A team of scientists from Italy have found that "macrocells" of electrical current can trigger or exacerbate the corrosion of steel and concrete structures in maritime environments.

The chlorine-related degradation of steel and concrete in marine environments is one of the greatest problems faced by engineers working on submerged hollow structures, such as tunnels and parking lots. This is particularly the case given the exorbitant cost of both building and maintaining these structures in such difficult operating environments.

Scientists in Italy say they have now isolated one of the key factors impacting the corrosion and degradation process via the use of sophisticated modelling techniques, which could be incorporated into design criteria in future to help extend the service life of underwater structures.

According to their findings, the “macrocell” phenomenon can trigger or significantly worsen the process of chloride-induced corrosion in fully submerged facilities made from reinforced concrete.

A macrocell refers to the circulation of electric current between the reinforcing steel bars situated on the external and internal sides of the concrete walls of a submerged structure. The macrocells arise when chlorides are present in the seawater on the external side of the wall and oxygen is present in the pocket of air on the corresponding internal side.

underwater corrision 1

Concrete structures are usually less prone to chlorine-based corrosion when fully submerged as opposed to when they are situated in tidal zones which are intermittently exposed to the atmosphere, since concentrations of oxygen in seawater are much lower.

According to Federica Lollini, assistant professor from the Department of Chemistry, Materials and Chemical Engineering at Politecnico di Milano, however, the occurrence of macrocells can trigger or exacerbate chlorine-induced corrosion in fully submerged hollow structures, which can drastically shorten their service life.

Lollini and her team used numerical models to simulate the electrochemical behaviour of real world structures, and concluded that the impact of macrocells indeed warrants far greater attention from engineers and architects during the design phase.

Now that the numerical models which Lollini and her colleagues devised have managed to successfully identify the kernel of the problem, they can also be applied to the task of aiding the development of electrochemical techniques for preventing or minimizing steel corrosion rates.