Argentum Solutions, Inc.
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David C. Silverman |
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Example of Use of the Technique (Nickel in Strong Acid) The polarization resistance method was used to estimate the corrosion rate of nickel in strongly acidic phosphoric/phosphorous acid solutions, the total acid content being 50 wt%. The procedure was to scan the potential between -20 mV and +20 mV at 0.1 mV/s after about 1 hour, after about 4 hours, and finally after 24 hours of exposure. The corrosion potential was stable during the generation of replicates at each time period. In several solutions, the potential did change by about 50 mV over the 24 hour period. The requirement of a stable corrosion potential was met fairly well under all conditions.This figure 
(2)and 
(3)The symbols are explained in the section entitled Summary of Polarization Resistance Technique in this tutorial. Note that the measured points have curvature even at the corrosion potential. A more detailed discussion is provided in Silverman, D. C., "Practical Corrosion Prediction Using Electrochemical Techniques", Uhlig's Corrosion Handbook, 2nd ed., ch. 68, p. 1179 (2000) 
(1800k).
The following table shows the calculated Tafel slopes and the corrosion rates estimated from the regression analysis and from mass loss. The Tafel slopes and polarization resistance values were averaged across the runs over the 24 hour period because the corrosion rate did not change over that period. The error shown is the standard deviation.
The difference between corrosion rates in the last two cases touches on several artifacts which can contribute to errors. First, the solution resistance was about 1 to 2 ohm-cm2 as measured by electrochemical impedance spectroscopy at 5000 Hz. The error introduced by ignoring solution resistance is very small in the first two cases but may account for at least 10% of the value in the last two. Second, though the standard deviation in the measurement of the polarization resistance is only about 20% of the average value, the difference between the extremes in the polarization resistance (measurement + standard deviation) - (measurement - standard deviation) ) is about 100% for those two cases. Seemingly small errors in relatively small polarization resistance values can lead to large errors in estimated corrosion rates. Third, the corrosion potential for those two cases was about -250 mV (SCE), close to the reversible potential for hydrogen under these extremely acidic conditions. Hydrogen evolution in the form of bubbles was observed further suggesting that the corrosion potential might have been close to the reversible potential for the hydrogen evolution reaction in this system. Such proximity could have led to errors in the calculated values because the assumption behind the Butler-Volmer equation that only one irreversible anodic and one irreversible cathodic reaction are present may have been violated. | |||||||||||||||||||||||||||||||||||||||
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David C. Silverman, Ph.D. - Primary Consultant |
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