Corrosion Testing Methods: Weight Loss, Electrochemical, and Salt Spray

Selecting the right corrosion test method is one of the most consequential decisions in materials qualification and service life assessment — because no single test technique captures all corrosion mechanisms, and applying the wrong method to the wrong problem produces data that is meaningless or actively misleading. This article provides a systematic technical treatment of the principal corrosion testing methods — weight loss immersion, potentiodynamic polarisation, electrochemical impedance spectroscopy (EIS), salt spray, cyclic corrosion testing, and standardised intergranular and SCC tests — covering the underlying electrochemistry, the governing ASTM and ISO standards, and the engineering criteria for method selection.

1. Weight Loss (Coupon) Testing — ASTM G31

Weight loss immersion testing is the simplest, most direct, and most interpretable of all corrosion test methods. A pre-weighed, dimensioned specimen of known area is immersed in the test solution for a defined period, cleaned of corrosion products by a standard procedure (ASTM G1), and reweighed. The mass difference is the total metal loss, from which a uniform corrosion rate is calculated. The calculator above implements this calculation to ASTM G1/G31.

1.1 Specimen Preparation and Cleaning (ASTM G1)

Specimen preparation is critical and often the largest source of error in weight loss measurements. ASTM G1 specifies:

• Surface condition: machine or grind to 120-grit finish minimum; degrease with acetone or alcohol; dry in desiccator; weigh to ±0.1 mg (analytical balance).
• Post-exposure cleaning: chemical cleaning using the alloy-specific reagent in ASTM G1 Table 1 (e.g., 15% HCl + 3.5 g/L hexamethylenetetramine inhibitor at 25°C for 1 min for carbon steel; Clarke’s solution for copper; 50% HNO₃ for stainless steel) to dissolve corrosion products without attacking the base metal.
• Blank correction: run a set of identical specimens through the same cleaning procedure without immersion exposure. Their mass change (typically <0.5 mg) is subtracted from the exposed specimen mass loss.
• Replicate specimens: minimum three coupons per condition per time interval; report mean and standard deviation.

1.2 Corrosion Rate Formulae

ASTM G1 / G31 Corrosion Rate Formulae:

  CR (mm/yr) = (87,600 × W) / (D × A × T)

  CR (mpy)   = (534 × W)    / (D × A × T)

  mdd        = (1,000,000 × W) / (A_dm2 × T_days)
             = (10,000 × W)  / (A_cm2 × T/24)

Where:
  W      = mass loss (g)           — from ASTM G1 cleaning procedure
  D      = material density (g/cm³)
  A      = exposed area (cm²)      — total wetted surface (faces + edges)
  T      = exposure time (hours)

Unit conversions:
  1 mm/yr = 39.37 mpy
  1 mpy   = 0.0254 mm/yr
  mm/yr → mdd:  mdd = CR(mm/yr) × D × 1000 / 365.25

Qualitative corrosion rate classification (NACE corrosion engineering):
  < 0.1 mm/yr   — Excellent (suitable for most corrosion-critical service)
  0.1–0.5 mm/yr — Good (acceptable with adequate corrosion allowance)
  0.5–1.0 mm/yr — Acceptable (monitor; specify corrosion allowance in design)
  1.0–5.0 mm/yr — Poor (short service life; materials change indicated)
  > 5.0 mm/yr   — Unacceptable (rapid deterioration; immediate action required)

1.3 Practical Considerations and Limitations

Weight loss testing gives total uniform mass loss averaged over the exposure area and time. It cannot detect localised corrosion (pitting, crevice) that may cause component perforation at rates far below the average calculated corrosion rate. A specimen with 10 pits losing 5% of its mass to pitting has a very different failure timeline than one losing the same mass uniformly. Always visually examine and optically measure pit dimensions on post-exposure specimens, reporting pit depth and density alongside the average corrosion rate. The relevant ASTM standard for pitting measurement on weight loss coupons is ASTM G46.

2. Electrochemical Testing Methods

Electrochemical techniques exploit the fact that corrosion is an electrochemical process — metal dissolution (anodic) is coupled with a cathodic reduction reaction (O₂ reduction or H⁺ reduction) and both are governed by electrode potential. By controlling potential or current in a three-electrode cell (working electrode = test specimen, reference electrode, counter electrode), the corrosion behaviour can be characterised orders of magnitude faster than immersion testing.

2.1 Potentiodynamic Polarisation (ASTM G5, G61, G150)

A potentiostat scans the specimen potential from cathodic to anodic values at a defined scan rate (typically 0.1–1 mV/s for quasi-steady-state data) and records the resulting current density. The resulting E–log|i| curve (Evans diagram, or polarisation curve) reveals:

• E_corr (open-circuit potential): the mixed potential at which anodic and cathodic currents are equal in magnitude. No thermodynamic significance for corrosion rate without i_corr.
• i_corr (corrosion current density): obtained by Tafel extrapolation of the linear anodic and cathodic portions of the log|i| vs. E plot to E_corr. Requires well-developed Tafel behaviour (>100 mV linear region) to be valid.
• Passive region: the potential range over which current density drops to i_pass (<10–100 μA/cm²) — indicates formation of a protective oxide film. The width and stability of the passive region are alloy and environment specific.
• E_pit (pitting potential): the potential at which current density suddenly increases sharply, indicating passive film breakdown and pit initiation. E_pit is used to rank pitting resistance of different alloys in the same environment, or one alloy across environments. Higher E_pit = better pitting resistance.
• E_rep (repassivation potential, also called E_pp): measured during the return scan, the potential at which current returns to passive levels. The hysteresis loop area between forward and reverse scans (E_pit – E_rep) quantifies the stability of propagating pits. Large E_pit – E_rep gaps indicate pits that, once initiated, are difficult to repassivate.

ASTM G150 (critical pitting temperature, CPT) uses galvanostatic or potentiostatic polarisation in a specifically temperature-controlled cell to determine the minimum temperature at which pitting initiates on stainless steel in 1 M NaCl at +700 mV (SCE). CPT is used to rank stainless grades for chloride pitting resistance alongside the PREN index discussed in our pitting corrosion article.

2.2 Tafel Analysis and Corrosion Rate from Polarisation

Tafel extrapolation:
  For E significantly anodic of Eₚᵓᵉᵉ:   log(i) = log(iₚᵓᵉᵉ) + (E - Eₚᵓᵉᵉ) / βₐ
  For E significantly cathodic of Eₚᵓᵉᵉ:  log(i) = log(iₚᵓᵉᵉ) - (E - Eₚᵓᵉᵉ) / βₚ

  where βₐ, βₚ = anodic and cathodic Tafel slopes (mV/decade)
  (typical values: βₐ = 60–120 mV/dec, βₚ = 120–180 mV/dec)

Converting iₚᵓᵉᵉ to corrosion rate (Faraday's law):
  CR (mm/yr) = (iₚᵓᵉᵉ × M × 3600 × 8760) / (n × F × D × 10000)
             = (iₚᵓᵉᵉ [A/cm²] × M [g/mol] × 3.272×10⁷) / (n × F × D)

  where:
    M = atomic / equivalent molar mass (g/mol); for steel, Mₚᵗ = 55.85
    n = number of electrons transferred (2 for Fe → Fe²⁺; 3 for Fe → Fe³⁺)
    F = Faraday's constant = 96,485 C/mol
    D = density (g/cm³)

Stern-Geary polarisation resistance method (ASTM G59):
  iₚᵓᵉᵉ = B / Rᵖ     where B = (βₐ × βₚ) / (2.303 × (βₐ + βₚ))
  B ≈ 26 mV for active systems (e.g., carbon steel in acid)
  B ≈ 52 mV for passive systems (e.g., stainless steel in neutral solution)

  Rᵖ = dE/di at E = Eₚᵓᵉᵉ — slope of linear polarisation curve (Ω·cm²)

2.3 Electrochemical Impedance Spectroscopy (EIS)

EIS applies a small sinusoidal voltage perturbation E(t) = E₀ sin(ωt) (typically 5–10 mV amplitude, which keeps the system in a quasi-linear regime) and measures the frequency-dependent current response I(t) = I₀ sin(ωt + φ), where φ is the phase shift. The complex impedance Z(ω) = E(t)/I(t) is measured across a frequency range of typically 10 kHz to 10 mHz (or lower for very slow diffusion-limited systems).

The data are displayed as a Nyquist plot (−Im(Z) vs. Re(Z)) or Bode plot (|Z| and phase angle φ vs. log frequency) and fitted to an equivalent circuit model. Common elements:

• R_s: solution (electrolyte) resistance — high-frequency intercept on Nyquist real axis.
• R_p (= R_ct): charge transfer resistance (= polarisation resistance). Low-frequency intercept − R_s on Nyquist real axis. Gives i_corr via Stern-Geary.
• C_dl (or CPE): double-layer capacitance — controls the arc diameter transition frequency. For rough or corroded surfaces, a constant phase element (CPE) replaces the ideal capacitor, with n-exponent measuring surface heterogeneity (n = 1 = ideal capacitor, n = 0.5 = Warburg diffusion element).
• Warburg impedance (W): mass-transport (diffusion) element appearing as a 45° line at low frequency on Nyquist plot, indicating diffusion-controlled kinetics (e.g., O₂ reduction on passive films).

EIS’s primary engineering advantage over DC polarisation is that it is effectively non-destructive — the small-amplitude perturbation does not significantly alter surface morphology, making it ideal for monitoring the same specimen or in-situ system over time. It is used for coating characterisation (film resistance R_f and coating capacitance C_c from two-time-constant equivalent circuits), corrosion inhibitor evaluation, and continuous in-process corrosion monitoring in chemical plant.

3. Salt Spray and Accelerated Atmospheric Testing

3.1 Neutral Salt Spray — ASTM B117 / ISO 9227

The neutral salt spray (NSS) test has been the dominant accelerated corrosion screening test for coated metals since its ASTM standardisation in 1939 (ASTM B117). The test environment is:

• 5 wt% NaCl solution, pH 6.5–7.2
• Temperature: 35°C ± 2°C
• Continuous salt fog spray: 1–2 mL/80 cm²/h collection rate
• Specimen angle: 15–30° from vertical, facing the spray nozzle

ASTM B117 does not define pass/fail criteria — it specifies only the test environment. Pass/fail and exposure duration are defined in product specifications. Typical durations: 96–240 h for hot-dip galvanised or zinc electroplate; 500–1000 h for high-build epoxy primers; 1000–4000 h for aerospace-grade primers + topcoats; 5000+ h for automotive OEM exterior panel systems.

3.2 Acetic Acid Salt Spray (CASS) and Copper-Accelerated Salt Spray (CASS) — ASTM B368

CASS (ASTM B368) adds 0.26 g/L CuCl₂ to the standard 5% NaCl solution and acidifies to pH 3.1–3.3 with acetic acid. The test temperature is 49°C ± 2°C. The copper ions and acid aggressively attack the zinc or nickel undercoat beneath chromate or lacquer layers, providing an accelerated test particularly relevant to decorative chromium-plated and anodised aluminium finishes. 16 h CASS is roughly equivalent to 96–168 h ASTM B117 for the same coating systems.

3.3 Cyclic Corrosion Testing — SAE J2334, ISO 11997, Volvo VCS

Cyclic corrosion tests alternate between salt wet, humid, and dry phases to simulate the wet-dry cycling of real atmospheric exposure. The SAE J2334 cycle is:

• Humidity phase: 100% RH at 50°C, 6 h
• Salt soak: immersion in 0.5% NaCl + 0.1% CaCl₂ + 0.075% NaHCO₃ at 25°C, 15 min
• Dry phase: 50°C at 50% RH, 17.75 h

This 24-hour cycle is repeated for 60–80 cycles (60–80 days) for automotive underbody and closure panels. Validation studies show SAE J2334 results correlate with 4–5 years of outdoor Michigan exposure on automotive steel systems, whereas ASTM B117 correlations are inconsistent. ISO 11997-1 (Cycle A) and ISO 11997-2 (Cycle B) are the international variants used in European automotive and marine coating qualification.

4. Immersion Testing Standards and Specialised Methods

5. Intergranular Corrosion Testing — ASTM A262 in Detail

Intergranular corrosion (IGC) testing is one of the most practically critical corrosion qualification tests for austenitic stainless steel fabrications used in chemical, pharmaceutical, and nuclear applications. ASTM A262 defines a hierarchy of five practices of increasing sensitivity and specificity, used to detect sensitisation (Cr₂₃C₆ precipitation from welding or elevated-temperature exposure — see our article on welding austenitic stainless steel).

5.1 Practice A — Oxalic Acid Etch (Screening)

Electrolytic etching at 1 A/cm² in 10% oxalic acid for 1.5 minutes at ambient temperature. Microstructural examination at 250× under optical microscope. Microstructural classification:

• Step structure: grain boundaries have no ditches — no Cr₂₃C₆ precipitation. Acceptable; no further testing required.
• Dual structure: some ditching present at grain boundaries but no grains completely surrounded by ditches. Further testing by Practice B, C, or E may or may not be required depending on specification.
• Ditch structure: continuous ditching; one or more grains completely surrounded by ditches. Indicates sensitisation — Practices B, C, or E must be performed; component is suspect.

5.2 Practice B — Streicher Test (FeSO₄-H₂SO₄)

Specimens are immersed in boiling ferric sulphate – sulphuric acid solution (25 g FeSO₄·7H₂O + 236 mL H₂SO₄ [98%] + water to 1 L) for 120 hours. Corrosion rate is measured by weight loss and compared to a sensitised reference specimen. Accepted when corrosion rate is less than a specified maximum (typically 10–30 g/m²/h depending on grade).

5.3 Practice E — Strauss Test (CuSO₄–H₂SO₄)

The most widely specified industrial test. Specimens are immersed in boiling copper sulphate – sulphuric acid solution (36 g CuSO₄·5H₂O + 35 mL H₂SO₄ [98%] + water to 1 L) for 15 hours. After exposure, each specimen is bent through 180° around a mandrel equal to its own thickness. Cracks on the outer bent surface, examined at 20× under a stereo microscope, indicate intergranular attack. The bent-beam test is more sensitive to IGC at lower penetration depths than weight loss alone. This test is routinely performed on weld procedure qualification test pieces for pressure vessels and piping per ASME VIII and process plant specifications.

6. EIS — Equivalent Circuit Modelling

7. Selecting the Right Corrosion Test

The most common error in corrosion testing is applying a convenient test rather than the mechanistically correct one. The selection framework below maps each mechanism to the appropriate primary standard and output parameter.

Understanding the corrosion mechanisms in your system is the prerequisite for test selection — our articles on corrosion mechanisms, stress corrosion cracking, and pitting corrosion provide the mechanistic foundation. Hardness testing (per our hardness testing methods article) remains critical for NACE MR0175 qualification alongside the electrochemical and immersion methods above.

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