Pourbaix Diagrams and Corrosion

Corrosion and Electrochemistry

• Equilibrium voltage (Eo) is the potential at which the oxidation and reduction reactions are in equilibrium.
• Overvoltage (η) is the difference between the real potential and the equilibrium potential, η = E - Eo.

Cathodic and Anodic Reactions

• Equilibrium potential for cathodic reaction is Eoc, and for anodic reaction is Eoa.
• Real potential (E) is the potential when the corrosion process takes place.
• Cathodic overvoltage (ηc) is E - Eoc < 0, and anodic overvoltage (ηa) is E - Eoa > 0.

Polarization

• Polarization is the displacement of each electrode potential from its equilibrium value, reducing the potential difference between anode and cathode.
• Polarization values affect the slopes of the curves, and corrosion rate decreases as polarization values increase.

Corrosion Process

• Me+2 ions reach the metal surface and adsorb to the surface.
• Me+2 ions receive electrons and turn into their metal form, Me°.
• Ions can diffuse into metal at the surface and be reduced.

Overvoltage Types

• Total energy amount in the system, ηT = ηD + ηA + ηc + ηcr + ...
• ηD: Diffusion overvoltage, ηA: Adsorption overvoltage, ηc: Charge overvoltage, ηcr: Crystal overvoltage
• The bigger overvoltage type controls the reaction rate.

Polarization Types

• Diffusion polarization occurs when the ion concentration on the electrode surface diverges from the concentration in the solution.
• Activation polarization is the formation of a layer containing absorbed hydrogen atoms that block the metal's surface from the corrosion process.
• Resistance polarization occurs when a conductive oxide film or adsorbed oxide film or adsorbed gas layer occurs on the electrode surface, increasing the resistance of the electrolyte-electrode road.

Measuring Corrosion Rate

• H = G / A.t, where H is the corrosion rate, G is the mass loss, A is the surface area, t is the time, and γ is the density.
• Corrosion rate classification: H < 0.15 mm/year, 0.15 mm/year < H < 1.5 mm/year, H > 1.5 mm/year.

Tafel Equations

• For activation polarization, the relationship between current density (i) and overvoltage (η) is given by the Tafel equations.
• Tafel constants (aa, ac, βa, βc) change according to the events that occur at the electrode and the environment.

Explore the concept of Pourbaix diagrams, which depict the stability of different ions in water and the corrosion behavior of metals. Learn how to interpret the diagrams to identify regions where metals are stable or prone to corrosion. Understand the importance of studying the ideal conditions for corrosion prevention.