Corrosion Testing Methods: A Comprehensive Guide to Evaluating Metal Degradation
Corrosion testing encompasses a remarkably diverse collection of laboratory and field methods — each designed to evaluate specific aspects of metal degradation behavior under defined conditions. Selecting the right corrosion testing method requires matching the test to the corrosion mechanism of concern, the material being evaluated, the service environment being simulated, and the information required — whether that is a simple pass/fail rating, a corrosion rate in mm/year, a pitting potential in volts, or a stress corrosion cracking threshold stress intensity. In the metals & infrastructure industry, understanding the available corrosion test methods and their respective strengths and limitations is essential for designing effective corrosion evaluation programs.
Immersion Testing
Weight Loss Coupon Testing — ASTM G31, G46
The simplest and most widely applicable corrosion test method — metal coupons of defined size and mass are immersed in a test solution for a defined period, removed, cleaned of corrosion products per ASTM G1 (acid descaling), dried, and reweighed. The mass loss is used to calculate the average corrosion rate:
Corrosion rate (mpy) = (Weight loss × K) / (Density × Area × Time)
Where K is a unit conversion constant appropriate for the time and area units used.
Weight-loss coupon testing is the universal baseline method for determining corrosion rate — simple, inexpensive, and applicable to virtually any metal-environment combination. Its primary limitation is that it measures average corrosion rate over the test period, potentially masking initial or final rate transients.
Electrochemical Testing Methods
Linear Polarization Resistance (LPR) — ASTM G59
LPR applies a small potential perturbation (typically ±10–20 mV vs. open circuit) to a metal specimen and measures the resulting current response. The ratio of potential to current (polarization resistance, Rp) is inversely proportional to the instantaneous corrosion rate through the Stern-Geary relationship:
icorr = B / Rp
Where B is the Stern-Geary coefficient (typically 13–26 mV for most systems), LPR provides non-destructive, continuous or periodic measurement of corrosion rate — making it the dominant method for real-time corrosion monitoring of probes in pipelines, process vessels, and storage tanks.
Potentiodynamic Polarization — ASTM G5, G61
Potentiodynamic polarization systematically scans the electrode potential from below the corrosion potential to above it (anodic scan) or from above to below (cathodic scan), measuring the resulting current density. The resulting polarization curve reveals:
- Corrosion potential (Ecorr) — the potential at which the metal freely corrodes
- Passive region — where current density is low despite increasing potential (passive film formation)
- Pitting potential (Epit) — where current increases sharply as pitting initiates (for passive metals)
- Tafel slopes — used to calculate Stern-Geary coefficient B for LPR calibration
Electrochemical Impedance Spectroscopy (EIS) — ASTM G106
EIS applies a small AC voltage perturbation over a wide frequency range and measures the impedance response, thereby separating solution resistance, charge-transfer resistance, and diffusion-related impedance components. EIS characterizes:
- Coating barrier performance and water uptake in organic coatings
- Passive film properties of corrosion-resistant alloys
- Corrosion inhibitor effectiveness
- Mechanism changes during long-term exposure
Electrochemical Noise (ECN)
ECN measures spontaneous current and potential fluctuations arising from corrosion processes — particularly localized corrosion (pitting, crevice) that produces characteristic current transients. ECN is particularly valuable for detecting the onset of localized corrosion before it becomes visible or measurable by other techniques.
Accelerated Atmospheric Corrosion Testing
Salt Spray (Fog) Testing — ASTM B117, ISO 9227
As discussed in Blog 44, salt spray testing is the most widely used accelerated corrosion test for coatings and materials in industrial practice. Specimens are exposed to a continuous 5% NaCl mist at 35°C, providing a severe, reproducible corrosion environment for comparative evaluation.
Prohesion (ASTM G85 Annex A5)
Prohesion uses alternating salt spray (0.05% ammonium sulfate + 0.35% NaCl) and drying cycles that better correlate with outdoor atmospheric corrosion for many coating systems than continuous ASTM B117 salt spray.
Kesternich Test (ISO 6988, DIN 50018)
The Kesternich test exposes specimens to SO₂ in high-humidity condensing conditions — simulating industrial atmospheric corrosion with acid rain and sulfurous pollution. Particularly relevant for the metals & infrastructure industry, where electrical infrastructure, transportation components, and building materials are exposed to sulfurous industrial atmospheres.
High-Temperature Corrosion Testing
Oxidation Testing — ASTM C633 (Modified)
High-temperature oxidation testing measures oxide scale growth on metallic specimens as a function of temperature and time — critical for alloys used in gas turbines, industrial furnaces, and chemical reactor components. Cyclic oxidation testing (heating and cooling cycles) evaluates oxide scale spallation resistance — the key durability parameter for thermal barrier coating systems and heat-resistant alloys.
Molten Salt Corrosion Testing
For materials used in concentrated solar power (CSP) plants, nuclear molten salt reactors, and high-temperature chemical processes, immersion in molten nitrate or chloride salts at 500–700°C is a key indicator of corrosion resistance in an environment that differs markedly from aqueous electrochemical corrosion and requires specialized containment and measurement techniques.
Specialized Corrosion Testing
Intergranular Corrosion — ASTM A262
ASTM A262 provides five practices for detecting susceptibility to intergranular corrosion in austenitic stainless steels — used to verify that welded or heat-treated components have not undergone sensitization that could cause grain-boundary attack in service.
Galvanic Corrosion — ASTM G82, G71
ASTM G71 and G82 provide methods for measuring galvanic current between coupled dissimilar metals and evaluating galvanic corrosion rates — essential for material selection in mixed-metal assemblies for marine, aerospace, and automotive applications.
Conclusion
Effective corrosion testing requires matching the method to the mechanism — weight loss coupons for baseline corrosion rates, electrochemical techniques for real-time monitoring and passive film characterization, accelerated atmospheric tests for coating qualification, and specialized methods for high-temperature and intergranular attack. No single test covers every scenario, and building a corrosion evaluation program around the right combination of methods yields actionable data for material selection, design validation, and asset life prediction.
Why Choose Infinita Lab for Corrosion Testing?
Infinita Lab provides the complete suite of corrosion testing methods — immersion testing (ASTM G31), electrochemical methods (LPR, EIS, potentiodynamic polarization per ASTM G5/G59/G106), salt spray (ASTM B117/ISO 9227), specialized methods (ASTM G48, A262, G71, NACE TM0177/TM0284), and high-temperature oxidation testing — serving the metals & infrastructure industry with comprehensive corrosion evaluation programs for material selection, coating qualification, failure investigation, and regulatory compliance. Our corrosion laboratory is staffed by experienced electrochemists and materials scientists who design and execute corrosion test programs that deliver actionable, defensible data. Contact Infinita Lab at infinitalab.com to discuss corrosion testing for your application.
Frequently Asked Questions
Which corrosion test method provides the most reliable prediction of field performance? No single method perfectly predicts field performance. The most reliable approach combines standardized laboratory testing for material ranking, electrochemical methods for mechanism understanding, and validation against outdoor exposure data. Methods replicating actual service temperature, chemistry, stress state, and geometry provide the best correlation.
How are corrosion test methods selected for a specific application? Method selection is driven by corrosion mechanism of concern, material class, service environment, and required output. Applicable industry standards from NACE, ASTM, and ISO for the specific application and material combination provide the most structured and defensible starting point for method selection.
What is the significance of solution deaeration or aeration in corrosion testing? Dissolved oxygen profoundly affects corrosion behavior in carbon steel, copper alloys, and stainless steels. Deaerated solutions simulate closed sealed systems while aerated solutions simulate open atmospheric systems. Testing under both conditions characterizes oxygen sensitivity relevant to service environments where oxygen content varies.
How is corrosion testing performed at elevated temperature under pressure? Autoclave systems — sealed pressure vessels maintaining defined temperature and pressure during specimen immersion — enable elevated temperature and pressure corrosion testing. Autoclave testing is standard for oil and gas, nuclear, and chemical process material qualification, simulating downhole and high-pressure reactor conditions impossible to replicate in open vessels.
What quality assurance measures are essential for reliable corrosion test data? Essential QA measures include calibrated instruments traceable to standards, certified reference solution compositions, solution analysis before and after testing, reference material testing alongside experimental specimens, and complete results.
