EXPERT TIP #33 – COUPONS FOR THE CORROSION INDUSTRY

In the corrosion industry, “coupons” are used in the fight against corrosion, but the term coupon is used
interchangeably in corrosion monitoring. In fact, there are two general types of corrosion monitoring
coupons used for entirely different purposes.

Cathodic Protection Coupons
These metallic coupons of a specific size are used to assess the effectiveness of cathodic protection (CP)
systems. They are typically made of the same metal as the structure being protected. CP coupons are buried
adjacent to the structure under protection, and a coupon cable or cables are routed to an above-ground
test station where CP professionals can conduct CP testing. The CP coupon, which is bare metal, represents
a coating defect or “holiday” in the structure’s protective coating. Tests conducted with CP coupons include:

1. Native potential (voltage) of the structure*
2. Protected potential of the structure*
3. Coupon disconnect potential*
4. AC & DC current density
*With respect to a separate reference electrode.

CP coupons, like other CP monitoring equipment, are simply another resource to help determine how
effective cathodic protection efforts are.

Corrosion Rate Coupons (also known as Corrosion Rate Probes)
These coupons or probes are used to monitor the rate of corrosion, internally or externally, of a metallic
structure or asset in a specific environment that may or may not be under cathodic protection. The bare
metallic coupon, made of various metals, shapes and sizes, is exposed to a corrosive environment where
they can be analyzed to assess the rate of corrosion of the metal in that environment. Corrosion rate tests
are conducted for a variety of reasons, including:

1. To provide information about the mechanisms of corrosion
2. To estimate the service life of process equipment

There is an entire industry and science related to corrosion rate coupons or probes used in various
industries. Using corrosion rate coupons/probes can assist equipment owners and operators in these
industries better assess and manage corrosion issues.