Experimental investigations on macro cell corrosion in chloride-contaminated concrete

Chloride-induced reinforcement corrosion in concrete is characterised by the action of so-called macrocells. The associated localised form of corrosion results from the strong electrochemical interaction between the relatively small pitting sites acting as anodes and the large passive steel areas acting as cathodes. The existence of macrocell corrosion poses severe problems in the interpretation of results obtained from corrosion experiments, especially regarding corrosion potential and galvanic current. To elucidate the mechanism of macroceU corrosion experimental investigations were conducted on reinforcement steel embedded in chloride-contaminated concrete. The macrocells were provoked by shortcircuiting actively corroding mild steel bars to passivated stainless steel, thereby intensifying the corrosion process. The experiments included measurement of corrosion potential, ionic concrete resistance, potential difference, corrosion rate and galvanic current. The magnitude of the galvanic current is significantly influenced by the resistance of passive steel to polarisation, and to a much lesser degree by the ionic concrete resistance. Results from potential mapping demonstrated that due to macrocell action the rest potential of the passive steel was shifted towards very negative values that, according to many codes of practice, would suggest active corrosion to take place