couponing

The coupon method or corrosion coupons is the simplest method of corrosion monitoring. Coupons are the oldest corrosion monitoring tool. Corrosion coupons are small pieces of metal or alloy of the material of interest that are immersed in the process medium and removed after a specified period of time and their weight or size loss is evaluated and examined. The samples are typically made in the form of a rectangular disk or strip. The figure below shows some examples of corrosion coupons with different dimensions.

The corrosion rate formula can be calculated as follows:

CR=C*K*WL/AT where the value of WL is calculated from the formula Wi – Wf = WL.

WL= (gr): Lost mass of coupon

Wi= (gr): Initial mass of coupon

Wf= (gr): Final coupon mass

C = Corrosion time factor

K = Density factor

A= (cm2): Surface area

T = Test time

Coupon design is an important part of any corrosion testing program in industrial units. The appropriate selection of coupon shape, surface finish, metallurgical conditions (coupon material), and coupon dimensions are planned based on the type of corrosion present in the system under study. The image below shows a sample of a corrosion coupon after performing a corrosion monitoring test.

A corrosion coupon sample after exposure to a corrosive environment

Advantages of the coupon method:

Low manufacturing cost, installation and ease of placement of corrosion coupons in industrial systems

Obtain the necessary information about the type and form of corrosion by using accurate analysis of corrosion products after exposure of the sample to the corrosive environment

Coupons can be designed to simulate conditions such as welding, residual stresses or grooves.

Coupons, because they are realistically placed within the process streams, reveal the intensifying effects of chemical compounds or contaminants.

Many coupons can be exposed to the process environment and tested simultaneously in duplicate or multiple copies (to measure data dispersion).

Many forms of localized corrosion such as crevice corrosion, stress corrosion cracking (SCC) and pitting require a sufficient period of time to begin their activity. In order to increase the confidence in the results obtained from these tests, coupons can be placed in the corrosive environment for long periods of time.

Disadvantages of the coupon method:

One of the disadvantages of this method is the long test time.

Removing coupons from the industrial unit or related equipment is very expensive and time-consuming and requires skilled personnel.

The corrosion rate calculated in this method is an average rate that is measured over a specific period of time. Therefore, sudden and short-term changes in corrosion processes (which are caused by events such as a decrease in the inhibitor power in the system, a change in the chemical composition of the solution, an inappropriate amount of additives to the process flow, etc.) cannot be detected.

Cleaning, drying, weighing and microscopic examination of coupons is a time-consuming and expensive task.

Some special forms of corrosion, such as abrasion corrosion and heat transfer effects, cannot be easily simulated by coupons.

The corrosion rate of industrial equipment is rarely equal to the rate calculated by coupons because simulating equipment by coupons is a very difficult task.

To place coupons in the process environment, the wall of industrial equipment generally has to be perforated. Therefore, couponing is considered a destructive method of corrosion monitoring.