Technical Resource
How to Choose the Right Corrosion Coupon
A step-by-step guide to selecting the optimal corrosion coupon for your monitoring program.
Step 1: Identify Your Corrosion Environment
The first step in coupon selection is understanding your process fluid chemistry — temperature, pressure, pH, dissolved gases (CO2, H2S, O2), flow velocity, and the presence of solids or multiphase flow. Each of these factors influences which coupon type and material will give you accurate, representative corrosion data.
Step 2: Choose Your Coupon Type
Weight loss (strip) coupons are the most common and economical choice for general corrosion rate measurement. Disc (flush) coupons are preferred where space is limited or flow disturbance must be minimized. Cylindrical coupons are used when directional corrosion data is needed. Erosion coupons feature angled surfaces to capture combined erosion-corrosion effects in high-velocity systems.
Step 3: Select the Right Material
The coupon material must match the pipeline or vessel material being monitored. Carbon steel coupons (A36, 1018, 1020) are used for carbon steel piping. If your system uses stainless steel, the coupon should be the same grade (304, 316, 316L, duplex). For aggressive environments, Hastelloy C276, Alloy 20, or Monel 400 coupons are appropriate.
Step 4: Determine Dimensions and Surface Finish
Standard strip coupons are 75 mm x 12 mm x 1.5 mm (3" x 0.5" x 0.06"), suitable for most applications. Surface finish per ASTM G1 is typically 120-grit for standard monitoring. Higher finishes (240, 320, 600-grit) are used when pitting corrosion measurement is the objective.
Step 5: Plan Your Holder and Access System
Consider how the coupon will be installed and retrieved. If the system can be depressurized, a simple threaded access point with a plug-type holder is sufficient. For live systems, you will need a full-opening access fitting and a retractable coupon holder with a retrieval tool.
Coupon vs ER Probe vs LPR — When to Use Which?
Weight loss coupons provide the most accurate long-term average corrosion rate but require physical retrieval. Electrical resistance (ER) probes provide semi-continuous data without retrieval but measure only general metal loss. Linear polarization resistance (LPR) probes give real-time instantaneous corrosion rates but are limited to conductive electrolytes. Many operators use coupons alongside probes — coupons as the primary benchmark and probes for trending between coupon retrievals.
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