Mechanisms and conditions for the formation of rusticles on steel immersed long-term in natural waters

Stalactite-like rust formations, known as 'rusticles' have been observed on some ocean shipwrecks usually after extended exposures and sometimes associated with microbiological influences. Herein that possibility is examined using field observations for some 40 different shipwrecks in seawaters and open freshwaters. Comparison is made to somewhat similar rust formations, known for more than 100 years as 'tubercles', that are mounds of highly non-uniform corrosion product found both in freshwaters and in seawaters. The data show that tubercles are widespread in occurrence but that rusticles form only in seawaters and that their typical stalactite-like formation is possible only in quiescent exposure conditions, caused by the extended build-up of rusts resulting from the oxidation of downward migration of ferrous chloride, itself generated by pitting corrosion under localized anaerobic seawater conditions. The processes in the formation of rusticles and tubercles are otherwise similar. Microbiological processes may be involved but are not essential.