Effect of thermal ageing on the mechanical and corrosion properties of a copper coated steel wire

Abstract In this study, pure shear and seawater corrosion properties of a low carbon steel wire with a diameter of 1.5 mm and covered by a 0.3 µm thick pure copper protective layer were tested at room temperature of 20 °C first. Then, the changes in the mechanical and corrosion behavior upon thermal ageing at 600 °C and 900 °C for 2 h were investigated. Torsion tests were performed on the wire specimens to generate torque and angle of twist data and establish the shear stress–strain relations. The corrosion behavior of the steel wire with and without thermal aging was evaluated by cyclic voltammetry and AC impedance tests in seawater. It has been found that the thermal ageing at 600 °C resulted in 30% loss in the shear strength of the steel wire. The shear strength decreased by nearly 70% after thermal ageing 900 °C. The modulus of rigidity (shear modulus) of the steel wire after thermal aged at 600 °C or 900 °C was only about one-thirds of that of the specimen tested at the room temperature. The decrease in rigidity and shear strength is due to both oxidation of the protective copper coating and strong annealing effect of the steel wire. Thermal aging also changes the corrosion behavior of the steel wire in seawater significantly. The higher the thermal aging temperature, the greater the impedance, indicating that the oxidation of the copper coating at the high temperature produced a semiconducting cupric oxide layer. This surface oxide layer contributes to the capacitive behavior of the steel wire in seawater.