Electrochemical impedance and current fluctuations analysis during slow strain rate test of a UNS S30400 stainless steel in low pH media

Journal of the Brazilian Chemical Society

Nogueira, Ricardo P.¹  Université Pierre et Marie Curie, Paris, France¹  Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil¹  Universidade do Estado do Rio de Janeiro, Nova Friburgo, Brazil¹  Ponciano, José A. C.¹  Bastos, Ivan N.¹ 

关键词: stress corrosion cracking;    stainless steel;    impedance spectroscopy;    electrochemical noise;   

DOI  :  10.1590/S0103-50532005000400009

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The susceptibility to stress corrosion cracking, SCC, is one of the main drawbacks of austenitic stainless steels mainly in chloride-rich environments. Specifically, there are two main situations where chloride limits the use of austenitic stainless steel: neutral pH at high temperature and room temperature with low pH.1,2Although being a widely studied phenomenon, the stress corrosion cracking dependence on pH, as well as the chloride threshold necessary to trigger the process, are not completely determined. Some papers have attempted to correlate the type of localized corrosion with pH and Cl- content,3,4 even delimiting the immunity region for stress corrosion cracking,4 but these attempts were not sufficient to definitevely clarify this phenomenon. As an example, both the UNS S30100 and the UNS S31000 stainless steels, in spite of the fact that their austenite phase are respectively unstable and stable, have very close empirical correlations (expressed by equations like pH > A x log(Cl –) + B) to describe the regions of pH where the SCC does not take place. As a matter of fact, empirical relations like the previous one are not able to encompass the complexity of SCC processes.Carranza and Galvele5 strained wires of austenitic stainless steel at high strain rate (~10 s-1) to find the range of electrode potentials where the kinetics of repassivation would be effectively related to SCC as suggested by Staehle.6 Bastos et al.7 employed this high strain rate to investigate the correlation between the electrolyte temperature and the susceptibility to SCC in rich-chloride environments. However, some problems arise when fast current transient occurs, as pointed out by Oltra et al.8 Besides the fact that SCC does not effectively take place in such high strain rates, they have illustrated the difficulties in gathering reliable information about these surface processes. The intrinsic uncertainty of measuring these transients, mainly after a single depassivation event, is due to the coupling of surface capacitances that provides, at first, the current necessary to repassivation. After this, the regulating device provides the same electrical charge to recover steady state conditions. In this situation the time constant of the measured transient does not correspond to the actual time constant but reflects the effects of unaffected area and the capabilities of current supply of the regulation device. Thus, according to Oltra et al.,8 these results indicated that conventional devices are not well adequate to study repassivation kinetics and its correlation with SCC.It is well known that the stress corrosion cracking is related to the corrosion of a localized region on the metallic surface whilst the rest of surface remains almost passivated. In this sense, SCC is generally accepted as being governed by the formation-rupture of a surface film on the metallic surface. Nevertheless, the literature is abundant in works where the stress corrosion cracking of austenitic stainless steel in room temperature occurs concomitantly with uniform corrosion, mainly at low pH values,3,9-12 even if this system is considered as being an exception to the one described above.9,10 These results have been recently confirmed by Nishimura and Maeda13 who proposed an active path dissolution on SCC of austenitic stainless steels in acidic chloride solutions. Therefore, it is presumed that the electric current flowing between the different regions of the metallic surface must contain information about the type of corrosive process taking place on the electrode. This is why the use of a zero resistance ammeter (ZRA) to detect the current flowing between different areas of the metallic surface seems to be an appropriate technique to detect this process. In this sense, some authors have attempt to identify the elementary transients related to SCC susceptibility by using a ZRA.14,15 Results obtained by Gomez-Duran and Macdonald15 are of particular interest since they have demonstrated that significant coupling currents flowed from the cracks and could be effectively picked up by the ZRA apparatus. It is worth noticing, however, that one should not neglect the fact that the overall electrochemical behavior of the surface has also a key role in such a complex phenomenon as SCC. The present work is therefore devoted to the coupling of a macroscopic average approach by means of electrochemical impedance measurements to the stochastic approach, which is based on the use of a ZRA to measure the electrochemical current noise. Both techniques, electrochemical noise-ECN and electrochemical impedance spectroscopy-EIS, were employed to investigate some aspects of the SCC of austenitic stainless steel at low pH at room temperature in the presence of chloride, mainly focusing on the identification of the type of corrosive attack. ExperimentalThe electrodes were prepared from commercial stainless steel UNS S30400 pretreated at 1323 K for 1 hour in argon atmosphere and subsequently quenched at room temperature water in order to obtain an austenitic microstructure free from carbide precipitation since the latter is known to affect the intergranular corrosion resistance. The samples used in slow strain rate test, SSRT, were machined in short transverse section of rolling. The surfaces were mechanically polished up to 600 grit with emery papers and stored in desiccator up to the moment of testing. The chemical composition of the alloy used is shown in Table 1.    The electrolytes were prepared from an aqueous solution of 1 mol L-1 NaCl to which different amounts of 1 mol L-1 HCl were added to adjust the pH values to 0.00 and 1.00 under natural aeration. The working electrodes, WE, were round samples of 4 mm in diameter and 16 mm in length tested at strain rate of 3x10-6 s-1.A 40 mm in diameter and 30 mm in length hollow cylinder of the same steel submitted to the same surface preparation was employed as a counter-electrode, CE, in the ECN measurements. The round WE and the CE were concentrically assembled inside the electrochemical.The stressed WE and the non-stressed CE were connected by a ZRA as depicted in Figure 1. The analogue output current signals were digitized by a dynamic signal analyzer HP 3562A with proper anti-aliasing low-pass filtering,16 after direct current (DC) value elimination and amplification. The power spectrum densities (PSDs) of the digitized signals were calculated after Hann windowing to avoid artefacts related to the Fast-Fourier-Transform algorithm that assumes an intrinsic periodicity of the measured signal.17 The PSDs were averaged 10 times for increasing the measurement accuracy.    Polarization curves and EIS measurements were carried out in a conventional three-electrode electrochemical cell, with a platinum wire as CE and a satured calomel electrode as the reference electrode. The EIS measurements were performed at the corrosion potential during the straining of the samples under SSRT. A Solartron SI 1280 device delivered a perturbation signal of 8 mVpp in the frequency range from 1mHz to 10 KHz and calculated the Z(f) transfer function. The values of the electrochemical impedance modulus |Z(f)|, were depicted in Ohm instead of Ohmcm2 due to the uncertainty of actual area of samples undergoing concomitant elongation and cracking. Results and DiscussionThe stress-strain SSRT curves at a strain rate of 3x10 –6 s –1 in air, as well as for samples immersed in pH 0.00 and pH 1.00 solutions are shown in Figure 2. The air is assumed to be an inert environment and the corresponding stress vs. strain curve was used in a comparative basis to evaluate how aggressive is the acid solution in each case. The corrosive environment obviously affects the mechanical properties of the steel when stress corrosion cracking takes place. As it was expected, in acidic solutions, the values obtained for maximum elongation and maximum stress were lower than the respective ones obtained in air. This mechanical behavior is a consequence of the stress corrosion cracking that reduces the strength of the specimen.    Figure 3 shows the polarization curves of the austenitic stainless steel on both electrolytes for the non strained samples (0 MPa) and at 375 MPa (constant load applied before the cell was filled with the electrolyte). These curves represent the mean stationary behaviour of the interface: they clearly show a marked difference of the interface behaviour depending on the pH. At 0.00 value the interface shows no clear passivation with a monotonically increasing current density before a saturation value is reached for an overpotential of about 200 mVsce. At pH 1.00 a net decrease in the current density indicates the existence of an active–passive transition which passivating property is soon lost in the

【 授权许可】

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