What is a corrosion rate of 1 mpy (mil per year) in mm/year?

1 mil per year (mpy) = 0.0254 mm/year = 25.4 µm/year. Conversely, 1 mm/year = 39.37 mpy. These are the most commonly used units in corrosion engineering. In oilfield and process industries, mpy (imperial) is widely used in North America, while mm/year and µm/year are preferred in European and ISO standards. Mass loss rate in g/m²/day is used when comparing different metals or when the corrosion product is being monitored in an effluent stream. The relationship: g/m²/day = CR (mm/year) × D (g/cm³) × 10⁷/365.25/10 = CR × D × 2739.7, where D is density in g/cm³.

25 March 2026· 17 min read· Calculator ASTM G1 / G31 Corrosion Science

Corrosion Rate Calculator — Weight Loss, LPR, and Unit Conversion (ASTM G1/G31/G96)

Q: What is the ASTM G1 formula for corrosion rate from weight loss?

The ASTM G1/G31 formula for corrosion rate from gravimetric weight loss is: CR (mm/year) = 87.6 × W / (D × A × T), where W is the weight loss in milligrams, D is the metal density in g/cm³, A is the total exposed surface area in cm², and T is the exposure time in hours. The constant 87.6 includes unit conversions to give the result in mm/year. For mils per year (mpy): CR (mpy) = 3445 × W / (D × A × T). For µm/year: multiply mm/year by 1000. The formula assumes uniform corrosion across the entire exposed surface — if corrosion is non-uniform (pitting, crevice), the calculated mm/year represents an average rate and underestimates the severity at localised attack sites.

Q: How should corrosion test coupons be prepared and cleaned per ASTM G1?

ASTM G1 specifies precise preparation and cleaning procedures for corrosion test coupons. Before exposure: coupons must be abraded to a uniform surface finish (typically 600-grit SiC paper), degreased with acetone, dried, and weighed to ±0.1 mg on an analytical balance. All surface area (including edges and holes) must be measured and recorded. After exposure: loose corrosion products are removed by scrubbing under running water, then the specific chemical cleaning solution for the metal is applied (e.g., Clark's solution — 20 g antimony trioxide + 50 g stannous chloride in 1 litre HCl — for carbon steel). Multiple cleaning cycles are performed until the weight stabilises within 0.1 mg between consecutive cleanings. A blank coupon (same material, same cleaning, no exposure) is used to determine the cleaning weight loss, which is subtracted from the test coupon weight loss.

Q: What corrosion rate is acceptable for carbon steel in process piping?

Acceptable corrosion rates for carbon steel in process piping depend on the design life and the corrosion allowance built into the pipe wall thickness. As general guidance: 1.0 mm/year (>40 mpy) is unacceptable for long-term service without aggressive inhibition, coating, or material upgrade. API RP 14E and NORSOK M-001 specify acceptable corrosion rates for oilfield production systems — typically <0.1 mm/year is targeted with chemical inhibition.

Q: What is the linear polarisation resistance (LPR) method for corrosion rate measurement?

Linear polarisation resistance (LPR) is an electrochemical technique that measures corrosion rate in real-time without removing metal from the surface. A small potential perturbation (±10–20 mV around the open-circuit potential) is applied to the working electrode, and the resulting current is measured. The polarisation resistance Rp = ΔE/Δi (Ω·cm²). The corrosion current density is then calculated using the Stern-Geary equation: i_corr = B/Rp, where B = βa×βc / (2.303×(βa+βc)) is the Stern-Geary constant (typically 13–52 mV for most metals). Finally: CR (mm/year) = i_corr × M / (n × F × D) × 3.27×10³, where M is atomic mass, n is valency, F is Faraday's constant, and D is density. LPR probes allow continuous corrosion monitoring in process streams, pipelines, and cooling water systems without plant shutdown.

Q: What is a corrosion allowance and how is it calculated?

Corrosion allowance (CA) is additional wall thickness added to a pressure vessel or pipe wall beyond the minimum thickness required for pressure containment, to accommodate metal loss over the design life without falling below the minimum safe thickness. CA = CR × design life (years), where CR is the expected corrosion rate in mm/year. For a 20-year design life at 0.1 mm/year: CA = 20 × 0.1 = 2.0 mm. API 579-1 / ASME FFS-1 and ASME B31.3 provide methods for assessing remaining service life when the actual corrosion rate is measured during in-service inspection: remaining life = (actual wall thickness − t_min) / CR, where t_min is the minimum required wall thickness from pressure calculations.

Q: How is corrosion inhibitor effectiveness measured from coupon data?

Corrosion inhibitor effectiveness (IE%) is calculated by comparing corrosion rates with and without inhibitor from paired coupon tests: IE% = (CR_uninhibited − CR_inhibited) / CR_uninhibited × 100%. For example, if uninhibited CR = 2.0 mm/year and inhibited CR = 0.15 mm/year: IE% = (2.0 − 0.15)/2.0 × 100% = 92.5%. ASTM G31 specifies that inhibitor coupon tests should be run in triplicate, with statistical analysis of results. The Langmuir adsorption isotherm is the most common model for interpreting inhibitor effectiveness as a function of inhibitor concentration. Process plant typically targets IE% > 90% at the specified inhibitor dosing rate before approving a new inhibitor formulation.

Q: What is the difference between uniform corrosion rate and pit depth?

The ASTM G1/G31 weight-loss formula calculates the average uniform corrosion rate — the equivalent depth of metal removed uniformly across the entire exposed surface. If pitting is present, the actual pit depth at the deepest point will be much greater than this average. The pitting factor (PF) = maximum pit depth / average uniform penetration depth quantifies the non-uniformity. A PF of 1 indicates perfectly uniform attack; PF > 5 indicates concentrated localised pitting. For pitting-susceptible alloys or environments, average corrosion rate from weight loss significantly underestimates the maximum penetration rate at individual pits. Pit depth measurement (dial gauge, optical profilometry, or ultrasonic) is required in addition to weight loss to fully characterise pitting attack. ASTM G46 covers examination and evaluation of pitting corrosion.

Quantifying corrosion rate is the foundation of materials selection, corrosion allowance design, inhibitor qualification, and remaining-life assessment for process plant, pipelines, and marine structures. This calculator implements three complementary methods: the gravimetric weight-loss formula per ASTM G1/G31 (the reference method for laboratory immersion testing), the linear polarisation resistance (LPR) method for converting electrochemical measurements to corrosion rate per ASTM G96/G59, and a unit conversion mode for converting between mm/year, mils per year (mpy), μm/year, and g/m²/day. A batch coupon comparison table allows up to eight coupons to be evaluated side by side, and an inhibitor effectiveness mode computes IE% from inhibited and uninhibited coupon pairs.

Key Takeaways

ASTM G1/G31 weight-loss formula: CR (mm/year) = 87.6 × W / (D × A × T), where W is weight loss (mg), D is density (g/cm³), A is exposed area (cm²), and T is exposure time (hours).
Unit conversions: 1 mm/year = 39.37 mpy = 1,000 μm/year. Mass loss rate (g/m²/day) = CR (mm/year) × D (g/cm³) × 2739.7.
Corrosion severity classification per NACE: <0.025 mm/year = excellent; 0.025–0.1 = good; 0.1–0.5 = fair; 0.5–1.0 = poor; >1.0 = unacceptable.
LPR method: CR (mm/year) = (B / R_p) × (M / (n × F × D)) × 3.27×10³. Use B = 26 mV (Stern-Geary constant) for most structural metals as a first estimate.
Corrosion allowance = CR × design life (years). For a 25-year design with CR = 0.1 mm/year: CA = 2.5 mm, added to the minimum pressure-design wall thickness.
Inhibitor effectiveness IE% = (CR_uninhibited − CR_inhibited) / CR_uninhibited × 100%. Process plant typically targets IE% ≥ 90% at specified inhibitor dosage.

Corrosion Rate Calculator

4 modes: weight-loss (ASTM G1) • LPR electrochemical • unit conversion • inhibitor effectiveness

Weight Loss
ASTM G1/G31

LPR / Stern-Geary
ASTM G96/G59

Unit Conversion
mm/yr ↔ mpy ↔ μm/yr

Inhibitor IE%
Effectiveness

Coupon label

Weight loss W (mg)

Before − after cleaning. Subtract blank coupon loss.

Exposure time T (h)

ASTM G31: minimum 96 h; 168–720 h recommended

Surface area A (cm²)

Total exposed area (all faces + edges)

Metal / Density

Custom density (g/cm³)

Design life (years)

Optional: calculates corrosion allowance

Please complete all required fields.

Batch Coupon Comparison

#	Label	W (mg)	A (cm²)	T (h)	mm/yr	mpy	g/m²/d	CA (mm)	Severity
No coupons added. Calculate then click “Add to Batch”.

Figure 1. Left: ASTM G31 laboratory immersion corrosion test setup. The coupon is suspended in test solution in a reflux flask with temperature control and a condenser to prevent concentration changes from evaporation. The ASTM G1 formula is shown at the base. Right: Fontana’s eight forms of corrosion with schematic cross-sections. Weight-loss calculations yield average uniform corrosion rates — separate assessment is required for pitting, SCC, intergranular, and erosion-corrosion damage modes. The LPR Stern-Geary plot shows how polarisation resistance R_p (the slope of the E vs i curve at OCP) is used to calculate corrosion current density. © metallurgyzone.com

The ASTM G1/G31 Gravimetric Method: Formula and Procedure

The gravimetric weight-loss method is the reference technique for quantifying uniform corrosion rate in laboratory immersion testing. It is inherently accurate, requires no calibration, and is directly traceable to mass measurement. ASTM G31 (Standard Guide for Laboratory Immersion Corrosion Testing of Metals) specifies the complete test procedure; ASTM G1 (Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens) governs coupon preparation, cleaning, and mass measurement.

The ASTM G1 Formula

CR (mm/year) = 87.6 × W / (D × A × T)

where:
  W  = weight loss [mg] = initial mass − final mass after cleaning − blank correction
  D  = metal density [g/cm³]
  A  = total exposed surface area [cm²] (all faces + edges + holes)
  T  = exposure time [hours]
  87.6 = dimensional constant (= 8.76×10¯ mm/cm × 10³ mg/g / (8760 h/year))

Alternative forms:
  CR (mpy)    = 3,445 × W / (D × A × T)     (1 mpy = 0.0254 mm/year)
  CR (μm/yr) = 87,600 × W / (D × A × T)
  CR (μm/h)  = CR(mm/yr) / 8.76

Mass loss rate (g/m²/day) = 10,000 × W / (A × T/24)
  = 24,000 × W / (A × T)  (A in cm², T in hours)

Unit Conversions

1 mm/year     = 39.370 mpy
              = 1,000 μm/year
              = 0.03937 inches/year
              = 1.140 μm/h
              = 8,760 μm/h × 1/1000 mm/μm × 8,760 h/year

Mass loss rate:
  g/m²/day = CR (mm/year) × D (g/cm³) × 2,739.7

Example verification: CR = 1 mm/year, D = 7.87 g/cm³:
  g/m²/day = 1 × 7.87 × 2739.7 = 21,571 g/m²/day  = 21.6 kg/m²/day

ASTM G1 Coupon Preparation and Cleaning

Before Exposure

ASTM G1 Section 4 specifies the following preparation sequence:

Dimensional measurement: Measure all dimensions to ±0.05 mm. Calculate total exposed area including all faces, edges, and holes (using appropriate geometric formulas). Record the area.
Surface preparation: Abrade to a uniform surface finish using 120-grit SiC paper followed by 320-grit, then 600-grit. Direction of final grinding marks must be consistent between coupons in the same test series. Annex A1 provides grade-specific surface preparation guidance.
Degreasing: Wipe with clean gauze moistened with acetone, then rinse with fresh acetone, then rinse with methanol or isopropanol. Air dry in a dessicator for 1 h.
Weighing: Weigh to ±0.1 mg on an analytical balance with a resolution of 0.0001 g. Record initial mass W₀. Weigh immediately before exposure, after final drying.
Mounting: Mount using glass, PTFE, or nylon hangers to prevent galvanic contact with other metals and to ensure full solution access to all faces. Maintain a minimum 25 mm separation between coupons.

After Exposure: Cleaning Procedure

The cleaning method removes corrosion products without attacking the underlying metal. ASTM G1 Table A1.1 specifies chemical cleaning solutions for each metal family:

Metal / Alloy	ASTM G1 Cleaning Solution	Conditions	Rinse
Carbon steel / cast iron	Clark’s solution: 20 g Sb₂O₃ + 50 g SnCl₂ in 1,000 mL HCl (conc.)	Room temp, 5–10 min, mechanical scrubbing	Water, methanol, dry
Copper and copper alloys	500 mL HCl (conc.) + 500 mL water	Room temp, 1–3 min	Water, methanol
Aluminium and alloys	70% nitric acid (HNO₃), undiluted	Room temp, 1 min	Water, methanol
Lead and alloys	5% acetic acid + 1% H₂O₂	Room temp, 5–10 min	Water, methanol
Nickel and alloys	15% HNO₃ + 5% H₂SO₄	Room temp, 5 min	Water, methanol
Stainless steels (passivated)	10% HNO₃ + 1% HF	20–30°C, 5 min (caution: HF)	Water, methanol
Zinc	200 g CrO₃ + 10 g AgSO₄ in 1,000 mL water	80–100°C, 5 min	Water, methanol
Repeat the cleaning cycle until the mass difference between consecutive cleanings is ≤ 0.1 mg (ASTM G1 section A1.3). A blank coupon (same material, same cleaning cycles, no immersion exposure) must be included to determine the cleaning metal loss correction.

Blank Coupon Correction

Chemical cleaning solutions inevitably dissolve a small amount of base metal in addition to corrosion products. The blank coupon correction accounts for this: a coupon of the same material, surface-prepared identically, is subjected to the same cleaning procedure but without any immersion exposure. Its mass loss is the “blank correction” B_c (mg). The corrected weight loss for the test coupon is:

Wᶜ𝔬𝕣𝕣 = W𝑡𝑒𝑠𝑡 − Bᶜ

where:
  W𝑡𝑒𝑠𝑡 = mass of test coupon before exposure − mass after cleaning [mg]
  Bᶜ    = mass loss of blank coupon under same cleaning procedure [mg]

For replicate testing, ASTM G31 recommends a minimum of 3 coupons per test condition.
Report: mean CR ± 1 standard deviation.
CV (coefficient of variation) > 15% on replicate coupons indicates non-uniform
corrosion or experimental error — investigate before reporting results.

Figure 2. Left: NACE corrosion severity classification (mm/year) with typical markers for carbon steel in various environments, and a bar chart showing corrosion allowance requirements for a 25-year design life at different corrosion rates. Right: carbon steel corrosion rates across eight environments (approximate, from published corrosion handbooks). The inhibitor example shows that 94% effectiveness is required to move uninhibited CO₂ brine (0.80 mm/year) into the “good” category (<0.10 mm/year). © metallurgyzone.com

The Electrochemical LPR Method (ASTM G96/G59)

Linear polarisation resistance (LPR) provides continuous, real-time corrosion rate measurements without removing metal from the surface. It is the standard technique for online corrosion monitoring in process streams, cooling water systems, and pipeline pigging tools.

The Stern-Geary Equation

Corrosion current density:
  iᶜ𝔬𝕣𝕣 = B / Rₚ

where:
  Rₚ   = polarisation resistance [Ω·cm²] = ΔE / Δi   at ΔE ≈ ±10–20 mV
  B    = Stern-Geary constant [V] = βₐ × βᶜ / (2.303 × (βₐ + βᶜ))
  βₐ   = anodic Tafel slope [V/decade]
  βᶜ   = cathodic Tafel slope [V/decade]

Typical B values:
  Active carbon steel:            B ≈ 26 mV   (βₐ = 60 mV, βᶜ = 120 mV)
  Passive stainless steel:        B ≈ 13–20 mV (βₐ small, βᶜ = 120 mV)
  General corrosion, first est:   B ≈ 26 mV
  Maximum corrosion, first est:   B ≈ 52 mV

Conversion to CR:
  CR (mm/year) = iᶜ𝔬𝕣𝕣 (μA/cm²) × M × 3.27×10¯ / (n × F × D)
               ≈ iᶜ𝔬𝕣𝕣 (μA/cm²) × 3.27×10¯ × EW / D

  EW (equivalent weight) = M/n = atomic mass / valency
  F  = 96,485 C/mol (Faraday’s constant)
  D  = density [g/cm³]

Simplified for iron (M=55.85, n=2, D=7.87 g/cm³):
  CR (mm/year) = iᶜ𝔬𝕣𝕣 (μA/cm²) × 0.01164

LPR vs Weight-Loss: Comparison

Feature	Weight-Loss (ASTM G1/G31)	LPR Electrochemical (ASTM G96/G59)
Time resolution	Average over exposure period (days–weeks)	Real-time (minutes); can detect transient events
Accuracy	±5–10% for uniform corrosion	±15–30% (depends on B value accuracy)
Corrosion type	All types (by mass loss); pitting requires separate assessment	Uniform corrosion only; unreliable for pitting/crevice
Reference standard	ASTM G1, G31	ASTM G96, G59, G5
Equipment	Analytical balance (±0.1 mg), timer	Potentiostat/galvanostat; 2 or 3 electrode probe
In-situ monitoring	No (requires specimen removal)	Yes — continuous online monitoring possible
Best for	Laboratory qualification; reference data	Plant monitoring; inhibitor performance trending
Corrosion product	Must be removed for cleaning — cannot examine in situ	Passive film condition inferred from B and R_p

Corrosion Allowance Design

Corrosion allowance (CA) is the additional wall thickness built into a pressure vessel or pipe wall to ensure the component remains structurally safe throughout its design life as metal is lost to corrosion. ASME B31.3 (Process Piping) clause 304.1.1 requires that the corrosion allowance be specified by the designer based on the actual or estimated corrosion rate in service:

CA = CR × Design life (years)

For a 20-year design with CR = 0.1 mm/year:
  CA = 0.1 × 20 = 2.0 mm

Minimum wall thickness with corrosion allowance:
  t𝑡𝑒𝑜𝑚 = t𝑚𝑖𝑛 + CA

where t𝑚𝑖𝑛 is the minimum pressure-design wall thickness per ASME B31.3 clause 304.1.2:
  t𝑚𝑖𝑛 = P×Dₒ / (2×(S×E + P×y))

Remaining service life (in-service inspection):
  RSL = (t𝓠𝒜𝓣𝓌𝒮𝒶𝓁 − t𝑚𝑖𝑛) / CR𝒾𝓞𝒾𝒸𝒾

Typical CA values by service and material: 1.5 mm (stainless steel in mildly corrosive service); 3 mm (carbon steel in treated cooling water); 6 mm (carbon steel in process service); 12–20 mm (carbon steel in sour crude service with poor inhibitor control). These are general industry defaults — the actual CA for any specific installation must be based on measured or predicted corrosion rates for the specific material/environment combination.

The Eight Fontana Corrosion Types and Their Detection

The gravimetric weight-loss method measures only the average uniform corrosion rate across the exposed surface. The other seven Fontana corrosion types require specific detection and assessment methods:

Corrosion Type	Mechanism	Primary Detection Method	Most susceptible materials
1. Uniform	Even electrochemical dissolution across all surfaces	Weight loss (ASTM G1/G31), UT wall thickness	Carbon steel in acids, brine, CO₂
2. Galvanic	Potential difference between dissimilar metals in same electrolyte	EMF series check; cathodic protection monitoring; visual inspection	Any two dissimilar metals in contact: Al/SS, Cu/Fe, Zn/Fe
3. Crevice	Stagnant electrolyte → O₂ depletion → acid + Cl⁻ concentration	Visual inspection; dye penetrant (PT); ASTM G48 Method B	Stainless steels under gaskets; duplex below CPT/CCT
4. Pitting	Passive film breakdown at chloride-rich surface defects	ASTM G46 pit depth measurement; optical profilometry; ASTM G48	Stainless (low PREN) in Cl⁻; Al alloys; Zn coatings
5. Intergranular	Sensitisation (Cr depletion at GBs) from improper heat treatment or welding	ASTM A262 (IGC test); nital etch metallography; corrosion coupon in ASTM G28	Sensitised 304/316; improperly welded duplex
6. Selective leaching	Selective dissolution of one element (Zn from brass, Si from cast iron)	EDAX/EDS composition mapping; hardness testing; metallography	Brass (dezincification); grey cast iron (graphitisation)
7. Erosion-corrosion	Synergistic mechanical erosion + electrochemical dissolution under flow	Weight loss + inspection of flow geometry; jet impingement test (ASTM G73)	Cu alloys in high-velocity water; carbon steel in sand-bearing crude
8. SCC	Crack growth under tensile stress + specific corrosive environment	Fractography (transgranular vs intergranular); ASTM G39 (C-ring); SSRT (ASTM G129)	Austenitic SS in hot Cl⁻; high-strength steels in H₂S; titanium in oxidising HCl
For all corrosion forms other than uniform attack, the weight-loss CR from ASTM G1/G31 is not representative and may significantly underestimate the true corrosion damage severity. Always conduct morphology examination alongside weight-loss testing.

Frequently Asked Questions

What is the ASTM G1 formula for corrosion rate from weight loss?

The ASTM G1/G31 formula is: CR (mm/year) = 87.6 × W / (D × A × T), where W is weight loss in milligrams, D is metal density in g/cm³, A is total exposed surface area in cm², and T is exposure time in hours. The constant 87.6 incorporates all unit conversions. For mils per year: CR (mpy) = 3,445 × W / (D × A × T). For μm/year: multiply mm/year by 1,000. The formula assumes uniform corrosion; if pitting or other localised attack is present, the average rate significantly underestimates the maximum penetration depth at local attack sites. See the pitting corrosion guide for assessment of localised attack.

How should corrosion test coupons be prepared and cleaned per ASTM G1?

ASTM G1 specifies: before exposure, abrade to 600-grit SiC, degrease with acetone, dry and weigh to ±0.1 mg. After exposure, scrub under running water, apply the metal-specific chemical cleaning solution (Clark’s solution for carbon steel: 20 g Sb₂O₃ + 50 g SnCl₂ in 1 L HCl), repeat until mass stabilises within 0.1 mg between cycles. A blank coupon (same material, same cleaning, no exposure) corrects for cleaning metal loss. Always subtract blank coupon weight loss from test coupon weight loss before applying the formula. The cleaned coupon surface should be inspected visually and by optical microscopy to assess the corrosion morphology (uniform, pitting, intergranular) before reporting the weight-loss rate.

What is a corrosion rate of 1 mpy in mm/year?

1 mil per year (mpy) = 0.0254 mm/year = 25.4 μm/year. Conversely, 1 mm/year = 39.37 mpy. In oilfield and process industries, mpy is standard in North America; mm/year and μm/year are used in European and ISO standards. For mass loss rate: g/m²/day = CR (mm/year) × D (g/cm³) × 2,739.7. The calculator above converts between all four units automatically. For context: 1 mpy (0.0254 mm/year) over 25 years = 0.64 mm total corrosion — less than a typical 3 mm corrosion allowance, which is why low-corrosion-rate environments require only modest CA.

What corrosion rate is acceptable for carbon steel in process piping?

By NACE severity classification: <0.025 mm/year (<1 mpy) is excellent; 0.025–0.1 mm/year (1–4 mpy) is good; 0.1–0.5 mm/year (4–20 mpy) is acceptable with standard 3–6 mm corrosion allowance; 0.5–1.0 mm/year (20–40 mpy) requires high CA or active inhibition; >1.0 mm/year is unacceptable for long-term service. API RP 14E targets <0.1 mm/year in inhibited oilfield systems. For uninhibited CO₂/H₂S systems, rates can exceed 3 mm/year — requiring either alloy upgrading to CRAs (corrosion-resistant alloys) or high-efficiency inhibitor injection. See the corrosion mechanisms guide for CO₂ and H₂S corrosion electrochemistry.

What is the linear polarisation resistance (LPR) method for corrosion rate measurement?

LPR applies a small potential perturbation (±10–20 mV around the open-circuit potential) to a working electrode in the corrosive medium and measures the resulting current. The polarisation resistance R_p = ΔE/Δi (Ω·cm²). The corrosion current is i_corr = B/R_p, where B is the Stern-Geary constant (typically 26 mV for active carbon steel as a first estimate). Corrosion rate is then calculated from i_corr using Faraday’s law. LPR probes allow continuous online corrosion monitoring in process lines — instantly detecting changes in corrosion rate from temperature, flow, chemistry, or inhibitor concentration changes. The method is unreliable for pitting or passive films; use it for uniform corrosion monitoring only.

What is a corrosion allowance and how is it calculated?

Corrosion allowance (CA) is additional wall thickness beyond the pressure-design minimum, to accommodate metal loss over the design life: CA = CR × design life (years). For 20 years at 0.1 mm/year: CA = 2 mm. API 579-1/ASME FFS-1 and ASME B31.3 provide remaining-life assessment methods when the actual in-service corrosion rate (from UT thickness measurement) differs from the design assumption: RSL = (measured thickness − t_min) / CR_measured. If the measured CR exceeds the design CR, remaining life is correspondingly reduced, and inspection intervals must be shortened. Typical CA values: stainless steel 1–3 mm; carbon steel in process service 3–6 mm; sour service carbon steel 6–12 mm. See the critical crack size calculator for combining corrosion allowance with fracture mechanics assessment.

How is corrosion inhibitor effectiveness measured from coupon data?

Inhibitor effectiveness IE% = (CR_uninhibited − CR_inhibited) / CR_uninhibited × 100%. The uninhibited and inhibited coupons must be tested under identical conditions (same solution, temperature, exposure time, flow velocity, coupon material and preparation). For example: uninhibited CR = 2.0 mm/year, inhibited CR = 0.15 mm/year → IE% = (2.0–0.15)/2.0 × 100 = 92.5%. ASTM G31 specifies triplicate coupons per test condition; report mean ± 1 standard deviation. Process plant typically requires IE% > 90% at the specified inhibitor dose before approving an inhibitor formulation. The Langmuir adsorption isotherm is commonly used to model IE% as a function of inhibitor concentration, enabling optimisation of dosing rates.

What is the difference between uniform corrosion rate and pit depth?

The ASTM G1/G31 weight-loss formula calculates average uniform penetration depth. If pitting is present, the deepest pit is far greater than this average. The pitting factor PF = maximum pit depth / average penetration depth quantifies non-uniformity. PF = 1 is perfectly uniform; PF > 5 indicates serious pitting. For example, a coupon with average CR = 0.05 mm/year but PF = 10 means the deepest pits are penetrating at 0.5 mm/year — 10 times more severe than the weight-loss number suggests. ASTM G46 (Examination and Evaluation of Pitting Corrosion) covers pit depth measurement by microscopy, profilometry, and dial gauge methods. For pitting corrosion in stainless steels, PREN (see PREN calculator) and critical pitting temperature testing (ASTM G48) are more relevant than weight-loss CR.

What are the eight forms of corrosion in the Fontana classification?

The Fontana and Greene (1967) classification identifies eight forms: (1) Uniform corrosion — even metal loss (measured by weight-loss CR); (2) Galvanic corrosion — accelerated attack of the less noble metal when two dissimilar metals are in electrical contact; (3) Crevice corrosion — concentrated attack in stagnant electrolyte zones under gaskets, flanges, and lap joints; (4) Pitting corrosion — highly localised deep holes penetrating the passive film; (5) Intergranular corrosion — preferential attack along grain boundaries from sensitisation; (6) Selective leaching — removal of one element (dezincification of brass, graphitisation of cast iron); (7) Erosion-corrosion — combined mechanical erosion and electrochemical corrosion under high-velocity flow; (8) Stress corrosion cracking (SCC) — crack growth under simultaneous tensile stress and specific corrosive environment. Weight-loss CR applies only to type 1; the other seven require separate assessment methods.

Key References

ASTM G1-03(2017) — Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. ASTM International.
ASTM G31-21 — Standard Guide for Laboratory Immersion Corrosion Testing of Metals. ASTM International.
ASTM G96-90(2018) — Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods).
ASTM G59-97(2020) — Standard Test Method for Conducting Potentiodynamic Polarisation Resistance Measurements.
Fontana, M.G. and Greene, N.D., Corrosion Engineering, 3rd ed. McGraw-Hill, 1986.
Jones, D.A., Principles and Prevention of Corrosion, 2nd ed. Prentice Hall, 1996.
NACE SP0169:2013 — Control of External Corrosion on Underground or Submerged Metallic Piping Systems. NACE International.

Recommended Technical References

Corrosion Engineering — Fontana & Greene (3rd Ed.)

The classic foundational text on all eight corrosion types, with electrochemical theory, testing methods, and industrial case studies.

View on Amazon

Principles and Prevention of Corrosion — Jones (2nd Ed.)

Comprehensive university-level treatment of electrochemical corrosion, mixed potential, Tafel slopes, LPR, and corrosion testing methods.

View on Amazon

Corrosion Test Coupons — ASTM G1/G31 Compatible

Pre-prepared carbon steel and stainless steel corrosion test coupons for weight-loss immersion testing in laboratory and plant environments.

View on Amazon

Digital Analytical Balance 0.0001 g / 0.1 mg Precision

ASTM G1 requires ±0.1 mg precision. A 0.1 mg or better analytical balance is essential for accurate corrosion coupon weight-loss measurements.

View on Amazon

Disclosure: MetallurgyZone participates in the Amazon Associates programme. If you purchase through these links, we may earn a small commission at no extra cost to you. This helps support free technical content on this site.

Corrosion Rate Calculator — Weight Loss Method (ASTM G1/G31)

Corrosion Rate Calculator — Weight Loss, LPR, and Unit Conversion (ASTM G1/G31/G96)

Key Takeaways

Corrosion Rate Calculator

The ASTM G1/G31 Gravimetric Method: Formula and Procedure

The ASTM G1 Formula

Unit Conversions

ASTM G1 Coupon Preparation and Cleaning

Before Exposure

After Exposure: Cleaning Procedure

Blank Coupon Correction

The Electrochemical LPR Method (ASTM G96/G59)

The Stern-Geary Equation

LPR vs Weight-Loss: Comparison

Corrosion Allowance Design

The Eight Fontana Corrosion Types and Their Detection

Frequently Asked Questions

Key References

Recommended Technical References

Corrosion Engineering — Fontana & Greene (3rd Ed.)

Principles and Prevention of Corrosion — Jones (2nd Ed.)

Corrosion Test Coupons — ASTM G1/G31 Compatible

Digital Analytical Balance 0.0001 g / 0.1 mg Precision

Further Reading & Related Topics

PREN Calculator

Pitting Corrosion

Corrosion Mechanisms

Hydrogen Induced Cracking

Austenitic Stainless Steel

Critical Crack Size

Pipe Thermal Expansion

Calculators Hub