Curriculum · Module 6 · Intermediate

Corrosion and Coating Evaluation

From corrosion rate to coating life: standards-compliant electrochemical corrosion measurements.

How is corrosion rate measured? Tafel extrapolation and polarisation resistance (LPR)

Frequently searched:corrosion rate measurementTafel extrapolationLPR polarisation resistanceStern-Geary

1 Theory

Mixed-potential theory

Corrosion is the simultaneous progress on the same surface of the anodic dissolution of the metal and the cathodic reduction of a species in the environment (O₂, H⁺). The potential at which the currents of the two reactions are equal is the corrosion potential E_corr, the common current the corrosion current i_corr. By Faraday's law i_corr is converted to the mass-loss rate and from there to the penetration rate (mm/year): CR = K·i_corr·EW/ρ. The advantage of electrochemical methods is that they give a measurement in minutes versus a mass-loss test lasting months.

Tafel extrapolation

A potentiodynamic scan is made ±250 mV around E_corr; the logarithm of |i| is plotted against potential. Extrapolation of the anodic and cathodic Tafel lines to E_corr gives i_corr and the Tafel slopes (β_a, β_c). A correct result requires at least one decade of linear region on each branch; passivation, diffusion limit and ohmic drop destroy the linearity. Since the scan alters the sample surface the method is semi-destructive.

Polarisation resistance (LPR)

The slope R_p = dE/di is measured in the range E_corr ±10–20 mV; with Stern–Geary i_corr = B/R_p. B is calculated from the Tafel slopes; if unknown, default values such as 26 mV (activation-controlled) or 52 mV (diffusion-controlled) are used with an error margin. Fast, non-destructive and suited to continuous monitoring; however, the measured R_p also contains the electrolyte resistance — in poorly conducting media R_u must be subtracted or separated by EIS.

Advanced methods

Harmonic analysis and variable-amplitude sinusoidal methods such as VASP/CASP derive both the Tafel slopes and i_corr from a single measurement and remove the assumption of the constant B. Electrochemical noise (EN) gives the noise resistance (R_n ≈ R_p) and localised-corrosion indicators from potential and current fluctuations without external excitation. In systems where mass transport is limiting (e.g. oxygen diffusion) there is no cathodic Tafel region; i_corr is read from the limiting current.

2 Application in measurement

Under small polarisation of ±10–20 mV around the corrosion potential Ecorr the current-potential relationship is linear; the slope is the polarisation resistance Rp and is converted to corrosion current by the Stern-Geary equation. Tafel extrapolation finds Icorr from the log|i| – E curve under wider polarisation (±250 mV); ASTM G59 and G102 define the measurement and calculation procedures.

Errors: ohmic drop (high-resistance media, concrete), mass-transfer limitation (oxygen reduction), change of the surface during the measurement. Sinusoidal techniques such as VASP and CASP give Rp and the Tafel coefficients in a single measurement.

The corrosion rate (mm/year) is calculated from Icorr by Faraday's law; equivalent weight and density are taken according to G102.

Key equations

Stern-Geary: Icorr = B/Rp, B = βa·βc/(2.303(βa+βc))

3 Applicability

Where it applies

Material/inhibitor screening, comparison of environment aggressiveness, field corrosion monitoring (LPR probes), testing to standards (ASTM G59, G102, G3).

Limits and caveats

On passive metals (stainless steel, Ti, Al) i_corr is very small and the Tafel regions are ill-defined; for localised corrosion (pitting) potentiodynamic cyclic polarisation (ASTM G61) is required.
Results are an average of the surface; they do not represent localised attack.

4 What did the application notes find?

Corrosion AN10 — Shows step by step the application of Tafel fit and R_p fit for iron in acid; the consistency of the i_corr obtained by both methods and the derivation of the constant B from the Tafel slopes.
Corrosion AN36 — Shows the principle of the VASP technique (variable-amplitude sinusoidal micropolarisation) and the extraction of β_a, β_c and i_corr from a single measurement; the results agree with classical Tafel.
Corrosion AN37 — Introduces the CASP (constant-amplitude sinusoidal micropolarisation) method; explains the determination of the Tafel parameters and i_corr by harmonic analysis.
Corrosion AN47 — Shows that the Stern method becomes invalid when the cathodic reaction is limited by mass transport, and the calculation of i_corr from the model corrected with the limiting current.
Corrosion AN48 — Quantifies the systematic error created by ohmic drop in i_corr determination and shows that correction is mandatory.

5 Which instruments perform this measurement?

SP-200 Electrochemical Cells

References

article M. Stern, A. L. Geary. Electrochemical Polarization I. A Theoretical Analysis of the Shape of Polarization Curves. J. Electrochem. Soc. 104, 56, 1957. doi:10.1149/1.2428496
article E. McCafferty. Validation of corrosion rates measured by the Tafel extrapolation method. Corrosion Science 47, 3202–3215, 2005. doi:10.1016/j.corsci.2005.05.046
standard ASTM International. ASTM G59 — Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements. ASTM, 2020. link
standard ASTM International. ASTM G102 — Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. ASTM, 2015. link

Coating evaluation with EIS; pitting, noise and localised-corrosion techniques

Frequently searched:coating EISpitting corrosion cyclic polarisationelectrochemical noiseSVET LEIS corrosion

1 Theory

EIS of organic coatings

An intact coating is a high-impedance dielectric: in the Nyquist plot an almost vertical capacitive line, in Bode a |Z| of 10⁸–10¹⁰ Ω·cm² at 0.01 Hz. The coating capacitance C_c = ε·ε₀·A/d depends on thickness and water uptake; the Brasher–Kingsbury relation calculates water uptake from the capacitance rise. When electrolyte reaches the metal through pores in the coating a second time constant (double layer + charge transfer) appears and |Z|_0.01 Hz drops. Below 10⁶ Ω·cm² is usually the sign of loss of protection; monitoring over time (immersion test) is the accelerated measure of coating life.

Passivity and pitting

On passive metals a thin oxide film stops corrosion; chloride breaks this film locally and a pit forms. Cyclic potentiodynamic polarisation gives the pitting potential (E_pit, sudden rise of the current) and the repassivation potential (E_rp, closure of the hysteresis on the reverse scan); the wider the gap, the greater the susceptibility to localised corrosion. Critical pitting temperature (CPT) and potentiostatic pitting tests are standard methods.

Electrochemical noise (EN)

Two identical working electrodes are connected through a zero-resistance ammeter and the potential is followed with a third, reference electrode. The ratio of the standard deviations of the current and potential fluctuations gives the noise resistance (R_n); transients are the signature of metastable pits. Since there is no external excitation the surface is not disturbed and the onset of localised corrosion is caught early; it must be proven that the instrument noise is below the signal according to ASTM G199.

Reinforced concrete and local techniques

Rebar corrosion in concrete is measured in a high-resistance medium; half-cell potential mapping, LPR (with guard ring) and EIS are used and ohmic-drop correction is mandatory. For coating defects and galvanic couples, scanning probe techniques (SKP, SVET, LEIS) give position-resolved information instead of an average (see Module 9).

2 Application in measurement

On organic coatings EIS follows the decrease of protectiveness with time through the coating capacitance (water uptake) and pore resistance; for high-impedance (GΩ) coatings the ULC option and shielding are required. Cyclic polarisation determines the pitting and repassivation potentials.

Electrochemical noise (ENA) is an excitation-free method; the noise resistance Rn is related to Rp and detects localised corrosion events (ASTM G199). Position-resolved techniques — SVET, LEIS, SECM, SKP — map corrosion at micron scale on the M470 platform.

For multi-sample parallel tests the VSP-300/VMP-3e; in reinforced concrete and high-resistance media IR correction is mandatory.

3 Applicability

Where it applies

Comparison of coating formulations and pre-treatments, quantitative evaluation after immersion/salt spray, ranking of localised corrosion in stainless/aluminium alloys, field noise monitoring.

Limits and caveats

On very high-impedance coatings (>10¹⁰ Ω) the instrument's input impedance and cable leakage set the limit; a Faraday cage and a dedicated cell are needed.
Standardised interpretation of EN analysis is limited; results should be supported by independent methods.

4 What did the application notes find?

Corrosion AN22 — Shows the evaluation of rebar in reinforced concrete by EIS and polarisation resistance in a high-resistance medium; separation of the concrete resistance and the effect of carbonation.
Corrosion AN25 — Shows simultaneous pitting tests with a multichannel potentiostat; determination of E_pit and E_rp and statistical comparison between samples.
Corrosion AN39-1 — Shows measurement and validation of the instrument's intrinsic noise against ASTM requirements; a prerequisite for meaningful EN.
Corrosion AN39-2 — Shows the execution of an EN measurement on a real corrosion system according to the ASTM protocol and the calculation of the noise resistance.
Corrosion AN39-3 — Shows the comparison of the noise resistance R_n with the polarisation resistance R_p on the same system and the use of the software's noise-analysis tools (statistics, spectral density).
Scanning Probes AN9 — Shows imaging of a coating defect on zinc-coated iron with SKP; four different regions (intact coating, damage, corrosion product, bare metal) are distinguished by work-function difference.
Scanning Probes AN6 — Shows that intermittent-contact SECM gives a true electrochemical activity map by excluding the topography effect in two corrosion examples.

5 Which instruments perform this measurement?

SP-200 VSP-300 M470

References

article F. Mansfeld. Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings. J. Appl. Electrochem. 25, 187–202, 1995. doi:10.1007/BF00262955
article R. A. Cottis. Interpretation of Electrochemical Noise Data. Corrosion 57, 265–285, 2001. doi:10.5006/1.3290350
article M. Stratmann, H. Streckel. On the atmospheric corrosion of metals which are covered with thin electrolyte layers — I. Verification of the experimental technique. Corrosion Science 30, 681–696, 1990. doi:10.1016/0010-938X(90)90032-Z
article D. Polcari, P. Dauphin-Ducharme, J. Mauzeroll. Scanning Electrochemical Microscopy: A Comprehensive Review of Experimental Parameters from 1989 to 2015. Chem. Rev. 116, 13234–13278, 2016. doi:10.1021/acs.chemrev.6b00067

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