5.2 Measuring corrosion rates

Start with the open-circuit potential

Immerse the sample and record the open-circuit potential (OCP) until it is stable, often 30 to 60 minutes, sometimes longer. A drifting OCP means the surface is still changing, for example a film is forming or dissolving, and any rate measured then will be uncertain. Report the final OCP and how long you waited.

Linear polarization resistance

Close to E_corr, within about ±10 to 20 mV, the net current is nearly proportional to the potential. The slope gives the polarization resistance R_p, and Stern and Geary (1957) showed how it relates to the corrosion current.

LPR is quick and barely disturbs the surface, so it can be repeated over hours or days, which is useful for following an inhibitor. Scan slowly, for example 0.1 to 0.2 mV/s as in ASTM G59, and correct for solution resistance in poorly conducting fluids, because R_s adds to the measured R_p.

Tafel extrapolation

A wider scan, typically from about −250 mV to +250 mV around E_corr, shows the Tafel regions of both reactions. Extrapolating the straight parts of log|i| against E back to E_corr gives i_corr, and the slopes give β_a and β_c.

• Fit only true straight lines that extend over at least a decade of current, as McCafferty (2005) stressed. Many anodic branches never show a clean Tafel region because of films or passivation, and then the cathodic branch alone is used.
• The large polarization can change the surface, so run the cathodic branch first, or use a fresh sample for each branch.
• The Tafel calculator works for corrosion data too.

Impedance in corrosion

An impedance spectrum at E_corr gives the charge-transfer resistance, which plays the role of R_p, without a DC sweep. For coated metals, the impedance at low frequency, for example |Z| at 10 mHz, and the appearance of a second time constant show how well the coating protects (Lesson 1.3).

From current to corrosion rate

Faraday's law turns i_corr into a thickness loss per year. ASTM G102 gives the standard form.

Compare electrochemical rates with mass loss from weighed coupons over the same exposure where you can, and give both.

Testing inhibitors

• Measure the blank and several inhibitor concentrations in the same way and on samples prepared the same way.
• Give the inhibition efficiency IE = (i₀ − i_inh)/i₀, or the same from R_p, IE = (R_p,inh − R_p,0)/R_p,inh.
• Repeat each condition on at least three samples and give the spread. Differences of a few percent in IE are often within it.
• Test in a fluid close to the real one, for example with the right salts, dissolved gases, temperature and flow, because inhibitors can behave very differently outside the lab conditions.
• If two labs disagree, check the instruments and the procedure first: the same sample measured on two potentiostats should give the same R_p within the scatter of repeated runs.

Check your understanding

References and further reading

1. M. Stern, A. L. Geary, Electrochemical polarization I. A theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–63.
2. E. McCafferty, Validation of corrosion rates measured by the Tafel extrapolation method, Corros. Sci. 47 (2005) 3202–3215.
3. ASTM G102, Standard practice for calculation of corrosion rates and related information from electrochemical measurements, ASTM International.
4. ASTM G59, Standard test method for conducting potentiodynamic polarization resistance measurements, ASTM International.
5. R. W. Revie, H. H. Uhlig, Corrosion and Corrosion Control, 4th ed., Wiley, 2008.