Stephen Turgoose

The effects of solution resistance on electrochemical noise resistance measurements: A theoretical analysis

The theoretical basis of electrochemical noise resistance measurements in the presence of significant solution resistance is examined with a simple linear circuit model. A shot noise model of the noise generation process is assumed to develop the dependence of electrochemical potential and current noise on corrosion rate, although the conclusions in respect to electrochemical noise resistance do not depend on this. It is concluded that the electrochemical noise resistance method measures essentially the same resistance as is measured by a conventional linear polarization resistance measurement, although it is found to be capable of making measurements in higher resistance systems.

The Estimation of Corrosion Rate from AC Impedance Measurements

The interpretationof electrochemical impedancespectrahasbeenre-examinedfrom first principles using a numerical model of the metal-solution interface, together with conventionalequivalentcircuit models. This hasshownthat thereis no singleparameter that canbe usedfor theestimationof corrosionratefrom impedancespectra,andobtaining a reliable estimaterequiresa knowledgeof the correctphysicalmodelof theinterface. INTRODUCTION The electrochemical impedancetechnique is well-establishedas a method for the investigation of electrochemicalreactions. However, the analysisof the resultant data remains difficult. The most common approachis that basedon the equivalent circuit concept,exemplifiedby themodel of RandlesEl]. Unfortunatelytheuseof this approach has tended to become divorced from a considerationof the specific electrochemical processesoccurring. Various authors[2][3][4] haveremediedthis situationby a detailed comparisonbetweenthereactionmechanismandtheresultantimpedancespectrum.While this is undoubtedly theoptimal approachto obtaining detailedmechanisticinformntion, it is rathercomplexfor applicationto the‘simple’ problemof estimatingthecorrosionrate, especiallyin therather ill-defined circumstancesobtaining in the real world. This paper attempts to bring together thesetwo approachesin order to provide guidance for the estimation of corrosion rate from ac impedancespectra. 111 previous work[5], the relationshipbetweenpolarization resistanceand ac impedancemeasurementshas been investigatedusing a physicalmodel of a very simple system. This revealedthat, for the particularsysteminvestigated,therateof reactioncould be relatedto thechargetransfer resistance,whereasit was not relatedto the polarization resistancewhen the latter was definedas thezero frequencylimit of the ac impedance, less anysolutionresistance. At thetime this work waspresented,the authorswere unableto presenta clear view of the general principles involved in extracting the corrosion rate from an ac impedance