SiliconDOI 10.1007/s12633-014-9202-6

ORIGINAL PAPER

Studies on Silicon Containing Nano-hybrid Epoxy Coatingsfor the Protection of Corrosion and Bio-Fouling on MildSteel

Saravanan Palanivelu · Duraibabu Dhanapal ·Ananda Kumar Srinivasan

Received: 11 September 2013 / Accepted: 1 May 2014© Springer Science+Business Media Dordrecht 2014

Abstract An epoxy nano-composite coating was developedusing amine functionalized nZnO (in the amount of 2.5 %,5.0 % and 7.5 wt %) as the dispersed phase and a com-mercially available epoxy resin as the matrix phase. Thestructural features of these materials were ascertained byFT-IR spectral studies. The anti-corrosive properties of theepoxy/nZnO hybrid coatings in comparison with a virgincoating were investigated by a salt spray test and electro-chemical impedance spectroscopy technique. The surfacemorphology determined by SEM, indicates that nZnO parti-cles were dispersed homogenously through the epoxy poly-mer matrix. The results showed improved antifouling andanticorrosive properties for epoxy-nZnO hybrid coatings.

Keywords Mild steel · Nano zinc oxide · Hybrid coating ·Antifouling · Salt-spray

1 Introduction

The ban on harmful substances in antifouling paints pavedan avenue to the development of new antifouling strate-gies. Alternative coatings should be as effective as con-ventional paints but with lower toxicity. Bio fouling is theaccumulation of micro-organisms, plants and animals onsurfaces [1] causing serious damage and fuel consumption,

S. PalaniveluDepartment of Chemistry, St. Joseph’s College of Engineering,Chennai, 600119, Tamil Nadu, India

S. Palanivelu · D. Dhanapal · A. K. Srinivasan (�)Department of Chemistry, Anna University, Chennai, 600025,Tamil Nadu, Indiae-mail: srinivanand@annauniv.edu; srinivanand@gmail.com

which may be as high as 7 % of GDP. To overcome thesedrawbacks of fouling and to further improve the perfor-mance of coatings, incorporation of nano-sized pigmentsand fillers in the coating is a recent practice. The mostcommon nano-particles used in coatings are SiO2, TiO2,ZnO, Al2O3, CaCO3, etc. nZnO due to its characteristicstructure possesses unique chemical, biological and semi-conductor properties. Because of its non-toxic nature andability to block UV radiation, it has found vast applicationsin cosmetics, textile and in many coating industries [2–4].

On the other hand, the dispersion of nanoparticles intoa polymer matrix is a challenging task, which often resultsin the agglomeration of nanoparticles leading to poor coat-ing performance. The surface of nanoparticles must there-fore be modified with suitable coupling agents to ensuregood dispersion and to prevent the nanoparticle agglom-eration nanoparticles. Many studies have been carried outon the organized fabrication of surface functionalization ofnanoparticles through organoalkoxysilanes [5–9]. Function-alization of nanoparticles prevents desegregations duringcuring, resulting in a more homogenous coating. More-over, nanoparticles tend to occupy small hole defects formedfrom local shrinkage during curing of the resin and act as abridge, as well interconnecting more molecules. Such mod-ifications result in an increase in the rigidity of coatingsleading to improved barrier properties required for corro-sion, fouling protection and reduce coating blister formationor delamination [10–13].

In this work, surface modification of nZnO was achievedwith a 3-aminopropyltriethoxysilane coupling agent andthe surface modified nZnO in different concentrations wasreinforced with epoxy resin to formulate nZnO reinforcedepoxy coatings. The reinforcing effect of nZnO particleswith epoxy resin towards corrosion and fouling resis-tance was investigated by several techniques including

Silicon

Fourier transform infra-red (FTIR) spectra, electrochemicalimpedance (EIS), salt-spray tests, SEM and field exposurestudies. The mechanical properties of the nZnO reinforcedepoxy coatings along with virgin epoxy coating were deter-mined by flexibility tests, cross-cut adhesion tests andimpact resistance tests [14–20]. The results of these stud-ies are discussed along with supporting evidence of thenanoparticle behavior.

2 Experimental

2.1 Materials

An epoxy resin GY250 representing diglycidyl ethers ofbisphenol-A (DGEBA) with an epoxy equivalent weight(EEW) 182–192 (E.wt/g) and a viscosity of approxi-mately 10,000 mPa was used for our study. Aradur HY951curing agent was obtained from Huntsman (India). TheSilica powders, red iron oxide pigment, nZnO and 3-aminopropyltriethoxysilane were purchased from � AldrichChemicals. Acetone, sulphuric acid, NaOH were purchasedfrom Sisco Research Laboratories, India. Rustoline oil andan, epoxy type adhesive were supplied by Hindustan Ciba-Geigy Ltd., India.

2.2 Preparation and Testing of Coatings

Mild steel specimens [of composition C-0.033 %; Si-0.005 %; Mn-0.235 %; P-0.011 %; S-0.005 %; Cr-0.046 %;Ti-0.003 %; Ni-0.043 %; Al-0.073 %, Co-0.007 %, Nb-0.005, Fe-balance] were used for our study. Five differentcoating systems used to protect steel structures againstcorrosion were chosen for the purpose of the research.The nomenclature of different coating systems is given inTable 1. The specimens were degreased with acetone toremove impurities from the substrate. Then the specimenswere subjected to sand blasting at a pressure of 100 psithrough the nozzle to get the appropriate surface roughness.

The particle size of the sand is was 82 mesh. The distancebetween the substrate and the blaster was maintained 2 feet.The specimens were kept in Rustoline oil for conditioning.The process of the preparation of hybrid coatings is shownin Figure S1.

Coatings were applied by a hand bar coater on com-mercially available mild steel plates (30 mm ×10 mm×1 mm) for chemical resistance tests, 70 mm ×50 mm×1 mm for salt spray tests. All the coated samples werecured at room temperature for 3 days and then kept indesiccators for at least 1 week before the tests were per-formed. Coating thickness was measured by a Mini test600FN, EXACTO-FN instrument. The thickness of thecoatings was found to be approximately 100 µm. Beforesubjecting them to various tests, the panels were edge-sealed to an extent of 5-8 mm from the edges using anepoxy type adhesive (supplied by Hindustan Ciba-GeigyLtd., India).

2.3 Surface Modification of ZnO Nanoparticles

The introduction of reactive NH2 groups onto the sur-face of ZnO nanoparticles was achieved through thereaction between 3- aminopropyltriethoxysilane and thehydroxyl groups on the ZnO nanoparticle surface. Typ-ically, 2.0 g ZnO nanoparticles and 2 ml 3- amino-propyltriethoxysilane in 40 ml O-xylene were kept at150 °C for 3 hrs under stirring and Argon protection.After that, the ZnO nanoparticles were collected by filtra-tion and rinsed three times with acetone. Afterwards, theamine functionalized ZnO nanoparticles were dried undervacuum for 12 hrs. The reaction sequence is given inScheme 1.

2.4 Preparation of Epoxy nano Composite

Prior to curing, the desired amount of epoxy resin, rediron oxide pigment, silica powder, leveling agent GLP503Aand nZnO were mixed together and crushed followed

Table 1 Nomenclature of coating system on mild steel

Coating Epoxy Additives Curing

system resin Pigment Extender Surface modified nZnO agent

2.5 % 5.0 % 7.5 %

Uncoated × × × × × × ×

Coating 1 DGEBA × × × × × TETA

Coating 2 DGEBA � � × × × TETA

Coating 3 DGEBA � � � × × TETA

Coating 4 DGEBA � � × � × TETA

Coating 5 DGEBA � � × × � TETA

Silicon

Scheme 1 Surface modification of nonmaterial

by sifting through 200 mesh screen. The mixing processwas performed in an oil bath (50-70 °C). The mixturewas then subjected to ultra-sonication for 8-12 h to dis-perse the nano particles of ZnO completely within theepoxy resin to fabricate the antifouling coating after theaddition of a stoichiometric amount of hardener to themixture. By this method, four coating formulations con-taining different amount of nZnO (2.5, 5.0 and 7.5wt.%) were prepared and the composite paint was appliedon mild steel coupons using a bar-coater to a thicknessof approximately 100 µm. The surface modification ofnZnO and its subsequent reaction with epoxy is illustratedin Scheme 2.

2.5 Spectroscopic Studies

A PerkinElmer 1750 FTIR spectrometer was utilizedto determine the chemical structure of virgin epoxyresin, nZnO modified epoxy resin and commercial epoxyresin. The spectra of epoxy resins cured with AradurHY951 as coatings were obtained by the followingmethod: a small portion of the cured epoxy resin wasground to a fine powder, mixed with KBr powder,and pressed into a pellet which is used to obtain thespectrum.

2.6 Mechanical Properties

Impact resistance, Flexibility and Cross-Cut adhesion testsare described in detail in the following references andsummarized herein [21–23].

1. Impact resistance: To determine the impact indentationon the coated metal panels, a steel ball of about 50 mmdiameter and weighing 500 ±10 g was dropped on thesurface of the panel freely from a height of 750 mmat randomly selected 10 different points on the surfaceavoiding nearness of edges as per ASTM D2794 andresults are given in Table 2.

2. Flexibility: This test which enables the measurement ofresistance to cracking (flexibility) was carried out on allcoated specimens as per ASTM D522 standard using aconical mandrel and results are given in Table 2.

3. Cross-Cut adhesion test: Square boxes of 1 mm2 weremade on a 1 cm ×1 cm square of the test specimen.These boxes were covered by adhesive tape (cellotape).Cellotape was then pulled off and the number of boxesremoved from the surface was counted to check theadhesion of the coating. Cross-cut adhesion was carriedout according to ASTM 3359-83B and results are givenin Table 2.

Scheme 2 The Schematic formula for the reaction between epoxy resin and surface modified ZnO nanoparticle

Silicon

Table 2 Mechanical characterization of Nano-hybrid coating systems on mild steel

Coating Dry film Impact resistance Cross-Cut Water Gel

systems thickness (Weight 1.8 kg and adhesion adhesion test content

(µm) height of fall 1.2 m) Flexibility test (%) (%)

Coating 1 ±100 No damage Passed (6 mm) 5B 12.5 47.5

Coating 2 ±100 No damage Passed (3 mm) 5B 11.4 50.2

Coating 3 ±100 No damage Passed (3 mm) 5B 9.4 63.3

Coating 4 ±100 No damage Passed (3 mm) 5B 7.3 77.4

Coating 5 ±100 No damage Passed (3 mm) 5B 5.2 92.5

2.7 Water Absorption and Gel Content Measurement

The water absorption (w %) measurements for differentepoxy/nZnO hybrid films of 3 cm ×1 cm size was calculatedvia vacuum oven desiccation for 24 h to determine their dryweight (Wa) and water absorption was measured by immers-ing the film in distilled water for 24 h to determine the wetweight (Wb). Then the water absorption was calculated bythe following formula [24].

W ( %) = Wb − Wa

Wa× 100

A known weight of oven-dried film (W1) was put into aSoxhlet extractor for continuous extraction with toluene for24 h at 100 °C. The polymer gel remained after extractionwas dried (W2) and the degree of crosslinking was measuredin terms of gel content (%). Three test specimens were usedfor each sample and the average was taken as gel content[25, 26].

Gel Content (%) = W2

W1× 100

2.8 Chemical Resistance Testing

Corrosion testing was performed in distilled water, acid (10wt. % HCl), alkali (5 wt. % NaOH) and distilled water byplacing the coated and uncoated specimens in individualplastic jars and dipping them in aforementioned media. Theduration of the test was 96 days at 38 °C. The degree ofadhesion and visual inspection of blister and cracks wereevaluated for the coated panels. An acetone double rub testaccording to ASTM D 4752 was carried out to evaluate thechemical resistance of the coated samples [27, 28].

2.9 Salt Spray Test

An accelerated salt spray corrosion test was carried out onall coated steel panels of 70 mm ×50 mm ×1 mm with andwithout diagonal scribes on coated panes. Salt spray testswere used to simulate the marine atmosphere where mild

steel was exposed according to ASTM B 117-3. 5 % NaClsolution was made and used as salt spray solution with a pHof 6.9, air pressure of 15psi. A scratch with 200 µm in widthand 40 mm in length was made. The temperature of the saltspray chamber was controlled between 33.8-35.1 °C, witha continuous test time of 1200 hrs. The method used forcleaning the Specimens were before loading and after com-pletion of testing as follows: a) Specimens were cleanedgently prior to loading: b) Specimens were then washed gen-tly in clean running water to remove salt deposits from theirsurfaces and dried immediately. The panels were carefullyobserved for the detection of rust spots on their surface andphotos were taken by An Olympus 4XWIDE digital cam-era. The time for the formation of brown rust and blisterswas noted [29–31].

2.10 Potentiodynamic Polarization Measurements

In the case of Tafel polarization, potential was scannedfrom-250 mV to +250 mV with respect to open circuitpotential (OCP) at a scan rate of 0.5 mV/Sec. From thisanodic and cathodic polarization curves, the Tafel regionswere identified and extrapolated to Ecor to get corrosionpotentials Icor and corrosion rates CR in mm per year [32].All measurements were carried out at ambient tempera-ture (25± 2 °C). The experimental data were analyzed byusing the commercial software package ZSimpWin [33–40].The interpretation of impedance data was performed afternumerical fitting using an equivalent circuit proposed inFigure S2(b).f

2.11 Electrochemical Impedance Spectroscopic Studies(EIS)

EIS measurements were performed on a three electrode cor-rosion cell by an ACM electrochemical impedance analyzer(Gill AC Serial No. 1634- Sequencer Version 4). A saturatedcalomel electrode (SCE) and platinum foil were used asreference and counter electrodes, respectively. For electro-chemical impedance spectroscopy experiments, mild steel

Silicon

substrates with the dimensions of 3 cm ×1.4 cm ×1 cmwere used. These panels were sand blasted and cleanedwith ethanol in an ultrasonic bath before painting. The sandblasted panels were painted by a hand bar coat applicatorand the coated specimens were dried for 12 h at ambienttemperature. The dry film thickness measured was found tobe approximately 100 µm for all the coated specimens. TheEIS test was performed in 3.5 % sodium chloride electrolytesolution. Only 1 cm2 area of the uncoated specimen, pureepoxy coated specimens and nZnO-epoxy coated specimenswere exposed to the electrolyte. The remaining areas weresealed by a masking tape. In electrochemical impedancespectroscopy experiment, AC signal of 10 mV amplitudeand various frequencies from 10000 Hz to 0.1 Hz at opencircuit potentials were impressed to the coated mid steelsamples. Figure S2(a) represents the typical test setup forthe EIS study of the coated specimens. From the impedanceplots, the charge transfer resistance, Rct and the double layercapacitance, Cdl values can be calculated.

2.12 Field Exposure test at Muttukadu, India

Mild steel specimens (of 8 cm ×5 cm ×1 cm) coated withunmodified epoxy, nZnO containing epoxy coating systemswere used for the fouling study. The study was carried outat coast of Bay of Bengal in Chennai, on the East Coast ofIndia. The specimens were suspended at a depth of 3 m fromthe lowest spring tide low water mark. The specimens hungvertically using nylon ropes with proper support in a pro-tected area for 365 days. The specimens were removed andthe distribution of fouling growth to observe the corrosionassociated with fouling. The barnacles, along with otherorganisms, which settled on the surface of specimens coatedwith different coating systems, where then, removed using ahard brush without damaging the specimen surface [41, 42].

2.13 SEM Analysis

The same samples were then coated with a thin layer ofgold by vaporization and the morphology of the antifoulingcoating panels was observed by scanning electron micro-scope (LEO 1455VP). Samples were examined before andafter degradation by immersion in the marine environment[43, 44].

3 Result and Discussion

3.1 FTIR Analysis of the Epoxy/ZnO nanocompositeCoatings

The structures of GY250 epoxy resin and HY951 polyamineadducts were confirmed by IR spectral analyses. The six

different coating formulations exhibit very different IRspectra. The IR spectrum of GY250 reveals the presence ofcharacteristic absorption bands for Ar–C=C–H stretchingand bending –CH2 and –CH3 asymmetrical and symmet-rical, –C–Ar–O–C stretching, and epoxy CH2–(O–CH–)ring stretching vibration. The presence of epoxy groupsin IR spectra was proved from the presence of strongbands at 3056 cm−1 (γ C-H epoxy) and 915 cm−1 (γ C-O epoxy). The 1, 4-substitution of aromatic ring was seenat 830 cm−1 for GY250 epoxy resin. There was a broadband with very low intensity at 3429 cm−1 correspond-ing to the vibration mode of water OH group indicatingthe presence of small amount of water adsorbed on thenZnO crystal surface. The band at 1601 cm−1 was due tothe OH bending of water. A strong band at 530 cm−1 isattributed to the nZnO stretching band which is consistentwith that reported before. The IR analysis carried out forHY951 polyamine adducts reveals the presence of char-acteristic absorption bands for N–H stretching and bend-ing vibration. The broad doublet peak observed between3340–3200 cm−1 may be due to the –NH2 vibration absorp-tion of amine compound. The aliphatic –CH2 and –CH3

vibrations were seen between 3000 and 2850 cm−1 forthe polyamine adduct. The most obvious distinguishingfeatures were that the polyamine adduct spectra had anintense broad N–H stretching absorption around 3300 cm−1

[46–54]. Fig. 1 shows the infrared spectra of all thecoatings analyzed on KBr disc between the zone 400–4000 cm−1.

Fig. 1 TIR of (a) Coating ‘1’, (b) Coating ‘2’, (c) Coating ‘3’, (d)Coating ‘4’, (e) Coating ‘5’ and (f) Nano ZnO

Silicon

3.2 Analysis of Mechanical Properties

Though flexibility testing is not quantitative in nature, it pro-vides qualitative results on the coating systems. Results ofthe impact test carried out on all coatings indicate that allthe coatings have passed the impact test. Results of coni-cal mandrel flexibility test conducted on all the coatings areshown in Table 2, bring out the fact that Coating ‘1’ failed at3 mm diameter of the cone where peeling and cracks werepredominantly found and passed at 6 mm diameter. All othercoatings have been found to possess good flexibility char-acteristics. The failure of coating ‘1’ may be attributed tothe formation of large free volumes, micro voids and stressconcentration during the film formation [55–57].

The adhesion of coating depends on the nature of sub-strates as well as on the coating formulations. Table 2 showsthe results of adhesion tests of virgin epoxy and epoxycoatings containing nZnO. From the test, we observed thatno squares were removed from the coated metal substratesexcept coating ‘1’ whose squares were removed (3-4 innumbers) during cross-cut adhesion test. These results indi-cate that the adhesion of epoxy/nZnO hybrid coatings wassuperior to the virgin epoxy coating [58]. The reason forthe improved adhesion offered by epoxy/nZnO hybrid coat-ings may be due to their enhanced integrity and defect-freenature.

3.3 Results of water absorption and gel content tests

The data in Table 2 show that the minimum water absorp-tion and maximum (%) of gel content for coatings correlatewith increasing nZnO content. This may be due to themore cross linked structures formed with increased nZnOcontent. The incorporation of nZnO particles into epoxyresins offers enhanced integrity and durability of such coat-ings, since these fine particles dispersed in coatings seemto have occupied cavities and caused crack bridging, crackdeflection and crack bowing. Functionalization of nanopar-ticles also prevented epoxy disaggregation during curing,resulting in a more homogenous coating. Moreover, nZnOparticles acted as a bridge, interconnecting more moleculesleading to a reduced total free volume and increased crosslinking density and rigidity [59–61].

3.4 Analysis of Chemical Resistance Test

In the previous section we have discussed the mechanicaltests of the cured nano-composite coatings ‘1’ to‘5’. In thepresent section, we have discussed the chemical resistanceof Nano-composite coatings ‘1’ to ‘5’. For the chemicalresistance test, the coated panels were subjected to adversechemical environments such as acid, alkali, solvent andwater in order to study the durability of such coatings. The

data obtained for cured coating systems (virgin and compos-ite coatings) from water, acid, alkali, and solvent exposurewere listed in Table 3. The uncoated panels underwentminimum weight loss in water while coated panels (mildsteel, Table 3) remained unaffected in aqueous medium (dis-tilled water) indicating that these coating materials render animpermeable barrier to the substrates of mild steel throughwhich water and other corrosive species could not enter andthus they successfully protected the substrate from corro-sion. Furthermore, the composite coated specimens wereimmersed in 5 wt % H2SO4 and 5 wt % NaOH for 96 hhad their films remaining intact with a slight loss in gloss.The better performance shown by nZnO reinforced epoxyamine cured coatings may be attributed to their increasedcross linking density, which was achieved by crack bridg-ing leading to enhanced integrity of such coatings [61, 62].The uncoated panels underwent maximum corrosion under5 wt % NaOH solution, which was evident from their weightloss measurements (Table 3). However, all coated sampleswere found to be quite resistant to water, acid, and alkalienvironments.

The use of polar solvents such as ketones is often usedto assess the degree of cure of a cross-linked compositionfor solvent resistance, methyl isobutyl ketone or acetoneis recommended. In addition to immersion testing, solventresistance may be assessed by a solvent rub rest. In thisrespect, acetone was used to determine the degree of cur-ing of the present coating systems by rub method. Failurewas determined either by distruption or dissolution of thecoating films from panels. The higher the degree of cross-linking the better the film properties, ideally required forcorrosion resistance. Hence solvent molecules can hardlypenetrate the cross-linked network at all. In this respect, itwere found that the all developed composite coatings passthe acetone double rub test and there were no defects onthe surface even after 27 double rubs of solvent, indicatingtheir superior performance when compared with uncoatedspecimens.

3.5 Evaluation of Corrosion Resistanceby Potentiodynamic Polarization Studies

Figure 2 shows typical potentiodynamic polarization curvesfor mild steel in 3.5 % NaCl with and without coatings.It was apparent that the polarization curves for coatedmild steel showed remarkable potential shifts to noble val-ues compared to the uncoated metal immersed in 3.5 %NaCl. The various polarization parameters such as corro-sion potential (Ecorr) and corrosion current density (Icorr)

obtained from cathodic and anodic curves by extrapolationof Tafel lines are given in Table S1. It should mentionedthat the Ecorr values increased significantly for mild steelcoated with the coatings ‘3’, ‘4’, ‘5’, ‘2’, ‘1’ and uncoated

Silicon

Table 3 Solvent and chemical resistance tests of Nano hybrid coating systems on mild steel

Coating Distilled water 5 % H2SO4 5 % NaOH Acetone a

system (WLb,mg) 96 h (WLb,mg) 96 h (WLb,mg) 96 h (DRs)

Uncoated 0.0193 0.3988 0.4005 -

Coating 1 A C C >25

Coating 2 A B B >27

Coating 3 A B B >27

Coating 4 A B B >27

Coating 5 A B B >27

mild steel. This observation clearly showed that mild steelapplied with these coatings control both cathodic and anodicreactions and thus act like mixed type inhibitors [63]. Forinstance, the corrosion rate for uncoated mild steel wasfound to be 1.0298 10−1 mm/y and it was minimized bycoating the metal with coating ‘3’ to a lower value of 1.4960×10−6 mm/y than the other coatings namely coating ‘1’, ‘2’,‘4’ and ‘5’ whose values are respectively 2.07677 ×10−3,9.3727 ×10−3, 3.8113 10−3, 2.5744 ×10−3. Moreover, theCR of coating ‘3’ is found to be very minimum i.e. 1.4960×10−6 mm/year, which is 10 % lower than that observedfor uncoated MS. Hence it was observed that the coatingsapplied on mild steel restrict the interaction between themetal and the electrolyte to a considerable extent. The extentof restriction to the entry of corrosive species was more inthe case of coating ‘3’ and least in the case of coating ‘1’and uncoated specimens. The reason for this behavior exhib-ited by coating ‘3’ may be due to the even distribution of2.5 wt % nZnO within the epoxy coating offering a barriereffect by interconnecting more molecules and leading to adefect free coating.

Fig. 2 Polarization response of different coating systems for 0 daysof immersion in 3.5 % NaCl solution. (a) Uncoated (b) Coating ‘1’ (c)Coating ‘2’ (d) Coating ‘3’ (e) Coating ‘4’ (f) Coating ‘5’

3.6 Evaluation of Corrosion Resistance by EIS

The Bode plots for the uncoated mild steel and coating sys-tems ‘1’, ‘2’, ‘3’, ‘4’ and ‘5’ are depicted in Fig. 3. EIStudies were carried out on mild steel panels coated withmodified epoxy resin containing with and without nZnO in3.5 % NaCl. It was interesting to note that with the exceptionof the uncoated mild steel and neat coating, all modi-fied epoxy coated samples with and without nZnO showedcorrosion resistance up to 103-108 Ohm.cm2 indicating amoderate corrosion taking place initially and the resultswere depicted in Table S2 [64]. The impedance was anal-ysed by using simple Randle’s circuit, since one semicircleis obtained in the Nyquist plot. Since polymer coatings werelow in many cases and the impedance behaviour representsthe charge transfer process of the mild steel dissolution.From the Fig. 3 and Table S2, it was clearly evident thatthe coating ‘3’ has a high corrosion resistance (1.071 ×108)

compared to the other coatings (uncoated, ‘1’, ‘2’, ‘4’ and‘5’) with and without nZnO and the order of corrosionresistance is found to be as follows:

3 > 4 > 5 > 2 > 1 > uncoated

This superior corrosion resistance offered by coating sys-tem ‘3’ may be due to the presence and even distribution ofsurface modified nano zinc oxide of optimum wt %, whichgave a flawless coating of low porosity, high crosslinkingdensity having improved coating integrity.

3.7 Salt spray Test Results

The coated specimens were scratched diagonally using asharp knife in order to expose the base metal to the con-tinuous salt fog chamber containing 3.5 % NaCl solution.The 1200 h salt spray test supports the results obtainedfrom EIS studies. The salt spray results of coated pan-els before and after exposure to salt fog atmosphere areschematically given in Fig. 4. At the end of the salt spraytest, no visible corrosion products were seen on the sur-face of the unscratched area of panels coated with modifiedepoxy coating ‘3’ and ‘4’. As observed in the case of

Silicon

Fig. 3 Bode plot of differentcoating systems for 0 days ofimmersion in 3.5 % NaClsolution. (a) Uncoated (b)Coating ‘1’ (c) Coating ‘2’ (d)Coating ‘3’ (e) Coating ‘4’ (f)Coating ‘5’

EIS, coating 1 containing no nZnO and other additivesoffered poor corrosion resistance than other four coatings(‘2’, ‘3’, ‘4’ and ‘5’) chosen for the study. Table 4 showsthe results of the observations made for the coatings ‘1’to ‘5’ after 600 and 1200 h of salt-spray testing. Theintact films showed neither corrosion nor coating defectson the main body of the panels. However, rusting with-out spreading was observed at the cross-scribe suggestingan impressive performance of the formulations against saltatmosphere [65, 66]. Hence all formulations gave goodresults without major loss of adhesion; blistering and rust-ing of the cross-cut even after 1200 h of salt-spray exposure

in the cabinet. However, coatings ‘1’ being virgin epoxyand coating ‘2’ exhibited poor salt-spray resistance. Theorder of corrosion resistance of all the coatings is givenbelow:

3 > 4 > 5 > 2 > 1

3.8 Antifouling Studies by Scanning Electron Microscopy(SEM)

The fouling resistance of these coatings was determinedby exposing the coated samples to the sea for a period

Fig. 4 Salt-spray test result ofcoating systems after 1200 hexposure of 3.5 % NaCl: (a)Photograph of coating panelsbefore salt-spray test (b)Photograph of coating panelsafter 1200 h in 3.5 % NaClSolution

Silicon

Table 4 Results of salt spray (fog) test

Coating

system Observation after 600 h Observation after 1200 h

Coating 1 pale yellow rust along the scribes, no blistering, no chalking light brown rust along the scribes, rust creep 2 mm along scribes

Coating 2 no pale yellow rust along the scribes, no blistering, no chalking light brown rust along the scribes, rust creep 1 mm along scribes

Coating 3 no pale yellow rust along the scribes, no blistering, no chalking no light brown rust along the scribes, no rust creep

Coating 4 no pale yellow rust along the scribes, no blistering, no chalking no light brown rust along the scribes, no rust creep

Coating 5 no pale yellow rust along the scribes, no blistering, no chalking light brown rust along the scribes, rust creep 2 mm along scribes

of 12 months at east coast of India, Tamil Nadu, Chen-nai (Muttukadu boat house). For SEM observation, sampleswere fixed in a glutaraldehyde solution then washed in aphosphate buffer. Dehydration was performed in differentalcohol/water solutions, followed by critical point dryingof samples. SEM photographs of the ‘1’, ‘2’, ‘3’, ‘4’ and‘5’ coated specimens attached with fouling organisms weretaken. The surface morphology of ‘1’, ‘2’, ‘3’, ‘4’ and ‘5’coated samples after immersion in the sea was examinedby SEM and the results are illustrated in Fig. 5. From the

fouling study, we observe that there was more corrosion onuncoated mild steel and more fouling attachment on coating‘1’, ‘2’, and ‘5’ respectively. On the other hand, the intensityof fouling organisms attached to ‘3’ and ‘4’ coated pan-els was minimal. Among the biocide loaded coated panels,specimens coated with coating ‘3’ (2.5 % nZnO) showedleast fouling organisms on its surface than ‘1’, ‘2’, ‘4’and ‘5’ coated specimens indicating its excellent antifoul-ing ability. This may be due to its superior antimicrobialactivity imparted by optimised coating formulation ‘3’ than

Fig. 5 SEM image taken before and after the panels were immersed in seawater: (a) SEM images of coating panels 0 days immersion in seawater(b) SEM images of coating panels after 12th month’s immersion in seawater

Silicon

Fig. 6 Photos taken after the panels were immersed in seawater: (a) Photograph of coating panels after 6th month’s immersion in seawater (b)Photograph of coating panels after 12th month’s immersion in seawater

the other concentration [67]. The mechanism of foul releasefrom amine functionalized nZnO surfaces is still not com-pletely understood. However, it is generally believed that theanti-biofouling properties are due to their low surface ener-gies. A similar observation was made by Chen et al. [68] forthe low surface energy non-toxic organo silicon nano-SiO2

antifouling coatings, which showed decreased adhesion ofthe biofouling organisms on the coating films. The order of

fouling resistance of all the coatings is given below:

3 > 4 > 5 > 2 > 1 > uncoated

Figure 6(a) shows the photographs of ‘1’, ‘2’, ‘3’, ‘4’ and‘5’ coated panels against fouling for 6 months immersion inthe seawater exposure.The coating system ‘3’ having bettercorrosion resistance than coating systems ‘1’, ‘2’, ‘4’ and‘5’ resisting the adhesion of microorganisms and biofilms

Fig. 7 Forming process of the coating films

Silicon

indicating their superior fouling resistance as well. Fig. 6(b)(‘1’, ‘2’, ‘3’, ‘4’ and ‘5’) shows the fouling resistance of thedifferent coating systems immersed in seawater for 1 year.From this study, coating systems ‘3’ was again determinedto have less fouling organisms on the coated panels indi-cating their excellent antifouling performance required formarine industry. It can be seen that the antifouling efficiencyof the coating film first is improved by adding 2.5 wt % ofnZnO and then the performance becomes worse when theamount of nZnO added is 5 wt % and 7.5 wt % respec-tively. The reason which caused this change of antifoulingproperty may be inferred from the results. When the addi-tion of nZnO is 2.5 wt %, it is well distributed withinthe coating, to effectively inhibit bio-fouling. On the otherhand, if the addition of nZnO is beyond 2.5 wt %, theexcess addition causes uneven distribution of nZnO in thecoating film which might have led to the pore forma-tion on the surface of the coating namely ‘4’ and ‘5’.This porous nature of the coating ‘4’ and ‘5’ favoredthe invasion by marine fouling organisms thus impart-ing inferior antifouling activity than coating ‘3’. Similarobservation was made from EIS study where the coat-ing ‘3’ containing 2.5 wt % of nZnO exhibited the bestcorrosion resistance indicating nZnO’s even distributionwithin the epoxy matrix offering better coating integrity.Moreover, the fouling organisms attached to coated film‘3’ were removed very easily after the fouling test wasover, and the panel looked fresh. This observation fur-ther confirms the better antifouling performance of coat-ing ‘3’ in comparison with other coatings of the presentstudy.

3.9 Mechanism of the Property Improvement

It has been shown that nZnO particles have been well dis-persed in the epoxy resin/nZnO composite through in situand inclusion polymerization, they can also effectively bedispersed in the resultant nZnO modified coatings. Afterflowing and curing, the nanoparticles can be thoroughlydispersed in the coating films. Fig. 7 shows the formingprocess of the nZnO modified coating films, and other nor-mal sized fillers are ignored in order to describe it moreclearly.

As shown in Fig. 7, the nZnO particles can form extrainteraction points with the epoxy resin, in addition to thecuring points, which can enhance the compactness of thecoating film and result in the improvement of neutral saltspray corrosion resistance of the coating films. When thecoating films were distorted or impacted, inorganic particleswill act as a stress concentrator and crazing will come intobeing at the interface. Because nanoparticles have advan-tages in quantity and specific surface areas at the sameloading content compared with micron-scale particles, more

crazing will take place and more energy can be absorbed.Furthermore, nano modification of the epoxy coatings leadsto increased levels of interaction forces (such as hydrogenbonding and Vander Waals interactions) between the nZnOnanoparticles and the epoxy resin matrix.

4 Conclusions

The present study involves the development and charac-terisation of modified epoxy resin with a nano zinc oxide(nZnO) additive. The nano additive of different weightpercentages was loaded with epoxy resin to formulatenano-hybrid coatings. The coated panels were evaluatedfor their corrosion resistance by means of electrochemicalimpedance studies, solvent rub tests and salt spray tests.The fouling resistance of these coatings was determined byantifouling studies subjecting the coated specimens to sea-water for a period of 12 months at the east coast of India,Tamilnadu, Chennai (Muttukadu boat house).

The nano-hybrid coating containing 2.5 wt % of nZnOhad shown excellent adhesion, impact resistance and flex-ibility which meet the requirements of a high performancecoating. This is mainly attributed to the uniform distribu-tion of nZnO (2.5 wt %) within the epoxy matrix offeringimproved coating properties ideally suitable for corrosionand fouling prevention.

The data resulted from corrosion studies clearly indicatethat the nZnO used at an optimum wt % as an antifoulantshowed superior an antifouling efficiency than the othercoatings. It can be concluded that the nZnO of optimumwt % may be used as an effective antifouling biocide incoatings.

Hence, we conclude that the barrier performance ofepoxy coatings can be enhanced substantially by the incor-poration of a second nano phase of inorganic filler particlesthat should be dispersed uniformly within the epoxy resinto form an epoxy nanocomposite coating, by decreasingthe porosity and restrain the diffusion path for delete-rious species. The incorporation of nZnO particles intoepoxy resins also offers environmentally benign solutionsby enhancing the integrity and durability of epoxy coat-ings by filling cavities and enabling crack bridging, crackdeflection and crack bowing leading to better performanceand longevity than the conventional epoxy coatings that arecurrently in use.

Acknowledgments Instrumentation facility provided under FIST-DST and DRS-UGC to Department of Chemistry, Anna University,Chennai are gratefully acknowledged. One of the authors Mr. P. Sar-avanan likes to thank the management of St. Joseph’s college ofengineering, Chennai for the infrastructure and moral support. Theauthors are grateful to Manjumeena Rajarathinam (Research scholar)

Silicon

CAS in Botany, University of Madras, Guindy campus for havingoffered timely support in proof correction of this manuscript.

References

1. Yebra DM, Kiil S, Dam-Johansen K (2004) Prog Org Coat 50:75–104

2. Wei-gang J, Ji-Ming H, Liang L, Zhang J-Q Chu (2006) Prog OrgCoat 57:439–443

3. Bonati S, Monteleone F (2001). Biocidal-Antifouling Agents withLow Eco toxicity Index. International Application Publication.Publication No. WO0128328

4. Hadfildk MG, Paul VJ (2001) Biofouling 12:9–295. Borisova D, Mohwald H, Shchukin DG (2011) ACS Nano 5:1939–

19466. Chang CC, Oyang TY, Hwang FH, Chen CC, Cheng LP (2012) J

Non-Cryst Solids 358:72–767. Chen CC, Lin DJ, Don TM, Huang FH, Cheng LP (2008) J Non-

Cryst Solids 354:3828–38358. Miyagawa H, Rich MJ, Drzal LT (2004) J Polym Sci B 42:4384–

43909. Becker O, Varley R, Simon G (2002) Polymer 43:4365–4373

10. Shi X, Nguyen TA, Suo Z, Liu Y, Avci R (2009) Surf Coat Technol204:237–245

11. Champ M (2000) Sci Total Environ 258:21–7112. Chang SI, Gray KA (2003) Met Organ Interact Environ Syst

43(1):529–3013. Kim J, Konstantinou I, Albanis T (2004) Environ Int 30:235–4814. Miriam P, Guillermo B, Monica G, Beatriz del A, Mirta S

(2006) Cupric tannate—A low copper content antifouling pigment44(4):311–315

15. Zhang L, Jiang Y, Ding Y, Povey M, York D (2010) J NanoparticleRes 9(3):479–489

16. Suwanboon S, Amornpitoksuk P, Bangrak P, Sukolrat A (2010) JCeram Process Res 11(1):547–551

17. Padmavathy N, Rajagopalan V (2008) Sci Technol Adv Mater9:035004–11

18. Auffan M, Rose J, Bottero JY, Lowry GV, Jolivet JP, Wiesner MR(2009) Nature Nanotechnol 4:634–641

19. Diego Meseguer Y, Soren K, Claus E, Weinell KimDamJ (2006)Prog Org Coat 56:327–337

20. Delio R, Dordick R (2002) Biotechnol Progress 18:551–521. Banoee M, Seif S, Nazari ZE, Jafari-Fesharaki P, Shahverdi HR,

Moballegh A, Moghaddam KM, Shahverdi AR (2010) E Publ11(3):77–79

22. Dhoke SK, Khanna AS (2012) Prog Org Coat 74:92–9923. Dhoke SK, Khanna AS (2009) Mater Chem Phys 117:550–55624. Yang JM, Lin HT, Lai WC (2002) J Membr Sci 208:10525. Bai CY, Zhang XY, Dai JB, Zhang CY (2007) Prog Org Coat

59:33126. Subramani S, Lee JM, Cheong IW, Kim JH (2005) J Appl Polym

Sci 98:62027. Ayman M, Ahmed F, Kafrawy EI, Morsy H, Abdel-Azim A (2007)

Prog Org Coat 58:13–2228. Sharif A, Naqvi F, Eram S, Kanak LV (2006) Prog Org Coat

55:268–27529. Ananda Kumar S, Balakrishnan T, Alagar M, Denchev Z (2006)

Prog Org Coat 55:207–21730. Berndt M, Berndt CC, Cramer SD, Covino BS (2003) Corrosion -

fundamentals, testing and protection. ASM international, USA 1331. Bagherzadeh MR, Mahdavi F (2007) Prog Org Coat 60:117–12032. Dong CF, Sheng H, An YH, Li XG, Xiao K, Cheng YF (2010)

Prog Org Coat 67:269–273

33. Sinebryukhov SL, Gnedenkov AS, Mashtalyar DV, GnedenkovSV (2010) Surf Coat Technol 205:1697–1701

34. Pebere N, Picaud T, Duprat M, Dabosi F (1989) Corros Sci29:1073–1086

35. Mansfeld F (1995) J Appl Electrochem 25:187–20236. Ananda Kumar S, Sankara Narayanan TSN (2002) Prog Org Coat

45:323–33037. Sankara Narayanan TSN, Subbaiyan M (1994) Met Finish

92(9):33–3638. Okazaki Y (2002) Biomaterials 23:2071–207739. Louis Floyd F, Avudiappan S, Gibson J, Mehta B, Smith P,

Provder T (2009) J Escarsega Prog Org Coat 66(1):8–3440. Hladky K, Callow LM, Dawson JL (1980) Brit Corros 15:20–2541. Ananda Kumar S, Sasi Kumar A (2010) Prog Org Coat 68(3):189–

20042. ASTM (2004). D3623, Standard Test Method for Testing

Antifouling Panels in Shallow Submergence43. Fabienne F, Isabelle L, Valerie L, Dominique H, Karine VR (2005)

Prog Org Coat 54:216–22344. Dhoke SK, Khanna AS, Mangal Sinha T J (2009) Prog Org Coat

64:371–8245. Aswini KM, Rama Shanker M, Ramanuj N, Raju KVSN (2010)

Prog Org Coat 67:405–41346. Marieta C, Remiro G, Garmendia G, Harismendy I, Mondragon

I (2003) Rphologies in toughened thermosetting matrices. EurPolym J 39(10):1965–1973, ISSN 0014-3057

47. Nikolic G, Zlatkovic S, Cakic M, Cakic S, Lacnjevac C, Rajic Z(2010) Sensors 10(1):684–696 ISSN 1424-8220

48. Ramezanzadeh B, Attar MM, Farzam M (2011) J Therm AnalCalorim 103:731–739

49. Rigail-Ceden A, Sung CSP (2005) Polymer 46:9378–938450. Chalmers JM, Everall NJ, Schaeberle MD, Levin IW, Lewis EN,

Kidder LH, Wilson J, Crocombe R (2002) Vib Spectrosc 30:43–4951. Poisson N, Lachenal G, Sautereau H (1996) Vib Spectrosc

12:237–24752. Mijovic J, Andjelic S (1996) Polymer 37:1295–130353. Billaud C, Vandeuren M, Legras R, Carlier V (2002) J Appl

Spectrosc 56:1413–142154. Zlatkovic S, Nikolic GS, Stamenkovic J (2003) Chem Ind (Serb)

57:563–56755. Krzysztof K, Tadeusz S (2008) Prog Org Coat 62:425–42956. Selvaraj R, Selvaraj M, Iyer SV (2009) Prog Org Coat 42:454–45957. Ayman MA, Abdou MI, Abdel A, Elsayed A, Mohamed E (2008)

Prog Org Coat 63:372–37658. Krishnan SM (2006) Prog Org Coat 57:383–39159. Lam K, Lau KT (2006) Compo Struc 75:553–55860. Hartwig A, Sebald M, Putz D, Aberle L (2005) Macromol Symp

221:127–13661. Dietsche F, Thomann Y, Thomann R, Mulhaupt R (2000) J Appl

Polym Sci 75:396–40562. Ayman MA, Shanker NO, Abdou MI, Abdelfatah M (2006) Prog

Org Coat 56:91–9963. Arthananareeswari M, Sankara Narayanan TSN, Kamaraj P,

Tamilselvi M (2012) J Coat Technol Res 9(1):39–4664. Ananda Kumar S, Alagar M, Mohan V (2002) JMEPEG 11:123–

12965. Ananda Kumar S, Alagar M, Mohan V (2001) Surf Coat Int B

Coat Trans 84:43–4866. Pedro de LN, Alexsander PA, Walney SA, Adriana NC (2008)

Prog Org Coat 62:344–35067. Anandakumar S (2009). A novel epoxy coatings for corrosion

and fouling resistance, in - Proceedings of Coatings ScienceInternational Conference, COSi

68. Chen M, Qu Y, Yang L, Gao H (2008) Structures and antifoul-ing properties of low surface energy non-toxic antifouling coatingsmodified by nano-SiO2 powder. Sci China Ser B Chem 51:848