Abstract
Twelve-year laboratory tests of rebar reinforced concrete beams partially submerged in artificial seawater have confirmed that steel corrosion may occur a few months after immersion and may continue for many years. An average steel potential of -71 mV (SCE) was established during the first few months of immersion. Pitting corrosion, marked by a sharp shift in potential of -200 to -300 mV, occurred when chlorides in sufficient concentration reached the rebar surface. Oxygen in air bubbles on the surface provided the cathodic reactant. The average potential reached a maximum negative potential of -547 mV (SCE) after four years immersion and subsequently increased to -360 mV after 12 years. The potential records are marked throughout by frequent spikes varying in amplitude from a fraction of a millivolt to more than 100 mV. It is believed the spikes reflect continuing competition between passivation in the highly alkaline pH of hydrated cement and corrosion in the acidic pH of active anodes. The tests have demonstrated that passive steel potentials are at least 200 mV more positive than corroding potentials, although this separation may not be evident in field measurements because of potential averaging effects.
