Polarization Behavior of Carbon Steel in Two Chloride Solutions

Pitting behavior of carbon steel in 0.5M NaCl and 0.5M NH4Cl solution was studied by means of polarization tests and AC impedance tests. Their corrosion characteristics had been determined by microscope and electron probe micro-analyzer (EPMA). The results suggested that carbon steel exhibited active feature in 0.5M NaCl and 0.5M NH4Cl solution. While once pH value of NH4Cl was increased and NaHCO3 solution was added to NaCl solution, carbon steel behaved characteristics of activation, activationpassivation transition, passivation and pitting corrosion. It could be observed from microscopic observation and EPMA that pitting corrosion was initiated by inclusions in carbon steel. With increase of pH value and concentration of NaHCO3 solution, the stability of passive film formed on carbon steel became stronger and pitting corrosion potential became nobler. According to the electrochemical reaction, formation of active peak for carbon steel in two chloride solutions was related to complex and the intermediate product.