An electrochemical sensor which incorporates a counter electrode (CE) and a reference electiode (RE) into an o-ring has been developed for in situ monitoring of crevice corrosion initiation and propagation. It is applicable to crevice comosion situations where the crevice is created by a gasket or an o-ring snd the attack occurs in that crevice. It is shown that
polarization resistance (Rp) values calculated from in situ electrochemical impedance measurements correlate with the onset of crevice attack and with its propagation for crevice corrosion of alloy 625 (UNS N06625) in 3.5% NaCl and in 10% FeCl3 solutions.