Analysis of the steel: concrete interface.

The mechanism of passivation and corrosion of steel reinforcement in concrete are inferred from analytical electron microscopy and X- ray diffraction studies of the reaction products formed at the steel:concrete interface boundary, and from the electrochemical and thermodynamic factors which control the formation of the observed products. The analysis outlines in some detail the effects of chloride ions and atmospheric CO "SUB 2" uptake on processes occurring at the interface. The applicability of some currently-held mechanistic ideas is questioned. In the absence of chlorides, a passivating layer consisting of spinel structured solid solution forms. In concretes made to low w/c ratio little formation of Ca(OH) "SUB 2" occurs at the steel:cement paste interface. As normal corrosion proceeds, possibly by discharge of H "SUB 2" , the corrosion product layer seems to thicken at an average rate of 1-2 mum yr-1. However, even very low levels of chloride appear to condition some concentration of chloride at the corrosion product-paste interface, where skageneite, betaFeOOH, develops together with goethite, alpha-FeOOH. Progressively higher chloride concentrations degrade the passivating layer; the changing chemical, mineralogical and transport characteristics of the system as a function of chloride ion concentration are discussed. (A)