Influence of steel composition on the mechanical properties and Zn-Mg-Al coating layer on the corrosion resistance of steel wire ropes for offshore applications

Abstract

Steel rope is generally used for mooring application. Recently, higher strength and corrosion resistance are required to avoid the weight penalty with increase of the sea-water depth in offshore oil production. This can be achieved by increasing the carbon content and adding alloy elements in zinc coating. Three objectives of the research were conducted as follows; to investigate the effect of composition on strength and corrosion resistance of galvanized wire, to investigate the degradation mechanism of high tensile strength (2,150 ~2,250MPa) galvanized steel rope in sea-water and to develop a Zn-Al-Mg ternary alloy coating to enhance the corrosion resistance of steel ropes. Microstructural characterisation on coated steel wires was carried out using optical microscope, field emission scanning electron microscope (FESEM) equipped with energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD). Both Tafel polarization and salt spray tests were conducted to investigate the corrosion resistance of Zn-Mg-Al alloy coatings. Torsion, fatigue and tensile tests were conducted. The best fatigue property was obtained at 2,167MPa tensile strength. Tensile strength above 2,167MPa, fatigue and torsion properties were reduced and the susceptibility to surface defect also was increased during fatigue testing, by which cracks can be easily created and grown at different planes at the same time. It is the main cause of fatigue degradation. Degradation of mooring rope is caused by combination of fretting and corrosion which normally occurs in wire rope assembly type known as Independent Wire Rope Core (IWRC). Excellent corrosion resistance over 3,000 hours based on salt spray test was obtained by addition of magnesium and aluminium elements in the range of 1.0~3.0wt% but no improvement for over 3wt% Mg, which might be attributed to coarsening of the phase grains. The multi-layer coatings in which each layer consists of several different alloy phases has an advantage because each phase has a unique function and they exert their synergic functions in corrosion environment. As a conclusion, applying Zn-Mg-Al coating to mooring rope improves life span of rope due to reduced crack in the coating and excellent corrosion resistance.