The “coupon technique” frequently is mentioned in the literature as a method of assessing corrosion. Little discussion, however, has been devoted to important details of procedure. As a result, those inexperienced in the art often draw misleading conclusions from reported data. It is the purpose of this paper to present a detailed description of one company’s method of using coupons. A clearer understanding of details will make this important tool more valuable.
The “coupon technique” is the only single method of study which measures the effect of both physical and chemical variables of the corrosion environment. Small metal plates, commonly called coupons, are weighed and otherwise prepared for exposure, exposed to the well fluids for a number of days, examined, cleaned of corrosion products and reweighed. Both qualitative and quantitative data are obtained. Careful examination and interpretation of the appearance of corrosion products adhering to the exposed coupons permits determination of the type attack which may be anticipated. The difference in weight before and after exposure places a numerical value on the corrosiveness.
This paper describes the coupon study procedure of one oil and gas producing company. Selection of coupon material, preparation of specimens, shipment and handling of coupons and the methods of exposing under various conditions are discussed in detail. In addition, techniques of evaluating and processing exposed coupons are considered. The construction of special equipment for processing and storage of coupons is described.
The coupon technique has been used to study corrosion under many conditions. A list of environments which have been investigated through the use of this method include gas lift and gas injection systems, water injection wells, gasoline plants, condensate wells, oil wells of all types, tanks, cooling systems, etc. In general, the results of studies of gaseous systems can be interpreted most literally. Studies of liquid systems have been very valuable, but, occasionally, variables not related to the corrosion process have influenced results. A brief resume of these and other pertinent factors concerned with the use and limitations of the coupon technique is given.
