The passive behaviour of 304 stainless steels in saturated calcium hydroxide solution under different deformation
Introduction
The use of stainless steel rebars is one of the most effective methods for ensuring the durability of concrete structures in highly aggressive environments [1], [2]. Castro-Borges et al. [3] reported that a stainless steel rebar constructed pier still performed satisfactorily after more than 60 years of service. Dimitri and Stewart [4] and Cramer et al. [5] concluded that using stainless steel rebar will reduce the cumulative costs by 50% for a concrete bridge exposed to a marine environment over 120 years.
Concrete structures are always subjected to a number of loads, which will ultimately degrade them. Anhvu et al. [6] studied stress corrosion cracking of precast steel wires and found that their service life was significantly reduced as the magnitude of the stress was increased. In addition, the results indicated that high stress increases mass loss in comparison to unstressed samples. On examining a cracked pre-stressed concrete line for water supply, Valiente [7] found that the strength and ductility of the steel wires were obviously diminished, whereas those of the concrete materials were not significantly degraded. Clearly, stress affects the corrosion behaviour of rebar in concrete.
Some authors also studied the effects of stress on the passive behaviour of metals in neutral or acid solutions. Yang and Luo [8] studied the passive behaviour of 304 stainless steel under tensile stress in a borate buffer solution. The results suggested that the critical chloride concentration, corresponding to rupture of the passive films, decreased with increasing tensile stress. Vignal et al. [9] studied the semiconducting behaviour of 316L stainless steel under elastic stress in an acid solution at pH 3 containing 0.02 M sodium chloride and found that high elastic stress caused the passive films to be heavily doped. In addition, the heavily doped films were sensitive to pitting corrosion. In another report, Zhang and Cheng [10] investigated the passive behaviour of pre-cracked X70 pipeline steel in carbonate/bicarbonate solution under tensile stress. The authors proposed that both the crack-tip and the region ahead of the cracks could be passivated. However, the results also indicated that the passive films at the crack-tip were less stable and susceptible to pitting corrosion. Studying the micro-electrochemical behaviour of 304L stainless steel in 0.5 M K2SO4 solution containing 10 mM K3Fe(CN)6, Sidane et al. [11] found that the kinetic constant for oxidation of the mediator on the substrate increased significantly with increasing stress magnitude. Zhu and Luo [12] studied the activity of Alloy 800 under stress in a ferrocenemethanol solution containing potassium chloride and thiosulfate, and found that both tensile stress and compression stress enhanced the surface activity. Phadnis et al. [13] studied the passive behaviour of cold-rolled 304 in 3.5 wt% NaCl solution and found that the Cr/Fe ratio of the passive film increased as the cold-rolling procedure. After investigating the metastable pitting of 17–4 precipitation hardening stainless steel in 3.5 wt% NaCl solutions, Nakhaie and Moayed [14] reported that the pit peak current and pit growth rate increased with increasing degree of cold working. Díaz et al. [15] studied the passivation of high-strength steel wires under constant stress in alkaline solutions containing different chloride ions. Their results indicated that stress will provoke the breakdown of passive films, even at low chloride concentrations solutions.
To the best of our knowledge, there has been little study on the effects of stress or strain on the passivation of stainless steel in alkali solutions. In the present study, the passive behaviour of deformed 304 stainless steels has been studied in saturated calcium hydroxide solution. The results suggest that the passive films on seriously deformed samples become heavily doped, and the space charge layers are thinned with increasing degree of deformation. Moreover, XPS results suggest that high deformation provokes oxidation of Fe2+ ions to Fe3+ ions in the passive films, whereas it does not visibly affect the oxidation state of the Cr cations.
Section snippets
Materials and experiments
Type 304 stainless steel, with the chemical composition listed in Table 1, was used in this study. Samples of the size shown in Fig. 1 were adopted for tensile testing. The stress–strain curve of the studied stainless steel is shown in Fig. 2. As can be seen, plastic deformation occurred when the strain exceeded 0.83%. The deformations presented in Table 2 were chosen to study the influence of the degree of deformation on the passivation of stainless steel. The designed deformations were
Potential dynamic polarization curves
Potentiodynamic polarization curves of differently deformed samples are depicted in Fig. 4. As can be seen, the magnitude of deformation did not noticeably influence the corrosion potential. On the other hand, the anodic current density slightly increased with the degree of deformation. This situation suggests that the passivity of the 304 stainless steel decreased with increasing deformation.
Semiconducting properties of the passive films
Fig. 5 shows examples of Mott–Schottky plots for the deformed stainless steel samples at different
Conclusions
The influence of deformation on the passive behaviour of 304 stainless steel in saturated calcium hydroxide solution has been studied. The results have shown that serious deformation can render to the passive films heavily doped. The donor and acceptor densities in the passive films were obviously increased with increasing degree of deformation. Moreover, the space-charge layer of passive films was thinned with increasing deformation. Simultaneously, the Fe2+ ion content in the films decreased
Acknowledgements
The authors would like to thank the National Natural Science Foundation of China (51301060, 51210001), the Fundamental Research Funds for the Central Universities (2013B03514), and the 111 Project (B12032) for supporting this work.
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