Angle-resolved XPS study of carbon steel passivity and chloride-induced depassivation in simulated concrete pore solution Authors: P. Ghods et al Year: 2012 Introduction Carbon steel rebar is generally protected against corrosion by a passive oxide film that is formed when this material is exposed to the high alkaline environment of concrete. However, the partial or complete loss of passive film, also called depassivation, may lead to excessive rates of corrosion. The presence of chlorides in high concentration can cause depassivation. The authors of this paper present non destructive angle resolved XPS investigation of the oxide films grown in a simulated concrete pore solution that includes the ionic species that are generally present in concrete. The effect of chloride on the oxide films was also studied by adding sodium chloride to the simulated concrete pore solution after the passivation of the specimens. It is expected that this study will provide reliable information about the nanostructure and properties of the passive film on carbon steel in the simulated environment that is comparable to that of rebar in concrete and its chloride-induced rupture. Experimental method Rebar was cut into four 8-mm-long specimens and XPS analyses were performed to obtain elemental composition of the samples. Table 1 shows the elements of the specimens and it is observed that the main components of samples are Fe, Mn, Si, C and Cu. The concrete pore solution (CP solution) with ph=13.3 was prepared and three specimens were immersed in the CP solution and kept there for two weeks to lead passivation. After this period the first specimen (labeled CP-0) was taken out of the passivating solution and transferred to another container with the same solution (CP). Sodium Chloride was added to the CP solution containing the two remaining specimens to obtain a chloride concentration of 0.45 M. After two weeks the second specimen (CP-1) was transfer to a different container with the same solution (CP + 0.45 M chloride). The solution with the remaining specimen was increased to a 3 M chloride concentration. All the specimens were kept in their respective solutions for four weeks until the XPS analyses. The specimens were analyzed using the Kratos Axis Ultra XPS with a monochromated Al X-ray source (hv=1486.6 ev). Ultra pure gold metal was used (Au 4f7/2 = 84.0 ev) to adjust the work function of the spectrometer.
Results The authors use a plot from the literature (Fig.2) with the average binding energies of various iron compounds classified into four major groups as Fe-1, Fe-2, Fe-3 and Fe-4, corresponding to Fe metal, Fe 3 C, Fe 3 O4/ FeO (Fe 2+ structure) and Fe 2 O 3 / FeOOH (Fe 3+ structure), respectively. This classification was used in this study to identify the compounds present in the oxide film of the three different specimens (CP-0, CP-1 and CP-2). Furthermore, an additional component (Fe-5) associated with Fe 2 O 3 -Satellite was used in curve fitting analysis of Fe 2p, as suggested in other studies. Fig. 1 shows XPS Fe 2p high resolution for being able to observe the double peak (Fe2p1/2 and Fe2p3/2) spectrum of sample CP-0 (i.e. the steel after being passivated). In Fig. 1 can see these five major iron compounds are found in this oxide film. Fig. 2. The peak binding energy position (Eb (ev)) data for six iron-incorporated compounds at the surface of steel obtained from NIST Standard Reference Database.
Figure 1. Fe 2p high resolution XPS spectra for the CP-0 specimen passivated in the CP solution at different emission angles (i.e.,0⁰,40⁰,50⁰ and 60⁰). Fig. 1 shows the Fe2P high resolution XPS spectrum for sample CP-0 at different emission angles (i.e. across the depth oxide film). It can see the intensity of peaks increases by increasing the emission angle. In other words, the amount of iron oxide is larger in the area near to the surface (60º, 4.1 nm depth) than the area close to the substrate (0º, 8.3 nm depth). Fig. 1. Typical curve fitting conducted for Fe 2p high resolution XPS spectrum that includes a doublet structure (2 peaks) due to multiplets splitting (i.e., Fe 2p3/2 and Fe 2p1/2). The Fe 2p spectrum is composed of five iron compounds; i.e., Fe-1, Fe-2, Fe-3, Fe-4 and Fe-5, respectively corresponding to Fe metal, Fe3C, Fe 3 O 4 /FeO (Fe(II)), Fe 2 O 3 /FeOOH (Fe(III)) and Fe2O3-Satellite.
Fig. 4 shows replica Fe 2p high resolution spectra obtained from two different spots for each of the three specimens (CP-0, CP1 and CP-2) at various emission angles (0⁰, 40⁰ and 60⁰) to determine the variation in the properties of iron oxide film. In Fig. 4a it can see that the spectra and its replica at the three different angles are nearly similar. This fact suggests that the oxide film formed on the surface of steel in the CP-0 is relatively uniform. On the hand, for CP-1 and CP-2 there are remarkable variations between the two spots at 40⁰ and 60⁰. These variations of the two spots areas indicate that the oxide film is unequally distributed on the surface of CP-1 and CP- 2. Moreover, the variation in CP-2 is larger than that one for CP-1. Therefore, this event suggests that specimens exposed to the higher chloride concentration (CP-2 3M Cl) lead grater variations in oxide film than those exposed to lower chloride concentration (in this case CP-1 0.45 M Cl). Fig. 4. The Fe 2p XPS spectra obtained from two different areas at the surface of specimens showing the variation of Fe 2p high resolution XPS spectrum data: (a) CP-0 (passive film before exposure to chloride); (b) CP-1 (passive film exposed to 0.45 M chloride); (c) CP-2 (passive film exposed to 3 M chloride). Fig. 5 shows a comparison Fe2p spectrum between the three specimens (CP-0, CP-1, and CP-2) at 0⁰, 40⁰ and 60⁰. Here the peaks are normalized with the iron metal peak. For this reason, the intensity of the peaks can be taking into account as an indicator for the amount of the iron oxide at different depths
of the oxide film (i.e. at different emission angles). Also, in fig. 5 it can be observed that the intensity of the peak in the area of iron oxide is higher for CP-0 than the peak for CP-1 and CP-2. This suggests the amount of iron oxide is larger for sample CP-0 than samples CP-1 and CP-2. This fact means that the thickness for samples CP-1 and CP-2 is smaller than sample CP-0; hence this success can be attributed to the dissolution of iron oxide into the solution containing chloride. Additionally, there is not a remarkable variation in CP-1 and CP-2 spectra (fig. 5), which can indicate that the thickness does not change with the concentration of chloride. Fig. 5. Comparison of the Fe 2p spectra for CP-0 (passive film before exposure to chloride), CP-1 (passive film exposed to 0.45 M chloride) and CP-2 (passive film exposed to 3 M chloride at: (a) 0⁰emission angle; (b) 40⁰ emission angle; (c) 60⁰ emission angle. Fig. 7 shows the ratio Fe(II)/Fe(III) of the three different specimens at various emission angles. The average Fe(II)/Fe(III) ratio decreases by increasing the emission angles. This suggests that the concentration of Fe(III) oxides is higher in the region close to the surface than the region close to the steel substrate. This ratio decreases by increasing the chloride concentration (i.e Fe(II)/Fe(III) ratio is the smallest for sample CP-2, 3 M chloride) at emission angles range 0⁰-50⁰, however this ratio is very similar for the three compounds at emission angles larger than 50⁰. This indicates that the concentration of chloride changes the stoichiometry of the passive film, in other words chloride affects the Fe 2+ oxides,
which are part of the oxides that protect steel carbon against corrosion. Hence, the rupture of the oxide film can be attributed to the further oxidation of protective Fe 2+ oxides to the less protective Fe 3+ oxides, peculiarly in the region closer to the steel substrate. The average thickness of the oxide films may be calculated from iron oxide/ iron ratio (assuming a uniform film) from two different spots at 0⁰ of the three specimens (CP-0, CP-1 and CP-2) it was found to be 5.1, 4.9 and 4.5 nm respectively. Conclusions XPS technique allowed to determine the thickness of passive film (CP-0, 5 nm approximately). Also it was possible to investigate the composition of the oxide film varying the emission angle (Fe 3+ oxides near to the free surface and Fe 2+ oxides nearby to the steel substrate). Additionally, it was determined that the increasing of chloride concentration decreases the thickness of the passive film and changes its stoichiometry since the ratio Fe 2+ /Fe 3+ decreases near to the steel substrate. In other words, depassivation of carbon steel can occur. It can be concluded that the rupture of the oxide film regarding chloride is may be due to the further oxidation of the passive film.