Wire ropes with a sheathed spiral strand are commonly used for mooring applications in offshore oil and gas production. Each strand comprises a bundle of galvanized steel wires with a blocking compound applied to the outer layer of the wire bundle to prevent seawater from contacting the internal strands of wire if there is a breach in the urethane cover. The blocking compound and a sacrificial zinc layer on each strand of wire are designed to protect the carbon steel.
Depending on the water depth where the wire rope is being used offshore, it can experience a wide range of temperatures—from 22°C at the surface to 4°C at the seabed. Corrosion behavior of wires at these temperatures is critical, in case of breach in the urethane cover.
Immersion and electrochemical corrosion testing was performed on subsea mooring line wire rope with and without blocking compound in synthetic seawater at 4°C, 13°C, and 22°C. Samples of galvanized wire with and without blocking compound at the lowest temperature (4°C) did not exhibit any iron corrosion products even after 160 days of exposure to synthetic seawater. The corrosion rate of galvanized steel with blocking compound remained less than 2 mpy, while that for galvanized steel without blocking compound remained less than 5 mpy.