The effect of thiosemicarbazide on corrosion resistance of steel reinforcement in concrete

Construction and Building

MATERIALS

Construction and Building Materials 21 (2007) 669–676

www.elsevier.com/locate/conbuildmat

The effect of thiosemicarbazide on corrosion resistance of steel reinforcement in concrete A. Ali Gu¨rten b

a,*

, Kadriye Kayakırılmaz a, Mehmet Erbil

b

a Department of Chemistry, Faculty of Science and Art, Nig˘de University, 51200 Nig˘de, Turkey Department of Chemistry, Faculty of Science and Art, C¸ukurova University, 01330 Adana, Turkey

Received 4 March 2005; received in revised form 21 November 2005; accepted 10 December 2005 Available online 31 January 2006

Abstract This study describes a laboratory investigation of the influence of thiosemicarbazide (TSC) on the corrosion of reinforcing steel and the compressive strength of concrete. The effect of TSC on the corrosion resistance of steel reinforced concrete was evaluated by carrying out electrochemical tests in NaCl and NaCl + TSC solutions for 60 days. Polarisation resistance (Rp) values of TSC added reinforced concrete were much higher than those without TSC. Similarly, AC impedance spectra revealed that the resistance of TSC mixed electrodes were also quite higher than those without. The compressive strength of concrete specimens containing TSC was measured and an increase of 20–25% was observed.  2005 Elsevier Ltd. All rights reserved. Keywords: Reinforcing steel; Corrosion; Thiosemicarbazide; AC impedance

1. Introduction It is generally agreed that embedded steel rebars in concrete are initially immune to corrosion because of the alkaline environment provided by the surrounding concrete. Corrosion occurs by the loss of alkalinity of concrete in the form of carbonates and thus causes cracks in the concrete that provides a direct route for chlorides to approach the reinforcing steel and prevent re-passivation reaction that leads to pitting corrosion. The accumulation of corrosion products such as iron oxides and hydroxides causes an internal stress within the concrete [1–7]. Various methods have been applied to protect corrosion of reinforced steel; these methods include variation of the concrete formulation, cathodic protection, surface treatment of rebars (e.g., epoxy coating), and addition of inhibitors. Corrosion inhibitors are chemical compounds that are widely used in

*

Corresponding author. Tel.: +90 388 225 2090; fax: +90 388 225 0180. E-mail address: [email protected] (A.A. Gu¨rten).

0950-0618/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.12.010

corrosion technology to protect structures from deterioration and offer an effective, reliable solution when concrete is exposed to environment containing chloride. They have no adverse effect on the properties of the fresh and hardened concrete admixtures and prevent corrosion of the embedded steel when added in an adequate amount [8]. Corrosion inhibitors are either added to concrete during its preparation or penetrated into the hardened concrete surface. Therefore, corrosion inhibitors offer protection by forming a protective film at the steel surface and reducing the ingress of aggressive ions into the concrete matrix [9,10]. The inhibition efficiency of homologous series of organic substances, containing different heteroatoms, are in the following sequence; P > Se > S > N > O. Some amides and derivatives such as urea (U), thiourea (TU), thioacetamide (TA) and thiosemicarbazide (TSC) have been found to be potential inhibitors. A mixture of nitrogen and sulphur compounds is often found to be better than either type alone. Several nitrogen and sulphur containing compounds have been reported to be effective inhibitors for different metals and the relationship between

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the amide molecular structures and their inhibition efficiencies have been studied in several research works. Aminoalcohols such as ethanolamine, dimethylethanolamine N,N-dimethylethanolamine can also be used for the protection of rebar in concrete in commercial corrosion inhibitors [11–14]. The objective of this paper is to study the effectiveness of thiosemicarbazide (CH5N3S), the simple molecule containing both nitrogen and sulphur atoms in its structure, in providing corrosion protection to reinforcing steel in 0.1 M NaCl solution. The effect of the inhibitor on compressive strength of concrete was also determined. The corrosion resistance was evaluated by measuring corrosion potentials, resistance to linear polarisation and AC impedance spectroscopy. 2. Experimental 2.1. Materials and specimen preparation procedures Fig. 1. Working electrode (test electrode).

The composition of the concrete is given in Table 1. Saturated dry surface density of aggregate was 2.59 · 103 kg m3 and their size ranged from 0.5–4 to 4–8 mm. Cylindrical reinforced concrete specimens of 70 mm long and 40 mm in diameter with embedded reinforcing steel bar were prepared for electrochemical studies and were connected by means of copper wires (Fig. 1). External solution and mixing water compositions are shown in Table 2. The water/cement (w/c) ratio was 0.5. Steel bars used for reinforcement had the chemical composition of 0.13% C, 0.65% Mn, 0.030% S, 0.013% P and 0.017% Si. Fifty millimeters long test electrodes with 17.27 cm2 contact area and 10 mm diameter were prepared by coating with epoxy paint and were mechanically polished by using 600–1200 grades wet SiC papers. The electrodes were post-cured for 24 h and then were placed in the external solution. Concrete specimens were divided into two groups. The first series was prepared using distilled water (specimen A–C) whereas the second was mixed with chloride solution (specimen A 0 – C 0 ). The concrete specimens were placed in 0.1 M NaCl solution. TSC inhibitor (Merck GR grade) was added to the mixing water in the specimen B and B 0 , and in the exposure solution of specimen C and C 0 . Detailed information about the external and mixing water environments are given in Table 2. Three types of tests namely; corrosion potential, polarisation resistance and compressive strength were carried out on three individual samples from each specimen.

Table 1 Composition of the concrete

Water Cement Aggregate Air

Mass (kg)

Density (kg/dm3)

Volume (dm3)

180.00 360.00 1731.10 –

1.00 2.96 2.59 –

180.00 120.96 668.38 30.00

Table 2 External solution and mixing water environmentsa Specimens

External solution

Mixing solution

0 A B

Distilled water 0.1 M NaCl 0.1 M NaCl

C

0.1 M NaCl + 1 · 103 M TSC 0.1 M NaCl 0.1 M NaCl

Distilled Distilled Distilled M TSC Distilled

A0 B0 C0 a

0.1 M NaCl + 1 · 103 M TSC

water water water + 1 · 103 water

0.1 M NaCl 0.1 M NaCl + 1 · 103 M TSC 0.1 M NaCl

Values are based on triplicate analysis.

2.2. Test methods 2.2.1. Linear polarisation resistance (LPR) The linear polarisation resistance, Rp, can be determined from the slope of the plot of applied potential against measured current. Steel was polarised to single cycle for each measurement to ±20 mV of the corrosion potential at a scan rate of 1 mV/s utilising EG&G Model 360 potentiostat/galvanostat with a wisidaq computer program that analyses the Rp parameters as kX cm2. Corrosion state of the steel bar in each specimen was studied by using the three electrode techniques. A saturated calomel electrode (SCE), platinum foil was used as reference and an auxiliary electrode, respectively [15–17]. Measurements were carried out weekly for a period of 60 days. 2.2.2. AC impedance spectroscopy Impedance measurements were carried out at the open circuit potential (Eoc), using a computer-controlled poten-

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tiostat and frequency response analyser and the three-electrode assembly. Prior to the impedance measurement, a stabilisation period of 30 min was allowed, which proved to be sufficient to attain a stable value for Eoc. The measurements carried out with applied sinusoidal potential waves of 5 mV amplitudes around corrosion potential, with frequencies ranging from 105 to 101 Hz. The measurements were carried out on the 1, 2, 7, 28 and 60th day of the curing period [18,19]. 2.2.3. Corrosion potential measurements The detection of corrosion by using potential measurements, provide information on the time to initiation of reinforcement corrosion under laboratory conditions, is one of the most typical procedures for the routine inspection of reinforced concrete structures. Corrosion of the specimens was monitored by measuring the corrosion potentials and recorded at regular intervals, until equilibrium conditions were established, using multimeter versus a SCE for a period of 60 days [20–22]. 2.2.4. Compressive strength In compressive experiments, cubic shaped concrete specimens (15 cm · 15 cm · 15 cm) were used. The compressive strength was determined after curing for 28 days in external solution (0.1 M NaCl, 0.1 M NaCl + 103 M TSC or distilled water) by using Tonitecnic compressive testing machine (Table 2). The average compressive strength of the three concrete cubes was determined by loading at a constant rate of 4–5 kN/s using a hydraulically operated digital compression machine of 3000 kN capacity according to ASTM C 39. The degree of deterioration was also evaluated by measuring the reduction in compressive strength [8]. The percentage reduction in compressive strength was calculated according to the equation below: % ¼ ½ðA  BÞ=A  100; where A is the average compressive strength of three specimens cured under distilled water, MPa; and B is the average compressive strength of three specimens exposed to the test solutions, MPa. 3. Results and discussions 3.1. Corrosion potential (Ecor) of embedded steel The effects of TSC addition on the corrosion behaviour of rebar in the concrete were monitored and the results are shown in Fig. 2(a) and (b). Ecor for the specimens in all media have varied from 0.202 to 0.638 V (versus SCE) for a period of 60 days (Fig. 2). These values are the averages of three measurements and the standard deviations were found to be in the range 0.023–0.036 for n = 3. Ecor values in the concrete specimen without corrosion inhibitor (specimen A) was more negative than the corrosion potentials of the steel in the concrete specimen prepared with TSC (specimen B) and less negative than the

Fig. 2. The variation of the corrosion potentials with time.

specimen prepared with distilled water and which was cured in 0.1 M NaCl containing 103 M TSC external solution (specimen C) as indicated in Fig. 2(a). Until the 35th day, corrosion potentials on steel bars in the concrete specimen prepared with 0.1 M NaCl and which was cured in 0.1 M NaCl external solution (specimen A 0 ) have showed small variation which has implied that the curves of Ecor of rebar are slightly more negative than the potentials of the concrete specimen prepared with 0.1 M NaCl incorporating TSC (specimen B 0 ) and less negative than the specimen prepared with 0.1 M NaCl, cured in 0.1 M NaCl containing 103 M TSC external solution (specimen C 0 ) (Fig. 2(b)). After the 35th day, the Ecor measured for specimen A 0 was more positive than that of specimen C 0 . On the average, the Ecor of the rebar in specimen A was 0.200 V more negative than that of rebar in specimen B as is shown in Fig. 2(a). This indicated that the rebar embedded in the concrete containing TSC (specimen B) had become more passive than the rebar in concrete without TSC (specimen A). The Ecor of the rebar in specimen A 0 was 0.080 V more negative than that the rebar in specimen B 0 (Fig. 2(b)). According to literature, it is reported that probability of corrosion would be greater than 95% if the observed corrosion potential is more negative than

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0.270 V/SCE and corrosion potential falls well below 0.320 V/SCE, indicating the initiation of corrosion [23– 26]. Hence, while the addition of TSC in concrete has reduced the corrosion probability but not into the distilled water as external solution (Fig. 2(a)). On the other hand, the addition of TSC in chloride solution could not be effective on corrosion resistance of rebar in concrete (Fig. 2(b)). Specimen A–C that contained less chloride ions than specimen A 0 –C 0 were found to be less corrosive just as expected. Chloride ion has the capability of diffusing through the concrete matrix and depassivating the film on the reinforced steel’s surface. The corrosion potential provides qualitative and probably indicates reinforcement corrosion. Quantitative and reliable information on reinforcement corrosion can be obtained by measuring the resistance or current density [8]. 3.2. Linear polarisation methods Fig. 3 shows the variation of the polarisation resistance versus exposure time (day) for the steel-reinforced concrete. It is very well known that when polarisation resistance

increases, corrosion rate decreases. The decreasing resistance with time must be due to anodic dissolution of the embedded steel surface [27]. During the 60 days period, polarisation resistance values have showed in all media, a very small variation during the first 20 days, followed by an increase to 113 kX cm2 as is shown in Fig. 3. These values are the averages of three measurements and the standard deviations were found to be in the range 4.8–9.6 for n = 3. The Rp values of specimen A–C showed that the addition of TSC has not influenced greatly the corrosion resistance during the first 20 days. Therefore, the values of specimens B and C were higher than that of specimen A (Fig. 3(a)). The polarisation resistance value for specimen A 0 was smaller than the values of specimens B 0 and C 0 (Fig. 3(b)). Polarisation resistance technique is used to determine corrosion rates of steel in concrete and is directly related to the corrosion rate. Thus, The reason for the Rp values of specimen A being lower than specimen B and C is probably due to the physical barrier effect of TSC on process [28]. Higher Rp values obtained in specimen B 0 and C 0 compared to that of specimen A 0 has showed a decrease in the corrosion of rebar (Fig. 3(b)). 3.3. AC impedance method

Fig. 3. The variation of polarisation resistance (Rp) with time.

The impedance measurements on steel bar in the concrete samples were carried out using platinum electrode as a counter electrode, SCE as a reference electrode and the steel bar embedded concrete as the working electrode (frequency range 0.1–105 Hz). Fig. 4(a)–(e) shows impedance diagrams (as Nyquist diagrams) of steel in all concrete specimens. As shown in Fig. 4(a), during the first day, the diagram for the specimens has exhibited a semicircular change which is Rs + Rct until the real impedance value has reached 36 X (21.93 kHz) during the first day in Table 3. This region, which is known as semicircle, can attain only one section because the surface structure of the steel is not disturbed at the applied maximum frequency. The solution resistance (Rs) can be obtained by extrapolating the semicircle curve to the real impedance axis (Z 0 /X). The region of the real impedance and the imaginary impedance, which increases continuously, is known as Warburg impedance. This resistance that is due to coating, accumulating of surface and concrete, causes shielding effect or resistance effect for the metal corrosion [29–32]. Charge transfer resistance is the rate determining resistance at the reinforcing steel corrosion. During the first day, while Rs + Rct value for specimen A which was made of steel embedded in concrete containing only distilled water as a mixing solution and 0.1 M NaCl as external solution, was 36 X, it has increased to 70 X for specimens B and C. However, the resistance of the specimen A 0 containing 0.1 M NaCl as mixing solution, was measured as 38 X, when 0.1 M NaCl solution was used, this value has increased to 44 X and 68 X for specimens B 0 and C 0 , respectively. These values indicate that specimen A 0 had already suffered from severe damage on its passive layer and that the addi-

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Fig. 4. Impedance spectra of reinforcing steel [time (day) (a) 1 (b) 2 (c) 7 (d) 28 (e) 60]. In all of the diagrams, the maximum frequency in 105 Hz and the minimum frequency values that correspond to Warburg region are given in Table 3. The minimum frequency values that belong to Warburg region are given within the diagrams. Explanations of legends are given in Table 2.

tion of TSC to the concrete, has formed a protective layer on the steel and concrete surface. However, when the electrodes (specimen A–C 0 ) were kept in the external solution, for the longer periods the resistance values have increased gradually. These increments in the resistance are due to

the diffuse layer effect, and the Rd resistance addition to the Rct + Rs. After 60 days, the values of Rs + Rct + Rd have showed an increment from 70 to 600 X for specimen B and from 70 to 540 X for specimen C. Electrodes containing chloride and TSC in their mixing and external

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Table 3 Measured Rct, Rs and frequencies values determined from Nyquist plots Specimen

Time (day)

Rs + Rct (X)

Frequency (kHz)

Specimen

Time (day)

Rs + Rct (X)

Frequency (kHz)

0

1 2 7 28 60

38 50 62 95 120

26.04 36.76 0.69 0.96 0.00012

A

1 2 7 28 60

36 46 60 74 85

21.93 36.76 9.62 0.96 0.69

A

B

1 2 7 28 60

70 100 170 80 600

4.63 0.96 21.93 0.96 1.23

B0

1 2 7 28 60

44 70 90 150 220

8.33 0.96 1.51 1.81 26.04

C

1 2 7 28 60

70 95 160 280 540

0.96 0.69 1.81 81.05 0.96

C0

1 2 7 28 60

68 95 180 310 640

0.96 0.96 1.81 96.15 1.17

solutions (specimen B 0 and C 0 ) have showed similar increment in resistances as 44–220 X and 68–640 X, respectively. It is worth mentioning here that in all media, Warburg impedance region has showed similar changes. Rct and Cdl that show resistance and capacitance and also Rcon, concrete resistance, Ccon, concrete capacitance is given within the equivalent circuit in Fig. 5. The rebar present in reinforced concrete has a pore structure where the capillary channels are in contact with external medium. When these capillary channels approach the metal ends, electrical double layer forms thus allowing charge transfer in this layer. Charge transfer resistance and double layer capacitance at equivalent circuit are shown as Rct and Cdl in Fig. 5. The outside concrete resistance (Rcon) and the proposed capacitance for the concrete (Ccon) were fixed onto the circuit. The outermost double layer formed within the capillary channels and the solution resistance (Rs) that arose between the outermost of the concrete and the potential point measurement is also shown in Fig. 5. According to the equivalent circuit; high frequency region of Nyquist diagram shows the charge transfer resistance and diffuse layer resistance. However, increasing imaginary impedance in the lower frequency region is also considered as Warburg impedance (Fig. 4). These imped-

ances are evaluated as a total resistances produced from both the concrete and corrosion products. At low frequency region (Warburg region), TSC influence was not observed and thus this can be attributed to changes in Rs + Rct by chemical adsorption effect. However, a weak influence of TSC indicates that a physical adsorption is present and thus the effect of TSC is considered to be in the form of increasing the diffuse layer resistance. Oxygen diffusion from the outer medium to the metal surface occurs through the capillary channels and pores in concrete. Since additives have blocked these capillary channels and pores, diffusion becomes more difficult and diffuses layer resistance increases [24]. The increase of the resistances in Nyquist diagrams and the decrease in shift of Warburg region to low frequencies supports this observation very well. Since the capillary channels in the concrete are not blocked, it is easier for the additive (TSC) to pass through and reach the metal surface from the external solution (solution without TSC) in chloride medium. When TSC in added to the mixing solution, molecules losses their mobility and thus the effect will be much less than that in the external solution. The resistance values of the specimens with TSC in the mixing water and those with TSC in the exposure solution are given in detail in Table 3. 3.4. Compressive strength

Fig. 5. Equivalent circuit for concrete (Rs, solution resistance; Rcon, concrete resistance; Rct, charge transfer resistance; Ccon, concrete capacitance; Cdl, diffuse layer capacitance).

The compressive strength values of the specimens that have been cured for 28 day are given in Table 4. The value of the blank specimen (both mixing and external solution in distilled water) was 19.4 MPa. Whereas, the values of the A–C 0 specimens were varied from 20.8 to 26.1 MPa and the concrete without additives was found to be the least resistible. Compressive strength of the concrete containing distilled water as mixing and external solution was observed at 19.4 MPa (specimen 0), while the sample containing distilled water only as mixing

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Table 4 Compressive strength of the concrete specimens at 28 daysa

Acknowledgement

Test series

Compressive strength (MPa)

Specimen Specimen Specimen Specimen Specimen Specimen Specimen

19.4 20.8 25.1 23.8 24.5 26.1 24.6

The authors thank the University of Nig˘de for supporting this research (NUAF FEB 2000/013).

0 A B C A0 B0 C0

a

Values are based on triplicate analysis with a maximum standard deviation of 0.5.

and chloride as external solution was found to be 20.8 MPa (specimen A). However the most resistible concrete was the one containing chloride with TSC as mixing solution (specimen B 0 ). Admixed chlorides to mortar tends to react with the cement aluminates during the hydration of cement thus, allowing a quicker ingress of chlorides that comes from the environment once the concrete has hardened [8,24]. These results show that there was an increase of 20–25% in the strength of concrete by adding TSC. 4. Conclusion The effect of thiosemicarbazide on the corrosion of steel embedded in concrete was examined and the addition of TSC to the mixing or external solution was found to have increased the Rct + Rs + Rd resistance and have minimised the corrosion of the rebar in concrete. However, the specimen that contained distilled water only as mixing solution and TSC as mixing or external solution, have showed similar increase in resistance. The corrosion of the rebar in concrete containing chloride was found to be inactive even when 0.1 M NaCl with TSC were used as external solution. The corrosion tendency is strongly dependent upon the corrosion potential and polarisation resistance. Polarisation resistance values indicated that addition of TSC to the mixing or external solution had significantly increased the values of Rp from 40% to 70%. While the addition of distilled water containing TSC to the concrete has greatly reduced the corrosion rate of steel, since chloride ion has the capability of diffusing through the concrete matrix and depassivating the film on the reinforced steel surface. Whereas, the addition of TSC containing chloride have affected weakly the corrosion resistance of rebar in concrete. Corrosion potential values indicated that although the addition of TSC to concrete prepared by distilled water was effective in delaying the initiation of reinforcement corrosion, the addition of TSC with chloride couldnot be as distinctly effective on corrosion resistance of rebar in concrete. Meanwhile, the increase in the strength varied from 20% to 25% in the concrete specimens admixed with TSC together with chloride ion. This study shows that TSC did not adversely affect the compressive strength of concrete.

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