Introduction: Corrosion—not as simple as "rusting"—is an electrochemical "silent war"
Corrosion of steel is not simply "iron + oxygen → rust", but rather anodic reaction (Fe→Fe²⁺+2e⁻) + cathodic reaction (O₂+2H₂O+4e⁻→4OH⁻) — two spatially separated electrochemical reaction pairs — between countless micro-anodes and micro-cathodes on the steel surface — where electrons conduct through the steel substrate and ions conduct through the water film — forming a microscopic corrosion cell. The anti-corrosion function of coatings — (1) Barrier — blocks the permeation of O₂ and H₂O — reduces cathodic reactants; (2) Inhibition — high-impedance coating reduces ionic conduction between anode and cathode — decreases corrosion current; (3) Cathodic protection (zinc-rich) — zinc acts as a sacrificial anode — preferentially corrodes to protect the steel. Electrochemical methods (Tafel/LPR/EN/EIS) are the only technique that can "real-time, non-destructively, quantitatively" measure the corrosion rate under coatings — the "gold standard" for evaluating coating anti-corrosion performance.
I. Comparison of Three Electrochemical Methods
| Method | Applied Perturbation | Measured Parameter | Output | Applicable Coating Condition | Test Time |
|---|---|---|---|---|---|
| Tafel Extrapolation | Strong polarization (±250mV) | log|I|-E curve | icorr/corrosion rate (mpy) | Coating failed (bare steel) | 5-15 min |
| LPR | Weak polarization (±10-30mV) | Rp=ΔE/ΔI | Polarization resistance/icorr | Coating degraded (impedance <10⁷ Ω·cm²) | 2-5 min |
| EN | Zero perturbation (natural fluctuation) | Potential/current noise | Pitting index PI/Rn | Coating intact → failed full cycle | 5-60 min |
FAQ
Q1: Why must the "linear region" of Tafel extrapolation be in the strong polarization region of >±100mV—is weak polarization not acceptable?The Tafel equation—η(E-Ecorr)=βa×log(I/Icorr) applies to the anode—η=βc×log(I/Icorr) applies to the cathode—in the strong polarization region of >±100mV—the anodic or cathodic reaction is overwhelmingly dominant (the other reaction is negligible)—the log|I|-E curve shows strict linearity and extrapolates to η=0 (i.e., E=Ecorr)—intersecting with the corrosion potential line → icorr. In the weak polarization region (±10-50mV)—the anodic and cathodic reactions affect the current simultaneously—the curve is non-linear—Tafel extrapolation is unreliable.
Q2: LPR (Linear Polarization Resistance) Stern-Geary equation — inverse relationship between Rp and icorr?Stern-Geary equationicorr=B/RpB=(βa×βc)/(2.3×(βa+βc))≈13-26mV (steel in NaCl/concrete). Rp (polarization resistance)=ΔE/ΔI (slope within ±10mV range) — the higher Rp = the lower the corrosion rate (the better the coating protection). LPR is only applicable to coatings with obvious water permeation (impedance <10⁷Ω·cm²). In the "intact" state of coatings Rp>10¹⁰Ω·cm² — the signal-to-noise ratio of LPR is extremely large — measurement is inaccurate.
Q3: How is the "pitting index" (PI) of electrochemical noise (EN) calculated?PI=σI/RMS(I)——σI=standard deviation of current noise——RMS(I)=root mean square of current noise. PI<0.01 uniform corrosion (uniform coating degradation / no localized pitting); PI>0.1 localized corrosion / pitting (local coating damage → pitting — high PI is an "urgent warning" signal of anti-corrosion coating failure). The zero-disturbance characteristic of EN is suitable for field online monitoring (pipelines / storage tanks) — corrosion status can be monitored without disconnecting the coating system — it is the best practice of "Structural Health Monitoring (SHM)" in the anti-corrosion field.
Q4: Selection of the "reference electrode" (RE) in a three-electrode system — Ag/AgCl vs SCE vs Cu/CuSO₄?In the lab, the saturated calomel electrode (SCE) has a stable potential (+0.241V vs SHE) — small temperature coefficient — and is the laboratory standard. For field/seawater, Ag/AgCl (seawater) resists chloride ion corrosion — stable potential (+0.197V vs SHE). For buried/soil, Cu/CuSO₄ (CSE) has a potential of +0.316V vs SHE — it is the industry standard reference electrode for buried pipeline CP (cathodic protection). When citing corrosion potentials in literature, you must specify the type of reference electrode used. The potential difference between different references exceeds 100mV — confusing references will lead to completely incorrect corrosion judgments.
Q5: Why do "two time constants" often appear in the EIS (Electrochemical Impedance Spectroscopy) of coatings, and what physical processes do they correspond to respectively?The Bode plot (phase angle-frequency) of coating EIS shows:High-frequency (>10⁴Hz) time constantCoating capacitance (Qc/coating itself)——reflects the coating's water absorption/dielectric constant change;Low-frequency (<1Hz) time constantDouble-layer capacitance (Qdl/coating/steel interface)——reflects the interfacial corrosion reaction beneath the coatingThe appearance of a low-frequency time constant = corrosion under the coating has already begun (even if the coating appearance remains intact)——this is the core advantage of EIS over visual inspection in detecting corrosion months or even years earlier.
Q6: Why is there a huge difference (>10-100 times) in corrosion rate (mpy/mils per year) between laboratory and actual service?Laboratory——solution (3.5% NaCl/constant temperature/constant oxygen)accelerated corrosion conditionscorrosion rate (>10 mpy/bare steel). Actual servicewet-dry alternation + temperature variation + low salt concentrationcorrosion rate (<1 mpy). Laboratory electrochemical datacannot be directly "translated" into actual service lifethe best use of electrochemistry is"relative comparison of different coatings" (ranking)rather than predicting absolute values.
Q7: The combined strategy of "accelerated corrosion test + EIS monitoring" in coating corrosion protection evaluation?During salt spray/immersion tests, periodically (every 100-500h) take samples for EIS measurement to track the change of |Z| at 0.01Hz over time—plot the "|Z|-time" decay curve. When |Z| drops below 50% and improves evaluation accuracy by >30%.
Q8: How to avoid "Shielding Effect" and "Edge Effect" in electrochemical testing?Shielding——The wire/fixture of the test electrode is in the solution——The electrode's potential signal is "shielded" with error >10mV. Avoid——Use heat-shrink tubing for insulation and minimize only the electrode surface area exposed in the solution. Edge——The steel substrate at the cut edge of the coated panel is exposed——The corrosion current at the edge "leaks" and the EIS measurement includes interference from edge corrosion. Avoid——(1) Seal the panel edge with paraffin/epoxy edge sealing (>5mm wide); (2) Use a "free film" with no substrate at the edge.
Q9: Effects of different electrolyte solutions (3.5% NaCl/artificial seawater/real seawater/soil leachate) on the EIS results of coatings?3.5% NaClStandard + simplebut lacks Ca²⁺/Mg²⁺ in real seawater (which may react with the coating to form calcium/magnesium soaps—potentially affecting the coating's "true" EIS). Artificial seawater (ASTM D1141)closest to real seawaterrecommended for marine coating evaluation. Real seawater—containsmicroorganisms + organic mattervaries greatly across different sea areas—poor repeatability. Soil leachatesimulates buried environmentthe "true" corrosiveness of soil is far higher than NaCl solution (SRB sulfate-reducing bacteria in soil produce H₂S with extremely strong corrosion)—this is a special challenge for EIS evaluation of buried pipeline coatings.
Q10: Limitations of electrochemical methods in "coating lifetime prediction" — does electrochemistry measure the "present" rather than the "future"?Electrochemical (EIS/EN) measurements reflect the coating's“here and now”corrosion state — the inference that "the coating is intact now = intact for >10 years in the future" is invalid without supporting accelerated aging data. The correct lifetime prediction method — (1) "accelerated aging + EIS monitoring" EIS at different aging times — plot the "|Z| vs. aging time" curve — extrapolate the curve to |Z|=10⁶Ω·cm² (failure criterion) — obtain the“predicted failure time”This is the "most scientific contribution" of electrochemical methods to lifetime prediction.
Related Reading
Summary
Three electrochemical corrosion methods—Tafel (strong polarization / i_corr), LPR (weak polarization / R_p), and EN (zero disturbance / pitting index)—cover the full-cycle monitoring of coatings from "intact" to "complete failure". EIS (low-frequency |Z|) is the earliest + most sensitive + most non-destructive "early-warning radar" for coating corrosion, detecting the "first signal" of under-coating corrosion months to years before visible rust appears. Kexin New Materials provides customers with a full suite of coating anti-corrosion electrochemical evaluation services and life-prediction model support.