Electrochemical behavior of Cr-Mo steel alloy in high temperature aqueous sodium chloride solution

The electrochemical behavior of a low alloy steel (Fe-2.25 Cr-1Mo) was investigated in 1 m NaCl over a range of temperatures (75-250°C) with various levels of contaminations with CuCl₂. The change in free corrosion potential with time and the anodic and cathodic potentiodynamic polarization were measured. Cyclic potentiodynamic polarization was also measured on a previously corroded electrodes for different times (7 and 72 hrs) either in the same or in a fresh electrolyte. The results revealed that the corrosion potential is shifted, at all temperatures, towards more noble values to an extent which increases with the concentration of CuCl₂. The cathodic current density also increases as the concentration of CuCl₂ increases. CuCl₂ acts as cathodic depolarizer. It undergoes electrodeposition leading to the formation of Cu metal onto the corrosion product, which becomes less adherent and less protective. Most of this Cu metal falls off the alloy surface along with the spalled corrosion product. The shift in the corrosion potential towards more noble values with the increase in the precorrosion time, in absence of Cu²⁺ ions, indicates that the corrosion product which forms protect the surface against further attack. In the presence of Cu²⁺ ions, the passivation occurred after the effect of copper is diminished.