Corrosion due to macrocells is a serious threat to certain types of structures, especially in the ocean. New developments have recently shed light on how macrocells form in hollow undersea structures and what engineers can do to mitigate their effects.

Macrocells develop whenever an electric current develops between widely spaced anodes and cathodes. Macrocells are becoming better understood as more industries take advantage of advances in corrosion technology to build more complex structures offshore.

After all, only 29 percent of the Earth's surface is above sea level. With apologies to Richard Feynman, there is plenty of room at the bottom of the ocean. Pipelines, Internet cables and other components of transportation infrastructure have needed to use the ocean floor for decades, but in the near future many more industries will have to explore the practicalities of building underwater.

The growing wind energy industry, for example, shows a global trend toward larger turbines being built in deeper waters. In another development, the U.S. Navy recently secured rights to build a 500-square-mile undersea training range in 2014 off the coast of Georgia. There are plans underway for more tunnels, parking structures and even hotels at the bottom of the ocean as surface real estate becomes impractically expensive.

One of the biggest challenges to this trend in construction is that undersea structures corrode easily and are extremely difficult to repair. In December, a study published in NACE International's journal Corrosion was able to get deeper insight into precisely how macrocells contribute to undersea structural failures.

In the report, Federica Lollini from the Politechnic University of Milan explained that undersea construction designers tend to neglect the corrosive effects of macrocells due to a simple lack of information on how they operate.

What Lollini found was that the external walls of hollow structures in the ocean are constantly exposed to chlorides in the seawater, while the oxygen trapped in the internal wall generates a macrocell between them. As a result, the submerged structures degrade due to the chloride-induced corrosion at a more rapid rate. The predictive useful life of the structure must be reduced, and maintenance should be scheduled earlier.

Another recently discovered protection from chloride-induced macrocell corrosion involves an original buffering process. Improper patch repair can result in additional corrosion and a significant underestimation of macrocell corrosion damage. It turns out that the magnitude of the corrosion is more dependent on the details of the individual kinetics of the repair than the specifications of the anode or cathode.

One of the organizations that has already benefited from this new understanding of how macrocells operate in the ocean is the Beruit International Marine Industry, which is currently building a floating island resort that will travel along the coast of Lebanon. The resort island will be supported by undersea cones and tubes of steel engineered for corrosion resistance. Their attention to anodic and cathodic protection will be critical, as developing macrocells could quickly eat away profits from this ultramodern marvel.