Corrosion rates from low polarization data calculated by a galvanostatic, non-linear curve fitting method

Abstract

Most methods for analysing low polarization (current, voltage) data to give corrosion current, I k, and anodic, cathodic Tafel slopes, b a, b c, are based on the potentiostatic method where errors are predominantly in measured current. A discussion on potentiostatic versus galvanostatic methods is given and the galvanostatic method is preferred experimentally. This choice causes a problem with the analysis routine, which is based on the minimization of current residuals for the usual potentiostatic analysis methods, and will lead to significantly greater errors in estimating I k, b a and b c. The analysis routine should preferably match the type of experiment used to collect the low polarization data. If the galvanostatic method is used experimentally, where errors are predominantly in measured voltage, then a galvanostatic analysis routine should be used to analyse the data, i.e. a method based on the minimization of voltage residuals. This will result in reduced errors in estimating I k, b a and b c. The galvanostatic analysis routine reported here allows the operator to collect any number of anodic or cathodic data pulses (four or more), in any order and with no fixed relationship between one pulse and the next. It has been applied experimentally to measure the corrosion characteristics of Zn in phthalate-buffered 0.1 M NaCl solution at pH 5.2 and 3.9, and comparisons are made with solution analysis derived mass loss.