Chlorides and carbonation in aquatic environments
How reinforcement loses its passive film in tanks, rafts and marine structures, and why the first signs are measurable before the cover cracks.
In sound concrete the pore solution is highly alkaline, with a pH of about 13. In that environment the steel is covered by a thin film of passive oxides. Two agents commonly destroy that film: carbon dioxide, which lowers the pH of the cover, and chloride ions from seawater or de-icing salts, which break the film down directly.
Once the steel is depassivated, expansive corrosion products form. They crack and then spall the cover, which accelerates the process. In some cases, flowing water washes the oxides away and no swelling is seen, even though section is being lost. That is why seepage water itself is worth analysing: dissolved iron is a sign that the reinforcement is already corroding.
On aquatic and marine structures this matters for tanks, basement rafts, coastal works, and the concrete that supports acrylic viewing panels. A leak can be the first public sign of a process that started inside the cover long before.
MAREX maps corrosion activity, measures cover, carbonation and chlorides, takes cores for the laboratory, and recommends protection. The mechanism and the field methods are set out on the reinforced concrete service page, with the supporting MAREX method sheets (including D1-1, B2-2, B2-3 and B2-8).
Those sheets are listed as method references. They are not offered as a public download on this site at this stage.