GREEN RUST, IRON SOLUBILITY AND THE ROLE OF CHLORIDE IN THE CORROSIONOF STEEL AT HIGH PH

The solubility of iron, as well as nature of the solid corrosion produ cts, influences greatly the kinetics and mechanism of reaction of stee l embedded in cement or concrete. At high pH, ferric iron has a very l ow solubility, <1mM/l, and spinel, which passivates steel, is obtained as the solid corrosion product. However, at pH approximately 13, low chloride ion concentrations are inimical to spinel formation; the solu bility-limiting solid corrosion product is instead ''green- rust'', a layer-structured hydrate containing both Fe2+ and Fe3+ and Cl-. In the range 10 - 15mM/1 chloride, iron solubility at pH approximately 13 in creases abruptly from near-zero to approximately 170 - 180mM/l. The en hanced solubility of iron, probably as an aqueous chlorocomplex, allow s it to be transported. The resulting mobility with subsequent precipi tation from the complex results in corrosion product being disseminate d within the paste.