Characterization of chloride-induced corrosion products that form in steel-reinforced cementitious materials.

摘要

The goals of this investigation were to identify the chloride-induced corrosion products that form from steel reinforcement in concrete, determine where they form, the corresponding corrosion rate, and relate these observations to the performance of steel-reinforced concrete in the field. This information is intended to be incorporated into structural service life models such that more accurate predictions of the remaining lifetime of a structure can be made. To accomplish these goals, experiments were conducted on various cementitious materials that ranged from steel in simulated pore solution to concrete of various types.; In situ electrochemical and Raman spectroscopy observations were performed on steel immersed in either a Type 10 or Type 50 white cement simulated pore solution that contained varying levels of chlorides up to 1 M NaCl. The composition of these pore solutions was developed from pore solutions expressed from 6 month old cement paste cylinders. The surface of the steel was either ground with 240 grit SiC paper or left in the as-received condition with a mill scale predominately composed of magnetite (Fe₃O₄). The effects of the type of pore solution, surface condition of the steel, and varying chloride exposure on the type and distribution of corrosion products were studied. The results indicated that the critical chloride/hydroxide ratios at which corrosion initiates depend upon the pH of the simulated pore solution, even within the narrow range of 12.9 to 13.4. Corrosion initiated with lower chloride levels and at lower applied potentials in the lower pH level of the white cement simulated pore solution than in the relatively higher pH Type 10 cement simulated pore solution. In addition, mill scale was observed to provide some enhanced corrosion protection by delaying the onset of corrosion. Once corrosion initiated, however, the corrosion rates of the as-received steel were similar to the ground steel surfaces. The corrosion products observed included magnetite, maghemite, Green Rust I, and hæmatite.; To coordinate with the aforementioned simulated pore solution-steel experiments, steel plates were cast into a 0.45 w/c cement paste which had been stabilized with 10% by mass of silica sand (henceforth referred to as modified cement paste). As before, both Type 10 and white cement pastes were studied as well as the effect of the surface finish of the steel, either ground or as-received. (Abstract shortened by UMI.)