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Effect of surface pre-treatment by silanization on corrosion protection of AA2024-T3 alloy by sol–gel nanocomposite coatings
Surface & Coatings Technology 240 (2014) 353–360
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Effect of surface pre-treatment by silanization on corrosion protection of AA2024-T3 alloy by sol–gel nanocomposite coatings R.V. Lakshmi ⁎, G. Yoganandan, A.V.N. Mohan, Bharathibai J. Basu Surface Engineering Division, CSIR — National Aerospace Laboratories, Bangalore 560017, India
a r t i c l e
i n f o
Article history: Received 12 August 2013 Accepted in revised form 19 December 2013 Available online 30 December 2013 Keywords: Sol–gel Corrosion Organic coatings EIS Passive film
a b s t r a c t Corrosion protection behavior of sol–gel coating doped with cerium nitrate was investigated on AA2024. The sol– gel layer composed of methyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane and colloidal silica. The performance of the sol–gel coating with and without surface silanization was evaluated using electrochemical techniques and neutral salt spray test. The morphology and composition of the coatings were examined by FESEM and EDX techniques before and after corrosion. Electrochemical studies indicated that cerium doped sol–gel coating with the additional APTEOS layer provided better corrosion resistance even after 168 h of exposure among all the developed coatings. Surface silanization provided additional protection to the metal surface. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aerospace aluminium alloy 2024 is susceptible to pitting corrosion in aggressive environments as a result of its heterogeneous microstructure. In this aspect, due to the toxicity and carcinogenic nature of hexavalent chromium, conventional chromate conversion coating needs to be replaced. Sol–gel based coatings are environment-friendly alternative coatings that have been studied for corrosion protection of metals and alloys [1–17]. Numerous reports have been made on surface protecting sol–gel coatings using silane with different functional groups. Of them, 3-glycidoxypropyltrimethoxysilane (GPTMS) is one of the potential precursor silanes containing an epoxide group which can crosslink with inorganic network like silica, zirconia, titania or alumina and result in improved scratch resistance, corrosion resistance and flexibility of the hybrid coatings [4–13]. Another advantage of sol–gel coatings with epoxy groups is their good compatibility and bonding with the epoxy based primer or topcoat typically used for the aircraft structure. Further, silanes with an epoxy functional group facilitate the chemical bonding between the sol–gel coating and the next layer. The surface preparation prior to sol–gel treatment is found to have a marked effect on the corrosion protection and adhesion of the coating on AA2024 specimen [18–20]. The corrosion resistance of a sol–gel hybrid coating is attributed to its physical barrier properties, which restrict the penetration of the electrolyte towards the metallic substrate, thus preventing contact between the metal surface and corrosive species. Sol–gel hybrid coatings blend the mechanical and chemical characteristics of the organic and inorganic networks and can provide a dense ⁎ Corresponding author. Tel.: +91 8025086473. E-mail address: [email protected] (R.V. Lakshmi). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.12.051
barrier film. The presence of the organic part makes the sol–gel network more flexible and less prone to cracking. Studies have proved that pores and defects in the sol–gel layer provide paths for electrolyte penetration and corrode the substrate. Although increasing the coating thickness can improve the performance of the barrier layer, they are prone to cracking when the critical thickness which is about 2 μm is exceeded [21–23]. In the present investigation, a sol–gel layer of a hybrid sol prepared from a mixture of methyltriethoxysilane (MTEOS), GPTMS and colloidal silica based coating is developed on AA2024 substrate with and without cerium nitrate inhibitor. An earlier reported work with MTEOS and colloidal silica on alclad 2024 showed excellent barrier property for the corrosion protection of the substrate [24]. It exhibited good adhesion with the substrate. However, it is known that methyl groups being hydrophobic in nature will not favor the adhesion of the pretreatment layer with the primer layer. Hence, a hybrid sol of MTEOS and colloidal silica along with GPTMS is prepared and studied. In order to improve the adhesion and corrosion resistance performance of the developed coating a new approach of surface silanization is adopted as an adhesive layer. The silanization of the substrate surface is carried out using a diluted ethanolic solution of aminopropyltriethoxysilane (APTEOS). APTEOS is known to be a very good adhesion promoter because of its amine groups. A comparative study is made between the without and with Ce inhibitor and also between single and bi-layer coating. Based on the electrochemical and salt spray results it is found that the adhesion promotion and Ce ion inhibitor play a significant role for improving the corrosion resistance performance of the present developed coating. The developed coatings are characterized by field-emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX), potentiodynamic polarization (Tafel analysis), electrochemical impedance spectroscopy (EIS) and neutral salt spray test as per the ASTM B-117 techniques.
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R.V. Lakshmi et al. / Surface & Coatings Technology 240 (2014) 353–360 Table 1 Tafel analysis of the PDS data of the sol–gel coated coupons. Sample name
icorr (μA/cm2)
Ecorr/SCE (mV)
Corrosion rate (mmpy)
Al uncoated-bare MG without Ce-1 h MG without Ce-72 h MG without Ce-168 h MGCe-1 h MGCe-72 h MGCe-168 h AMGCe-1 h AMGCe-72 h AMGCe-168 h
1.21 0.029 0.109 25.6 0.0186 0.928 13.32 0.0058 0.0395 1.214
−610 −631 −650 −550 −569 −540 −587 −597 −576 −1014
0.0132 0.0003 0.0012 0.279 0.0002 0.0101 0.1452 0.000063 0.00043 0.01323
2. Experimental 2.1. Materials and surface preparation AA2024-T3 with the size of about 25 × 15 × 1.2 mm3 sheet was used as the substrate. Its nominal composition after acetone degreasing was found to be Al 89.44%. Cu 5.82%, Mg 3.68%, Si 0.28%, Ti 0.44% and Zn 0.35% through EDX. MTEOS and APTEOS were purchased from Fluka. GPTMS and colloidal silica (LUDOX® LS, solid contents 30 wt.%, aqueous dispersion, particle size 12 nm, pH = 8.2) were procured from SigmaAldrich and used as received. Ethanol was procured from Merck Chemicals. Cerium(III) nitrate hexahydrate was procured from Aldrich. Aqueous solutions of cerium nitrate in Milli Q water were prepared and used for doping. The specimens were first degreased thoroughly with acetone and ground with 1200 grit emery paper followed by ultrasonication in acetone for 15 min. Then, the panels were immersed in a mixture of sodium carbonate and phosphate solution at 70 °C for 4 min followed by subsequent immersion in 0.5 M sodium hydroxide and 5.4% nitric acid for 2 min each at room temperature. This pretreatment was helpful to remove the smut over the surface and to increase the number of hydroxyl groups present on the surface. A thorough rinse with Milli Q water in between each step was carried out and finally after the pretreatment the specimens were dried with cold air.
2.2. Coating preparation
Fig. 1. (a) Potentiodynamic polarization curves of bare and MG coated substrates; (b) MGCe; (c) AMGCe coating in 3.5% NaCl solution.
The silane solution was prepared by the hydrolysis of 2 ml MTEOS, 2.2 ml GPTMS and 2 ml colloidal silica. 1 M acetic acid was used as catalyst and the water to precursor silane molar ratio was maintained about 3.8. The mixture was kept under stirring condition for 24 h in order to complete the hydrolysis reaction. The sol was then diluted with equal volumes of ethanol and stored in an air tight container in the refrigerator to arrest further condensation reactions. The sol was sprayed in two to three passes to get a uniform coverage on the surface using a spray gun and compressed air with an air pressure of 20 psi. All the coated specimens were cured at ambient conditions for 24 h followed by thermal treatment at 100 °C for 2 h. This coating is denoted as MG without Ce in the further discussion. For coatings with Ce(III) inhibitor, 2 ml of the freshly prepared sol was doped with 6.1 × 10− 3 mol l−1 concentration of cerium nitrate solution. It was mixed well to attain homogeneous phase and then this mixture was sprayed on the freshly prepared pretreated specimens. This coating is denoted as MGCe in the manuscript. All the thermal curing procedure was kept similar for both MG with and without Ce coatings. A thin uniform layer of APTEOS was developed on the freshly prepared pretreated specimens by dip process for 60 s using a 2% ethanolic solution. The dipping and withdrawing of samples were performed at a rate of 50 mm/min and 80 mm/min respectively. The samples were subsequently dried in the cool air for 1 h and heated at 60° C for
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Fig. 2. FESEM images (a, b) of the MGCe coating directly on chemical etched surface, (c) cross-sectional image of a typical sol–gel coating and (d) at higher magnification.
30 min. Over this layer MG with Ce mixture was sprayed as a second layer. This coating is denoted as AMGCe. 2.3. Characterization of sol–gel coating The surface morphology of the coatings was studied using FESEM, model Carl Zeiss Supra 40, equipped with an Energy Dispersive X-ray analysis (EDX), from Oxford Instruments for the elemental analysis. Thickness of the coating was known by cross-sectional FESEM and was also confirmed by a 3D profiler, model Nanomap 500LS. Hardness and scratch resistance of the coating were measured according to the ISO 15184 standard, using Pencil hardness tester, model Elcometer
501. Adhesion between the coating layer and the substrate was assessed according to the ASTM D3359 standard test method, using Cross Hatch Cutter, model Elcometer 107. The electrochemical studies on bare and coated specimens were conducted using IVIUM b40556. The test was carried out in de-aerated 3.5 wt.% (0.6 M) NaCl solution (200 ± 2 ml) using a conventional three electrode cell equipped with specimen as working electrode (1 cm2), platinum foil and Standard Calomel Electrode (SCE) as counter and reference electrodes respectively. The reference electrode was connected to a Luggin capillary and the tip of the Luggin capillary was placed closer to the surface of the working electrode. The test specimens were immersed in 3.5% NaCl solution for an hour in order to stabilize the
Fig. 3. FESEM images of MGCe coatings on chemical etched surface after 72 h immersion and polarization.
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Fig. 4. FESEM images of AMGCe coatings in the (a) as-prepared condition and (b–d) 72 h immersed and polarized conditions.
open circuit potential (Eocp). An ac voltage signal of 10 mV was applied in a frequency range from 100 kHz to 100 mHz. The impedance data was displayed as Bode plot and Nyquist plots and analyzed using ZSimpwin software. The system was then allowed to attain open circuit potential, the upper and lower potential limits of potentiodynamic polarization were set at ±200 mV with respect to the Eocp. The potentiodynamic polarization (Tafel) plot obtained was represented as Potential vs. log i. The corrosion current density (icorr) and corrosion potential (Ecorr) values were obtained from the intersection point of tangent drawn in cathodic and anodic regions (Tafel extrapolation method). The coated substrates were also evaluated by neutral salt spray (NSS) test according to standard ASTM B117 in a salt spray chamber, model Ascott Sxp 120. Reproducibility of the results was confirmed with at least 3 specimens in each case. 3. Results and discussion 3.1. Evaluation of sol–gel coatings by potentiodynamic polarization All the coated specimens were potentiodynamically polarized after different immersion times in 3.5% NaCl solution such as 1 h, 72 h and 168 h. The corresponding graphs are shown in Fig. 1(a), (b) and (c) and the obtained values are given in Table 1. The polarization result of bare specimen is also given for comparison. From Fig. 1(a) it can be observed that the value of icorr for the sol–gel coating without Ce ion inhibitor (MG without Ce, 2.9 × 10−8 A/cm2) is almost about two orders of magnitude lower than the bare specimen 1.2 × 10−6A/cm2. The result clearly indicates the protection rendered by sol–gel coating. Also, the coating shows an Ecorr value of − 640 mV/SCE after 1 h immersion. This negative shift in Ecorr value compared to the bare substrate indicates the cathodic protection behavior of the developed coating. From the cathodic region of the graph it can be seen that the hydrogen
reduction reaction rate is comparatively reduced for coated specimen than the substrate. In general the sol–gel coating acts as a barrier layer which retards the penetration of H+ ions to undergo reduction reaction. In the anodic domain, a passive region of about 110 mV (Epit − Ecorr) is observed for the MG without Ce coating. This indicates that the coating offers very good barrier for the electrolyte penetration to attack the substrate. After 72 h of immersion, there is no change in the cathodic reaction rate but only a small increase in the anodic reaction rate is observed. After 168 h of immersion the corrosion current density (icorr) value is increased by about 2 orders of magnitude compared to the 72 h immersion. This can be attributed to the continuous attack of Clions. This damages the coating easily and once when the electrolyte reach the substrate the corrosion attack becomes severe and consequently dissolution of metal takes place. Although an increased oxidation reaction rate is observed for MG without Ce coating, there is no considerable pitting reaction in the applied potential range which is observed in the bare specimen. Thus, it indicates the effective inhibition of localized attack of the substrate by the developed coating. However, in order to improve the long term corrosion protection of the coating, it was doped with Ce(III) ion inhibitor. The polarization graph of Ce(III) doped sol–gel coating is shown in Fig. 1(b). The coating with Ce(III) inhibitor shows marginal reduction in the icorr value compared to MG undoped coating after 1 h immersion in NaCl solution. But, it should be noted that the MG coating without Ce shows a passive region in its anodic curve compared to the Ce doped coating. This implies that Ce doping has affected the passive/barrier property of the coating marginally. It is known that inclusion of bigger Ce cations into the hybrid sol will induce partial destabilization of the sol–gel network and deteriorates the integrity of the coating. The defects thus created may allow the Cl- ion to penetrate through to attack the substrate and consequently the corrosion inhibition performance becomes poor [25]. This effect is evidently seen in the 72 h immersed
Table 2 EDX analysis results in the smooth area and pitted area of the AMGCe coating. Element (wt.%)
Al
Si
O
C
Cu
Cl
Ce
Ce/Si
As prepared coating Defect area after 72 h immersion and polarization
– 6.17
29.99 24.25
46.97 44.45
22.81 20.17
– –
– 4.44
0.22 0.53
0.011 0.0218
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sample. The Ce doped coating shows poor corrosion inhibition performance compared to the MG without Ce coating (Table 1). After 168 h of immersion, this coating also shows increase in icorr value which is about 11 and 21 times lower compared to MG with and without Ce inhibitor. The surface morphology of both MG coatings with and without Ce(III) was similar. Surface images of MGCe coatings are shown in Fig. 2. The surface in the as-prepared condition (Fig. 2(a)) was smooth, homogeneous and free from any sort of defects or cracks. However, at higher magnification (Fig. 2(b)) circular blister-like features of sizes ranging from 200 nm to 2 μm were seen. This indicates poor adhesion of the developed coating with the substrate. The thickness of the coating was in the range of 2–3 μm as seen by the cross-sectional FESEM shown in Fig. 2(c). It was also confirmed by the profilometer to be about 3.3 μm. The coatings at higher magnification in Fig. 2(d) showed the presence of silica nanoparticles present in the sol. FESEM images of the 72 h immersed and polarized surfaces are shown in Fig. 3(a)–(c). From the figure it can be seen that the coating had flaked in the form of tiny circular platelets throughout the surface. This explains the poor performance of the coating during immersion time. The multiple cracks formed on the surface due to coating flaking are a clear pathway for the electrolyte penetration and initiation of corrosion. Fig. 3(c) shows the magnified image on the flaked coating. The deposits seen on the surface may be the corrosion products. Hence, it can also be concluded that poor adhesion of the coatings with the substrate was also a reason for the poor corrosion inhibition performance of this coating in NaCl environment since water can stay for a longer time in the interfacial layer. Therefore, to improve the adhesion of the coating a thin layer of APTEOS was used as an adhesion promoter between the substrate and MGCe coating. The as-prepared AMGCe coating exhibited a good scratch resistance with a pencil hardness of 7H and good adhesion with a rating of 5B. The measurements were made as described in our earlier report [22]. Polarization results of AMGCe coating (Fig. 1(c) and Table 1) showed an icorr value of about three orders of magnitude lower than the uncoated substrate. The coating exhibited an icorr value of 0.0058 μA/cm2, which is almost 5 times lower when compared to MG without Ce coating and 3 times lower compared to MGCe coating. There was also an increase in the passive region by about 20 to 30 mV (Epit − Ecorr) compared to MGCe specimen. Thus, the APTEOS layer apart from increasing the adhesion of the MG coating, might also have acted as an additional barrier layer for the electrolyte to penetrate. The 72 h immersed and polarized specimen also exhibited enhanced corrosion protection compared to the other coatings. This indicates that the intermediate layer prevents the Cl- ion attack even after 72 h of immersion. Also, the coating chipping off problem during immersion into NaCl solution was not observed in this coating. After 168 h of immersion the coating showed higher corrosion reaction rate but is comparatively lesser than MGCe. Fig. 4 shows the FESEM images of AMGCe coatings in the as-prepared (Fig. 4(a)) and 72 h immersed and polarized (Fig. 4(b)–(d)) conditions. The asprepared coating showed a smooth defect-free surface unlike MGCe coatings. The coatings did not exhibit any blisters or micro-cracks even at higher magnifications (not shown). However, 72 h immersed and polarized coating showed tiny cracks developed on the surface as shown in Fig. 4(b). Also, there were 2–3 pits formed on the surface due to the chloride ion attack. The coating around the pit had wider cracks as shown in Fig. 4(c) and (d). This pitting attack can also be seen from the polarization graph (Fig. 1(c)), at the pitting potential at −500 mV. EDX analysis was carried out in the smooth area and pitted area of the coating to study the cerium concentration distribution on the surface and the results of EDX analysis are given in Table 2. The Ce/Si ratio present on the as-prepared surface was 0.011 which is closer to the calculated value of 0.0104. The Ce/Si ratio at the pit area showed an increased cerium concentration with Ce/Si ratio to be 0.0218. This is a clear indication of the cerium migration to the damaged spot trying to heal the defect. Corrosion rate [26] was calculated using Eq. (1) and is tabulated in Table 1. It is clearly seen that the corrosion rate has
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Fig. 5. (a) Bode plot, (b) phase angle and (c) Nyquist plot of coated MGCe and AMGCe specimens after 1 h, 72 h and 168 h of immersion in 3.5% NaCl solution.
drastically reduced for AMGCe coatings. This increased corrosion protection can be attributed to the additional APTEOS layer, which would have covered the surface defects. In addition, APTEOS layer would have also acted as an additional barrier for the electrolyte penetration. corrosion rateðmmpyÞ ¼
icorr M 3270 dV
ð1Þ
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where, icorr is the corrosion current density of the sol–gel coating in A/cm2, M is the atomic mass of the metal (Al) in grams (27 g), d is its density in g/cm3 (2.7 g/cm3) and V is its valence. 3.2. Evaluation of sol–gel coatings by EIS studies Electrochemical impedance spectroscopy (EIS) is a useful tool to determine the anticorrosive property of a coating with respect to immersion time. Electrochemical analysis of the coated MGCe and AMGCe specimens is performed after 1 h, 72 h and 168 h of immersion in 3.5% NaCl solution and is displayed as Bode plot, phase angle and Nyquist plot in (Fig. 5). Since MG without Ce shows deterioration in corrosion resistance during immersion into NaCl based on the polarization results the impedance analysis of single layer and bi-layer coatings with Ce(III) inhibitor are discussed in detail. From the Bode plot, it is seen that the phase angle curve of the MGCe coated specimens after 1 h immersion presents three maxima: one is in the higher frequency region at 100 kHz; the second is seen between 1 and 10 kHz and the third is in the lower frequency region in between 1 and 3 Hz. It exhibits two time constant behaviors in the higher frequency to medium frequency region and it implies that the coating is composed of two layers. In general a sol–gel coating will contain a thick porous outer layer and a thin inner barrier layer [27]. This porous outer layer is distinguishable because the rate of evaporation of solvent from the surface will be different from that of the inner layer. Therefore micro-cracks or pores at the outer layer and a relatively defect free compact inner layer will be expected. The surface layer plays a crucial role in the corrosion inhibition behavior. This is because it will act as the pathway for the electrolyte to attack the inner layer and then the substrate. The bode plot phase angle curve of 72 h immersed coating (MGCe-72 h) at higher to intermediate frequency region shows a difference wherein the clearly distinguishable two time constant behavior seen earlier (after 1 h) become a single time constant. It indicates no significant difference between the outer porous and inner compact layers and can be due to the attack of Cl- ion during immersion period. However, there is not much difference in the lower frequency region in both phase angle and modulus of impedance curve observed. This indicates the substrate is not fully attacked by the corrosive species. After 168 h immersion, the shallow peak in the phase angle curve at about 1600 Hz is shifted towards higher frequency region (10,000 Hz) when compared to the 72 h immersion. This shift indicates the penetration of Cl- ion through the defects or formation of a new corrosion product (oxide/hydroxide) layer due to corrosion. A similar observation can also be made in the lower frequency region. This indicates severe attack of the substrate surface and no passive layer exists to differentiate the electrolyte from the surface of the specimen. The two main reasons for phase angle shift towards higher frequency direction are the structural change of the coating due to water sorption and the surface state change of the substrate due to corrosion [28]. Thus, this indicates the occurrence of water uptake/electrolyte diffusion through the nanopores and defects present in the coatings, leading to its deterioration. In this context, it is important to know the porosity induced in the coating due to colloidal silica nanoparticle used in the sol. This is because, sol–gel films heavily loaded with inorganic particles can form a porous film. But, it is reported in the
Fig. 6. Electrochemical equivalent circuits (EECs) used to fit the EIS data.
literature that optimal particle concentration in sol–gel systems is approximately 20% by weight. Also, addition of inorganic nanoparticles has been reported to significantly decrease the porosity of the certain coatings [5,29]. The reduction in the particle size is reported to improve corrosion protection properties. The concentration of Ludox nanosilica added is very much within the range (~17%) and size is 20 nm. Hence colloidal silica nanoparticles may not affect the barrier property of the film significantly. In the case of AMGCe a similar trend is observed in the Bode phase angle and modulus of impedance curve with a small enhanced resistance and capacitance behavior at higher and lower frequency region compared to the MGCe coating after 1 h immersion. The three time constant behavior model of this coating may significantly differ from the previous coating at the second layer. In general the introduction of a new layer in between substrate and MGCe should exhibit some significant difference in the intermediate frequency region. But this coating shows minor difference in the medium frequency region and significant impact on the higher and lower frequency region. This suggests that the adhesion promoter indirectly enhances the resistance and capacitance behavior of the outer layer of the coating (MGCe) and also the double layer on a whole (at substrate–electrolyte interface). The impedance modulus at lower frequency range (0.1 Hz) can be used to compare
Table 3 Fit parameters obtained from fitting the experimental impedance spectra with the equivalent circuit. Sample name
Q1coat (μS sn/cm2)
n1coat
R1coat (kΩ-cm2)
Q2coat (μS sn/cm2)
n2coat
R2coat (kΩ-cm2)
Qdl (μS sn/cm2)
ndl
Rct (kΩ-cm2)
MGCe 1 h MGCe 72 h MGCe 168 h AMGCe 1 h AMGCe 72 h AMGCe 168 h
1.1 – 0.045 1.6 0.005 0.025
0.84 – 0.98 0.8 0.95 0.97
6.6 – 0.025 14 953 0.15
0.08 0.5 – 0.08 5.3 8.5
0.9 0.89 – 0.9 0.86 0.73
0.08 0.05 – 0.07 658 158
8.5 40 343 6.2 3.5 2.2
0.74 0.77 0.5 0.76 0.6 0.7
116 4.4 0.77 591 2.8 6.65
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the protection provided by different systems against corrosion. AMGCe coating exhibited impedance magnitude of about 5.9 × 105 Ω cm2 at low frequency region for 1 h immersion which is 4 times higher than the MGCe coating. After 72 h immersion this coating exhibited similar behavior with higher modulus of impedance in the higher to intermediate frequency region compared to the other coating. This enhanced impedance indicates that the Ce ion inhibitor effectively shows its corrosion resistance property in this coating. Since this coating is compact and highly adherent to the substrate the effective corrosion inhibition performance is clearly visible during immersion into NaCl solution. On the other hand though the Ce ion is present in the MGCe layer, due to poor adhesion the corrosion reaction is more random and the effect of Ce inhibitor becomes negligible or cannot be effectively seen. After 168 h of immersion also the surface protection continues with a small difference in the capacitance behavior at low frequency region. Therefore, the attack of the substrate surface is delayed due to the existence of the APTEOS layer at the surface of the substrate. This can be attributed to the effective corrosion inhibition performance of the intermediate oxide layer (typical Al\O\Si covalent bond formation due to the interaction between Al\OH groups of the substrate and Si\OH groups of the silane). The diameter of the semicircle in the Nyquist plot (Fig. 5(c)) is associated with the polarization resistance and thus the corrosion rate; the larger the semicircle diameter, lower is the corrosion rate. The diameter of the semicircles of all MGCe coatings was found to be smaller than AMGCe coatings. This suggests sooner loss in the coating integrity due to water uptake and penetration of electrolyte in the case of MGCe coatings. Thus improvement in the surface protection due to APTEOS layer is again evident. Increase in immersion times led to the depression in the diameter of the semicircle (inset) in both sets of coatings. The quantitative information obtained from the EIS experiment by electrical equivalent circuit is given in the Table 3. The constant phase
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element (CPE) is used in order to explain the deviation from ideal capacitor behavior. The constant phase element can be defined by ZCPE = 1 / Q(jω)n, where Q is the pseudo capacitance, j is the imaginary function (√−1), ω is the angular frequency and n is associated to the deviation from the ideal behavior of a pure capacitor. The EECs used to model the obtained impedance spectra are given in Fig. 6. In the equivalent circuit, Re is the solution resistance, R1coat and Q1coat are the coating resistance and constant phase element of the outer layer of the sol–gel coating respectively. Similarly R2coat and Q2coat are the resistance and constant phase element of the inner barrier (oxide) layer. Rct is the charge transfer resistance describing the corrosion of the metal and Qdl is the double layer pseudo capacitance. The fitted parameters of the models are given in Table 3. The value of R1coat is seen to be higher for AMGCe coating than the MGCe coating at 1 h immersion condition. This indicates AMGCe to be a better barrier than MGCe coating. After 72 h immersion MGCe is well fitted with double time constant equivalent circuit which is shown in Fig. 6(b). The AMGCe coatings, however still had three time constants. The AMGCe coating showed a value of oxide resistance almost by an order of magnitude higher after 72 h immersion. This can be attributed to the combination of barrier property and cerium effect. As explained above, Ce3+ ions may be precipitated in the form of their oxides and hydroxides at the cathodic sites of coating–substrate interface, promoting a stable oxide layer. No such improvement was observed in the MGCe coating which implies the presence of large number of conductive pathways towards the substrate. The corrosion reaction process at the interface between metal surface and electrolyte can be inferred from the Rct value. Prolonged immersion in the corrosive medium has reduced the Rct and increased the Qdl in all the developed coating. This indicates the initiation of corrosion activity at the interface between coating and substrate surface. However, it should be noted that the Rct value for AMGCe coating have remained higher than the MGCe
Fig. 7. Photographs of (a) 336 h salt spray tested coupons and (b) different areas of AMGCe under study.
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Table 4 EDX analysis of the 336 h salt spray tested AMGCe surface at different locations. Element (wt.%)
Al
Si
O
C
Cu
Cl
Ce
Ce/Si
On the scribe of salt spray tested sample On an area near the scribe On the pit of salt spray tested sample
29.1 16.3 11.9
2.95 14.82 19.94
46.9 47.5 46.7
19.69 20.06 15.05
– 0.27 2.89
– 0.95 2.60
1.27 0.03 0.93
0.43 0.002 0.047
coatings almost by 3 orders of magnitude even after 168 h of immersion. The increase in the Qdl is also in a lower proportion in the case of AMGCe when compared to the MGCe coatings. AMGCe coating hence proves to be a better coating than the other. The descending rate of Rct with immersion time is slower for AMGCe compared to that of MGCe may reveal that the better integrity of the sol–gel coating during the corrosion process [30]. 3.3. Salt spray test The coated specimens and bare substrate were subjected for the neutral salt spray exposure. Salt spray test conducted on these coatings also conveyed the similar information. Fig. 7 shows the photographs of 336 h salt spray subjected specimens. As seen from Fig. 7(a) the coating with APTEOS basecoat rendered a better protection to the substrate than the others. The performance of MGCe coating was almost similar to the bare substrate surface after salt spray exposure. The surface had completely tarnished with lots of pits and salt deposits. The scribe made on the surface was hardly seen due to deposition of corrosion products throughout the surface. This could be because of the multiple cracks forming tendency of the coating as seen earlier. EDX analyses of the AMGCe surface (Fig. 7(b)) showed some interesting results and are shown in Table 4. The Ce/Si ratio present on the as-prepared surface was 0.011 which is closer to the calculated value of 0.0104. Cerium content on the scribed area was high, Ce/Si = 0.43. It was very low in an area adjacent to the scribe (Ce/Si = 0.002). Similarly the EDX analysis on a pit formed by the intermetallics showed higher amounts of cerium inside the pit with Ce/Si ratio being 0.047. Thus migration of cerium(III) to the damaged area is evidently seen. 4. Conclusions Corrosion resistance characteristics of sol–gel single layer and with an adhesive layer coating system were investigated. Incorporation of active corrosion inhibitors was found to increase the corrosion resistance of the sol–gel coating. EDX analysis indicated that the concentration of cerium was higher at the cut and defect areas than the smooth undamaged areas, suggesting Ce(III) migration. A further improvement in the corrosion protection was observed when the sol–gel film was combined with an APTEOS base-coat. This extra layer acted as an adhesive layer as well as an additional barrier for the electrolyte penetration. It gave defect free coatings as seen from the micrographs. Thus an interesting synergistic effect was observed with the application of the two layers. The enhancement in the corrosion resistance of the underlying metal substrate might be due to the combined effect of barrier property and the effect of cerium ions.
Acknowledgment The authors are grateful to Mr. Shyam Chetty, Director, NAL and Dr. C. Anandan, Head, SED for their support. Thanks to Mr. Siju, SED for technical assistance with FESEM and EDX of the coated samples. Thanks to Photography section, NAL for the clear pictures provided. We acknowledge the financial assistance received from CSIR Supra Project under 11th Five Year Plan Scheme, Project No. SIP-SED-04 for carrying out this work. References [1] D. Wang, G.P. Bierwagen, Prog. Org. Coat. 64 (2009) 327–338. [2] Y.J. Du, M. Damron, G. Tang, H. Zheng, C.-J. Chu, J.H. Osborne, Prog. Org. Coat. 41 (2001) 226–232. [3] J.H. Osborne, K.Y. Blohowiak, S.R. Taylor, C. Hunter, G. Bierwagon, B. Carlson, D. Bernard, M.S. Donley, Prog. Org. Coat. 41 (2001) 217–225. [4] H. Wang, R. Akid, Corros. Sci. 49 (2007) 4491–4503. [5] M.L. Zheludkevich, R. Serra, M.F. Montemor, I.M. Miranda Salvado, M.G.S. Ferreira, Surf. Coat. Technol. 200 (2006) 3084–3094. [6] M.L. Zheludkevich, R. Serra, M.F. Montemor, K.A. Yasakau, I.M. Miranda Salvado, M.G.S. Ferreira, Electrochim. Acta 51 (2005) 208–217. [7] E. Roussi, A. Tsetsekou, D. Tsiourvas, A. Karantonis, Surf. Coat. Technol. 205 (2011) 3235–3244. [8] A. Wittmar, H. Caparrotti, M. Wittmar, M. Veith, Surf. Interface Anal. 44 (2012) 70–77. [9] M. Schem, T. Schmidt, J. Gerwann, M. Wittmar, M. Veith, G.E. Thompson, I.S. Molchan, T. Hashimoto, P. Skeldon, A.R. Phani, S. Santucci, M.L. Zheludkevich, Corros. Sci. 51 (2009) 2304–2315. [10] N.P. Tavandashti, S. Sanjabi, T. Shahrabi, Prog. Org. Coat. 65 (2009) 182–186. [11] A. Collazo, A. Covelo, C. Perez, Surf. Interface Anal. 42 (2010) 1201–1204. [12] H. Wang, R. Akid, M. Gobara, Corros. Sci. 52 (2010) 2565–2570. [13] S.K. Tiwari, Manju Tripathi, Raghuvir Singh, Corros. Sci. 52 (2012) 334–341. [14] K.A. Yasakau, M.L. Zheludkevich, O.V. Karavai, M.G.S. Ferreira, Prog. Org. Coat. 63 (2008) 352–361. [15] M. Pepe, S. Aparicio, A. Cere, J. Duran, Non-Cryst. Solids 348 (2004) 162–171. [16] L. Paussa, N.C. Rosero-Navarro, F. Andretta, Y. Castro, A. Duran, M. Aparicio, L. Fedrizzi, Surf. Interface Anal. 42 (2010) 299–305. [17] F. Andreatta, L. Paussa, A. Lanzutti, N.C. Rosero Navarro, M. Aparicio, Y. Castro, A. Duran, D. Ondratschek, L. Fedrizzi, Prog. Org. Coat. 72 (2011) 3–14. [18] P. Alvarez, A. Collazo, A. Covelo, X.R. Nóvoa, C. Pérez, Prog. Org. Coat. 69 (2010) 175–183. [19] A.S. Hamdy, D.P. Butt, A.A. Ismail, Electrochim. Acta 52 (2007) 3310–3316. [20] B.J. Basu, A. Srinivasan, J. Manasa, V.K.W. Grips, Surf. Eng. 28 (2012) 294–299. [21] S. Sakka, J. Sol–Gel Sci. Technol. 2 (1994) 451–455. [22] A. Atkinson, R.M. Guppy, J. Mater. Sci. 26 (1991) 3869–3873. [23] P. Innocenzi, M.O. Abdirashid, M. Guglielmi, J. Sol–Gel Sci. Technol. 3 (1994) 47–55. [24] R.V. Lakshmi, G. Yoganandan, K.T. Kavya, B.J. Basu, Prog. Org. Coat. 76 (2013) 367–374. [25] D. Raps, T. Hack, J. Wehr, M.L. Zheludkevich, A.C. Bastos, M.G.S. Ferreira, O. Nuyken, Corros. Sci. 51 (2009) 1012–1021. [26] R. Vennila, P. Kamaraj, M. Arthanareeswari, Chem. Sci. Trans. 2 (S1) (2013) S173–S180. [27] S.M.A. Hosseini, A.H. Jafari, E. Jamalizadeh, Electrochim. Acta 54 (2009) 7207–7213. [28] S.Y. Zhang, Y. king, Z.S. Zhang, X.Y. Zhang, J. Appl. Electrochem. 33 (2003) 1063–1068. [29] A. Conde, A. Duran, J.J. de Damborenea, Prog. Org. Coat. 46 (2003) 288–296. [30] H. Shi, F. Liu, E. Han, Mater. Chem. Phys. 124 (2010) 291–297.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of surface pre-treatment by silanization on corrosion protection of AA2024-T3 alloy by sol–gel nanocomposite coatings R.V. Lakshmi ⁎, G. Yoganandan, A.V.N. Mohan, Bharathibai J. Basu Surface Engineering Division, CSIR — National Aerospace Laboratories, Bangalore 560017, India
a r t i c l e
i n f o
Article history: Received 12 August 2013 Accepted in revised form 19 December 2013 Available online 30 December 2013 Keywords: Sol–gel Corrosion Organic coatings EIS Passive film
a b s t r a c t Corrosion protection behavior of sol–gel coating doped with cerium nitrate was investigated on AA2024. The sol– gel layer composed of methyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane and colloidal silica. The performance of the sol–gel coating with and without surface silanization was evaluated using electrochemical techniques and neutral salt spray test. The morphology and composition of the coatings were examined by FESEM and EDX techniques before and after corrosion. Electrochemical studies indicated that cerium doped sol–gel coating with the additional APTEOS layer provided better corrosion resistance even after 168 h of exposure among all the developed coatings. Surface silanization provided additional protection to the metal surface. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aerospace aluminium alloy 2024 is susceptible to pitting corrosion in aggressive environments as a result of its heterogeneous microstructure. In this aspect, due to the toxicity and carcinogenic nature of hexavalent chromium, conventional chromate conversion coating needs to be replaced. Sol–gel based coatings are environment-friendly alternative coatings that have been studied for corrosion protection of metals and alloys [1–17]. Numerous reports have been made on surface protecting sol–gel coatings using silane with different functional groups. Of them, 3-glycidoxypropyltrimethoxysilane (GPTMS) is one of the potential precursor silanes containing an epoxide group which can crosslink with inorganic network like silica, zirconia, titania or alumina and result in improved scratch resistance, corrosion resistance and flexibility of the hybrid coatings [4–13]. Another advantage of sol–gel coatings with epoxy groups is their good compatibility and bonding with the epoxy based primer or topcoat typically used for the aircraft structure. Further, silanes with an epoxy functional group facilitate the chemical bonding between the sol–gel coating and the next layer. The surface preparation prior to sol–gel treatment is found to have a marked effect on the corrosion protection and adhesion of the coating on AA2024 specimen [18–20]. The corrosion resistance of a sol–gel hybrid coating is attributed to its physical barrier properties, which restrict the penetration of the electrolyte towards the metallic substrate, thus preventing contact between the metal surface and corrosive species. Sol–gel hybrid coatings blend the mechanical and chemical characteristics of the organic and inorganic networks and can provide a dense ⁎ Corresponding author. Tel.: +91 8025086473. E-mail address: [email protected] (R.V. Lakshmi). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.12.051
barrier film. The presence of the organic part makes the sol–gel network more flexible and less prone to cracking. Studies have proved that pores and defects in the sol–gel layer provide paths for electrolyte penetration and corrode the substrate. Although increasing the coating thickness can improve the performance of the barrier layer, they are prone to cracking when the critical thickness which is about 2 μm is exceeded [21–23]. In the present investigation, a sol–gel layer of a hybrid sol prepared from a mixture of methyltriethoxysilane (MTEOS), GPTMS and colloidal silica based coating is developed on AA2024 substrate with and without cerium nitrate inhibitor. An earlier reported work with MTEOS and colloidal silica on alclad 2024 showed excellent barrier property for the corrosion protection of the substrate [24]. It exhibited good adhesion with the substrate. However, it is known that methyl groups being hydrophobic in nature will not favor the adhesion of the pretreatment layer with the primer layer. Hence, a hybrid sol of MTEOS and colloidal silica along with GPTMS is prepared and studied. In order to improve the adhesion and corrosion resistance performance of the developed coating a new approach of surface silanization is adopted as an adhesive layer. The silanization of the substrate surface is carried out using a diluted ethanolic solution of aminopropyltriethoxysilane (APTEOS). APTEOS is known to be a very good adhesion promoter because of its amine groups. A comparative study is made between the without and with Ce inhibitor and also between single and bi-layer coating. Based on the electrochemical and salt spray results it is found that the adhesion promotion and Ce ion inhibitor play a significant role for improving the corrosion resistance performance of the present developed coating. The developed coatings are characterized by field-emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX), potentiodynamic polarization (Tafel analysis), electrochemical impedance spectroscopy (EIS) and neutral salt spray test as per the ASTM B-117 techniques.
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R.V. Lakshmi et al. / Surface & Coatings Technology 240 (2014) 353–360 Table 1 Tafel analysis of the PDS data of the sol–gel coated coupons. Sample name
icorr (μA/cm2)
Ecorr/SCE (mV)
Corrosion rate (mmpy)
Al uncoated-bare MG without Ce-1 h MG without Ce-72 h MG without Ce-168 h MGCe-1 h MGCe-72 h MGCe-168 h AMGCe-1 h AMGCe-72 h AMGCe-168 h
1.21 0.029 0.109 25.6 0.0186 0.928 13.32 0.0058 0.0395 1.214
−610 −631 −650 −550 −569 −540 −587 −597 −576 −1014
0.0132 0.0003 0.0012 0.279 0.0002 0.0101 0.1452 0.000063 0.00043 0.01323
2. Experimental 2.1. Materials and surface preparation AA2024-T3 with the size of about 25 × 15 × 1.2 mm3 sheet was used as the substrate. Its nominal composition after acetone degreasing was found to be Al 89.44%. Cu 5.82%, Mg 3.68%, Si 0.28%, Ti 0.44% and Zn 0.35% through EDX. MTEOS and APTEOS were purchased from Fluka. GPTMS and colloidal silica (LUDOX® LS, solid contents 30 wt.%, aqueous dispersion, particle size 12 nm, pH = 8.2) were procured from SigmaAldrich and used as received. Ethanol was procured from Merck Chemicals. Cerium(III) nitrate hexahydrate was procured from Aldrich. Aqueous solutions of cerium nitrate in Milli Q water were prepared and used for doping. The specimens were first degreased thoroughly with acetone and ground with 1200 grit emery paper followed by ultrasonication in acetone for 15 min. Then, the panels were immersed in a mixture of sodium carbonate and phosphate solution at 70 °C for 4 min followed by subsequent immersion in 0.5 M sodium hydroxide and 5.4% nitric acid for 2 min each at room temperature. This pretreatment was helpful to remove the smut over the surface and to increase the number of hydroxyl groups present on the surface. A thorough rinse with Milli Q water in between each step was carried out and finally after the pretreatment the specimens were dried with cold air.
2.2. Coating preparation
Fig. 1. (a) Potentiodynamic polarization curves of bare and MG coated substrates; (b) MGCe; (c) AMGCe coating in 3.5% NaCl solution.
The silane solution was prepared by the hydrolysis of 2 ml MTEOS, 2.2 ml GPTMS and 2 ml colloidal silica. 1 M acetic acid was used as catalyst and the water to precursor silane molar ratio was maintained about 3.8. The mixture was kept under stirring condition for 24 h in order to complete the hydrolysis reaction. The sol was then diluted with equal volumes of ethanol and stored in an air tight container in the refrigerator to arrest further condensation reactions. The sol was sprayed in two to three passes to get a uniform coverage on the surface using a spray gun and compressed air with an air pressure of 20 psi. All the coated specimens were cured at ambient conditions for 24 h followed by thermal treatment at 100 °C for 2 h. This coating is denoted as MG without Ce in the further discussion. For coatings with Ce(III) inhibitor, 2 ml of the freshly prepared sol was doped with 6.1 × 10− 3 mol l−1 concentration of cerium nitrate solution. It was mixed well to attain homogeneous phase and then this mixture was sprayed on the freshly prepared pretreated specimens. This coating is denoted as MGCe in the manuscript. All the thermal curing procedure was kept similar for both MG with and without Ce coatings. A thin uniform layer of APTEOS was developed on the freshly prepared pretreated specimens by dip process for 60 s using a 2% ethanolic solution. The dipping and withdrawing of samples were performed at a rate of 50 mm/min and 80 mm/min respectively. The samples were subsequently dried in the cool air for 1 h and heated at 60° C for
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355
Fig. 2. FESEM images (a, b) of the MGCe coating directly on chemical etched surface, (c) cross-sectional image of a typical sol–gel coating and (d) at higher magnification.
30 min. Over this layer MG with Ce mixture was sprayed as a second layer. This coating is denoted as AMGCe. 2.3. Characterization of sol–gel coating The surface morphology of the coatings was studied using FESEM, model Carl Zeiss Supra 40, equipped with an Energy Dispersive X-ray analysis (EDX), from Oxford Instruments for the elemental analysis. Thickness of the coating was known by cross-sectional FESEM and was also confirmed by a 3D profiler, model Nanomap 500LS. Hardness and scratch resistance of the coating were measured according to the ISO 15184 standard, using Pencil hardness tester, model Elcometer
501. Adhesion between the coating layer and the substrate was assessed according to the ASTM D3359 standard test method, using Cross Hatch Cutter, model Elcometer 107. The electrochemical studies on bare and coated specimens were conducted using IVIUM b40556. The test was carried out in de-aerated 3.5 wt.% (0.6 M) NaCl solution (200 ± 2 ml) using a conventional three electrode cell equipped with specimen as working electrode (1 cm2), platinum foil and Standard Calomel Electrode (SCE) as counter and reference electrodes respectively. The reference electrode was connected to a Luggin capillary and the tip of the Luggin capillary was placed closer to the surface of the working electrode. The test specimens were immersed in 3.5% NaCl solution for an hour in order to stabilize the
Fig. 3. FESEM images of MGCe coatings on chemical etched surface after 72 h immersion and polarization.
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Fig. 4. FESEM images of AMGCe coatings in the (a) as-prepared condition and (b–d) 72 h immersed and polarized conditions.
open circuit potential (Eocp). An ac voltage signal of 10 mV was applied in a frequency range from 100 kHz to 100 mHz. The impedance data was displayed as Bode plot and Nyquist plots and analyzed using ZSimpwin software. The system was then allowed to attain open circuit potential, the upper and lower potential limits of potentiodynamic polarization were set at ±200 mV with respect to the Eocp. The potentiodynamic polarization (Tafel) plot obtained was represented as Potential vs. log i. The corrosion current density (icorr) and corrosion potential (Ecorr) values were obtained from the intersection point of tangent drawn in cathodic and anodic regions (Tafel extrapolation method). The coated substrates were also evaluated by neutral salt spray (NSS) test according to standard ASTM B117 in a salt spray chamber, model Ascott Sxp 120. Reproducibility of the results was confirmed with at least 3 specimens in each case. 3. Results and discussion 3.1. Evaluation of sol–gel coatings by potentiodynamic polarization All the coated specimens were potentiodynamically polarized after different immersion times in 3.5% NaCl solution such as 1 h, 72 h and 168 h. The corresponding graphs are shown in Fig. 1(a), (b) and (c) and the obtained values are given in Table 1. The polarization result of bare specimen is also given for comparison. From Fig. 1(a) it can be observed that the value of icorr for the sol–gel coating without Ce ion inhibitor (MG without Ce, 2.9 × 10−8 A/cm2) is almost about two orders of magnitude lower than the bare specimen 1.2 × 10−6A/cm2. The result clearly indicates the protection rendered by sol–gel coating. Also, the coating shows an Ecorr value of − 640 mV/SCE after 1 h immersion. This negative shift in Ecorr value compared to the bare substrate indicates the cathodic protection behavior of the developed coating. From the cathodic region of the graph it can be seen that the hydrogen
reduction reaction rate is comparatively reduced for coated specimen than the substrate. In general the sol–gel coating acts as a barrier layer which retards the penetration of H+ ions to undergo reduction reaction. In the anodic domain, a passive region of about 110 mV (Epit − Ecorr) is observed for the MG without Ce coating. This indicates that the coating offers very good barrier for the electrolyte penetration to attack the substrate. After 72 h of immersion, there is no change in the cathodic reaction rate but only a small increase in the anodic reaction rate is observed. After 168 h of immersion the corrosion current density (icorr) value is increased by about 2 orders of magnitude compared to the 72 h immersion. This can be attributed to the continuous attack of Clions. This damages the coating easily and once when the electrolyte reach the substrate the corrosion attack becomes severe and consequently dissolution of metal takes place. Although an increased oxidation reaction rate is observed for MG without Ce coating, there is no considerable pitting reaction in the applied potential range which is observed in the bare specimen. Thus, it indicates the effective inhibition of localized attack of the substrate by the developed coating. However, in order to improve the long term corrosion protection of the coating, it was doped with Ce(III) ion inhibitor. The polarization graph of Ce(III) doped sol–gel coating is shown in Fig. 1(b). The coating with Ce(III) inhibitor shows marginal reduction in the icorr value compared to MG undoped coating after 1 h immersion in NaCl solution. But, it should be noted that the MG coating without Ce shows a passive region in its anodic curve compared to the Ce doped coating. This implies that Ce doping has affected the passive/barrier property of the coating marginally. It is known that inclusion of bigger Ce cations into the hybrid sol will induce partial destabilization of the sol–gel network and deteriorates the integrity of the coating. The defects thus created may allow the Cl- ion to penetrate through to attack the substrate and consequently the corrosion inhibition performance becomes poor [25]. This effect is evidently seen in the 72 h immersed
Table 2 EDX analysis results in the smooth area and pitted area of the AMGCe coating. Element (wt.%)
Al
Si
O
C
Cu
Cl
Ce
Ce/Si
As prepared coating Defect area after 72 h immersion and polarization
– 6.17
29.99 24.25
46.97 44.45
22.81 20.17
– –
– 4.44
0.22 0.53
0.011 0.0218
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sample. The Ce doped coating shows poor corrosion inhibition performance compared to the MG without Ce coating (Table 1). After 168 h of immersion, this coating also shows increase in icorr value which is about 11 and 21 times lower compared to MG with and without Ce inhibitor. The surface morphology of both MG coatings with and without Ce(III) was similar. Surface images of MGCe coatings are shown in Fig. 2. The surface in the as-prepared condition (Fig. 2(a)) was smooth, homogeneous and free from any sort of defects or cracks. However, at higher magnification (Fig. 2(b)) circular blister-like features of sizes ranging from 200 nm to 2 μm were seen. This indicates poor adhesion of the developed coating with the substrate. The thickness of the coating was in the range of 2–3 μm as seen by the cross-sectional FESEM shown in Fig. 2(c). It was also confirmed by the profilometer to be about 3.3 μm. The coatings at higher magnification in Fig. 2(d) showed the presence of silica nanoparticles present in the sol. FESEM images of the 72 h immersed and polarized surfaces are shown in Fig. 3(a)–(c). From the figure it can be seen that the coating had flaked in the form of tiny circular platelets throughout the surface. This explains the poor performance of the coating during immersion time. The multiple cracks formed on the surface due to coating flaking are a clear pathway for the electrolyte penetration and initiation of corrosion. Fig. 3(c) shows the magnified image on the flaked coating. The deposits seen on the surface may be the corrosion products. Hence, it can also be concluded that poor adhesion of the coatings with the substrate was also a reason for the poor corrosion inhibition performance of this coating in NaCl environment since water can stay for a longer time in the interfacial layer. Therefore, to improve the adhesion of the coating a thin layer of APTEOS was used as an adhesion promoter between the substrate and MGCe coating. The as-prepared AMGCe coating exhibited a good scratch resistance with a pencil hardness of 7H and good adhesion with a rating of 5B. The measurements were made as described in our earlier report [22]. Polarization results of AMGCe coating (Fig. 1(c) and Table 1) showed an icorr value of about three orders of magnitude lower than the uncoated substrate. The coating exhibited an icorr value of 0.0058 μA/cm2, which is almost 5 times lower when compared to MG without Ce coating and 3 times lower compared to MGCe coating. There was also an increase in the passive region by about 20 to 30 mV (Epit − Ecorr) compared to MGCe specimen. Thus, the APTEOS layer apart from increasing the adhesion of the MG coating, might also have acted as an additional barrier layer for the electrolyte to penetrate. The 72 h immersed and polarized specimen also exhibited enhanced corrosion protection compared to the other coatings. This indicates that the intermediate layer prevents the Cl- ion attack even after 72 h of immersion. Also, the coating chipping off problem during immersion into NaCl solution was not observed in this coating. After 168 h of immersion the coating showed higher corrosion reaction rate but is comparatively lesser than MGCe. Fig. 4 shows the FESEM images of AMGCe coatings in the as-prepared (Fig. 4(a)) and 72 h immersed and polarized (Fig. 4(b)–(d)) conditions. The asprepared coating showed a smooth defect-free surface unlike MGCe coatings. The coatings did not exhibit any blisters or micro-cracks even at higher magnifications (not shown). However, 72 h immersed and polarized coating showed tiny cracks developed on the surface as shown in Fig. 4(b). Also, there were 2–3 pits formed on the surface due to the chloride ion attack. The coating around the pit had wider cracks as shown in Fig. 4(c) and (d). This pitting attack can also be seen from the polarization graph (Fig. 1(c)), at the pitting potential at −500 mV. EDX analysis was carried out in the smooth area and pitted area of the coating to study the cerium concentration distribution on the surface and the results of EDX analysis are given in Table 2. The Ce/Si ratio present on the as-prepared surface was 0.011 which is closer to the calculated value of 0.0104. The Ce/Si ratio at the pit area showed an increased cerium concentration with Ce/Si ratio to be 0.0218. This is a clear indication of the cerium migration to the damaged spot trying to heal the defect. Corrosion rate [26] was calculated using Eq. (1) and is tabulated in Table 1. It is clearly seen that the corrosion rate has
357
Fig. 5. (a) Bode plot, (b) phase angle and (c) Nyquist plot of coated MGCe and AMGCe specimens after 1 h, 72 h and 168 h of immersion in 3.5% NaCl solution.
drastically reduced for AMGCe coatings. This increased corrosion protection can be attributed to the additional APTEOS layer, which would have covered the surface defects. In addition, APTEOS layer would have also acted as an additional barrier for the electrolyte penetration. corrosion rateðmmpyÞ ¼
icorr M 3270 dV
ð1Þ
358
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where, icorr is the corrosion current density of the sol–gel coating in A/cm2, M is the atomic mass of the metal (Al) in grams (27 g), d is its density in g/cm3 (2.7 g/cm3) and V is its valence. 3.2. Evaluation of sol–gel coatings by EIS studies Electrochemical impedance spectroscopy (EIS) is a useful tool to determine the anticorrosive property of a coating with respect to immersion time. Electrochemical analysis of the coated MGCe and AMGCe specimens is performed after 1 h, 72 h and 168 h of immersion in 3.5% NaCl solution and is displayed as Bode plot, phase angle and Nyquist plot in (Fig. 5). Since MG without Ce shows deterioration in corrosion resistance during immersion into NaCl based on the polarization results the impedance analysis of single layer and bi-layer coatings with Ce(III) inhibitor are discussed in detail. From the Bode plot, it is seen that the phase angle curve of the MGCe coated specimens after 1 h immersion presents three maxima: one is in the higher frequency region at 100 kHz; the second is seen between 1 and 10 kHz and the third is in the lower frequency region in between 1 and 3 Hz. It exhibits two time constant behaviors in the higher frequency to medium frequency region and it implies that the coating is composed of two layers. In general a sol–gel coating will contain a thick porous outer layer and a thin inner barrier layer [27]. This porous outer layer is distinguishable because the rate of evaporation of solvent from the surface will be different from that of the inner layer. Therefore micro-cracks or pores at the outer layer and a relatively defect free compact inner layer will be expected. The surface layer plays a crucial role in the corrosion inhibition behavior. This is because it will act as the pathway for the electrolyte to attack the inner layer and then the substrate. The bode plot phase angle curve of 72 h immersed coating (MGCe-72 h) at higher to intermediate frequency region shows a difference wherein the clearly distinguishable two time constant behavior seen earlier (after 1 h) become a single time constant. It indicates no significant difference between the outer porous and inner compact layers and can be due to the attack of Cl- ion during immersion period. However, there is not much difference in the lower frequency region in both phase angle and modulus of impedance curve observed. This indicates the substrate is not fully attacked by the corrosive species. After 168 h immersion, the shallow peak in the phase angle curve at about 1600 Hz is shifted towards higher frequency region (10,000 Hz) when compared to the 72 h immersion. This shift indicates the penetration of Cl- ion through the defects or formation of a new corrosion product (oxide/hydroxide) layer due to corrosion. A similar observation can also be made in the lower frequency region. This indicates severe attack of the substrate surface and no passive layer exists to differentiate the electrolyte from the surface of the specimen. The two main reasons for phase angle shift towards higher frequency direction are the structural change of the coating due to water sorption and the surface state change of the substrate due to corrosion [28]. Thus, this indicates the occurrence of water uptake/electrolyte diffusion through the nanopores and defects present in the coatings, leading to its deterioration. In this context, it is important to know the porosity induced in the coating due to colloidal silica nanoparticle used in the sol. This is because, sol–gel films heavily loaded with inorganic particles can form a porous film. But, it is reported in the
Fig. 6. Electrochemical equivalent circuits (EECs) used to fit the EIS data.
literature that optimal particle concentration in sol–gel systems is approximately 20% by weight. Also, addition of inorganic nanoparticles has been reported to significantly decrease the porosity of the certain coatings [5,29]. The reduction in the particle size is reported to improve corrosion protection properties. The concentration of Ludox nanosilica added is very much within the range (~17%) and size is 20 nm. Hence colloidal silica nanoparticles may not affect the barrier property of the film significantly. In the case of AMGCe a similar trend is observed in the Bode phase angle and modulus of impedance curve with a small enhanced resistance and capacitance behavior at higher and lower frequency region compared to the MGCe coating after 1 h immersion. The three time constant behavior model of this coating may significantly differ from the previous coating at the second layer. In general the introduction of a new layer in between substrate and MGCe should exhibit some significant difference in the intermediate frequency region. But this coating shows minor difference in the medium frequency region and significant impact on the higher and lower frequency region. This suggests that the adhesion promoter indirectly enhances the resistance and capacitance behavior of the outer layer of the coating (MGCe) and also the double layer on a whole (at substrate–electrolyte interface). The impedance modulus at lower frequency range (0.1 Hz) can be used to compare
Table 3 Fit parameters obtained from fitting the experimental impedance spectra with the equivalent circuit. Sample name
Q1coat (μS sn/cm2)
n1coat
R1coat (kΩ-cm2)
Q2coat (μS sn/cm2)
n2coat
R2coat (kΩ-cm2)
Qdl (μS sn/cm2)
ndl
Rct (kΩ-cm2)
MGCe 1 h MGCe 72 h MGCe 168 h AMGCe 1 h AMGCe 72 h AMGCe 168 h
1.1 – 0.045 1.6 0.005 0.025
0.84 – 0.98 0.8 0.95 0.97
6.6 – 0.025 14 953 0.15
0.08 0.5 – 0.08 5.3 8.5
0.9 0.89 – 0.9 0.86 0.73
0.08 0.05 – 0.07 658 158
8.5 40 343 6.2 3.5 2.2
0.74 0.77 0.5 0.76 0.6 0.7
116 4.4 0.77 591 2.8 6.65
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the protection provided by different systems against corrosion. AMGCe coating exhibited impedance magnitude of about 5.9 × 105 Ω cm2 at low frequency region for 1 h immersion which is 4 times higher than the MGCe coating. After 72 h immersion this coating exhibited similar behavior with higher modulus of impedance in the higher to intermediate frequency region compared to the other coating. This enhanced impedance indicates that the Ce ion inhibitor effectively shows its corrosion resistance property in this coating. Since this coating is compact and highly adherent to the substrate the effective corrosion inhibition performance is clearly visible during immersion into NaCl solution. On the other hand though the Ce ion is present in the MGCe layer, due to poor adhesion the corrosion reaction is more random and the effect of Ce inhibitor becomes negligible or cannot be effectively seen. After 168 h of immersion also the surface protection continues with a small difference in the capacitance behavior at low frequency region. Therefore, the attack of the substrate surface is delayed due to the existence of the APTEOS layer at the surface of the substrate. This can be attributed to the effective corrosion inhibition performance of the intermediate oxide layer (typical Al\O\Si covalent bond formation due to the interaction between Al\OH groups of the substrate and Si\OH groups of the silane). The diameter of the semicircle in the Nyquist plot (Fig. 5(c)) is associated with the polarization resistance and thus the corrosion rate; the larger the semicircle diameter, lower is the corrosion rate. The diameter of the semicircles of all MGCe coatings was found to be smaller than AMGCe coatings. This suggests sooner loss in the coating integrity due to water uptake and penetration of electrolyte in the case of MGCe coatings. Thus improvement in the surface protection due to APTEOS layer is again evident. Increase in immersion times led to the depression in the diameter of the semicircle (inset) in both sets of coatings. The quantitative information obtained from the EIS experiment by electrical equivalent circuit is given in the Table 3. The constant phase
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element (CPE) is used in order to explain the deviation from ideal capacitor behavior. The constant phase element can be defined by ZCPE = 1 / Q(jω)n, where Q is the pseudo capacitance, j is the imaginary function (√−1), ω is the angular frequency and n is associated to the deviation from the ideal behavior of a pure capacitor. The EECs used to model the obtained impedance spectra are given in Fig. 6. In the equivalent circuit, Re is the solution resistance, R1coat and Q1coat are the coating resistance and constant phase element of the outer layer of the sol–gel coating respectively. Similarly R2coat and Q2coat are the resistance and constant phase element of the inner barrier (oxide) layer. Rct is the charge transfer resistance describing the corrosion of the metal and Qdl is the double layer pseudo capacitance. The fitted parameters of the models are given in Table 3. The value of R1coat is seen to be higher for AMGCe coating than the MGCe coating at 1 h immersion condition. This indicates AMGCe to be a better barrier than MGCe coating. After 72 h immersion MGCe is well fitted with double time constant equivalent circuit which is shown in Fig. 6(b). The AMGCe coatings, however still had three time constants. The AMGCe coating showed a value of oxide resistance almost by an order of magnitude higher after 72 h immersion. This can be attributed to the combination of barrier property and cerium effect. As explained above, Ce3+ ions may be precipitated in the form of their oxides and hydroxides at the cathodic sites of coating–substrate interface, promoting a stable oxide layer. No such improvement was observed in the MGCe coating which implies the presence of large number of conductive pathways towards the substrate. The corrosion reaction process at the interface between metal surface and electrolyte can be inferred from the Rct value. Prolonged immersion in the corrosive medium has reduced the Rct and increased the Qdl in all the developed coating. This indicates the initiation of corrosion activity at the interface between coating and substrate surface. However, it should be noted that the Rct value for AMGCe coating have remained higher than the MGCe
Fig. 7. Photographs of (a) 336 h salt spray tested coupons and (b) different areas of AMGCe under study.
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Table 4 EDX analysis of the 336 h salt spray tested AMGCe surface at different locations. Element (wt.%)
Al
Si
O
C
Cu
Cl
Ce
Ce/Si
On the scribe of salt spray tested sample On an area near the scribe On the pit of salt spray tested sample
29.1 16.3 11.9
2.95 14.82 19.94
46.9 47.5 46.7
19.69 20.06 15.05
– 0.27 2.89
– 0.95 2.60
1.27 0.03 0.93
0.43 0.002 0.047
coatings almost by 3 orders of magnitude even after 168 h of immersion. The increase in the Qdl is also in a lower proportion in the case of AMGCe when compared to the MGCe coatings. AMGCe coating hence proves to be a better coating than the other. The descending rate of Rct with immersion time is slower for AMGCe compared to that of MGCe may reveal that the better integrity of the sol–gel coating during the corrosion process [30]. 3.3. Salt spray test The coated specimens and bare substrate were subjected for the neutral salt spray exposure. Salt spray test conducted on these coatings also conveyed the similar information. Fig. 7 shows the photographs of 336 h salt spray subjected specimens. As seen from Fig. 7(a) the coating with APTEOS basecoat rendered a better protection to the substrate than the others. The performance of MGCe coating was almost similar to the bare substrate surface after salt spray exposure. The surface had completely tarnished with lots of pits and salt deposits. The scribe made on the surface was hardly seen due to deposition of corrosion products throughout the surface. This could be because of the multiple cracks forming tendency of the coating as seen earlier. EDX analyses of the AMGCe surface (Fig. 7(b)) showed some interesting results and are shown in Table 4. The Ce/Si ratio present on the as-prepared surface was 0.011 which is closer to the calculated value of 0.0104. Cerium content on the scribed area was high, Ce/Si = 0.43. It was very low in an area adjacent to the scribe (Ce/Si = 0.002). Similarly the EDX analysis on a pit formed by the intermetallics showed higher amounts of cerium inside the pit with Ce/Si ratio being 0.047. Thus migration of cerium(III) to the damaged area is evidently seen. 4. Conclusions Corrosion resistance characteristics of sol–gel single layer and with an adhesive layer coating system were investigated. Incorporation of active corrosion inhibitors was found to increase the corrosion resistance of the sol–gel coating. EDX analysis indicated that the concentration of cerium was higher at the cut and defect areas than the smooth undamaged areas, suggesting Ce(III) migration. A further improvement in the corrosion protection was observed when the sol–gel film was combined with an APTEOS base-coat. This extra layer acted as an adhesive layer as well as an additional barrier for the electrolyte penetration. It gave defect free coatings as seen from the micrographs. Thus an interesting synergistic effect was observed with the application of the two layers. The enhancement in the corrosion resistance of the underlying metal substrate might be due to the combined effect of barrier property and the effect of cerium ions.
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