3b (37)

4. Pourbaix Diagrams

Based on thermodynamic data of reactions between metal and water, Pourbaix developed potential versus pH diagrams which indicate thermodynamicalły stable phases, as a function of electrode potential and pH. The Pourbaix diagram for iron is shown in Figurę 4.

Figurę 4. Pourbaix diagram for the FeO-H₂0 system at 25°C.

There are three generał regions described as regions of corrosion, passivitv and immunity. A soluble product is formed under a rangę of acidic conditions and under narrow rangę of various alkaline conditions. These are regions of corrosion. Between these two regions an insoluble film is formed, and that region is referred as a passive region. The third region, immunity, shows that metal is thermodynamicalły stable and no corrosion will occur. Pourbaix diagrams provide a strong thermodynamic basis for understanding corrosion reactions. There are two main łimitations for use of the diagrams. One arises from lack of kinetic data and the other comes from the purity of environment. In practice, corrosion processes are take place in contaminated environments.

5. Consequences of Steel corrosion.

In the case of reinforcement corrosion, the simplest and most descriptive model for service life is shown in Figurę 5. This well known model considers:

-    An initiation period which consists of the time from the erection of the structure until the aggressive agent (either chlorides or the carbonation front) reaches the rebar and depassivates the Steel.

-    Apropagation period from the Steel depassivation until a certain unacceptabłe łevel of deterioration is developed in the structure.

Lifetimr ar time befcre repair

Figurę 5. Service life model for reinforcement corrosion.

6. Corrosion: structural effects. ₀,^c/j^    ^f

Ln case of non prestressed remforcements, the first direct effect of the reinforcement corrosion is its section " decrease due to the corroding process, as shown in Figurę 6. Iron oxides (rust) resułting from the corrosion process have a larger volumeThan the original Steel and this effect induces intemal stresses in the concrete which may lead