5 GCU Raphaël ROBINET - Solène LABIT- Anne GIBOIRE

1 – Corrosion : Definition

1 – The electrochemical’s basis of corrosion An example : the iron

The equation for the corrosion of the Iron •Anode : Fe(s) -> + Fe2+(aq) + 2e• Cathode O2(g) + 2H2O(l) + 4e- -> 4OH-(aq) • The overall cell reaction: Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s) •Rust reaction: (OH)2(s) + O2(g) -> 2Fe2O3 •H2O(s) + 2H2O(l)

1 – Nerst’s Equation

R: universal gas constant T: Absolute temperature n: number of electrons involved F: Faraday constant E is the standard potential of the metal [Men +] concentration of metal ions in the solution

The Nernst equation is a very important equation in electrochemistry because it relates the potential E of the cells with the standard potential E0 and activity / concentration of the electroactive ion

1-Galvanic Series

1- Kinetic of corrosion: Faraday’s law

W : Weight (g) A : Atomic weight n : number of electron i : electrical intensity (A) t : Time (s) F : Faraday constant → If you replace i by the density of current you can find the thickness which is corroded

1- Tafel’s Straights

Potential Cathode

Ecor

Anode

icor

Log i

1- Curve of polarization for passivables materials

E Transpassive domain

2

1

Passive

3 4

Active estate

Log i

2 – Corrosion of Reinforcement
8

• Anodic reaction : Fe

Fe2+ + 2e2OH-

• Cathodic reaction: ½ O2 +H2O+ 2e• Fe2+ + 2OHFe(OH)2 Rust

2.A - Consequences
9 • Reduction of strength capacity: reduced cross-sectional area of steel • Loss of bond between the steel and the concrete • Rust drip esthetic • Increasing of the volume due to rust cracking, spalling…

2.B – Factors influencing the Corrosion
10 • Presence of chlorides • Deterioration of concrete cover: physical, mecanical and chemical causes • Concrete porosity • CO2 in the atmosphere

2.C – Stages of Corrosion
11

• Initiation: depassivation • Propagation • Concrete cracking

(1) initiation