Propagation of reinforcement corrosion: principles, testing and modelling

Reinforcement corrosion is the risk most frequently cited to justify concrete durability research. The number of studies specifically devoted to corrosion propagation, once the object of most specialised papers, has declined substantially in recent years, whilst the number addressing initiation, particularly where induced by chlorides, has risen sharply. This article briefly describes the characteristics of steel corrosion in concrete that need to be stressed to dispel certain misconceptions, such as the belief that the corrosion zone is a pure anode. That is in fact seldom the case and as the zone is also affected by microcells, galvanic corrosion accounts for only a fraction of the corrosion rate. The role of oxygen in initiating corrosion, the scant amount required and why corrosion can progress in its absence are also discussed. Another feature addressed is the dependence of the chloride threshold on medium pH and the buffering capacity of the cement, since corrosion begins with acidification. Those general notions are followed by a review of the techniques for measuring corrosion, in particular polarisation resistance, which has proved to be imperative for establishing the processes involved. The inability to ascertain the area affected when an electrical signal is applied to large-scale elements is described, along with the concomitant need to use a guard ring to confine the current or deploy the potential attenuation method. The reason that measurement with contactless inductive techniques is not yet possible (because the area affected cannot be determined) is discussed. The method for integrating corrosion rate over time to find cumulative corrosion, P-corr, is explained, together with its use to formulate the mathematical expressions for the propagation period. The article concludes with three examples of how to use corrosion rate to assess cathodic protection, new low-clinker cements or determine the chloride threshold with an integral accelerated service life method.