Studies on the workability of tafel technique for high potential scan rate electro-chemical polarisation of steel and zinc-coated steels in 3.5% NaCl solution

Abstract

Corrosion processes involving both activation and diffusion controlled reduction exhibit 'slop break' preceeded by 'Knee' in their cathodic electrochemical polarisation curves. It has been observed that the Tafel extrapolation technique to evaluate corrosion current density (icorr) does not always work for polarisations of steel and zinc-coated steels in saline solutions, when the applied potential scan rate is high and happens to be in the vicinity of 5 mv/sec. It has been shown in this study that for polarisations of steel and zinc-coated steels in 3.5% NaCl, Tafel technique can be successfully made use of for graphical extraction of Icorr from 'E vs log i' plot even when the applied potential scan rate is as high as 30 times the ASTM standard, provided the distance between the 'knee' and the 'slop-break' along the linear portion is at least 184mV, in 'potential-length' i.e. length expressed in potential unit.