Effect of applied potential on corrosion fatigue of wire ropes in sea water

Abstract

The initiation and growth of corrosion fatigue cracks in wire rope materials immersed in natural sea water have been studied as functions of applied potential, stress range, number of stress cycles, and frequency. The work included bright and galvanized wires of different sizes and mechanical properties and ancillary measurements of electrochemical properties and hydrogen diffusivities in zinc and steel were undertaken. The most important single result from the work has been the demonstration that environment enhanced cracking occurs in two ranges of potential separated by a regime, some 300 mV in extent, in which sea water does not enhance cracking beyond that observed in air. Bright wire has an open circuit potential above this range of immunity and so suffers corrosion fatigue, but galvanized wire will take up a lower potential that is likely to be protective. However, the eventual consumption of the zinc suggests that potential control through the application of impressed current will have longer term advantages. To this end the potential drops occurring across wire bundles have been studied in view of the possibility that the potential measured at the outside of a rope may be appreciably removed from that obtaining at the centre. It has been shown that large potential drops only occur when the control potential requires the passage of relatively large currents. However, control of the outside of a 127 mm dia. rope at –1000 mV caused the centre of the rope to take up a potential of –800 mV and both of these potentials are within the range wherein environment enhanced cracking was not observed. The implication is that even large diameter ropes should be capable of protection, but caution should be exercised against overprotection in view of the possible incidence of hydrogen induced cracking of the wires near the outside.