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Characterization of stainless steel corrosion processes in mortar using various monitoring techniques
Construction and Building Materials 221 (2019) 604–613
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Characterization of stainless steel corrosion processes in mortar using various monitoring techniques Miha Hren, Tadeja Kosec ⇑, Andrazˇ Legat Slovenian National Building and Civil Engineering Institute, Dimicˇeva 12, 1000 Ljubljana, Slovenia
h i g h l i g h t s Stainless steel corrosion in concrete was monitored during 2-year period. 5 monitoring (physical, electrochemical) techniques were used and evaluated. The damages were estimated and correlated to corrosion currents measured. Limitations and advantages of each monitoring technique were assessed.
a r t i c l e
i n f o
Article history: Received 21 November 2018 Received in revised form 28 March 2019 Accepted 14 June 2019
Keywords: Stainless steel Corrosion Concrete Mortar Chloride induced corrosion Corrosion monitoring techniques
a b s t r a c t Monitoring of the corrosion on stainless steel type AISI 304 in highly porous mortar specimens was performed during a period of 2 years. The carbonated mortar specimens were cyclically wetted by a 3.5% NaCl solution, with drying periods in between. Five different monitoring techniques were applied: the galvanostatic pulse, the coupling current, the use of coupled multi-electrode arrays and electrical resistance probes and electrochemical impedance spectroscopy. At the end of the exposure period, comparisons were made between the results obtained by using these different techniques, as well as by taking into account actually observed damage by spectroscopic techniques. The advantages and limitations of each of the applied monitoring techniques were assessed. Ó 2019 Published by Elsevier Ltd.
1. Introduction Concrete cannot adequately protect the steel reinforcement from different types of corrosion processes. However, at the same time, phenomena such as carbonation, chloride penetration, and hydration, can continually change the concrete’s properties, which, in turn, can affect the corrosion behaviour of the embedded steel [1]. Many measures exist which can be used to reduce the extent of corrosion of steel in concrete. They range from proper concrete mix design and adequate rebar cover thickness, to the use of supplementary cementitious materials, inhibitors, coatings, and cathodic protection systems [2,3]. In more aggressive corrosion environments with high concentrations of chlorides, using stainless steel reinforcement bars can be a viable way of increasing protection against corrosion. Since the use of stainless steel generally causes higher upfront costs, this viability increases with increasing ⇑ Corresponding author. E-mail address: [email protected] (T. Kosec). https://doi.org/10.1016/j.conbuildmat.2019.06.120 0950-0618/Ó 2019 Published by Elsevier Ltd.
service life. The two best-known field examples of the use of stainless steel in concrete are a pier in Progreso, Mexico [4], and precast cladding panels in Wellington, New Zealand [5]. Both sites are located in marine or severe marine environments, and in both cases the stainless steel rebars have remained intact, whereas carbon steel bars in the same area show severe corrosion damage. When assessing the level of corrosion of steel in concrete or mortar, many parameters can be monitored using different techniques. Visual examinations can provide useful information about the state of steel based on concrete surface defects or accumulated corrosion products, whereas phenolphthalein can indicate carbonation depth, and the laboratory investigation of samples can provide data about chloride content and concrete permeability [6]. Visual examination has been used in a more scientific manner for the evaluation of corrosion damage to stainless steel in concrete at two test sites [7,8]. Both experiments lasted more than 10 years, and demonstrated the very good corrosion resistance of austenitic stainless steels in marine environments, where ferritic steels were unable to provide sufficient protection against corrosion.
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Commonly used electrochemical techniques include potentiodynamic techniques. Short-term laboratory potentiodynamic experiments are usually performed in simulated concrete pore solutions using standard three-compartment electrochemical cells [9–17], although sometimes concrete or mortar has been used [18,19]. The techniques are used to investigate corrosion potential and corrosion rate, pitting potential, Tafel slopes, and the ability of stainless steel to repassivate after pitting has begun. Another common technique involves half-cell potential measurements, or the more advanced galvanostatic pulse measurement technique, which usually results in a potential map across the concrete surface. This non-destructive technique is fast and therefore useful for on-site investigations [20,21], but it has been known to overestimate corrosion rates [22]. Another simple method for the measurement of corrosion rates involves the use of two coupled electrodes, where the potential drop across a resistor is measured. A more advanced technique involves the use of zero resistance ammeters to measure coupled currents across an array of electrodes [22–24]. This method provides both spatial and temporal information about the corrosion process. Electrochemical impedance spectroscopy, too, has been used in applications involving concrete [25]. This AC technique can provide additional information about all the layers present in a corroding system, including passive films, corrosion products, and concrete. As with potentiodynamic techniques, stainless steel has been investigated both in concrete pore solutions [10–12,26,27], and in mortar or concrete specimens [19,28–30]. Embedded sensors can also be used for the monitoring of corrosion activity in concrete structures [23,31], but their use in stainless steel applications is rare. Based on our previous studies [32] and the results of our specific experiments in simulated aqueous solutions, it was expected that the corrosion damage occurring to stainless steel should, despite aggressive corrosion environment, be minor. The goal of this study was to monitor and characterize the corrosion of stainless steel in concrete. The whole study lasted for 2 years, during which stainless steel bars embedded in different mortar specimens were periodically wetted with 3.5% NaCl, and dried in between. Five different electrochemical and physical techniques were used for corrosion monitoring. Special emphasis was placed on the evolution of corrosion processes over time. X-ray micro CT, SEM, and optical microscopy were used to validate the results obtained by the monitoring techniques. The extent of corrosion damage occur-
ring to the stainless steel was investigated by comparing measured corrosion rates, in combination with observations made by microscopical analysis. The advantages and limitations of the investigated corrosion monitoring techniques were identified and evaluated. 2. Material and methods 2.1. Preparation of the test specimens Four different types of mortar test specimens were prepared in order to investigate the corrosion behaviour of stainless steel in concrete (see Fig. 1). All the test specimens were made from CEM I 52.5 R cement with a water/binder ratio of 0.75 in order to reduce the corrosion initiation time. One part of cement and four parts of sand were used to make the mortar, according to the standard EN 196-1. All the working electrodes were made of AISI 304 steel, whose chemical composition is presented in Table 1. The thickness of the mortar cover was 5 mm. All the stainless steel rods had a diameter of 5 mm, and were in contact with the mortar over a length of 100 mm. Before installation, they were grinded with a 500 grit paper, and degreased with acetone. The specimens for the galvanostatic pulse measurements consisted of 100 30 30 mm mortar prisms (see Fig. 1a), with a single rod that was embedded 5 mm from the top (a total of 4 such specimens were prepared). The electrochemical impedance spectroscopy and galvanostatic pulse specimens (2 of each) were mortar prisms of the same dimensions into which two identical smooth rods were embedded, 5 mm and 20 mm from the top, respectively (Fig. 1b). These specimens were protected by an epoxy coating which was applied to all four sides and to the bottom of the specimen in order to limit carbonation of the mortar to the top steel bar only. The specimens for EIS were equipped with a 100 X resistor in order to measure the voltage drop occurring between the top and bottom bars. The two specimens of the third type had dimensions of 120 80 30 mm, and included an electrical resistance sensor (Fig. 1c). The fourth type of specimen had an embedded coupled multi-electrode array of sensors consisting of 25 stainless steel wires, each with a diameter of 0.8 mm, which were arranged in a 5 5 matrix in epoxy resin (Fig. 1d). A 5 mm thick layer of mortar was applied on top of the electrodes.
Fig. 1. The four different types of test specimens which were used in the experiment: (a) a typical specimen for the galvanostatic pulse measurements (4 specimens), (b) a typical specimen for electrochemical impedance spectroscopy (2 specimens) and for the coupling current measurements (2 specimens), (c) a typical specimen with an electrical resistance sensor (2 specimens), and (d) the specimen with a coupled multi-electrode array of sensors (1 specimen).
Table 1 Chemical composition of the stainless steel used for the Ø5 mm rods (%). C
Si
Mn
P
S
Cr
Mo
Ni
Co
Cu
N
Fe
0.039
0.412
1.46
0.066
0.033
18.51
0.331
7.81
0.157
0.512
0.084
70.4
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2.2. The period of exposure The mortar was cured for 28 days at a relative humidity of 100%. All the specimens were subsequently exposed to accelerated carbonation at 4% CO2 and 55% relative humidity. Accelerated carbonation was stopped once the top steel bar was completely covered by carbonated mortar. A carbonation depth, which was monitored by fenolphtalein indicator, of 11 mm was reached in 14 days. Cyclic exposure with 3.5% NaCl was performed once per week, with a two-day wetting period followed by a five day drying period at room temperature and controlled humidity at 60%. The entire exposure period lasted for 2 years (120 weeks) in the case of all the specimens except the coupled multi-electrode array, which had a shorter exposure period due to limited availability of the measuring equipment.
2.3. The used monitoring techniques The galvanostatic pulse (GP) measurements, electrochemical impedance spectroscopy, and electrical resistance measurements were performed once per week at the end of each wetting period, whereas the coupled multi-electrode array and the coupling current specimens were monitored continuously. This measurement method is a linear polarization technique, where a constant current is applied to a working electrode and the potential response is measured over a period of time [6,21]. In this experiment, a 50 mA current was applied for 10 s using a Force Galvapulse device. Based on the potential response, measured against an Ag/AgCl reference electrode, both the ohmic and the polarization resistance were determined. The latter was used, along with the SternGeary equation, to estimate the corrosion current density. A value of B = 52 mV was used [33]. All the corrosion rates were calculated using Eq. (1), which is derived from Faraday’s law of electrolysis, where a current density i is the input in nA/cm2. The following values and units were used as constants: atomic mass a = 55.1 g/mol, density q = 7.9 g/cm3, and the number of electrons lost per oxidized atom n = 2.
CR ¼
3:27 a i ½nm=year nq
ð1Þ
In the simple coupling current technique two identical electrodes are connected together across a resistor. A voltmeter is used to measure the voltage drop and Ohm’s law is used to calculate the corrosion current. In this study, the two embedded steel rods acted as electrodes. One was located 5 mm from the exposed surface, whereas the other 20 mm was beneath it. The technique cannot detect corrosion currents when matching anodic and cathodic sites are on the same electrode. The use of an electrical resistance sensor is a physical technique where the reduction in the thickness of an electrode results in increased resistance. The electrodes are produced in the form of thin and long wires which form the resistor components of a Wheatstone bridge. A constant current is passed through the circuit, and the voltage drops on the resistors are measured over time. ER probes were described in greater detail in the authors’ previous work [23]. Coupled multi-electrode arrays (CMEA) are used to measure the corrosion current between coupled multiple electrodes. In the given case the measurements were performed using a customdesigned zero resistance ammeter, where the resistors and power supply used allowed a maximum current of 50 mA to be detected. The frequency of data acquisition was set to either 1 Hz or 30 Hz, depending on the time-frame. Before connecting up the specimen, the internal device offsets for each ammeter were measured.
Electrochemical impedance spectroscopy (EIS) is an AC technique in which a potential is applied at different frequencies, and the current response is measured. In the case of this experiment, Gamry Reference 600 equipment was used to apply a 10 mV potential, with frequencies ranging from 100 kHz to 5 mHz, and 10 measurements per decade. The top rebar was used as a working electrode, whereas the bottom stainless-steel electrode was used as both a counter electrode and a reference electrode. 2.4. Post exposure examinations After exposure, the specimens were examined in detail using different microscopic techniques. Before the specimens were dismantled, they were 3D scanned using an Xradia MicroXCT-400 X-ray imaging system (Xradia, USA, 2010). The obtained microtomography images were processed and inspected for possible corrosion damage. After selected specimens had been dismantled, optical microscopy images (Tagarno HD Trend, 2016) of the electrodes were taken before and after they had been cleaned with a hydrochloric acid solution in order to remove the corrosion products. The cleaned specimens were then inspected by means of JEOL 5500 SEM to determine the extent and type of corrosion damage. 3. Results Four different monitoring techniques were used to characterize the behaviour of stainless steel in mortar samples over a 2-year exposure period. 3.1. Galvanostatic pulse measurements The results of these measurements, expressed as the measured corrosion current density over time, are presented in Fig. 2. All the specimens with just one embedded rod showed similar current values and similar trends (Fig. 2a). At the start of the exposure period, the corrosion currents were between 70 and 90 nA/cm2. Over the first 60 weeks, this range of values deceased to a range of between 40 and 60 nA/cm2. From 60 weeks on, the average current density values stabilised between 35 and 55 nA/cm2. The average corrosion rate during the entire exposure period was 0.6 mm/year. A similar trend with slightly higher current values can be observed in the case of the top bars in the specimens with two rebars (Fig. 2b). The current started at 100 nA/cm2 and the dropped to 70 nA/cm2 over the exposure period. For both of the bottom bars, the corrosion current density values varied between 150 and 200 nA/cm2 throughout the exposure to chlorides. This translates to roughly 1.7 and 2.3 mm/year, which is twice as high as in the case of the top bars. Here it should again be mentioned that the top bars were in carbonated mortar and had a 5 mm thick mortar cover, whereas the bottom bars were in non-carbonated mortar and had a 20 mm thick concrete cover. The results of optical observation showed only minor changes on the surface of the stainless steel rods (see the inset in Fig. 2a). Neither tomography nor the SEM images could confirm the occurrence of any corrosion damage (not shown). 3.2. Coupling current measurements The measured corrosion current density between the two stainless steel rods in the mortar specimens during the 2-year exposure period is presented in Fig. 3. A positive current indicates that the bottom electrode is anodic, whereas the top electrode is cathodic. In the case of a negative current, the opposite is true. At the beginning of the exposure period, it was found that that the bottom electrodes of both of the mortar specimens were more anodic, despite
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Fig. 2. Corrosion current density values as determined by the use of the galvanostatic pulse technique for: (a) the mortar specimens with one embedded rebar, and (b) the mortar specimens with two embedded rebars.
Fig. 3. Results of the coupling current measurements performed on the two mortar specimens.
the fact that the top electrode was located in carbonised mortar and exposed to faster chloride and oxygen migration. Since the measured current was very low, it was strongly affected by noise due to ambient temperature changes. The range of measured currents thus varied from 2 nA/cm2 to 14 nA/cm2. This translates to a corrosion rate of between 20 nm/year and 150 nm/year. After about 30 weeks, the corrosion current decreased and stabilised. At this time the average current density for the two specimens was 3 nA/cm2, and 1 nA/cm2. Throughout the exposure period, the start or the end of the wetting period did not have any short term effects on the results of the measurements. The corrosion activity detected by this technique did not translate into any visible damage which could be detected by microscopic techniques. 3.3. The coupled multi-electrode array measurements The entire exposure period of the coupled multi electrode array is presented in Fig. 4a, with the anodic current density displayed below (Fig. 4b), as well as details of the electrode activity during a typical wetting period (Fig. 4c and d). The positive currents indicate anodic activity, whereas the negative currents indicate cathodic behaviour. Each coloured line represents one electrode and its colour determines its location in the matrix (see the inset in Fig. 4a). The light-blue coloured sections represent the wetting intervals. The results are divided into three sections based on the
collected data (Fig. 4a). During the dormant periods the specimen was not exposed to cyclic wetting. The first and last sections show a significant increase in noise during the wetting periods. This indicates which electrodes started showing corrosion activity, and when. There was no indication of either of the electrodes becoming predominantly cathodic or anodic. Due to the high noise levels, the data acquisition sampling rate in the middle section was increased to 30 Hz and averaging was used to reduce the noise. The graphs presented in Fig. 4a show the anodic and cathodic activities of the 25 stainless steel electrodes during the 66 weeks of exposure. A more detailed view of one exposure cycle is presented in Fig. 4c. Electrode 4B showed the highest anodic activity during all the observed cycles, which is also indicated by the redcoloured electrodes shown in Fig. 4d and e. Fig. 4e shows the total anodic activity of the electrodes during the 2-year exposure period. Electrodes 2D and 3D were also anodic across multiple cycles, and, as the exposure continued, other electrodes in the centre became more anodic compared with electrode 4B. As the wetting cycles continued, some of the electrodes started to drift slowly towards the positive anodic side, and remained there even during the drying part of the cycles. The green-coloured electrodes, on the other hand, represent cathodic activity (see Fig. 4d). Most of the cathodic reactions occurred in the lower right section of the matrix, where electrode 5D was the most cathodic. It can be observed that, in most cycles, cathodic and anodic reactions occurred on different
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Fig. 4. (a) Results of the coupled multi-electrode array measurements for the entire exposure period, (b) the average anodic currents for the middle section, (c) a detailed wetting period during week 45, (d) average anodic (red) and cathodic (green) activity during week 45, and (e) the average anodic current density per electrode during the whole exposure period. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrodes, and these sites remained the same throughout the exposure period. Most of the electrodes remained either anodic or cathodic throughout individual cycles (Fig. 4c). The graph shown in Fig. 4b shows the total anodic activity as the sum of all the anodic activities. It can be directly correlated to the corrosion rate (Fig. 4b). Fig. 5a shows the total anodic activity as the sum of anodic events. These can be directly correlated to the instantaneous corro-
sion rates, which, during wetting periods, can be as high as 3.5 mm/ year. However, the average corrosion current density, presented in Fig. 5b can be rather low. Fig. 6 shows the most anodic electrode 4B throughout several cycles after exposure. SEM observations indicated three pitting corrosion locations on the electrode (Fig. 6b), whereas optical microscopy and microtomography did not show any corrosion damage (Fig. 6a and c). This was expected, since, during the
Fig. 5. (a) The total anodic activity measured on all the electrodes over time, and (b) the average anodic and cathodic activity per electrode.
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Fig. 6. The state of electrode 4B (Fig. 4): (a) after cleaning, (b) as a SEM image, and (c) tomography reconstruction. The scale applies to the 2D images only.
wetting cycles, the corrosion rates reached values of around 0.1 mm/year after noise had been eliminated. Sub-micron resolution is outside the scope of both of these measurement techniques. 3.4. Measurements with electrical resistance sensors Two identical specimens with embedded ER sensors were monitored over a total time-span of 120 weeks, as shown in Fig. 7. In both cases the initial thickness of 100 mm started decreasing at a slow rate during the first 20 weeks, and then stabilized at a reduction of 80 nm. During this stage the corrosion rate amounted to about 0.2 mm/year. Over the next 50 weeks, the thickness of the sensor was reduced by an additional 40 nm, resulting in a corrosion rate of 0.04 mm/year. After that, i.e. in the remaining 50 weeks, no change in thickness was detected. The average corrosion rate during the observed period was thus 0.05 mm/year. As was expected, no corrosion damage was observed by either by optical microscopy or by SEM (Fig. 7b). Tomography was attempted, but no useful information was obtained, since the lowest obtainable resolution, which was needed to capture any thickness reduction of the ER sensor, would be 30 mm. 3.5. Electrochemical impedance spectroscopy The results of the EIS measurements which were obtained during the 2-year exposure period are presented in the form of Nyquist and Bode diagrams in Fig. 8. The points represent the measured data, whereas the lines represent the fitted data. Although two specimens were exposed, the results are presented only for one specimen since both of the specimens showed similar behaviour.
The impedance response consisted of the x-axis intercept at high frequencies and from the middle- and low- frequency tail. The low frequency impedance remained stable throughout the exposure period. Within this frequency range the measured values were between 2 and 2.3 MO cm2. In order to obtain additional information from the measurements, the impedance spectra were fitted to an equivalent circuit. After considering the multiple circuits which are discussed in the literature [11,28,34,35], the circuit shown in Fig. 8 was used. It consists of two time constants with a resistor and constant phase element connected in parallel [30]. Rm represents the mortar resistance, and is related to the high frequency impedance response. The first time constant at intermediate frequencies reflects passive film properties, CPE1 as the capacitance of the passive film in parallel to R1 which represents ohmic resistance. The second time constant represents metal/passive film interface properties where CPE2 as the double layer capacitance is in parallel to R2, charge transfer resistance. The fitting results are presented in Fig. 9. Mortar resistance slowly increased in the case of both of the investigated specimens, from 2 to 5 kO cm2. The n1 and n2 values for both of the constant phase elements were between 0.9 and 0.95. CPE1 increases during the initial 30 weeks, followed by a slow decline. The resistance R1 also shows a rapid increase during the first 30 weeks, but increases at a slower pace, and with more dispersed values afterwards. The second time constant (R2 and CPE2) values were relatively constant throughout the 120-week exposure period (see Fig. 9). The resistance R2 was high, up to 400 MO cm2. Most values of CPE1 were between 8 and 12 mF/cm2 and for CPE2 3.5 and 5.5 mF/cm2, respectively. At the end of exposure period, the passive film thickness was evaluated from the impedance data. Based on the equation:
Fig. 7. (a) Loss of thickness of the ER sensors in the two mortar specimens; (b) a typical microscopic image of the specimen after exposure, and (c) a tomographic image.
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Fig. 8. The Nyquist and Bode diagrams for one specimen, for selected weeks, measured at the end of the wetting period.
lowest and highest current densities were calculated as 0.2 nA/ cm2 and 9.5 nA/cm2, respectively. This translates to corrosion rates of between 10 and 110 nm/year. These very low observed corrosion currents are in agreement with the results of microscopic observations, in which no corrosion damage was observed (not shown).
4. Discussion
Fig. 9. Values of the parameters deduced from the fitting of the EIS spectra to the equivalent circuit in Fig. 8 for stainless steel exposed in mortar.
d¼
ee0 A C eff
½nm
ð2Þ
where e is the value of the dielectric constant (e = 30), d is the thickness of the film, e0 has a value of 8.8510–14 F/cm, and A is the surface area (15.7 cm2), the thickness of the oxide layer can be estimated. Mohammadi et al. [36] have shown that, in the case of AISI 304 steel, passive film thickness can be best estimated from the effective capacitance, Ceff (Eq. (3)), where the imperfections of the capacitance and mortar resistance are taken into account. The variables in this equation have already been described. 1=n1
C eff ¼ CPE1
1 Þ=n1 Rð1n ½lF m
ð3Þ
The relative stable capacitance data values indicate a constant thickness of the passive layer. This value was estimated to amount to 3.4 and 4.2 nm at the end of exposure period for the two different specimens, respectively. With impedance spectroscopy, the corrosion current can be estimated based on the interfacial charge transfer resistance (R2), applying it to the Stern-Geary equation. In order to get a more conservative estimate, a value of B = 75 mV has been suggested in the literature [19,37]. The average corrosion current for both specimens was 0.9 nA/cm2, but due to high scatter of the results, the
The corrosion of stainless steel in mortar was investigated. Five different monitoring techniques were used to monitor the behaviour of cyclically wetted mortar specimens over a 2-year exposure period: the galvanostatic pulse technique, the coupling current method, the use of a coupled multi-electrode array and of electrochemical impedance spectroscopy, and the use of electrical resistance sensor. Table 2 shows an overview of the average, minimum, and maximum recorded corrosion rates determined by each of these techniques. As can be observed from this table, the corrosion rates for all the used techniques are within the same order of magnitude, apart from the galvanostatic pulse measurements. Although the same stainless steel was used for the investigation of all monitoring techniques, some disparity in the corrosion behaviour can be expected due to differences in the processing and treatment of the steel sheets, rods and wires, as well as the nonhomogeneity of the used mortar. The average corrosion rates were calculated over the entire 2-year exposure period, whereas the maximum and minimum rates were determined after noise and short-term spikes had been eliminated. It should be noted that the calculated and observed corrosion damage was not high enough for it to be possible to find any accurate correlations between the two. In this paper it was shown that the tested stainless steel had very stable and intact passive film throughout 2-year exposure. However, the corrugated bars made from stainless steel might have different properties when used in reinforced structures [32]. During forming process of the reinforced bars and when bending the stainless steel bars, strain induced martensite can form, which, according to literature data and our experience has more corrosion susceptible properties than austenitic microstructure [38]. This might lead to localized types of corrosion attack in real exposures. The results of the galvanostatic pulse (GP) technique showed significantly higher corrosion rates compared to those obtained
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M. Hren et al. / Construction and Building Materials 221 (2019) 604–613 Table 2 Average, minimum and maximum corrosion rates in nm per year, as measured by the different monitoring techniques. [nm/year]
Galvano-static pulse
Coupling Current
CMEA
ER sensor
EIS
Maximum corrosion rate Average corrosion rate Minimum corrosion rate
2880 1050 300
210 25 0
280 15 0
200 50 0
110 10 2
by using the other techniques. No visible corrosion damage was found during the post-exposure examinations. When compared to other techniques, it seems that the GP method overestimates the corrosion rates. Similar results were found for steel corrosion in concrete [22]. The discrepancy could be partly attributed to the fact that the measurements were performed at the end of the wetting cycles, when the corrosion rates could be higher than normal. The second reason is the simplification of using a Randles circuit to derive the corrosion rate [39]. In our case a two time constant circuit was used instead (Fig. 8). The tests indicated a drop in the corrosion rates over the first 60 weeks for all the bars which were positioned closer to the wetting surface. This could in part be related to the analysis mentioned above, in part to the development of a passive film, and in part to the changing properties of the mortar as it hydrates and gets saturated with chlorides. Surprisingly, the bars which were located further from the wetting surface had higher corrosion rates (Fig. 2b). As mentioned above, the bars were in the non-carbonated mortar and with limited oxygen access, which could have affected the formation of a passive film in this area. The coupling current measurements showed lower corrosion rates when compared to the galvanostatic pulse measurements, but were in line with the results obtained by the other methods and post exposure examinations. The average corrosion rate was most likely affected by the large electrode sizes, where microcell corrosion processes occur and cannot be detected by this technique. The most noteworthy detected process occurred in the first 30 weeks, when the corrosion current slowly decreased and stabilized at low values. The likeliest explanation is that a passive film developed on both electrodes, thus decreasing the potential difference and the current between the two. During this initial period, the bottom bar is more anodic compared to the top bar, in the case of both specimens. This could be related to the different passivation processes in the non-carbonized mortar, with less oxygen and fewer chlorides available. When compared to an uncarbonized pore solution, Bautista et. al. [40] showed that the charge transfer resistance (R2, Fig. 8) of AISI 304 is higher in simulated pore solutions at pH 9 and a low chloride content, which reflects the situation of the upper bar at the beginning of the exposure. As the chloride content in the carbonized mortar increases, the charge transfer resistance drops, and the upper bar becomes more anodic relative to the lower bar. This phenomenon can be observed in Fig. 3, where the measured macrocell corrosion current drops to low values after 40 weeks. Stainless steel in a chloride solution is susceptible to pitting corrosion, with carbonation further decreasing the pitting potential for AISI 304 steel in saline simulated concrete pore solutions [11]. The results from the coupled multi-electrode array are in agreement with these findings. Specifically, electrode 4B (Fig. 4c and d) remained highly anodic throughout the multiple wetting and drying cycles, which is indicative of pitting corrosion. This was reaffirmed by the fact that, during a single wetting cycle, the electrodes rarely switched between anodic and cathodic behaviour. Over time, corrosion pits seemed to spread more evenly across the multiple electrodes surrounding 4B. This probably happened at the expense of the neutral and barely anodic electrodes becoming more anodic, whereas electrode 4B became less anodic. This trend of multiple increasing anodic currents could suggest
general corrosion developing over a larger time-frame. The average corrosion rate across all the electrodes was among the lowest measured, at 15 nm/year. However, the ratio between the maximum and the average corrosion rates is the largest, confirming the technique’s ability to detect short and localized events (Table 2). The electrical resistance sensors showed the second highest average corrosion rates, with most of the detected corrosion activity happening in the first 20 weeks (Fig. 7). This technique provides very accurate values for the thickness reduction of the metal. After 70 weeks, the thickness reduction was 120 nm. The average corrosion rate throughout the exposure period was 0.05 mm/year (Table 2). The use of multiple techniques showed an initial decrease in the corrosion current density, which lasts between 20 and 40 weeks before stabilization. Looking at the impedance results (Fig. 9), the change in both the first time constant parameters (CPE1, R1) and the passive film thickness coincides with this time-frame. One interpretation of these parameters is related to the formation of a passive film and the accompanying redox reactions. As the chloride concentration in the vicinity of the steel increases and the mortar continues to hydrate, the thickness of the passive film changes thickness or else its composition changes, which was observed in the increasing of both its resistance and capacitance. Likewise, hydration makes the mortar more compact, increasing its resistance during the first 60 weeks. Since chlorides saturate the area between the working and the reference electrode in the second half of the exposure, the mortar resistance decreases. A similar phenomenon has been observed by other authors [19,34].
5. Conclusions The corrosion of stainless steel in mortar was monitored during a 2-year exposure period. The mortar was cyclically wetted and dried using 3.5% NaCl. Five different monitoring techniques were used on specifically prepared specimens in order to monitor the corrosion of stainless steel in mortar: physical (electrical resistance probe measurements) and 4 different electrochemical techniques (the coupling current technique, galvanostatic pulse measurements, electrochemical impedance spectroscopy, and the use of a coupled multi electrode array) were used. The microscopic techniques that were implemented after the end of the exposure period (X-ray computer tomography, optical microscopy and SEM) confirmed our expectations: only insignificant localized corrosion damage was found. The results of the monitoring techniques were, in general, in agreement with these observations, but certain differences between them were observed. It should be mentioned that the comparison of cumulative and temporal measurements in time and space is difficult, and can sometimes be unreliable. In the case of the galvanostatic pulse (GP) measurements, significantly higher corrosion rates were recorded in comparison with the other techniques. This discrepancy could be partly due to the fact that these measurements took place at the end of the wetting cycles, when corrosion rates could be higher than normal. The second reason is presumably the simplification of an equivalent electrical circuit using just one time constant (the Randles circuit). The estimation of the corrosion rate is based on the whole polarized surface area, so the detection of localized corrosion processes is
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problematic. Due to the low frequency of measurements, the detection of corrosion transients is also limited. The coupling current technique is a sensitive one, but since both anodic and cathodic processes could occur on the same electrode, the overall assessment of the corrosion rate is unreliable. This deficiency was not found in the case of the coupled multi-electrode array (CMEA) technique, where the surface area of individual electrodes is much smaller. Moreover, CMEA is a very powerful technique, which enables the monitoring of the spatio-temporal behaviour of corrosion processes, including the detection and localization of metastable corrosion events. The performed electrochemical impedance spectroscopy (EIS) measurements indicated that, after the initial period, the passive film thickness was relatively stable throughout the exposure period. This is in agreement with the physical method (i.e. ER - electrical resistance probes) which showed the highest corrosion rate in the first 20 weeks, and thereafter the corrosion rate decreased to values that were close to zero. Neither of these two techniques was able to detect localized or transient corrosion events. All the used monitoring techniques showed a slow reduction in the corrosion rate, from the beginning onwards, until a stable state was reached (from roughly 0.3 lm/year to a few 10 nm/year). In this stable phase with nearly negligible corrosion, certain shortterm transients were observed. It is believed that the first stage included the growth of a passive film, whereas the specific transients observed in the second stage were generated by the formation of metastable pits (i.e. initiation and repassivation events). It can be concluded that anti-corrosion properties of stainless steel in mortar are very favourable. It is uncertain, however, whether the duration of the exposure is sufficient to predict the long-term corrosion behaviour of stainless steel in mortar, especially in possible transitions from metastable to stable corrosion processes.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
Declaration of Competing Interest None. Acknowledgements The financial support received from the Slovenian Research Agency (SRA) during 2015–2017, within the scope of the Young Scientist program (ID37501), is hereby gratefully acknowledged.
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[23]
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Characterization of stainless steel corrosion processes in mortar using various monitoring techniques Miha Hren, Tadeja Kosec ⇑, Andrazˇ Legat Slovenian National Building and Civil Engineering Institute, Dimicˇeva 12, 1000 Ljubljana, Slovenia
h i g h l i g h t s Stainless steel corrosion in concrete was monitored during 2-year period. 5 monitoring (physical, electrochemical) techniques were used and evaluated. The damages were estimated and correlated to corrosion currents measured. Limitations and advantages of each monitoring technique were assessed.
a r t i c l e
i n f o
Article history: Received 21 November 2018 Received in revised form 28 March 2019 Accepted 14 June 2019
Keywords: Stainless steel Corrosion Concrete Mortar Chloride induced corrosion Corrosion monitoring techniques
a b s t r a c t Monitoring of the corrosion on stainless steel type AISI 304 in highly porous mortar specimens was performed during a period of 2 years. The carbonated mortar specimens were cyclically wetted by a 3.5% NaCl solution, with drying periods in between. Five different monitoring techniques were applied: the galvanostatic pulse, the coupling current, the use of coupled multi-electrode arrays and electrical resistance probes and electrochemical impedance spectroscopy. At the end of the exposure period, comparisons were made between the results obtained by using these different techniques, as well as by taking into account actually observed damage by spectroscopic techniques. The advantages and limitations of each of the applied monitoring techniques were assessed. Ó 2019 Published by Elsevier Ltd.
1. Introduction Concrete cannot adequately protect the steel reinforcement from different types of corrosion processes. However, at the same time, phenomena such as carbonation, chloride penetration, and hydration, can continually change the concrete’s properties, which, in turn, can affect the corrosion behaviour of the embedded steel [1]. Many measures exist which can be used to reduce the extent of corrosion of steel in concrete. They range from proper concrete mix design and adequate rebar cover thickness, to the use of supplementary cementitious materials, inhibitors, coatings, and cathodic protection systems [2,3]. In more aggressive corrosion environments with high concentrations of chlorides, using stainless steel reinforcement bars can be a viable way of increasing protection against corrosion. Since the use of stainless steel generally causes higher upfront costs, this viability increases with increasing ⇑ Corresponding author. E-mail address: [email protected] (T. Kosec). https://doi.org/10.1016/j.conbuildmat.2019.06.120 0950-0618/Ó 2019 Published by Elsevier Ltd.
service life. The two best-known field examples of the use of stainless steel in concrete are a pier in Progreso, Mexico [4], and precast cladding panels in Wellington, New Zealand [5]. Both sites are located in marine or severe marine environments, and in both cases the stainless steel rebars have remained intact, whereas carbon steel bars in the same area show severe corrosion damage. When assessing the level of corrosion of steel in concrete or mortar, many parameters can be monitored using different techniques. Visual examinations can provide useful information about the state of steel based on concrete surface defects or accumulated corrosion products, whereas phenolphthalein can indicate carbonation depth, and the laboratory investigation of samples can provide data about chloride content and concrete permeability [6]. Visual examination has been used in a more scientific manner for the evaluation of corrosion damage to stainless steel in concrete at two test sites [7,8]. Both experiments lasted more than 10 years, and demonstrated the very good corrosion resistance of austenitic stainless steels in marine environments, where ferritic steels were unable to provide sufficient protection against corrosion.
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Commonly used electrochemical techniques include potentiodynamic techniques. Short-term laboratory potentiodynamic experiments are usually performed in simulated concrete pore solutions using standard three-compartment electrochemical cells [9–17], although sometimes concrete or mortar has been used [18,19]. The techniques are used to investigate corrosion potential and corrosion rate, pitting potential, Tafel slopes, and the ability of stainless steel to repassivate after pitting has begun. Another common technique involves half-cell potential measurements, or the more advanced galvanostatic pulse measurement technique, which usually results in a potential map across the concrete surface. This non-destructive technique is fast and therefore useful for on-site investigations [20,21], but it has been known to overestimate corrosion rates [22]. Another simple method for the measurement of corrosion rates involves the use of two coupled electrodes, where the potential drop across a resistor is measured. A more advanced technique involves the use of zero resistance ammeters to measure coupled currents across an array of electrodes [22–24]. This method provides both spatial and temporal information about the corrosion process. Electrochemical impedance spectroscopy, too, has been used in applications involving concrete [25]. This AC technique can provide additional information about all the layers present in a corroding system, including passive films, corrosion products, and concrete. As with potentiodynamic techniques, stainless steel has been investigated both in concrete pore solutions [10–12,26,27], and in mortar or concrete specimens [19,28–30]. Embedded sensors can also be used for the monitoring of corrosion activity in concrete structures [23,31], but their use in stainless steel applications is rare. Based on our previous studies [32] and the results of our specific experiments in simulated aqueous solutions, it was expected that the corrosion damage occurring to stainless steel should, despite aggressive corrosion environment, be minor. The goal of this study was to monitor and characterize the corrosion of stainless steel in concrete. The whole study lasted for 2 years, during which stainless steel bars embedded in different mortar specimens were periodically wetted with 3.5% NaCl, and dried in between. Five different electrochemical and physical techniques were used for corrosion monitoring. Special emphasis was placed on the evolution of corrosion processes over time. X-ray micro CT, SEM, and optical microscopy were used to validate the results obtained by the monitoring techniques. The extent of corrosion damage occur-
ring to the stainless steel was investigated by comparing measured corrosion rates, in combination with observations made by microscopical analysis. The advantages and limitations of the investigated corrosion monitoring techniques were identified and evaluated. 2. Material and methods 2.1. Preparation of the test specimens Four different types of mortar test specimens were prepared in order to investigate the corrosion behaviour of stainless steel in concrete (see Fig. 1). All the test specimens were made from CEM I 52.5 R cement with a water/binder ratio of 0.75 in order to reduce the corrosion initiation time. One part of cement and four parts of sand were used to make the mortar, according to the standard EN 196-1. All the working electrodes were made of AISI 304 steel, whose chemical composition is presented in Table 1. The thickness of the mortar cover was 5 mm. All the stainless steel rods had a diameter of 5 mm, and were in contact with the mortar over a length of 100 mm. Before installation, they were grinded with a 500 grit paper, and degreased with acetone. The specimens for the galvanostatic pulse measurements consisted of 100 30 30 mm mortar prisms (see Fig. 1a), with a single rod that was embedded 5 mm from the top (a total of 4 such specimens were prepared). The electrochemical impedance spectroscopy and galvanostatic pulse specimens (2 of each) were mortar prisms of the same dimensions into which two identical smooth rods were embedded, 5 mm and 20 mm from the top, respectively (Fig. 1b). These specimens were protected by an epoxy coating which was applied to all four sides and to the bottom of the specimen in order to limit carbonation of the mortar to the top steel bar only. The specimens for EIS were equipped with a 100 X resistor in order to measure the voltage drop occurring between the top and bottom bars. The two specimens of the third type had dimensions of 120 80 30 mm, and included an electrical resistance sensor (Fig. 1c). The fourth type of specimen had an embedded coupled multi-electrode array of sensors consisting of 25 stainless steel wires, each with a diameter of 0.8 mm, which were arranged in a 5 5 matrix in epoxy resin (Fig. 1d). A 5 mm thick layer of mortar was applied on top of the electrodes.
Fig. 1. The four different types of test specimens which were used in the experiment: (a) a typical specimen for the galvanostatic pulse measurements (4 specimens), (b) a typical specimen for electrochemical impedance spectroscopy (2 specimens) and for the coupling current measurements (2 specimens), (c) a typical specimen with an electrical resistance sensor (2 specimens), and (d) the specimen with a coupled multi-electrode array of sensors (1 specimen).
Table 1 Chemical composition of the stainless steel used for the Ø5 mm rods (%). C
Si
Mn
P
S
Cr
Mo
Ni
Co
Cu
N
Fe
0.039
0.412
1.46
0.066
0.033
18.51
0.331
7.81
0.157
0.512
0.084
70.4
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2.2. The period of exposure The mortar was cured for 28 days at a relative humidity of 100%. All the specimens were subsequently exposed to accelerated carbonation at 4% CO2 and 55% relative humidity. Accelerated carbonation was stopped once the top steel bar was completely covered by carbonated mortar. A carbonation depth, which was monitored by fenolphtalein indicator, of 11 mm was reached in 14 days. Cyclic exposure with 3.5% NaCl was performed once per week, with a two-day wetting period followed by a five day drying period at room temperature and controlled humidity at 60%. The entire exposure period lasted for 2 years (120 weeks) in the case of all the specimens except the coupled multi-electrode array, which had a shorter exposure period due to limited availability of the measuring equipment.
2.3. The used monitoring techniques The galvanostatic pulse (GP) measurements, electrochemical impedance spectroscopy, and electrical resistance measurements were performed once per week at the end of each wetting period, whereas the coupled multi-electrode array and the coupling current specimens were monitored continuously. This measurement method is a linear polarization technique, where a constant current is applied to a working electrode and the potential response is measured over a period of time [6,21]. In this experiment, a 50 mA current was applied for 10 s using a Force Galvapulse device. Based on the potential response, measured against an Ag/AgCl reference electrode, both the ohmic and the polarization resistance were determined. The latter was used, along with the SternGeary equation, to estimate the corrosion current density. A value of B = 52 mV was used [33]. All the corrosion rates were calculated using Eq. (1), which is derived from Faraday’s law of electrolysis, where a current density i is the input in nA/cm2. The following values and units were used as constants: atomic mass a = 55.1 g/mol, density q = 7.9 g/cm3, and the number of electrons lost per oxidized atom n = 2.
CR ¼
3:27 a i ½nm=year nq
ð1Þ
In the simple coupling current technique two identical electrodes are connected together across a resistor. A voltmeter is used to measure the voltage drop and Ohm’s law is used to calculate the corrosion current. In this study, the two embedded steel rods acted as electrodes. One was located 5 mm from the exposed surface, whereas the other 20 mm was beneath it. The technique cannot detect corrosion currents when matching anodic and cathodic sites are on the same electrode. The use of an electrical resistance sensor is a physical technique where the reduction in the thickness of an electrode results in increased resistance. The electrodes are produced in the form of thin and long wires which form the resistor components of a Wheatstone bridge. A constant current is passed through the circuit, and the voltage drops on the resistors are measured over time. ER probes were described in greater detail in the authors’ previous work [23]. Coupled multi-electrode arrays (CMEA) are used to measure the corrosion current between coupled multiple electrodes. In the given case the measurements were performed using a customdesigned zero resistance ammeter, where the resistors and power supply used allowed a maximum current of 50 mA to be detected. The frequency of data acquisition was set to either 1 Hz or 30 Hz, depending on the time-frame. Before connecting up the specimen, the internal device offsets for each ammeter were measured.
Electrochemical impedance spectroscopy (EIS) is an AC technique in which a potential is applied at different frequencies, and the current response is measured. In the case of this experiment, Gamry Reference 600 equipment was used to apply a 10 mV potential, with frequencies ranging from 100 kHz to 5 mHz, and 10 measurements per decade. The top rebar was used as a working electrode, whereas the bottom stainless-steel electrode was used as both a counter electrode and a reference electrode. 2.4. Post exposure examinations After exposure, the specimens were examined in detail using different microscopic techniques. Before the specimens were dismantled, they were 3D scanned using an Xradia MicroXCT-400 X-ray imaging system (Xradia, USA, 2010). The obtained microtomography images were processed and inspected for possible corrosion damage. After selected specimens had been dismantled, optical microscopy images (Tagarno HD Trend, 2016) of the electrodes were taken before and after they had been cleaned with a hydrochloric acid solution in order to remove the corrosion products. The cleaned specimens were then inspected by means of JEOL 5500 SEM to determine the extent and type of corrosion damage. 3. Results Four different monitoring techniques were used to characterize the behaviour of stainless steel in mortar samples over a 2-year exposure period. 3.1. Galvanostatic pulse measurements The results of these measurements, expressed as the measured corrosion current density over time, are presented in Fig. 2. All the specimens with just one embedded rod showed similar current values and similar trends (Fig. 2a). At the start of the exposure period, the corrosion currents were between 70 and 90 nA/cm2. Over the first 60 weeks, this range of values deceased to a range of between 40 and 60 nA/cm2. From 60 weeks on, the average current density values stabilised between 35 and 55 nA/cm2. The average corrosion rate during the entire exposure period was 0.6 mm/year. A similar trend with slightly higher current values can be observed in the case of the top bars in the specimens with two rebars (Fig. 2b). The current started at 100 nA/cm2 and the dropped to 70 nA/cm2 over the exposure period. For both of the bottom bars, the corrosion current density values varied between 150 and 200 nA/cm2 throughout the exposure to chlorides. This translates to roughly 1.7 and 2.3 mm/year, which is twice as high as in the case of the top bars. Here it should again be mentioned that the top bars were in carbonated mortar and had a 5 mm thick mortar cover, whereas the bottom bars were in non-carbonated mortar and had a 20 mm thick concrete cover. The results of optical observation showed only minor changes on the surface of the stainless steel rods (see the inset in Fig. 2a). Neither tomography nor the SEM images could confirm the occurrence of any corrosion damage (not shown). 3.2. Coupling current measurements The measured corrosion current density between the two stainless steel rods in the mortar specimens during the 2-year exposure period is presented in Fig. 3. A positive current indicates that the bottom electrode is anodic, whereas the top electrode is cathodic. In the case of a negative current, the opposite is true. At the beginning of the exposure period, it was found that that the bottom electrodes of both of the mortar specimens were more anodic, despite
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Fig. 2. Corrosion current density values as determined by the use of the galvanostatic pulse technique for: (a) the mortar specimens with one embedded rebar, and (b) the mortar specimens with two embedded rebars.
Fig. 3. Results of the coupling current measurements performed on the two mortar specimens.
the fact that the top electrode was located in carbonised mortar and exposed to faster chloride and oxygen migration. Since the measured current was very low, it was strongly affected by noise due to ambient temperature changes. The range of measured currents thus varied from 2 nA/cm2 to 14 nA/cm2. This translates to a corrosion rate of between 20 nm/year and 150 nm/year. After about 30 weeks, the corrosion current decreased and stabilised. At this time the average current density for the two specimens was 3 nA/cm2, and 1 nA/cm2. Throughout the exposure period, the start or the end of the wetting period did not have any short term effects on the results of the measurements. The corrosion activity detected by this technique did not translate into any visible damage which could be detected by microscopic techniques. 3.3. The coupled multi-electrode array measurements The entire exposure period of the coupled multi electrode array is presented in Fig. 4a, with the anodic current density displayed below (Fig. 4b), as well as details of the electrode activity during a typical wetting period (Fig. 4c and d). The positive currents indicate anodic activity, whereas the negative currents indicate cathodic behaviour. Each coloured line represents one electrode and its colour determines its location in the matrix (see the inset in Fig. 4a). The light-blue coloured sections represent the wetting intervals. The results are divided into three sections based on the
collected data (Fig. 4a). During the dormant periods the specimen was not exposed to cyclic wetting. The first and last sections show a significant increase in noise during the wetting periods. This indicates which electrodes started showing corrosion activity, and when. There was no indication of either of the electrodes becoming predominantly cathodic or anodic. Due to the high noise levels, the data acquisition sampling rate in the middle section was increased to 30 Hz and averaging was used to reduce the noise. The graphs presented in Fig. 4a show the anodic and cathodic activities of the 25 stainless steel electrodes during the 66 weeks of exposure. A more detailed view of one exposure cycle is presented in Fig. 4c. Electrode 4B showed the highest anodic activity during all the observed cycles, which is also indicated by the redcoloured electrodes shown in Fig. 4d and e. Fig. 4e shows the total anodic activity of the electrodes during the 2-year exposure period. Electrodes 2D and 3D were also anodic across multiple cycles, and, as the exposure continued, other electrodes in the centre became more anodic compared with electrode 4B. As the wetting cycles continued, some of the electrodes started to drift slowly towards the positive anodic side, and remained there even during the drying part of the cycles. The green-coloured electrodes, on the other hand, represent cathodic activity (see Fig. 4d). Most of the cathodic reactions occurred in the lower right section of the matrix, where electrode 5D was the most cathodic. It can be observed that, in most cycles, cathodic and anodic reactions occurred on different
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Fig. 4. (a) Results of the coupled multi-electrode array measurements for the entire exposure period, (b) the average anodic currents for the middle section, (c) a detailed wetting period during week 45, (d) average anodic (red) and cathodic (green) activity during week 45, and (e) the average anodic current density per electrode during the whole exposure period. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrodes, and these sites remained the same throughout the exposure period. Most of the electrodes remained either anodic or cathodic throughout individual cycles (Fig. 4c). The graph shown in Fig. 4b shows the total anodic activity as the sum of all the anodic activities. It can be directly correlated to the corrosion rate (Fig. 4b). Fig. 5a shows the total anodic activity as the sum of anodic events. These can be directly correlated to the instantaneous corro-
sion rates, which, during wetting periods, can be as high as 3.5 mm/ year. However, the average corrosion current density, presented in Fig. 5b can be rather low. Fig. 6 shows the most anodic electrode 4B throughout several cycles after exposure. SEM observations indicated three pitting corrosion locations on the electrode (Fig. 6b), whereas optical microscopy and microtomography did not show any corrosion damage (Fig. 6a and c). This was expected, since, during the
Fig. 5. (a) The total anodic activity measured on all the electrodes over time, and (b) the average anodic and cathodic activity per electrode.
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Fig. 6. The state of electrode 4B (Fig. 4): (a) after cleaning, (b) as a SEM image, and (c) tomography reconstruction. The scale applies to the 2D images only.
wetting cycles, the corrosion rates reached values of around 0.1 mm/year after noise had been eliminated. Sub-micron resolution is outside the scope of both of these measurement techniques. 3.4. Measurements with electrical resistance sensors Two identical specimens with embedded ER sensors were monitored over a total time-span of 120 weeks, as shown in Fig. 7. In both cases the initial thickness of 100 mm started decreasing at a slow rate during the first 20 weeks, and then stabilized at a reduction of 80 nm. During this stage the corrosion rate amounted to about 0.2 mm/year. Over the next 50 weeks, the thickness of the sensor was reduced by an additional 40 nm, resulting in a corrosion rate of 0.04 mm/year. After that, i.e. in the remaining 50 weeks, no change in thickness was detected. The average corrosion rate during the observed period was thus 0.05 mm/year. As was expected, no corrosion damage was observed by either by optical microscopy or by SEM (Fig. 7b). Tomography was attempted, but no useful information was obtained, since the lowest obtainable resolution, which was needed to capture any thickness reduction of the ER sensor, would be 30 mm. 3.5. Electrochemical impedance spectroscopy The results of the EIS measurements which were obtained during the 2-year exposure period are presented in the form of Nyquist and Bode diagrams in Fig. 8. The points represent the measured data, whereas the lines represent the fitted data. Although two specimens were exposed, the results are presented only for one specimen since both of the specimens showed similar behaviour.
The impedance response consisted of the x-axis intercept at high frequencies and from the middle- and low- frequency tail. The low frequency impedance remained stable throughout the exposure period. Within this frequency range the measured values were between 2 and 2.3 MO cm2. In order to obtain additional information from the measurements, the impedance spectra were fitted to an equivalent circuit. After considering the multiple circuits which are discussed in the literature [11,28,34,35], the circuit shown in Fig. 8 was used. It consists of two time constants with a resistor and constant phase element connected in parallel [30]. Rm represents the mortar resistance, and is related to the high frequency impedance response. The first time constant at intermediate frequencies reflects passive film properties, CPE1 as the capacitance of the passive film in parallel to R1 which represents ohmic resistance. The second time constant represents metal/passive film interface properties where CPE2 as the double layer capacitance is in parallel to R2, charge transfer resistance. The fitting results are presented in Fig. 9. Mortar resistance slowly increased in the case of both of the investigated specimens, from 2 to 5 kO cm2. The n1 and n2 values for both of the constant phase elements were between 0.9 and 0.95. CPE1 increases during the initial 30 weeks, followed by a slow decline. The resistance R1 also shows a rapid increase during the first 30 weeks, but increases at a slower pace, and with more dispersed values afterwards. The second time constant (R2 and CPE2) values were relatively constant throughout the 120-week exposure period (see Fig. 9). The resistance R2 was high, up to 400 MO cm2. Most values of CPE1 were between 8 and 12 mF/cm2 and for CPE2 3.5 and 5.5 mF/cm2, respectively. At the end of exposure period, the passive film thickness was evaluated from the impedance data. Based on the equation:
Fig. 7. (a) Loss of thickness of the ER sensors in the two mortar specimens; (b) a typical microscopic image of the specimen after exposure, and (c) a tomographic image.
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Fig. 8. The Nyquist and Bode diagrams for one specimen, for selected weeks, measured at the end of the wetting period.
lowest and highest current densities were calculated as 0.2 nA/ cm2 and 9.5 nA/cm2, respectively. This translates to corrosion rates of between 10 and 110 nm/year. These very low observed corrosion currents are in agreement with the results of microscopic observations, in which no corrosion damage was observed (not shown).
4. Discussion
Fig. 9. Values of the parameters deduced from the fitting of the EIS spectra to the equivalent circuit in Fig. 8 for stainless steel exposed in mortar.
d¼
ee0 A C eff
½nm
ð2Þ
where e is the value of the dielectric constant (e = 30), d is the thickness of the film, e0 has a value of 8.8510–14 F/cm, and A is the surface area (15.7 cm2), the thickness of the oxide layer can be estimated. Mohammadi et al. [36] have shown that, in the case of AISI 304 steel, passive film thickness can be best estimated from the effective capacitance, Ceff (Eq. (3)), where the imperfections of the capacitance and mortar resistance are taken into account. The variables in this equation have already been described. 1=n1
C eff ¼ CPE1
1 Þ=n1 Rð1n ½lF m
ð3Þ
The relative stable capacitance data values indicate a constant thickness of the passive layer. This value was estimated to amount to 3.4 and 4.2 nm at the end of exposure period for the two different specimens, respectively. With impedance spectroscopy, the corrosion current can be estimated based on the interfacial charge transfer resistance (R2), applying it to the Stern-Geary equation. In order to get a more conservative estimate, a value of B = 75 mV has been suggested in the literature [19,37]. The average corrosion current for both specimens was 0.9 nA/cm2, but due to high scatter of the results, the
The corrosion of stainless steel in mortar was investigated. Five different monitoring techniques were used to monitor the behaviour of cyclically wetted mortar specimens over a 2-year exposure period: the galvanostatic pulse technique, the coupling current method, the use of a coupled multi-electrode array and of electrochemical impedance spectroscopy, and the use of electrical resistance sensor. Table 2 shows an overview of the average, minimum, and maximum recorded corrosion rates determined by each of these techniques. As can be observed from this table, the corrosion rates for all the used techniques are within the same order of magnitude, apart from the galvanostatic pulse measurements. Although the same stainless steel was used for the investigation of all monitoring techniques, some disparity in the corrosion behaviour can be expected due to differences in the processing and treatment of the steel sheets, rods and wires, as well as the nonhomogeneity of the used mortar. The average corrosion rates were calculated over the entire 2-year exposure period, whereas the maximum and minimum rates were determined after noise and short-term spikes had been eliminated. It should be noted that the calculated and observed corrosion damage was not high enough for it to be possible to find any accurate correlations between the two. In this paper it was shown that the tested stainless steel had very stable and intact passive film throughout 2-year exposure. However, the corrugated bars made from stainless steel might have different properties when used in reinforced structures [32]. During forming process of the reinforced bars and when bending the stainless steel bars, strain induced martensite can form, which, according to literature data and our experience has more corrosion susceptible properties than austenitic microstructure [38]. This might lead to localized types of corrosion attack in real exposures. The results of the galvanostatic pulse (GP) technique showed significantly higher corrosion rates compared to those obtained
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M. Hren et al. / Construction and Building Materials 221 (2019) 604–613 Table 2 Average, minimum and maximum corrosion rates in nm per year, as measured by the different monitoring techniques. [nm/year]
Galvano-static pulse
Coupling Current
CMEA
ER sensor
EIS
Maximum corrosion rate Average corrosion rate Minimum corrosion rate
2880 1050 300
210 25 0
280 15 0
200 50 0
110 10 2
by using the other techniques. No visible corrosion damage was found during the post-exposure examinations. When compared to other techniques, it seems that the GP method overestimates the corrosion rates. Similar results were found for steel corrosion in concrete [22]. The discrepancy could be partly attributed to the fact that the measurements were performed at the end of the wetting cycles, when the corrosion rates could be higher than normal. The second reason is the simplification of using a Randles circuit to derive the corrosion rate [39]. In our case a two time constant circuit was used instead (Fig. 8). The tests indicated a drop in the corrosion rates over the first 60 weeks for all the bars which were positioned closer to the wetting surface. This could in part be related to the analysis mentioned above, in part to the development of a passive film, and in part to the changing properties of the mortar as it hydrates and gets saturated with chlorides. Surprisingly, the bars which were located further from the wetting surface had higher corrosion rates (Fig. 2b). As mentioned above, the bars were in the non-carbonated mortar and with limited oxygen access, which could have affected the formation of a passive film in this area. The coupling current measurements showed lower corrosion rates when compared to the galvanostatic pulse measurements, but were in line with the results obtained by the other methods and post exposure examinations. The average corrosion rate was most likely affected by the large electrode sizes, where microcell corrosion processes occur and cannot be detected by this technique. The most noteworthy detected process occurred in the first 30 weeks, when the corrosion current slowly decreased and stabilized at low values. The likeliest explanation is that a passive film developed on both electrodes, thus decreasing the potential difference and the current between the two. During this initial period, the bottom bar is more anodic compared to the top bar, in the case of both specimens. This could be related to the different passivation processes in the non-carbonized mortar, with less oxygen and fewer chlorides available. When compared to an uncarbonized pore solution, Bautista et. al. [40] showed that the charge transfer resistance (R2, Fig. 8) of AISI 304 is higher in simulated pore solutions at pH 9 and a low chloride content, which reflects the situation of the upper bar at the beginning of the exposure. As the chloride content in the carbonized mortar increases, the charge transfer resistance drops, and the upper bar becomes more anodic relative to the lower bar. This phenomenon can be observed in Fig. 3, where the measured macrocell corrosion current drops to low values after 40 weeks. Stainless steel in a chloride solution is susceptible to pitting corrosion, with carbonation further decreasing the pitting potential for AISI 304 steel in saline simulated concrete pore solutions [11]. The results from the coupled multi-electrode array are in agreement with these findings. Specifically, electrode 4B (Fig. 4c and d) remained highly anodic throughout the multiple wetting and drying cycles, which is indicative of pitting corrosion. This was reaffirmed by the fact that, during a single wetting cycle, the electrodes rarely switched between anodic and cathodic behaviour. Over time, corrosion pits seemed to spread more evenly across the multiple electrodes surrounding 4B. This probably happened at the expense of the neutral and barely anodic electrodes becoming more anodic, whereas electrode 4B became less anodic. This trend of multiple increasing anodic currents could suggest
general corrosion developing over a larger time-frame. The average corrosion rate across all the electrodes was among the lowest measured, at 15 nm/year. However, the ratio between the maximum and the average corrosion rates is the largest, confirming the technique’s ability to detect short and localized events (Table 2). The electrical resistance sensors showed the second highest average corrosion rates, with most of the detected corrosion activity happening in the first 20 weeks (Fig. 7). This technique provides very accurate values for the thickness reduction of the metal. After 70 weeks, the thickness reduction was 120 nm. The average corrosion rate throughout the exposure period was 0.05 mm/year (Table 2). The use of multiple techniques showed an initial decrease in the corrosion current density, which lasts between 20 and 40 weeks before stabilization. Looking at the impedance results (Fig. 9), the change in both the first time constant parameters (CPE1, R1) and the passive film thickness coincides with this time-frame. One interpretation of these parameters is related to the formation of a passive film and the accompanying redox reactions. As the chloride concentration in the vicinity of the steel increases and the mortar continues to hydrate, the thickness of the passive film changes thickness or else its composition changes, which was observed in the increasing of both its resistance and capacitance. Likewise, hydration makes the mortar more compact, increasing its resistance during the first 60 weeks. Since chlorides saturate the area between the working and the reference electrode in the second half of the exposure, the mortar resistance decreases. A similar phenomenon has been observed by other authors [19,34].
5. Conclusions The corrosion of stainless steel in mortar was monitored during a 2-year exposure period. The mortar was cyclically wetted and dried using 3.5% NaCl. Five different monitoring techniques were used on specifically prepared specimens in order to monitor the corrosion of stainless steel in mortar: physical (electrical resistance probe measurements) and 4 different electrochemical techniques (the coupling current technique, galvanostatic pulse measurements, electrochemical impedance spectroscopy, and the use of a coupled multi electrode array) were used. The microscopic techniques that were implemented after the end of the exposure period (X-ray computer tomography, optical microscopy and SEM) confirmed our expectations: only insignificant localized corrosion damage was found. The results of the monitoring techniques were, in general, in agreement with these observations, but certain differences between them were observed. It should be mentioned that the comparison of cumulative and temporal measurements in time and space is difficult, and can sometimes be unreliable. In the case of the galvanostatic pulse (GP) measurements, significantly higher corrosion rates were recorded in comparison with the other techniques. This discrepancy could be partly due to the fact that these measurements took place at the end of the wetting cycles, when corrosion rates could be higher than normal. The second reason is presumably the simplification of an equivalent electrical circuit using just one time constant (the Randles circuit). The estimation of the corrosion rate is based on the whole polarized surface area, so the detection of localized corrosion processes is
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problematic. Due to the low frequency of measurements, the detection of corrosion transients is also limited. The coupling current technique is a sensitive one, but since both anodic and cathodic processes could occur on the same electrode, the overall assessment of the corrosion rate is unreliable. This deficiency was not found in the case of the coupled multi-electrode array (CMEA) technique, where the surface area of individual electrodes is much smaller. Moreover, CMEA is a very powerful technique, which enables the monitoring of the spatio-temporal behaviour of corrosion processes, including the detection and localization of metastable corrosion events. The performed electrochemical impedance spectroscopy (EIS) measurements indicated that, after the initial period, the passive film thickness was relatively stable throughout the exposure period. This is in agreement with the physical method (i.e. ER - electrical resistance probes) which showed the highest corrosion rate in the first 20 weeks, and thereafter the corrosion rate decreased to values that were close to zero. Neither of these two techniques was able to detect localized or transient corrosion events. All the used monitoring techniques showed a slow reduction in the corrosion rate, from the beginning onwards, until a stable state was reached (from roughly 0.3 lm/year to a few 10 nm/year). In this stable phase with nearly negligible corrosion, certain shortterm transients were observed. It is believed that the first stage included the growth of a passive film, whereas the specific transients observed in the second stage were generated by the formation of metastable pits (i.e. initiation and repassivation events). It can be concluded that anti-corrosion properties of stainless steel in mortar are very favourable. It is uncertain, however, whether the duration of the exposure is sufficient to predict the long-term corrosion behaviour of stainless steel in mortar, especially in possible transitions from metastable to stable corrosion processes.
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Declaration of Competing Interest None. Acknowledgements The financial support received from the Slovenian Research Agency (SRA) during 2015–2017, within the scope of the Young Scientist program (ID37501), is hereby gratefully acknowledged.
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