Hollow wire corrosion of stainless steel ropes in a marine mooring system and its relation to microstructure

Indications of corrosion were observed on ropes made of stainless steel type 316 in mooring systems of floating platforms in shallow tropical ocean water and doubts about the material quality came up. Routine investigations could confirm the type of alloy and the absence of sensitization, but the corrosion pattern was found unusual: individual wires of the ropes were found as hollow tubes many centimeters in length, which we refer to as hollow wire corrosion (HWC). Comparative investigations with a rope of the same specification from an arbitrarily chosen alternative producer revealed very similar susceptibility to chloride-induced pitting but a significant difference in repassivation behavior of the two products. An electrochemical test was designed which could reproduce HWC under realistic conditions with one product while the other repassivated readily. By electron backscattered diffraction, the different susceptibility to HWC could be related to the very different textures of the wire materials of the two products, resulting from different manufacturing technologies.