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The sensitization and passivation characteristics of AL-6XN heat-treated for up to 24 h at 800 °C were evaluated through double loop electrochemical potentiodynamic reactivation (2 M H2SO4 + 0.01 M KSCN +2 M NaCl solution) and cyclic potentiodynamic polarization (3.5% NaCl solution) test methods. Furthermore, the dependence between the characteristic values (Epit, Erep, and degree of sensitization) was discussed. The σ phase was precipitated at the intergranular and intragranular in AL-6XN at 800 °C. When the heat treatment time exceeded 1 hour, AL-6XN occurred rapid sensitization due to the precipitation of σ phase. In a 3.5% NaCl solution, the passivation characteristics of AL-6XN with sensitivity were very poor and depended much on the degree of sensitization.

This study investigated the effect of temperature and time on the sensitization behavior of alloy 600. A doubleloop electrochemical potentiokinetic reactivation test was conducted to evaluate the degree of sensitization (DOS) in 0.1 M H2SO4 + 0.001 M potassium thiocyanate solution. The microstructure and surface damages of the aged specimens were characterized by optical microscope, scanning electron microscope, and energy dispersive X-ray spectroscopy. As a result of the experiment, the sensitization behavior was significantly influenced by the expansion of the Cr-depleted zone and Cr-diffusion rate depending on the aging temperature. In particular, the alloy 600 aged for 300 h or longer at 650 • C represented a very low DOS value of 3% or less, and its resistance against intergranular corrosion greatly improved.