Effect of hydrogen on the breakdown of passivating film on pure iron has been investigated in 0.15 $N Na_2B_4O_7 - 0.15 N H_3BO_4$ solution (pH 8.4) under applied potential of 300 $mV_{SCE}$ by using the gas phase hydrogen charging and electrochemical detection technique. Pitting initiation times have been measured as a function of hydrogen input pressure, $Cl^-$ ion concentration, and time to $Cl^-$ ion addition after the stop of hydrogen injection. With increasing hydrogen input pressure an $Cl^-$ ion concentration, the pitting initiation time increases, suggesting that the introduction of hydrogen into passivating film accelerates the breakdown of passivating film caused by $Cl^-$. The pitting initiation time increases as the time to $Cl^-$ ion addition after the stop of hydrogen charging increases. Capacitance of passivating film has been measured as a function of time during the injection of hydrogen by using the AC impedance method. As hydrogen is injected into the passivating film, the capacitance of passivating film increases first and then slightly decreases. The changes of passive current resulting from the potential changes within the passivating range of potential have been measured as a function of ageing time of passivating film with and without hydrogen injection. Cathodic current is passed for a few moments on potential drop from 650 $mV_{SCE}$ to 300 $mV_{SCE}$. As the aging time of passivating film increases, the charge passed for 1 minute after the potential jump decreases and the negative charge passed for 1 minute after the potential drop increases. Above experimental findings have been discussed in terms of the structural change of the passivating film resulting from the introduction of hydrogen.
