Similarity between atmospheric/underground corrosion and reinforced concrete corrosion.

In the present paper the similarities and differences between the systems steel/atmosphere or soil and steel/concrete are analyzed. The differences are related to the passive state of reinforcement embedded in non-carbonated and chloride free concrete. The similarities, when carbonation or chloride ions break down the passive film, are discussed. In these conditions the environmental relative humidity plays a decisive part in the corrosion kinetics of the reinforcing steel. The effect of environmental RH on steel corrosion has been well established. The existence of a critical RH for atmospheric or underground corrosion demonstrates the electrochemical nature of the corrosion phenomenon. As in atmospheric corrosion, it seems that the electrolyte supply is the main factor in determining the corrosion rate of active reinforcements. When the concrete moisture amount is very low, the electrode efficiency is also low and the corrosion process is under mixed control. With an increase of the water layer thickness, the corrosion rate increases. However after exceeding a certain water layer thickness, the process starts to be diffusion (oxygen) controlled and the corrosion rate decreases. The mechanism of atmospheric and soil corrosion of steel is reviewed and used as a basis for comparison of controlling factors. (A)