Surface corrosion evaluation of steel cable for suspension bridge using electromagnetic method

Abstract

The cables of suspension bridges are normally made of galvanized steel wires. Corrosion may happen with the galvanized steel wires from the surface after serving for a long time. We developed a compact, low-cost electromagnetic system to evaluate the corrosion of steel cables. AC magnetic field was produced by an excitation coil and eddy current was induced in the steel wires. A detection coil was used to measure the magnetic field produced by the eddy current. In our experiments, the excitation coil was 100 turns with a diameter of 3 cm, the detection coil was 60 turns with a diameter of 1 cm and the excitation frequency was about 80 kHz. A lock-in amplifier was used to get the X signal (same phase signal with the excitation field) and Y signal (90-degree phase different signal with excitation field). From the slope of the X–Y graph, the degree of corrosion can be judged. To make the system portable, the excitation coil, detection coil, amplifier, lock-in amplifier, and AD converter were put in a small box with the size of 85 mm × 170 mm × 60 mm; only a USB cable was used for the power and data transferring. The electric power required for the system was less than 1 W. We prepared some galvanized steel wires with and without corrosion and did some experiments in the laboratory. The degree of corrosion of galvanized steel wires was successfully evaluated using the system.