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Chemicals - Applications |
| The Effects of Calcium Nitrite and Mix Design on the Corrosion Resistance of Steel in Concrete | |
| N. S. Berke, Part I: Paper #273 in Corrosion '85, Boston,
1985 Part II: Paper #132 in Corrosion '87, San Francisco, 1987 Cement factor, water-cement ratio, addition rate of corrosion inhibitor (DCI™, calcium nitrite), C3A levels and time of immersion in 3% NaCl solution were varied for a series of twelve hundred 3" x 6" concrete cylinders with imbedded #3 rebars. Part I of this study details the composition and strength of the sixteen concretes employed and explains the test procedures, which included measuring corrosion potentials, polarization resistance (Rp) and AC impedance. Salt bridges (Pyrex tubes set in the concrete close to the steel to eliminate concrete resistance effects) were effective in linear polarization experiments for seven months, but then became inoperative, so I.R effects were corrected for by use of AC impedance (at 20 kHz) and current-interrupt circuitry. Some of the rebars were coated with epoxy to leave small bands of steel surface exposed at "Top" (above the solution level), "Middle" (at the solution level), or "Bottom" (below the solution level). No effect of the difference in position was observed, even though chloride analyses did vary with position, being higher at the "Top" rebar position (because salt solution wicked up through the concrete and dried). It was found that calcium nitrite is an effective corrosion inhibitor, which always improves corrosion resistance, but which gives especially good performance at water-cement ratios below 0.5. Polarization conductance (= 1/polarization resistance = 1/Rp) measurements were confined to a ± 20 mv range about the corrosion potential. At corrosion rates where 1/Rp > 20 micromho/cm2, rusting of the rebar occurred, leading to cracking after a total value of 1000 for time/Rp (mho . months/cm2) was reached. If the polarization conductance was measured to be less than 15 mho/cm2 (corresponding to 0.39 microamperes/cm2), the rebars were non-corroded and rust-free. The small current, which does occur even in the absence of rust formation, is probably related to maintenance of the passive film and eliminating defects. High polarization resistance values (low polarization conductance) correspond to low corrosion rates and corrosion potentials in the range of -100 to -300 mv versus SCE, while rusting was associated with more negative corrosion potentials of -350 to -500 mv versus SCE. AC impedance measurements are more complicated to perform than polarization resistance but can shed light on the corrosion process. The absence of a region at a phase angle of 45 degrees indicates that a diffusion limiting reaction (such as O2 diffusion to the catholic sites) is not present. Steel in concrete with calcium nitrite showed a charge resistance at least an order of magnitude higher than for steel in concrete without Ca(NO2)2, confirming the conclusions obtained from polarization resistance measurements. Nitrite analyses show that calcium nitrite protects to chloride/nitrite ratios of 1.5, based on original nitrite content. Lowering the w/c ratio significantly reduces chloride levels. Nitrite analyses, performed at 3 months into the study, show that nitrite is neither lost nor consumed. Even after corrosion begins (at high chloride levels), steel in calcium nitrite concrete corrodes at a slower rate than steel in concrete without calcium nitrite. Corrosion potentials, corrosion rates (corrosion conductance, 1/Rp), chloride concentrations and rebar appearance are tabulated for selected experiments at elapsed times up to 24 months. |
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