The effectiveness of zinc galvanised coatings in reducing fretting corrosion damage in steel ropes

RoundTable – ENDURANCE RESEARCH OF WIRE ROPE AND ITS PRACTICAL APPLICATIONS – Glasgow 1985

Fretting wear tests have been carried out in air and sea water on uncoated and hot-dipped galvanised 5mm diameter wire as is used in certain mooring ropes. Considerable wear damage occurs to the zinc coating due to its low hardness (80 VHN) and the zinc tends to be extruded from the fretting contact. The equilibrium potential of the zinc coating is -1.0v vs. S.C.E. which is below the value of the impressed potential (-950 mV vs. S.C.E.) which has been shown to give complete protection against fretting damage of the steel in sea water. Fretting the uncoated wire against the galvanised wire results in all the damage occurring on the zinc coating, primarily of a mechanical nature, and no damage to the steel wire. Galvanising is therefore an efficient method of eliminating fretting damage in steel ropes. Deposits of calcium and magnesium salts are formed due to the reduction in pH in the cathodic areas surrounding the fretting contact. It is possible that these could give rise to three-body wear although this has not been encountered in this work.

Author(s): R.B. Waterhouse, R.B. Pearson & R. Smallwood