and provides long lasting protection of the metal within a variety of different environments.
The inventors have identified that a sol-gel derived coating incorporating polyaniline is effective to protect an underlying metal substrate from corrosion.
Polyaniline (PANI) is a conductive polymer which has unique electrical and optical properties; is relatively cheap, easy to synthesize and stable under a wide variety of experimental conditions. Following initial reports that polyaniline provides good corrosion protection to stainless steel (David W. DeBerry, J. Electrochem. Soc. 132(5) (1985) 1022), polyaniline has been investigated as an additive in corrosion protection systems for ferrous alloy substrates and for the corrosion protection of the light alloys, e.g., aluminium (S Sathiyanarayanan, S Azim, G Venkatachari, J. App. Poly. Sci. 107(4) (2008) 2224; G Williams, H N McMurray, Electrochim. Acta 54 (2009) 4245).
According to a first aspect of the present invention there is a metal substrate comprising: an acid catalysed organic-inorganic hybrid sol-gel derived coating chemically bonded to the substrate; the coating comprising a polysiloxane based network having a Si—O backbone; and polyaniline dispersed within the coating.
In particular, the coating may be divided into at least two compositionally different regions including an outer region relative to the substrate and an intermediate bonding region positioned between the outer region and the substrate; the intermediate region comprising a composition resultant from the reaction of the coating and the substrate relative to a composition of the outer region; the coating chemically bonded to the substrate via the intermediate region.
According to a second aspect of the present invention there is provided a substrate comprising: a metal base layer; an acid catalysed organic-inorganic hybrid sol-gel derived coating chemically bonded to the substrate, the coating having a polysiloxane based network comprising a Si—O backbone; and polyaniline dispersed within the coating
According to a third aspect of the present invention there is provided a method of protecting a metal substrate from corrosion comprising: generating an acid catalysed organic-inorganic hybrid sol-gel; providing a polyaniline solution; combining the sol-gel with the polyaniline solution; incorporating a polysiloxane within the organic-inorganic hybrid sol-gel, the polyaniline solution or the combined sol-gel and polyaniline solution; coating the substrate with the polysiloxane sol-gel polyaniline solution; and curing the coating at the substrate, such that the coating is chemically bonded to the substrate, the coating comprising a polysiloxane based network having a Si—O backbone.
Optionally, the method comprises activating the anticorrosion properties of the coating by subjecting the coating to alkaline conditions.
In particular, the pH of the initial sol-gel may typically be less than 6 and is preferably less than 3.
The resultant coating may be considered to be divided into at least two regions or ‘layers’ including an outer layer relative to the coating and an intermediate bonding layer positioned between the outer layer and the substrate. Reference to ‘regions’ or ‘layers’ within the specification refers to in the region of the coating that comprises a composition resultant from the reaction of the sol-gel layer with the substrate. This region may be considered to have a smooth or tapering compositional change with regard to the remaining coating region in a direction away from the substrate and towards the solid-air interface. Reference to an ‘intermediate layer’ is not restricted to a compositional different layer having defined compositional boundaries with respect to other regions of the coating. That is, the concentration of the composition (resultant from the reaction of the sol-gel layer with the substrate) may decrease smoothly in a non-stepwise manner away from the substrate towards the solid-air interface of the coating, perpendicular to the substrate. In particular, the coating may comprise a smooth compositional change from the ‘outer layer’ through the ‘intermediate layer’ such that the regions diffuse into one another or merge seamlessly with regard to their composition. Accordingly, in certain embodiments there is no defined boundary or interface between the ‘layers’.
In particular, the sol-gel polyaniline hybrid system may be considered to offer a ‘self-healing’ capacity at damaged sites in the coating in turn providing comprehensive anticorrosion properties in a wide variety of otherwise corrosive environments.
Optionally, the volume ratio of the polyaniline sol to the sol-gel is in the range 20:1 to 1:1.
Optionally, the step of activating the anticorrosion properties comprises applying an alkaline electrolyte solution to the coating and allowing the solution to diffuse into the coating. In particular a saline solution may be used for activation wherein the substrate and coating are dipped into the saline solution or the saline solution is applied, in particular sprayed onto the coating so as to diffuse through the coating thickness to the underlying substrate.
Preferably, the method and apparatus further comprises doping the polyaniline solution with silica nano-particles prior to combining the sol-gel and the polyaniline solution.
Dispersing silica nano-particles within the coating has been found to increase the scratch resistance of the coating and to increase adhesion at the substrate when exposed to mechanical shearing forces.
Optionally, the method may comprise pre-treating the substrate prior to coating with the sol-gel and polyaniline solution by any one or a combination of the following: applying one or more chemical compounds to the substrate; applying mechanical abrasion to the substrate to roughen the substrate surface.
Optionally, the sol-gel may comprise any one or a combination of the following: a silica-sol; a titanium sol and/or a zirconia sol. Optionally, the sol-gel comprises an organic-inorganic hybrid sol. The hybrid sol-gel is typically formed from at least one organic precursor and at least one inorganic precursor such that the organic and inorganic components are hybridised in the resulting coating structure. That is, the organic and inorganic components are chemically bonded together to create a fully hybridised organic-inorganic structure. The inventors have identified that forming the sol-gel from organic and inorganic precursors enables control of the coating thickness and the required curing temperature. In particular, the present coating may be room temperature cured. The curing temperature is determined, in part, by selecting the type and relative concentrations of the organic and inorganic sol-gel precursors. Using a hybrid inorganic-organic sol-gel system also increases the available methods of coating the substrate.
The inventors provide a coating that is optimised to allow the creation of a thick coating that adheres firmly to the substrate. This is due, in part, to the coating comprising enhanced flexibility over known systems by incorporation of the polysiloxane component.
The inventors have realised that by forming the coating via a sol-gel process using and/or incorporating a polysiloxane, optionally with further precursors such as a silane and/or a silicate, a porous network is created that is facilitates mobility of the polyaniline once the coating is cured at the substrate. Utilising a polysiloxane precursor, in contrast to a siloxane monomer is advantageous for a number of reasons. In particular, the polysiloxane precursor provides control of coating thickness; increased bonding strength to the substrate; improved flexibility of the coating; a controlled porosity of the network; tailoring of curing temperature and hydrophobicity; and a crack free, non-brittle coating.
The polysiloxane effectively reduces the extent of the condensation reaction and accordingly loss of solvent during the sol-gel process. This provides enhanced coating flexibility and a crack free, non-brittle structure. The coating thickness is controlled as the polysiloxane precursor may be readily cross linked with other components of the system forming part of the resultant network. Cross linking agents and curing agents may be incorporated at the sol-gel stage so as to facilitate network formation during gelation.
The long-chain polysiloxane, being substantially linear, enables control of the porosity. Utilising linear polysiloxane also facilitates cross linking between the Si—O backbone. The physical, mechanical and/or electrochemical characteristics of the coating may be tailored by variation of any one or a combination of i) the concentration of polysiloxane in the coating/sol-gel (and the resulting coating network); ii) the chain length of the polysiloxane; and iii) the extent and nature of functional groups extending from the polysiloxane.
The polysiloxane may comprise function groups/or functionalised side chains extending from the main Si—O backbone. These functional groups and side chains may comprise any oxygen or nitrogen based groups with functionalised side chains comprising for example, acrylic, epoxy or other functionalised groups including organosilanes and/or hybrid organic-inorganic silicate, siloxane and silane compounds.
Optionally, the polysiloxane backbone may comprise the following structure:
—O—[—Si(R)2—O—Si(R)(R1)—]m—O—[—Si(R)2—O—Si(R)(R2)—]n—O—
wherein R, R1 and R2 are organic groups and/or organic polymers and m is 5 to 1000 and n is 5 to 1000.
Preferably, the —Si—O—Si—O—Si—O— bonding is repeated and linear
The synergistic combination of the Si—O and Si—C bonds provides for the possibility of creating thick porous coatings of the order of 100 μm or multiple 100s of μm.
Preferably, the thickness of the coating on the substrate is greater than 0.1 μm. Preferably, the thickness of the coating is between 0.1 μm to 100 μm. According to specific implementations, the intermediate region may comprise a thickness in the range 5 to 100 nm. Alternatively, the intermediate region may comprise a thickness in the range 10 to 50 nm.
Optionally, the coating may comprise a dopant species captured or chemically bonded at the hybrid organic-inorganic oxide network. In particular, the present coating is preferably formed by incorporating nano particles at the sol-gel stage of coating formation. The nano particles may comprise a silane, a silicate and/or other dopant particles such as γ-Al2O3 and hydroxyapatite. The linear polysiloxanes, within the network are preferably chemically bonded to one another by cross linking agents. The cross linking agents may comprise non-functionalised organic hydrocarbons or functionalised hydrocarbons or other organic, inorganic and/or organic-inorganic cross linking agents. The nano particles incorporated in the sol-gel phase chemically bond to the polysiloxane during the condensation process. The resulting network comprises substantially linear polysiloxane with Si—O repeating units and organic side chains extending from the main Si—O backbone. The organic side chains may comprise any alkyl, aryl and/or mixed alkyl-aryl groups. These alkyl or aryl groups may be substituted with additional functionalised groups along the Si—O backbone, where the functionalised groups comprises any elements selected from periodic table groups 5 to 7 including in particular nitrogen, phosphorus, oxygen, sulphur and chlorine.
Where the organic side group, directly bonded to the Si—O backbone is alkyl, the alkyl group may comprise between 1 to 20 carbon atoms. Optionally, the alkyl, aryl and/or mixed alkyl-aryl groups that are attached directly to the Si—O backbone may be functionalised by comprising nitrogen, phosphorous, oxygen, sulphur and/or chlorine atoms. Optionally, the Si—O backbone may comprise at least one functional side chain bonded directly to either the Si—O backbone or at least one of the alkyl, aryl or alkyl-aryl side groups.
According to the preferred implementation, the polysiloxane is substantially linear. Alternatively, the polysiloxane may be branched at more than one region along the length of the main Si—O backbone. Optionally, the polysiloxane may be a copolymer and in particular an alternating, periodic, statistical, random, block, branched, linear, or graft copolymer formed from polysiloxanes as described herein.
Further, the polysiloxane component may comprise a form of polysiloxane including in particular an organopolysiloxane.
In addition to polysiloxane, the present coating may also comprise anyone or a combination of the following additional precursors incorporated within the coating network during the sol-gel phase: any organically modified silane selected from the group consisting of alkylsilanes; methyltrimethoxysilane; methyltriethoxysilane; dimethyldiethoxysilane; trimethylethoxysilane; vinyltrimethoxysilane; vinyltriethoxysilane; ethyltriethoxysilane; isopropyltriethoxysilane; butyltriethoxysilane; octyltriethoxysilane; dodecyltriethoxysilane; octadecyltriethoxysilane; aryl-functional silanes; phenyltriethoxysilane; aminosilanes; aminopropyltriethoxysilane; aminophenyltrimethoxysilane; aminopropyltrimethoxysilane; acrylate functional silanes; methacrylate-functional silanes; acryloxypropyltrimethoxysilane; carboxylate; phosphonate; ester; sulfonate; isocyanate; epoxy functional silanes; chlorosilanes; chlorotrimethylsilane; chlorotriethylsilane; chlorotrihexylsilane; dichlorodimethylsilane; trichloromethylsilane; N,O-Bis(trimethylsilyl)-acetamide (BSA); N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA); hexamethyldisilazane (HMDS); N-methyltrimethylsilyltrifluoroacetamide (MSTFA); N-methyl-N-(t-butyldimethylsilyl)trifluoroacetamide (MTBSTFA); trimethylchlorosilane (TMCS); trimethylsilyimidazole (TMSI); and combinations thereof.
The polysiloxane may comprise any one or a combination of the following groups: an alkyl; a substituted alkyl, a halosubstituted, an alkenyl, an alkynyl, a halosubstituted alkynyl, a phenyl, a substituted phenyl, a hydroxylic compound. In particular, the polysiloxane may comprise an organofunctionalised group including in particular a hydroxyl, epoxy alkoxy, silanol, amino or isocyanate group. The polysiloxane may comprise a single repeat unit or may be formed as a two, three, four or five component polysiloxane having different respective repeater units forming part of the Si—O part of the backbone. Specifically, and my way of example, the polysiloxane may comprise any one or a combination of the following compounds: Poly[dimethylsiloxane-co-[3-(2-(2-hydroxyethoxy)ethoxy)propyl]methylsiloxane]; Poly(dimethylsiloxane), bis(3-aminopropyl) terminated; Poly(dimethylsiloxane), diglycidyl ether terminated; Poly(dimethylsiloxane)-graft-polyacrylates; Poly[dimethylsiloxane-co-methyl(3-hydroxypropyl)siloxane]-graft-tetrakis(1,2-butylene glycol); Poly[dimethylsiloxane-co-(2-(3,4-epoxycyclohexypethypmethylsiloxane]; Poly[dimethylsiloxane-co-(3-aminopropyl)methylsiloxane; Poly[dimethylsiloxane-co-methyl(stearoyloxyalkyl)siloxane]; Poly[dimethylsiloxane-co-[3-(2-(2-hydroxyethoxy)ethoxy)propyl]methylsiloxane]; (methacryloxypropyl)methylsiloxane-dimethylsiloxane copolymer; and/or (3-hydroxypropyl)methylsiloxane-dimethylsiloxane copolymer.
The polysiloxane preferably comprises a minimum repeat number of ten and may comprise ten, a hundred, a thousand or tens of thousands of repeat units within a single polymer backbone.
The term ‘alkyl’ refers to a linear, branched, cyclic, or any combination thereof hydrocarbon. The term ‘substituted alkyl’ refers to one or more of the hydrogens on the alkyl group being replaced by another substituent, such as cyano, alkyl, nitro, mercapto, alkylthio, halo, alkylamino, dialkylamino, alkoxy, and tri alkoxysilyl. The term ‘Substituted phenyl’ refers to one or more of the hydrogens on the aromatic ring being replaced by another substituent, such as cyano, alkyl, nitro, mercapto, alkylthio, halo, alkylamino, dialkylamino, and alkoxy.
Optionally, the present coating may also comprise any one or a combination of the following additional precursors: tetraethoxy orthosilicate (TEOS); methyltriethoxy orthosilicate (MTEOS); phenyltriethoxy orthosilicate (PTEOS); octyltriethoxy orthosilicate (OTEOS); dimethyldiethoxy orthosilicate (DMDEOS); methyltrimethoxy orthosilicate (MTMOS); phenyltrimethoxy orthosilicate (PTMOS); tetramethoxy orthosilicate (TMOS). Optionally, the step of curing the coating comprises curing the coating approximately at room temperature. Optionally, the step of curing the coating may comprise heating the coating above room temperature and below 350° C.
Optionally, the metal substrate comprises any relatively lightweight metal and metal alloy and particularly those used for the aerospace and aviation industries. In particular, the metal substrate may comprise any one or a combination of the following: aluminium; aluminium alloy; magnesium; magnesium alloy; steel; 2024 aluminium; stainless steel; zinc or zinc alloy or titanium or titanium alloy.
Importantly, the polyaniline is dispersed within the as-formed coating such that the majority, if not all of the polyaniline is not chemically bonded to the organic-inorganic network and is therefore mobile within the coating structure. Additionally, the polyaniline component is not polymerised or is not formed as a composite within the coating which would otherwise inhibit its mobility and/or diffusion capability within the coating. Additionally, the polyaniline, being uncoordinated within the coating, is capable of undergoing redox reactions and in turn providing the anti-corrosion property.
The present coating finds particular application for coating an aircraft fuselage and other aircraft components. The coating is also suitable to coat other substrates exposed to weathering corrosion including marine vessels and structures.
A specific implementation of the present invention will now be described, by way of example only and with reference to the accompanying figures in which:
a is a high resolution XPS N s1 core level spectra for aluminium AA 2024 coated with a polyaniline coating;
b is a high resolution XPS N s1 core level spectra for glass with a polyaniline coating;
c illustrates XPS high resolution Al 2p core level spectra with sputtering time;
d illustrates individual Al 2p core level spectra across the substrate and coating interface;
Preparation of Hybrid Sol-Gel Coatings
Chemical oxidation of aniline was performed by oxidation with ammonium persulphate in 1 M hydrochloric acid solution. 5.0 ml of 0.107 M aniline was dissolved in 300 ml of 1 M HCl at 0 to 5° C. and stirred for 1 hour. A solution of 5.6 g ammonium persulphate in 1 M 100 ml HCl was added dropwise to aniline solution over a period of 15 min with vigorous stirring. The solution was then continually stirred at 0 to 5° C. for 6 hours. The precipitate was collected with a Buchner funnel and washed with four portions of 50 ml 1 M HCl. The precipitate was dried at 60° C. for 24 hours, the HCl doped PANI (Emeraldine Salt, ES) was obtained as a green powder. The basic form of PANI (Emeraldine Base, EB) was obtained by stirring the ES with 1 M NH4OH for 3 hours followed by filtration and drying. Organic-inorganic matrix silica-based sols, catalysed by HNO3 acid, were prepared and used. The mixture of the PANI-doped sol was prepared in two ratio levels, denoted low and high. To increase the scratch resistance of the hybrid sol-gel/PANI coating, 1.0 wt % silica nano-particles (10 to 20 nm) were added in the final sol.
Die-cast magnesium alloy AZ31 (containing approx. 3 wt. % Al and approx. 1 wt. % Zn) was immersed in an acidified Industrial Kleen IKB 501 cleaning solution (Concoat Limited Company) for about 30 to 60 seconds to clean the surface. The coating was then applied to one side of the AZ31 alloy surface using a spray method. The coated magnesium alloy was oven dried in air at 75° C. for 5 min and then spray-coated again. Finally the coated AZ31 sample was left in the oven to be dried in air at 75° C. for no less than 3 hours. For comparison, one commercially available pre-treatment/coating (‘Alodine 1200’ pretreatment solution (Henkel) and three different sol-gel samples were investigated; i.e. sol-gel coatings without PANI (denoted as SGO), sol-gel coatings with high doped ratio of PANI (denoted as SGH), and sol-gel coatings with low doped ratio of PANI (labelled as SGL).
Experimental Techniques
Coatings were immersed in either 3.5 wt. % NaCl solution or dilute Harrison's solution (0.35 wt. % (NH4)2SO4 and 0.05 wt. % NaCl) at 20° C. Corrosion tests were carried out using a conventional three-electrode cell arrangement with the sample (area: 41 cm2) as the working electrode, a ‘saturated’ calomel reference electrode and a platinum counter electrode. Electrochemical impedance measurements were obtained at the measured open circuit potential (Eocp) values applying ±10 mV perturbation, in the frequency range 105 to 10−2 Hz. EIS data was recorded for samples immersed in Harrison's or 3.5 wt. % NaCl solution.
Salt spray testing (ASTM B 117) was carried out at 35° C. in a 5 wt. % NaCl spray at 100% humidity. Images were recorded every 120 hours to compare the performance of samples throughout the test period. To assess adhesion a sellotape™ test was carried out as follows: the samples were scratched using a razor blade to form a cross hatch. Sellotape™ tape was applied to the cross-hatch and fixed well before peeling it from the surface. The sellotape™ was then examined for signs of detachment of the coating. The thickness of the coating was estimated through the cross section image of the coated sample using a scanning electron microscope, Philips XL40, at 20 kV. Thin sputtered carbon films were applied to coatings in order to prevent surface charging during SEM examination.
Corrosion Assessment of Bare and Coated AZ31
Corrosion Characteristics of AZ31 Magnesium Alloys
Bare AZ31 showed significant corrosion activity within several minutes after immersion in the 3.5 wt. % NaCl solution as seen by the generation of hydrogen bubbles. Characteristic filiform corrosion was observed on samples subject to a commercial surface pretreatment, notably ‘Alodine 1200’ chromate conversion coating. Optical photos of typical surfaces pre-treated with the ‘Alodine’ conversion coating are shown in
Corrosion Behaviour of the Sol-Gel Coating without PANI (SGO)
A non-doped (without PANI) sol-gel coating on AZ31 showed significant corrosion activity, particularly at defect sites in the coating where hydrogen bubbles were observed at the surface of the coating. This behaviour was typical when inhibitor-free sol-gel coatings were immersed in 3.5% NaCl solution for just 1 to 2 hours. For this reason it is not possible to carry out EIS testing in 3.5% NaCl hence a dilute Harrison's solution was chosen to assess the corrosion behaviour of this particular coating system.
On immersion in Harrison's solution filiform corrosion 201 is observed beneath the SGO coating after only 2 to 5 hours, as shown in
Corrosion Resistance of the Hybrid Sol-Gel Coating with High-Ratio of PANI (SGH)
It was noted that the high PANI ratio SGH coating suffered from poor adhesion of the coating to the magnesium alloy substrate, resulting in delamination after 24 hours immersion in 3.5 wt. % NaCl solution. Despite this poor adhesion, it was considered worthwhile to explore the “self-repairing” functionality of the doped PANI using dilute Harrison's solution. Referring to
In summary, the SGH coating provides good corrosion protection when immersed in Harrison's solution; however, it exhibited poor adhesion particularly when immersed in a severely corrosive environment such as 3.5 wt. % NaCl solution indicating that the high content of PANI in the composition of the coating hinders the formation of a chemical bond to the substrate which has been observed through the network of the silica-based sol-gel matrix. On this basis, the concentration of PANI in the coating was reduced and adjusted to an appropriate ratio in order to obtain a satisfactory chemical bond to the substrate.
Corrosion Performance of the Sol-Gel Coating with Relatively Low-Ratio Doped PANI (SGL)
Evolution of Bode plots for the SGL hybrid sol-gel coating is shown in
Detailed interpretation of the EIS spectra was performed by numerical fitting of the Bode plots using the equivalent circuits presented in
The corrosion resistance of this sol-gel coating has also been demonstrated by conducting salt spray tests at 35° C. within a 5 wt. % NaCl spray at 100% humidity.
SEM Characterisation
A back-scatter electron image of the cross section of the hybrid sol-gel coating is shown in
Adhesion and Scratch Resistance
The adhesion of the sol-gel coating was assessed following 500 hours immersion in 3.5 wt % NaCl solution by using the sellotape™ test previously mentioned. No signs of detachment of the coating were observed, indicating good adhesion of the coating to the substrate. The coating also showed a pencil hardness greater than 6H with good scratch resistance. This is comparable with a commercial fusion bonded epoxy powder coating.
Corrosion Assessment of 2024 Aluminium Alloy (AA2024)
Activation of the Polyaniline
Examination of the sol-gel and polyaniline coating on AA2024 revealed pitting in the scratched area during salt spray testing. However, no pits appeared away from the scratched area. Images of saline solution treatment of samples and visual inspection noted that the pits initiated in the first few hrs (before 72 hrs) then propagated at these positions as time elapsed without the development of undercutting or delamination. Importantly, no new pits appeared after 72 hrs. These results suggest the coating has one or more of the following properties: i) the coating has a cathodic potential with respect to aluminium 2024; ii) the coating acts as a barrier coating; and iii) the assumed passive 704 film was not completely formed before being subject to treatment by saline solution.
If the coating is cathodic to the substrate or has barrier properties, pitting, general corrosion, undercut and delamination would appear at the scratch as a result of the salt electrolyte treatment however, none of these phenomena occurred. Moreover, scratch test results showed that the coating exhibits “self-healing” behaviour.
The third property above would appear to be the most plausible. To investigate this a saline solution test was conducted as follow: the sol-gel and polyaniline coating (on an aluminium substrate) sample was pre-immersed in 3.5% NaCl solution for 5 day then scratched before exposure to saline solution for 72 hrs. From
This result combined with immersion test results suggest that the present sol-gel-polyaniline coatings require a few days of immersion in a solution to be “activated” so as to form a protective layer at the coating/metal interface. The inventors suggest that this behaviour occurs when sufficient electrolyte is present at the metal/polymer interface.
The post-treatment procedure with an alkaline electrolyte appears to have two advantages: i) it heals any defects that are produced during the spray deposition of the coating by forming a homogeneous thin film; and ii) it enhances the initial corrosion resistance of the sol-gel polyaniline coating.
Visual inspection of the post-treated sample noted a change in the sample colour from blue (EB) to greenish blue (ES) after the post-treatment process. This change in colour may be due to a change in oxidation state of polyaniline from the base form to the doped form.
Furthermore, the electrical conductivity of the polyaniline after immersion in deionised water at 60° C. for 5 min was significantly improved, 10−6 S·cm−1 with respect to that before immersion (10−9 S·cm−1). A similar value of conductivity was obtained when a free standing film of polyaniline was immersed in 1% NaCl solution for 3 days. XPS analysis of the free standing film of polyaniline, after immersion in 1% NaCl for 3 days, identified that the change in electrical conductivity was mainly due to an increase of doped species (—NH+) at the expense of N═C species i.e.[—NH+]/[N═C] changes from 0.3 to 2.7.
The Interfacial Passive Layer
XPS results for the coated pure and 2024 aluminium alloy, identified three components of aluminium. Two of these components, notably aluminium metal and aluminium oxide, were detected at all stages of depth profile. However, a new component (Al—X) was detected only at the coating-substrate interface at a binding energy of 74.6±0.15 eV fwhm=1.5±0.1. Furthermore, the concentration of Al—X appears to increase as the depth profile progresses from the aluminium substrate towards the coating. It is suggested that the compound formed at the coating-substrate interface 704 is, in part, responsible for corrosion protection. The binding energy of 74.6±0.15 eV may relate to one or more of the following: Al2O3; Chemisorbed oxygen on Al surface; Substoichiometric aluminum oxide due to ion bombardment; AlOOOH; Al—O—C; Al—O—N complex;
Based on XPS results, it is possible that one or more of the above compounds can exist at the coating-substrate interface 704. Polyaniline (EB) consists of two group; an oxidised group (containing N═C) and a reduced group (containing N—C), the oxidised group is significantly more reactive with metal atoms than the reduced group. Therefore, reaction between polyaniline and the aluminium substrate is most likely via this oxidised group. This is supported by a change of relative concentration of [N═C]/[N—C] across the coating-substrate interface where N—C concentration increased as the depth profile goes towards aluminium, at the expense of N═C. The imine group, N═C, seems to be reduced to a state equivalent to an amine group, N—C.
To elucidate the self-healing functionality of the present coating, XPS analysis was carried out on aluminium and glass substrates. The high resolution N1s core level of PANI coated AA2024 shown in
To detect any change in PANI oxidation state related to interaction with aluminium substrate, the high resolution XPS N s1 core level of PANI coated 2024 (
The XPS results suggest that PANI is reduced when applied to the Al substrate. This postulation is not valid unless the Al substrate or other species, in the PANI coating system, is oxidised to complete the electrochemical reaction. Therefore in order to study the Al substrate beneath the PANI coating, XPS depth profiling was conducted. Pure aluminium (99.99) was used to avoid any other interaction between the PANI and the substrate alloying elements. Aluminium was evaporated under vacuum (10−5 torr) on a free-standing film of PANI (EB) at 60±5° C. The evaporation chamber was left under vacuum for one hour before evaporation to remove surface gases. The evaporated aluminium thickness was adjusted to ≈50 nm and the XPS depth profile was carried out from the aluminium side to polymer.
The change of the Al 2p core level with sputtering time is shown in
Detailed analysis of individual Al 2p core level spectra, as shown in
Moreover, it can be seen that the oxygen content gradually decreases from the surface to the PANI/Al interface region where it increases sharply and then decreases to a near zero value. Correspondingly the aluminium content decreases gradually at the interface region while, the carbon and nitrogen content gradually increases from the interface up to the end of the sputtering process, where the sum of their content was over 99%. Note: carbon appears on the surface of the aluminium before sputtering, however, it is not present after sputtering and is an artefact being due to atmospheric contamination.
Accordingly, the inventors suggest that polyaniline (EB) interacts with aluminium to form a complex layer or region resultant from the reaction between the underlying substrate and the polyaniline and/or sol-gel. The oxidised part of polyaniline (N═C) interacted with Al atoms creating an equivalent state to the amine group. This interaction seems to be enhanced by ionic species that diffused in the coating-substrate interface.
The inventors have determined that polyaniline can protect AA2024. However, it is easily delaminated from the metal substrate. Similarly, a silica sol-gel coating only provides corrosion protection when inhibitors are present in the sol-gel. However by combining both polyaniline and an organic-inorganic hybrid sol-gel it is possible to successfully prepare and deposit a highly protective coating on AA2024. The corrosion protection performance of the coating was found to be dependent upon the ratio of the PANI/sol-gel ratio. An EIS study showed that the PANI/sol-gel combination successfully protected AA2024 in both acidic and neutral 3.5% NaCl solutions for long periods up to 2 and 24 months respectively. Self-healing behaviour was noted for samples subjected to damage in the form of a scratched surface with SVET and EIS analysis showing recovery of the corrosion resistance within hours of the coating being damaged. The corrosion performance of the coating system is concluded to be due to the unique combination of the barrier properties of the sol-gel, the redox capacity of the polyaniline and the formation of a new nanolayer at the PANI/sol-gel and substrate interface.
Preparation of Hybrid Sol-Gel Coatings Containing a Polysiloxane
The organic-inorganic hybrid coating was prepared according to Example 1. However in place of silica nano-particles, polysiloxane [poly(dimethylsiloxane) (PDMS)] was added to the hybrid sol-gel prior to gelation and combination with the PANI solution.
Preparation of Hybrid Sol-Gel Coatings Containing a Polysiloxane
The organic-inorganic hybrid coating was prepared according to Example 1. However in place of silica nano-particles, (methacryloxypropyl)methylsiloxane-dimethylsiloxane copolymers was added to the hybrid organic-inorganic sol-gel prior to combination with the PANI solution.
Preparation of Hybrid Sol-Gel Coatings Containing a Polysiloxane
The organic-inorganic hybrid coating was prepared according to Example 1. However, the hybrid sol was prepared by mixing the silane-based precursors and (3-hydroxypropyl)methylsiloxane-dimethylsiloxane copolymers before adding the water and acid to start the hydrolysis and condensation. The hybrid sol-gel with the polysiloxane was then combined with the PANI solution to form a coating mixture prior to final coating on the substrate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1203711.5 | Mar 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/050494 | 2/28/2013 | WO | 00 |
