The report on structures in the Czech Republic, commissioned by the Czech Ministry of Transport states: "On almost all structures, areas were found that had been seriously damaged by pitting and lamellar corrosion."
Weathering steel is often used in bridges as it eliminates the need for painting by forming a thin permanent layer of protective rust.
It has become a popular material for structures in the UK based in locations unsuitable for regular maintenance.
However, Mott MacDonald's investigation of Czech structures found that the formation of a protective layer generally occurred locally, not
uniformly on the whole surface, so it did not protect the whole structure.
This made it vulnerable to the corrosive effects of de-icing salts.
Poor detailing around jointing allowing wet salty dirt to accumulate were also to blame.