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AC-induced corrosion - Towards a nonlinear impedance model P. Linhardt G. Ball Vienna University of Technology Institute for Testing and Research in Materials Technology (TVFA) Karlsplatz 13, 1040 Vienna, Austria ECHEM Center of Competence for Applied Electrochemistry Viktor-Kaplan-Straße 2 2700 Wiener Neustadt, Austria Introduction AC induced corrosion is a problem typically affecting pipelines influenced by railways, power-transmission lines etc. AC voltages: up to > 100 V are reported AC-currents: up to 1000 A/m² are reported at technical frequencies (50, 60 & 16 2/3 Hz) AC influence 16 2 / 3 Hz Laboratory experiments in soil boxes [1] pSPICE modeling model Tafel-behavior by analog behavior modeling (ABM) [2] set up & analyze electrical equivalent circuits by transient analysis [3] Problem Assess electrode processes by electrochemical measurements during AC-corrosion experiments in soil boxes Approach APPLY sinusodial AC to specimen galvanostatically – MEASURE the distorted potential response – ANALYZE based on a nonlinear impedance model Conclusions • pSPICE appears as a powerful tool to set up nonlinear impedance models and to carry out large signal transient analysis. • The analysis of nonlinear distortions is a promising tool for non invasive assessment of electrochemical processes during AC corrosion experiments. • The distortion residual can be extracted regardless of IR-drop, making it an attractive source of information about the electrode processes. • Futher work will address the development of nonlinear impedance modeling including diffusion phenomena and systematically explore the correlation between the distortion residual with the corresponding electrode processes. + - ~ = IAC+DC IAC IDC CE WE QRE soil box Emeasured References [1] P. Linhardt, G. Ball, AC induced corrosion: Results from laboratory investigations and from a failure analysis, CORROSION 2006, paper 06160. [2] pSPICE Reference Guide, Cadence Design Systems Inc., USA, 2005. [3] pSPICE Student Version Release 9.1, Orcad Inc. 2000. EXAMPLE 20 bar gas transmission line bitumenous coating cathodic protection Case history: [1] ~ = IAC+DC Emeasured electrolyte metal EIRdrop Eelectrode nonlinear impedance Rel -5 0 5 10 -50 -40 -30 -20 -10 0 10 20 30 40 50 IAC (mA/cm²) Eelectrode (V) 0 10 20 30 40 50 60 70 80 90 100 -5 0 5 10 15 -50 -40 -30 -20 -10 0 10 20 30 40 50 -50 -40 -30 -20 -10 0 10 20 30 40 50 t (ms) Eelectrode (V) Emeasured (V) IAC (mA) 16 2 / 3 Hz 0 10 20 30 40 50 60 70 80 90 100 -5 0 5 10 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 -50 -40 -30 -20 -10 0 10 20 30 40 50 t (ms) Eelectrode (V) Emeasured (V) IAC (mA) 16 2 / 3 Hz -5 0 5 10 -50 -40 -30 -20 -10 0 10 20 30 40 50 IAC (mA) Eelectrode (V) 0 10 20 30 40 50 60 70 80 90 100 -5 0 5 10 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 -50 -40 -30 -20 -10 0 10 20 30 40 50 t (ms) Eelectrode (V) Emeasured (V) IAC (mA) 16 2 / 3 Hz -5 0 5 10 -50 -40 -30 -20 -10 0 10 20 30 40 50 IAC (mA) Eelectrode (V) -5 0 5 10 15 10 -5 10 -4 10 -3 10 -2 10 -1 |I| (A) Eelectrode (V) -5 0 5 10 15 10 -5 10 -4 10 -3 10 -2 10 -1 |I| (A) Eelectrode (V) working electrode (WE) steel specimen, 1cm² Pt-wire quasi reference electrode (QRE) required to avoid AC-distortions drawback: missing DC potenial AC-Galvanostat ±70 V peak , ±100 mA peak , 0-100 μA DCcathodic Example of data, measured during specific phases of long term testing (>30 days) Numeric IR-drop correction, based on R el estimated by impedance spectroscopy (EIS) simulation results from transient analysis Parameters empirically chosen DC behavior as Tafel-plots model A strong cathodic activity wide anodic passive range model B cathodically and anodically active includes electrode surface coverage effects model A model B model A model B characteristic features of the Lissajous-figure are better mimiced by model B • Fourier analysis allows to separate the linear part from the potential signal, yielding the distortion residual. • The distortion residual reflects the nonlinear part of the electrode impedance. • The distortion residual is not influenced by the IR-drop. Excursion: the distortion residual soil box Counter Electrode (CE) stainless steel sample holder
