Cathodic protection of soil buried steel pipelines - a critical discussion of protection criteria and threshold values

This paper represents the consensus reached during a panel discussion held at the CEOCOR International Congress in Stockholm, Sweden, June 2-5, 2015. It reviews the state-of-the-art about the generally accepted understanding of the working mechanism of cathodic protection of ground buried steel structures. Based on this, limitations of existing protection criteria are discussed, particularly the difficulties related to measuring and interpreting current-applied potentials, instant-off potentials, and IR-free potentials. A set of protection criteria and their interpretation in accordance with the understanding of the working mechanism of cathodic protection is proposed. The main idea is that it is distinguished between the following two situations: 1) conditions that permit an increase in pH at the steel surface as a result of the applied protection current (good bedding conditions inhibiting convection) and thus provide corrosion protection by achieving passivity; and 2) situations where this is not possible (e.g. in streaming soft water) or where it is unknown if this is possible, and thus effective corrosion protection can only be achieved by polarizing the steel into the immunity zone.