Design method for steel structures in marine environment including the corrosion behaviour

The general corrosion process of steel structures in marine environment was studied and modelled in order to integrate it in the design procedure according to the semi-probabilistic approach. A statistical predictive model of corrosion was developed on a multi-factorial basis accounting for a wide range of environment and structural parameters. The corrosion loss was modelled by a depth-related exponential function in each zone. The strongest effects on corrosion were the age and the cluster of water environment. A part from classical influences, the multi-factorial model brought some surprising conclusions, e.g. a decrease of corrosion with the oxygen content of water, which differ from the conclusions of the electrochemical laboratory trials on non-corroded samples. A simplified statistical model was also developed for temperate sea water with the age of the structure as unique prediction parameter. A design case was calculated for a sheet-piling structure at 50 years marine service life. At the initial stage, i.e. without any corrosion, the deterministic and probabilistic approaches were consistent and safe. At service life, both approaches were still safe and consistent provided pitting corrosion was evicted from the statistics. This eviction was justified because pitting corrosion was shown not to impair the load-bearing capacity of a full-size sheet-piling wall. The design value for the semi-probabilistic approach was the average corrosion loss from the simplified model. This value is 25 % lower than the recommendation in Eurocode EN1993-5 which then appeared conservative; generalising this conclusion however needs further investigations on structural issues.