Mechanisms in Long-Term Marine Corrosion of Steel Reinforcement in Concretes

This paper is concerned with the mechanisms governing reinforcement corrosion in concretes in marine environments and how they influence the manner of local failure of the concrete. Despite the high pH of the concrete, air voids from inadequate concrete compaction can, under chloride conditions, produce localized pitting corrosion of adjacent steel bars. This may continue, under the hydrogen evolution cathodic reaction with the build-up of rusts causing localized concrete failure, followed by exposure of the steel to the environment, removal of the elevated concrete pH, and a subsequent much higher rate of corrosion. A completely separate deterioration process is the gradual dissolution and loss of concrete alkalis with time. This can lower the concrete pH sufficiently to permit general corrosion of steel to be thermodynamically feasible, it increases concrete permeability and it facilitates access to the environment to permit corrosion by oxygen reduction. The two processes produce different types of concrete failure. Examples drawn from actual reinforced concrete structures are given and the mechanisms explained, including the often-observed build-up of FeOOH-type rusts on the outside of magnetite rust layers well inside concretes. The implications that follow and research needs are discussed.