The weight-loss coupon is the most direct way to measure the corrosion rate inside a pipeline or vessel. This animation walks through the full laboratory method, from preparation to report.
STEP 1 – COUPON PREPARATION
Carbon steel strip with 2 holes for PTFE insulating washers. Dimensions measured to calculate the exposed area (A = 40.1 cm² in the example). Degreased (acetone), dried and kept in a desiccator. Handled only with clean gloves.
STEP 2 – INITIAL WEIGHING (W0)
Analytical balance, 0.1 mg resolution, calibrated and checked with reference weights. Reweigh until constant mass. Example: W0 = 42.3185 g, recorded against the coupon ID.
STEP 3 – FIELD EXPOSURE
Installed in the flowline under pressure, mounted with PTFE insulating washers to isolate the coupon from the holder (no galvanic effect). Installation and retrieval dates are recorded to get the exposure time T. Example: 90 days (2,160 h).
STEP 4 – CLEANING AFTER RETRIEVAL
Photographed as received, then corrosion products removed with inhibited acid (ASTM G1), rinsed, dried and returned to the desiccator.
STEP 5 – FINAL WEIGHING (W1)
Same balance, same conditions. Example: W1 = 41.8685 g → mass loss ΔW = 450.0 mg.
STEP 6 – GENERAL CORROSION RATE
CR (mm/y) = 87.6 × ΔW / (A × T × ρ), with ΔW in mg, A in cm², T in h, ρ in g/cm³ (7.86 for carbon steel).
CR = 87.6 × 450.0 / (40.1 × 2160 × 7.86) = 0.058 mm/y (≈ 2.3 mpy) → MODERATE per NACE SP0775 Table 2. Typical action: review inhibitor dosage.
STEP 7 – PITTING RATE & REPORT
PR = maximum pit depth / exposure time = 0.040 mm / 0.247 y = 0.16 mm/y (≈ 6.4 mpy), classified per NACE SP0775 Table 2.
LIMITS TO KNOW
• The result is an average rate over the exposure period: it does not show when corrosion happened. Continuous monitoring (ER/LPR probes) complements coupons.
• The coupon represents conditions at its location only (position in the line, flow regime, water phase).
• Pitting must be evaluated separately: a low general rate can hide severe localized attack.
Illustrative example values. Ref.: NACE SP0775-2023, ASTM G1-03(2017)e1.
📧 contact@petrodyzer.com | 🌐 www.petrodyzer.com
#PETRODYZER #CorrosionMonitoring #CorrosionCoupon #WeightLoss #NACE #ASTMG1 #OilAndGas #AssetIntegrity
source