Passivation of iron in pH buffered solutions with and without chloride ions

Interaction with the chloride ions can significantly cripple the service life of steel structures. It is therefore necessary to understand the impact of chloride ions on the kinetics of iron passivation. In order to firstly estimate the pH-potential region for iron passivation, potentiodynamic polarisation tests were carried out in borate buffer solutions at different pH. The optimum condition to achieve passivation was subsequently found to be in a solution having pH 9 with the electrode potential in the range 0.5 to 0.8 V_SCE. The influence of chloride ions on iron passivation was then studied under these conditions by performing chronoamperometry. These experiments were carried out in different solutions at pH 9 with/without borate buffer, both in the presence and absence of chloride ions. In general, passivity, which is manifested by the drop of the chronoamperometric currents from millamperes to few microamperes, was immediately acheived in chloride-free borate solutions (in a few seconds). In non-borate solutions with chlorides, passivity was not achieved even after many hours of chronoamperometry. In borate buffered solutions with the chlorides added, the chronoamperometric currents dropped to a few 100s of microamperes with time, implying that a less protective passive film forms on steel. These results have been discussed in the context of passivity breakdown in the presence of chlorides and local acidification.