C666 Journal of The Electrochemical Society, 162 (12) C666-C676 (2015)

Ureasilicate Hybrid Coatings for Corrosion Protection ofGalvanized Steel in Chloride-Contaminated Simulated ConcretePore SolutionRita B. Figueira,a,b,∗,z Carlos J. R. Silva,b and Elsa V. Pereiraa

aLNEC, Laboratorio Nacional de Engenharia Civil, 1700-066 Lisboa, PortugalbCentro de Quımica, Universidade do Minho, 4710-057 Braga, Portugal

Organic-inorganic hybrid (OIH) sol-gel coatings based on ureasilicates (U(X)) have promising properties for use as eco-friendlycoatings on hot dip galvanized steel (HDGS) and may be considered potential substitutes for pre-treatment systems containingCr(VI). These OIH coatings reduce corrosion activity during the initial stages of contact of the HDGS samples with highly alkalineenvironments (cementitious media) and allow the mitigation of harmful effects of an initial excessive reaction between cement pastesand the zinc layer. However, the behavior of HDGS coated with U(X) in the presence of chloride ions has never been reported.In this paper, the performance of HDGS coated with five different U(X) coatings was assessed by electrochemical measurementsin chloride-contaminated simulated concrete pore solution (SCPS). U(X) sol-gel coatings were produced and deposited on HDGSby a dip coating method. The coatings performance was evaluated by electrochemical impedance spectroscopy, potentiodynamicpolarization curves measurements, macrocell current density and polarization resistance in contact with chloride-contaminated SCPS.The SEM/EDS analyses of the coatings before and after the tests were also performed. The results showed that the HDGS samplescoated with the OIH coatings exhibited enhanced corrosion resistance to chloride ions when compared to uncoated galvanized steel.© The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited. [DOI: 10.1149/2.0661512jes] All rights reserved.

Manuscript submitted April 13, 2015; revised manuscript received September 3, 2015. Published September 17, 2015.

Corrosion of the reinforcing steel is one of the most importantcauses of degradation in reinforced concrete structures (RCS).1 Thepresence of chloride, carbonation of the concrete or the low qualityof the concrete cover causes serious damage to RCS. The entranceof chloride ions through the concrete from marine environments ordeicing salts can cause rupture of the passive layer, allowing thesteel surface to act as a coupled anodic and cathodic reaction cell inwhich corrosion processes take place.1–3 Therefore, the presence ofchlorides in concrete can seriously shorten the service life of RCS.3,4

The expanded volume of corrosion products deposited in the interfacebetween the concrete and the steel imposes expansive stresses, leadingto delamination, cracking or spalling and eventually to the collapse ofthe RCS.1

Several methods to improve the corrosion resistance of RCS havebeen proposed.1,5–7 In the last few years, the use of hot-dip galva-nized steel (HDGS) has been recognized as an effective measure toincrease the service life of RCS.8–11 The galvanized coating (zinclayer) acts as a physical barrier, hindering the contact of aggressiveagents with the steel substrate and the zinc acts as a sacrificial an-ode protecting the steel against corrosion. In addition, the formedzinc corrosion products have a smaller volume than those producedfrom iron, thus reducing the corrosion-induced delamination, crack-ing or spalling of the concrete.8 Additionally, the galvanized steelreinforcement can withstand exposure to chloride ion concentrationsseveral times higher than the chloride level that causes corrosion insteel reinforcement.8,12 However, when the HDGS is in contact withconcrete pore solution, (which is essentially a Ca(OH)2 saturated so-lution containing significant amounts of KOH and NaOH) the zinccorrodes.8–10,13,14 The pH of this medium is typically above 12.5. Thisinitial corrosion process, which may vary from hours to days, may leadto zinc consumption until either the formation of a passivation layeror until all the zinc layer is consumed.8,13–16 To minimize this cor-rosion process, measures such as increasing the chromate content byadding water-soluble chromates into the concrete mixture or the use ofchromate conversion layers have been widely implemented. Neverthe-less, due to the adverse health effects of the hexavalent chromium ions(Cr(VI)), hard restrictions on the use of chromates have been imposed.As a result, the growing interest in developing new materials to replacechromate conversion layers has led to the synthesis and testing of sev-eral organic-inorganic hybrid (OIH) materials, generally referred as

∗Electrochemical Society Student Member.zE-mail: rmfigueira@lnec.pt; rita@figueira.pt

ORMOCER (Organic Modified Ceramics) or ORMOSILS (OrganicModified Silanes), as corrosion protective coatings on several metallicalloys, such as aluminum,17–20 carbon steel,21–23 stainless steel,21,24–26

galvanized steel27–29 and magnesium.30,31 These OIH films are re-ported to be environmentally friendly alternatives to replace chromateconversion layers.32 Their synthesis is based on the sol–gel processwhich involves the hydrolysis of metal alkoxides to produce hydroxylgroups in the presence of stoichiometric water (generally catalyzedby an acid or base) followed by polycondensation of the resultinghydroxyl groups and residual alkoxyl groups, forming thin, dense andchemically inert films on the metallic substrates with controlled phys-ical shape and dimensions. Precursors containing non-hydrolysablegroups are used to incorporate the organic part in the coating. Thisorganic component provides flexibility and reduces defects while theinorganic constituent provides superior adhesion to the metal surfaceand improves mechanical resistance.33–36 The sol-gel method is a ver-satile and simple method to produce silicate based gel materials underroom temperature. The gel is aged for a period of time to allow thegel network to strengthen and then it is dried under atmospheric con-ditions. Materials prepared in this way are called xerogels.37 Smallmolecules resulting from the condensation process remain trappedwithin the formed network even after the post-gelation drying/curingprocess is completed.38 The OIH gels exhibit a biphasic structure thatcombines a rigid and hydrophilic silicate backbone linked to amor-phous, malleable and hydrophobic organic (polymeric) spacer. Thecoexistence of the two distinct phases enhances the dispersion of alarge variety of hosted species and allows control of the physical prop-erties of the gel (transparency, porosity, wettability, hydrophobicity,etc.).

The term ureasilicate (U(X)) refers to OIH sol-gel coatings ob-tained from a reaction between a functionalized siloxane (3-isocyanatepropyltriethoxysilane) and a di-amino functionalized polyether (Jef-famine) with different molecular weights (ranging from 230 to 2000g mol−1).27 Since urea bonds are formed between the two precursormolecules, the term U(X) has been used to identify this type of OIHnetwork where “U” refers to the type of bond established and “X” tothe molecular weight of Jeffamine used. U(X) coatings have been ex-tensively studied in contact with cementitious media27,39 showing thatthese coatings provide barrier properties that withstand the high pH ofthe electrolyte, protecting the HDGS when it first comes into contactwith cementitious media. The lowest corrosion rates obtained, after127 days of contact with a mortar, are given by U(230) and U(400)and are respectively 30 and 31 times lower than the control samples.39

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Journal of The Electrochemical Society, 162 (12) C666-C676 (2015) C667

Later and considering these promising results,27,39 studies on the in-fluence of the residence time, curing process between each dippingstep and thickness of U(230) and U(400)40 were performed in highlyalkaline environments. The glow discharge optical emission spec-troscopy results show that the coating thickness of U(400) globallyincreases both with the number of dipping steps and with residencetime.40 However, the SEM/EDS results point to the conclusion thatfull coverage is seldom achieved even when three layers are depositedon HDGS with residence time with the curing process between eachdeposition.40,41

The present work is focused on study the corrosion performanceof five U(X) sol-gel coatings (U(230), U(400), U(600), U(900) andU(2000)) in chloride-contaminated alkaline solutions which simulatethe aqueous phase present in concrete pores. This paper describesthe modifications that the presence of chloride ions has on the corro-sion behavior of coated HDGS samples in highly alkaline solutions.Bare HDGS samples were used for comparison purposes. The OIHcoatings were prepared according to the literature.27,39,40 The coat-ings were deposited on HDGS surfaces by dip-coating using one orthree consecutive dip steps. The novelty of the proposed work re-lies on the fact that, to the best of the authors’ knowledge, no studyis available in the literature where U(X) OIH sol-gel coatings de-posited on HDGS have been tested in chloride-contaminated SCPS.The morphology of the OIH films was assessed by scanning elec-tron microscopy (SEM). The corrosion properties of the OIH coat-ings were evaluated by macrocell current density (igal), polarizationresistance (Rp), electrochemical impedance spectroscopy (EIS) andpotentiodynamic polarization methods in chloride-contaminated sim-ulated concrete pore solutions (SCPS). igal and Rp measurements werecarried out since they offer several advantages for corrosion monitor-ing in new and repaired RCS.7,42–45 igal measurements allowed forreal-time and continuous monitoring, providing semi-quantitative in-formation about the corrosion rate and detected the instant wherethe construction steel depassivates.44,46 The Rp measurements wereperformed periodically since these allowed for measurement of in-stantaneous corrosion current density in order to assess the conditionof the embedded steel reinforcement related to its corrosion.47 EIS isa reliable, non-destructive and fast method providing accurate resultsabout corrosion protection behavior of the OIH sol-gel coatings. EISdata were interpreted on the basis of electrical equivalent circuits con-sisting of a combination of resistances and capacitances associatedin series or in parallel providing the same electrical response as thestudied electrochemical interface.23,48–51 The potentiodynamic polar-ization curves were used to compare the corrosion resistance of coatedHDGS samples.52

Experimental

Materials.— Five di-amino functionalized polyethers (hereafterreferred to as Jeffamine) with different molecular weights (MWs)were used. Jeffamine with five different molecular weights providedby Fluka (Jeffamine D-230, Jeffamine D-400, Jeffamine ED-600, Jef-famine ED-900 and Jeffamine ED-2000) and 3-isocyanate propyltri-ethoxysilane (ICPTES, 95%, Aldrich) were stored, protected fromlight and used as supplied. Absolute ethanol (EtOH, absolute 98%,Riedel-de-Haen) and citric acid monohydrate (Merck) were also usedas delivered. Ultra-pure water (0.055–0.060 μS/cm) obtained from aPurelab Ultra System (Elga) was used.

Sol-gel synthesis procedure of OIH ureasilicate coatings.— Theexperimental steps involved in the synthesis of ureasilicate matricesto produce the coatings on HDGS samples were performed accordingto the literature.27,39,40 Five different materials were prepared as coat-ings on HDGS, i.e. U(230), U(400), U(600), U(900) and U(2000). Thematrices were obtained using 1:2 stoichiometric molar ratio of eachJeffamine molecular weight and 3-isocyanate propyltriethoxysilane.These two reagents were mixed and stirred at 700 rpm for 20 min in aglass container. In the second step, 0.22 M of citric acid ethanolic solu-tion was added, setting the citric acid/3-isocyanate propyltriethoxysi-

lane molar ratio equal to 0.094. The mixture was stirred and after15 min distilled water was added until the total volume of reactionmedia equaled 8 mL. The final mixture was left to react for a further15 min.

Preparation of the coatings.— The HDGS samples were obtainedfrom commercially available plates and cut with dimensions of 5.0 ×1.0 × 0.1 (in cm). The HDGS samples with an average Zn thicknessof 16 μm on both sides were degreased with acetone. Coated HDGSsamples were prepared by dipping the metallic plates of HDGS in thesynthesized sol mixture using a dip coater (Nima, model DC Small).The OIH coatings were deposited by one and three consecutive dipsteps at a withdrawal speed of 10 mm min−1 without residence time.Previous studies have shown that the thickness of the U(X) depositedon HDGS by one and three dip steps ranged between 2.5–12.7 μm and3.6–24 μm, respectively39 depending on the MW of Jeffamine used.Therefore, the choice in producing samples coated by one and threeconsecutive dip steps was mainly to assess the corrosion performanceof thinner and thicker coatings in chloride contaminated simulatedconcrete pore solutions. Coated HDGS samples were subsequentlyplaced in an incubator-compressor (ICP-400, Memmert) and kept at40◦C for 15 days.

Preparation of chloride-contaminated simulated concrete poresolution.— The corrosion behavior of HDGS coated samples withthe different OIH coatings was studied in solutions simulating theconcrete interstitial electrolyte (simulated concrete pore solutions -SCPS) and contaminated with 1 wt% of chlorides (SCPS + 1 wt%Cl−). SCPS were prepared according to the literature10,53 by the addi-tion of analytical reagent grades 0.2 M KOH to a Ca(OH)2 saturatedsolution previously prepared with distilled water. A final solution witha pH = 13.2 was obtained and 1 wt% of chlorides was added as sodiumchloride. This medium was prepared in order to induce the corrosionof the substrate. According to the literature,54 the critical chloride con-centration reported to induce corrosion of reinforcing steel in SCPS,with pH values of 12.5 and 13.9, was of 0.02 and 1 wt%, respectively.Considering that the pH of the SCPS prepared in this work was above12.5, a value of 1 wt% was used to ensure that the chloride contentwas above the critical chloride concentration.

Electrochemical studies.— In this work different electrochemicaltechniques were used, namely: electrochemical impedance spec-troscopy (EIS), potentiodynamic polarization curve, polarizationresistance (Rp) and macrocell current density (igal). Two distinct ap-proaches were used to assess the electrochemical behavior of the OIHsol-gel coatings applied on HDGS. In the first approach, short-termstudies on HDGS coated with the different OIHs were conducted inchloride-contaminated simulated concrete pore solutions (SCPS +1 wt% Cl−) using EIS and potentiodynamic polarization measure-ments. In the second approach, the barrier stability of the OIHcoatings was monitored through igal and Rp measurements during 16days in SCPS and on the 8th day 1 wt% of Cl− was added as sodiumchloride. For comparison purposes, uncoated samples (control) werealso studied.

Electrochemical impedance spectroscopy (EIS) and potentiody-namic polarization curves measurements.— EIS and potentiodynamicpolarization curves measurements were carried out on HDGS coatedsamples by one and three consecutive dip steps without residence timein SCPS + 1 wt% Cl−. All the measurements were made in triplicateto check data reproducibility. These measurements were conducted inorder to study the influence of the number of dip steps used to coat theHDGS samples on the barrier protection in the presence of aggres-sive species such as chloride ions. The EIS measurements on HDGScoated samples were performed in the first instants of immersion inSCPS + 1 wt% Cl−. The potentiodynamic curves measurements wereperformed after 2h of exposure to SCPS + 1 wt% Cl−.

The EIS and potentiodynamic polarization curve measurementswere performed at room temperature in a Faraday cage. A glass cell

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C668 Journal of The Electrochemical Society, 162 (12) C666-C676 (2015)

Figure 1. Schematic representation of the system used to perform the igal measurements.

with a saturated calomel electrode (SCE) and a platinum foil (exposedarea ≈ 8 cm2) were used respectively as reference electrode (RE) andcounter electrode (CE). The exposed surface area of the working elec-trode (WE) (HDGS coated sample) in the electrolyte was ≈2 cm2.EIS studies were accomplished by applying a 20 mV (peak-to-peak,sinusoidal) electrical potential within a frequency range from 1 ×105 Hz to 0.01 Hz (10 points per decade) at open circuit potential(OCP). Measurements were performed using an Impedance/Gain-Phase Analyzer (Model 1260A, Solartron-Schlumberger) and a po-tentiostat/galvanostat (Model 1287A, Solartron-Schlumberger) con-trolled by a PC using Zplot software (Solartron-Schlumberger, ver-sion 2.9c). The frequency response data of the studied electrochem-ical cells were displayed in a Nyquist plot, using ZView software(Solartron-Schlumberger, version 2.9c) that was also used for data fit-ting purposes. For comparison purposes, cells prepared with uncoatedHDGS WE electrodes were used as the control.

Polarization resistance (Rp).— The Rp values were estimated bythe potentiostatic method using a potentiostat/galvanostat (VoltalabPGZ 301) using a three-electrode electrochemical cell system.45,46,55

The Rp values were calculated from the transients due to the appli-cation of a 10 mV anodic potential step for 100 s. A stainless steel(SS, type 316L) plate was used as a CE and HDGS coated with thedifferent OIHs was used as WE. Both electrodes had an active averagearea of 2 cm2. The edges of both the CE and WE plates, as well thenon-active areas and connecting zones were protected with a dual-component epoxy resin (Araldite). Titanium-activated wire (Ti/TiO2)with a length of 1 cm was chosen to be used as RE due to its lowcost and suitability to be embedded in real RCS. The electrodes wereconnected to an isolated copper cable and the cutting zone of the tip ofthe RE was covered with epoxy resin. For comparison purposes, cellswith non-coated HDGS WE electrodes were prepared and used as thereference (hereafter referred generically as control) and to check datareproducibility triplicate cells were assembled. The Rp measurementswere performed once a day during 16 days on samples coated byone dip step of U(X). After the first eight days of exposure to SCPS,1 wt% of chloride ions were added as sodium chloride.

Macrocell current density (igal).— The igal measurements were per-formed using a system based on two parallel electrodes (rectangularmetal plates with dimensions of 5.0 × 1.0 × 0.1 cm – Figure 1)according to the literature.27,44 For comparison purposes, cells pre-pared with uncoated HDGS WE electrodes were used as controland triplicate cells were assembled to check data reproducibility. Toassemble the electrochemical cells, the SCPS was transferred to a100 mL polyethylene flask. The electrodes were subsequently im-

mersed and the flask closed (Figure 1). Using an automatic data ac-quisition system (Datataker DT505, series 3), igal measurements ofthe prepared cells, were performed by reading the potential differenceto the terminals (shunted with a 100 � resistor) according to ASTMG109.56 Measurements were performed at one-minute intervals during16 days on samples coated by one dip step. After the first eight daysof exposure to SCPS, 1 wt% of chloride ions were added as sodiumchloride. The measurements were performed at the same periodicityfor more eight days.

Scanning electron microscopy (SEM/EDS).— The morphology ofthe OIH sol-gel coating surface applied on HDGS specimens was an-alyzed with a scanning electron microscope (SEM, JEOL JSM-6400)coupled with an EDS detector (Inca-xSight Oxford Instruments). Thesurface of the samples was covered with an ultrathin coating of golddeposited by sputter coating. SEM investigations of the surfaces werecarried out by using the back-scattered electron (BSE) detector inorder to emphasize the contrast for the different metallic phases.The SEM/EDS studies of the HDGS coated samples were performedon the substrate before and after 16 days in contact with chloride-contaminated SCPS.

Results and Discussion

Electrochemical impedance spectroscopy (EIS).— EIS measure-ments were performed in order to assess the electrochemical behaviorof HDGS coated by one (1 layer) and three (3 layers) consecutive dipsteps of OIH sol-gel coatings in chloride-contaminated SCPS (highlyalkaline media, pH = 13.2). Figure 2 shows representative Bode plotsobtained for control, U(230) and U(400) in the instant of exposure(≈ 10 minutes once the OCP has been established in SCPS + 1 wt%Cl−).

The Nyquist plot obtained for control samples (Figure 3a) in theinstant of exposure consist of one depressed capacitive loop (one time-constant). The high frequency (HF) capacitive loop is associated tothe charge transfer process of the metal corrosion and the double layerbehavior.48,49 For the coated HDGS samples, the impedance spectraconsist of two partially overlapped capacitive semicircles (two time-constants). Samples coated by one or three layers, with the lowestMWs of Jeffamine (U(230) and U(400) – Figures 2 and 3) revealthe presence of two time-constants in first instants of immersion. Theloop at HF, between 102 and 105 Hz, is generally assigned to the OIHsol–gel film capacitance19,57 and the one at low frequency (LF) can beassigned to the corrosive process by charge transference.21,22

The Nyquist plots obtained for samples coated with OIHs syn-thesized using higher MWs of Jeffamine, namely U(600), U(900)

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Journal of The Electrochemical Society, 162 (12) C666-C676 (2015) C669

Figure 2. Representative Magnitude and Phase angle Bode plots obtained forcontrol, U(230) and U(400) in the instant of exposure (≈ 10 minutes once theOCP has been established in SCPS + 1 wt% Cl−).

and U(2000) exhibited also two time-constants at high and mediumfrequency indicating at least two electrochemical processes with sim-ilar relaxation time constants, while one inductive loop was observedat the LF end (Figure 4). This type of impedance spectra have beenreported58–62 for galvanized surfaces due to the surface relaxation pro-cesses of adsorbed species (such as corrosion products) on the HDGSsurface. Considering the high pH of the chloride-contaminated SCPSand the mechanism proposed by Liebau,8,63 the results suggest that theadsorbed species (such as Zn2+, Zn(OH)2, Ca(Zn(OH)3)2·2H2O) maypossibly be the reason for the LF inductive loop found in all HDGSsamples coated with U(600), U(900) and U(2000).

According to the literature, the inductive loop found in the Nyquistplots (Figure 4) can be explained by considering the anodic reaction

Figure 4. Nyquist plots obtained for U(600), U(900) and U(2000) in theinstant of exposure to SCPS + 1 wt% Cl−, (≈ 10 minutes once the OCP hasbeen established).

separately from the cathodic.64–66 A possible cause could be the ad-sorption or desorption of activating or blocking species with increas-ing overpotential of each process. Two situations can be considered:(i) If the anodic adsorption (therefore cathodic desorption) of a block-ing species occurs the inductive loop appears due to the dominanteffect in the cathodic process; or (ii) if the anodic desorption (there-fore cathodic adsorption) of a blocking species for the reactions (OIHlayer) occurs then the observed inductive loop results as the dominanteffect in the anodic component.64 This behavior at LF range suggeststhat these OIH coatings are not entirely protective and may changewhen in contact with the electrolyte, thus exhibiting defects. Barelycoated regions also allow the electrolyte to penetrate across coatingcracks and reach the substrate. Additionally, in a chloride environ-ment, corrosion products such as zinc hydroxychlorides are formedon the surface of zinc.67 The LF capacitive loop67–69 can be also at-tributed to the diffusion of the electrolyte in the pores of the coatingsand the dissolution of the zinc layer. For samples coated with U(230)and U(400), no inductive loops in the LF region were found. Thisindicates that these OIH coatings confer good corrosion protectionand that the corrosion processes in the first instants of contact withchloride-contaminated SCPS are effectively delayed comparativelyto U(600), U(900) and U(2000) coatings. This behavior can be re-lated either to the breakdown of the former protective surface film orrelaxation of adsorbed species in the mechanism of zinc dissolution.65

Figure 3. Representative Nyquist plots and the respective fitting results obtained for a) control (uncoated HDGS); b) U(230) 1 layer; c) U(230) 3 layers; d) U(400)1 layer in the instant of exposure to SCPS + 1 wt% Cl−, (≈ 10 minutes once the OCP has been established). EECs used for the numerical fitting of the: e) controlsample; f) samples coated with OIHs.

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C670 Journal of The Electrochemical Society, 162 (12) C666-C676 (2015)

Detailed information on the behavior of the different OIH sol-gel coated samples can be extracted from the fitting of the spectrausing electrical equivalent circuits (EEC). The selected EECs havebeen widely used for analysis of impedance spectra of metals coatedwith sol-gel films.23,30,33,70 The interpretation of impedance spectrawas based on the EECs that include contributions from the sol-gelcoating and the corrosion process itself. Capacities were replaced byCPEs to improve the quality of the fitting. The physical origin of theCPE behavior is not completely understood, however, it is generallyaccepted that it is assigned to inhomogeneous physical propertiesof the system.71,72 Two different EECs (Figure 3) were used to fit theexperimental data. Figure 3e shows the EEC used to fit the control dataresults and Figure 3f shows the EEC used to fit the results obtained forsamples coated. The solid lines represent the results of the fitting. Forsamples coated by three consecutive dip steps only the fitting obtainedfor U(230) is displayed. The Bode plots obtained for samples coatedwith 1 layer of U(600), U(900) and U(2000) and 3 layers of U(600)are shown in Figure 5. To avoid overloading of Figure 5 only thefitting line for U(600) 1 layer and the results for the sample coatedwith 3 layers of U(600) are shown. The fitted region for these sampleshas been obtained by excluding the range of frequencies associatedto the inductive process and the EEC used is the same as depictedin Figure 3f. For all the EECs, the electrical elements Rs, CPEdl Rdl,

Rcoating and CPEcoating, are associated respectively, with: the electrolyteresistance; double layer capacitance at the metal–electrolyte interface;the charge transfer resistance of zinc; resistance of the sol–gel coatingand capacitance of the sol–gel coating. The fitting parameters areshown in Tables I and II and considering the χ2 obtained for each oneit can be assumed that a good fit is shown in Figures 3 and 5.

All the coatings show comparable responses immediately aftertheir immersion in chloride-contaminated SCPS, exhibiting low ca-pacity and high resistance (Table I and Figure 6a), showing a beneficialresistive behavior compared to control. Figure 6a shows that as theMW of Jeffamine increases the Rcoating and the CPEcoating decrease andincrease respectively. Generally, samples coated by three consecutivedip steps show higher resistances and lower capacities than samplescoated by one dip step (Figure 6). The highest Rcoating was given bysamples coated with U(230) and the lowest by samples coated withU(2000) (Table I and Figure 6). In general, the increase of the MWof Jeffamine induced an increase of CPEdl and a decrease of the Rdl

values (Figure 6b). These variations confirm the poorer barrier proper-

Figure 5. Representative magnitude and phase angle Bode plots for U(600) 1layer and the respective fitting results, U(600) 3 layers and U(900) and U(2000)1 layer in the instant of exposure to SCPS + 1 wt% Cl− (≈ 10 minutes oncethe OCP has been established).

ties provided by the OIH coatings when higher MWs of Jeffamine areused. The reason for this behavior may be explained considering thestructure of these materials. These OIHs can be described as a rigidinorganic (silicate based backbones) that provide enhanced mechan-ical properties which are spaced by flexible organic polyether chainslinked by urea bonds.27,39 These polyether chains increase when theMW of Jeffamine increases. Therefore, the organic component of the

Table I. EIS data fitting parameters and errors in percentage obtained in SCPS + 1 wt% Cl− for control and samples coated with U(230) andU(400) using the EECs shown in Figures 4e) and 4f).

Rs CPEdl Rdl Goodness of fitSamples (� cm2) (sα �−1 cm−2) α dl (� cm2) (χ2)

Control 4.2 (± 5.2%) 1.7 × 10−4 (± 20%) 0.69 1.5 × 102 (± 16%) 6.3 × 10−2

Rs CPEcoating Rcoating CPEdl Rdl Goodness of fit(� cm2) (sα �−1 cm−2) αcoating (� cm2) (sα �−1 cm−2) αdl (� cm2) (χ2)

U(230) 1 Layer 12 (± 8.3%) 3.9 × 10−6 (± 3.0%) 0.70 2.0 × 105 (± 9.3%) 3.8 × 10−5 (± 19%) 0.78 8.7 × 104 (± 2.8%) 7.0 × 10−3

3 Layers 23 (± 3.5%) 4.3 × 10−6 (± 2.0%) 0.67 2.0 × 105 (± 8.1%) 1.8 × 10−5 (± 14%) 0.81 1.0 × 105 (± 1.8%) 4.0 × 10−3

U(400) 1 Layer 10 (± 6.1%) 9.4 × 10−6 (± 5.0%) 0.67 1.1 × 103 (± 7.2%) 2.6 × 10−6 (± 16%) 0.86 1.2 × 105 (± 1.0%) 3.0 × 10−3

3 Layers 18 (± 6.4%) 4.9 × 10−6 (± 5.3%) 0.76 4.4 × 103 (± 7.1%) 3.4 × 10−6 (± 17%) 0.96 3.4 × 104 (± 1.1%) 7.0 × 10−3

Table II. EIS data fitting parameters and errors in percentage obtained in SCPS + 1 wt% Cl− for samples coated with U(600), U(900) and U(2000)using the EECs shown in Figure 5d).

Rs CPEcoating Rcoating CPEdl Rdl Goodness of fitSamples (� cm2) (sα �−1 cm−2) αcoating (� cm2) (sα �−1 cm−2) αdl (� cm2) (χ2)

U(600) 1 Layer 7.9 (± 1.3%) 2.0 × 10−5 (± 10%) 0.84 2.4 × 102 (± 9.5%) 3.6 × 10−5 (± 21%) 0.95 56 (± 17%) 3.8 × 10−2

3 Layers 8.1 (± 2.6%) 8.7 × 10−6 (± 9.6%) 0.88 3.4 × 102 (± 8.2%) 1.5 × 10−5 (± 16%) 0.97 26 (± 14%) 4.6 × 10−2

U(900) 1 Layer 6.5 (± 1.6%) 3.2 × 10−5 (± 5.1%) 0.82 2.4 × 102 (± 4.3%) 1.6 × 10−4 (± 27%) 0.91 25 (± 23%) 1.9 × 10−2

3 Layers 5.5 (± 1.9%) 2.1 × 10−5 (± 5.9%) 0.84 2.9 × 102 (± 7.2%) 6.1 × 10−5 (± 14%) 0.88 78 (± 29%) 2.4 × 10−2

U(2000) 1 Layer 8.2 (± 6.0%) 3.3 × 10−5 (± 13%) 0.82 1.6 × 102 (± 9.5%) 6.4 × 10−4 (± 9.4%) 0.92 58 (± 24%) 2.8 × 10−2

3 Layers 7.5 (± 1.9%) 4.3 × 10−5 (± 6.5%) 0.79 2.4 × 102 (± 5.7%) 1.7 × 10−4 (± 28%) 0.99 37 (± 24%) 7.0 × 10−3

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Figure 6. Variation of the a) RCoating and CPEcoating and b) Rdl and CPEdl values obtained from impedance data for HDGS coated by one dip step (one layer) andthree consecutive dip steps (three layers) according to the MW of Jeffamine in the instant of exposure to SCPS + 1 wt% Cl−.

coating is higher for U(600), U(900) and U(2000) materials whencompared to U(230) and U(400). In highly alkaline environments,the results suggest that the organic chains are partially damaged bythe electrolyte leading to rupture of the coating in certain areas. Thisrupture is significantly higher in samples coated with OIHs preparedwith higher MWs of Jeffamine. Yet, the role of Jeffamine is crucialto minimize cracking of the deposited gels during the curing processdue to network stress as the formation of silicate regions involves therelease of ethanol molecules. Therefore, in highly alkaline environ-ments, a settlement between the organic and inorganic componentsshould be taken in account.

Potentiodynamic polarization studies.— The potentiodynamic po-larization methods can provide a direct measure of the corrosion cur-rent, which can be related to corrosion rate and can be used to examinethe passivation of a metal in an electrochemical system. Figure 7 showsthe polarization curves for uncoated and coated HDGS with one (1layer) and three (3 layers) dip steps of U(230), U(400), U(600), U(900)and U(2000) recorded after 2 h of immersion in chloride-contaminatedSCPS (1 wt% of Cl− was added to SCPS).

The potentiodynamic polarization curves obtained for HDGScoated samples with U(230) and U(400) are appreciably differentfrom the control (Figure 7). First, the OCP of this OIH sol–gel coat-ings is significantly higher than that of the bare HDGS. Secondly, apassivation region with a rather low passivation current densities isobserved, which indicates that U(230) and U(400) coatings providea physical barrier by blocking the electrochemical process. HDGScoated by one or three consecutive dip steps of U(600), U(900) andU(2000) show OCP values very similar to the control, yet low passiva-tion current densities were recorded. Moreover, the control exhibited apolarization curve with a passive region comparatively narrower thanall the OIH-coated HDGS.

Quantitative information on corrosion currents and corrosion po-tentials can be extracted from the slope of the curves, using the Stern-Geary equation,73 as follows :

icorr = 1

2.303 Rp

(βa × βc

βa + βc

)[1]

Table III shows representative electrochemical parameters obtainednamely the Tafel slopes (βc and βa) and the polarization resistance(Rp). The corrosion current density (icorr) and corrosion potential(Ecorr) were determined by analysis of Tafel curves and are shownin Figures 8 and 9, respectively. The protection efficiency (PE%) was

calculated by using the following equation:

P E (%) = icorr − i∗corr

icorr× 100 [2]

where icorr and i∗corr are the corrosion current densities obtained for

uncoated and coated HDGS, respectively and is shown in Figure 8.Generally, coated HDGS samples in the presence of 1 wt% of Cl−

show improved results when compared to the control (Table III andFigure 7).

Figure 7. Representative potentiodynamic polarization curves of the HDGScoated by: a) one dip step (one layer); b) three consecutive dip steps (threelayers) of U(230), U(400), U(600), U(900) and U(2000) after being exposedin SCPS + 1 wt% Cl− during 2h. The curve for uncoated HDGS was includedfor comparison purposes.

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Table III. Representative electrochemical parameters (Tafelslopes), polarization resistance values and respective errors inpercentage obtained from potentiodynamic polarization curves ofHDGS coated with the different OIH exposed to SCPS + 1 wt%Cl− during 2h.

ßa ßc Rp

Sample (10 mV vs SCE) (mV vs SCE) (� cm2)

Control 9.3 49 3.6 × 102 (± 2.4%)U(230) 1 Layer 1.5 41 1.9 × 105 (± 6.1%)U(230) 3 Layers 2.5 62 8.3 × 104 (± 1.6%)U(400) 1 Layer 3.6 46 3.2 × 104 (± 7.3%)U(400) 3 Layers 2.3 28 6.9 × 104 (± 2.3%)U(600) 1 Layer 13 51 4.3 × 102 (± 2.7%)U(600) 3 Layers 14 45 4.8 × 102 (± 7.0%)U(900) 1 Layer 12 31 7.3 × 102 (± 2.7%)U(900) 3 Layers 9.9 19 5.0 × 102 (± 6.2%)U(2000) 1 Layer 11 26 5.5 × 102 (± 2.8%)U(2000) 3 Layers 10 31 4.3 × 102 (± 3.4%)

Coatings based on low MWs of Jeffamine (U(230) and U(400))show enhanced corrosion protection compared to OIHs based on highMWs of Jeffamine. The icorr values (Figure 8) obtained for control(uncoated HDGS) were about 1–3 orders of magnitude higher thancoated samples. Figure 8 shows that the icorr values obtained for coatedsamples ranged between 0.03–24.27 μA cm−2. The highest icorr value(Figure 8) was given by samples coated with three consecutive dipsteps of U(2000) and the lowest by samples coated by one dip step ofU(230). Figure 8 also shows that the highest and lowest PEs were givenby samples coated with U(230) and U(2000), respectively. Figure 9clearly shows that the Ecorr of samples coated with U(230) and U(400),either by one or three layers, was significantly higher than that of thebare HDGS. This might be due to the effective suppression of thecathodic reaction from water hydrolysis. It can also be observed thatthe Ecorr reduces as the MW of Jeffamine increases which indicatesthat coatings synthesized with higher MWs of Jeffamine are not soeffective in suppressing the cathodic reaction when compared to coat-ings produced with lower MWs of Jeffamine. Although the Cl− is astrong anodic activator due to its small radius and volume the results

Figure 8. icorr values and protection efficiency (PE) of the HDGS coated byone dip step (one layer) and three consecutive dip steps (three layers) of U(230),U(400), U(600), U(900) and U(2000) after being exposed in SCPS + 1 wt%Cl− during 2h. The icorr value for uncoated HDGS (control) was included forcomparison purposes.

Figure 9. Variation of the Ecorr values obtained for HDGS coated by one dipstep (one layer) and three consecutive dip steps (three layers) of the differentOIH U(X) after being exposed in SCPS + 1 wt% Cl− during 2h. The dashed redline represents the Ecorr value obtained for uncoated HDGS (control), whichwas included for comparison purposes.

obtained for samples coated with U(230) and U(400) indicate thatthe Cl− cannot reach the surface of the substrate after 2h of immer-sion. These findings suggest that the diffusion of Cl− ions across thecoating layer is prevented and the Cl− ions remain entrapped withinthe OIH network being unable to reach the surface of the metallicsubstrate. Furthermore, the potentiodynamic polarization curves arein agreement with EIS measurements.

Macrocell current density (igal).— Figure 10 shows the igal col-lected from the different prepared electrochemical cells involving theHDGS coated samples and the control that were immersed in SCPSduring 16 days. After 8 days of immersion, 1 wt% of Cl− was added

Figure 10. Plot of the variation of the igal and laboratory temperature withtime recorded for the HDGS sample cells coated with one layer of U(230),U(400), U(600), U(900) and U(2000) immersed in SCPS during 16 days(1 wt% of Cl− were added on the 8th day).

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into the SCPS. Generally, the coated samples showed high igal valuesin the first two days due to the corrosion of zinc. An exception to thistendency can be seen for the igal results obtained in the first day of im-mersion for samples coated with U(230) and U(400). These samples,when compared to U(600), U(900) and U(2000) showed the lowestigal values. After two, and in certain cases almost three days (U(600)),an oxide layer was formed in the working electrode surface and adecrease in the igal values was recorded, which is consistent with theliterature.27 The zinc in contact with SCPS (pH = 13.2), was oxidizedand the cathodic reaction arisen from water hydrolysis with hydrogenevolution took place on the galvanized surface:9,12,15,74,75

Anodic dissolution of zinc:

Zn → Zn2+ + 2e− [3]

Cathodic reaction from water hydrolysis:

2H+ + 2e− → H2 (g) ↑ [4]

The global process can be described as:

Zn + 2H2O → Zn(OH)2 + H2 (g) ↑ [5]

After 5 days of immersion, samples coated with U(600) and U(2000)dropped to lower igal values compared to samples coated with U(230)and U(900) which showed steady behavior until the addition of chlo-ride ions. Lower values for samples coated with U(230)27 were ex-pected after 5 days of immersion. This behavior suggests that the oxidelayer formed did not fully cover the HDGS leading to the passivationof the working electrode surface; however no further conclusion canbe drawn. The comparison of the igal values before and after Cl−

addition shows that for the coating U(2000) the igal values increasedremarkably compared to the other coated samples. A similar behavior,however, less obvious, was found for samples coated with U(400) andU(600). Samples coated with U(230) and U(900) showed a slight de-crease of the igal values after Cl− addition. Excluding samples coatedwith U(400), which reached lower igal values on the 12th day increasingafter that to higher values, the igal measurements (Figure 10) showedthat after Cl− addition a steady behavior along time was found foreach OIH coating. Lower igal values represent improved corrosionbehavior.

Coated HDGS samples showed lower values than the control dur-ing all the study period, therefore showed improved corrosion behav-ior. The results also indicated that the igal cells in SCPS were sensitiveto the external laboratory temperature variation, to the presence ofchloride ions and to the composition of the OIH material depositedwhich is also according to the literature.44,46

Polarization resistance (Rp).— Figure 11 shows the Rp values ob-tained for the different electrochemical cells prepared with HDGScoated by one dip step (1 layer), that were immersed in SCPS dur-ing 16 days. Results for control samples are also presented. After 8days of immersion, 1 wt% of Cl− was added into the SCPS. The Rp

measurements were performed once a day.Figure 11 shows that the Rp data are generally in agreement with igal

measurements. The results also indicate that the Rp cells in SCPS aresensitive to the presence of chloride ions and to the composition of theOIH material deposited which is also according to the literature.7,46,76

HDGS coated samples during all the period of study, in general,displayed higher values than the control. Therefore, showed improvedcorrosion behavior. During the first days of immersion, the Rp valuesdecreased suggesting that zinc corrosion occurred, which is also inagreement with the igal results and literature.27 After chloride addition(on the 8th day) the Rp values of samples coated with U(600), U(900)and U(2000) dropped to lower values, however, remained alwaysabove the control.

Improved results were obtained by samples coated with U(230)and U(400) when compared to the other samples and the Rp valuesare also in agreement with the EIS, potentiodynamic measurements,and igal data.

Figure 11. Plots of the of normalized polarization resistance (Rp) variationrecorded for HDGS sample cells coated with one layer of U(230), U(400),U(600), U(900) and U(2000) immersed in SCPS during 16 days (1 wt% ofCl− were added on the 8th day).

Morphology of the coatings.— Surface morphology of coatedHDGS samples was assessed by SEM/EDS analysis before (Figure 12)and after contact with SCPS chloride-contaminated (Figures 14 and15). Preliminary observations of the HDGS surface after being inchloride-contaminated SCPS during 16 days (1 wt% of chloride wasadded on the 8th day) were performed using a stereo-zoom micro-scope and are presented in Figure 13. SEM analysis (Figure 12) re-vealed that OIH coatings covered the HDGS substrate and as the MWof Jeffamine increased an improved coverage was obtained. This isdue to the increase of the sol viscosity since as the MW of the Jef-famine increases the viscosity of the obtained OIH coatings increases.

Figure 12. SEM images of HDGS surfaces coated by one dip step of a) U(400)and b) U(20000) before being embedded in mortar with the localization ofEDS analysis including semi-quantitative analysis before immersion in theelectrolyte.

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C674 Journal of The Electrochemical Society, 162 (12) C666-C676 (2015)

Figure 13. Stereomicroscopic observation of uncoated and coated HDGS surfaces, magnified twenty times, after being immersed in SCPS for 16 days (1 wt% ofchloride was added on the 8th day).

Additionally, during the curing process the action of gravity led to thedisplacement of the gel from the top to the bottom. This is particularlysignificant for samples coated with U(230) and U(400). Figure 12aclearly demonstrates that samples coated by one dip step of U(400)have several regions, barely coated within the same sample, whencompared to samples coated by one dip step of U(2000). The EDSanalysis shows that the OIH coating correspond to high peaks of C,Si and O. Uncoated areas only show the presence of high peaks of Zn(Figure 12).

The preliminary observations of the HDGS surface (working elec-trode used for igal measurements) after being in chloride-contaminatedSCPS during 16 days (1 wt% of chloride ions were added on the 8th

day) were performed using a stereo-zoom microscope. The imagesobtained for the control sample, and samples coated by one dip stepof the different OIH coatings are shown in Figure 13. The controlsamples were HDGS samples where no OIH coating was deposited.The visual observation clearly shows a difference between coated anduncoated (control) samples. The control sample shows severe corro-sion regions where the presence of iron oxide (rusty deposits) is clear.White deposits (zinc oxide) and calcium hydroxide zincate (CaHZn)crystals are also visible.8 However, due to the high pH of SCPS thesize of CaHZn crystals are so large that they cannot completely coverthe HDGS surface. As a consequence of this, small regions of thesubstrate are left exposed and thus without protection. Under theseconditions, the passivation of the HDGS surface is not possible. Athigh pH values the concentration of Ca2+ ions in solution is depleted,therefore the dissolution of the zinc is not mitigated and as a resultthe galvanized coating may totally dissolve8 leading to the corrosionof the steel underneath.

The appearance of high CaHZn crystals on the surface of HDGSsamples coated by one dip step of U(230), U(400) and U(600) maybe explained by the poor coverage obtained when these OIHs are de-posited. The barely or uncoated areas are attacked by the electrolyteleading to the formation of CaHZn crystals with higher dimensionsthan the ones formed on the surface of HDGS coated with U(900)and U(2000). In spite of the presence of a few crystals of CaHZnon the surface of coated HDGS, all the coated samples show com-pact corrosion product films. Furthermore, the presence of iron oxidewas not observed. The corrosion product film formed, consisting ofmany needle-like particles, is compact and covers the HDGS surfacecompletely. The formation of these films is due to the dissolution reac-

tion of the zinc layer in chloride-contaminated SCPS.8 Figure 13 alsoshows that the corrosion product film becomes more compact whenthe coatings used were synthesized with higher MWs of Jeffamine(U(900) and U(2000)). This may be explained by the high coveragethat was achieved with these OIHs (Figure 12b). Once the passive filmof calcium hydroxyzincate is formed, its stability is not altered.8

EDS analysis for control samples (Figure 14) show, after 16 daysin SCPS (1 wt% of Cl− was added on the 8th day) high intensity peaksof iron. These results indicate that the substrate was attacked by theelectrolyte and in certain areas the entire zinc layer was completelyeroded (due to zinc corrosion). Coated HDGS samples showed im-proved results and in the same experimental conditions the presenceof U(230) and U(400) was found which is justified by the existence ofhigh intensity peaks of C, Si and O (Figure 14). Moreover, the pres-ence of iron was not found on the surface of the coated HDGS. Theabsence of Si peaks in the EDS data obtained for U(600) and U(2000)(Figure 15) (similar results were obtained for U(900)) suggest that thecoatings synthesized with higher MWs of Jeffamine suffer partial dis-solution and or destruction after 16 days of immersion in SCPS (1 wt%of Cl− was added on the 8th day). This behavior may be explained bythe rupture of the OIH coatings during the formation of the corrosionproduct. Nevertheless, the formation of a compact corrosion productfilm is allowed and the surface of the HDGS is passivated. In spite ofthese OIHs (U(600), U(900) and U(2000)) suffering partial dissolu-tion/destruction, the results suggest that all coated samples mitigatethe zinc corrosion and the hydrogen evolution in the conditions stud-ied. The EDS obtained for coated samples also show (Figures 14 and15) the presence of chloride suggesting that the Cl− ions remainedentrapped within the OIH network, stopping them from reaching thesurface of HDGS and thus protecting the substrate from Cl− attack.

In summary, the electrochemical results obtained (igal, Rp, EISand potentiodynamic curves) are generally in agreement and point tothe same outcome showing that improved anti-corrosion performancewas given by samples coated with U(230) and U(400) (produced withlower MWs of Jaffamine). Inferior results were provided by samplescoated with U(600), U(900) and U(2000) (produced with high MWsof Jeffamine). This behavior may be explained by the increase of theorganic chains, which increase when the MW of Jeffamine increases.In highly alkaline environments, as mentioned previously the organicchains are partially damaged by the electrolyte leading to rupture ofthe coating in certain areas. This rupture is significantly higher in

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Journal of The Electrochemical Society, 162 (12) C666-C676 (2015) C675

Figure 14. SEM images of: a) control (HDGS without any OIH coating); and HDGS coated by one dip step of b) U(230) and c) U(400) with the localization ofEDS analysis including semi-quantitative analysis after being immersed in SCPS for 16 days (1 wt% of Cl− was added on the 8th day).

Figure 15. SEM images of: HDGS coated by one dip step of a) U(600) andb) U(2000) with the localization of EDS analysis including semi-quantitativeanalysis after being immersed in SCPS for 16 days (1 wt% of Cl− was addedon the 8th day).

samples coated with OIHs prepared with higher MWs of Jeffamineleading to inferior results compared to samples coated with lowerMWs of Jeffamine. The SEM/EDS results are also in agreement withthese findings.

Conclusions

The present work reported the electrochemical study of U(X) sol-gel based coatings on HDGS in chloride-contaminated SCPS. It wasdemonstrated that the U(X) coatings prevent chloride ions reaching bydiffusion to the surface of the metallic substrate by immobilizing themwithin the most exterior regions of the OIH network of the coating.

The analysis of the results obtained from electrochemical studies(igal, Rp, EIS and potentiodynamic curves) allowed to conclude thatimproved anti-corrosion performance was given by samples coatedwith U(230) and U(400). The poorer results, still better than the con-trol, were given by samples coated with U(2000). SEM/EDS results,in agreement with the igal and Rp results, pointed to the conclusionthat full coverage was seldom achieved which is consistent with theliterature.39,40

In conclusion, besides the barrier effect introduced by U(X) coat-ings by hindering the zinc corrosion activity during the initial stagesof contact of the HDGS samples with SCPS (highly alkaline environ-ment), these coatings protect the HDGS from Cl− attack and may beconsidered potential substitutes for chromate conversion layers andsystems containing Cr(VI). Therefore, these U(X) sol-gel coatingscan be employed as pre-treatments to reduce the corrosion in the firstinstants of immersion in SCPS and protect the HDGS from a furtherattack of Cl−.

Acknowledgments

The authors would like to gratefully acknowledge the financialsupport from Fundacao para a Ciencia e Tecnologia (FCT) for thePhD grant SFRH/BD/62601/2009 and the financial support by Cen-tro de Quımica [project F-COMP-01-0124-FEDER-022716 (ref. FCTPest-/Qui/UI0686/2011)-FEDER-COMPETE] and EU COST actionMP1202: HINT – “Rational design of hybrid organic-inorganic inter-faces: the next step toward functional materials.”

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