Metal Corrosion Calculator: Rate, Allowance & Galvanic Effects
Corrosion costs industries billions annually through unplanned failures, over-engineered designs, and compliance headaches. This professional-grade corrosion calculator eliminates manual unit conversions and calculation errors by applying industry-standard formulas (ASTM G1/G31, API 570, ASME B31.3) in one tool. Calculate corrosion rate from weight-loss or UT thickness data, estimate remaining service life, assess galvanic and pitting risk, and design corrosion allowances — all with instant results in mm/year, mpy, and more.
Corrosion Calculator Pro
Professional corrosion rate, remaining life & material integrity tool for engineers, inspectors & researchers
⚖️ Corrosion Rate — Weight-Loss (Gravimetric) Method
Corrosion Rate (Weight-Loss Method)
$$CR\,(\text{mm/y}) = \frac{87.6 \times W}{\rho \times A \times t}$$ $$CR\,(\text{mpy}) = \frac{534 \times W}{\rho \times A \times t}$$⚗️ Environmental Context (Optional)
📊 Results — Weight-Loss Method
📏 Thickness-Loss & Remaining Life Calculator
Thickness-Loss Corrosion Rate
$$CR = \frac{\Delta T}{\Delta t} = \frac{T_1 - T_2}{t_2 - t_1}$$Remaining Service Life (RSL)
$$RSL = \frac{T_{current} - T_{min}}{CR}$$🔮 Future Projection
📊 Results — Thickness & Remaining Life
⚡ Galvanic Corrosion Estimator
Galvanic Potential Difference
$$\Delta E = E_{cathode} - E_{anode}$$Galvanic Corrosion Rate Acceleration (Area Effect)
$$CR_{galvanic} = CR_{base} \times \left(1 + k \cdot \frac{A_c}{A_a}\right)$$| Material | Potential (V vs SHE) | Classification |
|---|
⚡ Galvanic Corrosion Assessment
🔬 Pitting Corrosion Analysis
Pitting Factor
$$PF = \frac{d_{max}}{d_{avg}}$$Average Uniform Penetration
$$d_{avg} = CR \times t$$Pit Growth Rate
$$r_{pit} = \frac{d_{max}}{t}$$🔬 Pitting Corrosion Results
🏗️ Corrosion Allowance & Design Life Calculator
Required Corrosion Allowance
$$CA = CR \times t_{design} \times SF$$Minimum Required Wall Thickness
$$T_{min} = T_{calc} + CA + T_{mfg}$$Remaining Corrosion Allowance
$$CA_{rem} = T_{current} - T_{min,structural}$$🏗️ Corrosion Allowance Results
🔄 Corrosion Unit Converter
Key Conversion Factors
$$1\,\text{mm/y} = 39.37\,\text{mpy} = 1000\,\mu\text{m/y} = 0.03937\,\text{in/y}$$ $$\text{mm/y} \approx \frac{87.6}{\rho}\,\text{g/m}^2/\text{d} \quad (\rho\,\text{in g/cm}^3)$$| mm/year | mpy | µm/year | in/year | Classification |
|---|---|---|---|---|
| < 0.025 | < 1 | < 25 | < 0.001 | Outstanding |
| 0.025–0.10 | 1–5 | 25–100 | 0.001–0.004 | Excellent |
| 0.10–0.51 | 5–20 | 100–510 | 0.004–0.02 | Good |
| 0.51–1.27 | 20–50 | 510–1270 | 0.02–0.05 | Fair |
| 1.27–3.18 | 50–125 | 1270–3180 | 0.05–0.125 | Poor |
| > 3.18 | > 125 | > 3180 | > 0.125 | Unacceptable |
Reference: NACE/AMPP corrosion severity guidelines
📄 Calculation Report Generator
📚 Corrosion Engineering Reference
Weight-Loss (Gravimetric): Best for lab coupon tests (ASTM G1, G31). Provides average uniform corrosion rate. Requires accurate area measurement and proper cleaning. Misses localized pitting.
Thickness-Loss (UT Method): Used in field inspection on installed equipment. Directly reflects real operating conditions. Multiple readings over time improve accuracy. Required for API 570 assessments.
⚠️ Short exposure coupon tests may not represent steady-state corrosion. Always aim for >168 hours exposure (ASTM G31) for meaningful results.
| CR (mm/y) | CR (mpy) | NACE Rating | Typical Action |
|---|---|---|---|
| < 0.025 | < 1 | Outstanding | No action required; excellent material selection |
| 0.025–0.10 | 1–5 | Excellent | Continue monitoring; suitable for most applications |
| 0.10–0.51 | 5–20 | Good | Monitor annually; consider inhibitors at upper range |
| 0.51–1.27 | 20–50 | Fair | Increase inspection frequency; inhibitors recommended |
| 1.27–3.18 | 50–125 | Poor | Urgent attention; consider material upgrade or coating |
| > 3.18 | > 125 | Unacceptable | Immediate action; shutdown risk; replace or upgrade material |
| Application | Typical CA (mm) | Design Life | Reference |
|---|---|---|---|
| Carbon Steel Process Piping | 1.5 – 6.4 | 20–30 years | ASME B31.3 |
| Pressure Vessels (mild service) | 1.6 – 3.2 | 20 years | ASME Sect. VIII |
| Storage Tanks (API 650) | 1.0 – 3.0 | 30 years | API 650 / 653 |
| Offshore Pipelines | 3.0 – 12.5 | 25 years | DNV-ST-F101 |
| Refinery Piping (aggressive) | 3.0 – 6.4 | 15–20 years | API 570 |
| Structural Steel (marine) | 2.0 – 5.0 | 25 years | ISO 12944 |
- Area measurement errors: Masked edges or holes can inflate/deflate rates by 10–30%.
- Cleaning method: Under-cleaning (scale remaining) gives low CR; over-cleaning (base metal removal) gives high CR. Follow ASTM G1 cleaning procedures.
- Short exposures: < 168 h captures transient, not steady-state. Rates may not represent long-term behavior.
- Pitting risk: Weight-loss averages mask local pitting. Always check for pitting separately.
- Environmental changes: pH, temperature, velocity changes during service affect real rates vs. calculated rates.
- This tool provides engineering estimates per established formulas. Always verify with qualified corrosion engineers for critical decisions.
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Corrosion Calculator Pro — Complete User Guide
Step-by-step instructions for every calculator tab — all formulas explained with units, worked examples, common mistakes, and accuracy notes so you can trust every result.
🚀 How the Calculator Works — 5-Step Overview
Choose method
Fill all fields
Press button
CR, life, risk
Copy / Print
Results from all tabs compile into one formatted report via the Report tab.
📘 Section 1: Understanding Corrosion Rates — Concepts & Units
A corrosion rate quantifies how fast a material loses thickness due to chemical or electrochemical reactions with its environment. It directly determines how long a component will last and when it must be inspected or replaced.
Corrosion rate is expressed as a penetration rate — how many millimetres (or mils) of material surface are consumed per year. A pipe wall corroding at 0.1 mm/y will lose 1 mm over 10 years.
📐 Corrosion Rate Units Explained
| Unit | Full Name | How It Works | Common In | "Poor" Threshold |
|---|---|---|---|---|
| mm/y | Millimetres per year | Direct penetration depth per year | Europe, ISO, ASME | > 1.27 mm/y |
| mpy | Mils per year (1 mil = 0.001 inch) | 1 mpy = 0.0254 mm/y | USA, NACE/AMPP, API | > 50 mpy |
| μm/y | Micrometres per year | 1 μm = 0.001 mm | Academic, thin films | > 1270 μm/y |
| g/m²/d | Grams per m² per day | Mass flux — depends on density | Atmospheric tests | Density-dependent |
| ipy | Inches per year | 1 ipy = 25.4 mm/y | Legacy US standards | > 0.05 ipy |
| Key: 1 mm/y = 39.37 mpy = 1,000 μm/y = 0.03937 in/y | ||||
🎯 NACE/AMPP Corrosion Severity Rating Scale
| Rating | mm/year | mpy | μm/year | Typical Action Required |
|---|---|---|---|---|
| Outstanding | < 0.025 | < 1 | < 25 | No action. Excellent material. Monitor at normal intervals. |
| Excellent | 0.025–0.10 | 1–5 | 25–100 | No action. Annual monitoring. |
| Good | 0.10–0.51 | 5–20 | 100–510 | Monitor. Consider inhibitors at upper range. |
| Fair | 0.51–1.27 | 20–50 | 510–1270 | Attention needed. Increase inspection frequency. |
| Poor | 1.27–3.18 | 50–125 | 1270–3180 | Urgent. Material upgrade or coating needed. |
| Unacceptable | > 3.18 | > 125 | > 3180 | Immediate action. Shutdown risk. Replace or upgrade now. |
⚖️ Section 2: Tab 1 — Weight-Loss (Gravimetric) Method
The weight-loss (gravimetric) method uses the mass lost from a metal coupon after exposure to calculate corrosion rate. It is the standard laboratory technique specified in ASTM G1 (specimen preparation/cleaning) and ASTM G31 (immersion testing).
📋 Step-by-Step Instructions
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1Select Material — Density Auto-Fills
Choose the metal from the Material dropdown. Density (g/cm³) fills automatically. For custom alloys, select Custom… and type density manually. A wrong density is the #1 source of error in this calculation.
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2Enter Initial Weight W₁ — Before Exposure
Weigh the coupon on a calibrated analytical balance before immersion. Select mg or g from the unit dropdown first, then enter the value. Milligrams is preferred for laboratory coupons — it gives more decimal precision.
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3Enter Final Weight W₂ — After Exposure and Cleaning
Clean the coupon per ASTM G1 (acid pickling or mechanical cleaning to remove corrosion products without removing base metal), then weigh again. Must use the same unit as W₁. If W₂ ≥ W₁, the calculator alerts you.
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4Enter Exposed Surface Area A
Measure only the wetted (exposed) area — exclude masked edges, mounting holes, and unexposed faces. Select unit: cm², in², or m². The calculator converts to cm² internally.
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5Enter Exposure Time t — Select Correct Unit
Select the time unit first (hours, days, months, or years), then enter the value. All units convert to hours internally. Entering 720 with the unit set to "years" instead of "hours" will give a vastly wrong result.
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6Optional: Add Environmental Context
Fields for Medium, Temperature, pH, and Chloride Conc. are optional. They appear in the generated report for traceability but do not change the weight-loss formula result.
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7Click "Calculate Corrosion Rate" — Read Results
Results: CR in mm/y, mpy, μm/y, g/m²/day; total metal loss; NACE severity badge; bar chart showing which category your result falls in. Click "Copy Results" to send to clipboard or "Print / PDF" to export.
📐 Formulas: Weight-Loss Corrosion Rate (ASTM G31)
| Symbol | Variable | Required Unit | Notes / Common Values |
|---|---|---|---|
| W | Weight (mass) loss | mg (milligrams) | W = W₁ − W₂. Must be positive. Calculator converts g→mg automatically. |
| ρ | Metal density | g/cm³ | Carbon steel: 7.85 | SS 316: 7.98 | Al: 2.70 | Cu: 8.96 |
| A | Exposed surface area | cm² | True wetted area only. Calculator converts in² and m² to cm² automatically. |
| t | Exposure time | hours | Calculator converts days, months, years to hours (using 8760 h/y, 30.44 d/mo). |
| CR | Corrosion rate | mm/y or mpy | The primary output. Also displayed in μm/y and g/m²/day. |
🧮 Worked Example — Carbon Steel Coupon in Seawater
Given inputs:
- Material: Carbon Steel — ρ = 7.85 g/cm³
- W₁ (initial weight) = 150,250 mg
- W₂ (final weight after cleaning) = 149,850 mg
- Weight loss W = 150,250 − 149,850 = 400 mg
- Area A = 50 cm²
- Time t = 30 days = 720 hours
Step 1 — Corrosion rate in mm/year:
Step 2 — Corrosion rate in mpy:
✔️ Input Validation Rules for Weight-Loss Tab
| Field | Valid Range | What Happens if Wrong |
|---|---|---|
| Density | 0.1 – 25 g/cm³ | Too low (e.g. 0) causes division error or absurd result. Check material selection. |
| Initial Weight | Must be > Final Weight | Calculator alerts: "Initial Weight must be greater than Final Weight." |
| Area | > 0 in selected unit | Wrong unit selection (e.g. 50 entered as in² when measured in cm²) inflates area by 6.45× → CR drops by same factor. |
| Time | > 0.1 in selected unit | Wrong unit (e.g. 720 hours entered as "years") gives a rate 8,760 times too low. |
📏 Section 3: Tab 2 — Thickness-Loss & Remaining Life Calculator
This tab uses Ultrasonic Thickness (UT) readings taken at two different dates to calculate the real, in-service corrosion rate and predict when the component reaches its minimum safe thickness. This is the primary method in API 570 (Piping Inspection) and API 510 (Pressure Vessel Inspection).
📐 Formulas: Thickness-Loss Method
Given: Carbon steel pipe • T₁ = 12.7 mm • T₂ = 10.9 mm • Tmin = 7.6 mm • Elapsed = 4 years
Rating: Good (approaching Fair). Per API 570 half-life rule: schedule next inspection in 3.6 years.
⚡ Section 4: Tab 3 — Galvanic Corrosion Estimator
Galvanic corrosion occurs when two metals with different electrochemical potentials are in electrical contact in an electrolyte. The less noble (active) metal becomes the anode and corrodes; the noble metal is the cathode and is protected. The larger the potential difference and the larger the cathode-to-anode area ratio, the more severe the attack.
📐 Galvanic Corrosion Formulas
Ac/Aa = cathode-to-anode area ratio (larger = more severe) | δΔE = normalised potential factor = min(ΔE / 0.5 , 3)
📋 Step-by-Step Instructions
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1Select Anode (Active / Corroding) Metal
The metal that will corrode. It has a lower (more negative) potential in the galvanic series table shown below the calculator. If unsure which metal is more active, consult that table — the calculator warns you if the pair appears reversed.
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2Select Cathode (Noble / Protected) Metal
The metal that gains protection. Higher (more positive) potential = more noble = cathode. The calculator alerts you if the same material is selected for both or if the pair appears to be reversed.
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3Enter Actual Exposed Areas for Each Metal
Enter the wetted surface area of each metal in cm². The Ac/Aa ratio is the dominant factor in severity. Enter realistic areas — equal areas (ratio = 1) represents a best-case scenario.
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4Select Electrolyte and Enter Base Corrosion Rate
Choose the medium (sets conductivity factor ke). Enter the base corrosion rate of the anode material without galvanic coupling — use a value from Tab 1 (weight-loss) or literature.
🔬 Section 5: Tab 4 — Pitting Corrosion Analysis
Pitting is highly localised corrosion that can perforate a wall even when average metal loss is small. A pipe can fail from pitting while its average corrosion rate rates as "Good." The Pitting Factor (PF) quantifies how localised the attack is — the higher the PF, the more dangerous pitting is relative to what weight-loss data alone suggests.
📐 Pitting Corrosion Formulas
PF = 1.0 means uniform corrosion. PF > 10 = extreme localisation. Most practical cases: PF 1–8.
📊 Pitting Factor Interpretation
| Pitting Factor (PF) | Classification | What It Means | Recommended Action |
|---|---|---|---|
| 1.0 – 1.9 | Uniform | Attack is essentially uniform | Weight-loss CR is reliable for life prediction |
| 2.0 – 4.9 | Moderate Pitting | Some pits notably deeper than average | Include pit depth in inspection; CR alone insufficient |
| 5.0 – 9.9 | Severe Pitting | Pits significantly deeper — wall perforation risk | Use pit growth rate for life prediction; consider alloy upgrade |
| ≥ 10 | Extreme Pitting | Highly localised — average CR is dangerously misleading | Immediate inspection; NDE required; pit depth governs |
🏗️ Section 6: Tab 5 — Corrosion Allowance & Design Life
Corrosion Allowance (CA) is deliberate extra material thickness added during design to account for expected metal loss over the service life. It is required by ASME B31.3, ASME Section VIII, API 570, and most piping/vessel codes.
📐 Corrosion Allowance Formulas
📋 Typical Corrosion Allowance by Code & Service
| Application | Typical CA (mm) | Design Life | Code |
|---|---|---|---|
| CS process piping (mild) | 1.5 – 3.2 | 20–25 y | ASME B31.3 |
| CS process piping (aggressive) | 3.2 – 6.4 | 15–20 y | ASME B31.3 / API 570 |
| Pressure vessels (CS) | 1.6 – 3.2 | 20 y | ASME Sect. VIII UG-25 |
| Storage tanks (bottom plate) | 1.5 – 3.0 | 25–30 y | API 650 / 653 |
| Offshore subsea pipelines | 3.0 – 12.5 | 25 y | DNV-ST-F101 |
| Refinery crude service piping | 3.0 – 6.4 | 15–20 y | API 570 Class 1 |
| Structural steel (marine) | 2.0 – 5.0 | 25 y | ISO 12944 |
| Stainless steel piping | 0 – 1.6 | 20+ y | Usually 0 if passive film intact |
| These are indicative values. Always verify with your applicable project code and qualified engineer. | |||
🔄 Section 7: Tab 6 — Unit Converter
The Unit Converter tab converts any corrosion rate, thickness, or temperature value between all common engineering units in real time. Just type in any one field — all others update instantly. No Calculate button needed.
📐 Corrosion Rate Conversion Formulas
🌡️ Temperature Conversion Formulas
📋 Quick Corrosion Rate Conversion Reference
| mm/year | mpy | μm/year | in/year | Note |
|---|---|---|---|---|
| 0.025 | 1.0 | 25 | 0.001 | Outstanding / Excellent threshold |
| 0.10 | 3.94 | 100 | 0.004 | Excellent / Good threshold |
| 0.51 | 20.1 | 510 | 0.020 | Good / Fair threshold |
| 1.00 | 39.4 | 1,000 | 0.039 | 1 mm lost per year |
| 1.27 | 50.0 | 1,270 | 0.050 | Fair / Poor threshold |
| 3.18 | 125.2 | 3,180 | 0.125 | Poor / Unacceptable threshold |
| 25.4 | 1,000 | 25,400 | 1.000 | Extremely severe (1 inch/year) |
📄 Section 8: Tab 7 — Report Generator
The Report tab compiles all calculation results from every tab into a single formatted engineering report. Copy it to an email, Word document, or inspection system, or export as PDF via your browser's print function.
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1Run All Desired Calculations First
Navigate to each needed tab and click Calculate. Only tabs where calculations have been run will appear in the compiled report.
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2Fill Project Information Fields
Enter Project Name, Equipment Tag, Location, Engineer, Date, and Standard. These appear as the report header for audit traceability.
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3Add Notes / Assumptions
Document any assumptions (e.g. "Short-term test — rate may not represent steady state" or "Density estimated from handbook"). Good engineering always documents its assumptions.
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4Generate Report → Copy or Print
Click "Generate Report" → formatted text appears in the box. Click "Copy to Clipboard" to paste anywhere. Use "Print / Export PDF" to save a printable version.
📐 Section 9: Complete Formula Reference Card
Every formula used by the calculator, consolidated in one place with full variable definitions and units.
| # | Formula Name | Expression | Units Required | Standard |
|---|---|---|---|---|
| F1a | Corrosion Rate (mm/y) | CR = (87.6 × W) ÷ (ρ × A × t) | W: mg | ρ: g/cm³ | A: cm² | t: hours | ASTM G31 |
| F1b | Corrosion Rate (mpy) | CR = (534 × W) ÷ (ρ × A × t) | Same as F1a | ASTM G31 |
| F2 | CR from Thickness | CR = (T₁ − T₂) ÷ (t₂ − t₁) | T: mm | t: years | API 570 |
| F3 | Remaining Service Life | RSL = (Tcurrent − Tmin) ÷ CR | T: mm | CR: mm/y | API 570 |
| F4 | Inhibitor-Corrected Rate | CReff = CR × (1 − η/100) | η in % | NACE |
| F5 | Future Thickness | Tfuture = Tcurrent − CReff × t | T: mm | t: years | — |
| F6 | Galvanic Potential | ΔE = Ecathode − Eanode | V vs. SHE | ASTM G82 |
| F8 | Pitting Factor | PF = dmax ÷ davg | d: mm | ASTM G46 |
| F9 | Pit Growth Rate | rpit = dmax ÷ t | d: mm | t: years | ASTM G46 |
| F10 | Time to Perforation | tperf = (Twall − dmax) ÷ rpit | T,d: mm | r: mm/y | — |
| F11 | Corrosion Allowance | CA = CR × tdesign × SF | CR: mm/y | t: years | ASME B31.3 |
| F12 | Min Wall Thickness | Tmin = Tcalc + CA + Tmfg | All in mm | ASME B31.3 |
| F13 | CA Consumed % | CA% = (CR × telapsed) ÷ CA × 100 | CR: mm/y | t: years | — |
📋 Section 10: Unit Conversion & Material Reference Tables
🛡️ Material Density Reference Table
| Material | Density (g/cm³) | Density (kg/m³) | Common Uses |
|---|---|---|---|
| Carbon Steel | 7.85 | 7,850 | Pipes, vessels, structures |
| Stainless Steel 304 | 7.90 | 7,900 | Food, pharma, mild chemical |
| Stainless Steel 316 | 7.98 | 7,980 | Marine, chloride environments |
| Duplex SS 2205 | 7.80 | 7,800 | Offshore, high-chloride |
| Aluminum (1100) | 2.71 | 2,710 | Atmospheric, mild service |
| Copper | 8.96 | 8,960 | Plumbing, heat exchangers |
| Brass (70/30 Cu-Zn) | 8.50 | 8,500 | Valves, fittings |
| Cast Iron | 7.15 | 7,150 | Flanges, pump casings |
| Titanium (Grade 2) | 4.51 | 4,510 | Seawater, aggressive chemicals |
| Inconel 625 | 8.44 | 8,440 | High-temp, sour service |
| Zinc (galvanised) | 7.13 | 7,130 | Sacrificial anode, coating |
⌛ Time Conversion Reference
| Unit | = Hours | = Days | Notes |
|---|---|---|---|
| 1 Day | 24 | 1 | — |
| 1 Week | 168 | 7 | Minimum exposure per ASTM G31 |
| 1 Month | 730.5 | 30.44 | Calculator uses 30.44 d/month average |
| 1 Year | 8,760 | 365.25 | Calculator uses 365.25 d/y (leap-year corrected) |
⚠️ Section 11: 10 Common Mistakes & How to Avoid Them
These mistakes can significantly skew your results. Check each one before running a critical calculation.
✓ Fix: Always set the unit dropdown before typing the number.
✓ Fix: W₁ = weight BEFORE exposure (heavier). W₂ = weight AFTER cleaning (lighter).
✓ Fix: Clean per ASTM G1 (acid pickling + mechanical cleaning per material specification).
✓ Fix: Measure ONLY the true wetted surface. Subtract masked areas from total coupon area.
✓ Fix: Minimum 168 h (7 days) per ASTM G31. Report as "initial rate" if shorter is unavoidable.
✓ Fix: Always verify density from material certificate or Section 10 density table.
✓ Fix: T₁ = EARLIER, THICKER. T₂ = LATER, THINNER. Check UT report dates.
✓ Fix: Always run the Pitting tab alongside. If PF > 5, pitting rate governs life prediction.
✓ Fix: Enter actual exposed wetted areas. A 10:1 Ac/Aa ratio dramatically amplifies corrosion.
✓ Fix: Apply a field multiplier (1.5–3×) or use UT-derived field rates (Thickness tab) for critical life predictions.
✅ Section 12: Accuracy Statement & Engineering Limitations
🛡️ Accuracy & Trust Note
All formulas in this calculator are drawn from internationally recognised standards — ASTM G1, G31, G46, API 570, ASME B31.3, ASME Section VIII, and NACE/AMPP severity guidelines. Unit conversions are exact, not approximated. Every formula is shown explicitly in the calculator and in this guide so you can verify each step independently.
Typical accuracy for well-prepared inputs: Weight-loss calculations are accurate to within ±5% when specimen cleaning, weighing, and area measurement follow ASTM G1/G31. UT-based calculations are limited by instrument accuracy (typically ±0.1–0.25 mm).
What this tool does NOT replace: This calculator provides engineering estimates. It is not a substitute for a full Fitness-for-Service (FFS) assessment per API 579 / ASME FFS-1, a Risk-Based Inspection (RBI) study, or qualified corrosion engineer review for critical decisions.
📌 Per-Tab Limitations
| Tab | Limitation | Workaround |
|---|---|---|
| Weight-Loss | Assumes uniform corrosion — misses pitting | Always run Pitting tab for chloride or acidic environments |
| Weight-Loss | Short tests (<168 h) give transient, not steady-state rates | Use minimum 7-day exposure; label result as "initial rate" if shorter |
| Thickness / RSL | Assumes constant future rate (conservative) | Acceptable for planning; recalculate with each new UT inspection |
| Galvanic | Empirical model only — not first-principles electrochemistry | Use as severity indicator; LPR or EIS data gives more precision |
| Pitting | Assumes linear pit growth — real pits often decelerate | Conservative lower bound; use ASTM G46 Extreme Value Statistics for critical service |
| Corrosion Allowance | Based on single-point corrosion rate — does not model future rate changes | Review CA every 5 years or after any significant process change |
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