Corrosion Propagation under Moderate Accelerated Conditions

Once corrosion of the reinforcing steel embedded in concrete has initiated, the corrosion propagation period is typically assumed to last five to ten years for carbon steel reinforcement. However, the duration of the corrosion propagation period could be significantly longer depending on the exposure environment, reinforcement diameter, size of the corroding site, concrete cover, and concrete composition (e.g., w/cm, total cementitious content, presence of supplementary cementitious materials), and resistivity (affected by temperature and moisture content). A better understanding of how corrosion propagates could provide better guidance when conducting the assessment and control of corrosion for structures in which corrosion has initiated. Corrosion propagation was investigated on instrumented reinforced concrete pipe segments, after corrosion of the reinforcement had initiated. The specimens were exposed to lab humidity and temperature for several months, after corrosion initiated. During this time, corrosion continued. It was then decided to apply a current equivalent to 0.5 μA/cm2 via a galvanostat (in some cases this current density was later increased to 1 and 2.5 μA/cm2) in addition to the naturally occurring corrosion current density. The applied current density assumed that half of the steel area under the reservoir was undergoing corrosion. The current was applied: typically current-on for 11 days and then disconnected for 3 days, and the process was then repeated. The specimens during the accelerated corrosion period were stored in high humidity, and after several cycles, selected specimens were covered with saturated sand while others remained under the high humidity exposure. During the disconnected periods, electrochemical measurements such as corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy were performed; the latter two tests at least two days after removal of the applied current. Selected specimens were terminated and gravimetric weight loss measured. These weight loss values were compared to the mass loss calculated by using Faradays law obtained from the measured corrosion current.