The corrosion experiment of iron was performed in boiling 55% $Ca(NO_3)_2$ solution (115℃) to elucidate kinetics of intergranular corrosion(IGC) of iron. After the specimen was exposed for 15 min to 24 h to the IGC test solution at $1000 mV_{SHE}$ (passive region) and the time-current curve was recorded, the specimen was then examined for IGC by optical microscopy. The time-current curves and optical micrographic observations suggested that IGC occurred after passivation and passivation is necessary for occurrence of IGC of iron. Normalized specimens (<0.001, 0.002-0.004, 0.02 wt.% C and 0.014-0.02 wt.%N) show no intergranular attack, while the annealed and tempered Armco iron (0.02 wt.% C) specimens show a susceptibility to IGC. The excess dissolved nitrogen or nitride at the grain boundaries (gbs) does not lead to a definite IGC. The presence of fine carbides at the gbs may cause IGC but a critical amount of carbon at the gbs may be necessary for ocurrence of IGC.
