The effects of phosphorus segregated at grain boundary on the intergranular corrosion ( IGC ) and the stress corrosion ( SC ) crack initiation and propagation of mild steel in 55% $Ca(NO_3)_2$ solution at 60℃ have been investigated. IGC tests were carried out at the passive potential of +$1000mV_{SHE}$. Electrical potential ( electric resistivity ) method was introduced to differentiate the SC crack initiation from propagation time at both +$1000mV_{SHE}$ and +$100mV_{SHE}$ using constant load device. The results showed that phosphorus leads the susceptibility to IGC, and IGC occurs after the repassivation stage of current density vs. time curve. The time to failure due to stress corrosion cracking ( SCC ) filled almost entirely the SC crack propagation time, irrespective of applied potentials. The effects of grain boundary segregation of phosphorus on SCC were detrimental at +$1000mV_{SHE}$, but not +$100mV_{SHE}$. Average SC crack propagation rates were found to be about $10^{-5}~10^{-4} mms^{-1}$.
