BS ISO 22426:2020 pdf download – Assessment of the effectiveness of cathodic protection based on coupon measurements 4 Assessment of CP effectiveness The assessment of the effectiveness of CP in accordance with ISO 15589-1 is based on an IR-free potential measurement. The determination of the IR-free potential on the cathodically protected structure is only possible based on combined direct current voltage gradient and close-interval potential survey measurements. This method is called “intensive measurement” and is described in EN 13509. This method requires, however, significant measurable voltage gradients associated with individual coating defects in order to allow for a reliable assessment of their IR-free potential and demonstrating conformity to ISO 15589-1. As a consequence, the determination of IR-free potential and demonstrating conformity to ISO 15589-1 is no longer possible on today’s structures with high-quality coating systems. While it is still possible to determine instant-off potentials on many structures and use this reading as an approximation to the IR-free potential in certain cases, the increasing level of AC interference is preventing the separation of the earthing systems connected through decoupling devices from the cathodically protected structures for safety reasons. Similarly, in the presence of DC interference conditions, the determination of both IR-free potentials and instant-off potentials is not possible. As a consequence, on an increasing number of structures neither IR-free potentials nor instant-off potentials can be determined in order to demonstrate conformity to ISO 15589-1. The only remaining technology for demonstrating effectiveness is the use of coupons that are connected to the structure under investigation. The use of coupons is further required by ISO 18086. The determination of the effectiveness of CP under AC interference is only possible based on a current density measurement on coupons. The validity and accuracy of data obtained on coupons depend on a number of factors, such as location, geometry and bedding conditions. This document provides guidance on these aspects. 5 Application principles 5.1 IR-free potential measurements The traditional coupon measurement technique has been used to demonstrate conformity of the coupon polarization, which is taken to be representative of the structure coating defects in accordance with the requirements of ISO 15589-1. There are several situations where the use of coupons is a feasible alternative to IR-free potential measurements directly on the structure. In particular, when accurate measurements directly on the structure itself are problematic. Examples include: — in areas affected by traction stray currents and telluric currents;...
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