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Highlights

Corrosion Science xxx (2015) xxx–xxxSynergistic inhibition of carbon steel corrosion in seawater by cerium chloride andsodium gluconate

Franjo Ivusic∗, Olga Lahodny-Sarc, Helena Otmacic Curkovic, Vesna Alar

• Significant synergistic effect was determined for cerium and gluconate.• The mixture showed significant corrosion inhibition of carbon steel in seawater.• Predominant anodic inhibition mechanism was observed.• The presence of cerium ions incorporated in the protective layer was confirmed.

Please cite this article in press as: F. Ivusic, et al., Synergistic inhibition of carbon steel corrosion in seawater by cerium chloride andsodium gluconate, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.05.017

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Synergistic inhibition of carbon steel corrosion in seawater by ceriumchloride and sodium gluconate

1

2

Franjo Ivusic a,∗Q1 , Olga Lahodny-Sarca, Helena Otmacic Curkovic b, Vesna Alarc3

a The Institute for Corrosion Research and Desalinization, Croatian Academy of Sciences and Arts, Vlaha Bukovca 14, 20000 Dubrovnik, Croatia4b Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10000 Zagreb, Croatia5c Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lucica 5, 10000 Zagreb, Croatia6

7

a r t i c l e i n f o8

9

Article history:10

Received 21 October 201411

Accepted 11 May 201512

Available online xxx13

14

Keywords:15

Carbon steelQ316

Rare earth elements17

EIS18

Polarization19

SEM20

Neutral inhibition21

a b s t r a c t

In this research the effect of cerium (III) chloride heptahydrate (CC) and sodium gluconate (SG) on thecorrosion inhibition of carbon steel C45 (1531) in natural seawater has been evaluated using electro-chemical methods and scanning electron microscopy (SEM). The results show that substantial corrosioninhibition (94.98%) using CC and SG can be obtained in synergistic manner. Surface analysis confirmed thepresence of cerium ions incorporated in the protective layer of carbon steel specimen. SG acts predomi-nantly as anodic inhibitor whereas CC acts as a mixed type inhibitor. Using both inhibitors predominantmechanism of anodic inhibition is observed.

© 2015 Elsevier Ltd. All rights reserved.

22

1. Introduction23

Seawater and brines are increasingly used in industrial practice24

(cooling water systems, desalination plants, seawater injection, fire25

fighting, ballast, pressure testing of pipelines) [1].26

A 3.5% NaCl solution is used frequently for seawater simulation27

in laboratory, but it is known to be more aggressive toward carbon28

steel than natural seawaters [2,3]. Although the corrosiveness of29

the natural seawater depends on many variable factors, the main30

cause of the corrosiveness of both natural seawater and 3.5% NaCl31

solution are chlorides that promote various corrosion processes,32

particularly at elevated temperatures [4].33

Due to low cost and ease of fabrication carbon and low-alloy34

steels are, despite pronounced tendency toward corrosion, the35

most widely used materials for marine environments regarding36

both structural components and pressure-retaining applications37

[5]. In saline conditions carbon steels are susceptible to general,38

localized and galvanic corrosion. Their protection is accomplished39

using protective coatings, corrosion inhibitors, galvanic protection40

or combination.41

Since environmental regulations are becoming more and more42

stringent regarding disposal and emission of toxic substances,43

application of chemical substances for corrosion inhibition is44

∗ Corresponding author. Tel.: +385 922624309; fax: +385 2042311.Q2E-mail address: fivusic@hazu.hr (F. Ivusic).

turning “green” too. The major improvement is in the area of 45

eliminating toxic compounds such as chromates, dichromates and 46

nitrites [6]. In spite of their toxicity, chromates are still used in 47

the manufacture and maintenance of aircraft. The health problems 48

associated with the use of chromates and the increased costs asso- 49

ciated with their safe use and disposal, has led to efforts in finding 50

suitable alternate inhibitors [7]. 51

Environmentally friendly or, more precisely, environmentally 52

acceptable inhibitors are those inhibitors which, in applied con- 53

centrations, do not disturb ecological balance (stable balance in the 54

numbers of each species in an ecosystem) or take negative impact 55

on human health. They can be divided on ad hoc basis in three 56

groups: inorganic substances such as rare-earth metal (REM) salts, 57

borates, silicates, molybdates; organic compounds such as thiogly- 58

collates, phosphonates, sulfonates, carboxylic acids and their salts 59

(amino acids, fatty acids, gluconates), vitamins, pigments, antibiotic 60

or antifungal drugs (e.g. imidazole compounds), alkaloids (nicotine, 61

caffeine); and true “green” inhibitors such as various herbal extracts 62

(water, alcohol or acid extracts). 63

The effectiveness of rare-earth metals as corrosion inhibitors 64

was described in a 1984 patent of Goldie and McCaroll [8]. Since 65

then, many studies have shown that rare earth compounds are effi- 66

cient corrosion inhibitors for different metals and alloys [9–24]. 67

Their advantage is effectiveness, low toxicity [25,26] and economic 68

competitiveness [27]. Some studies have shown synergy between 69

rare earth salts and organic carboxylate compounds such as sali- 70

cylate [6,28], cinnamate [29,30], octylthiopropionate [31], dibutyl 71

http://dx.doi.org/10.1016/j.corsci.2015.05.0170010-938X/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: F. Ivusic, et al., Synergistic inhibition of carbon steel corrosion in seawater by cerium chloride andsodium gluconate, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.05.017

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phosphate [32], hexadecanoate [33], tartrate [34] and mercaptoac-72

etate [35–37] in corrosion inhibition.73

Gluconic acid and its salts are environmentally suitable non-74

toxic compounds having also useful applications in medicine. For75

example, calcium gluconate is used as a health strengthening sub-76

stance as well as for the treatment of calcium deficiency. Zinc77

gluconate is used for common cold treatment and for better wound78

healing. The efficiency and the mechanism of the corrosion inhi-79

bition by gluconates, either as single compound or in a mixture,80

have been described in a number of recent studies [2,38–45]. A suc-81

cessful inhibition of carbon steel was obtained in the seawater or82

solutions containing different chloride concentrations by gluconate83

[1,2,40,45–49].84

The efficiency of gluconate inhibition depends on inhibitor con-85

centration, pH value and chemical composition of investigated86

solution, the nature of cations introduced in the solution as glu-87

conate or already present in water and the state of the metal surface88

[39,49]. In previous researches the anodic inhibition mechanism of89

mild steel corrosion using sodium gluconate has been shown [2,38].90

The influence of bivalent ions (Ca2+ and Zn2+) on the decrease of91

the cathodic reaction rate was recognized in solutions of their glu-92

conate salts [49]. Gluconates are part of the successful commercial93

corrosion inhibitors and are recommended in mixture with water94

soluble polymeric dispersant, organophosphonate and silicate [50].95

Although both gluconates and rare earth salts have been previ-96

ously studied as corrosion inhibitors their joint effect has not been97

examined. The aim of this research was to evaluate the capabil-98

ity of cerium chloride (CC) and sodium gluconate (SG) as corrosion99

inhibitors for carbon steel in chloride environments, to optimize100

their concentrations in inhibitor mixtures and to determine possi-101

ble synergetic effect between these inhibitors.102

2. Material and methods103

The electrochemical experiments were performed using carbon104

steel specimens C45 of following composition (wt%): C45 (C 0.45%,105

P 0.03%, S 0.03%, Si 0.22%, Cr 0.06%, Mn 0.57%, Cu 0.07%). Work-106

ing electrode area was 1 cm2. Before measurements the specimen107

was abraded with emery paper (400, 600, 800 and 1200 grade)108

degreased with ethanol and rinsed with demineralized water.109

Afterwards the working electrode was immersed in electrochemi-110

cal cell containing 600 mL of investigated medium. All experiments111

were performed at ambient temperature (22 ± 2 ◦C). The electro-112

chemical cell was equipped with graphite auxiliary electrode and113

a reference saturated calomel electrode, which was connected to 114

the working electrode over Luggin capillary. Electrochemical mea- 115

surements were performed on Potentiostat/Galvanostat EG&G PAR, 116

Model 273 A using software SoftCorr III. The corrosion potential 117

(Ecorr) was measured as a function of time in order to understand 118

the corrosion behavior of the sample in the electrolyte. As soon as 119

the sample was immersed into the electrolyte, the initial potential 120

of the sample was noted and monitored as a function of time until 121

the sample attained a constant potential (typically 30–60 min). 122

Afterwards, the quasi potentiostatic polarization was carried out 123

recording polarization curves in the range ± 250 mV vs. Ecorr with 124

a polarization rate 0.166 mV s−1. From polarization curves follow- 125

ing corrosion parameters were calculated by Tafel extrapolation 126

method: the corrosion current density (jcorr) and corrosion poten- 127

tial (Ecorr). The corrosion rate was calculated from the corrosion 128

current density according to Faraday’s law 129

vcorr = jcorrAr

�zF(1) 130

where Ar is the iron atomic weight, � is the steel density, z is the 131

number of electrons exchanged, and F is the Faraday constant. 132

The inhibition efficiency (�i) was calculated using Eq. (2) 133

�i = 100vni − vinh

vni(2) 134

where �i is the inhibition efficiency (%), vni is the corrosion rate in 135

natural seawater, and vinh is the corrosion rate in inhibitor contain- 136

ing seawater. 137

All experiments were performed in natural seawater from 138

Dubrovnik region. The composition of the natural seawater from 139

same region was described elsewhere [2]. For electrolyte (media) 140

preparation cerium(III) chloride heptahydrate (p.a. Acros) and 141

sodium gluconate were used (p.a. Alfa Aesar). Synergism parameter 142

(�s) was calculated using following relations [51] 143

�s = 1 − �1+2

1 − �′1+2

(3) 144

�1+2 = (�1 + �2) − �1�2 (4) 145

where �1 is the surface coverage of inhibitor 1, �2 is the surface 146

coverage of inhibitor 2, and �1+2 is the combined surface coverage 147

in presence of inhibitors 1 and 2. 148

This calculation is based on the assumption that the surface 149

coverage can be related to inhibiting effect by the Eq. (5). When 150

Fig. 1. Tafel polarization curves of carbon steel C45 in seawater – influence of cerium chloride addition (values in the legend represent concentrations in mmol L−1).

Please cite this article in press as: F. Ivusic, et al., Synergistic inhibition of carbon steel corrosion in seawater by cerium chloride andsodium gluconate, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.05.017

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Table 1Corrosion parameters of carbon steel C45 in seawater – influence of cerium chloride addition.

CC (mmol L−1) Ecorr (mV/SCE) ˇA (mV dec−1) −ˇC (mV dec−1) vcorr (mm y−1) jcorr (�A cm−2) �i (%)

0 −661 94 166 0.2803 23.98 00.04 −684 89 164 0.229 19.63 18.140.1 −683 71 178 0.2352 20.12 16.10.2 −680 78 164 0.2331 19.94 16.850.4 −687 64 174 0.1045 8.942 62.711 −637 75 141 0.0835 7.138 70.232 −622 56 133 0.0487 3.496 85.424 −630 52 198 0.0436 3.733 84.43

10 −560 87 129 0.112 10.87 54.67

Fig. 2. Tafel polarization curves of carbon steel C45 in seawater – influence of sodium gluconate addition (values in the legend represent concentrations in mmol L−1).

a synergistic effect exists between two inhibitors, the synergism151

parameter will be greater than 1.152

� = �i

100(5)153

Electrochemical impedance spectroscopy (EIS) was performed154

at Ecorr in the frequency range 100 kHz–10 mHz with a 10 mV155

rms amplitude using a PAR 263A potentiostat/galvanostat and fre- 156

quency response detector PAR 1025. 157

The surface of the steel specimen was examined after 10 days 158

of exposure of samples to studied media by stereomicroscope Leica 159

MZ6. The morphology and elemental composition of the steel spec- 160

imen were determined using scanning electron microscope (SEM) 161

VEGA TESCAN TS 5136 MM equipped with energy dispersive spec- 162

troscopy (EDS) analyzer (Oxford Instruments INCA). 163

Fig. 3. Tafel polarization curves of carbon steel C45 in seawater – cerium chloride and sodium gluconate addition (values in the legend represent concentrations in mmol L−1).

Please cite this article in press as: F. Ivusic, et al., Synergistic inhibition of carbon steel corrosion in seawater by cerium chloride andsodium gluconate, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.05.017

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Table 2Corrosion parameters of carbon steel C45 in seawater – cerium chloride and sodium gluconate addition.

SG (mmol L−1) CC (mmol L−1) Ecorr (mV/SCE) ˇA (mV dec−1) −ˇC (mV dec−1) vcorr (mm y−1) jcorr (�A cm−2) �i (%) �S (−)

0 0 −661 94 168 0.2803 23.98 0 –1 0 −597 69 177 0.1852 15.84 34 –1 0.4 −602 85 119 0.1063 9.089 62.1 0.6491 1 −587 79 200 0.0917 7.843 67.3 0.6011 2 −604 70 143 0.0303 2.595 89.18 0.8892.25 0 −555 60 110 0.1545 13.21 44.9 –2.25 0.4 −549 58 102 0.1050 8.97 62.59 0.5492.25 1 −563 93 148 0.0572 4.89 79.6 0.8042.25 2 −573 70 134 0.0180 1.54 93.58 1.2514.5 0 −543 69 104 0.1590 13.59 43.33 –4.5 0.4 −540 55 166 0.0526 4.497 81.25 1.1274.5 1 −585 64 195 0.0195 1.666 93.05 2.4274.5 2 −599 68 110 0.0141 1.203 94.98 1.646

3. Results and discussion164

3.1. The effect of cerium chloride addition on corrosion inhibition165

in seawater166

The effect of cerium chloride addition on protective properties167

of carbon steel C45 in seawater was evaluated using cerium chlo-168

ride heptahydrate in various concentrations. In order to evaluate169

inhibitor performance an experiment without inhibitor addition170

was also performed. Concentration range of cerium chloride varied171

from 0.04 to 10 mmol L−1. Tafel polarization curves for character-172

istic experiments are presented in Fig. 1. The values of corrosion173

parameters evaluated from electrochemical studies (Ecorr, vcorr, jcorr,174

�i) are given in Table 1. Lower cerium chloride concentrations (less175

than 0.4 mmol L−1) have tendency to shift Ecorr value slightly in the176

negative direction when compared to uninhibited curve, achieving177

minor reduction of the corrosion rate, mostly related to reduction178

of the cathodic current density. Higher cerium chloride concen-179

trations (up to 2 mmol L−1) mainly showed tendency to shift Ecorr180

value in the positive direction and achieved considerable corrosion181

rate reduction (Table 1). Concentration that resulted with the high-182

est corrosion resistance was 2 mmol L−1. Both anodic and cathodic183

current densities were reduced with slightly higher impact on the184

anodic process. At concentrations ≤2 mmol L−1 cerium chloride185

acts predominately as mixed inhibitor in natural seawater which is186

in good agreement with previous researches [6]. However, higher187

inhibitor concentration (10 mmol L−1) showed diminished corro-188

sion rate reduction (Table 1, Fig. 1) and even increased cathodic189

current density. Such behavior may be explained by the recent find-190

ings that the addition of rare earth metal (REM) cations can result191

in modification of cathodic process on carbon steel. Besides oxygen192

reduction reaction, the reduction of water bonded in REM(III)- aqua193

complex is observed as well [52,53]. At high Ce(III) concentrations194

water reduction becomes important leading to a higher corrosion195

current densities and shift of the corrosion potential toward more196

positive values. The rate of reduction of the aqua-ligand follows a197

Tafel law with a slope of a 0.12 V dec−1, which is observed in this198

study for the highest Ce concentration (Table 1).199

3.2. The effect of CC and SG addition on corrosion inhibition200

in seawater201

The inhibiting effect of SG on carbon steel was also examined202

by polarization measurements. Concentrations of SG were cho-203

sen according to the previous findings on gluconate corrosion204

inhibition [1,2]. Tafel polarization curves for the characteristic205

experiments are presented in Fig. 2. Sodium gluconate alone206

showed to be relatively weak anodic corrosion inhibitor. This fact207

has reflected in a shift of the corrosion potential toward more208

positive potentials with the increase in SG concentration. The209

maximum inhibiting efficiency obtained was 44.9% for 210

2.25 mmol L−1 SG solution. For the highest inhibitor concen- 211

tration examined (4.5 mmol L−1) decrease in inhibiting efficiency 212

was observed. Similar behavior was also reported by Touir et al. 213

[38] who proposed formation of soluble SG complex. It is also 214

important to notice that SG acts as a scale inhibitor [38] thus 215

reducing the formation of scale that in blank solution might 216

partially block cathodic sites and accordingly reduce cathodic 217

current densities. 218

In order to determine possible synergistic effect on corrosion 219

inhibition, CC was supplemented with SG in two component sys- 220

tem (Table 2). All of the inhibitor combinations (CC + SG) showed 221

a)

b)

Fig. 4. Nyquist plots for carbon steel in blank seawater and with the addition of stud-ied inhibitors. Measurements performed after: (a) 3 h and (b) 13days of immersion.(Symbols represent experimental data and lines represent fitted data).

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a)

b)

c)

Fig. 5. Bode plots for carbon steel in blank seawater and with the addition of studied inhibitors. Measurements performed after: (a) 3 h (b) 1 day and (c) 13 days of immersion.(Symbols represent experimental data and lines represent fitted data).

substantial corrosion rate reduction compared to uninhibited222

medium (Table 2, Fig. 3). Moreover, all inhibitor combinations had223

tendency to shift Ecorr value in the positive direction when com-224

pared to uninhibited curve. Even though both anodic and cathodic225

current densities decreased in the presence of inhibitor mixture, the226

anodic inhibition was much more pronounced. Consequently, with227

the increase of inhibitor concentration, corrosion rate decreased228

significantly.229

Accordingly, experiments with 1 mmol L−1, which was low-230

est examined concentration of SG, showed pronounced corrosion231

inhibition only at higher concentrations of CC (at 2 mmol L−1 corro-232

sion inhibition was 89.18%). Experiments with 2.25 mmol L−1 of SG233

showed even more expressed corrosion inhibition at high CC con-234

tent, as well as experiments with 4.5 mmol L−1 of SG. Therefore,235

experiments with highest CC addition showed improved corro-236

sion inhibition properties, with efficiency that reached around237

90% at lowest SG addition, or higher than 90% at higher SG addi-238

tions. Also experiment with highest SG addition and 1 mmol L−1 CC239

showed significant corrosion inhibition (93.05%). The best result240

was accomplished using both CC and SG at highest levels resulting241

in 94.98% inhibition.242

Fig. 6. Electrical equivalent circuits representing carbon steel C45 in: (a) blank sea-water and with addition of SG (b) with the addition of CC + SG and (c) with theaddition of CC.

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Fig. 7. Time evolution of impedance parameters for carbon steel exposed to blank seawater and with the addition of studied inhibitors.

From data in Table 2. synergism parameter (�s) was calculated.243

The highest value of �s was 2.42, which indicates that significant244

synergistic behavior between CC and SG exists in terms of corrosion245

inhibition.246

Electrochemical impedance spectroscopy was used to follow the247

corrosion behavior of protected and bare carbon steel in time. Three248

inhibitor mixtures that gave the highest inhibiting efficiency and249

the highest synergistic effect were selected for EIS measurements as250

well as one concentration for each inhibitor alone. For the inhibitor251

mixture 4.5 mmol L−1 SG and 1 mmol L−1 CC formation of corro-252

sion products on one part of the electrode was observed after few253

days of immersion indicating that electrode was not completely254

protected, unlike for other two synergistic inhibitor concentra-255

tions. For this reason EIS spectra are shown only for two inhibitor256

mixtures (Figs. 4 and 5). EIS measurements have shown that 257

impedance of carbon steel increases during the first day of immer- 258

sion in both blank and inhibited seawater. This may be explained by 259

formation of corrosion products in blank seawater or adsorption of 260

inhibitor in inhibited solutions. Latter on impedance in blank sea- 261

water remained almost constant in time while that of the sample 262

in seawater containing SG started to decrease. In all other solutions 263

impedance was increasing in time. 264

The impedance spectra of carbon steel obtained in blank seawa- 265

ter and with SG exhibit only one time constant that may be related 266

to the corrosion process occurring at the metal surface. These spec- 267

tra can be represented by equivalent circuit given in Fig. 6a. The 268

impedance spectra for samples in inhibited solution exhibit two 269

time constants (Fig. 6b) except for the CC alone where three time 270

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Fig. 8. Surface of carbon steel specimens C45 after 10 days immersion in seawater.

constants were observed (Fig. 6c). The Rf–Cf circuit is observed in 271

the high frequency domain. The Rf corresponds to the resistance 272

due to an ionic flow and the Cf represents the capacitance of the 273

surface film associated with the dielectric property of this film. In 274

a case of CC alone two time constants were observed at higher fre- 275

quencies. It may be assumed that a thick film composed of porous 276

outer layer of Ce(OH)3 (Rf,o–Cf,o) and more compact inner layer of 277

Ce2O3 (Rf,i–Cf,i) is formed as proposed by Aramaki [24]. The Rct–Cdl 278

circuit is observed at lower frequencies. These elements correspond 279

to the charge transfer resistance Rct and double layer capacitance 280

Cdl. Capacitive elements in electrochemical systems are usually rep- 281

resented by constant phase elements CPE. The impedance of CPE is 282

defined as 283

(6)Z(CPE) = 1Q (jω)n where Q is the constant of the CPE element, 284

j imaginary number, ω is the angular frequency, and n is the CPE 285

exponent where −1 ≤ n ≤ 1. The value of n is associated with the 286

non-uniform distribution of current as a result of roughness and 287

surface defects. The n values of nearly one suggest nearly ideal 288

capacitive behavior. The values of pseudocapacitance Cf and Cdl 289

were calculated using relationship C = (QRb1–n)1/n according to Brug 290

et al. [54]. 291

Results obtained by fitting the experimental data to selected 292

equivalent circuits are given in Fig. 7. It can be observed that charge 293

transfer resistance Rct increased in time for all inhibitor combina- 294

tions except for SG alone. This indicates that corrosion reaction was 295

slowed down in time in the presence of inhibitors. For SG corrosion 296

was only slowed down for shorter immersion times, up to 2 days, 297

while latter on the dissolution of SG complex and adsorption of 298

Cl− ions occurred resulting in an increase of double layer capaci- 299

tance. From EIS data for SG + CC combination it appears that besides 300

the inhibitor adsorption on active places on metal surface, which 301

reflects in increase of charge transfer resistance, thin surface film is 302

formed as well. The film resistance is relatively low but it increased 303

slightly in time. In literature [6,7] this surface film is commonly 304

described as cerium oxide/hydroxide layer that forms slowly in 305

solutions containing cerium(III) salts. These results are in accor- 306

dance with those obtained by polarization measurements (Fig. 3) 307

showing that in the presence of inhibitor mixture both anodic and 308

cathodic current densities decreased but no significant change in 309

Tafel slopes was observed as it would be expected for the thick 310

inhibitor film with good barrier properties. 311

For CC alone outer protective layer became thicker (lower 312

Cf,o values) and rougher (decrease of nf,o) in time, while inner 313

layer showed to be thinner (higher Cf,i values) but much more 314

Fig. 9. Scanning electron micrograph of the surfaces of unprotected carbon steel specimen in seawater and the results of EDS analysis.

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Fig. 10. Scanning electron micrograph of the surfaces of carbon steel specimen in seawater with addition of 2 mmol L−1 CC + 4.5 mmol L−1 SG and the results of EDS analysis.

protective (higher Rf,i). Formation of thick surface layer has also315

resulted in a decrease of ndl indicating that corrosion process was316

under diffusion control at longer immersion times.317

Both synergistic inhibitor compositions offered significant318

corrosion protection that remained in time. However, as the319

impedance of sample exposed to CC solution increased rapidly in320

time, after several days of immersion it became similar to that321

observed in SG + CC solution.322

Corrosion behavior of carbon steel in studied solutions can323

be more easily understood by examining the surface of unpro-324

tected and protected surfaces of steel specimens after 10 days of325

exposure to seawater as shown in Fig. 8. Bulk corrosion product326

formation is visible on unprotected carbon steel specimen (Fig. 8a),327

whereas the surface of a specimen protected by synergistic mixture328

is homogenous and practically corrosion products free (Fig. 8c). For329

both CC + SG inhibitor combinations specimen surface had similar 330

appearance. On sample that was exposed to CC solution formation 331

of thick deposits was noticed (Fig. 8b). As observed by EIS the thick 332

outer layer is porous and signs of corrosion products are visible 333

inside the pores. Although from EIS measurement it appears that 334

after several days of immersion corrosion resistance of carbon steel 335

in 2 mmol L−1 CC solution is of the same order of magnitude as for 336

mixtures of CC and SG, from optical microscopy it is evident that 337

addition of SG has beneficiary effect. It prevents the formation of 338

rough and thick surface deposits that could increase drag in flow- 339

ing systems and reduces the possibility of localized corrosion under 340

the deposits. 341

In Figs. 9–11 the scanning electron micrographs of the sur- 342

faces of unprotected and protected carbon steel specimens and 343

the results of EDS analysis after 10 days immersion in seawater 344

Fig. 11. Scanning electron micrograph of the surfaces of protected carbon steel specimen in seawater with the addition of 2 mmol L−1 CC and the results of EDS analysis.

Please cite this article in press as: F. Ivusic, et al., Synergistic inhibition of carbon steel corrosion in seawater by cerium chloride andsodium gluconate, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.05.017

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F. Ivusic et al. / Corrosion Science xxx (2015) xxx–xxx 9

are shown. The difference between samples is evident, unpro-345

tected sample exhibits distinctively non-homogenous surface with346

clusters of corrosion products (Fig. 9.). Accordingly, EDS analysis347

confirmed the presence of 31.06 wt% oxygen that is incorporated348

in corrosion product species. The protected sample has a homoge-349

nous surface, without considerable corrosion products formation350

(Fig. 10). EDS analysis revealed presence of cerium at fairly high351

concentrations. Moreover, similar to previous researches, forma-352

tion of islands of rare-earth oxide is noted. These islands are thought353

to be associated with the more active anodic and cathodic sites354

within the metal microstructure [6]. Higher content of carbon and355

lower content of oxygen (compared to uninhibited experiment)356

suggests that carboxylate species are part of the protective layer357

as well, which is in agreement with previous findings [6].358

In Fig. 11 images of steel surface exposed to CC solution are pro-359

vided. Part of the surface is covered with dense needle like structure360

of cerium compounds. The smaller parts of the surface exhibited361

different morphology, instead of cerium oxide/salt crystals, traces362

of corrosion are observed which is also confirmed by high amounts363

of O and Fe in EDS spectra.364

4. Conclusions365

(1) Studies performed in this work confirm that both cerium366

chloride and sodium gluconate inhibit mild steel corrosion in367

natural seawater. Cerium chloride acts predominantly as mixed368

inhibitor in natural seawater. Its optimal concentration for369

carbon steel C45 was 2 mmol L−1. Sodium gluconate provides370

lower inhibiting effect than cerium chloride and it behaves as371

anodic corrosion inhibitor.372

(2) Inhibitory effect was further improved by supplementation373

of cerium chloride with sodium gluconate. Their mixtures374

acted predominantly as anodic inhibitors. Optimal concen-375

tration determined in this work was 2 mmol L−1 of cerium376

chloride with 4.5 mmol L−1 of sodium gluconate. That combi-377

nation reached efficiency of about 95%. Moreover, significant378

synergistic effect was determined for cerium chloride and379

sodium gluconate at applied concentrations.380

(3) EDS analysis revealed the abundance of cerium ions which are381

incorporated in the protective layer of carbon steel specimen.382

Also, higher content of carbon and lower content of oxygen383

(compared to uninhibited experiment) suggests that carbox-384

ylate species are part of the protective layer as well.385

(4) Inhibitory action of cerium chloride and sodium gluconate was386

also confirmed by electrochemical impedance spectroscopy. EIS387

measurements showed that full protective effect is obtained388

after several hours of immersion and that it remains stable in389

time.390

Acknowledgements391

The authors are grateful to the “Croatian Science Foundation” forQ4392

the financial support of the project 9.01/253 “Ecological protection393

of metal constructions exposed to marine environment”.394

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