Methods for Electrochemical Corrosion Testing: Techniques & ASTM Standards
What Is Electrochemical Corrosion Testing?
Electrochemical corrosion testing uses controlled electrical measurements to characterize the corrosion behavior of metals and alloys in aqueous or other electrolytic environments. By measuring electrode potentials, current densities, and impedance responses, electrochemical techniques provide quantitative information about corrosion rate, mechanism, passivation behavior, pitting susceptibility, and coating integrity—often in hours or days rather than the months required for traditional immersion testing.
Electrochemical corrosion testing is fundamental to materials selection, corrosion protection design, and quality assurance in the oil and gas, chemical processing, marine, automotive, aerospace, and nuclear industries.
Fundamental Electrochemical Concepts
Corrosion Cell
A corrosion cell consists of an anode (where metal oxidizes: M → Mⁿ⁺ + ne⁻) and a cathode (where reduction occurs: typically O₂ + 2H₂O + 4e⁻ → 4OH⁻ or 2H⁺ + 2e⁻ → H₂). The corrosion current between anode and cathode directly represents the metal dissolution rate.
Corrosion Potential (Ecorr)
The open-circuit potential at which anodic and cathodic current densities are equal. Ecorr is measured by monitoring the potential of the working electrode (metal under study) vs. a reference electrode (SCE, Ag/AgCl, Cu/CuSO₄) in the test solution at zero applied current.
Passive Film
Many metals (stainless steel, titanium, aluminum) form a thin, adherent oxide film that dramatically reduces the corrosion rate. The passive region appears as a flat, low-current plateau on the anodic polarization curve.
Key Electrochemical Test Methods
Potentiodynamic Polarization (ASTM G5, G61, G150)
The working electrode potential is swept from below Ecorr to progressively more positive values while measuring the current density. The resulting polarization curve (Tafel plot) reveals:
- Corrosion potential (Ecorr)
- Corrosion current density (icorr): Extrapolated from the Tafel region; directly converts to corrosion rate (mm/year or mpy)
- Passivation potential (Epass) and passive current density (ipass): Indicates formation of protective passive film
- Pitting potential (Epit / Eb): The potential at which pitting corrosion initiates (current suddenly increases from the passive region)
- Protection potential (Eprot): The potential below which existing pits repassivate
| Parameter | Electrochemical Method | Significance |
| Ecorr | OCP measurement | Nobility of metal in environment |
| icorr | Tafel extrapolation | Corrosion rate |
| Epit | Potentiodynamic polarization | Pitting susceptibility |
| Passive region width | Polarization curve | Passivation stability |
Electrochemical Impedance Spectroscopy (EIS) (ASTM G106)
A small-amplitude AC perturbation is applied to the working electrode at a range of frequencies (0.01 Hz to 100 kHz). The impedance response is modeled with an equivalent electrical circuit to characterize:
- Solution resistance (Rs)
- Charge transfer resistance (Rct): Inversely proportional to corrosion rate
- Double layer capacitance (Cdl): Related to active surface area
- Coating resistance and capacitance: For coated substrates
EIS is particularly powerful for non-destructive monitoring of coating degradation and for mechanistic studies of passivation and inhibitor performance.
Linear Polarization Resistance (LPR) (ASTM G59)
A small potential perturbation (±10–20 mV around Ecorr) is applied and the resulting current measured. The polarization resistance (Rp = ΔE/ΔI) is inversely proportional to corrosion rate. Fast, real-time corrosion rate monitoring—used in corrosion inhibitor testing and continuous pipeline monitoring systems.
Zero Resistance Ammetry (ZRA)
Measures the galvanic coupling current between dissimilar metals under zero applied potential. Quantifies galvanic corrosion risk in bimetallic assemblies.
Why Choose Infinita Lab for Electrochemical Corrosion Testing?
Infinita Lab offers comprehensive electrochemical corrosion testing including potentiodynamic polarization, EIS, LPR, galvanic corrosion, and crevice and pitting evaluation through its nationwide accredited corrosion testing laboratory network. Our corrosion specialists design testing programs that accurately represent your material-environment system.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
How is corrosion rate calculated from the corrosion current density (icorr)? Corrosion rate (CR) in mm/year = (icorr × M × K) / (n × ρ), where M is the atomic weight of the metal, n is the number of electrons transferred per atom, ρ is the metal density, and K is a conversion factor (3.27 × 10⁻³ for mm/year with icorr in µA/cm²). This Faradaic conversion is valid for uniform corrosion; it overestimates average penetration for localized corrosion modes.
What is the significance of the pitting potential (Epit) in stainless steel testing? Epit (also called the breakdown potential Eb) is the potential at which the passive film on stainless steel is locally breached and stable pits initiate. A noble (more positive) Epit indicates better pitting resistance. The gap between Ecorr and Epit is the passivity window—the range of potential over which the passive film is stable. A large passivity window indicates more robust pitting resistance.
What test solution is typically used for electrochemical testing of stainless steels per ASTM G61? ASTM G61 uses a 3.5% NaCl aqueous solution at room temperature as the test electrolyte for cyclic potentiodynamic polarization of iron, nickel, and cobalt alloys. This simulates seawater chloride concentration—a common aggressive service environment. Other solutions (H₂SO₄, HCl, NaOH) may be specified for applications in other chemical environments.
How does EIS differ from DC polarization for evaluating coating performance? DC polarization requires sufficient corrosion activity to produce measurable currents and can accelerate degradation of the coating being tested. EIS uses non-destructive, small-amplitude AC perturbation and can characterize intact coatings with very high resistance (Rcoating > 10¹⁰ Ω·cm²) that would show no measurable DC corrosion current. EIS tracks the progressive degradation of coating resistance and capacitance over time, providing early warning of coating failure before visible corrosion appears.
What is the Tafel slope and why is it important in corrosion rate determination? Tafel slopes (ba for anodic, bc for cathodic) are the slopes of the linear (Tafel) regions of the semi-logarithmic polarization curve (log|i| vs. E). They are required for accurate Tafel extrapolation to determine icorr. Tafel slopes for common metal-electrolyte systems range from 60–120 mV/decade. Significant deviation from linearity in the Tafel region (due to mass transport, passivation, or mixed control) increases the uncertainty of Tafel extrapolation.
