Long-term immersion corrosion of irons and steel in seawaters with calcareous deposition

The marine immersion corrosion of irons and steel under calcareous deposition (principally calcium carbonate) is known to be relatively low for shorter exposures (e.g. a few years). Herein the effect of calcareous deposition on corrosion is considered for exposures up to 1300 years. The data are derived from archaeological steel and iron shipwrecks, cast iron cannons and cannonballs, and wrought iron anchors in locations where there was direct evidence, in and on the corrosion products, of calcareous deposition. Such deposition promotes formation of calcium and ferrous carbonate layers of low permeability on and within rusts. These tend to inhibit both early and long-term corrosion rates. The data show that up to about 200y exposure corrosion losses as a function of time can be approximated closely by a linear function of time. Longer exposures follow a moderate power-law function, consistent with diffusion considerations. Comments are made about the likely interplay between calcareous deposition and microbiological corrosion.