Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method

• A new method for atmospheric corrosion monitoring is proposed.
• Atmospheric corrosion can be monitored by an electrochemical impedance method.
• Impedance at 10 mHz Z10mHz(Av) is well correlated with corrosion rate Icorr(Av).
• The correlation equation is given by Icorr(Av) = (1.7 × 10−5) (Z10mHz(Av))−0.16.

Weathering steel corrosion was monitored for one to two years under natural atmosphere by an electrochemical impedance technique. Two identical comb-shape weathering steel sheets embedded in epoxy resin were used as monitoring probe electrodes at two different bridges in Japan. Impedances at 10 kHz (Z10kHz) and 10 mHz (Z10mHz) were automatically measured every hour. Coupons (50 × 50 × 2 mm3) prepared from the same steel sheets were exposed together to measure the corrosion mass loss. The average (Z10mHz)−1 value for half to one year exposure correlated well with the average corrosion rate determined from the corrosion mass loss.