Corrosion Science 67 (2013) 82–90

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Corrosion Science

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Hybrid epoxy–silane coatings for improved corrosion protection of Mg alloy

Fabiola Brusciotti ⇑, Darya V. Snihirova, Huibin Xue, M. Fatima Montemor, Svetlana V. Lamaka,Mario G.S. FerreiraInstituto Superior Técnico, ICEMS, Av. Rovisco Pais, 1049-001 Lisbon, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 May 2012Accepted 13 October 2012Available online 24 October 2012

Keywords:A. MagnesiumA. AlloyA. Organic coatingsB. EIS

0010-938X/$ - see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.corsci.2012.10.013

⇑ Corresponding author. Tel.: +34633447322; fax: +E-mail address: fb118@columbia.edu (F. Brusciotti

New hybrid epoxy–silane coatings, with added functionalities for improved performance and durability,were designed to increase the corrosion protection of magnesium alloys. The corrosion behavior of thecoated AZ31 was studied through electrochemical impedance spectroscopy (EIS) in 0.05 M NaCl. Themorphology and surface chemistry of the samples were also investigated through scanning electronmicroscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) before and after immersion inthe electrolyte. The new hybrid silane coatings showed a high resistance to corrosion that persistedthroughout one-month immersion in a pH-neutral NaCl solution.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Magnesium alloys are characterized by a unique set of proper-ties (lightweight, high thermal conductivity, dimensional stabilityand damping characteristics, recyclability,. . .) [1–5], which makethem valuable materials for many industrial applications, such asautomotive and aerospace components, sporting goods, electronicsand bio-compatible implants.

In aeronautics the use of magnesium alloys is particularly grow-ing, mostly driven by the increasing importance of fuel economyand reduction of CO2 emissions, hence the need to reduce theweight of the aircraft [6,7]. In this perspective, Mg alloys representa good candidate because of their high strength-to-weight ratio.

However, a major drawback in the use of magnesium alloys istheir high susceptibility to corrosion, caused by internal galvaniccouples due to second phases or impurities and the nature of thehydroxide film on the surface, which is porous and poorly protec-tive [3,5,8].

Literature on protective coatings for Mg alloys is still poor ifcompared to aluminum and steel. Anodizing [9] has proved to im-prove the corrosion resistance of the alloys under aggressive envi-ronments, as anodic oxidation produces an oxide film with goodcorrosion resistance and reasonable adhesion properties. Chro-mates-based coatings are also efficient corrosion protection sys-tems. Both strategies, though, present major drawbacks, such ascost issues (anodizing) and undesirable environmental and healtheffects (chromates).

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351 218419771.).

In this perspective, sol–gel based silanes have attracted signifi-cant interest as versatile coatings, easy to process and apply, withpotential to replace chromate-based corrosion protection surfacetreatments for Mg alloys, as stated in some of the most relevantwork on the matter [10–22]. The performance of such surfacetreatments is mostly dependant on their ability to form a densebarrier against the penetration of water and corrosion precursors,and on their adhesive bond strength to the substrate, which canbe achieved through introduction of various organo-functionalgroups into the silane matrix, thus tailoring their chemical compo-sition. Hence, sol–gel processes represent an effective and environ-mentally friendly route to prepare films on metallic substrates atlow cost. In addition, they have a simple application procedure,easily adaptable within industry. From the point of view of synthe-sis, the sol–gel route offers versatile ways to synthesize effectivecoatings with specific properties. Functionality is optimized by var-iation of experimental parameters such as chemical structure,composition and ratio of precursors and complexing agents, rateand conditions of hydrolysis, synthesis media, embedding of addi-tional active species, aging and curing conditions and depositionprocedure [4].

Hybrid silane based coatings exhibit increased flexibility andthickness as compared to their inorganic counterparts. In general,these sol–gel derived coatings have been found to provide goodcorrosion resistance to metal substrates due to their barrier prop-erties, strong adhesion, chemical inertness, versatility in coatingformulation, and ease of application under ambient temperatureconditions [13,14].

Often, corrosion inhibitors are added to the system in order toprovide added functionalities for improved performance and dura-bility, with self-repair ability for damage recovery [1,3,4,20,23–27].

F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90 83

The presence of additives should further decrease the corrosionrate. The concept is based on damage recovery from corrosion at-tack via controlled release of corrosion inhibitors stored in the or-ganic coating with a specially designed assembly [20,23–26,28,29]or simply dispersed in the polymeric matrix [1,3,4,27,30].

US Patents by Ostrovsky [16,17] disclosed a treatment for im-proved surface corrosion resistance of Mg and its alloys. The pat-ents describe the acid pickling solutions for surface pretreatmentand the compositions of water/organic solutions of hydrolyzedsilanes.

Khramov et al. developed sol–gel processed organic–inorganichybrid coatings with phosphonate functionalities [11,15]. Due tothe chemical interaction between the phosphonate groups andthe surface of magnesium substrate, these specially designed org-ano-silicate barrier coatings are expected to generate protectivelayers with improved adhesion and corrosion resistance for mag-nesium materials. The organo-silicate sols were prepared viaacid-catalyzed hydrolysis and condensation of a mixture of Tetra-ethoxySilane (TEOS) and Diethylphosphonatoethyl-triethoxySilane(PHS) in ethanol/water solution at low water-to-silane ratio. Thesecoating systems showed improved results compared with previousliterature, but their performance was proved only in not aggressiveenvironment (dilute Harrison solution) and for a short time inter-val (2 weeks).

Zhang et al. elaborated a novel sol–gel process, where appropri-ate additives were used to stabilize and disperse uniformly theinorganic salts precursors (such as Ce); the sols with appropriatepH could be applied directly on Mg alloys [10]. Also in this case,the improved corrosion resistance was proven only for a few-hourtime.

Montemor et al. [30] investigated the protective behavior of Bis-[TriEthoxySilysPropyl]Tetrasulfide (BTESPT) modified by the addi-tion of cerium nitrate or lanthanum nitrate in order to introduceactive corrosion protection in the silane film. Lamaka et al. [4]developed hybrid organic–inorganic sol–gel coatings synthesizedby copolymerization of epoxy-siloxane and titanium or zirconiumalkoxides. Addition of Tris(trimethylsilyl) phosphate significantlyimproved the corrosion protection of the Mg alloy. In both casesgood results were achieved in terms of corrosion resistance, butthe performance was evaluated during only one- and two-weekimmersion time, respectively. In addition the electrolyte usedwas 0.005 M NaCl, which is considered diluted, if compared tothe actual trends of industry and to what has been used for thepurpose of this work.

Overall, even though all the above newly-developed coatingsshow some improvement in terms of more environmentallyfriendly corrosion protection for Mg alloys, they still present somelimitations as they provide low durability and poor resistance tocorrosion even in presence of active additives.

Very good results were recently reported by Wang et al. [31].The hybrid sol–gel/polyaniline coating applied to AZ31 alloy with-stood immersion in 3.5% NaCl with the value of the low frequencyimpedance above 106 X cm2 after 27 days of immersion. However,the thickness of the coating was around 53 lm, which is relativelyhigh. In addition, these coatings were doped with corrosion inhib-itors, otherwise filiform corrosion was observed after few hours ofimmersion in diluted Harrisons solution.

Epoxy resins show very good properties such as superiorchemical and corrosion resistance, but at the same time some dis-advantages such as poor mechanical properties, which limit theirapplication. Improved corrosion protection has been obtainedthrough the application of epoxy-based coatings [19,32–34]. Luet al. [33] developed a Mg-rich epoxy coating with remarkable cor-rosion protection (a low frequency impedance above 108 X cm2 forthe first 10 days of immersion in 3 wt.% NaCl), but degradationalready started after 15 days. Zhang et al. [34] also developed

protective epoxy-based coating with high performance againstcorrosion attack, although a decrease of protection can be noticedduring the 300 h immersion time in NaCl.

For corrosion protection of magnesium alloys, one of the mostinteresting ways of modifying epoxy-based coatings is throughthe development of hybrid epoxy–silane coatings. Silane function-ality in this case improves the adhesion and mechanical propertiesof chemically and corrosion resistant epoxy coating. Meanwhile,such hybrid epoxy–silane coatings, which are known and appliedfor steel protection for years [35–38] were not used for protectionof magnesium alloys. Kartsonakis et al. [19] developed very re-cently cross-linked epoxy with organically modified silicatesand incorporated there nanocontainers enriched with ceriummolybdate as corrosion inhibitor. Coated coupons of magnesiumalloy ZK10 were exposed to 0.5 M NaCl solution for 2–4 months.However, the protective properties of the developed coatings canhardly be evaluated at their true value as the Bode plots presentedin [19] show impedance modulus and phase angle data only athigh frequency range, from 105 to 20–50 Hz.

Coatings provide a barrier against corrosion attack; however,once the water reaches the metal/coating interface, delaminationof the film might occur, together with formation of a defect dueto gas evolution, which considerably decreases the protectiveproperties. Hence, a pre-treatment that removes the impurities, in-creases the hydrophobicity of the metal surface and enhancesadhesion is necessary. Surface preparation of the substrate alsoplays a very important role in the overall performance of the coat-ing assembly, as the presence of impurities on Mg alloys has a cru-cial effect on their corrosion behavior [27,39,40]. Hence coatingtreatments are always preceded by a cleaning procedure thatmight consist of a simple grinding and polishing process, or of achemical process such as acid pickling. Among the acids used forcleaning, hydrofluoric acid (HF) has received particular attention,due to the formation of a protective layer on the metal surface,which improves its corrosion resistance [41,18].

The objective of this work is to design a new coating systemable to protect magnesium alloys from corrosion attack, moreeffective than what has been proposed so far in the literature. Hy-brid epoxy–silane coatings have been developed, with the additionof an amine component, which helps creating a better crosslinkedstructure during the curing process [42]. The corrosion protectionperformance of this set of coatings is evaluated by immersing thecoated Mg AZ31 alloy in NaCl for a long period of time and assess-ing the outcome through electrochemical impedance spectroscopy(EIS). EIS is one of the most intensively used and powerful tech-niques for investigation and prediction of corrosion protection. Itprovides an indication of changes in coating and metal interfaceperformance long before visual changes can be observed duringtraditional exposure tests. In particular, EIS can be used to rankthe corrosion resistance provided by the different coatings and toestimate the corrosion protection efficiency of the coatings. TheEIS results were complemented with analytical and morphologicalcharacterization, infra-red spectroscopy and scanning electronmicroscopy, respectively.

The results showed that the structure of the silane moleculesplay an important role in the overall barrier properties and corro-sion resistance.

2. Materials

2.1. Substrate

The magnesium alloy studied in this project is AZ31 (chemicalcomposition: 3 wt.% Al, 0.83 wt.% Zn, 0.31 wt.% Mn, 0.01 wt.% Si,0.003 wt.% Fe, 0.001 wt.% Cu, 0.001 wt.% Ni and the rest is Mg),

84 F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90

mechanically ground in water with silicon/carbon paper withmesh sizes 220, 500 and 1200 and chemically etched in 12% hydro-fluoric acid (HF) for 15 min. This etching treatment is very efficientas it helps removing impurities on the alloy surface and formingMg hydroxides, oxides and fluorides at the metal surface [41]. Itis crucial for the formation of an efficient protective coating.

2.2. Coating preparation

The coating solution consists of three main ingredients: a silane,an epoxy component (Poly (bisphenol A-co-epichlorohydrin), glycidylend-capped) and an amine (Diethylenetriamine, DETA), all purchasedfrom Sigma–Aldrich. Four different silanes were tested: Amino-PropylTriEthoxySilane (APTES), AminoPropylTriMethoxySilane (APT-MS), 3-(GlycidoxyPropyl)TriMethoxySilane (GPTMS) and Tris(TriMethylSilyl)Phosphate (TMSPh). The chemical structure of the si-lanes and the epoxy component are reported in Fig. 1. The silaneand epoxy components were first prepared separately, by dilutingthem in ethanol and acetone, and let stirring for 1 h. Then the twosolutions were mixed and the amine was added. The final concen-tration of the main components is 3 wt.% silane, 34.9 wt.% epoxyand 3.8% amine. The solution was stirred for 6 h before depositionon the substrate. All samples were coated on the same day, as thesolution has a limited stability.

The coating was deposited on the magnesium alloys by dippingthe substrate into solution by means of a laboratory dip coater typeRDC15 from Bungard Elektronik, with adjustable speed settings(insertion and withdrawal speeds) via SIEMENS SPS computer con-trol. Several trials were performed to set the parameters leading tobest performance. Unless stated otherwise, the results presented inthis work concern samples treated under the following conditions:immersion of substrates into the coating solution 3 times for 5 seach, at a speed of 36 cm/min. After dipping, the samples werecured in the oven at 150 �C for 1.5 h.

Coating thickness measurements were performed with a digitalElcometer 355 coating thickness gauge, using the appropriategauge and calibration standards for non-ferrous materials. Mea-surements were taken at a minimum of five points on the samplesurface in order to have a reliable estimate of the coating thickness.

2.3. Electrochemical investigation

The protective properties of the final product (coated Mgalloy) were evaluated through electrochemical impedancespectroscopy (EIS) measurements, performed at the open circuitpotential (OCP) in the 105–10�2 Hz frequency range, using anAUTOLAB PGSTAT302N in potentiostatic mode. The amplitude

Fig. 1. Chemical structure of the main coating components.

of the perturbation was 10 mV rms. The cell consisted of athree-electrode setup: a saturated calomel electrode (SCE) refer-ence, a platinum coil as a counter electrode, and the workingelectrode, which is the coated magnesium alloy under study (ac-tive area: 3.14 cm2). The tests were performed in aqueous pHneutral 0.05 M NaCl. EIS is used to gain valuable informationregarding the barrier properties of the coatings, the corrosionresistance and integrity of the film. The corrosion protectionmay be assessed by various parameters such as the resistanceof the coating, the capacitance of the double layer or the chargetransfer resistance.

2.4. FEG-SEM/EDS

A JEOL-JSM7001F field emission gun scanning electro-microscope (FEG-SEM) equipped with an energy dispersivespectroscopy (EDS) microanalysis hardware was used in order toexamine the morphology and the chemical composition of the sur-face before and after exposure to the chloride-containing solutions.

2.5. FTIR

Fourier transform infrared (FTIR) spectroscopy was performedby means of a NICOLET 5700 FT-IR from Thermo Electron Corpora-tion, with a mid-IR configuration (7400–350 cm�1), equipped witha DTGS TEC detector and a KBr Beam splitter.

3. Results and discussion

3.1. Coating morphology

The surface of the AZ31 alloy coated with the different silane-based epoxy solutions described above is homogeneous, withoutapparent defects on the film surface, independently of the silaneused.

The thickness of the coatings ranged from 8 to 14 lm, as mea-sured with an Elcometer. These measurements were confirmed byobservation with the FEG-SEM at the coatings cross section. EDSanalysis was performed on the cross section images in order toidentify the different layers and make an estimation of the coatingthickness, which is around �12 lm for the APTES coating.

3.2. Corrosion protection properties

The performance of the coatings was studied by EIS on the sam-ples immersed in 0.05 M NaCl. The EIS results are depicted in Fig. 2as Bode plots for all the different coating compositions tested forthis work.

The APTES and APTMS based coatings (Fig. 2a and b) present thebest performance. The two coatings provide a good corrosion resis-tance, which does not show relevant changes after about 1 monthof immersion time in 0.05 M NaCl. It is worth noticing that the lowfrequency impedance after immersion was ca. 109 X cm2, that isconsiderably higher than what has been reported in literature sofar [4,30,31] for silane-based coatings.

On the other side, the GPTMS and TMSPh based coatings (Fig. 2cand d) gave worse results, as the impedance modulus decreasessteadily from the first day of immersion. However, for both sam-ples the impedance values seem to stabilize and show a gradualdecrease at about 2 weeks; after 1 month the impedance modulusis around 107 X cm2, still higher or comparable value to what is re-ported in literature.

These results are consistent with the images in Fig. 3, whichshows the optical micrographs of all samples after exposure toNaCl and the EIS measurements (Fig. 2). A blank sample (AZ31treated with HF) is also shown (Fig. 3e) for comparative purposes.

Fig. 2. EIS Bode plots of coated AZ31 during immersion in 0.05 M NaCl: (a) APTES; (b) APTMS; (c) GPTMS and (d) TMSPh based epoxy coatings.

F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90 85

After 1 month immersion, the blank AZ31 alloy is completely cov-ered by a layer of corrosion product and the impedance modulusdecreased below approximately 104 X cm2.

The GPTMS and TMSPh based coatings (Fig. 3c and d) presentsome pits and larger corroded areas on the surface, in agreementwith the decrease of the impedance modulus with immersion time

(b)(a)

(c)

Bare AZ31

APTES APTMS

TMSPh GPTMS

(d) (e) Fig. 3. Optical images of samples after 1 month immersion in 0.05 M NaCl. (a) APTES, (b) APTMS, (c) GPTMS, (d) TMSPh based epoxy coatings and (e) Bare AZ31.

coating surface

alloy surface

Fig. 4. FEG-SEM image of an APTMS based coating after one-month immersion in0.05 M NaCl. The coating has been partially detached in order to show the patternon the alloy surface underneath.

86 F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90

(Fig. 2c and d). However, especially for the TMSPh film, the level ofdegradation is much lower than that of the uncoated alloy.

The surface of the APTES based coating (Fig. 3a) looks intactafter 1 month immersion in 0.05 M NaCl, consistent with its behav-ior as measured by EIS (Fig. 2a).

The surface of the APTMS based coating (Fig. 3b) presents sev-eral white spots, which could resemble pits at a first sight, and thisseems inconsistent with the EIS results (Fig. 2b). However, furtheranalysis showed that the surface of the coating is indeed intact, asthe EIS data would reveal, and the white spots are the results ofsome reaction taking place under the coating surface. This is clearlyshown in the FEG-SEM image of Fig. 4, where the coating has beenpurposely detached in order to show the two layers: the intactcoating surface on top and the round patterns on the alloy surfaceunderneath. This particular feature will be analyzed in further de-tails below. Surprisingly for highly electrochemically active Mg,once circle-like features on Mg surface appeared, they did notgrow. This may suggest either low aggressiveness of penetratingelectrolyte or blockage of the pathways by the corrosion products,or inhibiting action of the components of the coating.

Overall, the EIS results prove that all the proposed coatings havea high performance compared to those reported in the literaturedescribed above, where EIS experiments were performed in thesame or more diluted NaCl concentration.

Among the coatings developed in this work, APTES and APTMSbased coatings display the best behavior and a more detailed anal-ysis of the processes taking place during their immersion in NaCl isdiscussed below.

A first explanation of the different behavior between the fourcoatings could be found on the organic structure of the silane usedin the formulation (see Fig. 1). The presence of the amine groups in

APTES and APTMS (not present in GPTMS and TMSPh) is probablyresponsible for a denser and better crosslinked polymeric coatingdue to the bonding with the amine groups in DETA and the glycidylgroup in the epoxy component. This would confer to the two coat-ings (APTES and APTMS based) better barrier properties.

A careful analysis of the EIS spectra for APTMS shows some rel-evant information with time. During the first 7 days, the spectra re-veal the presence of one time-constant at about 3 � 103 Hz, which

Fig. 5. FT-IR on APTMS coated AZ31, before and after immersion in 0.05 M NaCl.

F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90 87

is attributed to the coating properties. After 1 week (about thesame time in which the appearance of white spots was first no-ticed), a second time-constant at about 100 Hz is present. Bothrelaxations occur at medium/high frequencies, which suggestsmore a phenomenon linked to the coating properties than a mag-nesium corrosion process, as the overall resistance of the coatingdoes not decrease significantly during one-month immersion time.As mentioned above, in spite of the white spots visible at the end ofthe EIS measurements, the surface of the coating seems corrosionfree. This is also confirmed by the results of the FT-IR measure-ments (Fig. 5), as there is no significant difference between thespectra obtained before and after immersion of the sample in NaCl.

(a)

(b)

Fig. 6. FEG-SEM images of the alloy surface under the coating after one-mo

However, once the coating was detached at the end of the EIS mea-surements, those white spots are clearly visible with FEG-SEM(Fig. 4 and Fig. 6a). They are spread uniformly all over the alloy sur-face, and the EDS analysis revealed the presence of Mg and O,hence they are attributed to the formation of corrosion product(Mg oxides and/or hydroxides), which then protected the alloy sur-face from further corrosion attack and coating degradation. Notethat there is no Cl peak in the EDS spectra confirming water pene-tration through the coating rather than electrolyte solution. This isalso in a good agreement with the high values of impedance, whichwould be lower in the case of 0.05 M NaCl electrolyte uptake.Although the film surface looks intact, the FEG-SEM image inFig. 7b reveals the presence of small holes (indicated with circles),probably representing the paths for water uptake.

The EIS spectra for the sample treated with APTES (Fig. 2a) arecharacterized by three time-constants, all present from the firstday of immersion. The first one, at about 3 � 103 Hz, can be attrib-uted to the coating properties, as it was observed for the APTMScoating. Another one at 102 Hz can be attributed to the formationof Mg oxides/hydroxides at the alloy surface (as in the case of APT-MS, more details on this will follow ahead in the paper) and finallyone at 1 Hz might be related to some intermediate species formingunder the coating. In the case of APTES, explanation of the pro-cesses taking place during immersion in NaCl is more complex ifcompared to APTMS. In fact, the APTES film surface stays intactduring immersion (Fig. 7a), and no holes indicating the passageof electrolyte are visible as in the case of APTMS. However, eventhough there are no white spots visible on the film surface, thecoating was forcedly detached after 1 month immersion in NaCland examined under FEG-SEM. As a result, the surface of the alloyunder the coating presented some patterns (Fig. 6b), similar incomposition to the ones observed on the APTMS samples, but lessfrequent and in smaller amount. Note that only small area of thecoating could be detached at once with the sharp scalpel as thecoatings adhered to the surface very well.

In order to have a better understanding, a more detailed analy-sis of the EIS spectra for the APTES and APTMS coatings (Fig. 2a and

nth immersion in 0.05 MNaCl: (a) APTMS and (b) APTES based coating.

(b)(a)

Fig. 7. APTES (a) and APTMS (b) coated AZ31 after 1 month immersion in 0.05 M NaCl.

(a) (b)

(c)

Fig. 8. Equivalent electrical circuit for modeling the EIS data. (a) and (b) For APTMSbased coatings and (c) for APTES ones.

88 F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90

b) was performed by modeling the data using the equivalent cir-cuits shown in Fig. 8 with one, two and three time-constants, tofollow the interpretation of the plots described above. These equiv-alent circuits include constant phase elements (CPE) to simulatethe capacitive response of the spectra. The CPE is associated toan exponent (a) and it represents a circuit parameter with limitingbehavior as an ideal capacitor if a = 1 and a resistor if a = 0. The cir-cuits in Fig. 8a and b relate to the APTMS spectra, which can bemodeled with one and two time constants depending on immer-sion time in NaCl, while the circuit in Fig. 8c is associated withthe APTES spectra, characterized by three time constants. CPEcoat

and Rcoat refer to the capacitive response and resistance of the coat-ing itself, CPEox-hyd and Rox-hyd to the formation of Mg oxide and/orhydroxide, as described above, and finally the CPEint and Rint are re-lated to the third relaxation in the model with 3 time constants.

Figs. 9 and 10 show how the resistance and capacitance param-eters vary with immersion time. As the two coatings above are

Fig. 9. Evolution of resistance (left) and constant phase element (right) for APTES coatin0.84 in all cases.

modeled with different equivalent circuits (the curves have differ-ent shapes), a direct comparison between them is not feasible.However, it can be observed that the Rcoat is constantly higherfor APTMS compared to APTES, pointing to better coating perfor-mance. At the same time, Rcoat for APTMS slightly decreases withimmersion time, probably due to the intake of electrolyte for theformation of corrosion product under the surface. In the case ofAPTES, the Rcoat is two orders of magnitudes lower, but a slight,constant increase with immersion time can be observed, possiblydue to better crosslinking of the polymer as suggested below.

Both Rox-hyd and Rint for APTES, associated to oxide/hydroxideand intermediate species formed on the alloy surface, have highvalues and can be considered constant with immersion time. Thisis probably due to the very slow formation of corrosion products,which in their turn have a protective action.

Although both coatings show a remarkable behavior in terms ofcorrosion protection of the Mg alloy, it is not easy to explain whatis happening under the coating surface with the elements gatheredso far. A deeper analysis of the physical processes taking place dur-ing immersion in NaCl is on going. As reported in [43], the imped-ance data can be strongly influenced by the nature of the polymerused in the coating and not necessarily by the phenomenaoccurring at the metal/coating interface. The two coatings beingcompared have similar general structure (Fig. 1), with the onlydifference being the presence of triethoxy (APTES) versus trimeth-oxy (APTMS) groups. According to [44], trimethoxy compounds aremore reactive and should bind more easily to the –OH groups onthe oxide-hydroxide/fluoride layers formed on the metallic sub-strate after etching with HF. However, the APTES-based coatingshowed very good adhesion to the substrate, as after 1 monthimmersion in NaCl it was impossible to detach from the surfaceunless using a sharp scalpel, which removed only a very smallcoating area. On the other side, it was less hard to detach the

gs during immersion in 0.05 M NaCl. The exponent (a) associated to CPE was above

Fig. 10. Evolution of resistance (left) and constant phase element (right) for APTMS coatings during immersion in 0.05 M NaCl. The exponent (a) associated to CPE was above0.96 for CPEcoat and above 0.65 for CPEox-hyd. The double cross lines in the two plots mark the change in electrical circuit (from one to two time constants).

F. Brusciotti et al. / Corrosion Science 67 (2013) 82–90 89

APTMS-based coating, probably due to the presence of the patternsformed underneath the coating surface, behaving as blisters. Oneexplanation for this might be that the lower reactivity and higherflexibility (less stiffness) of the ethoxy groups in APTES accountsfor an easier rearrangement of the molecule during curing time,allowing a more dense and crosslinked structure and better adhe-sion to the substrate (hence slower formation of corrosion producton the alloy surface due to a slow diffusion of oxygen through thecoating, explaining the third time constant in the equivalentcircuit). This could be investigated during future work by varyingthe coatings curing conditions and check how this affects the for-mation of those spots underneath the film. Also, substrate pre-treatment is quite crucial in the formation of an efficient protectivecoating, hence future work should be performed in optimizing thisparameter and understanding the process taking place at theinterface.

4. Conclusions

New hybrid coatings, epoxy-based and with a silane compo-nent, were developed in order to protect magnesium alloy AZ31from corrosion attack. The four suggested coating compositionsdiffer by the type of silane used.

The coated samples were tested for corrosion performanceusing electrochemical impedance spectroscopy during immersionin 0.05 M NaCl for 1 month. The results show that the four pro-posed hybrid coatings have a higher corrosion resistance thanthose reported in the literature, with a considerable improvementshown by the APTMS and APTES based ones, as no relevant degra-dation was observed after one-month immersion. In addition, theAPTES coating showed a very good adhesion to the substrate.

The organic structure of silane plays an important role in thehybrid coating formulation. The presence of the amine groups inAPTES and APTMS (not present in GPTMS and TMSPh) is probablyresponsible for a denser and better crosslinked polymeric coatingdue to the bonding with the amine groups in DETA and glycidylgroup in the epoxy component.

Both APTES and APTMS based coatings showed high perfor-mance in terms of corrosion protection, but the physical processestaking place during immersion are different, probably due to thedifferent behavior of the ethoxy vs. methoxy groups, which charac-terize them.

Further work is being performed in order to better understandthe coatings behavior in aggressive environment.

Acknowledgments

The authors would like to acknowledge Prof. Luis Santos fromInstituto Superior Tecnico for his help with FTIR measurement

and the Portuguese Foundation for Science and Technology (FCT),projects PTDC/CTM-MET/112831/2009, PTDC/ECM/69132/2006and D.V. Snihirova’s PhD grant SFRH/BD/72497/2010.

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