The electrochemical properties of various iron oxide scales on steel exposed in saturated calcium hydroxide solutions were investigated. The iron oxide scales were manufactured by different heat treatments and grinding processes and characterized using X-ray diffraction and scanning electron microscope. The electrochemical properties of the scales were assessed by measuring the corrosion potential and using electrochemical impedance spectroscopy and potentiodynamic polarization curves. It was found that wustite and magnetite are less noble compared to hematite but are more effective as cathodic surfaces. The results show that the electrochemical properties of the mill scale can be an important contributing factor in the corrosion of steel in concrete.
Chloride induced corrosion of steel in concrete is an important deterioration mechanism for concrete structures. Many studies have been performed to determine a critical chloride threshold level and a wide range of critical chloride threshold levels has been reported [
The exact mechanism the mill scale has on corrosion of steel in concrete is not fully understood. Ghods et al. [
The mill scale originating in the steel production process consists of mainly three types of iron oxides: wustite FeO, magnetite Fe3O4, and hematite Fe2O3 [
During annealing of steel a mill scale is formed on the steel surface which consists of three types of iron oxides. The part of the scale closest to the steel surface is dominated by wustite, FeO, the middle part is dominated by magnetite, Fe3O4, and the outermost part is dominated by hematite, Fe2O3.
Steel samples were cut from a hot rolled plate, S235 JR (EN 10025-2:2004), to the dimensions 25 x 25 mm and 2 mm thick. The composition of the steel can be seen in Table
Chemical composition of the steel bars (% mass).
C | Mn | P | S | N | Cu |
---|---|---|---|---|---|
0.17 | 1.4 | 0.035 | 0.035 | 0.12 | 0.55 |
The mill scale on the hot rolled samples was removed by grinding with 1200 grit paper and thereafter cleaning in an ultrasonic bath for 10 minutes in 50% ethanol and 50% acetone. After cleaning, the steel samples were heat-treated in order to produce an oxide scale dominated by a certain iron oxide and in the following the samples are therefore named after the intended dominating oxide: wustite, magnetite, and hematite. The manufacturing procedure of the different scales can be seen in Table
Manufacturing procedure of the different scales.
Sample name | Procedure |
---|---|
Steel | Mill scaled removed; ground and degreased |
Wustite | 850°C, 10 min |
Magnetite | 850°C, 10 min |
Hematite | 850°C, 8 min |
Bare steel samples were made for comparison with the manufactured oxide scale samples. The original mill scale was removed by wet grinding with 600 grit paper and then rinsed in alcohol, dried, and stored in a desiccator until the start of the electrochemical experiments.
All samples, except steel which were used as a reference, were annealed at 850°C for 8-10 minutes and then cooled with different rates depending on the desired oxide layer. Wustite is stable in temperatures higher than 570°C and decomposes to magnetite and iron below 570°C according to the reaction:
To produce a magnetite-rich scale the samples were annealed and then cooled in the oven to 600°C leaving the lid open, followed by air cooling to room temperature. This was followed by a second heat treatment at 400°C for 50 minutes. 400°C was chosen to obtain a fast transformation rate of wustite to magnetite and iron. After the heat treatment the samples were wet ground until the red water color disappeared as described earlier indicating that magnetite was the dominating oxide in the oxide scale. The samples were then rinsed in alcohol, dried, and stored in a desiccator.
To produce a hematite rich layer, the samples were annealed at two temperatures. The first annealing temperature was at 850°C as for the other samples and then the temperature was lowered to 550°C and the second heat treatment was performed for 3 hours. The relatively long heat treatment at 550°C was chosen to oxidize the magnetite without obtaining thick scales. After the second heat treatment, the samples were air cooled and rinsed with alcohol and dried before being stored in a desiccator.
To control if the desired oxides had been formed on the steel samples, the samples were analyzed with X-ray diffraction (XRD) and scanning electron microscope in combination with electron backscatter diffraction (SEM/EBSD). The XRD measurements were performed on crushed oxide scales with a Bruker D8 using CuK
The sample preparation for the SEM/EBSD investigations were performed by first sputtering a thin gold layer onto the oxide scale surface, typically a few nm, and thereafter applying a nickel layer, typically 2
The electrochemical experiments were performed using an Avesta cell in order to avoid crevice corrosion [
A schematic figure of the Avesta cell.
The main cell was filled with 120 ml of either saturated Ca(OH)2 solution or a saturated Ca(OH)2 solution with 0.6M Cl- added as NaCl. The pumping rate of the saturated Ca(OH)2 into the cell was 3.3 ml/h which means that when the main cell was filled with the chloride containing solution the chloride concentration was slowly diluted during the experiments. The reason why a chloride free solution was pumped into the cell was to eliminate problems with corrosion which would occur on steel samples below the filter paper when a chloride containing solution was used. To avoid chloride concentration gradients in the cell, air was bubbled into the cell through a gas dispenser.
The reference electrode used was a double junction Ag/AgCl sat KCl electrode immersed in a salt bridge with a saturated K2SO4 solution. A salt bridge was used as the Ag/AgCl electrode can become inaccurate in high pH solutions. A platinum wire was used as counter electrode. The measurements were performed using a Solartron 1286 potentiostat and a Solartron 1255 frequency analyzer.
The following procedure was performed for each sample. The open cell potential (OCP) was measured for 10 minutes. Electrochemical impedance spectroscopy (EIS) measurements were then performed in the frequency interval 105-10−3 Hz and amplitude ± 20 mV vs. OCP. Finally, a potentiodynamic polarization scan was made in either anodic or cathodic direction with the OCP as starting point. The scan rate was 0.2 mV per second and the samples were polarized to 0.8 V in the anodic direction and to -0.8 V in the cathodic direction. In total 16 samples (4 types of oxides exposed in saturated Ca(OH)2 water with either 0 or 0.6M NaCl and polarized in either anodic or cathodic direction) were tested.
The manufactured oxide scales were analyzed with XRD and SEM/EBSD (SEM in combination with electron backscatter diffraction). The calculated composition from the XRD intensity spectra of crushed oxide scales can be seen in Table
Calculated composition from XRD spectra of the samples [%].
Phase | Wustite | Magnetite | Hematite |
---|---|---|---|
Fe | - | 4 | - |
FeO (wustite) | 88 | 17 | 26 |
FeO(OH) | 9 | - | - |
Fe3O4 (magnetite) | 2 | 77 | 39 |
Fe2O3 (hematite) | 1 | 2 | 35 |
There is a fourth iron oxide, maghemite
One cross section sample of each oxide scale type was analyzed with SEM (in black and white) and EBSD (in color); see Figure
SEM images (black and white) and EBSD phase maps (color) on cross sections on the different oxide scale samples where a and b are on wustite, c and d are on magnetite, and e and f are on hematite.
SEM and EBSD images of a cross section of the magnetite sample can be seen in Figures
SEM and EBSD images of a cross section of the hematite sample can be seen in Figures
The OCP was measured for 10 minutes to investigate the corrosion potentials of the samples and the average potential of two samples can be seen in Table
Corrosion potential vs. Ag/AgCl sat. KCl [mV]: average value of two samples during 10 minutes.
Solution | Steel | Wustite | Magnetite | Hematite |
---|---|---|---|---|
sat. Ca(OH)2, 0M Cl | -180 | -186 | -101 | 80 |
sat. Ca(OH)2, 0.6M Cl | -440 | -183 | -217 | 2 |
In the solution containing chlorides the steel sample had a relatively large potential drop of approximately 250 mV lower than in the solution without chlorides. The potential difference of the iron oxides measured in the two solutions differed by approximately 100 mV. The steel sample had the least noble potential followed by magnetite and the wustite. The hematite sample had the noblest potential. The magnetite-rich sample contained iron particles which may lower the corrosion potential in the chloride containing solution if the iron particles are exposed to the solution.
Theoretically, if a steel surface with mill scale is connected to a steel surface without mill scale, the steel surface would act as an anode and the mill scale as a cathode. It is likely that this galvanic cell would affect the passivation behavior of the steel, especially in a chloride containing solution where the potential difference in this study is measured to be about 440 mV between steel and hematite. The distinctly different potentials of the oxide, as compared to the raw steel surface, also indicate that the oxides are relatively pore-free.
Avila-Mendoza et al. [
A representative Bode plot of each sample type, obtained from electrochemical impedance spectroscopy (EIS) measurements, can be seen in Figure
Average total impedance of two samples at 1 mHz [106 Ohm.cm2].
Solution | Steel | Wustite | Magnetite | Hematite |
---|---|---|---|---|
sat. Ca(OH)2, 0M Cl | 0.9 | 0.15 | 1.8 | 1.8 |
sat. Ca(OH)2, 0.6M Cl | 0.015 | 0.3 | 0.7 | 1.9 |
Example of Bode plots obtained in a sat. Calcium hydroxide solution.
Example of Bode plots obtained in sat. Calcium hydroxide solution with 0.6 M Cl-.
In the solution containing chlorides, the iron oxides had similar Bode plots relative to the plots obtained in the solution without chlorides. The iron oxides are not strongly affected by chlorides. The impedance remained high for magnetite and hematite and relatively low for wustite. The steel sample had significantly lower impedance which is attributed to a destabilized passive layer. This is in agreement with reported studies in the literature where the impedance modulus is lower for steel exposed in chloride containing alkaline solutions compared to solutions without chlorides [
The EIS results were further analyzed by fitting the data to the equivalent circuit in Figure
Proposed equivalent electrical circuit for fitting EIS data.
In the equivalent circuit,
The results from the equivalent circuit simulation can be seen in Table
Parameter data obtained from the equivalent circuit fitting for samples exposed in sat. Ca(OH)2. The data values are an average of two samples.
Parameter | Steel | Wustite | Magnetite | Hematite |
---|---|---|---|---|
| 30 | 110 | 9 | 20 |
| 0.004 | 0.002 | 0.02 | 0.01 |
| 29 | 111 | 15 | 18 |
| 1.22 | 0.14 | 14 | 59 |
Parameter data obtained from the equivalent circuit fitting for samples exposed in sat. Ca(OH)2 with 0.6 M Cl-. The data values are an average of two samples.
Parameter | Steel | Wustite | Magnetite | Hematite | |
---|---|---|---|---|---|
| [ | 53 | 101 | 21 | 21 |
| [MΩ.cm2] | 4 | 0.002 | 0.002 | 0.01 |
| [ | 15 | 130 | 11 | 17 |
| [MΩ.cm2] | 0.012 | 0.25 | 2 | 124 |
Not many studies have been reported in the literature with EIS data comparing steel with mill scale and steel without mill scale. Shi et al. [
All oxide scales and steel samples were assessed with PDP exposed to saturated calcium hydroxide solution with and without chlorides. Figure
Anodic and cathodic potentiodynamic polarization curves for a steel exposed in a saturated Ca(OH)2 solution and a steel sample in saturated Ca(OH)2 solution with 0.6 M Cl-.
Anodic and cathodic potentiodynamic potential curves for the iron oxides exposed in saturated Ca(OH)2 solution.
Anodic and cathodic potentiodynamic potential curves for the iron oxides exposed in saturated Ca(OH)2 solution with 0.6 M Cl-.
The PDP curves for wustite, magnetite, and hematite exposed in saturated calcium hydroxide can be seen in Figure
The anodic polarization reversed at 0.8 V vs Ag/AgCl to the cathodic direction. The reversed current was at the same level or lower than the current in the anodic direction. This means that the underlying steel is not corroding and that the increased current at 0.7 V is due to electrolysis of water.
Figure
The anodic polarization reversed at 0.8 V vs Ag/AgCl to the cathodic direction. The reversed current was in the same level or lower than the current in anodic direction. This means that the underlying steel is not corroding and is protected by the mill scale.
Figure
Cathodic curve for the iron oxides and an anodic curve for the steel sample.
Only a few studies have been reported in the literature where PDP curves from samples in an as-received condition have been compared with those from oxide free samples and there is a lack of quantitative information about the compositions of the mill scale in those studies. Mahalatti et al. [
In this study a combination of heat treatment and grinding processes was used to manufacture steel samples with three different synthetic mill scales: one dominated by wustite (FeO); one dominated by magnetite (Fe3O4); and one with hematite on the surface. Based on the results from electrochemical measurements the following differences can be observed: In chloride containing solutions the untreated steel sample had the least noble potential followed by samples with scales dominated by magnetite and wustite. The noblest potential was observed for the sample with hematite on the surface. The difference in potential between samples could be as high as 440 mV. When polarizing samples in the cathodic direction, the measured currents for the samples with scales dominated by magnetite and wustite were significantly higher than those for the sample with hematite.
In conclusion, wustite and magnetite are less noble but more effective as cathodic surfaces than hematite. The results show that the electrochemical properties of the mill scale can be an important contributing factor in the corrosion of steel in concrete.
The XRD data used to support the findings of this study are included within the supplementary information file. The SEM/EBSD images used to support the findings of this study are included within the article. The OPC data used to support the findings of this study are included within the article. The EIS data used to support the findings of this study are included within the article. The simulated EIS parameters used to support the findings of this study are included within the article. The PDP curves used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
The supplementary material files show X-ray diffraction spectra of crushed mill scale from the samples called wustite, magnetite, and hematite.