What is a corrosion coupon and how is it used?

• A. A device to measure pH of corrosion environment.
• B. A small metal sample placed in service to monitor corrosion rate by measuring weight loss over time.
• C. A coupon used to apply paint color.
• D. A sample for tensile strength testing.

Answer: (B)

A corrosion coupon is a small metal sample placed in service to monitor corrosion rate by measuring weight loss over time. By exposing the coupon to the same environment as the equipment, you capture the actual corrosion conditions you’re interested in. After a defined period, you retrieve the coupon, clean it according to standard procedures, and weigh it. The weight lost, divided by the exposed surface area and by the exposure time, yields the corrosion rate, typically expressed as mils per year or millimeters per year. This provides a direct, practical measure of how quickly material is corroding in that location and lets you compare different environments, materials, or coatings. It also helps indicate whether corrosion is uniform or localized, though a coupon may not perfectly represent localized attack if conditions differ at other spots. Coupons are not used to measure pH, apply paint, or test tensile strength.