Mechanistic study on initial passivation and surface chemistry of steel bars in nano-silica cement pastes
Introduction
The chloride threshold value (CTV) of steel bars is recognized as a most decisive parameter governing the durability design and service life prediction of concrete structures in chloride bearing environment [1,2]. While the CTVs obtained in the literature over the last fifty years scattered significantly, and no agreement of definitions and test methods of CTV has been reached [[3], [4], [5], [6], [7]]. One of the most critical reasons is the lack of the knowledge of the corrosion mechanism of steel bar in concrete induced by chloride ions [1,8], which remains as a critical scientific problem for the service life prediction of the concrete structures in chloride bearing environment.
Once the steel bars are embedded in fresh concrete, a spontaneous passivation process of the steel occurs regardless of the presence of mill scale or not [9,10]. This process can also be recognized as an initial corrosion process of steel in the highly alkaline environment provided by the concrete to form a passive film on the steel surface. After an extended period of hydration of the binders in concrete, the internal environment of the concrete, including the solid hydration products, alkaline pore solution, air and capillary voids/pores, aggregates, etc. tend to be stabilized [11,12]. Subsequently, the passive film would be in a continual state of breakdown and repair, and the stability and kinetic of chemical processes of passive film are governed by the surrounding concrete environment [13,14]. It is well known that the corrosion of steel bars in concrete induced by chloride ions occurs in the form of the breakdown of passive film when the chloride concentration exceeds a critical value on the surface of steel bar [9,11]. The quality of passive film formed during passivation process plays a critical role in affecting the chloride threshold value. The passivation of steel in concrete reflects the intrinsic corrosion resistance of steel bars embedded in concrete, which excludes the influence of corrosion resistance of concrete matrix. This contributes to revealing the corrosion mechanism induced by chloride ions and the fundamental mechanism controlling the durability of the concrete structures [15].
A considerable amount of studies on the passivation of steel were conducted in simulated pore solutions [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. It will take several days for the steel to reach a passive state in the simulated pore solution according to the evolution of the corrosion rate [10,16]. Wanees and coworkers [17,18] proposed that the open circuit potential of steel in saturated Ca(OH)2 solutions varies linearly with the logarithm of the immersion time at the very early age according to: E = a1 + b1 log t, where a and b are constants, t is the immersion time, and the final potential and the pH of the solution are inversely proportional according to a relation: Efinal = a2 – b2 pH. Ferreira and coworkers [25] showed that the thickness of the passive film and the Fe3+ species in the outermost layers increased with the presence of fly ash. There is a general view that the passive film formed in simulated concrete pore solutions is consisted of an inner Fe2+-rich layer and an outer Fe3+-rich layer with a typical thickness of 3 nm–15 nm [22]. Ghods [19,20] indicated that the thickness of the passive film formed in the simulated concrete solutions was in the range of 4–13 nm, and the passive film was thicker in the solution with a higher pH value. The Fe3+/Fe2+ ratio of the outer layer of passive film decreased from ca. 1.6 to ca. 1.1 when the immersion time was increased from 2 d to 9 d. The addition of Cl− decreased the thickness of the passive film, changed the stoichiometry of the passive film and increased the Fe3+/Fe2+ ratio in the inner layer of the passive film. Koga et al. [23] studied the passive film of steel formed in the pore solution produced by an ex-situ leaching procedure and calculated the thickness of the passive film (4.6 nm and 5.1 nm after 1 d and 2 d immersion respectively).
However, many studies [10,25,27] showed that the passive films formed on the steel bar were significantly different in concrete and in simulated pore solutions, and they were even dependent on the chemistry of the simulated pore solution. Page [28] suggested that the anodic passivation process of carbon steel in concrete was facilitated by an alkaline buffer effect of the lime-rich steel/concrete interface. Sagoe-Crentsil and Glasser [15] summarized that the passivity of steel in concrete, which was basically an electrochemical state, was mainly controlled by the availability of oxygen, electrode potential and properties of the cement paste. They also stated that it was the resistivity of the steel/concrete interface that determined the passivation rate of steel in concrete. Andrade [29] studied the passivation of steel in concrete using the EIS method and found that a time constant representing the reversible transformation of Fe3O4/γ-Fe2O3 in the passive film was evident in the very low frequency range. Macphee and Cao [30] proposed that the passivation of steel in concrete was inhibited by blast furnace slag incorporation due to the consumption of oxygen in the concrete.
Investigations focusing on the passivation of steel in the real concrete environment are much fewer compared to those in simulated concrete pore solutions, and the passivation process has not been thoroughly studied. The studies in real concrete were generally conducted by electrochemical test methods [10,[29], [30], [31]] and the chemistry/nanostructure of the passive film formed in the real concrete environment still remains unknown. Revealing this will contribute to the knowledge of the mechanism of the corrosion initiation of steel bars in concrete. In addition, with the development of the concrete technology, various supplementary cementitious materials (SCMs) were incorporated in concrete. Nano-silica, a kind of ultra-fine supplementary cementitious material, was widely studied to improve the performance of concrete at the nano-scale level [[32], [33], [34], [35]]. Two effects may occur when it is used in the concrete: pozzolanic reaction with the cement hydration products (Ca(OH)2) and the filler effect inducing more nucleation site for the hydration of cement [[36], [37], [38]]. Therefore, the addition of nano-silica may affect the dynamic variation of the internal environment of concrete, including the composition and the pH value of the pore solution. It may also potentially affect the passivation of steel bars in concrete, which was rarely reported.
Therefore, in the present work, the early-age passivation of carbon steel in the cement paste prepared with and without nano-silica was studied. The aim is to investigate the passivation process, characterize the passive film formed on the traditional carbon steel bar in real concrete environment and then reveal the influence of the SCMs (e.g. nano-silica) on the intrinsic corrosion resistance of the steel bar. The passivation process was studied by electrochemical test methods, including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The surface morphology of the steel was analyzed by Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (EDS) and the passive films formed were characterized by using X-ray Photoelectron Spectroscopy (XPS).
Section snippets
Materials and sample preparation
The carbon steel used in the present work were cut into a size of 10 × 2 mm from the S275J0 (EN 10025–2: 2004) hot rolled steel round bar and the chemical composition is provided in Table 1. A copper wire was soldered to one end of the cross section. The surface condition is one of the essential factors influencing the corrosion of steel in concrete [39]. A well-polished surface was expected to contain much fewer defect points on the steel thus obtaining reproducible results. Therefore, in the
Alkalinities of pore solutions in cement pastes
The chemistry of the cement paste, especially the alkalinity, is a dominating factor influencing the passivation of the steel bar in concrete. The changes in the alkalinity of the pore solutions at the early age of cement hydration will affect the dynamic formation process and the chemistry/nanostructure of the passive film on the steel bar. The evolutions of the pH values of the pore solutions in cement pastes prepared with and without nano-silica are shown in Fig. 2. For the Blank group, the
Conclusions
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The passivation process for the carbon steel in the cement pastes was different from that in the simulated pore solutions. Besides the factors of experimental set-up, such as the chemical composition of steel bars, surface finishing and geometry, the variations of the chemistry and microstructure of the cement paste were the main causes, such as the alkalinity, oxygen availability, porosity, relative humidity etc. These variations would be difficult to be considered in the simulated pore
Declaration of competing interest
The authors declare that they have no conflict of interest that could influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the financial support of Hong Kong Branch of the National Engineering Research Center for Steel Construction. The last author also acknowledges the support of the National Natural Science Fund for Distinguished Young Scholars (51525903).
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