Environmental factors affecting the corrosion behaviour of reinforcing steel. VI. Benzotriazole and its derivatives as corrosion inhibitors of steel

Accepted Manuscript Environmental factors affecting the corrosion behaviour of reinforcing steel. VI. Benzotriazole and its derivatives as corrosion inhibitors of steel S.M. Abd El Haleem, S. Abd El Wanees, A. Bahgat PII: DOI: Reference:

S0010-938X(14)00299-6 http://dx.doi.org/10.1016/j.corsci.2014.06.043 CS 5914

To appear in:

Corrosion Science

Received Date: Accepted Date:

25 October 2013 13 June 2014

Please cite this article as: S.M. Abd El Haleem, S. Abd El Wanees, A. Bahgat, Environmental factors affecting the corrosion behaviour of reinforcing steel. VI. Benzotriazole and its derivatives as corrosion inhibitors of steel, Corrosion Science (2014), doi: http://dx.doi.org/10.1016/j.corsci.2014.06.043

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Environmental factors affecting the corrosion behaviour of reinforcing steel. VI. Benzotriazole and its derivatives as corrosion inhibitors of steel 1

1, 2,*

S. M. Abd El Haleem S. Abd El Wanees

1, 3

, and A. Bahgat

1

Chemistry Department, Faculty of Science, Zagazig University, Zagazig 44519, Egypt 2 Chemistry Department, Faculty of Science, Tabuk University, Tabuk, Kingdom of Saudi Arabia 3 Center for Advanced Materials, Qatar University, Doha 2713, Qatar Abstract Benzotriazole and some of its derivatives cause inhibition of corrosion of reinforcing steel in saturated naturally aerated Ca(OH)2 solutions contaminated by Cl-. The inhibition efficiency of these compounds depends on both the inhibitor type and concentration, as well as, on the chloride ions content. Inhibition by these compounds is assumed to take place through the formation of iron-chlorobenzotriazoles complexes and/or the adsorption of the deprotonated species on the steel surface according to Langmuir's isotherm. The thermodynamic parameters Kads and ΔG°ads for the adsorption process are calculated and discussed. Benzotrizoles shift the pitting corrosion potential into the noble direction accounting for increased resistance to pitting. Keywords: A. Reinforcing steel, B. Open circuit potential, C. Oxide film repair C. Potentiodynamic polarisation, C. Oxide film destruction, C. Inhibition efficiency, C. Benzotriazole *Corresponding author, E mail: [email protected] 1 

1-Introduction Reinforcing steel is always under a passive state caused by the high alkalinity of surrounding concrete environment in presence of oxygen which stabilizes the passive oxide film on the steel rebar [1-5]. The passive film is not invulnerable; it can be damaged chemically or mechanically. Chemical damage is always attributed to carbonation and/or attack by chloride and sulphate ions. Chloride ingress could be arising from seawater, de-icing salts, unwashed sea sand and admixtures [6]. The risk in corrosion of reinforcing steel increases as the ratio of concentration of chloride ions to that of the inhibitive hydroxyl ion in solution increases [7]. Chloride ions usually lead to localized corrosion which might cause accelerated loss of the steel section and rapid failure of the reinforcement bar [8]. Sulphate ions, on the other hand, are known to be actively participated in corrosion of reinforcing steel especially in the area of Middle East [9, 10] One of the most efficient methods used to control corrosion of reinforcing steel is the addition of corrosion inhibitors to the concrete mixture which should not change the concrete characteristics [11]. Inhibitors need also to be compatible with the concrete. Several types of inorganic and organic compounds are searched as corrosion inhibitors for reinforcing steel [12].

2 

Of the most effective anodic inorganic inhibitors of corrosion of carbon steel are nitrite ions [13-16]. However, there are some limitations to their use including possible toxic effects, mechanical losses and eventually risky effects when not added in sufficient amounts [8, 17, 18]. LiNO3, Li2CrO4 and Li2MoO4 were used by Samiento-Bustos et al. [19] as corrosion inhibitors for carbon steel. Their inhibition efficiencies increased with concentration. Sodium molybdate and sodium nitrite were used also as corrosion inhibitors for reinforcing steel in saturated Ca(OH)2 solution polluted with sulphuric and nitric acids [9]. The two compounds displayed similar inhibitory effect within a high range of inhibitor concentration. Abd El Haleem et al. [8, 10], on the other hand, used several inorganic anions as corrosion inhibitors for reinforcing steel in Ca(OH)2 solutions polluted by chloride ions. The inhibition efficiency increased with the additive concentration in the order: CrO42- < HPO42- < WO42- < MoO42-. Varieties of organic compounds were reported as effective corrosion inhibitors for carbon steel [6, 20-27]. The development of organic corrosion inhibitors is based on compounds containing nitrogen, oxygen and sulphur atoms, in addition to multiple bonds in the molecules that facilitate their adsorption on the metal surface [6, 28]. Adsorption of organic compounds on metal surfaces depends

3 

on some physicochemical properties of the molecules including their function groups, possible steric effects and electron density of donor atoms [6, 28]. Amines and alkanolamines were used largely as constituents in commercial products employed as corrosion inhibitors for reinforcing steel in concrete although their poor inhibition efficiency [6, 29]. Carboxylate compounds, on the other hand, especially poly-carboxylates showed very good inhibition effectiveness in concrete [22, 29]. Amino alcohols were also used to inhibit corrosion of steel in simulated pore solution and mortar specimens [30, 33]. The inhibition efficiencies of these compounds were found to depend on the nature of the environment and the way of use. Sawada et al. [34] and Kubo et al. [35] electrochemically injected ethanolamine and guanidine into saturated specimens of carbonated and noncarbonated concretes from external electrolytes under the influence of an electrical field applied between embedded steel cathodes and the external anode. Inhibition by these compounds was found to depend on their degree of ionization in both media. N-heterocyclic organic compounds were also used as inhibitors for corrosion of reinforcing steel [36-38]. Benzotriazole (BTA) and its derivatives formed very strong complexes with transitions metals and most widely used as corrosion inhibitors for copper and 4 

copper containing alloys [39 - 43]. They were also used in aircraft and roadway deicing fluids, antifreeze, brake fluids, lubricating oils and industrial cooling systems [38]. BTA and some of its derivatives were also used by several authors to inhibit the corrosion of iron in NaCl solutions [44] and steel in acid solution [45, 46]. Their corrosion inhibition efficiency was found to depend on additive type and concentration and attributed to the formation of insoluble complexes that protected the metal surface. Some other investigators used BTA and some of its derivatives to inhibit the corrosion of reinforcing steel in simulated pore solution. Mennucci et al. [47] investigated the use of BTA as corrosion inhibitor for carbon steel in simulated pore solution contaminated by chloride ions. They concluded that BTA is a potentially attractive alternate to nitrites for inhibiting corrosion of reinforcement steel in concrete. The increased inhibition efficiency of this compound was attributed to the formation of a complex layer that covered the steel surface [48]. Ababneh et al. [38], on the other hand, used BTA and one of its derivatives to inhibit steel corrosion in solution simulated carbonated concrete. The study indicated the applicability of these compounds for steel corrosion protection in reinforced concrete structures.

5 

In most of the previous works on corrosion and corrosion inhibition of reinforcing steel in concrete, several solutions were selected by many authors to simulate the concrete pore environments. Thus, Page and Treadaway [7] argued the high alkalinity of the concrete pore solution to the presence of the sodium and potassium oxides beside Ca(OH)2. However, in silicate fume cement pastes Page and Vennesland [49] showed that the concrete pore solution is composed of Ca(OH)2 in presence of silicates and aluminates. Ghods et al. [2] used saturated Ca(OH)2 in presence of NaOH, KOH and CaSO4 as a simulated concrete pore solution. Gadadbar Sahoo and Balasubramaniam [50] and Wen Chen et al. [51], on the other hand, used saturated Ca(OH)2 as a test medium. Recently, we studied the corrosion behaviour of reinforcing steel in naturally aerated saturated Ca(OH)2 as the principle constituent of concrete pore solution [8, 10, 52, 53] . The aim of the present investigation is to highlight the effect of benzotriazole and it derivatives, 5-nitrobenzotriazole and 5-chlorobenzotriazole on the rate of passive film destruction on reinforcing steel and the consequent initiation of pitting corrosion, by chloride ions, in saturated naturally aerated Ca(OH)2 solution. This was carried out by following the open circuit potential and potentiodynamic anodic polarization of the steel electrode as function of additives

6 

concentration. Further inspection of the attacked and inhibited steel surfaces was carried out by SEM and EDS analysis. 2-Experimental The steel electrodes were made from steel samples produced by the Egyptian Steel Mill Co. (Helwan, Cairo), used always as reinforcement, and has the following chemical composition in accordance with the Egyptian ASTM A615 Standard [54]: C

Si

Mn

P

S

Fe

0.32

0.24

0.89

0.024

0.019

98.507 mass %

The steel electrodes were cut from actual rebar materials and mechanically polished to fine short rods of 10 mm diameter and 5 cm length. The electrodes were fixed to borosilicate glass tubes with epoxy resin so that only the bottom surface area (0.79 cm2) was exposed to the solution. Electrical contacts were achieved through copper wires soldered to the ends of the steel rods not exposed to the solution”. Prior to each experiment, the surfaces of the steel electrodes were grounds with SiC grinding papers successively to 600 grits using a grinding machine (model Jean Wirtz TG 200, Germany). The electrodes were then degreased with acetone and finally washed with bi-distilled water. The potential of 7 

the steel electrode was measured versus time to the nearest mV on a Wenking potentiometer type PPT 70 relative to the saturated calomel electrode (SCE). The scatter in electrode potential measurements was evaluated as less than +10 mV. The steady-state potential was considered as the value which did not change by more than 1 mV in 10 min. All measurements were conducted in saturated naturally aerated Ca(OH)2 solutions (pH 12.9) prepared from analytical grade chemicals and triply-distilled water. Benzotriazole (BTA) and its derivatives, 5nitrobenzotriazole (5-NO2-BTA) and 5-chlorobenzotriazole (5-Cl-BTA) were obtained from Merck, Schuchardt, Germany. They have the following chemical formula: O2 N

N N N H

Benzotriazole

Cl

N

N

N N H

5-Nitrobenzotriazole

N N H

5-Chlorobenzotriazole

Different concentrations of these compounds were used as inhibitors. The electrolytic cell has a double wall jacket through which water at the adjusted temperature was circulated. The main joint of the cell contains openings for both the steel electrode and the reference half cell (SCE) to ensure keeping the oxygen content of the solution unchanged throughout the time of the experiment. 8 

Measurements were carried out at a constant temperature, 25 + 1oC. The cell temperature was controlled using an ultra-thermostat type, Poly Science (USA). Potentiodynamic anodic polarization of the steel electrode was carried out using a conventional three electrodes system. A platinum sheet was used as an auxiliary electrode; the working electrode was the steel electrode and a saturated calomel electrode (SCE) as a reference electrode. Before carrying out polarization, the steel electrode was subjected to cathodic pretreatment for 20 min at –1.5 V (SCE). Anodic polarization was carried out at a scanning rate of 1mV/s using a Wenking Potentioscan, Type POS 73. The current density-potential curves were recorded on X-Y recorder, Type Advance HR 2000. Experiments were carried out in fresh portions of solution and with newly polished electrodes. Attacked and inhibited steel surfaces were examined with a scanning electron microscope, SEM, coupled with energy dispersion spectroscopy analysis, EDS. A Jeol scanning electron microscope, JSM-5410 (Japan), with accelerated voltage of 25 kV and a working distance of 20 mm was used.

9 

3. Results and discussion 3. 1. Effect of Benzotriazoles Concentration: Benzotriazole (BTA) and its derivatives, 5-NO2-BTA and 5-Cl-BTA are used as corrosion inhibitors for the reinforcing steel electrode in saturated naturally aerated Ca(OH)2 solutions contaminated by chloride ions. The action of these additives is found to depend on the concentration of the chloride ion, on one hand, and on inhibitor type and concentration, on the other hand. Thus, the curves of Figs 1, 2 and 3 represent the variation of the open circuit potential, E, of the steel electrode in saturated naturally aerated Ca(OH)2 solutions, containing 0.0001 M NaCl, as function of time in presence of different concentrations of BTA, 5-NO2BTA and 5-Cl-BTA, successively. The curves of Figs 4, 5 and 6 represent the behaviour of the steel electrode, in presence of higher chloride ions concentration (0.05M), under the same experimental conditions. Inspection of the curves of Figs (1-6), reveals that in organic-free Ca(OH)2 solution, the steady-state potential of the steel electrode is reached from negative values following the immersion of the electrode in the solution. The ennobling of the electrode potential denotes that the pre-immersion oxide film carried by the steel surface is not sufficient to impart passivity to the metal [52, 53]. Oxide film healing and thickening continue as the immersion time increases till steady-state 10 

potential is reached [8, 52, 55]. During this process of oxide film healing and thickening, reduction of oxygen is assumed to be the probable partial cathodic reaction [55-57]. The electrons necessary for the reduction reaction are furnished from the ionization of Fe atoms entering the passive oxide phase under the influence of anodic current which polarizes the steel electrode and shifts its potential in the positive direction [8, 56]. The steady-state potential, Est, is attained within ~240 min of electrode immersion in the test solution. Further inspection of the curves of Figs.1-6 reveals two different types of behaviour of the steel electrode in presence of the organic additives, depending on the chloride ions content in the medium. Thus, in presence of 0.0001 M Cl-, Figs 13, the steady state potentials, Est, are approached from negative values and become more noble the higher the additives

concentration. This behaviour could be

attributed to increased rate of oxide film healing by the organic additives [52, 57]. However, in presence of higher chloride ions concentration (0.05 M), the steady state potentials, Est, are approached from negative values and become less active the higher the organic additives concentration. This behaviour could be attributed to a decreased rate of oxide film destruction by the added benzotriazoles [52, 57]. The curves of Figs 7 (A and B) show the variation of the steady state potentials, Est, of the steel electrode with the logarithm of the molar concentration of the 11 

organic additives in presence of 0.0001 M and 0.05M Cl-, respectively. As could be seen from these curves, the steady state potentials, Est, vary, in both cases, with the logarithm of the concentration of the organic additives according to segmented S-shaped curves. This behaviour indicates that the added organic compounds act as inhibitors for the chloride induced pitting corrosion by way of an adsorption mechanism [52, 57] as will be seen later. Fig 8 represents the scanning electron micrograph of the steel sample after immersion for a period of 4 hours in saturated naturally aerated Ca(OH)2 solution containing 0.05 M NaCl. Clear large pits are formed surrounded by some corrosion products. However, the micrograph of Fig 9 shows the SEM of the steel electrode surface in saturated Ca(OH)2 solution containing 0.05M NaCl in presence of 0.001M 5-Cl-BTA, as an example of the other additives. This micrograph shows small fine pits with some corrosion products spread on the steel surface in comparison with the large pits formed in additive-free solution, Fig 8. On the other hand, Fig 10 (A&B) and Table 1, show the SEM complementary EDS analysis of the corrosion products formed on the steel electrode surface immersed in naturally saturated Ca(OH)2 solution containing 0.05M NaCl for 4 hrs

in

absence, Fig 10A and in presence of 0.001M 5-Cl-BTA, Fig10B , as an example of the other added benzotriazoles. Inspection of Fig 10 A & B and Table 1 indicates 12 

that the atomic percent of Fe in the scanned area (which is connected with steel corrosion) decreases relatively in presence of the added organic additives than in their absence. This could be attributed to the effective action of the added benzotriazoles as inhibitors for pitting corrosion of steel. To follow the effect of the used benzotriazoles on the rate of oxide film repair and/or destruction by chloride ions, the open circuit potential, E, is plotted as function of the logarithm of immersion time, t. The curves of Figs 11 A, B and C show such behaviour in saturated naturally aerated Ca(OH)2 solutions containing 0.0001 M NaCl in presence of increasing concentrations of BTA, 5-NO2-BTA and 5-Cl-BTA, successively. The curves of Figs 12 A, B and C, on the other hand, show the behaviour of the steel electrode in Ca(OH)2 solutions containing 0.05 M NaCl in presence of BTA, 5- NO2-BTA and 5-Cl-BTA, successively. Inspection of the curves of Figs 11 and 12 reveals the following: i) - In both Cl- concentrations, the open circuit potential, E, varies with the logarithm of the immersion time, t, according to straight line relationships indicating that oxide film repair and/or destruction in absence and presence of the organic additives follow a direct logarithmic law and depend on both the chloride and organic additives concentrations.

13 

ii) In presence of low chloride ion concentration (0001M), Figs 11 (A, B and C) the open circuit potential, E, varies with log t, until reaching the final steady state potentials, according to the relation: E = Į1 + ȕ1 log t

(1)

where Į1 and ȕ1 are constants. As could be recognized from the straight lines of these figures, the slope, ȕ1, increases with increasing the additives concentrations. This behaviour could be attributed to increased rate of oxide film repair in presence of the organic additives. iii) In presence of the higher chloride ion concentration (0.05 M), Fig 12 (A, B and C), the behaviour noted is quite different. The open circuit potential, E, of the steel electrode varies with the immersion time, t, until reaching the final Est according to the relation: E = Į2 – ȕ2 log t

(2)

where Į2 and ȕ2 are constants. As could be seen from the curves of Fig 12 (A, B and C) the slope ȕ2 of the straight lines decreases markedly on increasing the concentration of the organic additives. This behaviour could be attributed to a decreased rate of oxide film destruction by chloride ions upon the addition of these organic compounds [52, 57]. 14 

From the previously noted behaviour of the steel electrode in presence of low and high chloride ions concentrations, it is clearly noted that the order of action of the organic compounds as inhibitors for the chloride induced pitting corrosion of the steel is always the same either in increasing oxide film repair or in decreasing oxide film destruction: (weak) 5-nitrobenzotriazole < benzotriazole < 5chlorobenzotriazole (strong). The linearity of the open circuit potential with the logarithm of time during oxide film growth on metal surfaces was investigated by several authors. Oxide film growth was supposed to be controlled by diffusion of ions and transfer of electrons under the influence of gradients in their concentration and electric potential [53, 58-60]. Cabrera and Mott [61] were of the opinion that cations migration occurred always under the influence of a potential gradient built up across the growing oxide film. Using the guillotined Al electrode, Burstein [62] showed that the corrosion potential of the generated Al surface in buffered phosophate solution (pH 6.9), after dropping to very low value, started to increase continuously with time toward the steady-state. The form of the transient, which describes the repassivation of the metal in open circuit, showed that Ecorr raised linearly with the logarithm of time with a (dEcorr)/(dt) = 0.13V/decade. In our study, the corrosion potentials varied also continuously and linearly with the 15 

logarithm of time till steady state values were reached without any transient at the early stage of potential measurements. In a consequent study, Burstein and Cinderey [63] explained quantitatively the linear dependence of the free corrosion potential of a freshly generated Al surface on the logarithm of time during the repassivation by oxide film growth. Chao and Liu [64] and Sato and Cohen [65], on the other hand, proposed the direct logarithmic relation to describe the kinetic form of the linear increase of potential with time, under galvanostatic anodic polarization. Iron oxides were visualized to be cubic close packed arrays of oxide ions with certain number of Fe2+ and/or Fe3+ ions distributed among the octahedral and tetrahedral interstitial sites in the oxide lattice [8, 53, 66, 67]. It is, therefore, assumed that the growth of oxides on steel surface occurs by way of migration of Fe2+ and/or Fe3+ ions to the oxide/solution interface [8, 53, 69, 73]. Under the influence of a strong electric field, of the order of magnitude of RT/nF per atom layer of the oxide, can detectable ionic current flow [8, 55]. In this case, Ohm’s law does not apply and ion transport is assumed to be governed by the familiar Guntherschulze and Betz relationship [70]. Under conditions of open circuit, the flowing current is assumed to originate from the specific adsorption of anions on the oxide covered metal surface [55]. This causes creation of image charges of the 16 

same magnitude but of opposite sign at the oxide/metal interface which facilitate the transfer through the oxide to the film/solution interface under high electric field [55]. The field strength depends only on the nature and charges of the present anions, and to a lesser extent on their concentration and could be substituted by the term E/į, where E is the measured potential and į is the thickness of the oxide film. As the process of oxidation of steel in saturated naturally aerated Ca(OH)2 solution leads exclusively to the increase in the thickness of the oxide films on the steel surface, it necessitates a corresponding equivalent change of the potential, E, so as to keep the field strength constant. With these assumptions, the open circuit potential, E, is found to change with the immersion time, t, according to [8, 52, 55]: ‫ܧ‬

ൌ …‘•–Ǥ ൅

ଶǤଷ଴ଷఋ ష ఉ

Ž‘‰ ‫ݐ‬ሺ͵ሻ

where į- is the rate of oxide film thickening per unit decade of time, and ȕ is a constant which is identified as [60]: ߚ

ൌቀ

௡ி ோ்

ቁ ߙߜ ᇱ ሺͶሻ

where Į is the transfer coefficient and į' is the width of the energy barrier surmounted by the ion during transfer.

17 

Assuming accordingly that the thickening of the oxide film on the steel surface occurs by way of diffusion of Fe2+ ions to the oxide/ metal interface [71], so that ‘‘n” in Eq. 4 amounts to 2, Į acquires a value of 0.5 and į' the value of 1 nm, the constant ȕ should acquire the value of 39 nm/V at 25ºC. From the slopes, ȕ1, of the straight lines of the E-log t curves of Fig 11 A, B and C, the values of the rate of oxide film repair, į-1, in saturated naturally aerated Ca(OH)2 solutions containing 0.0001M NaCl, in presence of increasing concentrations of BTA, 5-NO2-BTA and 5-Cl-BTA, have been calculated [8, 52, 55]. The rate of oxide film healing or repair is found to vary linearly with the logarithm of the concentration of the added organic compounds as shown in Fig 13A, according to the relation [57]: į-1 = a1 + b1 log Cinh

(5)

where a1 and b1 are constants. The constant a1 represents the rate of oxide film repair in presence of 1M of the added organic compounds and amounts to 2.64, 2.95, and 3.87 nm/unit decade of time for NO2-BTA, BTA and Cl-BTA, successively. At the same time, the values of the constant b1 amount to 0.44, 0.46 and 0.65 nm/unit decade of time/unit decade of concentration, for 5-NO2-BTA, BTA and 5-Cl-BTA, successively. From the values of a1 and b1, it is quite clear that the present organic compounds act as inhibitors for the chloride induced 18 

pitting corrosion of steel with their inhibiting action increases, as previously noted, in the order: 5-nitrobenzotriazole < benzotriazole < 5-chlorobenzotriazole. From the values of the constant, ȕ2, of the straight lines of Fig 12 A, B and C, one can determine the rate of oxide film destruction, į-2, at high chloride concentration in presence of the organic additives. į-2 is found to vary with the logarithm of the inhibitors concentration, as shown in Fig 13B, according to the relation [52, 57]: į-2 = a2 - b2 log Cinh

(6)

where a2 and b2 are constants. The constant a2 represents the rate of oxide film destruction in presence of 1 M of the added organic compounds and found to be 0.13, 0.11 and 0.09 nm/unit decade of time for 5-NO2-BTA, BTA and 5-Cl-BTA, successively. At the same time, the values of the constant b2 amount to - 0.047, - 0.037 and - 0.029 nm/unit decade of time/unit decade of concentration, successively. From the values of the constants a2 and b2, it is also clear that the inhibition action of the used organic additives, in presence of

high Cl-

concentration, in decreasing the rate of oxide film destruction, increases in the order: 5-NO2-BTA < BTA< 5-Cl-BTA.

19 

3.3. Inhibition efficiency and adsorption isotherm The adsorption of BTA inhibitors on the metal surface can occur either directly via donor–acceptor interactions between the ʌ-electrons of the heterocyclic compounds and the vacant d-orbitals of iron atoms or via interactions of triazoles with already adsorbed chloride ions [71, 72]. Specific adsorption of anions having smaller degrees of hydration, such as chloride ions, is expected to be more pronounced [71]. On the other hand, strong adsorption of organic molecules is not always a direct binding of the molecules with the metal surface. In some cases, adsorption occurs through the already adsorbed chloride ions which interact with the subsequently adsorbed organic molecules [71]. Therefore, the molecular structure of the organic compound is important in synergistic inhibition. The greatest synergistic inhibition is to be expected for an anion–cation pair in which both ions have appreciable tendencies toward covalent binding [71]. Abdulrahman et al. [6], on the other hand, showed that adsorbed molecules with a negatively charged substituent or a lone pair of electrons develop a repulsive action towards Cl-; avoiding chloride to be in contact to the carbon steel passive layer. In the present investigation, the surface coverage, ș, is determined for the studied benzotriazoles, in presence of the dilute Cl- (0.0001 M), is determined from 20 

the variation of the rate of oxide film destruction in absence and presence of the added inhibitors, by suggesting that the rate of oxide film destruction is inversely proportional to the rate of oxide film repair, į-1. However, in presence of the high Cl- concentration (0.05M), ș is directly determined from the variation of the rate of oxide film destruction, į-2, in absence and presence of the added inhibitors. In both cases, the values of surface coverage, ș, and the percentage inhibition efficiency, Ș, can be calculated from the relations: ș

Ș

ൌ ቂͳ െ

ൌ  ቂͳ െ

ሺఋమష ሻ౟౤౞ ሺఋమష ሻι

ሺఋమష ሻ౟౤౞ ሺఋమష ሻι

ቃሺ͹ሻ

ቃ ͳͲͲሺͺሻ

where (į-2)º and (į-2)inh are the rates of oxide film destruction of the passive oxide film on the steel surface in absence and presence of the added benzotriazole derivatives, respectively. In Table 2 values of the percentage inhibition efficiency, Ș, are depicted as function of the inhibitors concentration. Inspection of the data of Table 2 (A & B) reveals the following: 1) Ș is dependent on the inhibitor type and concentration, as well as, on Clconcentration. Thus, for one and the same benzotrizole compound and chloride ions concentration, Ș increases with increasing the inhibitor

21 

concentration and the inhibiting action of the used benzotriazoles follows the order: 5-Cl-BTA > BTA > 5-NO2-BTA. 2) In presence of dilute chloride ions concentration (0.0001M), Table 2A, Ș reaches the values 93.3%, 90.9% and 68.8% in presence of 0.0005M of 5-ClBTA, BTA and 5-NO2-BTA, successively. These values reflect the decreased efficiencies of these additives as pitting corrosion inhibitors for reinforcing steel in presence of such low concentration of chloride ions. 3) In presence of higher chloride ions concentration (0.05M) and in presence of the same concentration of these inhibitors (0.0005M), Table 2B, Ș reaches the values 71.0%, 62.3% and 57.4% for 5-Cl-BTA, BTA and 5-NO2-BTA, successively. These relatively low values of Ș could be attributed to the high aggressiveness of the corrosive medium in presence of such high concentration of chloride ions. Several adsorption isotherms are attempted to fit the surface coverage, ș, including that of Frumkin, Temkin, Freundlich and Langmuir isotherms.

It is

found that the experimental data fit the Langmuir adsorption isotherm. Accordingly, the surface coverage, ș, is related to the inhibitor concentration, Cinh, by the following equation [73, 74];



ș ଵିș

ൌ  ‫ܭ‬ୟୢୱ ‫ܥ‬୧୬୦ ሺͻሻ 22



or





஼౟౤౞ ఏ

ൌ

ଵ ௄౗ౚ౩

൅‫ܥ‬୧୬୦ ሺͳͲሻ

where Kads is the equilibrium constant of the adsorption process.

A plot of

஼౟౤౞ ஼೔೙೓ ఏ

ఏ

versus Cinh for the studied benzotriazole compounds in

presence of dilute and high chloride ions concentrations is represented by Figs 14 and 15, respectively. As could be seen from these figures, all used inhibitors fit straight line relationships with a slope close to unity and strong correlation coefficient (R2 > 0.99), as shown in Table 3 which includes also the values of the adsorption parameters, Kads and ΔG°ads. The values of Kads are calculated from the intercepts of the straight lines of Figs 14 and 15. The standard free energy of adsorption, ΔG°ads, in kJmol-1 of the various inhibitors is related to Kads by the following equation [75]: ‫ܭ‬ୟୢୱ  ൌ 

ଵ ହହǤହ

‡š’

ι οீ౗ౚ౩

ோ்

ሺͳͳሻ

where 55.5 is the concentration of water in the solution in mol/L, R is the universal gas constant in J mol-1 deg-1, T is the absolute temperature. As could be seen from Table 3, the values of ΔG°ads depend to a higher extent on the Cl- concentrations and to a lesser extent on the additive type. Thus, for the dilute Cl- concentration (0.0001 M), ΔG°ads acquires values between -36.17 kJ mol-1 and -40.78 kJ mol-1, 23 



while in presence of the high Cl- concentration (0.05 M), ΔG°ads acquires values between – 32.17 kJ mol-1 and -37.47 kJ mol-1 for the studied inhibitors. These values are consistent with a chemisorption process involving sharing or transfer of electrons from the organic inhibiting species to the metal surface to form a metal bond [76, 77]. 3.4. Potentiodynamic anodic polarization: In the present part of the work, the potentiodynamic anodic polarization of the reinforcing steel is performed in saturated naturally aerated Ca(OH)2 solution contaminated with 0.01M NaCl, as pitting corrosion agent, in presence of increasing concentrations of the used benzotriazoles, at a scanning rate of 1mV/s. The curves of Figs 16, 17 and 18 represent the potentiodynamic I/E curves in absence and presence of increasing concentrations of BTA, 5-NO2-BTA and 5-ClBTA, successively. Inspection of the curves of these figures reveals the following: 1) In organic compounds-free solutions, as the potential of the working electrode is shifted into the positive direction, the flowing current density increases continuously to reach the passive region, in which the current density remains more/or less unchanged before its sudden increase due to the destruction of the passive film and initiation of pitting corrosion. Initiation of pitting corrosion occurs at the critical pitting potential, Epit [78]. 24 

2) Addition of low concentrations of the used benzotriazoles causes the shift of the critical pitting potential, Epit, slightly into the noble direction accounting to increased resistance to the initiation of pitting corrosion [78]. 3) Further increase in the concentration of the added organic compounds causes marked shift of the critical pitting potentials in the noble direction as shown in Fig 19 which represents the relation between the critical pitting potential, Epit and the concentration of the added benzotriazoles. In these concentration ranges of benzotriazoles, Epit varies linearly with log Cinh according to the relation [78]: Epit = a3 + b3 log Cinh

(12)

where a3 and b3 are constants. Further inspection of the lines of Fig 19 reveals that the minimum concentration of the added compounds, which causes a marked inhibition of pitting, decreases in the order: 5-NO2-BTA > BTA > 5-Cl-BTA, while the value of the slope, b3, increases in the reverse order. Both factors indicate the increased tendency of these benzotriazoles to act as inhibitors for pitting corrosion of reinforcing steel and are in parallel with the previously reported results under open circuit conditions. 3.4. Mechanism of inhibition by benzotriazoles:

25 

Benzotriazole and its derivatives are effective inhibitors for the corrosion of metals like copper, zinc and copper alloys [38-43, 73, 79-84]. Two different inhibition mechanisms of BTA for copper are known. One is based on the formation of a polymeric complex with Cu+ of the type [Cu (I) (BTA)]n [81, 85]. The second involves the adsorption of benzotriazole either in its molecular form or in its protonated form. Adsorption depends on potential, pH of solution and BTA concentration [82]. Little is known about the effect of benzotrizoles on iron and steel inhibition [44, 45, 86, 87]. As it is well known benzotriazole exists in aqueous solution in either the neutral molecule (BTAH) at pH between 2 and 8 or in the deprotonated anionic species (BTA-) at pH > 8 or in the protonated cationic species (BTAH2+) at pH < 1, according to:





In acidic and NaCl media, benzotriazole and its derivatives bind strongly to metals by forming bonds via the triazole ring [88- 92]. The resulting protective film is only of the order of molecule dimension and acts as a barrier which reduces the transport of the corrosion ions to the metal surface [93, 94].

26 

In alkaline media, Yao et al. [86], using Raman spectra, studied the adsorption of BTA on Fe surface and indicated that each Fe atom may coordinate with several BTA- ions through N atoms. Due to steric hindrance, some BTA- ions can be attached to the Fe surface with their triazole rings, and others with benzene rings. However, Mennucci et al. [47], using the same technique, indicated that the protection of Fe in alkaline chloride solution by benzotrizole involves the formation of a complex. This process depends on the electrode potential, pH of solution and the concentration of benzotrizole. Thus in alkaline media, benzotrizole which is present in its deprotonated form, BTA- can form the complex [Fen(Cl)p(BTA)m] in presence of chloride ions. Mennucci et al. (47) were also of the opinion that adsorption of BTA might occur by coordination of the pairs of free electrons from nitrogen atoms with iron [47, 86]. The complex layer spreads over the steel surface and acts as a partial barrier to the access of environment aggressive species to the metallic substrate. It has also been commonly recognized that organic inhibitors usually promote the formation of a chelate on the metal surface through the transfer of electrons from the organic compounds to the metal to form a coordinate covalent bond [95, 96]. In this way, the metal acts as an electrophile whereas the nucleophile centers of the inhibitor molecule are normally hetero atoms with free 27 

electron pairs, which are ready available for sharing to form a bond. Therefore, the effect of substituents in related molecules, such as on the inhibition efficiencies has been correlated with a number of molecule properties such as orbital energies, dipole moment, charge densities, heat of formation and ionization potentials [97102]. In this context, the inhibition efficiency of benzotrizole for the corrosion of reinforcing steel might be improved by the introduction of a proper substituent in its molecule [103-109]. Therefore, it is the object of the present study to investigate the effect of some substituents such as an electron acceptor (-Cl) and an electron donor (-NO2) groups on the protective characteristics of benzotrizole on reinforcing steel in saturated naturally aerated Ca(OH)2 solution contaminated with the aggressive chloride ions. As could be seen from the data of Table 2, the inhibition efficiency, Ș, of the used benzotriazoles, in presence of two concentrations of chloride ions in saturated Ca(OH)2 solution, decreases in the order: 5-chlorobenzotriazole > benzotriazole > 5-nitrobenzotriazole In such high alkaline medium, benzotriazoles are mainly found in their deprotonated forms BTA- [47] which might acquire their inhibition, of the chloride induced pitting corrosion of reinforcing steel, through the formation of their corresponding complexes on the metal surface [47]. Chloride ions are assumed to 28 

take part in this process to yield complex compositions of the form [Fen(Cl)p(BTA)m] [47, 86]. Adsorption of these complexes on steel surface might occur by coordination of pair electrons of the nitrogen atom of the triazole ring with iron [47, 86]. Similar views reported by Mennucci et al. [47] who supported the hypothesis that the film formed on steel electrode in simulated concrete pore solution in presence of BTA is due to a complex formed between the corrosion products and solution components such as BTA and chloride ions. The decreased inhibition efficiency of the studied benzotriazoles in the order previously reported could be attributed to their abilities to form the corresponding chloro-complexes. Similar results were reported by Aramaki et al.[110], who studied

the

mechanism

of

adsorption

and

complex

formation

of

5-

chlorobenzotriazole, 5-methylbenzotriazole and 5-nitrobenzotriazole in sulphate medium. All these derivatives readily formed protective complex polymers at high pH, high benzotrizole concentrations and high noble potentials (85, 97-102). The inhibition effectiveness of these derivatives decreased also in the order: 5chlorobenzotriazole

>

benzotriazole

~

5-methylbenzotriazole

>

5-

nitrobenzotriazole The reported relatively high protection efficiency of 5-chlorobenzotriazole in comparison with the other additives is assumed to be associated with the high 29 

electron acceptor character of the chloro-substituent and the dependence of the equilibrium (14) on the concentration of species in solution [110, 111]:

BTAH

BTA- + H+ ;

K = 6.31 x 10-9

(14)

The presence of a chloro- substituent in position 5 of the benzotriazole molecule reduces the bonding energy of H attached to the N-atom in position 2, by a factor of two [95]. This predicts an increase in the acidity of the first proton in the benzotriazole molecule and the displacement of the equilibrium (14) to the BTAH form which facilitates the formation of the surface complex [95]. The electron withdrawal character of the -NO2 group, on the other hand, is assumed to deactivate benzotriazole and reduces the possibility of formation of the surface complex [Fen(Cl)p(BTA)m]. Deactivation of BTA could be attributed to the increase of the bonding energy of the hydrogen atom attached to N atom in position 2 [95]. Consequently, the relative low inhibition efficiencies of benzotriazole and 5-nitrobenzotriazole, in comparison with 5-chlorobenzotriazole, is supposed to be associated with their lightly weak chemisorption on the steel surface as could be noted from the low values of ΔG°ads of these two compounds, Table 2, especially in presence of the high chloride ions concentration [95].

30 

4. Conclusions The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)2 solutions contaminated with Cl- is investigated in absence and presence of

benzotriazole

and

its

derivatives, 5-chlorobenzotriazole

and

5-

nitrobenzotriazole. This is assessed by following the open circuit potential as function of time, SEM examination and by potentiodynamic anodic polarization. From the experimental data obtained, the following conclusions could be drawn:

1- The rate of oxide film destruction by Cl- ions in absence and presence of the added organic compounds follows a direct logarithmic law. 2- The inhibition efficiency of benzotriazoles depends on the concentrations

of both the Cl- and the organic additives and increases in the order: 5nitrobenzotriazole < benzotriazole < 5-chlorobenzotriazole. 3- Benzotriazoles inhibit pitting corrosion by way of adsorption and/or precipitation of their chloro-complexes on steel surface. 4- Adsorption of these species on steel surface obeys Langmuir's adsorption isotherm. The calculated values of ΔG°ads reveal a chemisorption mechanism. 31 

5- Presence of these benzotriazoles causes the shift of the critical pitting potential in the noble direction accounting for increased resistance to initiation of pitting corrosion. 5-References 

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[102]N. Khalil, Quantum chemical approach of corrosion inhibition, Electrochim. Acta 48 (2003) 2635-2640. [103] J. S. Wu and K. Nobe, Effects of substituted benzotriazoles on the electrochemical behaviour of copper in H2SO4, Corrosion 37(1981)223-225. [104] L. Tommesani G. Brunoro, A. Frignani, C. Monticelli, M. Dal Colle, On the protective action of 1, 2, 3-benzotriazole derivative films against copper corrosion, Corros. Sci. 39 (1997) 1221-1237. [105] A. Frignani, M. Fonsati, C. Monticelli, G. Brunoro, Influence of the alkyl chain on the protective effects of 1, 2, 3-benzotriazole towards copper corrosion: Part I: inhibition of the anodic and cathodic reactions, Corros. Sci. 41(1999) 12051215 [106] A. Frignani, M. Fonsati, C. Monticelli, G. Brunoro, Influence of the alkyl chain on the protective effects of 1, 2, 3-benzotriazole towards copper corrosion. Part ii: formation and characterization of the protective films, Corros. Sci. 41(1999) 1217-1227. [107] O. Hollander, R.C. May, The chemistry of azole copper corrosion inhibitors in cooling waters, Corrosion 41(1985) 39-45. [108] C. Tornkvist, D. Thierry, J. Bergman, B. Liedberg, C. Leygraf, Methyl substitution in benzotriazole and its influence on surface structure and corrosion inhibition, J. Electrochem. Soc. 136(1989) 58-64. [109] V. Otieno-Alego, S. E. Bottle, D.P. Schweinsberg, In: Proceeding 13th international corrosion conference, Melbourne, Australia, 25–29 Nov (1996) 3: 320. [110] K. Aramaki, T. Kiuchi, T. Sumiyoshi, H. Nishihara, Surface enhanced Raman scattering and impedance studies on the inhibition of copper corrosion in sulphate solutions by 5-substituted benzotriazoles, Corros. Sci. 32 (1991) 593-607. [111] John E. Fagel Jr., Galen W. Ewing, The ultraviolet absorption of benzotriazole, J. Amer. Chem. Soc. 73 (1951) 4360–4362. 43 

Legend of Figures Fig 1. Variation of the open circuit potential, E, with the immersion time for reinforcing steel in sat. Ca(OH)2 containing 0.0001 M NaCl in presence of different concentrations of benzotriazole. Fig 2. Variation of the open circuit potential, E, with the immersion time for reinforcing steel in sat. Ca(OH)2 containing 0.0001 M NaCl in presence of different concentrations of 5-nitrobenzotriazole. Fig 3. Variation of the open circuit potential, E, with the immersion time for reinforcing steel in sat. Ca(OH)2 containing 0.0001 M NaCl in presence of different concentrations of 5-chlorobenzotriazole. Fig 4. Variation of the open circuit potential, E, with the immersion time for reinforcing steel in sat. Ca(OH)2 containing 0.05 M NaCl in presence of different concentrations of benzotriazole. Fig 5. Variation of the open circuit potential, E, with the immersion time for reinforcing steel in sat. Ca(OH)2 containing 0.05 M NaCl in presence of different concentrations of 5-nitrobenzotriazole. Fig 6. Variation of the open circuit potential, E, with the immersion time for reinforcing steel in sat. Ca(OH)2 containing 0.05 M NaCl in presence of different concentrations of 5-chlorobenzotriazole. Fig 7. Variation of the steady-state potential, Est, of the reinforcing steel with the logarithm of the molar concentration of the inhibitor in sat. Ca(OH)2 containing:(A) 0.0001 M NaCl and (B) 0.05 M NaCl. Fig 8. SEM micrograph of steel sample after immersion for 4 hours in saturated naturally aerated Ca(OH)2 solution containing 0.05 M NaCl. Fig.9. SEM micrograph of steel sample after immersion for 4 hours in saturated naturally aerated Ca(OH)2solutions containing 0.05 M NaCl in presence of 0.001M 5-chlorobenzotriazole.

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Fig 10. EDS analysis of corrosion products formed on steel surface in saturated naturally aerated Ca(OH)2 solutions in presence of 0.05 M of NaCl in (A) absence and (B) in presence of 0.001 M 5-chlorobenzotriazole. Fig 11. Variation of the open circuit potential of reinforcing steel with the logarithm of immersion time, t, in sat. Ca(OH)2 solution containing 0.0001M NaCl in presence of different concentrations of: (A) BTA, (B) 5-nitro-BTA and (C) 5chloro-BTA. Fig 12. Variation of the open circuit potential of reinforcing steel with the logarithm of immersion time, t, in sat Ca(OH)2 solution containing 0.05 M NaCl in presence of different concentrations of: (A) BTA, (B) 5-nitro-BTA and (C) 5chloro-BTA. Fig 13A. Variation of the rate of oxide film repair of reinforcing steel with the logarithm of inhibitor concentrations in presence of0.0001M NaCl. Fig 13B.Variation of the rate of oxide film destruction of reinforcing steel with the logarithm of inhibitor concentrations in presence of0.05 M NaCl. Fig14. Langmuir adsorption isotherms for reinforcing steel immersed in saturated Ca(OH)2 solution containing 0.0001M NaCl in presence of benzotriazole, 5-nitrobenzotriazole and 5-chlorobenzotriazole. Fig15. Langmuir adsorption isotherms for reinforcing steel immersed in saturated Ca(OH)2 solution containing 0.05M NaCl in presence of various concentrations of: (A) benzotriazole, (B) 5-nitrobenzotriazole and 5chlorobenzotriazole. Fig 16. Poentiodynamic anodic polarization curves of reinforcing steel in saturated Ca(OH)2 solutions containing 0.01M NaCl, in absence and presence of different concentrations benzotriazole. Fig 17. Poentiodynamic anodic polarization curves of reinforcing steel in saturated Ca(OH)2 solutions containing 0.01M NaCl, in absence and presence of different concentrations 5-nitrobenzotriazole.

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Figs 18. Poentiodynamic anodic polarization curves of reinforcing steel in saturated Ca(OH)2 solutions containing 0.01M NaCl, in absence and presence of different concentrations 5-chlorobenzotriazole Fig 19. Variation of Epit.of reinforcing steel electrode with the logarithm of inhibitors concentration, Cinh.



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Table

Table 1.EDS analysis of the reinforcing steel in saturated Ca(OH) 2 solution in 0.05M NaCl in absence and presence of 1 x 10-3M 5- Chloro BTA. Element C O Ca Fe

NaCl 16.41 6.54 0.44 76.61

Atomic % 5- Chloro BTA 15.41 8.01 2.56 74.02

Table

Table 2. The values of surface coverage, θ, and the percentage inhibition, η, of inhibitors in presence of: (A) 0.0001M NaCl and (B) 0.05M NaCl. (A) In presence of 0.0001M NaCl Cinh, mol/L -6

1x10 M 5x10-6 M 1x10-5 M 5x10-5 M 1x10-4 M 5x10-4 M

5-Chloro BTA θ 0.583 0.655 0.746 0.826 0.877 0.933

η 58.3 65.5 74.6 82.6 87.7 93.3

5-Nitro BTA θ 0.406 0.560 0.630 0.595 0.850 0.909

η 40.6 56.0 63.0 75.0 85.0 90.9

BTA θ 0.176 0.210 0.430 0.538 0.595 0.688

η 17.6 21.0 43.5 53.8 59.5 68.8

(B) In presence of 0.05M NaCl Cinh, mol/L -5

1x10 M 5x10-5 M 1x10-4 M 5x10-4 M 1x10-3 M 5x10-3 M

5-Chloro BTA θ 0.249 0.530 0.625 0.710 0.752 0.765

η 24.9 53.0 62.5 71.0 75.2 76.5

5-Nitro BTA θ 0.423 0.555 0.623 0.673 0.730

η 42.3 55.5 62.3 67.3 73.0

BTA θ 0.30 0.47 0.57 0.63 0.70

η 30.0 47.0 57.4 63.0 70.0

Table

Table 3. The values of Kads and 'Gqads for different inhibitors in presence of dilute, 0.0001 M Cl-, and high, 0.05 M Cl- concentrations, at 25°C. R2

Inhibitor 0.0001 M

Kads, Lmol-1 0.05 M

0.0001M 0.05 M

-'Gqads, kJmol-1 0.0001 M 0.05 M

5-Chloro-BTA

0.999

0.999

252720

66500

40.78

37.47

BTA

0.999

0.999

87110

36000

38.14

35.95

5-Nitro-BTA

0.999

0.998

39390

7840

36.17

32.17

1

Highlights - The rate of oxide film repair follows a direct logarithmic law. -The inhibition efficiency depend on Cl- and the organic additives concentrations. - Benzotriazoles inhibit pitting corrosion by way of adsorption. -ΔG°ads values for organic additives reveal a chemisorption mechanism.

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