ELECTROCHEMICAL-BEHAVIOR OF THE SIMULATED HEAT-AFFECTED ZONE OF A516 CARBON-STEEL IN H2S SOLUTION

Electrochemical behavior of the simulated grain-coarsened heat-affecte d zone (HAZ) of A516 carbon steel in an H2S containing acidic chloride solution was investigated by electrochemical impedance spectroscopy ( EIS) measurement at open circuit potential at 25 degrees C. EIS measur ement was made for the specimens, prepared with three different heat i nput conditions (15, 30 and 45 kJ . cm(-1)) by thermal simulation with a Gleeble machine, in the H2S containing solution. The results show t hat the corrosion process of the simulated grain-coarsened HAZ can be divided into two stages regardless of the heat input condition. At an early stage, the impedance which is characterized by one large semicir cle in the Nyquist plot increases with immersion time. At a later stag e, the precipitation of sulfide film modifies the surface properties a nd leads to a variation in the EIS data by revealing a capacitive effe ct in the low frequency region. The polarization resistance initially increases with immersion time but then decreases gradually. Equivalent circuits containing two or three time constants are proposed to repre sent the corrosion processes at different stages.