How can chemistry added to fresh concrete protect a bridge years later?
Calcium nitrite helps inhibit corrosion of reinforcing steel. Concrete's alkalinity normally protects steel with a passive film, but chloride from seawater or deicing salts can break it down.
Nitrite helps maintain or restore a protective oxidized surface.
That matters because rust expands, creating pressure that can crack and spall concrete. Calcium nitrite works at the steel-concrete interface long before corrosion becomes visible.
Global Chemical Database