porated aluminium alloy sacricial

Indi

icibe sresporveainfn oreo

ial anoiniumof steetroches not snoble

the alloy through the formation of b-phase [6].Modication of Al+5%Zn alloy anode is essential due to its non-

columbic loss and low galvanic efciency. Moreover the surface ofthe anode may attack by microbial fouling if the anode is in contactwith aqueous environments containing microorganisms [7]. Theinclusion of metal oxides can signicantly improve the metallurgi-

2. Experimental details

2.1. Synthesis of nano cerium oxide

Nano crystalline cerium oxide powder was synthesized by theprecipitation method [13]. Ammonia solution of pH 8 was addedto cerium nitrate (Ce (NO3)3 6H2O) solution, heated at 80 C underconstant stirring. Then the mixture was kept at that temperaturefor 2 h. The precipitate was collected by ltration, washed and -nally calcined at 350 C in a mufe furnace for 2 h in presence ofair.

* Corresponding author. Tel.: +91 471 2418782 (Off.), +91 92498 63611 (Res.).

Corrosion Science 50 (2008) 22322238

Contents lists availab

Corrosion

.e lE-mail address: smashibli@yahoo.com (S.M.A. Shibli).due to the formation of passive oxide lm on the surface. The suc-cess of the Al anode depends upon the alloying of certain metalswhose role is to prevent the formation of a continuous adherentand protective oxide lm on the alloy, thus permitting continuousgalvanic efciency. In order to promote activation, Al is usually al-loyed with small quantities of elements such as Zn, Hg, In, Sn, Bi, Tiand Mg [15]. Most of the works in this eld were carried out on Alrich Zn sacricial anodes and the concentration of Zn in Al alloysacricial anodes has been optimized to 5 wt% due to highimprovement in metallurgical and electrochemical properties of

ium oxide has been reported elsewhere [10,11]. Yan Yanping et al.developed a cerium-containing sacricial anode of Al alloy of mul-tiple elements for marine applications [12]. But no work has yetbeen reported regarding nano cerium oxide incorporation for theactivation of Al alloy sacricial anodes. Hence the present workcan be benecially considered for developing a sacricial anodewith high efciency and biocidal activity for effective use in marineenvironments.1. Introduction

Cathodic protection using sacricnique for corrosion control. Alumwidely used in cathodic protectionmerits such as low density, large elecability and reasonable cost. Pure Al iodes because it exhibits a relatively0010-938X/$ - see front matter 2008 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.06.017de is an effective tech-sacricial anodes arel structures due to itsmical equivalent, avail-uitable for galvanic an-potential in sea water

cal characteristics of the anodes. Literature reports that ZnOAl2O3mixed oxide composite has been used for this purpose [8]. In thiscontext nano cerium oxide (CeO2) was selected for the presentwork to develop reinforced Al alloy sacricial anodes. Cerium oxidehas long been considered as one of the most important oxidematerials because of its desirable properties such as high refractiveindex, good transmission, adhesion, high stability against mechan-ical abrasion and catalytic activity [9]. The biocidal activity of Cer-B. EISC. Cathodic protection considerably which enables the anodes to be used in aggressive marine conditions.

2008 Elsevier Ltd. All rights reserved.Development of nano cerium oxide incoranode for marine applications

S.M.A. Shibli a,*, S.R. Archana a, P. Muhamed Ashraf b

aDepartment of Chemistry, University of Kerala, Thiruvananthapuram, Kerala 695 581,bCentral Institute of Fisheries Technology, Cochin, Kerala 682 029, India

a r t i c l e i n f o

Article history:Received 17 December 2007Accepted 9 June 2008Available online 21 June 2008

Keywords:A. AluminiumA. Alloy

a b s t r a c t

Aluminiumzinc alloy sacrthe sacricial anodes canaluminium matrix. In the pfrom 0 to 1 wt% were incortrochemical test results respectroscopy revealed thecaused effective destructioformance of the anode. Mo

journal homepage: wwwll rights reserved.a

al anodes are extensively used for cathodic protection. The performance ofignicantly improved by incorporation of microalloying elements in theent work nano cerium oxide particles of different concentrations, rangingated for activating and improving the performance of the anode. The elec-led the increased efciency of the anode. The electrochemical impedanceormation that the presence of nano cerium oxide in the anode matrixf the passive alumina lm, which facilitated enhancement of galvanic per-ver, the biocidal activity of cerium oxide prevented the bio accumulationle at ScienceDirect

Science

sevier .com/locate /corsc i

electrolyte.

n SciTo study about the stability of the crystalline phase of nano cer-ium oxide, the particles were heated to 720 C for 2 h in the mufefurnace in presence of air and then subjected to X-ray diffractionanalysis using Cu Ka radiation. The average particle size was deter-mined from the broadening of the XRD line. The size of the parti-cles, DXRD was calculated using Scherrer equation [14].

DXRD 0:9kb cos h ;

where k is the wavelength of radiation, h is diffraction angle and b isthe full width half maximum in radians. The average particle sizewas also conned by TEM using 2000FX-11, Transition ElectronMicroscope. JEOL, Japan.

2.2. Anode casting

Commercially available Al (99.75%) and Zn (99.95%) ingots wereused for casting Al+5 wt% Zn alloy. This combination favors the for-mation of b-phase of the crystallographic state during casting. Thealloy ingots were cut, weighed and melted in a clay-graphite cruci-ble in a mufe furnace at temperature of 720 C. Different amountsof nano CeO2 particles were added into the melt and stirred using aSiC rod to homogenize it. The melt was again kept in the mufefurnace for another 15 min at the same temperature and thenpoured into a preheated graphite die of dimension 5.5 3.5 0.5 cm.

2.3. Physico-chemical evaluation

The anodes were subjected to Vickers micro indentation hard-ness test as per ASTM-E 384-05 using a Shimadzu HMV-2000instrument. For the present study the test load applied was 50 gffor an indentation time of 14 s at 25.5 C. The microscopic struc-tures such as grain size and grain boundaries of the anodes werecharacterized by scanning electron microscope of Hitachi S-2400.The electrodes were polished by using different grades of emerypaper down to 1000, rinsed with dilute NaOH and distilled water.SEM micrographs at different magnications were compared toanalyze the morphological characteristics.

2.4. Electrochemical characterization

2.4.1. PolarizationLinear sweep voltammetry was carried out using an Autolab 80

plus FRA2 corrosion system. The electrolyte used was aerated in3.5% NaCl solution. Ag/AgCl, Pt and the coupon having 1 cm2 ex-posed area were used as reference, counter and working elec-trodes, respectively. The coupons were polished with differentgrades of emery paper up to 1000, degreased with acetone andrinsed with distilled water. The coupons were then immersed in3.5% NaCl for 1 h prior to polarization studies at a scan rate of0.005 V/S at 30 2 C.

2.4.2. Galvanic efciencyThe test anode and a steel cathode having surface area 1 cm2

and 10 cm2, respectively were coupled and immersed in 3% NaClsolution at 30 2 C for a period of 1 month. The current owinginbetween the mild steel cathode and sacricial anode was contin-uously measured as a function of time, by using a zero resistanceammeter. For this purpose the galvanic couple was provided witha parallel connection having an ammeter and then the original cir-cuit was disconnected prior to the measurements in each time. Theactual current produced by the anode was determined from the

S.M.A. Shibli et al. / Corrosioplot of current vs. time. The area under the graph should be exactlyproportional to the actual charge delivered by the anode. Theweight of the anode before and after immersion of the respective2.5. Bio analysis

The anodes were evaluated for biological corrosion by immers-ing in marine water. The coupons, having different percentages ofnano cerium oxide, were immersed in subsurface water of Vizhin-jam Port. After 3 days, the anodes were taken out from the sea fordetermining the bio accumulation. The marine condition wasmaintained by keeping the anodes in a pool of simulated sea watertill they were transferred to the laboratory. The total viable countof the biolm formed was determined by standard plate countmethod. This was done simultaneously with coupon retrieved.Sterilized cotton swabs were employed to remove the surfacialgrowth from the anodes and the same was aseptically transferredin peptone water. The mixture was shaken for 5 min so that allorganisms were dispersed uniformly into the medium. After30 min, the samples were serially diluted with sterile water toget 104 dilution. Sample (0.1 ml) from the 104 dilution were sep-arated on to the Zobell Marine Agar plates. Incubation of the plateswas carried out for 24 h at 37 C.

Colony forming units (CFU) were enumerated and originalgalvanic couple was determined after cleaning the anode by fol-lowing a standard procedure (ASTM G 31). From the weight lossmeasured, the theoretical current to be produced by the anodewas calculated as

Galvanic efficiency A=B 100;where A is the actual current produced by the anode and B is thetheoretical current to be produced by the system as per Faradayslaw.

2.4.3. Self-corrosionThe anodes were immersed in 3% NaCl solution for a period of

30 days. The electrolyte was kept stagnant at 30 2 C. The anodeswere cleaned using a hot mixture of 20 g potassium dichromateand 50 ml phosphoric acid in 1 l water. The anodes were rinsedwith distilled water, dried and then weighed. The difference inweight of the metal before and after immersion was measuredand used to calculate the self-corrosion rate as given below

Corrosion rate Weight loss g cm2h1

Surface area time

2.4.4. OCP and CCP variationThe open circuit potential (OCP), the potential difference be-

tween the test anodes with respect to standard calomel electrode(SCE) was continuously monitored for a period of 1 month com-mencing from introduction of the anode into the electrolyte (3%NaCl kept at 30 2 C). The closed circuit potential (CCP) of the testanodes was monitored after coupling with mild steel cathodes hav-ing the surface area in the ratio 1:10. The current density generatedat the anode surface was maintained constant during CCP mea-surements using a controlled variable resistance.

2.4.5. Electrochemical impedance characteristicsElectrochemical impedance spectroscopy (EIS) was carried out

by using an electrochemical analyzer [Autolab PG STAT 30 plusFRA 2]. The electrolyte used was 3.5% NaCl. Ag/AgCl, Pt and theanode having 1 cm2 exposed were used as reference, counterand working electrodes, respectively. The impedance analysiswas carried out at the frequency range of 1 MHz to 0.1 Hz withreference to OCP after 30 min exposure of the coupons in the

ence 50 (2008) 22322238 2233counts were calculated from the dilution factor

Microbial counts Number of CFU dilution factor:

3. Results and discussion

3.1. Synthesis and morphology of nano cerium oxide

The probable precipitation reaction for the synthesis of nanocerium oxide is as follows [13]. Cerium nitrate was hydrolyzedwith NH4OH. The hydrated Ce4+ ions can form complexes withH2O molecules or OH ions. Polymers of this hydroxide,CeH2OxOH4yy , can then serve as the precursors of the oxide.In aqueous solution, H2O as a polar molecule tends to take protonsaway from coordinated hydroxide and the reaction can be ex-pressed by equation [13,15]

CeH2OxOH4yy H2O! CeO2 nH2OH3O

The calcination temperature was xed at 350 C since therewere reports showing that calcinations at higher temperature re-sults in the micro size cerium oxide formation [16].

The powder X-ray diffraction patterns for cerium oxide calcinedat 350 C and 720 C are shown in Fig. 1. In the 2h range of 2080,the ve typical peaks (1,1,1), (2,0,0), (2,2,0), (3,1,1) and (1,1,2) canbe indexed as F.C.C phase of cerium oxide [16]. The XRD pattern ofcerium oxide with uorite structure depends on annealing temper-ature since the phase transformation would take place when thetemperature increases above 300 C [13]. The d-spacing matchedclosely with those of cubic cerium oxide phase at 720 C also(JCPDS 81-0792). The crystalline size of cerium oxide at 350 C, cal-culated from the Scherrer formula using the (1,1,1) diffraction peakwas 15 nm. The width of the peaks gradually decreases withincreasing calcination temperature [16]. The XRD analysis was also

revealing the particle size of

Fig. 3. SEM micrograph of Al+5%Zn anodes at diffe

S.M.A. Shibli et al. / Corrosion Sci3.3. Evaluation of galvanic performance

The trend of anode potential (OCP) against time when the an-odes were immersed in a 3% NaCl solution is reported in Fig. 5.The initial OCP of the nano cerium oxide incorporated anodeshowed more negative value than the bare anode. The initial OCPvalue of the Al+5%Zn anode was found to be 0.944 V. After 1month of immersion the potential changes to 0.986 V. The OCPvalues of different concentration of nano cerium oxide incorpo-rated anodes were found to lie in the range from 0.953 V to0.967 V and after 1 month of immersion it shifted in the rangefrom 0.986 V to 0.989 V. As time goes on the OCP values slowlyshifted to more cathodic region. There was no marked difference inpotential among the anodes after 1 month. OCP cannot be consid-ered as a sole factor determining the anodic performance, further

Fig. 4. SEM micrograph of nano cerium oxide incorporated Al+5%Zn

Fig. 5. Variation of OCP with time of nano cerium oxide incorporated Al+5% Zn alloyanode. [() 0%, (h) 0.05%, (N) 0.1%, (e) 0.2%, (j) 0.5%, (4) 1% nanocerium oxide].rent magnications [(A) 500 and (B) 1.5 k].

ence 50 (2008) 22322238 2235analysis were conducted to asses the performance of anodes indetail.

The closed circuit potential (CCP) of the Al+5%Zn alloy anodesincorporated with different amounts of nano cerium oxide werealso compared (Table 1). An active CCP is desirable because a rela-tively noble potential could indicate the presence of passivation.The Al+5%Zn alloy anodes with 0.2% nano cerium oxide incorpora-tion shows a more cathodic CCP value of 0.987 V and is leastpolarized (Fig. 6). Anodes must also possess high galvanic ef-ciency in order to avoid frequent anode replacement. Duplicateexperiments were conducted and the average values of the ef-ciency of 0.05%, 0.1%, 0.2%, 0.5% and 1% nano cerium oxide incorpo-rated anodes were 38.4%, 63.9%, 78.6%, 62.6% and 48.5%,respectively and the efciency of Al+5%Zn anode was 44.4%. Therewere variations of below or around 1% of the efciency values.Thus the galvanic performance of anodes was much improved bythe incorporation of nano cerium oxide. The overall galvanic per-formances of the nano cerium oxide incorporated anodes are com-pared in Table 1. From the data it is clear that lower self-corrosionvalues were observed for higher amount of cerium oxide incorpo-ration. The reduction in self-corrosion values of the anodes couldbe attributed to the reduction in grain boundary corrosion. The0.2% nano cerium oxide incorporated anodes showed least self-cor-rosion value. The cerium oxide addition offered better reinforce-ment to the Al+5%Zn alloy matrix caused very low metaldissolution during long-term exposure.

3.4. Potentiodynamic polarization

The effect of nano cerium oxide on the polarization behaviour ofaluminium alloy sacricial anode is shown in Fig. 7. Addition ofnano cerium oxide to the anode alloy shifts the corrosion potentialto more negative values, which is desirable for the cathodic protec-

anodes at different magnications [(C) 500 and (D) 1.5 k].

Table 1The galvanic performance of Al+5 wt% Zn incorporated with nano cerium oxide (Electrolyte: 3% NaCl, temp: 30 2 C, stagnant condition)

Sl no. Amount of nano cerium oxide (%) OCP V vs. SCE CCP V vs. SCE at different current densities (mA cm2) Self-corrosion 106 g cm2 h1 Efciency (%)1 10 15

1 0 0.944 0.965 9.922 0.910 19.01 44.42 0.05 0.953 0.983 0.935 0.932 20.83 38.43 0.1 0.957 0.972 0.943 0.940 22.14 63.94 0.2 0.961 0.987 0.954 0.941 14.14 78.6

0.960.95

2236 S.M.A. Shibli et al. / Corrosion Science 50 (2008) 223222385 0.5 0.955 0.982 6 1 0.967 0.974 tion systems. The presence of nano cerium oxide decreased thepolarization resistance (Rp) and increased the corrosion potentialEcorr in the negative direction (Table 2). The corrosion rate and Icorrwere maximum and Rp value was minimum for 0.2% nano ceriumoxide incorporated Al alloy sacricial anode. Though the potentialvariations are not more than few mV, they were comparable andfrom those results, the optimum concentration of cerium oxide

Fig. 6. Variation of CCP with time of nano cerium oxide incorporated Al+5% Zn alloyanode. [() 0%, (h) 0.05%, (N) 0.1%, (e) 0.2%, (j) 0.5%, (4) 1% nanocerium oxide].

Fig. 7. Polarization behaviour of Al+5%Zn anodes incorporated with nano ceriumoxide. [(A) 0%, (B) 0.05%, (C) 0.1%, (D) 0.2%, (E) 0.5% nano cerium oxide].

Table 2The LSV parameters of nano cerium oxide incorporated Al alloy sacricial anode in 3.5% N

Percentage of cerium oxide Ecorr (V) Icorr (A cm2) 105 Rp0 0.924 0.644 790.05 0.935 2.839 790.1 0.910 1.795 370.2 0.931 15.86 160.5 0.924 1.527 411 0.930 1.209 89was revealed. Duplicates also showed similar results. The above re-sults revealed that compared to other concentrations of ceriumoxide, 0.2% nano cerium oxide imparted better performance tothe anode.

3.5. Electrochemical impedance spectroscopy (EIS) measurements

AC impedance spectroscopic studies were carried out to getinformation about the electrochemical and physico-chemical phe-nomena associated with the electrode reactions during galvanicdissolution process. The EIS plots of Al alloy sacricial anodesincorporated with nano cerium oxide are shown in Fig. 8. Theimpedance spectra of all the anodes, studied in the present workhave centre lies under the real axis, which is the characteristicbehaviour of AlZn alloys undergoing uniform galvanic dissolution[18]. The high frequency plot has been associated with the chargetransfer process and the low frequency plot with mass transferprocess. The semicircle at the high frequency was found to havesimilar behaviour in spite of the variation in the cerium oxide con-tent. The second semicircle can be attributed to the formation of aZn(OH)2 and Al(OH)2 layers on the anode surface due to the oxida-tion of Zn and Al [19]. The depression and pseudo inductive behav-iour of the second semicircles can be attributed to activedissolution [19]. Depressed semicircular shape of the compleximpedance plane is due to the inhomogenities of the anode surface[20].

The experimental data can be described using a simple equiva-lent circuit. In this equivalent circuit, Rs is the solution resistance,Rp is the polarization resistance, A constant phase element(CPE) is introduced for better data tting instead of an ideal capac-itance parameter. The impedance expression of CPE is dened as

ZCPE Ajwn1;where A and n are frequency independent t parameter, j = (1)1/2and w = 2kf, the frequency. Depending on the values of n, the CPEcan represent resistance (n = 0 and A = R), capacitance (n = 1,A = C), inductance (n = 1, A = L) and Warburg impedance (n = 0.5and A =W). CPE is related to some inhomogenities on the surfaceof the anodes. The objective of impedance analysis was to measure

4 0.942 16.21 62.66 0.950 18.34 48.5the total polarization resistance (Rp) that constitutes the main prac-tical parameter useful for the understanding of the anode dissolu-tion rate. The double layer capacitor in real cells often behaveslike a CPE instead of like a capacitor. Several theories have been pro-

aCl at 30 2 C

(X cm2) bc V dec1 ba V dec1 Corrosion rate mm/year

9.1 0.385 0.011 0.0621.4 0.142 0.012 0.2761.2 0.059 0.012 0.1748.6 0.056 0.029 1.5384.9 0.216 0.014 0.1482.1 0.289 0.023 0.117

the hydroxyl radicals themselves also could produce H2O2. The hy-droxyl radical and hydrogen peroxide are responsible for the bio-cidal effect. Fig. 9 shows the photos of the microorganismsgrown on nutrient agar developed from the biolm. Table 4 indi-cates the bacterial count in different samples. From these data itis clear that as the amount of composite in the anode increasesthe microbial count decrease from 3.8 104 to 9.9 102. Thus bythe incorporation of nano cerium oxide, a biocide, in Al alloy sacri-cial anode the growth of microorganisms on the anode surface issignicantly reduced.

1 2.8 10

S.M.A. Shibli et al. / Corrosion Sciposed to account for the non-ideal behavior of the double layer butnone has been universally accepted. In most cases n is treated as anempirical constant and not have much physical basis.

It is possible by EIS to study the behaviour of the oxide lm onthe anode surface when it is exposed to an electrolyte. Rp value isan indication of the effective interaction between the oxide lmand the substrate, which lowers the surface resistance, a requisiteto sacricial anodes [5]. Duplicate experiments were conductedand the average values were compared. The Rp value of nano cer-ium oxide incorporated alloys are in the order 0.2% < 0.5%