The long term effects of cathodic protection on corroding, pre-stressed concrete structures: Hydrogen embrittlement of the reinforcing steel.

摘要

Assessment of the effect of cathodic protection on a chloride contaminated bridge pile involves the definition of the hydrogen embrittlement behavior of the pearlitic reinforcement combined with quantification of the local (i.e., at the steel/concrete interface) chemical and electrochemical conditions, both prior to and throughout the application of cathodic protection.; The hydrogen embrittlement behavior of the reinforcement was assessed through a combination of Devanathan/Stachurski permeation experiments to quantify subsurface hydrogen concentrations, C{dollar}sb{lcub}rm H{rcub},{dollar} as a function of the applied hydrogen overpotential, {dollar}eta,{dollar} and crack initiation tests for bluntly notched and fatigue pre-cracked tensile specimens employing elastic-plastic finite element analysis and linear elastic fracture mechanics, respectively. A threshold mobile lattice hydrogen concentration for embrittlement of {dollar}2times10sp{lcub}-7{rcub}{dollar} mol/cm{dollar}sp3{dollar} was established for bluntly notched and fatigue pre-cracked specimens. Crack initiation occurred by the formation of shear cracks oriented at an angle approaching 45{dollar}spcirc{dollar} from the tensile axis, as proposed by Miller and Smith (Miller, 1970), in regions where both the longitudinal and shear stresses were maximized (i.e., near the notch root). These Miller cracks then triggered longitudinal splitting which continued until fast fracture of the remaining ligament occurred.; Instrumented laboratory scale piles were constructed and partially immersed in ASTM artificial ocean water. With time, localized corrosion (crevicing) was initiated along the reinforcement, and was accompanied by an acidic shift in the pH of the occluded environment due to ferrous ion hydrolysis. Cathodic protection current densities from {dollar}-{dollar}0.1 {dollar}mu{dollar}A/cm{dollar}sp2{dollar} to {dollar}-{dollar}3.0 {dollar}mu{dollar}A/cm{dollar}sp2{dollar} were applied via a skirt anode located at the waterline. Current densities as low as 0.66 {dollar}mu{dollar}A/cm{dollar}sp2{dollar} were sufficient to deplete the dissolved oxygen concentration at the steel/concrete interface and result in the observance of hydrogen production within regions near the waterline where the pH had decreased locally due to ferrous ion hydrolysis.; By combining the effect of local cathodic protection level as a function of position along the reinforcement on hydrogen absorption with the information on the hydrogen embrittlement characteristics of the reinforcement as a function of hydrogen concentration, safe windows for the application of cathodic protection may be identified. Although hydrogen production and absorption were detected at {dollar}-{dollar}0.66 {dollar}mu{dollar}A/cm{dollar}sp2,{dollar} concentrations which were of sufficient magnitude to be considered embrittling were not realized until {dollar}-{dollar}1.33 {dollar}mu{dollar}A/cm{dollar}sp2.{dollar} Local hydrogen concentrations were compared to the 100 mV, 200 mV, and {dollar}-{dollar}780 mV{dollar}sb{lcub}rm SCE{rcub}{dollar} absolute potential cathodic protection criteria. With the exception of the 100 mV depolarization/decay criteria, it was not possible to sufficiently protect the high corrosion rate splash zone of the piling without exceeding the threshold hydrogen concentration for embrittlement at some zone within the reinforcement.