THE PROGRAMME IS AIMED AT STUDYING THE CORROSION OF CARBON STEEL REINFORCEMENT IN CONCRETE UNDER THE ANAEROBIC CONDITIONS WHICH ARE EXPECTED TO DEVELOP IN REPOSITORY NEAR-FIELD AREAS AFTER A FEW YEARS. THIS CORROSION IS IMPORTANT BECAUSE THE PRINCIPAL PRODUCTS, H2 AND LOW SOLUBILITY FERROUS OXY-COMPOUNDS, MAY ACCUMULATE AT THE METAL/CONCRETE INTERFACE. SUCH ACCUMULATIONS COULD ULTIMATELY CAUSE CRACKING OF THE CONCRETE NEAR-FIELD BARRIER.
THE AIMS OF THE WORK ARE TO EVALUATE THE RATE OF ANAEROBIC CORROSION AT DIFFERENT HYDROGEN OVER-PRESSURES AND TO INVESTIGATE THE NATURE AND DISTRIBUTION OF SOLID CORROSION PRODUCTS. THESE DATA CAN BE USED WITH EXISTING MASS TRANSPORT MODELS TO ASSESS THE LIKELY ACCUMULATION OF SOLID CORROSION PRODUCTS AND THE BUILD UP OF A HYDROGEN PRESSURE IN THE NEAR-FIELD.
An anaerobic cement preparation and storage facility was constructed for monitoring the corrosion potential and corrosion rate of samples using electrochemical methods. Samples were finally broken down to identify the corrosion product and assess the average corrosion rate using weight loss measurements. Experiments were also carried out in autoclaves using simulated cement pore water solutions at a range of hydrogen overpressures. Electrochemical monitoring methods were used to assess the effect on corrosion rate of the levels of hydrogen overpressure likely to exist in a repository.
Electrochemical potential monitoring of the samples, prepared in the anaerobic glovebox facility, showed that most of the samples achieved anaerobic potentials within a few weeks of casting. On breaking down these samples, weight loss measurements indicated that the corrosion rates were lower than those measured by electrochemical methods.
Corrosion product analysis of the samples showed either no corrosion product or a very thin black corrosion product film which was identified as magnetite by laser Raman spectroscopy.
Large quantities of carbon steel are likely to be used in repositories for intermediate and low level wastes both for containment and structural purposes. In many cases the environment surrounding the steel will be cementitious, consisting of structural concrete, cemented waste or cement grout backfilling. Because these materials have a finite permeability, it is inevitable that the steel will be in contact with water and hence subject to some corrosion. Initially this water will be that used to mix the cement, but in later years it will be altered by the natural waters which permeate the repository site.
The investigations covered:
the rate of anaerobic corrosion of steel in concrete;
the nature of the corrosion products formed on carbon steel which is embedded in concrete under anaerobic conditions;
the effect of hydrogen overpressures on the rate of corrosion in simulated cement pore water electrolytes.
All experiments were carried out under ambient conditions (20 C to 30 C).
1. INVESTIGATION OF THE INFLUENCE OF HYDROGEN OVER-PRESSURES UP TO 100 ATMOSPHERES ON THE ELECTROCHEMICAL KINETICS OF THE ANODIC AND CATHODIC REACTIONS CONSTITUTING THE CORROSION CELL.
2. LONG TERM CORROSION TESTS UNDER OXYGEN FREE CONDITIONS WITH CARBON STEEL SPECIMENS CAST INTO 6 DIFFERENT CEMENT OR CONCRETE MIXES WITH SUBSEQUENT CHARACTERIZATION OF THE SURFACE CORROSION PRODUCTS BY XRD.