# Materials Corrosion

## Pourbaix Diagram Module

Predict materials corrosion resistance using the Pourbaix Diagram Module included in all Thermo-Calc installations.

## About the Pourbaix Diagram Module

The Pourbaix Diagram Module is a module included in all Thermo-Calc installations that makes it easy to calculate Pourbaix diagrams, also known as Eh-pH diagrams. A Pourbaix diagram is based on thermodynamic equilibrium under varying electrochemical conditions. For a certain system, it shows the stability boundaries for solid, gaseous, and aqueous interactions.

Such data gives important information about the corrosion resistance of metal alloys, but also has much wider applicability. In fact, it gives important information for all situations where water interacts with solids to form aqueous solutions, for example, in geology and environmental science.

The plot shows a Pourbaix diagram for austenitic steel with 7 at% Cr and 5 at% Ni and NaCl 3 mol/liter, at 200°C and 100 bar (from Shi et al. 2011).

## Predict Materials Corrosion Resistance

Aqueous corrosion occurs when a solid material reacts with water and dissolves into water. It occurs almost everywhere and may lead to serious material damages and unexpected failures. It represents tremendous economic cost and environmental degradation. Consequently, scientists and engineers must conduct expensive and time-consuming corrosion experiments as part of failure analyses, risk evaluations, quality improvements, and application enhancements.

The Pourbaix Diagram Module in Thermo-Calc can significantly reduce or sometimes even eliminate the need for these experiments by generating the data needed to predict corrosion resistance in materials.

• How do acidity (pH) and oxidizing/reducing (Eh) conditions affect the corrosion of a given alloy?
• What aqueous species are formed under different electrochemical conditions (pH-Eh) for a given alloy?
• What solid phases are most stable under different electrochemical conditions (pH-Eh)?
• How does the range of passivity vary for different alloy compositions?
• How does the temperature and pressure affect the corrosion of a given alloy?
• What aqueous species are expected when water of a given pH is equilibrated with a deposit of a given chemical composition?
• How would the specification vary with the local Eh value (the depth) of the water reservoir?

## The Pourbaix Diagram Module Allows You to Calculate

• A full Pourbaix diagram after defining your system by answering some simple and necessary questions
• Concentration of aqueous species along the phase boundaries in the calculated Pourbaix diagram, and plot them easily and quickly

The Pourbaix diagram is an ordinary potential phase diagram for H2O and selected contents of the additional components. The axes are the thermodynamic potentials: pH and Eh. The diagram is sectioned at the temperature and pressure of interest, usually 25°C and 1 bar, and projected down on the pH-Eh plane. If there were no other components in addition to H2O, the diagram thus would have the same topology as an ordinary potential phase diagram, in other words, areas represent one-phase regions, lines two-phase, and points three-phase equilibrium, as shown in the figure. When there are additional components, areas may represent more than one phase and lines more than two phases, and so on.

It is common in literature to include in the Pourbaix diagram information that the dominating aqueous species is on in different parts of the diagram. This is known as a so-called predominance-area diagram. One would then add lines denoting when two species have the same concentration. Such lines are not included in the Pourbaix diagrams calculated in Thermo-Calc. However, it is fairly easy to plot the content of different species along the calculated phase boundaries with the Pourbaix Diagram Module.

In an area where only the aqueous solution is stable, one expects full corrosion. Areas in which the metallic alloy is stable represent immunity, in other words, the aqueous phase cannot dissolve the alloy and there is no corrosion. Areas where there are also various solid compounds represent different degrees of corrosion resistance. It is known that some oxides, for instance, Hematite and Corundum, form protective oxide scales that stop further corrosion. This is called passivation.

Pourbaix diagram for pure water at 25°C and 1 bar

## Databases

The Pourbaix Diagram Module requires at least one database containing an aqueous-solution phase with thermodynamic data for water and various aqueous solutes. The database PAQ2, also known as the “Public Aqueous Solution,” based on the Specific Interaction Theory (SIT) works well. This model may be regarded as the combination of Debye-Hückel Limiting Law and the regular-solution model. Thus, other types of interactions are taken into account in addition to the Coulomb interaction.

## Availability

Learn how Thermo-Calc can be used to make a Pourbaix diagram for a 304 Stainless Steel in this free lesson from our Learning Hub

Corrosion: Pourbaix Diagrams

Thermodynamic investigations on materials corrosion in some industrial and environmental processes

Pourbaix Diagrams

See examples of how the Pourbaix Diagram Module is used in applications for corrosion.