inhibition effect of sodium nitrite

CORROSION SCIENCE SECTION

CORROSION—Vol. 67, No. 12 125001-1ISSN 0010-9312 (print), 1938-159X (online)

11/000151/$5.00+$0.50/0 © 2011, NACE International

Submitted for publication April 11, 2011; in revised form, July 15, 2011.

‡ Corresponding author. E-mail: [email protected]. * State Scientific Research Institute, Centre for Physical Sciences

and Technology, Institute of Chemistry, Goštauto 9, LT-01108, Vilnius 2600, Lithuania.

Inhibition Effect of Sodium Nitrite and Silicate on Carbon Steel Corrosion in Chloride-Contaminated Alkaline Solutions

O. Girciene·,‡,* R. Ramanauskas,* L. Gudavici –ute·,* and A. Martušiene·*

ABSTRACT

The ability of some corrosion inhibitors to protect carbon steel against corrosion was investigated using the voltammet-ric and electrochemical impedance spectroscopy (EIS) tech-niques. The surface analysis of samples was carried out by x-ray photoelectron spectroscopy (XPS). EIS measurements have shown that prolongation of steel exposure to solution containing sodium nitrite (NaNO2) from 0.5 h up to 240 h leads to an approximately tenfold increase in charge-transfer resis-tance (Rt) values. After 240 h of exposure to the solution with the NaNO2 + sodium silicate (Na2SiO3) mixture, Rt values were about threefold higher as compared to those obtained in the solution containing only one inhibitor, NaNO2. The XPS data revealed that an up to 4 nm thick passive layer was formed after 240 h of carbon steel exposure to the solution with NaNO2. An additional introduction of Na2SiO3 into the same solution increased the thickness of the oxide layer >8 nm. In the outer part of the oxide layer, Fe was detected in the form of ferrous oxide (FeO), while magnetite (Fe3O4) was the main constituent of the inner part of the oxide layer.

KEY WORDS: alkaline solution, carbon steel, chloride ions, corrosion, corrosion inhibitor

INTRODUCTION

Corrosion of iron or steel embedded in concrete is the subject of extensive research and discussion with regard to instrumental techniques and data interpre-

tation.1-4 Concrete-reinforced structures are usually very resistant to corrosion because the alkaline con-crete environment (pH ~13) passivates steel by forma-tion of the iron passive layer. Passive oxide films have been described to have a bilayer structure consist-ing of an inner layer that grows directly on the metal surface and an outer layer precipitated via the hydro-lysis of cations ejected from the inner layer.4-7 Corro-sion of the embedded steel occurs if this protective film is disrupted as in, for instance, the presence of chlorides from deicing salts. The mechanism by which steel embedded in concrete corrodes in the presence of chlorides is not fully understood in spite of the fact that the local breakdown of the passive layer formed under highly alkaline conditions of the concrete is the most frequent reason for reinforcement corro-sion. It is assumed that corrosion initiates when the chloride concentration reaches its critical value at the rebar surface. This critical chloride concentration is known as the chloride threshold level.8-12 The chlo-ride thresholds determined are expressed as total and free chlorides as well as the chloride/hydroxide ratio. The chloride threshold is one of the key parameters needed for service life prediction of concrete struc-tures. The main difficulty that the determination of this parameter encounters is that the chloride thresh-old-inducing active corrosion is not a unique value and depends on several variables.12

The corrosion of steel in concrete is difficult to investigate, mainly because of experimental prob-lems such as high resistivities, highly porous materi-als, cell design, and macrocells. An alternative way of tackling the problem is to use solutions that simulate


125001-2 CORROSION—DECEMBER 2011

the chemical environment present in the pores of con-crete. Since the pore solutions in cement and concrete are thought to consist mainly of aqueous potassium hydroxide (KOH), sodium hydroxide (NaOH), and cal-cium hydroxide (Ca[OH]2), it is possible that the stud-ies of the cyclic voltammetry of iron in these solutions could be useful as a basis for understanding the elec-trochemical corrosion behavior of steel reinforcement in porous concrete structures.

The electrochemistry and corrosion of iron and carbon steel in hydroxide solutions have been stud-ied by many authors.4-6,13-18 Corrosion and passivation of Fe in alkaline solutions are relatively complex pro-cesses that are not yet fully understood because the composition and structure of the passive layer are still not completely understood. This is partly because the passive films are so thin and because the struc-ture should ideally be studied in the wet electrochemi-cal environment in which these films are formed. The composition of the passive layer (oxides or hydroxides) and its structure (crystalline or amorphous) remains controversial, despite the use of in situ and ex situ techniques.15 The electrochemical and ellipsomet-ric responses of Fe electrodes were studied in alka-line electrolytes (NaOH and saturated Ca[OH]2) under potentials at which the passive layer is formed.5-6 These results suggest that the passive layer has a composite structure involving a inner layer that is dif-ficult to electroreduce, probably similar to magnetite (Fe3O4) in composition, and a gelatinous iron hydrox-ide outer layer, where a reversible Fe2+/Fe3+ reaction has been detected by cyclic voltammetry.6

Many approaches can be used to mitigate the cor-rosion of reinforcing steel, among which protective coatings, cathodic protection, concrete realkaliniza-tion, and corrosion inhibitors are commonly used. The use of corrosion inhibitors is probably more attrac-tive from the point of view of economics and ease of application. The most commonly used inhibitors are formulated on the basis of nitrite ions.19-24 Studies also showed that the nitrite ion, an anodic inhibitor,21 modifies the oxide film on the steel bar in concrete. The nitrite ions compete with the chloride ions for fer-rous ions produced in concrete and incorporate them into a passive layer on the iron surface.21 When the passive state can be compromised because of the presence of chlorides, the corrosion risk is determined by the chloride content, usually evaluated as the chlo-ride/hydroxyl ratio.8-12 On the other hand, the effi-ciency of nitrite as inhibiting agent in the presence of chloride is evaluated in terms of the nitrite/chlo-

ride ratio. There is no clear agreement on the thresh-old value of this ratio. Different authors23-24 have given values ranging from 0.34 to more than 1 as those nec-essary to prevent corrosion in concrete.

Along with calcium nitrite (Ca[NO2]2), which is a traditional commercial inhibitor used for application in reinforced concrete structures, many substances have been tested as inhibitors against the corrosion of carbon steel in the alkaline media.21,25-31 Sodium sili-cate (Na2SiO3) has been used as a corrosion inhibitor for years.25-30 It is greatly attractive in terms of non-toxicity, cost, and availability. A number of authors have discussed the mechanism of formation and the nature of protective layers formed on metals in solu-tions of sodium silicate.25-30 The film formed on iron in the presence of silicate is usually described as a two-layer deposit, the inner layer being composed of corro-sion products and the outer one of a conglomerate of adsorption compounds of silica, meta-hydroxide, and silica gel.28 Surveys of the behavior of several inhibi-tors in a single and mixed manner also have been reported.29,31

In this paper, the effects of chloride ions on the breakdown of passive films formed on carbon steel in a 0.1 M NaOH solution have been studied, and sodium nitrite, sodium silicate, and their synergistic effect on the corrosion prevention of carbon steel have been discussed.

EXPERIMENTAL PROCEDURES

The corrosion behavior of carbon steel was inves-tigated in an aerated stagnant 0.1 M NaOH solution containing 0 to 1 M sodium chloride (NaCl) at 25°C. The substances tested as corrosion inhibitors were sodium nitrite (NaNO2, SN), Na2SiO3 (SS), and their mixture (SN+SS). All solutions were prepared from analytical grade chemicals and deionized water.

The working electrode was a carbon steel sample, the composition of which is listed in Table 1, with an area of 4 cm2, whose nonworking surface was masked with epoxy resin. The steel samples were polished with no. 600 silicon carbide (SiC) paper and rinsed with distilled water.

All electrochemical measurements were made using a standard three-electrode system with a Pt counter electrode and a saturated silver/silver chlo-ride (Ag/AgCl) reference electrode. All potentials are reported vs. a saturated Ag/AgCl reference electrode.

The polarization measurements were performed using a potentiostat with potential scan rates of

TABLE 1Chemical Composition of the Steel (wt%)

Steel C Mn Si Cr Ni Cu P S

Carbon steel 0.21 1.2 0.6 ≤0.3 <0.3 <0.3 <0.04 <0.045


CORROSION—Vol. 67, No. 12 125001-3

0.002 V s–1 and 0.05 V s–1. The cyclic voltammetric (CV) scans started at an initial open-circuit poten-tial. Before voltammetry experiments, the open-circuit potential of electrodes in the solution was monitored for 0.5 h. The potential generally was cycled between –1.25 VAg/AgCl and 0.35 VAg/AgCl. The electrodes were cycled at least 20 times, and the twentieth cycle was recorded. The corrosion current densities (icorr) were determined by Tafel line extrapolation.

The breakdown potential was determined from anodic polarization curves since the intercept of the tangents drawn at the point of sudden current density increase in the passive region. The data on corrosion potential (Ecorr) and pitting potential (Epit) values were the average of five measurements.

The inhibition efficiencies (IE%) for the steel elec-trode were calculated from icorr using the following equation:24,29

IE i i icorr corr corr% [( –i i( –i i( –i i( –i icorr( –corri icorri i( –i icorri i )/ ]= ×i= ×icorr= ×corr= ×i i= ×i icorr= ×corr= ×% [= ×% [( –= ×( –i i( –i i= ×i i( –i ii icorri i( –i icorri i= ×i icorri i( –i icorri i )/= ×)/ ]= ×]0 0i i0 0i i i0 0i( –0 0( –i i( –i i0 0i i( –i i )/0 0)/= ×0 0= ×i= ×i0 0i= ×i= ×0 0= ×i i= ×i i0 0i i= ×i ii i( –i i= ×i i( –i i0 0i i( –i i= ×i i( –i i )/= ×)/0 0)/= ×)/ 100 (1)

where i0corr and icorr are the corrosion current values of the specimens in solutions without and with the addi-tion of an inhibitor.

IE% was also calculated from the charge-transfer resistance (Rt) by:24-25,29

IE R R Rt tR Rt tR R t% [( –R R( –R Rt t( –t tR Rt tR R( –R Rt tR R )/ ]= ×R= ×Rt= ×t= ×R R= ×R Rt t= ×t tR Rt tR R= ×R Rt tR R% [= ×% [( –= ×( –R R( –R R= ×R R( –R RR Rt tR R( –R Rt tR R= ×R Rt tR R( –R Rt tR R )/= ×)/ ]= ×]0= ×0= ×100 (2)

where Rt and R0t are the charge-transfer resistance

values in the base solution with and without inhibi-tors, respectively.

Electrochemical impedance spectroscopy (EIS) tests were performed at the open-circuit potential after 0.5 h and 10 days (240 h) of aging. The EIS measurements were carried out in a frequency range from 20 kHz to 0.001 Hz using an impedance spec-trum analyzer. The perturbation signal amplitude was 10 mV at the open-circuit potential. The EIS data were fitted using commercial software.

The XPS spectra were recorded by a spectrome ter using x-radiation of MgKa (1,253.6 eV, pass energy of 20 eV). To obtain depth profiles, the samples were etched in the preparation chamber by ionized argon at a vacuum of 5 × 10–4 Pa. An accelerating voltage of ca. 1.0 kV and a beam current of 20 μA cm–2 were used, which corresponded to an etching rate of ca. 2 nm min–1.

RESULTS AND DISCUSSION

Potentiodynamic PolarizationCyclic voltammetry has proven to be a useful

technique for studying various aspects of the electro-chemical behavior of iron and steel in alkaline solu-tions.1-3,6,13-18 Typical CV (cycles 1 through 5 and 20) for carbon steel in a 0.1 M NaOH solution, when the

potential generally was cycled between –1.25 VAg/AgCl and 0.35 VAg/AgCl, are shown in Figure 1. Equilibrium potentials (vs. Ag/AgCl) for some possible reactions on iron in 0.1 M NaOH (pH = 13) were calculated from the equations given by Misawa,32 and they are marked in Figure 1 by vertical lines. Line 1 represents the po-tential for the formation of iron(II) hydroxide (Fe[OH]2), lines 2 and 3 represent the potentials for the forma-tion of Fe3O4, and lines 4 through 7 represent the po-tentials for the formation of a-FeOOH (goethite) and γ-FeOOH (lepidocrocite).14 The voltammograms that are shown in Figure 1 are similar in appearance to those previously reported, with some variations in the number and shape of the peaks.1,13-18 A sharp anodic peak (ap) at ~ –0.64 VAg/AgCl and one cathodic peak (cp) at ~ –0.99 VAg/AgCl can be identified. Additionally, two anodic shoulders, a1 (–0.97 VAg/AgCl ÷ –1.07 VAg/AgCl) and a2 (–0.7 VAg/AgCl ÷ –0.18 VAg/AgCl), and one cathodic shoulder, c2 (–0.85 VAg/AgCl ÷ –0.69 VAg/AgCl) were ob-served on the anodic and cathodic sweeps. Shoulder a1 was observed at –1.04 VAg/AgCl ÷ –0.82 VAg/AgCl, i.e., where Hugot-Le Goff, et al.,14 identified two anodic peaks on iron in a deaerated 1 M NaOH solution. Peak ap grew with the number of cycles, as did peak cp, in-dicating a direct relationship between the two peaks (Figure 1). The observed relationship between anodic and cathodic peaks shows that the product formed at peak ap is reduced at peak cp. Shoulder c2 is consid-ered to be a reverse (reduction) reaction correspond-ing to an anodic reaction at shoulder a2.13

It has been shown14 that Fe3O4 is formed in a wide range of anodic polarization values, starting

FIGURE 1. Cyclic voltammograms (cycles 1 through 5 and 20) for carbon steel in a 0.1 M NaOH solution; 0.05 V s–1, 25°C. Vertical lines refer to equilibrium potentials calculated from the equations given by Misawa32 for electrochemical couples.



from the potentials of shoulder a1. The passivating film was suggested to be composed of an inner Fe3O4 layer and an outer layer containing other products of anodic iron oxidation. On this inner Fe3O4 layer can grow deposits of products such as Fe(OH)2, magnetite (Fe3O4) (from the transformation of other compounds), iron(III) hydroxide (Fe[OH]3), d-FeOOH, a-FeOOH, and iron(II) hydroxide (Fe2O3).

14

The detrimental effect of chloride ions on the pas-sive layer formed on iron exposed to alkaline environ-ments has been reported extensively in literature.8-12,24 Therefore, a sequence of voltammograms was carried out in a solution contaminated with varying amounts of chloride ions (Figure 2). As can be seen from Fig -ure 2(a), after addition of 0.01 M NaCl to a 0.1 M NaOH solution ([Cl–]/[OH–] = 0.1), the height of an ap increases fourfold from 0.001 A/cm2 to ~0.004 A/cm2. A further increase in chloride concentration in the solution from 0.01 up to 0.1 M ([Cl–]/[OH–] = 1) changes the height of ap only slightly (Figure 2[a]). Extensive depassivation upon exposure to 0.2 M NaCl in the 0.1 M NaOH solution ([Cl–]/[OH–] = 2) is evident, the voltammogram being extremely distorted with a very large current, especially in the normally passive region at the potentials of ap (Figure 2[b]). The insta-bility of the currents at a high ap might be the result of localized corrosion. The influence of Cl– ions on the passivity breakdown of carbon steel can be interpreted as a balance between two processes competing on the metal surface: stabilization of the passive film by OH– adsorption and disruption of the film by Cl– ions adsorption. When the activity of chlorides overcomes that of hydroxyls, pitting occurs.

To reduce the corrosion of carbon steel, an inhibi-tor can be used. The nitrites are the most commonly used inhibitors.19-25 The nitrite ion, which acts as an anodic inhibitor, modifies the oxide film on steel to be more protective than the film that naturally is formed in concrete.20-21 From the polarization curves pre-sented in Figure 3(a), it is seen that the anodic and cathodic current densities decrease in the presence of inhibitor 0.1 M NaNO2 in the 0.1 M NaOH solu-tion. A set of polarization curves with [NO2

–] = 0.1 M was recorded in a solution contaminated with various amounts of chloride ions and the [NO2

–]/[Cl–] ratios ranging between 0.1 and 1, as presented in Figure 3. Low Cl– concentrations such as 0.1 M and 0.2 M ([NO2

–]/[Cl–] = 1 and 0.5) produced no significant dif-ference in the polarization curves when they were compared with those in a chloride-free solution (Fig-ure 3[b], curves 1 and 2). When the concentration of chloride ions in the solution is increased up to 0.75 M ([NO2

–]/[Cl–] = 0.13), the polarization curves recorded (Figure 3[b], curves 3 and 4) practically coincide with the curves recorded in 0.1 M NaOH without the inhib-itor (Figure 3[a], curve 1). However, in the presence of 1 M NaCl, [NO2

–]/[Cl–] = 0.1 (Figure 3[b], curve 5), the rates of anodic and cathodic processes increase almost two-fold and steel undergoes general corro-sion. The attack of chlorides on the passive film is marked by a sharp increase in current.

An aggressive solution, 0.1 M NaOH + 1 M NaCl (base), was selected for further studies. The inhibi-tion effect of NaNO2 (SN), Na2SiO3 (SS), and their mix-ture (SN+SS) on steel corrosion was investigated. The protective properties of the passive film formed

(a) (b)FIGURE 2. Cyclic voltammograms for carbon steel in the solution (a): 1- 0.1 M NaOH, 2- 0.01 M NaOH + 0.01 M NaCl, 3- 0.1 M NaOH + 0.05 M NaCl, 4- 0.1 M NaOH + 0.1 M NaCl; (b): 5- 0.1 M NaOH + 0.2 M NaCl; cycle 20, 0.05 V s–1, 25°C.



on the steel surface were investigated measuring the film breakdown potential (Epit) induced by chloride ions. Oxide layers were formed by immersion of the steel electrode in the working solution for 0.5 h or for 10 days (240 h), where anodic polarization was later measured. Anodic polarization curves of preliminary passivated samples in the working solution without/with inhibitors are presented in Figure 4. The val-ues of Ecorr and Epit were determined and the differ-ences between them in various solutions are listed in Table 2. The scatter of the Ecorr data was no more than ±0.025 V and of the Epit data was no more than ±0.015 V.

The studies performed have shown that after 0.5 h of immersion in the base solution, the difference of Epit – Ecorr was ~0.14 V (Figure 4, curve 1), while the highest detected value of difference was ~0.4 V in the presence of 1 M NaNO2 (Figure 4, curve 5). Whereas, in the presence of 0.1 M Na2SiO3 in the base solu-tion, the Epit – Ecorr value was ~0.2 V, i.e., only ~0.05 V higher than that in the solution free from the inhibitor (Figure 4, curve 2).

Polarization measurements performed using a steel electrode after 240 h of exposure to the stud-ied solution have shown that in the presence of 1 M NaNO2, the difference Epit – Ecorr increases about 10% up to ~0.44 V (Figure 4, curve 6). Long-term stud-ies in the base solution without inhibitor and in the presence of SS were not performed because steel cor-rosion actively proceeds under such conditions. The most positive Epit potentials were detected for carbon

steel electrodes after 240 h of immersion in the base solution with the inhibitor’s mixture (Figure 4, curves 7, 8). In solutions of such composition, the measured difference Epit – Ecorr markedly increases and reaches 0.8 V (Table 2).

Corrosion currents (icorr) of the carbon steel sam-ples investigated after immersion in the base solution for 0.5 h and 240 h with various inhibitors and their mixtures were determined from Tafel line extrapola-tion (Figure 4). The values of icorr and the IE% obtained from electrochemical measurements are listed in Table 2. As seen from the data presented in Table 2, the lowest icorr values after immersion for 0.5 h in the studied solution were determined when 1 M NaNO2 was added to the solution, IE = 94%, while the IE% of Na2SiO3 was about 55%. When the exposure time was extended to 240 h in solution with a SN value of icorr = 1.8 × 10–7 A/cm2, with SN+SS mixture the values of icorr declined to 6.4 × 10–8 A/cm2 (Table 2) and IE% reached ~98%.

The data obtained imply that the highest detected values of the difference of Epit – Ecorr and the smallest icorr were detected for carbon steel electrodes after 240 h of immersion in the base solution in the pres-ence of the SN and SN+SS inhibitor mixture.

Electrochemical Impedance Spectroscopy Measurement

The EIS diagrams for carbon steel samples exposed for 0.5 h and 240 h to the base 0.1 M NaOH + 1 M NaCl solution without and with corrosion inhib-

(a) (b)FIGURE 3. Cyclic voltammograms for carbon steel in the solution (a): 1- 0.1 M NaOH, 2- 0.1 M NaOH + 0.1 M NaNO2; (b): 1- 0.1 M NaOH + 0.1 M NaNO2 + 0.1 M NaCl, 2- 0.1 M NaOH + 0.1 M NaNO2 + 0.2 M NaCl, 3- 0.1 M NaOH + 0.1 M NaNO2 + 0.5 M NaCl, 4- 0.1 M NaOH + 0.1 M NaNO2 + 0.75 M NaCl, 5- 0.1 M NaOH + 0.1 M NaNO2 + 1 M NaCl; cycle 20, 0.05 V s–1, 25°C.



itors are given in Figure 5. The impedance data were fitted using two equivalent circuit models that are used generally to describe corrosion processes, which are shown in Figure 6.18,25

The impedance spectra recorded on the carbon steel samples exposed for 0.5 h to the base solution without/with the inhibitor indicated the presence of only one capacitive time constant (Figure 5, curves 1

through 3). Curves 1 through 3 were modeled by an equivalent circuit consisting of one Rt-CPE (charge-transfer resistance-constant phase element) and uncompensated ohmic resistance (RΩ) in series (Figure 6[a]). CPE consists of the capacitance (C, μF cm–2) and frequency dispersion (n), a dimensionless parameter (n ≤ 1), which usually is correlated with the surface roughness.33 If n is close to 1, this means CPE is most capacitive. The exponent n of the CPE was between 0.77 and 0.95 for all experiments (Table 3).

For fitting the data, all capacitances in the equiv-alent circuit had to be replaced by a CPE34 to adapt for nonideal behavior. The appearance of the CPE is often related to the electrode roughness or to the inhomogeneity in conductance or dielectric con-stant.35-36 The corrected values of the capacitances were obtained from the fitting program of Zahner elec-tric†, which uses the equation ZCPE = 1/[ωoV(jω/ωo)

n] for calculation.36-38 According to Schiller and Strunz,38 CPE is similar to the capacitive element, but with an absolute phase angle of less than 90°. The CPE was extended through a normalization factor ω/ω0, to enable the use of the parameter V with the dimension F (Farad). Setting ω0 to 1,000 Hz adjusts the trans-fer function to the impedance of a capacitor with the same (Farad) value in the center of the typical fre-quency range of a double-layer capacitance (Ct). All capacitance values used in this paper were normal-ized in this way.

Curve 1 (Figure 5) shows a typical set of Bode plots for uninhibited carbon steel electrode in base solution. As we can see, the addition of sodium sil-icate (SS) to the base solution (Figure 5, curve 2) produced conditions that are very similar to those obtained in the base solution without the inhibitor. The addition of an effective inhibitor such as sodium nitrite (SN) produced passive conditions (Rt increases from 2.2 kΩ cm2 to 6.5 kΩ cm2, i.e., almost three-fold), but only one capacitive time constant was obtained throughout 0.5 h of exposure (Figure 5, curve 3). This

TABLE 2Electrochemical Parameters for Carbon Steel Obtained from the Polarization Curves

Immersion Time icorr Ecorr Epit (Epit – Ecorr) Solution (h) (A cm–2) IE% (VAg/AgCl) (VAg/AgCl) (V)

0.1 M NaOH + 1 M NaCl 0.5 3.8×10–6 — –0.365 –0.225 0.140 0.1 M NaOH + 1 M NaCl + 10–1 M Na2SiO3 0.5 1.7×10–6 55 –0.365 –0.170 0.195 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 0.5 2.3×10–7 94 –0.171 0.225 0.396 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 240 1.8×10–7 95.3 –0.140 0.295 0.435 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 10–3 M Na2SiO3 240 8.1×10–8 97.7 –0.032 0.638 0.67 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 10–2 M Na2SiO3 240 7.4×10–8 98 –0.025 0.705 0.73 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 10–1 M Na2SiO3 240 6.4×10–8 98.3 –0.005 0.785 0.80

FIGURE 4. Tafel polarization curves measured after immersion of the carbon steel electrode for 0.5 h in the solution: 1- 0.1 M NaOH + 1 M NaCl, 2- 0.1 M NaOH + 1 M NaCl + 0.1 M Na2SiO3, 3- 0.1 M NaOH + 1 M NaCl + 0.1 M NaNO2, 4- 0.1 M NaOH + 1 M NaCl + 0.5 M NaNO2, 5- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 and for 240 h in the solution: 6- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2, 7- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 0.01 M Na2SiO3, 8- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 0.1 M Na2SiO3; 0.002 V s–1.

† Trade name.



can be from the presence of a passive film, whose electrical characteristics are described by the param-eters Rt and CPE. The fitting parameters are summa-rized in Table 3.

A 10-day (240 h) exposure of carbon steel sam-ples to the base solution with SN led to a great increase in impedance values and a clear appearance of a second capacitive time constant at the low-fre-quency domain of the phase diagram (Figure 5, curve 4). Curve 4 was modeled by an equivalent circuit con-sisting of two R-CPE elements and uncompensated RΩ in series (Figure 6[b]). The first in series Rf-CPEf com-bination should correspond to a passive film adherent to the steel electrode; the second Rt-CPEt is related to the electrochemical properties of the corroding steel electrode. The EIS data in Figure 5 and Table 3 pro-vide information that in the base solution with SN the Rt values increase with the immersion time from 6.5 kΩ cm2 after 0.5 h to 66.9 kΩ cm2 after 240 h.

Finally, the co-inhibition of SN and SS inhibitors has been considered. To determine the optimal con-centration of inhibitors in the mixture, EIS measure-ments were carried out after 240 h of exposure to the base solution with the addition of 1 M NaNO2 together with 0.001, 0.01, and 0.1 M Na2SiO3 (Table 3). Al-though there are some differences in concentrations of inhibitors, all of them correspond well to an equiva-lent circuit consisting of two R-CPE elements and RΩ in series (Figure 6[b]). As a result of the complexity of the systems, this equivalent circuit only represents a simplified manner to describe the electrochemical in-terface and fitting results in Figure 5, curves 4 and 5, made by this model. The comparison of the EIS mea-

surements data (Table 3) shows that after 240 h of ex-posure to the solutions with SN and the mixture of inhibitors SN+SS, the double-layer capacitance, Ct, comprises 63.9 μF/cm2 to 97.1 μF/cm2, i.e., increases by a factor of 1.5 to 2 as compared to the data ob-tained after 0.5 h exposure to the solution with one inhibitor SN (Ct = 52.3 μF/cm2). Simultaneously Rt values increase from 66.9 kΩ cm2 (in the presence of one inhibitor, NaNO2) to 240.5 kΩ cm2 (in the pres-ence of the mixture of inhibitors), i.e., increase by a factor of 10 to ~37 as compared to the data obtained after 0.5 h exposure in the solution with one inhibi-tor SN (Rt = 6.5 kΩ cm2). When the electrode exposure to the solution with SN was prolonged from 0.5 h up to 240 h, the IE% increased from 66.2% to 96.7%. In

(a) (b)

(a)

(b)

FIGURE 6. Equivalent circuits used for simulation of experimental data.

FIGURE 5. Bode plots of the impedance spectra after immersion of the carbon steel for 0.5 h in the solution: 1- 0.1 M NaOH + 1 M NaCl, 2- 0.1 M NaOH + 1 M NaCl + 0.1 M Na2SiO3, 3- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 and for 240 h in the solution: 4- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2, 5- 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 0.1 M Na2SiO3.



the solution with the SN+SS mixture, the IE% reached 99%.

The passive films on the carbon steel after 240 h of immersion in solution with SN (Cf = 192.2 μF/cm2) are thinner in comparison with the film formed in the solution with the inhibitor mixture (Cf = 85.7 ÷ 116.8 μF/cm2). Rf values, which denote the resistance of the passive layer, are 2.9 kΩ·cm2 to 5.4 kΩ·cm2 (Table 3).

As the results demonstrate, the inhibition effi-ciency increases as a result of the synergistic effect of two co-inhibitors. The total impedance values of car-bon steel samples increase with the immersion time in the solution with the SS+SN mixture.

X-Ray Photoelectron Spectroscopy Surface Analysis

The composition and thickness of oxide films formed on the carbon steel surface were examined using XPS measurements. Studies were carried out by exposing samples to the base solution with the SN and SN+SS mixture for 10 days and to the base solution with the SN+SS mixture for 1 year. Fe 2p3/2, O 1s, N 1s, and Si 2p peaks were analyzed after surface sput-tering with Ar+ ions for an increasing period of time.

Fe 2p3/2 and O 1s spectra of the steel sample ex-posed to the base solution with 1 M NaNO2 for 10 days are presented in Figures 7(a) and 8(a), respectively. It is evident that both spectra are composed of asym-metric peaks. Fe 2p3/2 spectra prior to Ar+ sputtering exhibit a peak at 710.8 eV and a shoulder at 706.5 eV (Figure 7[a]), which may be assigned to Fe3+ and me-tallic Fe0 (Table 4). With an increase in sputtering

depth, the intensity of a higher binding energy peak decreases while that of a lower binding energy in-creases. If the position of the latter peak is constant during the whole sputtering time, the peak at 710.8 eV shifts to 709.3 ± 0.1 eV in the depth range between 1 nm and 2 nm and to 708.2 eV for deeper layers. The obtained data indicate that the inner part of the oxide film contains Fe3O4 (corresponding binding energy of Fe, 708.2 eV), while the outer part of the passive film contains FeO (corresponding binding energy of Fe, 709.3 eV) (Table 4).

To determine respective binding energies and rel-ative intensities and to quantify the contribution of each chemical species that comprise the spectra, the deconvolution of Fe 2p3/2 and O 1s peaks was per-formed and some examples of this procedure are pre-sented in Figure 9.

The ratio of the Fe oxides to metallic Fe (Feox/Fe0) at each sputtered depth was determined by propor-tioning the total intensity of Fe oxides to the intensity of metallic Fe, similarly as it was done earlier.39 The obtained results are presented in Figure 10, curve 1. For the sample exposed to the base solution with SN, the variation Feox/Fe0 below 4 nm is relatively con-stant, suggesting that the oxide film is approximately 4 nm thick. The obtained data on the film thickness are in agreement with the other studies, which indi-cated that the thickness of oxide film on the surface of mild steel in a 0.1 M NaOH solution does not exceed 6 nm and the presence of chloride ions decreases the thickness by 14%.39-40

The oxygen O 1s spectra of the same samples exhibited a broad peak, with two components at

TABLE 3Electrochemical Impedance Spectroscopy Parameters Obtained by Fitting the Bode Plots Shown in Figure 6

with Equivalent Circuits Shown in Figure 7 for the Carbon Steel

Immersion Time RΩ Cf Rf Ct Rt Solution (h) (Ω·cm2) (µF cm–2) (kΩ·cm2) (µF cm–2) (kΩ·cm2) IE%

0.1 M NaOH + 1 M NaCl 0.5 6.5 — — 49 2.2 — (n=0.79) 0.1 M NaOH + 1 M NaCl + 0.5 6.6 — — 49.9 2.5 14 0.1 M Na2SiO3 (n=0.77) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 0.5 4.1 — — 52.3 6.5 66.2 (n=0.77) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 240 3.5 192.2 3.2 77.6 66.9 96.7 (n=0.97) (n=0.86) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 240 3.4 116.8 5.2 63.9 197.1 98.9 10–3 M Na2SiO3 (n=0.96) (n=0.87) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 240 3.6 85.7 5.4 66.2 240.5 99.1 10–2 M Na2SiO3 (n=0.87) (n=094) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 240 3.7 107.6 2.9 97.1 227.4 99 10–1 M Na2SiO3 (n=0.84) (n=0.95)



(a)

(a)

(b)

(b)

FIGURE 7. XPS spectra for Fe 2p3/2 region for different sputtering times after immersion of sample for 10 days in the solution: (a) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 and (b) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 0.1 M Na2SiO3.

FIGURE 8. XPS spectra for O 1s region for different sputtering times after immersion of sample for 10 days in the solution: (a) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 and (b) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 0.1 M Na2SiO3.

~530.0 ± 0.3 eV and ~531.9 ± 0.3 eV, indicating two oxide components O2– and OH–, respectively. These assignments do not contradict the data of the Fe spec-tra (Figure 8[a]), indicating that the inner part of the oxide film contains Fe3O4, while the outer part of the passive film contains FeO.

Results of XPS studies performed with the sam-ples exposed for 10 days to the solution with the inhibitor SN+SS mixture have highlighted several dif-ferences in the composition of the surface layer as compared with the samples exposed to solution con-taining only SN. According to the Feox/Fe0 variations



(Figure 10, curve 2), the thickness of this layer was detected to be higher than 8 nm. Fe 2p3/2 and O 1s spectra (Figures 7[b] and 8[b]) reveal that the compo-sition of Fe oxide phase was similar to that obtained in the solution without SS (Table 4), with the excep-tion that Fe0 was not detected in the top of the film. In addition, Si was detected at all the depths of the layer (Table 5). The determined Si 2p binding energies in the compounds were 103.1 eV and 103.5 eV, which corresponds to silicon dioxide (SiO2)/Si. The binding energy values of Fe 2p3/2 imply that the outer oxide film can be formed of FeO (709.3 eV), while the inner film contains Fe3O4 (corresponding binding energy of Fe is 708.4 eV). Inclusions of N compounds were detected in the outer layer of the oxide film, and the quantities of that element also depended on the com-position of oxide formation solution. In the passive layer formed on the steel surface exposed to the solu-tion without SS, 2 at% to 1.4 at% N was detected at a depth of 1 nm, while the quantity of nitrogen detected on the surface exposed to the solution con-taining silicate at the same depth was less, namely, 0.4 at% to 0.6 at% (Table 5). The binding energy val-ues N 1s in the compounds dependent on sputter-ing depth were determined to be 402.0, 400.6, 399.9, and 398.8 eV; however, we failed to identify the corre-sponding compounds.

TABLE 4X-Ray Photoelectron Spectroscopy Peak Parameters for Carbon Steel After Immersion for 10 Days

in the 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 Solution Without/With 0.1 M Na2SiO3

Sputtering Depth (nm) Element Peak Position (eV) Assignment Peak Position (eV) Assignment

0 Fe 2p3/2 706.5 Fe0 — — Fe 2p3/2 710.8 FeOOH/Fe2O3 710.6 FeOOH O 1s 529.9 O2– 530.8 O2– O 1s 531.5 OH– 532.6 H2O 1 Fe 2p3/2 706.5 Fe0 706.5 Fe0 Fe 2p3/2 709.3 FeO 709.8 FeO O 1s 530.1 O2– 531.6 OH– O 1s 531.7 OH– 533.2 SiO2 2 Fe 2p3/2 706.5 Fe0 706.5 Fe0 Fe 2p3/2 709.2 FeO 709.5 FeO O 1s 530.1 O2– 531.6 OH– O 1s 531.7 OH– 533.1 SiO2 4 Fe 2p3/2 706.5 Fe0 706.5 Fe0 Fe 2p3/2 708.2 Fe3O4 709.1 Fe3O4 O 1s 529.9 O2– 531.7 OH– O 1s 531.7 OH– 533.0 SiO2 6 Fe 2p3/2 706.5 Fe0 706.6 Fe0 Fe 2p3/2 708.2 Fe3O4 708.4 Fe3O4 O 1s 530.0 O2– 530.6 O2– O 1s 532.0 OH– 532.3 OH– 10 Fe 2p3/2 706.6 Fe0 706.6 Fe0 Fe 2p3/2 708.2 Fe3O4 708.4 Fe3O4 O 1s 529.9 O2– 530.7 O2– O 1s 531.9 OH– 532.3 H2O 16 Fe 2p3/2 706.6 Fe0 706.5 Fe0 Fe 2p3/2 708.2 Fe3O4 708.5 Fe3O4 O 1s 529.9 O2– 530.5 O2– O 1s 531.9 OH– 532.5 H2O

With Na2SiO3Without Na2SiO3

Studies of a steel sample exposed to the solution with a mixture of SN+SS for 1 year have shown that the thickness of the surface layer markedly increases and is >350 nm (Table 6). This layer is too thick to be considered a passive film and should be regarded as a precipitated coating. The quantity of Si at all the depths of the passive layer comprises about 16.5 at% to 19.9 at% and Si is detected mainly in the form of SiO2. In the outer part of the layer (depth: 1 nm) the quantity of Fe detected was minor, ~0.13 at%, while at a depth of 350 nm the quantity of Fe enlarged to 6.5 at%. Nitrogen, unlike the data obtained after 10 days of exposure, was detected up to the depth of 350 nm. Its quantity decreased from ~0.4 at% on the surface to 0.13 at% in the inner part of the oxide layer (Table 6). The outer part of oxide layer formed in such solution was composed mainly of SiO2 com-pounds, while in the inner part of oxide layer Fe3O4 was detected along with SiO2.

In a previous paper,30 a transparent silicate coat-ing was formed on the surface of hot-dip galvanized steel by immersing in SS solution. The authors deter-mined that the silicate coatings mainly are composed of zinc oxides/hydroxides, zinc silicate, and SiO2. It is considered that the coatings may be a kind of net-work structure with cross-linked Si–O–Si and Si–O–Zn bonds. It can be assumed that in our case a precipi-



tated coating of analogous structures, Si–O–Si and Si–O–Fe, can be formed; however, the structure was not investigated.

CONCLUSIONS

v The inhibition effect of NaNO2, Na2SiO3, and their mixture was evaluated after different immersion times of carbon steel samples in chloride-contami-nated solutions, which simulated the pore solution in concrete. Electrochemical test results obtained have shown that the breakdown potential values were more positive and corrosion currents were the lowest ones for the steel electrodes exposed to the solutions con-taining NaNO2 and the NaNO2 + Na2SiO3 mixture.v EIS measurements have shown that prolongation of steel exposure to solution containing NaNO2 from 0.5 h up to 240 h leads to an approximately tenfold increase in Rt values. After 240 h of exposure to the solution with the NaNO2 + Na2SiO3 mixture, Rt values were about threefold higher as compared to those obtained in the solution containing only one inhibitor, NaNO2.v Analysis of the XPS data obtained shows that in the 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 solution, a passive layer up to 4 nm in thickness is formed after 10 days on the surface of carbon steel. An addi-tional introduction of 0.1 M Na2SiO3 into this solu-tion increased the thickness of the oxide layer to more than 8 nm. At all depths of the oxide layer formed, Si was detected as SiO2. In both cases, Fe was detected in the form of FeO in the outer part and as Fe3O4 in

the inner part of the oxide layer. When the duration of steel exposure to the solution with the NaNO2 + Na2SiO3 mixture was prolonged up to 1 year, the thickness of the passive layer increased markedly and was >350 nm. The outer part of the oxide layer formed

(a) (b)FIGURE 9. Decomposition of peaks for (a) Fe 2p3/2 and (b) O 1s regions for the carbon steel after immersion of sample for 10 days in the 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 solution (1 nm depth).

FIGURE 10. Comparison of the Feox/Fe0 at the selected sputtered depths after immersion of carbon steel sample for 10 days in the solution: (a) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 and (b) 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 + 0.1 M Na2SiO3.



TABLE 5Data of X-Ray Photoelectron Spectroscopy Depth Analysis of the Passive Layer, Which was Formed on the Carbon Steel

Surface After 10 Days’ Exposure to the 0.1 M NaOH + 1 M NaCl + 1 M NaNO2 Solution Without/With 0.1 M Na2SiO3

Sputtering Depth (nm) Fe (at%) O (at%) N (at%) Fe (at%) O (at%) N (at%) Si (at%)

0 10.4 48.1 2 0.39 52.5 0.55 6.9 1 27.9 58.1 1.3 3.1 63.5 0.39 11.4 2 34.3 56.9 — 5.2 68.8 — 11.2 4 51.5 39.5 — 10 69 — 10.6 6 61.2 31 — 25.8 58.9 — 6.2 10 67.5 23 — 50.2 37.9 — 4.1 16 76.9 16.6 — 77.6 13.7 — —

TABLE 6Data of X-Ray Photoelectron Spectroscopy Depth Analysis

of the Passive Layer, Which was Formed on the Carbon Steel Surface After 1 Year Exposure to the 0.1 M NaOH +

1 M NaCl + 1 M NaNO2 + 0.1 M Na2SiO3 Solution

Sputtering Depth (nm) Fe (at%) O (at%) Si (at%) N (at%)

0 — 77.2 18.7 0.51 50 0.13 79.2 19.9 0.39 150 0.6 77.4 19.9 0.32 250 0.9 74.8 19.9 0.36 350 6.5 71.1 16.5 0.13

Without Na2SiO3 With Na2SiO3

in such solution is composed mainly of the SiO2 com-pound, while in the inner part of the oxide layer, Fe3O4 was detected along with SiO2.v To summarize the results of electrochemical and XPS measurements, the mixture of NaNO2 + Na2SiO3 inhibitors ensured the most pronounced inhibition of the carbon steel corrosion in the investigated solu-tion 0.1 M NaOH + 1 M NaCl. The inhibition efficiency increases as the result of the synergistic effect of two co-inhibitors.

ACKNOWLEDGMENTS

This research was supported by the Research Council of Lithuania under Grant no. MIP – 77/2010. The authors thank A. Sudavicius (Institute of Chem-istry, Vilnius, Lithuania) for his assistance performing XPS measurements.

REFERENCES

1. J.T. Hinatsu, W.F. Graydon, F.R. Foulkes, J. Appl. Electrochem. 19 (1988): p. 868.

2. J.T. Hinatsu, W.F. Graydon, F.R. Foulkes, J. Appl. Electrochem. 20 (1990): p. 841.

3. F.R. Foulkes, P. McGrath, Cem. Concr. Res. 29 (1999): p. 873. 4. M. Sánchez, J. Gregori, M.C. Alonso, J.J. García-Jareño, F.

Vicente, Electrochim. Acta 52 (2006): p. 47. 5. H. Oranowska, Z. Szklarska-Smialowska, Corros. Sci. 21 (1981):

p. 735.

6. O.A. Albani, J.O. Zerbino, J.R. Vilche, A.J. Arvia, Electrochim. Acta 31 (1986): p. 1403.

7. D.A. Koleva, Z. Guo, K. van Breugel, J.H.W. de Wit, Mater. Corros. 60 (2009): p. 704.

8. L. Li, A.A. Sagüés, Corrosion 57, 1 (2001): p. 19, doi: 10.5006/1.3290325.

9. D. Izquierdo, C. Alonso, C. Andrade, M. Castellote, Electrochim. Acta 49 (2004): p. 2731.

10. M.F. Hurley, J.R. Scully, Corrosion 62 (2006): p. 892, doi: 10.5006/1.3279899.

11. K.Y. Ann, H.-W. Song, Corros. Sci. 49 (2007): p. 4113.12. U. Angst, B. Elsener, C.K. Larsen, Ø. Vennesland, Cem. Concr.

Res. 39 (2009): p. 1122.13. S. Juanto, J.O. Zerbino, M.J. Miguez, J.R. Vilche, A.J. Arvia,

Electrochim. Acta 32 (1987): p. 1743.14. A. Hugot-Le Goff, J. Flis, N. Boucherit, S. Joiret, J. Wilinski, J.

Electrochem. Soc. 137 (1990): p. 2684.15. S.T. Amaral, E.M.A. Martini, I.L. Müller, Corros. Sci. 43 (2001): p.

853.16. M. Sánchez, J. Gregori, M.C. Alonso, J.J. García-Jareño, H.

Takenouti, F. Vicente, Electrochim. Acta 52 (2007): p. 7634.17. S. Joiret, M. Keddam, X.R. Nóvoa, M.C. Pérez, C. Rangel, H.

Takenouti, Cem. Concr. Compos. 24 (2002): p. 7.18. L. Freire, X.R. Nóvoa, M.F. Montemor, M.J. Carmezim, Mater.

Chem. Phys. 114 (2009): p. 962.19. C. Monticelli, A. Frignani, G. Trabanelli, Cem. Concr. Res. 30

(2000): p. 635.20. H. Saricimen, M. Mohammad, A. Quddus, M. Shameem, M.S.

Barry, Cem. Concr. Compos. 24 (2002): p. 89.21. J.M. Gaidis, Cem. Concr. Compos. 26 (2004): p. 181.22. K.Y. Ann, H.S. Jung, H.S. Kim, S.S. Kim, H.Y. Moon, Cem. Concr.

Res. 36 (2006): p. 530.23. M.B. Valcarce, M. Vázquez, Electrochim. Acta 53 (2008): p. 5007.24. M.B. Valcarce, M. Vázquez, Mater. Chem. Phys. 115 (2009): p. 313.25. El-Sayed M. Sherif, R.M. Erasmus, J.D. Comins, Electrochim.

Acta 55 (2010): p. 3657.26. O. Lahodny-Šarc, L. Kaštelan, Corros. Sci. 21 (1979): p. 265.27. R.D. Armstrong, S. Zhou, Corros. Sci. 28 (1988): p. 1177.28. S.T. Amaral, I.L. Müller, Corros. Sci. 41 (1999): p. 747.29. M. Salasi, T. Shahrabi, E. Roayaei, M. Aliofkhazraei, Mater. Chem.

Phys. 104 (2007): p. 183.30. M.R. Yuan, J.T. Lu, G. Kong, Surf. Coat. Tech. 204 (2010): p. 1229.31. J.-R. Chen, H.-Y. Chao, Y.-L. Lin, I.-J. Yang, J.-C. Oung, F.-M.

Pan, Surf. Sci. 247 (1991): p. 352.32. T. Misawa, Corros. Sci. 13 (1973): p. 659.33. U. Rammelt, G. Reinhard, Electrochim. Acta 35 (1990): p. 1045.34. K. Jütnerr, Electrochim. Acta 35 (1990): p. 1501. 35. H. Göhr, Ber. Bunsen Ges. Phys. Chem. 85 (1981): p. 274.36. A. Ficus, U. Rammelt, W. Plieth, Electrochim. Acta 44 (1999): p.

2025.37. U. Rammelt, S. Köhler, G. Reinhard, Electrochim. Acta 53 (2008):

p. 6968.38. C.A. Schiller, W. Strunz, Electrochim. Acta 46 (2001): p. 3619.39. P. Ghods, O.B. Isgor, J.R. Brown, F. Bensebaa, D. Kingston, Appl.

Surf. Sci. 257 (2011): p. 4669.40. F. Miserque, B. Huet, G. Azou, D. Bendjaballah, V.L. Hostis, J.

Phys. IV 136 (2006): p. 89.

inhibition effect of sodium nitrite

Documents