Environmental factors affecting the corrosion behavior of reinforcing steel. IV. Variation in the pitting corrosion current in relation to the concentration of the aggressive and the inhibitive anions

Corrosion Science 52 (2010) 1675–1683

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Environmental factors affecting the corrosion behavior of reinforcing steel. IV. Variation in the pitting corrosion current in relation to the concentration of the aggressive and the inhibitive anions S.M. Abd El Haleem, S. Abd El Wanees *, E.E. Abd El Aal, A. Diab Chemistry Department, Faculty of Science, Zagazig University, Zagazig, Egypt

a r t i c l e

i n f o

Article history: Received 11 September 2009 Accepted 16 January 2010 Available online 22 January 2010 Keywords: A. Reinforcing steel A. Ca(OH)2 A. Aggressive anions C. Inhibition C. Pitting corrosion current C. Repassivation

a b s t r a c t The pitting corrosion current of reinforcing steel is measured under natural corrosion conditions in 2  2 2 as Ca(OH)2 solutions in presence of Cl as aggressive ions and CrO2 4 ; HPO4 ; NO2 ; WO4 and MoO4 inhibiting anions. The corrosion current starts to flow after an induction period which depends on solution composition (concentration, pH and presence or absence of the aggressive and the inhibiting anions). The limiting corrosion currents increase with increasing the Cl ion concentration and decrease with increasing the pH and inhibiting ions concentration. The inhibition efficiency of the studied inhibiting 2  2 2 ions increases in the following order: CrO2 4 < HPO4 < NO2 < WO4 < MoO4 , and depends on the way by which the inhibitor is added to the solution. Injection of the inhibiting anions in solution causes repassivation of the pre-formed pits through competition with Cl ions for adsorption sites on metal oxide surface. The adsorbability constant and the free energy of repassivation of the inhibiting anions are calculated. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Reinforcement steel in concrete normally acquires a permanent state of passivity due to the high alkalinity of the environment [1– 4]. pH values higher than 12 result from the liberation of Ca(OH)2 and the presence of potassium and sodium hydroxides ensuring continuous protection of the steel [5,6]. Presence of oxygen stabilizes the passive film on the surface of the embedded steel. Although the precise nature of the passive film is unknown, several views have been advanced in this context. A spinel a-Fe3O4–cFe2O3 solid solution is proposed to form the passive film on steel [7–9]. However, Cao et al. [10,11] proposed the formation of a dense-3D non porous iron (III) layer adherent to the steel surface. Contamination of concrete with ions like Cl causes breakdown of passivity and initiation of localized attack [12]. Chloride ions can enter the concrete from de-icing salts, from sea water in marine environments, from chloride containing admixtures and/or from mixing water [1]. Above a certain threshold chloride ion concentration, the passive layer on the reinforcing steel breaks down as a result of chloride induced pitting of steel [13–16]. The continuous or periodic monitoring of corrosion of reinforcement is mainly based on electrochemical measurements. These measurements are referred both to corrosion potential, Ecorr [17– 20] and corrosion current, Icorr [17]. Ecorr is related only to the * Corresponding author. Tel.: ++20 552362504; fax: +20 552362501. E-mail address: [email protected] (S. Abd El Wanees). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.021

likelihood that reinforcements suffer active corrosion and therefore fails to provide accurate measurement of corrosion rate. Corrosion current, Icorr, on the other hand, which is measured under polarization conditions, does not reflect the actual natural corrosion rate [17,21]. It is, therefore, of interest to measure the corrosion current (rate of corrosion) under natural corrosion conditions using a non destructive technique. A simple H-divided cell was previously used to measure the pitting corrosion current of reinforcing steel [22], Zinc [23–25], Fe [26] and steel [27]. The purpose of this work is to investigate the effect of concentration of both the aggressive and inhibiting anions on the values of pitting corrosion current (corrosion rate). Two ways of introducing the inhibiting anions to the corrosive medium are conducted to highlight their influence on the inhibition efficiencies of the used inhibitors. In this study, concrete was simulated by naturally aerated Ca(OH)2 solutions, which are polluted by Cl as aggressive 2 2 2  ions [3,22,28,29]. CrO2 4 , HPO4 , WO4 , MoO4 , and NO2 are used as corrosion inhibitors. Performance of steel in this medium is assessed by measuring corrosion current, Icorr, using the simple H-divided electrolytic cell which was described previously [22], followed by SEM examination. 2. Experimental The simple electrolytic cell was previously described [22]. It is consisted of a 250 ml borosilicate glass beaker containing 100 ml of Ca (OH)2 solution as a passivating agent, one of steel electrode

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and a magnetic bar stirrer. The second steel electrode was enclosed in a borosilicate glass tube ended with a fine porosity fritted centered glass disc G4. The two steel electrodes were short-circuited through a nanoamperometer (Siemens type N-5536). The steel electrodes were made from steel samples produced by the Egyptian Steel Mill Company (Helwan – Cairo), used always as reinforcement, and have the chemical composition, shown in Table 1, in accordance with the ASTM A615 Standard [30]: Each electrode was fixed to a borosilicate glass tube with epoxy resin so that the total exposed surface area is 1.33 cm2. The electrodes were abraded into uniform surfaces by a grinding machine (Jean Wirtz TG 200, Germany) with successive grades of metallographic SiC emery papers. They were then degreased with acetone and finally washed with tridistilled water. Since the surface treatment and the area of the two electrodes were the same, and the composition of the electrolyte in the cell compartments was identical {the two sides of the cell were filled with the same solution of naturally aerated Ca(OH)2}, no current could be detected on connecting the electrodes through the nanoamperometer. This process took 20 min. When this condition was established, a weighed amount of Cl anion as NaCl salt was added to the solution of the main compartment and the current flow was recorded as a function of time until steady-state values were established. Since the electrode was initially in the passive state, the conditions are suitable for the initiation and propagation of pitting corrosion after the addition of the aggressive anions to the main compartment [8]. The pitting corrosion current density is defined here as: (total current flowing in the circuit)/(total area of the electrode undergoing attack). The effect of the inhibiting anions on the pitting corrosion current of the steel electrode was conducted following two different procedures. In procedure I, after the attainment of zero current in the naturally aerated Ca(OH)2 solution, weighed samples of both the aggressive and the inhibiting salts, corresponding to definite concentrations of the two counteracting anions, were added to the main compartment of the cell and the current was measured as a function of time until steady-state values were reached. However, in procedure, II, the current was first measured as a function of time in solution of Ca(OH)2 containing a definite concentration of Cl ions until reaching a steady value, i1. Following then, a certain amount of the inhibitor anion, corresponding to a definite concentration, was injected in the solution in the form of a small volume of a concentrated salt solution. This process took only few seconds and the change in volume was 61%. The current was then measured with time until a new steady-state value was reached in the presence of additive, i2. From the current values i1 and i2 measured in absence and presence of the inhibiting ions, respectively, using both procedures, the inhibition efficiency, IE, can be calculated as:

  i2 IE ¼ 1   100 i1

to measure the pH of the solution. All chemicals were of analytical grade qualities. Solutions were prepared using tridistilled water. Each experiment was carried out with freshly prepared electrodes and with new portions of the solutions. For each solution concentration, duplicate measurements of the flowing current were carried out to ensure reproducibility. Error in corrosion current measurements was evaluated at less than 10%. Measurements were carried out at a constant temperature 25 ± 0.1 °C. The cell temperature was controlled using an ultra thermostat type Polyscience (USA). Scanning electron microscopy of the tested electrodes was carried out using a Jeol Scanning Microscope JSM- T100 (Japan).

3. Results and discussion 3.1. Variation in the corrosion current with Cl ions concentration The steel electrodes were first equilibrated in naturally aerated solutions of Ca(OH)2. When a state of permanent passivity occurred, no current could be detected in the cell. This process took about 20 min. Passivity was attributed to oxide film formation, the nature of which is contradictory [8,9]. When this state of affairs occurs, weighed amounts of NaCl, corresponding to increasing concentration of Cl ions, are added to the main compartment of the cell and the current is followed with time till steady-state values are reached. The curves in Fig. 1 represent the variation of the logarithm of the corrosion current density (lA/cm2) with time, t in minutes in naturally aerated 1  103 M Ca(OH)2 solution in presence of increasing concentrations of NaCl. Inspection of the curves of Fig. 1 shows that in presence of Cl ions with concentrations below 5  105 M, no current flows in the cell. These concentrations of Cl ions are not sufficient to influence the passive character of the steel surface, or can be tolerated by the passivating agent. However, on slight increase in the concentration of the aggressive Cl ions, up to a threshold concentration [14–16] which depends on Ca(OH)2 concentrations, corrosion current starts to flow after the elapses of an induction period, s, which depends on the concentration of Cl ions. The flowing currents reach limiting values which become higher the higher the concentration of Cl ions. Following then, currents start to decrease once again either to reach zero value in presence of 5  105 M Cl ions, or to reach slightly higher values that depend also on the Cl ion

0.5 M 0.3 M

0.75M NaCl -0.4 -0.6

ð1Þ

0.1M -2 5x10 M -2 2.5x10 M -2 1x10 M

-0.8

log I, µ Acm-2

Naturally aerated calcium hydroxide solution was used as a passivating agent, simulating the concrete pore solution under natural corrosion conditions [3,22,28,29], while NaCl was used as the attacking agent. On the other hand, Na2CrO4, Na2HPO4, NaNO2, Na2WO4 and Na2MoO4 were used as inhibitors. The pH of the Ca(OH)2 solutions was adjusted by dropwise addition of NaOH solution. Orion research expandable ion Analyzer EA 920 was used

1 x10-3 M Ca(OH)2 + x M NaCl

-0.2

-3 5x10 M

-1.0

-4 5x10 M

-1.2

-4 1x10 M

-1.4 -1.6

-5 5x10 M

-1.8 -2.0 0

Table 1 The chemical composition of steel samples.

20

40

60

80

100

120

140

Time, min

C

Si

Mn

P

S

Fe

0.32

0.24

0.89

0.024

0.019

Bal. 98.507 mass%

Fig. 1. Variation of the logarithm of pitting corrosion current density with time for steel electrode immersed in solutions of 1  103 M Ca(OH)2 containing increasing concentrations of NaCl, at 25 °C.

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concentration (1  104 to 1  102 M Cl ions). This behavior could be attributed to the repassivation of the pre-formed pits [22,26,31–34]. During repassivation, metastable pits with very limited life times are assumed to be generated. In the process of repassivation, the aggressive anions at pit surface should be replaced by the passivating anion [35]. As the concentration of the halogen ions is further increased, above 1  102 M, the induction period becomes increasingly shorter. These features are the usual characteristics of the induction period associated with the initiation of pitting corrosion [22–25]. This period is attributed to the time taken by the aggressive anions either to penetrate through the passivating film or to compete with the inhibiting (passivating) anions for the active sites on the metal or metal oxide surface [22–25]. The dependence of the induction period, s, on the concentration, C, of the Cl anions, in 1  103 M Ca(OH)2 solution, can be seen from the plots in Fig. 2. This figure represents the relationship between the two variables on a double logarithmic scale. A straight line relationship is obtained satisfying the following equation [22– 27]:

stant a1 represents the logarithm of the corrosion current density in

lA/cm2 in presence of 1 M Cl ion. Eq. (3) was derived theoretically on the basis of competitive adsorption of the two counteracting anions on the surface of the metal or metal oxide and its effect on the structure of the electrical double layer at the metal/solution interface [23,27]. Comparison between the experimental values of a1 and b1 with the corresponding terms of the theoretical equation was realized in different cases [23,27]. According to the theoretical derivation, the constant b1 (the slope of the log Ipitting  log C Cl relation) should amount to the value a1n1/z1, where a1 is the transfer coefficient of the anodic reaction, n1 is the number of electrons involved in the oxidation reaction and z1 is the valency of the aggressive anion. The transfer coefficient, a1, usually acquires values between 0.3 and 0.7 for reactions governed by transfer across a simple energy barrier. These values depend on the nature of the electrode reaction, as well as, on the type and concentration of the supporting electrolyte [22,27]. Thus, for an anodic reaction governed by a single charge transfer reaction of the kind [22,27]:

Fe2þ ! Fe3þ þ e

log s ¼ a  b log C Cl

ð2Þ



where a and b are constants. The values of the constants a and b are 0.43 (log time in minutes) and 0.45 (log time in minutes/decade), respectively. Following the induction period, the current density increases steadily to reach steady values which depend also on the concentration of the aggressive anions. Since the area of the anodically formed pits is not known and the study is of a comparative nature, the pitting corrosion current density is defined here as the total current following in the circuit/total area of the steel electrode. The increase in Cl anions content is associated with a corresponding increase in the magnitude of the steady-state current flowing between the passive electrode and that undergoing corrosion. In Fig. 2, the relation between the pitting corrosion current density, Ipitting in lA/cm2, and the concentrations of Cl anions, C Cl , in mol 11, at a constant passivating concentration is also shown. A straight line relationship is obtained satisfying the equation [22– 27]:

log Ipitting ¼ a1 þ b1 log C Cl

ð3Þ

where a1 and b1 are constants. The values of a1 and b1 are 0.35 (in log lA/cm2) and 0.17 (in log lA/cm2/decade), respectively. The con-

ð4Þ

where n1 = 1 and z1 for Cl ion = 1, the term a1n1/z1 assumes the values 0.3 as a lower limit and 0.7 as an upper limit. In this investigation a value of 0.23 is obtained for the slope of the log Ipitting  log C Cl relation which is nearly in accordance with an anodic reaction of the steel electrode governed by one electron transfer [22]. 3.2. Variation in the corrosion current with the pH of solution The curves of Fig. 3 represent the variation of the logarithm of the pitting corrosion current density of steel electrode with time in naturally aerated 1  103 M Ca(OH)2, in presence of 7.5  101 M Cl ions, at different pH values. pH of the solution is adjusted by dropwise addition of NaOH solution. As the pH of solution increases, the induction period associated with the initiation of pitting corrosion increases while the limiting steady-state current corresponding to the propagation of pitting corrosion decreases. This behavior could be attributed to the sustaining of the passive film growth on the steel electrode as the alkalinity of the medium is increased [7,22,36]. The curves of Fig. 4 represent the variation of the induction period, s and the steady corrosion current density with the pH of the solution. Invariably straight line relationships are obtained as follows:

-0.4

2.0

-0.4

pH=10.8 pH=11.47 pH=11.85 pH=12.2 pH=12.9

-0.6 -0.8

1.6 2

log τ, min.

1.2 -0.8 0.8 -1.0

0.4

log I, µAcm-2

-1.0

log Icorros, µ A/cm .

-0.6

-1.2 -1.4 -1.6 -1.8 -2.0 -2.2 -2.4

0.0

-1.2 -4

-3

-2

-1

0

log CCl-, M Fig. 2. Variation of each of the induction period, s, and the current density, Icorros, with Cl anions concentration on double logarithm scale, at 25 °C.

-10

0

10

20

30

40

50

60

70

80

90 100 110 120 130

Time, min Fig. 3. Variation of the logarithm of pitting corrosion current density with time for steel electrode immersed in 1  103 M Ca(OH)2 solution containing 7.5  101 M NaCl at different pH values, at 25 °C.

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1.2

-0.4

log induction time, τ, min

-0.6 0.8 -0.8 0.6

-1.0

0.4

0.2

log current density, μΑ/cm 2

1.0

-1.2

0.0 10.5

11.0

11.5

12.0

12.5

13.0

-1.4 13.5

pH Fig. 4. Variation of the logarithm of each of the induction period, s, and the steady corrosion current density with the pH of the solution, at 25 °C.

log s ¼ a2 þ b2 pH

ð5Þ

and

log Icorros ¼ a3  b3 pH

ð6Þ

inhibitors causes the elongation of the induction period associated with the initiation of pitting corrosion, while the steady-state current associated with pitting propagation is decreased. This behavior indicates that these additives act as good inhibitors for pitting corrosion of the reinforcing steel. The curves in Fig. 6 show the relation between the induction period for pit initiation, s, and the concentration of the used inhibitors, Cinh, on a double logarithmic scale. Straight lines are obtained satisfying the relation:

log s ¼ a4 þ b4 log C inh

ð7Þ

where a4 (in log time in minutes) and b4 (in log time in minutes/ decade) are constants. The curves in Fig. 7A and B represent the variation in the logarithm of the current density (lA/cm2) with time (min) as obtained using procedure II of experiments, in solutions of 1  103 M Ca (OH)2 + 7.5  101 M Cl ions to which NaNO2 and Na2MoO4 are injected, respectively, following the initiation of pitting corrosion. Inspection of the curves in Fig. 7A and B show that following the injection of the inhibitor in the corrosive solution, the steady current reached in Cl ions-containing solution, i1, decreases directly with time to approach new steady-state pitting corrosion currents, i2, which decrease in values with increasing the inhibitor concentration. No induction periods are recorded with these measurements. This behavior could be easily attributed to the

where a2 (a3) and b2 (b3) are constants. 3.3. Variation in the corrosion current with the inhibitors concentration

A

-3

-

1x10 M Ca(OH)2+ 7.5 x10-1M Cl + x M NaNO2 Free -4

-0.3

5.0 x10 MNaNO2 1.0 x10-3 M -3 2.5x10 M -3 5.0 x10 M -2 1.0 x10 M -2 2.5 x10 M -2 5.0 x10 M

-0.4 -0.5 -0.6 -0.7

log I, μAcm-2

Chromate, phosphate, tungstate, molybdate and nitrite ions are used as inhibitors of the pitting corrosion of the steel electrode in 1  103 M naturally aerated Ca(OH)2 in presence of 7.5  101 M Cl, as an aggressive ion (blank solution). The effect of these additives is recognized through their influence on both the induction period and the pitting corrosion current measured in presence of Cl ions. Two procedures are followed up to investigate the effect of the inhibiting anions.

-0.8 -0.9 -1.0 -1.1 -1.2 -1.3

3.3.2. Procedure II When the current flowing between the two steel electrodes in 1  103 M Ca(OH)2 solution reaches the zero value, a weighed amount of NaCl, corresponding to 7.5  101 M Cl ions, is added to the main compartment of the cell and the current is followed with time until steady values, i1, are reached. Following then, small volumes of concentrated solutions of each of the tested inhibiting anions are quickly injected in the solution which is being stirred. This process takes only few seconds and the change in volume is 61%. The current is then followed with time until new steady values, i2, are reached. The curves in Fig. 5A and B represent the variation of the logarithm of the current density (lA/cm2) with time, t in minutes, in solutions containing 1  103 M Ca(OH)2 + 7.5  101 M Cl ions in presence of increasing concentrations of NaNO2 and Na2MoO4, respectively, as examples of the other used inhibitors, using procedure I of experiments. Inspection of the curves of this figure indicates that the presence of increasing concentrations of the used

-1.4 -1.5 0

20

40

60

80

100

Time, min -3 1 x10 M Ca(OH)2+ 7.5 x10-1M Cl + x M Na2MoO4

B -0.4

Free -4 5x 10 M Na2MoO4 -3 1.0 x10 M -3 2.5 x10 M -3 5.0 x10 M

-0.6 -0.8

log I, µAcm-2

3.3.1. Procedure I Following the attainment of zero current in the Cl ions-free Ca(OH)2 solution, weighed amounts of NaCl and the sodium salts of the respective inhibitive anions, corresponding to definite concentrations of each, are simultaneously added to the main compartment of the cell and the current is followed with time until steady values are reached, i2.

-2 1.0 x10 M

-1.0

-2 2.5 x10 M

-2 5.0x10 M

-1.2 -1.4 -1.6 -1.8 0

20

40

60

80

100

Time, min Fig. 5. Variation of the logarithm of pitting corrosion current density with time for steel electrode immersed in 1  103 M Ca(OH)2 + 7.5  101 M NaCl in the absence and presence of different concentrations of (A) NaNO2 and (B) Na2MoO4 (set I of experiments), at 25 °C.

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Na2 MoO4 1.0

A

Na2 WO4 Na NO2

-0.4

Na2 HPO4

0.8

Na2 CrO4

0.01M Ca(OH)2 + 0.75 M NaCl +

-0.5

x M NaNO2

0.6

log I, μAcm-2

log τ, min

Injection of NaNO2

-0.3

0.4

0.2

0.0 -3.5

0.3 M NaNO2

-0.6

0.25 M 0.2 M 0.15 M

-0.7

0.12 M 0.1 M 0.05 M free

-0.8

-0.9

-3.0

-2.5

-2.0

-1.5

-1.0

,

log Cinh M

-1.0 0

Fig. 6. Variation of the induction period, s with the inhibitor concentration, Cinh on a double logarithmic scale.

20

40

60

80

100

120

Time, min Injection of Na2MoO4

(i) Irrespective of the way by which the inhibiting anions are added to the corrosive solution, the inhibition efficiency of each inhibitor increases with increasing its concentration. (ii) At one and the same additives concentration, IE increases in 2  2 the following order: CrO2 4 < HPO4 < NO2 < WO4 < , which is the same sequence of increasing the inhibMoO2 4 itive actions of these anions towards pitting corrosion of the steel [36–39].

B

-0.3

-0.4

0.01M Ca(OH)2 + 0.75 M NaCl + -0.5

x M Na2MoO4

-0.6

0.25 M

-2

0.3

log I, μAcm

repassivation of the already formed pits. The extent of pit repassivation increases with increasing the concentration of the added inhibitors [22,31–34]. The curves in Fig. 8A and B represent the relations between the pitting corrosion current density and the concentration of the inhibitors anions, on double logarithmic scales, using, procedures I and II, respectively. Nearly segmented S-shaped curves are obtained which could be explained on the promise that these inhibitors act by adsorption on the metal surface [8]. These inhibitors are assumed to compete with Cl ions for adsorption sites on the metal oxide covered surface [8]. The scanning electron micrographs, SEM, of the steel electrode samples after 3 h immersion in solution containing 1  103 M Ca (OH)2 + 7.5  101 M Cl ions in absence and presence of 5  102 M of NaNO2, respectively, are shown in Fig. 9. It is clear that in absence of NaNO2, the aggressive anions causes the formation of clear pits, some of which contain inside traces of the corrosion products. Few fine pits are also noted in the micrographs. The large corroded areas might probably have been formed from smaller pits expanded equally laterally, as well as, inwardly so that it takes the shape of large attacked areas, Fig. 9A. However, in presence of NO2 ions, Fig. 9B, the SEM micrograph shows small numbers of more fine pits with no large corroded areas as those observed in inhibitor-free solution. The inhibiting action of the used additives, as pitting corrosion inhibitors, could be detected from the values of the inhibition efficiency (IE), calculated from the pitting corrosion current densities measured in absence and presence of different concentrations of each of the used anions, according to Eq. (1). Tables 2 and 3 show the values of IE for different concentrations of the inhibiting anions, using procedure I and II, respectively. Inspection of the values of IE of Tables 2 and 3 reveals the following conclusions to be drawn:

M Na2MoO4

0.15 M 1.00 M

0.125 M 0.10 M 0.050 M Free

-0.7

-0.8

-0.9

-1.0 0

20

40

60

80

100

120

Time, min Fig. 7. Variation of the logarithm of pitting corrosion current density with time for steel electrode immersed in 1  103 M Ca(OH)2 + 7.5  101 M NaCl before and after injection of different concentrations of (A) NaNO2 and (B) Na2MoO4 (set II of experiments), respectively, at 25 °C.

(iii) Considering the way by which the inhibitors are added to the corrosive medium, Tables 2 and 3 indicate that the inhibition efficiency of each respective inhibiting anions, IE, is markedly high when the inhibitors are added before immersing the electrodes in the corrosive medium, procedure I, than that recorded when the inhibitor is injected following the initiation of the pits, procedure II. Thus, in presence of, for example, 5  102 M Na2MoO4, the inhibition efficiencies reported using procedures I and II are found to be 85.76% and 9.02%, respectively. Therefore, to get better inhibition efficiency, the reinforcing steel should be immersed directly in solutions already containing both the aggressive and the inhibitive anions from the start of the corrosion process. The inhibition efficiency of these oxy-anions may be associated with their lower polarizability [38,40]. Whatever their actual

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A

-3

-0.4

1 x 10 M Ca(OH)2 +0.75 M NaCl

-0.5

log I corr, mA/cm

2

-0.6 -0.7 -0.8 -0.9

Na2MO4

-1.0

Na2WO4 NaNO2

-1.1

Na2HPO4 -1.2 -1.3 -3.5

Na2CrO4 -3.0

-2.5

-2.0

-1.5

-1.0

,

log Cinh M

B

-3

1 x 10 M Ca(OH)2 +0.75 M NaCl

0.40 0.38

log I corr, mA/cm

2

0.36 0.34

1 2 3

Na2CrO4

4

Na2WO4

5

Na2MoO4

Na2HPO4 NaNO2

0.32 0.30 Fig. 9. SEM micrographs of steel electrode surfaces immersed for 3 h in 1  103 M Ca(OH)2 + 7.5  101 M Cl (A) in absence of NaNO2 and (B) in presence of 1  102 M NaNO2, respectively.

0.28 1 2 34 5

0.26 0.24 0.22 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

log Cinh, M. Fig. 8. Variation of the corrosion current density with the concentration of the inhibitors, on double logarithmic scales, using procedures I (A), and II (B), respectively, at 25 °C.

inhibiting action, these anions are assumed to be firstly adsorbed on the active sites of the oxide film covered steel surface in competition with Cl ions [37–40]. Nitrite ions behave as an anodic inhibitor which oxidizes Fe2+ to Fe3+ with the formation of the protective Fe2O3 in alkaline solutions [39,41,42]. Refaey et al. [43] assumed þ the probable reduction of NO 2 ions to NH4 ions with a subsequent oxidation of steel by the residual oxygen to give Fe2O3. On the other hand, when HPO2 4 ions present in higher concentrations, they act as an anodic inhibitor [39,44]. The protective film formed on the steel surface is a mixture of c-Fe2O3 and FePO32H2O [39,44]. The inhibiting action of WO2 4 ions was attributed by Ose et al. [45] to be due to their partial reduction. Lower valence tungstate oxides are believed to be incorporated in the corroding metal oxides to yield a protective passive film. However, Cew et al. [46] and Jabreera et al. [47] assumed that tungstate ions oxidize Fe2+ ions to higher valence ferric state that facilitated more passivity of steel. Roberston [48], on the other hand, assumed the primary adsorption of the WO2 4 ions on the steel surface, followed by the forma-

Table 2 Inhibition efficiency, IE, calculated using procedure I. Concentration (M)

5.5  104 1.0  103 2.5  103 5.0  103 1.0  102 2.5  102 5.0  102

%IE Na2CrO4

Na2HPO4

NaNO2

Na2WO4

Na2MoO4

4.76 10.95 17.33 22.61 38.80 42.38 47.14

6.66 13.10 19.76 27.21 45.23 50.26 57.61

10 15 24.28 34.80 51.19 57.61 62.14

12.04 17.80 30.26 43.80 60.00 66.66 76.35

15.23 24.50 39.35 50.88 66.66 80.28 85.76

tion of iron tungstate that blocks the active sites on the steel surface. The inhibition efficiency of chromate ions is attributed to their high oxidizing properties which involve the reduction of Cr6+ to Cr3+ with subsequent formation of Cr2O3 which is incorporated in the passive film already formed on the steel surface in alkaline solutions [49,50]. Refaey et al. [43] attributed the inhibition action of chromate ions to their lower polarizability in comparison with Cl ions. Due to their higher specific adsorption, CrO2 4 ions can displace the adsorbed Cl ions on the steel surface with subsequent increase in the protectiveness of the passive layer. The mechanism of corrosion inhibition of steel by MoO2 4 ions is well interpreted by Mustafa et al. [51]. Their inhibition action may be attributed to the reduction of Mo6+ to Mo3+ with the formation of MoO2 which is incorporated in the passive film on the steel

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S.M. Abd El Haleem et al. / Corrosion Science 52 (2010) 1675–1683 Table 3 Inhibition efficiency, IE, and surface coverage, heq, calculated using procedure II. Na2HPO4

NaNO2

it io

Na2MoO4

%IE

heq

%IE

heq

%IE

heq

%IE

heq

%IE

0.07 0.21 0.25 0.27 0.34 0.37 0.40

6.5 21.4 25.0 27.4 34.0 37.4 38.8

0.07 0.22 0.26 0.29 0.35 0.39 0.40

7.1 21.9 26.2 28.6 35.2 38.6 39.8

0.08 0.23 0.27 0.30 0.36 0.39 0.41

7.59 23.21 27.4 29.8 36.4 39.7 41.0

0.08 0.26 0.29 0.31 0.38 0.41 0.42

8.2 25.6 28.6 31.0 37.6 40.9 42.1

0.09 0.28 0.30 0.32 0.40 0.42 0.43

9.02 28.1 29.6 31.5 40.0 42.1 43.3

surface. The reduction of the molybedate anions could provide additional oxide anions that interfere with the ability of Cl ions to reach the metal/metal oxide film interface, possibly by blocking sites through which Cl ions preferentially penetrate the film [44]. The inhibiting action of MoO2 4 ions may also be due to the formation of a thin film of molybedate on the steel surface ions have higher [44,52]. This may explain, in part, why MoO2 4 ions in this work. However, the inhibition efficiency than CrO2 4 low inhibition efficiency of CrO2 4 ions observed in our study, relative to the other inhibiting anions, could be attributed to the danions when present in gerous oxidizing properties of CrO2 4 insufficient concentration [50], specially in the presence of the high concentration of Cl ions (7.5  101 M Cl ions) in this case. A similar view was reported by Refaey [44], who found that the inhibition efficiency of the pitting corrosion of mild steel in HCl solutions by inorganic anions decreases in the order: HPO2 4 > 2 MoO2 4 > CrO4 . These anions act as inhibitors for pitting corrosion by competitive adsorption with Cl ions for adsorption sites on the steel surface [44]. Furthermore, and referring to the curves in Fig. 7A and B, and the like, it is quite clear that the injection of the inhibiting anions, following the initiation of pitting corrosion, results in an initial rapid decrease of the current with time. This is followed by a gradual decrease of the current to approach finally new steady-state values which decrease with increasing the inhibitors concentration. Similar current transients were reported by other investigators under polarization conditions [53–56]. In this investigation, the rate of decrease of current density with time following the injection of the inhibitors in solution could be used to estimate the rate constant for the repassivation of the pre-formed pits, while the steady-state currents are analyzed towards the determination of the extent of pitting repassivation by the used inhibiting anions. This could be realized in the light of a competitive adsorption model of the inhibitive anions and Cl ions for adsorption sits on the passive oxides on the metal surface [44,48,50,57]. The fraction of oxide area that is covered by the inhibiting anion produces only a negligible current compared to that produced by the fraction of oxide area uncovered by the inhibitor anions or covered by the aggressive Cl ions [58]. This model assumes the degree of surface coverage, h, as

ht ¼ 1 

Na2WO4

heq

ð8Þ

where ht is the degree of surface coverage of the metal oxide by the inhibitive anion at time t, it is the current density supported by the steel electrode in the presence of the inhibiting anions after time t of the injection of the inhibiting anion in solution and io is the steady-state current density supported by the electrode in the inhibitorfree solution. The curves in Fig. 10A and B show the variation of ht with time for the steel electrode in 103 M Ca (OH)2 + 7.5  101 M Cl ions, in presence of increasing concentrations of NaNO2 and Na2MoO4,

respectively. As could be seen from these curves, and the likes, as the concentration of the inhibiting anions increases ht increases until reaching steady-state values, heq. As could be seen from Table 3, the values of heq increase with increasing the concentration of the inhibitive anions. To correlate heq to the concentration of the different inhibiting anions used, a plot of heq/(1  heq) with the concentration, Cinh, on a double logarithmic scale, Fig. 11, gives rise to straight lines with slopes of about unity which obey the Langmuir adsorption isotherm [59]:

log heq =ð1  heq Þ ¼ log K ads þ log C inh

-3

A

ð9Þ

-1

-

1 x10 M Ca(OH)2 + 7.5 x 10 M Cl + x MNaNO2 0.3 M NaNO2

0.4

0.25 M 0.2 M

Surface coverage, θt

5.00  102 1.00  101 1.25  101 1.50  101 2.00  101 2.50  101 3.00  101

Na2CrO4

0.15 M 0.125M 0.1 M

0.3

0.2

0.1

0.05M 0.0 0

5

10

15

20

25

Time, min -3

-1

-

1 x10 M Ca(OH)2 + 7.5 x 10 M Cl + x M Na2MoO4

B

0.3 M Na2MoO4 0.25 M 0.2 M

0.4

Surface coverage, θt

Concentration (M)

0.15 M 0.125M 0.1 M

0.3

0.2

0.1

0.05M

0.0 0

5

10

15

20

25

Time, min Fig. 10. Variation of ht with time for the steel electrode in 103 M Ca(OH)2 + 7.5  101 M Cl ions, in presence of increasing concentrations of (A) NaNO2 and (B) Na2MoO4, respectively, at 25 °C .

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S.M. Abd El Haleem et al. / Corrosion Science 52 (2010) 1675–1683

-0.05 -0.10 -0.15 -0.20

log[θ/ (1−θ)]

-0.25 -0.30 -0.35

Na2MoO4

-0.40

Na2WoO4

-0.45

NaNO2

-0.50

Na2HPO4 Na2CrO4

-0.55 -0.60 -1.0

-0.9

-0.8

-0.7

-0.6

-0.5

,

log Cinhibitor M Fig. 11. A plot of heq/(1  heq) against the concentration of the inhibitor, Cinh, on a double logarithmic scale.

Table 4 The values the standard free energy of repassivation, DGrep , and the adsorbability constant, Kads, for different inhibitors.

DGrep (kJ/mol) Kads (mol1 L)

Na2CrO4

Na2HPO4

NaNO2

Na2WO4

Na2MoO4

11.83

12.51

12.80

13.08

13.10

2.14

2.81

3.16

3.53

3.57

where Kads is the adsorbability constant of the inhibiting anions on the steel surface. The free energy of repassivation, DGrep , is derived from the relation:

DGrep ¼ 2:303RT logð55:5K ads Þ

ð10Þ

The values of both DGrep and Kads at 25 °C are shown in Table 4. Inspection of Table 4, shows that as the value of the standard free energy of repassivation, DGrep , decreases that of the adsorbability constant, Kads, increases in the direction of increasing the inhibition efficiency of the used inhibiting anions in the order: 2  2 2 CrO2 4 < HPO4 < NO2 < WO4 < MoO4 , which is the same order reported before [37]. A value of DG of 40 kJ/mol is usually adopted as a threshold value between chemi- and physi-sorption processes [60]. The calculated values for the repassivation of the pre-formed pit on the reinforcing steel in Ca(OH)2 solutions by the added inhibitive anions, in the range 11.83 to 13.11 kJ/ mol indicate that these anions act by physical adsorption on the oxide film in competition with Cl ions without any chemical interaction between these ions and the metal surface [36,44,48,50,57]. 4. Summary 1. Pitting corrosion current of reinforcing steel is measured in naturally aerated Ca(OH)2, solution as function of Cl ions concentration, pH and presence of inhibiting anions using a simple Hdivided cell. 2. Corrosion current starts to flow after an induction period which depends on the solution composition. 3. The limiting corrosion current density varies with the Cl ion concentration and pH of solution according to straight line relationships. 4. In presence of the inhibiting anions, the pitting corrosion current reaches steady-state values which depend on the way by which the inhibiting anions are introduced in solution.

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