Review of Electrochemical Principles as Applied to Corrosion of Steel in a Concrete or Grout Environment
Appears on pages(s):
chlorides; corrosion; electrolysis; galvanic corrosion; pH; reinforced concrete; reinforcing steels; Materials Research
Corrosion of steel in concrete proceeds at a far greater rate in the presence of chloride ions. Most researchers agree that chloride ions act as an essential part of the corrosion cell by 1) lowering the pH of the concrete pore water in contact with the steel, thereby dissolving the passive oxide film on the steel surface, or 2) penetrating the film to react with and trans-port the metallic iron into the electrolyte. In the latter process, the resulting iron chloride complex ion combines with hydroxyl ions to form ferrous hydroxide in solution and lower the pH. This, in turn, thins out the oxide film and speeds penetration of chloride ions. Eventually the continuing cor-rosive action results in a pit. To initiate corrosion, a threshold concentration of chloride is needed in excess of the amount immobilized by reaction with tricalcium aluminate in the cement. Investigation of the chloride ion content in concrete adjacent to corroding reinforcing steel shows the concentration to be 1.0 to 1.4 lb of chloride ion per cubic yard of concrete (0.59-0.83 kg/m3).