Double-layer effects corrosion techniques

The error of the idealized polarization-resistance technique can be calculated from Eq. (26). The error, presented in Fig. 9, can be expressed as a function of two dimensionless parameters that include all variables [( o)c /2.3v i r][6 6 /(6 -hfe )] and d(j>2ldE. Parts of the curves are shown with a dotted line, indicating the conditions under which the polarization curve of the corroding metal is distorted by the double-layer effect to such a degree that this becomes obvious to the experimenter. As described in Section III.2, the overall appearance of the polarization curves often remains normal even when the double-layer effect causes a significant error in the corrosion measurements. However, under extreme conditions the effect becomes obvious. The dotted lines in Fig. 9 indicate the conditions for which the polarization curve is... 

Figure 9. Error in corrosion current density due to the neglect of double-layer effect for idealized polarization-resistance technique. ...

Figure 12. Error in corrosion current density due to the neglect of double-layer effect for a corrosion system with oxygen reduction (solid lines) or hydrogen evolution (dashed lines) as the cathodic process for idealized polarization-resistance technique. = 0.03 V, = 0.12 V, 1 1 electrolyte, Ch-2S ii cm (zo) /v = 0.0 (solid) or =1.0 (dashed). ...

The effect of changing system parameters (e.g., corrosion potential, Tafel slopes, etc.) has been treated qualitatively. The effect of double-layer charging time has also been dis-cussed. " In most cases, the charging time is negligible compared to the usual measurement times, but unreasonably long measurement times, on the order of hours, may be required for systems with very low conductivity solutions and for low corrosion rates (that is, when the rc time constant of the double layer is large). For transient experimental techniques, the time-dependent effects manifest themselves as frequency or scan-rate dependence of the results. - -... 

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