201 (2007) 5973–5978www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Copper modified chitosan for protection of AA-2024

O. Lundvall b, M. Gulppi a, M.A. Paez a,⁎, E. Gonzalez a,J.H. Zagal a, J. Pavez a, G.E. Thompson c

a Facultad de Química y Biología, Departamento de Química de los Materiales and Departamento de Ciencias Química,Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, Chile

b KTH Industrial Engineering and Management, Royal Institute of Tecnology, Stockholm, Swedenc Corrosion and Protection Centre, University of Manchester, M60 1QD, Manchester, UK

Received 22 May 2006; accepted in revised form 8 November 2006Available online 12 December 2006

Abstract

The aim of this study is to examine chitosan, which is a good film former, as a corrosion protective coating for AA-2024-T3 aluminium alloy.The aluminium samples were first anodized to increase adhesion of the subsequently dip coated film from a chitosan–acetic acid solution. Tofurther increase the protective ability of the chitosan coating, the coated samples were immersed in a copper ion solution for 24 h. The copper saltsused were sulphate and acetate. The chitosan membranes exposed to copper ion solutions revealed a reduced permeability in comparison with theunexposed samples, and an increase in stability in aqueous solutions, as revealed by a steady state and a high open circuit potential in boraxsolution. From UV–Vis spectroscopic measurement of the film on ITO glasses, the reduction in permeability of the chitosan film modified bycopper ions appears to be associated with copper cross linking within the chitosan structure.© 2006 Elsevier B.V. All rights reserved.

Keywords: Aluminium; Chitosan; Corrosion protection

1. Introduction

Aluminium alloy AA-2024-T3 is extensively used in theaerospace industry where the strength-to-weight ratio andrelatively low cost are considered paramount [1,2]. In generalthe 2000 series alloys present susceptibility to localizedcorrosion associated with the behaviour of the alloying elements,mainly copper, during surface treatments [3,4]. Further, thepresence of second phase material contributes to a high cathodicactivity when the metal is immersed in an electrolyte. Both, thealloying element behaviour and the local electrochemicalprocesses at second phase regions limits chemical andelectrochemical procedures to protect the aluminium alloy. Inview of the need for compliance with the required level ofprotection against corrosion, the alloy is protected by twosequential processes: anodizing or, conversion coating, in acidelectrolytes containing chromate species, followed by applica-tion of epoxy primers and paint coatings. The chromate base

⁎ Corresponding author. Tel.: +56 2 6812575; fax: +56 2 6812108.E-mail address: mpaez@lauca.usach.cl (M.A. Paez).

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.11.005

treatments, promotes (in the case of anodizing a porous anodicfilm, and in the case of conversion coating) a scalloped film–electrolyte interface, ensuring a good adhesion of primers andcoatings. The resulting organic film, of low permeability towater and limited reduction of the fatigue strength, gives a goodcorrosion resistance to the aluminium alloy. However, chromatespecies have become the subject of increasing environmentalregulation due to their toxic and carcinogenic properties. Thus, anumber of alternative treatments have been explored by bothcommercial and academic sectors. Several alternatives are welldocumented and tested, and some of these are commerciallyavailable through chemical suppliers. Others are still in thedevelopment phase. In attempting to replace protection systemscontaining environmentally harmful ingredients, this work isfocused on the evaluation of a natural polysaccharide polymers,chitosan, as a coating to protect AA-2024-T3.

1.1. General aspects of chitosan

Chitosan is a non-toxic, biodegradable, and functionalbiopolymer consisting primarily of β linked 2-amino-2-deoxy-

Fig. 1. Diagram showing the chitosan structure of chitosan.

Fig. 2. Diagram showing two types of chitosan structures (A and B) modified bycopper.

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β-D-glucopyranose units (Fig. 1). It has been used for diverseapplications, such as substrates for drug and enzyme immobili-zation [5], separationmembranes [6], andmetal adsorbants for theremoval of Hg(II) [7,8], Cu(II) [9–12], Cr(III and VI) [13–15],Ag(I) [16], Fe(III) [17,18], Mo(VI) [19] and Cd(II) [20] fromground and waste water.

An undesirable property of chitosan as the corrosion-mitigating coating films is that it absorbs a large amount ofmoisture in an atmospheric environment, and then forms ahydrogel [21]. Thus, the emphasis of previous work has beenfocused on modifying the molecular structure chitosan byorganic or, bioorganic compounds. Some promising resultshave been reported by Sugama and Milian-Jimenez [22] withcorn-starch derived dextrine modifying chitosan. The Al-6063samples covered by the modified chitosan film, prepared from aprecursor solution with chitosan/dextrine ratio of 70/30, offereda lesser sensitivity to moisture, and improved pore resistance byan order of magnitude higher than those of the single chitosan,coatings, and conferred salt-spray resistance for 720 h.

A different way to modify the chitosan structure is by usingthe natural capability of CH to adsorb metal ions, which, aspreviously mentioned, has been used to remove differentmetallic cations from ground and waste water. Many factorsdetermine chitosan's capacity for absorption of metals includingpH, ion concentration, temperature, percent deacetylation,interaction time and chain length. According to the literature,the metallic cations bind to amines on chitosan [23]. Thenitrogen electrons present on the amino and N-acetylaminogroups can establish dative bonds with transition metal ions.Some hydroxyl groups in these biopolymers may function asdonors and can be involved in the coordination with metal ions[24]. Low pH would favour protonation of the amino sites andwould greatly diminish the metal chelating ability of chitin andchitosan, suggesting that at a neutral pH the metal ions shouldbe coordinated to a greater extend. Different models have beenproposed to explain the mechanism of coordination in theformation of complexes [25]. The called bridge model considersthe metallic ion bound to several nitrogen atoms, from the sameor, different chains [26]. The called pendant model supposes themetallic ions bound to amino group as a pendant [25,27–29].An example of the formation of a chitosan chelate with Cu ionsaccording to the pendant model is shown in Fig. 2, where twoindependent chitosan binding sites for Cu(II), create fourcoordination sites: two amines and two alcohols.

Considering the importance of reducing the permeability ofchitosan to be used in protection of metals, in this work we alsoevaluate the influence of copper incorporation in the perfor-mance of chitosan films in protecting the aluminium alloy.

1.2. Surface treatment for improving aluminium–organic adhesion

In general the environmentally friendly processes haveproblems to match the advantages of chromating in providingexcellent adhesion and corrosion protection. For alternativesurface treatments some investigations have demonstratedgood filiform corrosion properties when using anodizing inphosphoric acid [30] and sulphuric acid prior to apply organiccoating [31]. However, the former pretreatment includes in theprocedure prior to anodizing a degreasing step followed byetching in a mixture of chromic–sulphuric as the standardcleaning and pickling procedure used by Stork AerospaceAESP. On the other hand, for the anodizing pretreatmentusing sulphuric acid, its effectiveness was studied in extrudedEN AW-6060 in the artificially aged T6 condition and in anAlSi15 foundry alloy, both containing copper in the range of0.01 and 0.08 wt.%. The copper content of the alloy isrelevant to the surface treatment selected, since duringchemical cleaning and anodizing this alloying element isenriched at the alloy/film interface and locally dissolved atsurface regions of high copper content, mainly second phaseregions. The local dissolution generally comes in hand withcopper redeposition, giving rise to additional surface regionsof local high copper content.

Regarding the limitations of pretreating AA-2024 T-3, whichhas a copper content of about 5 wt.%, in this work we evaluateDC anodizing in phosphoric acid solution as a pretreatment toincrease aluminium–chitosan adhesion.

2. Experimental details

2.1. Materials

The chitosan with a degree of deacetylation of at least 85%,was obtained from Sigma–Aldrich. The 0.28 M chitosansolution was prepared by dissolving under stirring 9 g of thecompound in 200 mL of 0.1 M acetic acid at room temperature,until all particles were dissolved. This produced a clear andviscose liquid with a pH of about 4, thereafter the solution wasstored in a sealed container.

Fig. 3. Scanning electron micrograph showing the anodized aluminium alloysurface coated by chitosan.

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The AA-2024-T4 samples, provided by ENAER (Chileanaircraft industry), of a composition (wt.%) of 91.15–93.45 Al,3.8–4.9 Cu, 1.2–1.8 Mg, 0.3–0.9 Mn, 0.5 (max) Fe, 0.5 (max)Si and 0.25 (max) Zn plus trace elements, were precut with adimension of 50×20×3 mm.

2.2. Sample preparation

The aluminium samples to be modified by chitosan filmswere prepared in accordance with the following generalsequence: 1) mechanical polishing with 1200 and 2400 gritpaper; 2) anodizing in acid solution; 3) immersion in chitosansolution.

Anodizing was carried out at a constant voltage of 12 V in10% phosphoric acid for 30 min, following the Boeing process[1,2]. After anodizing, the samples were rinsed in distilled waterand dried in a cold stream.

The anodized aluminium samples were dipped into a soakingbath of chitosan solution at room temperature and kept in thesolution for 1 min, and then withdrawn slowly. After dipping inthe chitosan solution, the samples were left to hang dry for 6 h; asmooth and transparent layer, covering the metal surface, wasvisually observed. One set of these samples was thereafterimmersed in CuSO4 or Cu(CH3COO)2, of varying concentra-tion, for 24 h [4].

2.3. Adhesion and film morphology

Adhesion was measured using the ASTM D3359 standardtest, followed by its examination in an optical microscope inorder to observe the degree of peeling along the cuts.

The aluminium surfaces were examined in a JEOL 5410scanning electron microscope. The oxide–chitosan layermorphology was further studied from examination of ultra-microtomed sections in a JEOL-2000, FX-II, TransmissionElectron Microscope.

2.4. Electrochemical test

To elucidate the protective characteristic of the chitosanlayer, the open circuit potentials (OCP) of the coated anduncoated aluminium specimens we recorded for 2 or, 4 h in0.1 M Na2B4O7 (borax), pH 9.21. A borax solution was selectedas electrolyte because of the high stability of chitosan films atthat pH. In general chitosan coatings resulted unstable at pHlower than 8. The electrochemical testing was carried out in anelectrochemical cell using the aluminium samples as theworking electrode and a saturated calomel electrode as thereference electrode. Before testing, the aluminium samples werecovered in resin leaving only 1 cm2 exposed.

2.5. Spectroscopic evaluation of chitosan film modified withcopper acetate

In order to study copper incorporation into the chitosan film,spectroscopic measurements in the UV–Vis region were carriedout. The chitosan films were prepared from an aqueous solution

of the polysaccharide (10 g/L) in six Petri dishes containing ITOglasses, which were left at room temperature for 24 h to dryness.Petri dishes number 2 to 5 (number 1 was the blank) were filledwith a copper acetate aqueous solution at pH 5.4 maintainingcontact with the chitosan films for 6 h at room temperature. Theexcess of copper acetate solution was eliminated and the filmsin the Petri dishes number 2, 3, 4 and 5 were immersed for 2 h inNaOH solution at pH values 5.8, 9.8, 10.8, and 11.8 respectively.After this, the films were separated from the alkaline solutionand exhaustively rinsed with bidistilled water and dried for 24 h.The thickness of the chitosan film modified with copper acetatewas about 300 μm determined with a ruler under a lens of highmagnification, using a Leica optical microscope. UV–Visspectra of modified chitosan films coated on glass were recordedon a Cary 100E UV–Vis spectrophotometer.

3. Results and discussion

3.1. Surface and film morphologies

A scanning electron micrographs of an anodized samplesurface coated by chitosan is presented in Fig. 3. A texturedappearance, typical of chemically or, electrochemically treatedaluminium samples is revealed, indicating that the generalmorphology given by the surface treatment does not change bythe coating. Thus, in the micrograph troughs of different sizeslocated preferentially along the rolling grooves are observed,which are associated with a non-uniform surface dissolution andoxide formation during the pretreatment [1,2].

A SEM micrograph showing a section of the coated samplesis presented in Fig. 4. An undulating metal–oxide interface,with a thin aluminium oxide of about 0.7 μm in thickness, isobserved. The chitosan film is revealed as a uniform dark bandat the top of the oxide layer, with a thickness of approximately5.8 μm.

Fig. 5 shows a transmission electron micrograph of anultramicrotomed section of the anodized sample coated withchitosan. An inner textured band of oxide material of about763 nm, corresponding to a regular porous film, and an outer

Fig. 4. Scanning electron micrograph showing a section of the anodizedaluminium alloy surface coated by chitosan.

Fig. 5. Transmission electron micrograph showing an ultramicrotomed sectionof the anodized AA-2024-T-3 coated by chitosan.

Fig. 6. Open circuit potential behaviour of the differently pretreated AA-2024-T-3 alloy samples.

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dark band, of smooth appearance, are observed. The dark bandwas associated with a chitosan layer, since the carbon profile(EDX analysis) from the metal–film to the film–air interfacesdetermined a progressive increase in the carbon content and, asudden increase when reaching the film–air interface. Fromexamination of the oxide layer, a barrier layer thickness andpore diameter of 16 and 35 nm were estimated, giving ananodizing ratio of 13 Å V−1, which is in good agreement withprevious data for anodizing aluminium under the selectedconditions [2].

3.2. Adhesion of the chitosan film

In general, the adhesion test for the untreated AA-2024sheets modified with chitosan revealed a bad adhesion. Fromexamination in an optical microscope when the tape is removedthe chitosan peels off leaving nothing on the surface. In contrast,the aluminium samples modified by chitosan and preceded byanodizing revealed no peeling at all after removing the tape.This confirms the role of anodizing in increasing the aluminiumsurface–organic adhesion [30,31], which most likely isassociated with the oxide surface roughness as revealed in themicrographs of Fig. 5. The high porosity of the oxide, togetherwith pore branching, which is a feature of porous filmdevelopment on aluminium copper containing alloys, wouldfavour an increase of the surface area permitting chitosan–oxidesurface interaction, since chitosan may penetrate into the pores.On the other hand, besides the increase of the surface area forinteracting with chitosan, the role of hydroxide material at theouter layer of the alumina film is considered of majorimportance. Chitosan may interact with hydroxyl groups,which in the presence of moisture give rise to hydrogel [6]. Itis possible that the partially hydrated alumina film, covering thealuminium substrate, ensures a link between the chitosan andthe aluminium surface.

3.3. Open circuit potential (OCP)

Fig. 6 illustrates the behaviour of the open circuit potential(OCP) for the coated and uncoated anodized aluminium sample

in 0.1 M Borax solution. For the uncoated aluminium samplethe initial potential of about −0.437 V decreases rapidly to avalue of −0.480 mV. Thereafter, the OCP increases up to−0.460 V, and then, decreases progressively to −650 mV,remaining at this potential value for about 2 h until the end ofthe experiment. For the chitosan coated aluminium sample, theinitial potential, of approximately −0.265 V, remains constantfor about 40 min and then decreases and achieving a steady statevalue of −0.704 V. The more positive value for the initial OCPof the coated aluminium sample with respect to that of theuncoated samples is possibly associated with a higher resistancefor ionic conduction attributed to the presence of the chitosanfilm. On the other hand, the decrease in the OCP with theimmersion time indicates an increase in the cathodic activitypossibly related to local dissolution of the film material. For theanodized sample, local dissolution is expected to proceed at filmregions above flaws and/or, at film regions at pore base, whilefor the coated chitosan anodized sample, local dissolutionconsiders dissolution of both the chitosan layer and the oxidefilm. From the previous results, chitosan promotes an additionalprotection to the anodized aluminium sample, acting as a barrier

Fig. 7. Open circuit potential behaviour of the AA-2024 coated by chitosanmodified from different copper salt solutions, showing the influence of copperincorporation into the chitosan film.

Fig. 8. Open circuit potential behaviour of the AA-2024 coated by chitosanmodified using copper acetate solutions of different concentrations.

Fig. 9. Visible spectra of chitosan films modified with copper at different pH.

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for electrolyte incorporation into the porous film. However, it isevident that for immersion times longer that 30 min theprotection is strongly reduced, which is an expected behaviourconsidering the relatively high permeability of chitosan.Chitosan permeability is the major factor influencing thereduced protection of chitosan film for immersion times longer30 min, as shown later. The permeability of the organic filmwould allow the electrolyte to reach the anodic oxide–electrolyte interface, and then, the partial or complete localdissolution of the oxide since the solubility of alumina increasesat pH values higher than 8 and lower than 7.

3.4. Chitosan film modified with copper

In order to reduce the permeability and the watersusceptibility of the chitosan film, the formed film on theanodized aluminium surfaces were modified differently byimmersing the chitosan coated samples in copper sulphate or,copper acetate solutions for 24 h.

As can be seen in Fig. 7, the incorporation of copper in thechitosan coating shifts the initial OCP in the anodic direction inapproximately −0.200 mV with respect to that of theunmodified chitosan film. However, with increasing theimmersion time, the OCP behaviour is different for thedifferently modified aluminium alloy samples; while the OCPof chitosan films modified by copper sulphate decaysimmediately after 10 min of immersion in the borax solution,the OCP of chitosan films modified in a copper acetate solutionremains constant for more that 4 h. The major reason of this isbelieved to be the difference in pH of the copper sulphate andacetate, that affects chitosan solubility during copper incorpo-ration. The Cu-sulphate has a stable pH of about 3–3.5, becauseof its buffering capacity, whereas the Cu-acetate has a pH ofabout 5.5. Since the solubility of chitosan is relatively high atlow pH, during modification the chitosan film is partiallydissolved. This implies that the chitosan film in the coppersulphate solution does complexate with copper, but at the same

time it dissolves slowly, leaving a chitosan–Cu complex filmthinner than the original chitosan film. Conversely, in copperacetate solution the original chitosan film does not dissolve or itdissolves very little. From the previous analysis the thickness ofthe modified chitosan layer apparently determines the quality ofprotection given to the aluminium surface, associated mainlywith a high resistance for ionic conduction through the film.

Fig. 8 illustrates the OCP behaviour of chitosan filmsmodified from copper acetate solutions of different concentra-tions. The salt concentration affects the OCP behaviour veryslightly, suggesting that copper incorporation is governedmainly by the chitosan capacity of incorporating copper, limitedpossibly by the number of amine groups in the film [32].

In order to get further insight into the role of solution pH oncopper incorporation into the chitosan film, the organic filmswere deposited on glass surfaces and modified with copper fromsolutions at different pH. In Fig. 9 are presented the Vis spectraof the modified chitosan films supported on glass surfaces as a

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function of pH. The pH increase is accompanied by adisplacement of the wide band at high wavelength towardsmaller values. This absorption band is attributed to d–d-electronic transition [33]. The d orbitals of the free cupric ionshave the same energy. With increasing pH, the colour of the filmturns from pale blue to purple, which has been interpreted by thefact that the amino chromophore groups of the chitosan enter thecoordination sphere of copper, leading to a variation of theenergy of the 3d orbitals of Cu2+ ions and giving rise to adisplacement of the d–d transition to smaller wavelength [34].

The previous analysis and the spectra in Fig. 9 suggest thatcopper can be coordinated at pHN3. At pH lower than 3 copperions enter the chitosan film as part of the electrolyte because ofthe high permeability of chitosan, but these are coordinated to alower extent. Considering the high initial OCP of the chitosanfilm modifying from copper sulphate solution, the completeabsence of copper coordination is discarded.

4. Conclusions

1. Chitosan produces smooth and homogenous coatings onanodized AA-2024 T-3, but the high permeability of theorganic film reduces their protective characteristic.

2. For short immersion times, the OCP behaviour of theanodized AA-2024 modified by chitosan reveals potentialvalues more positive than that of the anodized sample.However, with increasing the exposure time the potential isshifted rapidly into the cathodic direction, suggestingdissolution of the chitosan film.

3. Copper incorporation into the chitosan film, from copperacetate solutions, reduces its permeability and dissolutionvery strongly, as revealed from the OCP behaviour of thecorresponding samples. For these alloy samples, the OCPvalues are higher and more stable than those coated withchitosan with no copper in their structure. The probablereason why copper acetate is more effective than coppersulphate is that it has a rather high pH of 5.5, compared toaround 3 for copper sulphate in aqueous solution, whichpromotes chitosan dissolution during modification of thechitosan film.

4. From spectroscopic measurements, the pH of the copper saltsolution plays an important role for copper incorporation andchanges in the chitosan structure providing a betterperformance for corrosion protection. It is apparent thatsolution pHs higher than 4 favour coordination of copperions through the amine groups of the chitosan, giving rise tocross-linking in the chitosan structure and hence, to adecrease in the permeability of the organic film.

Acknowledgments

The authors are grateful to Fondecyt (Grant: 1060050) and toDICYT (USACH) for the financial support.

References

[1] P.G. Sheasby, R. Pinner, sixth ed., The Surface Treatment and Finishing ofAluminium and its Alloys, vol. 1, ASM International, Materials Park, OH,2001.

[2] G.E. Thompson, G.C. Wood, in: J.C. Scully (Ed.), Treatise on MaterialsScience and Technology, vol. 23, London, New York, 1983, p. 205.

[3] H. Habazaki, M.A. Páez, K. Shimizu, P. Skeldon, G.E. Thompson, G.C.Wood, X. Zhou, Corros. Sci. 38 (1996) 1033.

[4] M.A. Paez, T.M. Foong, C.T. Ni, G.E. Thompson, K. Shimizu, P. Skeldon,G.C. Wood, Corros. Sci. 38 (1996) 59.

[5] R. Vazquez-Duhalt, R. Tinoco, D. Antonio, L. Tirmmie, G. Payne,Bioconjug. Chem. 12 (2001) 301.

[6] T. Uragami, T. Matsuda, H. Okuno, T. Miyata, J. Membr. Sci. 88 (1994)243.

[7] K. Ohga, Y. Kuraughi, H. Yanase, Bull. Chem. Soc. Jpn. 60 (1987) 444.[8] K. Kurita, T. Sannan, Y. Iwakura, J. Appl. Polym. Sci. 23 (1979) 511.[9] F.-C. Wu, R.-L. Tseng, J. Ruey-Shin, Ind. Eng. Chem. Res. 38 (1999) 270.[10] E. Onsoyen, O. Skaugrud, J. Chem. Technol. Biotechnol. 49 (1990) 395.[11] H. Blair, T.-C. Ho, J. Chem. Technol. Biotechnol. 31 (1980) 6.[12] K. Kurita, Y. Koyama, A. Taniguchi, J. Appl. Polym. Sci. 31 (1986) 1169.[13] R. Maruca, B. Suder, J. Wightman, J. Appl. Polym. Sci. 27 (1982) 4827.[14] Z. Modrzejewska, W. Kaminski, Ind. Eng. Chem. Res. 38 (1999) 4946.[15] T. Vincent, E. Guibal, Ind. Eng. Chem. Res. 40 (2001) 1406.[16] T.Asakawa,K. Inoue, T. Tanaka,KagakuKogakuRonbunshu26 (2000) 321.[17] S. Bhatia, N. Ravi, Biomacromolecules 1 (2000) 413.[18] B. Gamblin, J. Stevens, K. Wilson, Hyperfine Interact. 112 (1998) 117.[19] E. Guibal, C. Milot, J. Tobin, Ind. Eng. Chem. Res. 37 (1998) 1454.[20] I. Jha, L. Iyengar, A. Prabhakara Rao, J. Environ. Eng. Div. Am. Soc. Civ.

Eng. 114 (1988) 962.[21] I. Gabrielii, P. Gateholn, Proceedings of theAmerican Chemical Society,

Division of Polymeric Materials: Science and Engineering, vol. 79, 1998,p. 459.

[22] T. Sugama, S. Milian-Jimenez, J. Mater. Sci. 34 (1999) 2003.[23] V.W.D. Chui, K.W. Mok, C.Y. Ng, B.P. Luong, K.K. Ma, Environ. Int. 22

(1996) 463.[24] J. Lerivrey, B. Dubois, P. Decock, J. Micera, H. Kozlowski, Inorg. Chim.

Acta 125 (1986) 187.[25] K. Ogawa, K. Oka, T. Yui, Chem. Mater. 5 (1993) 726.[26] S. Schlick, Macromolecules 19 (1986) 192.[27] A. Domard, Int. J. Biol. Macromol. 9 (1987) 98.[28] J.M. Nieto, C. Peniche Covas, J. Del Bosque, Carbohydr. Polym. 18

(1992) 221.[29] W. Kaminski, Z. Modrzejewska, Sep. Sci. Technol. 32 (1997) 2659.[30] L. Velterop, ATB Metall. 43 (2003) 284.[31] O. Øystein, B. Steinar, A. Bjørgum, J. Mardalen, M. Hallenstvet, Corros.

Sci. 46 (2004) 2081.[32] R. Schmuhl, H.M. Krieg, K. Keizer, Water S.A. 27 (2001) 1.[33] B.J. Hathaway, D.E. Billing, Coord. Chem. Rev. 5 (1970) 143.[34] M. Rhazi, J. DesBriere, A. Tolaimate, M. Rinaudo, P. Vottero, A. Alagui,

Polymer 43 (2002) 1267.