Pourbaix Diagram

Definition - What does Pourbaix Diagram mean?

A Pourbaix diagram provides information about the stability of a metal as a function of pH and potential. These diagrams are available for over 70 different metals. Pourbaix diagrams have several uses, including in corrosion studies.

A Pourbaix diagram is also known as a potential/pH diagram, equilibrium diagram, EH-pH diagram, a pE/pH diagram on an E-pH diagram.

Corrosionpedia explains Pourbaix Diagram

Pourbaix diagrams are plotted by using the Nernst equation (an equation used to calculate electrode potential). As the Nernst equation is derived entirely from thermodynamics, the Pourbaix diagram can be used to determine which species (metals or alloys) is thermodynamically stable at a given electrode potential (E) and pH.

Nernst equation: E = E0 - (0.059/n) × ln (Cion)

Where: E = electrode potential V; E0 = standard electrode potential, V; n = number of electrons transferred; Cion = molar activity (concentration) of ions

Pourbaix diagrams provide information on three states of metals:

  • Corrosion: active state
  • Passivity: forming passive layers inhibiting the corrosion process on the surface of the metal
  • Immunity: thermodynamic stability

Characteristics of Pourbaix diagram:

  • Horizontal lines represent electron transfer reactions. They are pH-independent, but potential-dependent.
  • Vertical lines are potential-independent but pH-dependent and not accompanied by any electron transfer.
  • Sloping, straight lines give the redox potentials of a solution in equilibrium. This equilibrium indicates electron transfer as well as pH.
  • The diagram is computed for the equilibrium conditions at 77°F (25°C).

Objectives of the Pourbaix diagrams:

  • To show the directions of the various reactions at given pH and potential
  • To make a basis for estimation of the corrosion product compositions at various pH and potential combinations
  • To show which environmental pH and potential changes will reduce or prevent corrosion

Limitations of Pourbaix diagram

  • The validity of the diagrams is limited to reactions between pure metals, pure water and the species that can be formed from these. Small amounts of impurities and alloying elements may change the diagram.
  • These diagrams are purely based on thermodynamic data and do not provide any information on the reactions and not possible to measure the corrosion rates.
  • Consideration is given only to equilibrium conditions in specified environment, and factors such as temperature and velocity are not considered, which may seriously affect the corrosion rate.

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