ISO 22426:2020 Assessment of the effectiveness of cathodic protection based on coupon measurements.
5.3 Spread resistance
In relation to coupons, the spread resistance is the ohmic resistance from the exposed metallic surface of the coupon towards remote earth. This is the resistance that controls the DC or AC current through a coating defect for a given DC or AC voltage. Determining the spread resistance on coupons allows for assessing acceptable on-potentials (DC) and AC voltages.
5.4 Corrosion rate measurements
Various types of coupons and probes have been designed for the purpose of quantifying corrosion and the corrosion rate. Examples are weight loss coupons, perforation probes and ER probes. Refer to Annex A for more details.
6 Design considerations
6.1 General
The coupon design should reflect the purpose of the coupon measurement. The purpose may be:
— a detailed and comprehensive assessment of the CP effectiveness;
— an assessment of the effectiveness of CP under DC interference;
— an assessment of the effectiveness of CP under AC interference.
The information obtained with coupons depends on the geometry and size of the coupon. In the case of assessing the effectiveness of CP, the critical aspects are associated with insufficient cathodic current. In that case, a coupon with a design that results in a highest relative spread resistance, e.g. Figure 1 a), represents a worst case. In contrast, the most critical conditions in the case of AC and DC interference occur on small coating defects with a design that results in lowest spread resistance, e.g. Figure 1 c). As a consequence, these influencing parameters shall be considered. The fundamental concept of a coupon is the mimicking of a coating defect on the structure. These coating defects can have various shapes and sizes. Therefore, the coupon geometry should be adapted to an assumed coating defect geometry and size present on the structure. The relevant parameters are discussed in the following clauses.
6.2 Geometry of the defect
The case of a coating defect with vertical side walls is shown in Figure 1 a). This represents the case where the coating was locally damaged resulting in parallel walls going through the coating. The resistance of the electrolyte within the defect gives a contribution to the spread resistance and results in a homogeneous current distribution on the metal surface. This type of coupon, see Figure 1 a), is least sensitive to the total surface area in the case of large values of z/y. The value y represents the diameter of the coating defect and z represents the coating thickness. The reason for this is the parallel current distribution caused by the constrained electrical field. The calculated average current density is identical to the current density on the edges of the coupon. This configuration represents a conservative assessment of the effectiveness of CP, since typical coating defects do not have vertical parallel sides and permit higher average current densities on the steel within the coating defect.ISO 22426 pdf download.