Abstract High rates of corrosion had led to severe material loss in a low-alloy medium carbon steel Tether Chain from an offshore mooring system after only 5 years in service. The material losses were caused by extensive corrosion pitting and wear damage from adjacent links. Substantial levels of biomass were detected in the seabed soil and seawater at depths below 20 meters. The corrosion products on the chain link surfaces also tested positive for slime forming microorganisms indicating mature biofilm formation with inclusion of microenvironments which enabled proliferation of diverse aerobic, facultative and anaerobic heterotrophic populations. Those identified by molecular microbiological methods employed included Acid-Producing, Iron-Oxidising, Sulphate-Reducing and Sulphur-Reducing populations. The findings from the analysis confirmed a mechanism of Microbiologically Induced Corrosion (MIC), with wear rates further exacerbated by the action of rubbing wear continually exposing fresh steel surfaces which corroded in the bacteria rich environment. The repetitive, cyclical nature of the combination of rubbing and corrosion resulted in unexpected, accelerated corrosion rates.