Improving the corrosion protection properties of organically modified silicate–epoxy coatings by incorporation of organic and inorganic inhibitors

Progress in Organic Coatings 72 (2011) 653–662

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Improving the corrosion protection properties of organically modified silicate–epoxy coatings by incorporation of organic and inorganic inhibitors A.C. Balaskas a , I.A. Kartsonakis a,∗ , D. Snihirova b , M.F. Montemor b , G. Kordas a a b

Sol-Gel Laboratory, Institute of Materials Science, NCSR “DEMOKRITOS”, 15310 Agia Paraskevi, Greece ICEMS, Instituto Superior Tecnico, Technical University of Lisbon, Portugal

a r t i c l e

i n f o

Article history: Received 6 April 2010 Received in revised form 15 July 2011 Accepted 20 July 2011 Keywords: ORMOSIL AA2024-T3 Corrosion Inhibitors Impedance

a b s t r a c t Organically modified silicate (ORMOSIL)–epoxy coatings were formed on aluminium alloy 2024-T3 in order to protect the substrates from corrosion. Organic and inorganic compounds, all known for their corrosion inhibition ability, were incorporated into the polymer matrix for the improvement of the corrosion resistance. The ORMOSIL–epoxy coatings were deposited via the dip-coating process. The properties of the coatings were investigated after 24 h of curing at 90 ◦ C. The morphology of the coatings was examined by scanning electron microscopy (SEM). Their composition and structure were investigated by Fourier transform infrared spectroscopy (FT-IR) and energy dispersive X-ray analysis (EDX). The corrosion resistance of these coatings was investigated using electrochemical impedance spectroscopy (EIS). The results showed that the most effective corrosion protection ability was provided by the inorganic inhibitors and that an important role can be attributed to the cation in the nitrate salt. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Corrosion of aluminium alloys (AA) has an enormous economic impact. Chromate surface treatments and chromate-containing epoxy primers are often used for corrosion control of AA. However, many efforts have been focused on the development of new techniques for the replacement of hexavalent chromium. Alternative protection of AA is urgent since hexavalent chromium provokes human diseases [1,2] and many environmental problems. ORganically MOdified SILicates (ORMOSILs) are promising coatings for corrosion protection of AA. These compounds are hybrid organic–inorganic materials, formed by sol–gel method, via hydrolysis and condensation of organically modified silanes with traditional alkoxide precursors [3]. ORMOSILs coatings can be used to mitigate corrosion activity on AA due to their improved adherence to the substrate, since they form stable covalent bonds, and very good barrier properties [4–8]. In addition, these coatings can be loaded with organic and inorganic inhibitors, which will enhance their protection ability. Yang and collaborators synthesized and used a sol–gel conversion coating for aluminium corrosion protection. The sol–gel coating consisted of SiO2 and ZrO2 . They concluded that their coating inhibited local dissolution of the aluminium surface in Harrison’s solution [9]. Addition of SiO2 nanoparticles was also reported by

∗ Corresponding author. Tel.: +30 2106503302; fax: +30 2106547690. E-mail address: [email protected] (I.A. Kartsonakis). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.07.008

Montemor et al. [10] as beneficial additives in silane coatings for the protection of galvanized steel. Pathak and co-workers prepared a 3-glycidoxypropyltrimethoxysilane, methyltrimethoxysilane and hexamethoxy-methylmelamine sol–gel solution. Aluminium substrates were dip coated in those solution and the results revealed that the coatings act as a protective barrier in the corrosive electrolytes [11]. Khramov et al. synthesized a sol–gel derived hybrid coating including corrosion inhibitors and they examined the developed coatings on AA 2024-T3. They concluded that the corrosion protection could be improved by means of direct addition of the corrosion inhibitors [12] to the protective coating. The anti-corrosion resistance properties of ORMOSIL coatings on 2024-T3 aluminium alloys were studied by Metroke et al. and have been reported as a promising treatment for aluminium alloys. They prepared coatings with different concentrations (up to 16.6 vol.%) of alkyl-modified silane, Xn · · ·Si(OR)4−n , where X = methyl, dimethyl, n-propyl, n-butyl, i-butyl, n-hexyl, noctyl, or i-octyl. They concluded that the best overall corrosion resistance was observed for coating systems containing ≥10.4 vol.% alkyl-modified silane [8]. Furthermore, Metroke et al. synthesized spray and dip-coated 3-glycidoxypropyltrimethoxysilane (GLYMO)-tetraethoxysilane (TEOS) ORMOSIL films. Films prepared from high organic contents (25 and 67 mol% GLYMO) and low hydrolysis water content values exhibited the best corrosion resistance when applied by dip coating. These coatings comprise a dense network structure with organic groups dispersed throughout the film, providing a hydrophobic barrier coating capable of repelling water and corrosion initiators. Generally the coating

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Table 1 The % (w/w) nominal composition of AA2024-T3. Element

Al

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

% (w/w)

90.7–94.7

<0.5

<0.5

3.8–4.9

0.3–0.9

1.2–1.8

<0.1

<0.25

<0.15

thicknesses were in the 6–16 ␮m range, with the thickest coatings being observed for the highest concentrations of alkyl-modified silane [13]. These are, however, slightly thicker for pre-treatment purposes and the corrosion tests were made in potassium sulphate electrolytes or in salt spray chambers. Generally, these films are applied as “blank” coatings on the aluminium substrates with promising results. Several different formulations can be used with different results on the overall corrosion performance [8,11–14]. Following these works, we propose a novel approach that consists in the modification of thin ORMOSIL coatings (2–4 ␮m) with organic and inorganic corrosion inhibitors. This was already done, with success, for thin silane coatings (0.5–2 ␮m) and thicker sol–gel films (4–6 ␮m) [10,12]. The direct addition of inhibitors to a coating must fulfil a number of requirements: the inhibitor must be effective for the specific metal to be protected; the inhibitor shall not damage the barrier properties and cannot be early leached from the coating. This has been demonstrated in previous works. Organic inhibitors such as 8-hydroxyquinoline (8HQ) have been added in sol–gel films to protect aluminium alloy 2024-T3 from corrosion providing selfhealing ability to the film without influence its barrier properties [14]. Also it has been used in sol–gel films for protection of magnesium alloy AZ31 without affecting the barrier properties of the film [15]. The inhibiting action of 8HQ on AA 2024-T3 by the formation of a thin organic layer of insoluble complexes on the surface of the alloy preventing the dissolution of Mg and Al as well as dissolution and redeposition of Cu has been studied by Lamaka et al. [16]. The inhibition effect of 8HQ on pure aluminium has been discussed by Garrigues et al. [17] and it was found that the organic compound is absorbed on the passive alumina layer preventing the absorption of chloride anions and pitting activity. The inhibiting action of 8HQ on the copper corrosion process by the formation of Cu(II)–8hydroxyquinoline complexes has been discussed by Ciccileo et al. [18]. Lamaka et al. [19,20] used benzotriazole in nanoresevoirs in sol–gel coating for protection of AA 2024-T3 providing selfhealing ability. 2-Mercaptobenzothiazole has also been examined for protection of AA 2024-T3 by decreasing the number of pits and reducing the degree of corrosion attack [16]. Also, Zheludkevich et al. studied the effect of 2-mercaptobenzothiazole in protection of AA 2024-T3. It was found that due to 2-mercaptobenzothiazolethe rate of both the anodic and cathodic processes is decreased and the dealloying copper-rich particles are hindered [21]. Previous works [22] showed that addition of cerium nitrate to sol–gel films improves the corrosion protection properties of the film but increased concentrations of cerium nitrate lead to degradation of the barrier properties of the sol–gel matrix. Identical results were observed by Trabelsi et al. on silane coatings modified with cerium nitrate [23]. However, storage of cerium ions on CeO2 or SiO2 nanoparticles can have a much better effect as demonstrated by Montemor et al. [24,25]. The beneficial effect of cerium (III) ions has been related with increased crosslinking of the silane rich phase [24,25] and to inhibition of oxygen reduction reactions by formation of cerium-rich films over copper containing intermetallics which act as local cathodic sites [26]. Manganese has been studied by Danilidis et al. [27,28] for anticorrosive properties purposes due to some similarities with chromium (multiple oxidation states). Cobalt (II) ions were also found by Presuel-Moreno et al. [29] to be effective reducing the oxygen reduction reactions on AA 2024-T3 by formation of insoluble oxides when the surface becomes locally alkaline.

In the present work, we report the corrosion protection ability of ORMOSIL hybrid coatings based on amine cured organically modified silicate that were synthesized through a sol–gel technique and formed onto AA 2024-T3 via dip coating. Cerium nitrate, cobalt nitrate, manganese nitrate, 8-hydroxyquinoline, benzotriazole and mercaptobenzothiazole were the inhibitors incorporated into the polymer matrix. The effectiveness of the resulting coatings against corrosion was examined by EIS. The surface morphology and the chemical composition were evaluated by scanning electron microscopy (SEM) and infrared spectroscopy (FT-IR), respectively. Inhibitors cobalt and manganese nitrate are used for the first time into ORMOSIL–epoxy coatings for corrosion protection. Furthermore, the effectiveness between organic and inorganic inhibitors is studied too. 2. Experimental details 2.1. Materials and reagents Aluminium alloy 2024-T3 panels were used for all the experiments with the nominal mass composition depicted in Table 1. All chemicals were of analytical reagent grade: 2-mercaprobenzothiazole (2MB, Sigma–Aldrich, St. Louis, USA), benzotriazole (BTA, Sigma–Aldrich, St. Louis, USA), 8-hydroxyquinoline, (8HQ, Sigma–Aldrich, St. Louis, USA), (PVP, average molecular weight: polyvinylpyrrolidone 55,000, Sigma–Aldrich, St. Louis, USA), cobalt nitrate (Co(NO3 )2 , Sigma–Aldrich, St. Louis, USA), cerium nitrate (Ce(NO3 )3 , Sigma–Aldrich, St. Louis, USA), manganese nitrate (Mn(NO3 )2 , Sigma–Aldrich, St. Louis, USA), N-(2-Aminoethyl)-3(trimethoxysilyl)propylamine (Z 6020, Sigma–Aldrich, St. Louis, USA), epoxy resin “Araldite GY 257” (GY 257, Ciba-Geigy), 2,2 diaminodiethylamine (HY 943,Sigma–Aldrich, St. Louis, USA) were used without further purification. 2.2. Preparation of ORMOSIL–epoxy coating The process for the preparation of the ORMOSIL–epoxy coating includes 5 steps (Table 2). First of all, N-(2-aminoethyl)-3(trimethoxysilyl)propylamine was hydrolyzed in absolute ethanol for 1 h (solution A). Then, resin Araldite GY 257 was dissolved in mixture of absolute ethanol and acetone (solution B). Solutions A and B were intermixed providing solution C. A new solution was prepared by dissolving 2.2 -diaminodiethylamine in 25 ml acetone (solution D). Finally, solution C containing Epoxy and ORMOSIL and solution D containing the crosslinker were intermixed providing solution E. The final solution E was kept under vigorous stirring for 12 h. The appropriate quantity of inhibitor (1.5%, w/w) was added

Table 2 Conditions of preparation of ORMOSIL–epoxy 1.5% (w/w) inhibitor solution. Material

Quantity (g)

Z 6020 GY 257 HY 943 Ethanol Acetone Inhibitor

3.00 34.74 3.70 100.00 75.00 3.29

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in the solution E under vigorous stirring 1 h before the dip coating process. 2.3. Dip coating process The panels were dip coated into the ORMOSIL–epoxy solution for six times with a withdraw speed of 32 cm/min. There was no time waiting between consecutive immersion steps. The panels remained in the solution for 1 min. Then, the coated panels were cured at90 ◦ C for 24 h. The AA 2024-T3 panel had been previously cleaned, under specific conditions. These conditions include the insertion of the panel into 2% (w/w) NaOH for 3 min at 40 ◦ C. After that, the panel was rinsed with distilled water and was inserted into 4.33 M HNO3 for 30 s at room temperature. Finally, the panel was rinsed with distilled water. 2.4. Characterization and corrosion evaluation The morphology of the coatings was determined by SEM using a PHILIPS Quanta Inspect (FEI Company) microscope with W (tungsten) filament 25 kV equipped with EDAX GENESIS (AMETEX Process & Analytical Instruments). Furthermore, the coatings were characterized by reflectance infrared spectroscopy using a PerkinElmer universal ATR sampling accessory spectrum 100 FT-IR spectrometer. The corrosion resistance of these coatings was studied by electrochemical impedance spectroscopy, in 0.05 M NaCl, using a SI 1287 Solartron Electrochemical interface connected with a SI 1260 Impedance/gain-phase analyser. The experiments were performed at room temperature, in a Faraday cage, at the open circuit potential, using a three-electrode electrochemical cell, consisting of working electrode (≈3.15 cm2 of exposed area), saturated calomel electrode (SCE) as reference and platinum as counter electrode. The measuring frequency ranged from 100 kHz down to 5 mHz. The rms voltage was 10 mV. Spectra were treated using the Z-view Software using the adequate equivalent electric circuits. The number of frequency points was 10 points per decade. 3. Results and discussion 3.1. Morphology of the coatings The SEM and EDX analysis of the above coated samples before exposure at 0.05 M NaCl solution at room temperature is presented in Figs. 1–7. For all the coated samples, the surface morphology presents some common features. The darker zones seem to correspond to the presence of thicker coated regions, formed on

Fig. 1. SEM and EDX analysis of ORMOSIL–epoxy coating on AA2024-T3.

Fig. 2. SEM and EDX analysis of ORMOSIL–epoxy–Ce(NO3 )3 coating on AA2024-T3.

Fig. 3. SEM and EDX analysis of ORMOSIL–epoxy–Co(NO3 )2 coating on AA2024-T3.

depressions present at the alloy surface. These depressions generally develop after surface finishing and are related with dissolution of intermetallic inclusions. Epoxy coating presents no evident cracks or defects on its surface. Moreover, no significant features could be observed on the surface morphology as result of the addition of organic or inorganic inhibitors. The white dusts in the surface are agglomerations of the silane dioxides leading to the formation of aggregates. These particles are likely to be formed due to remnants of solution on the surface after dip coating, since the excess of solution was not removed. The EDX analysis for all samples shows carbon and nitrogen (from the coating and inhibitor, respectively), aluminium and cop-

Fig. 4. SEM and EDX analysis of ORMOSIL–epoxy–Mn(NO3 )2 coating on AA2024-T3.

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Fig. 5. SEM and EDX analysis of ORMOSIL–epoxy–8HQ coating on AA2024-T3.

Fig. 8. Cross-section SEM of AA2024-T3 coated with ORMOSIL–epoxy.

Fig. 6. SEM and EDX analysis of ORMOSIL–epoxy–BTA coating on AA2024-T3.

per from the substrate and oxygen from the coating and from the substrate. The detection of both copper and aluminium suggests that the coating presents thinner coated areas and/or defects, which allow detection of secondary electrons from the bare metal. The metal elements of the inorganic inhibitors: manganese, cerium and cobalt are also present in the corresponding coatings (Table 3). For the coatings ORMOSIL–8HQ and ORMOSIL–BTA, there is an increase of the nitrogen content, which is due to nitrogen from the 8HQ and BTA inhibitors, respectively (Table 3). Furthermore, the EDX analysis of ORMOSIL–2MB coating presents an increase of sulphur, corresponding to sulphur species from 2MB inhibitor

(Table 3). In spite of increasing the % (w/w) of nitrogen and sulphur elements via the presence of the corresponding inhibitors into the coating, the amount of these elements cannot be accurate due to their low value. In other words, the presence of the inhibitors into the coatings can be proved by EDX analysis due to the presence of their corresponding nitrogen and sulphur elements, but the quantification of these elements are not fully accurate because the peak to background ratio was very low. Furthermore, precise quantitative analysis of data of elements C and Si is difficult to be made because the variation of data for Al and Cu (both come from AA2024-T3) throw off the percentage values for C and Si. On the other hand, the EDX data do confirm that metal ions from various inhibitors are indeed incorporated into the film. A cross section of the ORMOSIL–epoxy coating with no inhibitor incorporated into the polymer matrix is depicted in Fig. 8. The thickness of the coating ranges from 1.0 to 4.6 ␮m. Glue was used in order the sample to be in a vertical position on the holder for the SEM measurement. The incorporation of inorganic or organic inhibitors into the coatings did not change the range of the thickness. 3.2. FT-infrared spectroscopy analysis

Fig. 7. SEM and EDX analysis of ORMOSIL–epoxy–2MB coating on AA2024-T3.

Fig. 9 shows the FT-IR spectra of ORMOSIL–epoxy coating. The FT-IR characterization was performed on films deposited on the substrate. The peaks of GY 257, Z 6020 and HY 943 are attributed at Table 4 [30–34]. Figs. 10–15 present the FTIR spectra of ORMOSIL–epoxy–8HQ, ORMOSIL–epoxy–2MB, ORMOSIL–epoxy–BTA, ORMOSIL–epoxy–Ce(NO3 )3 , ORMOSIL– epoxy–Co(NO3 )2 and ORMOSIL–epoxy–Mn(NO3 )2 . All these spectra depict the characteristic bands of the ORMOSIL–epoxy coating. Furthermore, the spectra of ORMOSIL–epoxy–Ce(NO3 )3 , ORMOSIL–epoxy–Co(NO3 )2 and ORMOSIL–epoxy–Mn(NO3 )2 present the characteristic peaks between 1350 up to 1410 cm−1 due to asymmetrically NO3 stretching vibrations from their inorganic nitrate salts. On the other hand, the characteristic peaks of the organic inhibitors are overlapped on their corresponding spectra, and peaks assignments are presented in Table 5.

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Table 3 Percent (w/w) element concentration of AA2024-T3 coatings. Coatings

C

Si

S

N

Al

Cu

Ce

Co

Mn

ORMOSIL–epoxy ORMOSIL–epoxy–8HQ ORMOSIL–epoxy–2MB ORMOSIL–epoxy–BTA ORMOSIL–epoxy–Ce(NO3 )3 ORMOSIL–epoxy–Co(NO3 )2 ORMOSIL–epoxy–Mn(NO3 )2

71.72 55.03 52.90 50.57 59.14 62.08 67.66

0.27 0.25 0.12 0.18 0.19 0.23 0.21

– – 0.43 – – – –

0.95 1.86 1.06 1.85 – 2.43 0.59

23.23 35.48 39.12 40.71 32.17 27.67 24.45

2.08 1.93 2.22 2.29 2.04 1.34 1.51

– – – – 1.57 – –

– – – – – 0.71 –

– – – – – – 0.49

Fig. 9. FT-IR spectra of ORMOSIL–epoxy coating.

3.3. Electrochemical study The corrosion behaviour of the ORMOSIL–epoxy coatings modified with the different corrosion inhibitors was assessed by electrochemical impedance spectroscopy under immersion in 0.05 M NaCl solution. The EIS spectra obtained for the blank coating are depicted in Fig. 16. The spectra are characterized by the presence of two time constants, one for frequencies between 1 Table 4 FT-IR characteristic peaks of GY 257, Z 6020 and HY 943. Compound

Ranges (cm−1 )

Comment

GY 257

550–600

GY 257

730–755

GY 257 GY 257

966 2917, 2952, 2813, 2885

GY 257 GY 257 GY 257 HY 943 HY 943 HY 943 Z 6020 Z 6020

3045 1380–1388 1363–1367 900–934 3170–3280 1020–1095 791–835 2847

Z 6020

3378–3410

Z 6020 Z 6020, HY 943 Z 6020, HY 943 Z 6020, HY 943

800–850, 1170–1210 755 1032 1100–1120

Z 6020, HY 943 Z 6020, HY 943 Z 6020, HY 943

1285–1304 1450–1465 1507–1520

C-phenyl out of plane ring deformation vibration CH2 vibration and vibration of ring Epoxy ring R–CH3 asymmetric stretching vibration Phenyl vibration –CH3 vibrations C–H vibrations N–H out of plane vibration –NH2 vibration Stretching vibration of Si–O –NH2 and Si–O–CH3 vibrations –NH2 and Si–O–CH3 asymmetric vibration CH3 –NH symmetric and asymmetric stretching vibrations Si–O–CH3 symmetric vibration –NH2 and C–H2 C–C and NH2 Vibrations of firstly and secondary amines To C–H2 and C–NH2 vibrations Firstly amine vibration Secondary amine vibrations

Fig. 10. FT-IR spectra of ORMOSIL–epoxy–8HQ.

and 10 Hz and another one in the low frequency range. The first one is assigned to the response of the aluminium oxide layer, including double layer effects. The impedance spectra shows a low frequency time constant, at about 0.01 Hz that can be assigned to mass diffusion controlled processes in the active pits. This part of the spectrum is usually fitted using the porous bounded open Warburg impedance element or one Rpit –CPEpit element association as proposed elsewhere [35] when diffusion occurs trough a finite diffusion layer. The time constant that could be assigned to the coating and that should appear in the high frequency range is not detected, revealing that the barrier properties of the coating are poor and deteriorate very fast during immersion. The global impedance values are below 10 k cm2 . These values are very similar to those obtained for uncoated samples immersed in the same

Fig. 11. FT-IR spectra of ORMOSIL–epoxy–2MB.

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Fig. 15. FT-IR spectra of ORMOSIL–epoxy–Mn(NO3 )2 .

Fig. 12. FT-IR spectra of ORMOSIL–epoxy–BTA.

Table 5 FT-IR characteristic peaks of inhibitors 8HQ, BTA and 2MB.

Fig. 13. FT-IR spectra of ORMOSIL–epoxy–Ce(NO3 )3 .

conditions (0.05 M NaCl) as published in previous works [16]. This result reveals that the blank ORMOSIL–epoxy coating does not improve the corrosion resistance. The ORMOSIL–epoxy coating modified with cerium nitrate revealed a different spectra (Fig. 17). The total impedance is

Fig. 14. FT-IR spectra of ORMOSIL–epoxy–Co(NO3 )2 .

Compound

Ranges (cm−1 )

Comment

8HQ

Ring stretching

8HQ 8HQ BTA

785, 823, 1059, 1154, 1512, 1581 and 1626 1286 and 1273 1231 and 1222 920 and 1000

BTA

1170–1240

BTA BTA BTA

1500 1500–1600 740

2MB 2MB 2MB

2550–2600 685–710 1260–1330

C–N stretching C–O stretching C–H aromatic in plane deformation vibrations Stretching deformation vibrations of C–N, Aromatic vibrations of C C NH In-plane vibrations of the ring of nitrogen S–H stretching vibration C–S stretching vibration Stretching of the phenyl carbon–nitrogen bond

nearly constant and the maximum values reach 100 k cm2 , which are one order of magnitude higher than those of the blank ORMOSIL–epoxy coating or blank AA2024 in 0.05 M NaCl. Moreover, a high frequency time constant, due to the coating response is clearly observed, being kept for all the immersion period. This reveals that the coating behaves as a barrier layer, thus providing

Fig. 16. EIS Bode plots for the blank ORMOSIL–epoxy coatings.

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Fig. 17. EIS Bode plots for the ORMOSIL–epoxy coatings modified with Ce(NO3 )3 .

Fig. 19. EIS Bode plots for the ORMOSIL–epoxy coatings modified with Co(NO3 )2 .

extra protection comparatively to the blank coating. Nevertheless, the low frequency time constant, indicative of pitting attack is always present, suggesting that corrosion activity could not be effectively inhibited. This trend reveals that the presence of cerium enhances the barrier properties of the ORMOSIL coating and increases the overall impedance, but does not fully suppress pitting. Identical trends were reported by Ref. [36] for sol–gel coatings enriched in SiO2 particles doped with low amounts of cerium nitrate in 3.5% NaCl. Increased impedance values in the presence of cerium ions comparatively to blank coatings are also reported for ZrO2 modified sol–gel films. Incorporation of zirconia nanoparticles leads to improvement of the barrier properties of the organosiloxane hybrid sol–gel coatings and additional protection is conferred by doping the sol–gel film with cerium-based inhibitor [22]. The EIS spectra obtained for the coating modified with Mn(NO3 )2 (Fig. 18) are similar to the spectra obtained for the Ce modified coating. The barrier properties, as well as the overall impedance values are identical to those measured for the Ce(NO3 )3 modified coating. For both coatings, the midterm frequency range reveals the presence of two time constants, and not only one, as observed for the blank coating. This impedance response indicates the presence of two phenomena related to the interfacial behaviour. Although double layer effects through defects formed on the oxide

layer can affect the EIS response in the same range, the comparison with the blank system suggests that this split of the phase angle is related to the presence of inhibitor. This can arise either from a “conversion” like layer formed as consequence of the presence of Ce or Mnor an inhibition film due to nitrate. Previous results suggesting formation of an inhibitive layer on AA 2024 were previously reported in Ref. [16,17]. In both works a time constant was observed and assigned to a protective layer of adsorbed inhibitors. The same effect is observed in the present work. The addition of Co(NO3 )2 (Fig. 19) also reveals the above mentioned features, however the impedance shows a more pronounced drop. Moreover, the presence of two time constants in the medium frequency range is not as evidenced as for the previous two coatings therefore, revealing that the additional time constant is more related with the inhibitive cation (Ce3+ or Mn2+ ) rather than the anion (NO3 − ), which is common for the three inorganic inhibitors. The addition of organic inhibitors leads, in general, to a decrease of the barrier properties, since the high frequency time constant characteristic of the coating could not be observed as shown in Figs. 20–22. Among the three organic inhibitors 2-mercaptobenzothiazole (Fig. 20) and benzotriazole (Fig. 22) revealed very similar spectra and identical impedance values.

Fig. 18. EIS Bode plots for the ORMOSIL–epoxy coatings modified with Mn(NO3 )2 .

Fig. 20. EIS Bode plots for the ORMOSIL–epoxy coatings modified 2MB.

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Fig. 21. EIS Bode plots for the ORMOSIL–epoxy coatings modified 8HQ.

The impedance values show a more marked decrease for the 8-hydroxyquinoline inhibitor (Fig. 21). The phase angle, in the medium frequency range, is broader in the presence of the organic inhibitors comparatively to the blank coating, suggesting again two overlapping processes in this frequency range. This reveals that the presence of inhibitor also reinforces the oxide barrier layer. However, the effect is not as marked as observed for the inorganic inhibitors. An identical trend was found in blank silane coatings applied on AA2024 coupons. The broadening of the phase angle was attributed to a more stable interfacial layer [37] in which a layer of inhibitor was adsorbed. In Ref. [17] the authors report the formation of a hydroxiquinoline adsorbed layer in the alloy surface as responsible for this phase angle broadening. Identically, a time constant was assigned to this new layer in the interpretation of the EIS results. For a more detailed interpretation of the EIS spectra the results were fitted using equivalent circuits (EC) (Fig. 23). In these equivalent circuits the capacitive component was replaced by a constant phase element (CPE), which simulates deviations from a non-ideal capacitive behaviour. More details about the nature and interpretation of this element can be found elsewhere [38]. For the coatings modified with the inorganic inhibitors, an equivalent circuit composed of 4 time constants is proposed: the

Fig. 22. EIS Bode plots for the ORMOSIL–epoxy coatings modified BTA.

Fig. 23. Equivalent circuits used to fit the experimental results obtained for the ORMOSIL–epoxy coatings modified with inorganic (a) and organic (b) inhibitors. Fitting results for the Mn(NO3 )2 coating immersed for 24 h (c). The equivalent circuit was the one depicted in (a). 2 ∼ 1E−4.

high frequency one (Rcoat /CPEcoat ) simulates the high frequency response, i.e., the coating barrier properties. Rcoat can be assigned to the resistance of the pores/defects in the coatings and CPEcoat to the intact protective barrier coating. R1oxide /CPE1oxide represent the extra time constant formed at frequencies above 10 Hz and the R2oxide /CPE2oxide relates with the time constant at slightly lower frequencies (0.1–1 Hz), which is expected from the response of the aluminium oxide layer. Finally the low frequency behaviour is characteristic of AA2024 substrates showing pitting activity. In many situations, to account for these effects an open Warburg is included in the circuit [16]. However, this time constant can also be simulated with a Rpit /CPEpit association, to account for the response from active pits dissolving the metallic substrate and leading to the formation of a porous layer of corrosion products, through which diffusion of charged species will occur [35]. In fact, the low frequency CPE exponent values for this time constant were in the range 0.62–0.5, suggesting an important contribution of finite length diffusion effects as expected due to the deposition of corrosion products that affect the transport properties in the active pits. The exponent (n) value for the Rcoat /CPEcoat was typically in the range 0.75–0.85 and for the CPEs associated with the oxides it was between 0.62 and 0.74. An example of the fitting goodness is presented in Fig. 22 for the Mn(NO3 )2 coating after 24 h of immersion. The fitting was also tried with only three time constants as proposed in previous works [22], but the fit goodness was at least two orders of magnitude lower. For coating modified with the organic inhibitors, the high frequency time constant was removed, since the

A.C. Balaskas et al. / Progress in Organic Coatings 72 (2011) 653–662 Table 6 Fitting parameters (resistances) for the coatings modified with inorganic inhibitors. Time

Rcoat ( cm2 )

R1oxid ( cm2 )

R2oxid ( cm2 )

Rpit ( cm2 )

Ce(NO3 )2 24 520 48 584 72 601

3520 850 840

25,360 48,652 75,623

39,520 75,632 88,653

Mn(NO3 )2 24 604 549 48 72 574

6061 752 767

45,167 51,576 64,514

48,142 69,546 62,880

Co(NO3 )2 24 662 551 48 396 72

3100 521 –

32,500 16,850 7668

36,580 22,350 7050

coating barrier properties were not visible in the EIS spectra. The best fitting parameters for the different resistances are depicted in Tables 6 and 7. The capacitances are not depicted in the table, but are discussed in the text. The resistance of the coating pores, for all the coatings modified with inorganic inhibitors are between 600  cm2 and 500  cm2 and show a small increase for the Ce and Mn nitrate doped coatings. At early immersion stages, this resistance is slightly higher for the Co(NO3 )2 modified coating. However, for this system the coating resistance suffers an important drop, revealing that it contains more defects and therefore poorest barrier properties. The admittance of the CPE is always in the range of 10−7  cm2 s−n and remains almost constant for the Ce and Mn containing coatings, increasing after 2 days of immersion, for the cobalt modified coating for about one order of magnitude. The resistance R1oxide decreases with time, for all the coatings. However, for the Co(NO3 )2 coatings, after 3 days it is no longer observed. In fact this behaviour confirms that this resistance is cation dependent and that its value is related with the formation of a layer, probably of metal hydroxide, on the top of the original aluminium native oxide layer. Thus, a more protective interfacial layer is formed in the presence of cerium and manganese, revealing that these two cations reinforce the protective interfacial properties. The admittance of the CPE is generally below 30 × 10−5  cm2 s−n and slightly decreases for Ce and Mn but increases for Co. The resistance R2oxid , which is associated to the aluminium oxide increases in the presence of cerium and manganese cations, reaching values of 75.6 k cm2 and 64.5 k cm2 , respectively, revealing that these two cations reinforce the protective character of the interfacial aluminium oxide layer. The admittance of the CPE decreases from 2 × 10−5  cm2 s−n to values around 5 × 10−6  cm2 s−n . For the coating containing cobalt, this resistance decreases and reaches values that are similar for those of the blank coatings. Concomitantly the admittance of the CPE increases. Table 7 Fitting parameters (resistances) for the coatings modified with organic inhibitors. Time

R1oxid ( cm2 )

R2oxid ( cm2 )

Rpit ( cm2 )

2MB 24 48 72

218 215.1 200.3

25,265 24,719 20,978

33,631 33,914 47,554

BTA 24 48 72

61.56 197.3 170.2

23,767 22,825 11,000

29,283 46,675 21,500

8HQ 24 48 72

169.5 167.2 152.8

10,474 7451 6480

15,807 8837 8054

661

The R2oxide values are much higher than those of R1oxide , being in the range expected for aluminium oxide as shown in previous works [39]. The low frequency resistance, which is closely related with the corrosion activity in the pits formed on the oxide film increases with time, being the final values higher in the presence of Ce(NO3 )3 (88.6 k cm2 ) and Mn(NO3 )2 (62.8 k cm2 ). This resistance suffers an important decrease in the presence of cobalt from 36.5 k cm2 down to 7 k cm2 . The corresponding admittance of the CPE increases, reaching the highest values (>185 × 10−5  cm2 s−n ) for the Co containing coating. These admittances of the CPE values are too high for being related to the double layer processes and can be attributed to mass transport phenomena across the corrosion products formed inside the active pits. The CPE exponent was in the range 0.62–0.5. For the organic inhibitors, the resistance of the coating pores could not be detected, revealing that the coating barrier properties were strongly damaged. The evolution of the first oxide resistance R1oxid is characterized by a common decrease for all the organic inhibitor modified coatings, the values are slightly higher for 2MB (∼218  cm2 ). BTA revealed values lower than those of 8HQ (160–150  cm2 ). Generally these values are much lower than those observed for the inorganic inhibitors, revealing that the new interfacial layer formed is less protective in the case of the organic inhibitors. The admittance of the CPE generally increased with time. The second resistance R2oxide decreases for all the coatings modified with organic inhibitors and those modified with 8HQ show the strongest drop for about one half of the initial values, being very close to the values measured for the blank coating. The coating modified with 2MB is the most stable one. Again these values are much lower than those determined in the coating containing inorganic inhibitors, showing that the aluminium oxide layer is more attacked in these coatings. The resistance for the active pits tends to decrease for the BTA and 8HQ coatings, revealing more intense corrosion activity. Slightly higher values could be determined for the 2MB coating. For the 8HQ the values are about 5 times lower, revealing that this inhibitor has little effect on the development of pitting attack and deposition of corrosion products. In previous works [14,15] it was observed that these organic inhibitors provided effective corrosion inhibition ability in sol–gel coatings but, in this system, the very poor barrier properties of the coating most probably accelerate inhibitor leakage from the coating and therefore the loss of effective corrosion inhibition. The EIS measurements reveal some important features for the ORMOSIL–epoxy coatings modified with different corrosion inhibitors, which are reported as effective for corrosion protection of AA2024. When mixed in the ORMOSIL–epoxy coating none of the tested inhibitors could completely suppress pitting activity, since a low frequency time constant could be detected in all the coatings. However, the addition of inorganic inhibitors reveals a beneficial impact on the ORMOSIL–epoxy coating resistance, since the barrier properties are enhanced comparatively to the blank ORMOSIL coating. A possible explanation is that the interfacial layer is reinforced by the formation of metallic hydroxides or by the formation of more resistive aluminium oxide film, thus delaying pitting attack; the effect being more marked for cerium and manganese nitrate. In the case of the cerium ions the beneficial effects on the coating properties can be due to enhanced cross-linking reactions [24,25]. The increased interfacial resistances of the Ce modified coatings, which reveal effective protection concerning localized corrosion attack, are probably due to precipitation of highly insoluble deposits over the cathodic intermetallics present on the AA2024-T3 [40,41]. Mn(NO3 )2 also revealed interesting results. It has been reported that permanganate-based conversion pre-treatments can perform well on corrosion protection of aluminium alloys [28] but little

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has been discussed regarding manganese nitrate. Nevertheless, literature reports Mn2+ ions can attenuate local acidification on Zn dissolution, promoting the interfacial oxide layer stability [42]. Nevertheless the precipitation of Mn(OH)2 over the cathodic sites can also contribute to improved stability of the increased interfacial and polarization resistances. The corrosion protection ability is closely related to the nitrate salt cation, being less effective for cobalt. The corrosion inhibition ability of cobalt ions has been compared with that of Ce ions and it was concluded that Co reduces the oxygen reduction rate in more alkaline solutions (pH 9.5), while the Ce pretreatment works better in slightly less alkaline solutions (pH 7–8.5) [29]. Although AA2024T3 pitting attack is characterized by important acidification in the anodic zones and alkalinization in the cathodic ones, it may happens that the pH does not reach sufficient alkaline values to activate the precipitation of Co(OH)2 . The EIS results show that the inhibiting performance depends upon the cation and that the effect of nitrate, which is also known to inhibit corrosion, seems negligible in the present situation. It is known that nitrate salts are soluble, so that the small amounts incorporated into the coatings are likely to leak quickly into the NaCl, reducing the nitrate to chloride ratio. Identical explanation has been proposed by Danilidis et al. [28]. Generally the organic inhibitors tested, which are known as effective for aluminium alloys, are not suitable to modify the ORMOSIL–epoxy coatings, since the barrier properties are damaged and, therefore, the effectiveness of these inhibitors is most lost in these conditions. The inorganic inhibitors used either as direct additives improve the anti-corrosion properties of the ORMOSIL–epoxy coatings. Since they improve the barrier properties of the organic coatings they are promising candidates to be encapsulated in reservoirs for enhanced coating compatibility and controlled release and, therefore more effective and smart corrosion inhibition ability. 4. Conclusions The ORMOSIL coatings revealed an uneven surface characterized by a coating thickness ranging from 1.0 to 4.6 ␮m. The presence of the organic and inorganic inhibitors was confirmed by EDX analysis. The ORMOSIL coatings modified with the inorganic corrosion inhibitors Ce(NO3 )3 and Mn(NO3 )2 result in improved coating barrier properties, comparatively to the blank coating. They also result in higher interfacial oxide resistances, thus delaying pitting attack and propagation. A new time constant, accounting for the inhibitor protection ability, is clearly evidenced in the EIS spectra. The addition of Co(NO3 )3 is not detrimental for the coating but the positive impact on the interfacial oxide layer is easily lost and, finally, it results in higher corrosion activity. The organic inhibitors do not enhance the barrier properties of the ORMOSIL–epoxy coatings. Among the three inhibitors tested the most effective one seems 2-mercaptobenzothiazole and the less effective one is 8-hydroxyquinoline. Inorganic inhibitors can be used as additives to improve the corrosion resistance of the ORMOSIL coatings but the organic ones are not suitable for direct modification this type of coatings. Acknowledgements The authors want to thanks “EADS” Deutschland for providing the samples of AA2024-T3. The abbreviation EADS

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