Measuring the Corrosion Potential

The potential of a corroding metal, often termed is probably the single most useful variable measured in corrosion studies or for corrosion monitoring. It is readily measured by determining the voltage difference between a metal in its environment and an appropriate reference electrode. 

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In this laboratory, you will construct a number line for reference electrode conversions, measure the corrosion potentials of several alloys in a salt water solution, construct a galvanic series with two different reference electrodes, and convert the two galvanic series to the NHE scale to determine if they agree (as they should). This lab will demonstrate some of the concepts discussed in Chapter 2. 

Measure the open circuit potential of each of the five electrodes relative to the SCE and record these values in the table provided under Results. Convert each of these to the NHE scale with the help of the number line. Now, measure the corrosion potential of each of the five electrodes relative to the Hg/Hg2S04 and record these values in the table. Convert each of these to the NHE scale with the help of the number line. Compare the values of each of the electrodes vs. NHE as determined by the two different reference electrodes. It should not make any difference which reference electrode was used, as a reference electrode is simply a reference point or benchmark. 

Open circuit potential As defined by the McGraw-Hill Dictionary of Scientific and Technical Terms, open circuit potential (OCP) is the steady-state or equilibrium potential of an electrode in absence of external current flow to or from the electrode. OCP measures the corrosion potential of a corroding metal with regard to a reference electrode. For instance, increased susceptibility of stainless steels to pitting and crevice corrosion in seawater has been attributed to increase in OCP, which could partly be due to biofllm formation. Monitoring of OCP spectra can be used to rank the corrosion vulnerability of metals in comparison with each other. 

ASTM C 876 Describes a technique to measure the corrosion potential of steel in concrete. In the past researchers have used absolute corrosion potential values to give probabilities of the existence of corrosion and to estimate corrosion rates. This is no longer the case and current thoughts are that the contours are an indication of corrosion activity for larger structures, but absolute corrosion rates must be determined by some other method. 

In pitting attack studies, it is fundamental to measure the corrosion potential ( corr) of a corroding aluminum in the natural state. Eco , which is interpreted as the intersection of the anodic and cathodic polarization curves, reflects the electrochemical characteristics of both anodic and cathodic conditions. The contribution of anodic and cathodic behavior to the corrosion process is categorized into three groups ... 

Stratmann showed in several articles published around 1987 that for metals with a thin moisture layer the Volta potential difference was proportional to the corrosion potential of that system [100]. Using a calibration, it is therefore possible to measure the corrosion potential of a corroding system through an air gap and without making contact. 

It is important to remember that the potential of the different constituents only shows the driving force of the process that is to take place, whereas nothing is said about the kinetics, which, as will be shown, are dependent on a number of factors. Usually potentiodynamic measurements are used to investigate the kinetics of an alloy and of synthetically prepared phases (51, 52, 58). However, as is the case when measuring the corrosion potential, potentiodynamic measurements also require a bulk electrolyte. To get a more comprehensive idea about the atmospheric corrosion behaviour, the surface itself needs to be studied and other techniques have to be utilised. 

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