-
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: [email protected] (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%