Pourbaix Diagram

About Pourbaix Plots

•This is a graphical representation of electrochemical stability for different redox states of an element as a function of pH.

•These are essentially phase diagrams that map the conditions of potential and pH (most typically in aqueous solutions) where different redox species are stable.

•Areas in the Pourbaix diagram mark regions where a single species (Fe2+(aq), Fe3O4(s), etc.) is stable. More stable species tend to occupy larger areas.

•Lines, mark places where two species exist in equilibrium.

•Pure redox reactions are horizontal lines - these reactions are not pH-dependent

•Pure acid-base reactions are vertical lines - these do not depend on potential

•Reactions that are both acid-base and redox have a slope of -0.0592 V/pH x # H+⁄# e-).

Pourbaix Plots

Equilibria in the iron Pourbaix diagram (numbered on the plot):

1.Fe2++2e−⟶Fe(s) (pure redox reaction - no pH dependence)

2.Fe3++e−⟶Fe2+ (pure redox reaction - no pH dependence)

3.2Fe3++3H2O⟶Fe2O3(s)+6H+ (pure acid-base, no redox)

4.2Fe2++3H2O⟶Fe2O3(s)+6H++2e− (slope = -59.2 x 6/2 = -178 mV/pH)

5.2Fe3O4(s)+H2O⟶2H++2e− (slope = -59.2 x 2/2 = -59.2 mV/pH)

•The water redox lines have special significance on a Pourbaix diagram for an element such as iron.

•Recall that liquid water is stable only in the region between the dotted lines.

•Below the H2 line, water is unstable relative to hydrogen gas, and above the O2 line, water is unstable with respect to oxygen.

•For active metals such as Fe, the region where the pure element is stable is typically below the H2 line.

This means that iron metal is unstable in contact with water, undergoing reactions:

•Fe(s)+2H+⟶Fe2+(aq)+H2 (in acid)

•Fe(s)+2H2O⟶Fe(OH)2(s)+H2 (in base)

•Iron (and most other metals) are also thermodynamically unstable in air-saturated water, where the potential of the solution is close to the O2 line in the Pourbaix diagram. Here the spontaneous reactions are:

•4Fe(s)+3O2+12H+⟶4Fe3++6H2O (in acid)

•4Fe(s) + 3O2- ⟶2Fe2O3 (s) (in base)

Corrosion

Diagram Overview:

The diagram represents the electrochemical behaviour of iron (Fe) as a function of pH (x-axis) and electrode potential (y-axis, in volts).

It is divided into three main zones:

🔴 Corrosion,

🟢 Passivation,

🟡 Protection.

2. Corrosion Region (Red zone):

Found mostly at low to moderate pH and higher potentials.

Iron exists as Fe²⁺ and Fe³⁺ ions here.

The metal is actively corroding as it dissolves into the solution.

Typical condition where iron loses electrons and forms soluble ions.

3. Passivation Region (Green zone):

Found at moderate to high pH and potential.

Iron forms protective oxide/hydroxide films like Fe₂O₃ (s), Fe₃O₄ (s), and Fe(OH)₂ (s).

These passive films act as a barrier to further corrosion.

This region is preferred for the long-term stability of iron structures.

4. Protection Region (Yellow zone):

Found at lower potentials, often in reducing environments.

Iron remains in its metallic state (Fe) and does not dissolve.

This zone represents cathodic protection, where the potential is shifted negatively to avoid corrosion.

5. Arrow Markings:

Blue arrow (Anodic Passivation):

Shifting potential upward (more positive) can lead to the formation of passive oxide layers.

This is used for anodic protection, promoting passive layer formation to prevent corrosion.

The red arrow (Cathodic Protection): Shifting potential downward (more negative) moves the system into the protection zone. It prevents iron from oxidising, keeping it in a metallic state (Fe).

6. Point ‘X’ (Transition Point):

Represents a critical potential-pH condition where iron could transition either Upward into passivation (safe),

Downward into cathodic protection (safe),

Or remain in corrosion (unsafe).

7. Solid Phase Regions:

Areas labelled with species like Fe₂O₃ (s), Fe₃O₄ (s), Fe(OH)₂ (s) indicate the stability zones of solid iron oxides/hydroxides.

These solids are key in passivation behaviour.

8. Practical Application:

Engineers and corrosion scientists use this diagram to determine Safe operating pH and potential for iron-based materials.

Conditions to apply anodic or cathodic protection in pipelines, bridges, ships, etc.

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