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1.Axes
pH is plotted on the x-axis.
Potential is plotted on the y-axis in units of volts relative to the standard hydrogen electrode (SHE). Potential can be thought of as the oxidising power of the solution.
Potential is plotted on the y-axis in units of volts relative to the standard hydrogen electrode (SHE). Potential can be thought of as the oxidising power of the solution.
2. Water Equilibria
For corrosion to occur a cathodic reaction (consumes
e−) must take place to balance the anodic reaction
(produces e−) of metal dissolving.
In pure water there are two possible cathodic reactions*:
1. O2 + 4H+ + 4e− = 2H2O
Oxygen dissolved in water is in equilibrium with water.
Ee = 1.223 - 0.0591pH
2. 2H+ + 2e− = H2
Water is in equilibrium with gaseous hydrogen.
Ee = - 0.0591pH
* others may be possible if the water contains oxidating species such as CrO42−, NO3−, etc
In pure water there are two possible cathodic reactions*:
1. O2 + 4H+ + 4e− = 2H2O
Oxygen dissolved in water is in equilibrium with water.
Ee = 1.223 - 0.0591pH
2. 2H+ + 2e− = H2
Water is in equilibrium with gaseous hydrogen.
Ee = - 0.0591pH
* others may be possible if the water contains oxidating species such as CrO42−, NO3−, etc
2. Water Equilibria
Above the line the oxidised species is stable, below
the line it can be reduced if a suitable anode is available.
The lines representing these reactions have the same positions on all Pourbaix Diagrams.
1. O2 + 4H+ + 4e− = 2H2O
2. 2H+ + 2e− = H2
The lines representing these reactions have the same positions on all Pourbaix Diagrams.
1. O2 + 4H+ + 4e− = 2H2O
2. 2H+ + 2e− = H2
3. Metal Equilibria
Zinc can oxidise in four different ways. The four stable
oxidation products are Zn2+, Zn(OH)2, HZnO2−
and ZnO22−.
Appropriate Nernst equations are written for each of these (at an arbitrarily chosen concentration of 10−6 mol dm−3) in equilibrium with solid zinc.
Appropriate Nernst equations are written for each of these (at an arbitrarily chosen concentration of 10−6 mol dm−3) in equilibrium with solid zinc.
3. Metal Equilibria
1. Zn2+ + 2e−
= Zn
Ee = -0.763 + 0.0295 log [Zn2+]
2. Zn(OH)2 + 2H+ + 2e− = Zn + 2H2O
Ee = -0.439 - 0.0591 pH
3. HZnO2− + 3H+ + 2e− = Zn + 2H2O
Ee = 0.054 - 0.0886 pH + 0.0295 log [HZnO2−]
4. ZnO22− + 4H+ + 2e− = Zn + 2H2O
Ee = 0.441 − 0.1182 pH + 0.0295 log [ZnO22−]
The Nernst equation for reaction 1 shows that it is independent of pH, it is therefore horizontal when plotted.
The other reactions all vary linearly with pH, so they are plotted as straight lines with negative gradients.
Ee = -0.763 + 0.0295 log [Zn2+]
2. Zn(OH)2 + 2H+ + 2e− = Zn + 2H2O
Ee = -0.439 - 0.0591 pH
3. HZnO2− + 3H+ + 2e− = Zn + 2H2O
Ee = 0.054 - 0.0886 pH + 0.0295 log [HZnO2−]
4. ZnO22− + 4H+ + 2e− = Zn + 2H2O
Ee = 0.441 − 0.1182 pH + 0.0295 log [ZnO22−]
The Nernst equation for reaction 1 shows that it is independent of pH, it is therefore horizontal when plotted.
The other reactions all vary linearly with pH, so they are plotted as straight lines with negative gradients.
4. Metal Stability
Above an anodic line the oxidised product is
stable.
Below an anodic equilibrium line solid zinc is stable relative to the oxidised product.
Therefore if zinc is in conditions that correspond to a position on the Pourbaix diagram that is below* all the anodic reaction equilibrium lines the zinc is said to be immune to corrosion: it cannot be dissolved.
*However, a piece of zinc in water cannot lie in this region without an externally applied potential because there is no anodic reaction.
Below an anodic equilibrium line solid zinc is stable relative to the oxidised product.
Therefore if zinc is in conditions that correspond to a position on the Pourbaix diagram that is below* all the anodic reaction equilibrium lines the zinc is said to be immune to corrosion: it cannot be dissolved.
*However, a piece of zinc in water cannot lie in this region without an externally applied potential because there is no anodic reaction.
5. Oxidised Species Stability
Above an anodic line the oxidised product is
stable.
If zinc is under conditions that correspond to a position above more than one anodic line the oxidised product stable at the lowest Ee is the one present.
The Pourbaix diagram can be divided into four regions, corresponding to different oxidation products being stable.
If zinc is under conditions that correspond to a position above more than one anodic line the oxidised product stable at the lowest Ee is the one present.
The Pourbaix diagram can be divided into four regions, corresponding to different oxidation products being stable.
6. Oxidised Species Behaviour
The lines between regions of stablitlity of different
oxidised products also have equations that can be calculated by considering
K, the equilibrium constant.
1.Zn(OH)2 + 2H+ = Zn2+ + 2H2O
pH = 8.5
2. Zn(OH)2 = HZnO2− + H+
pH = 10.7
3. HZnO2− = ZnO22− + H+
pH = 13.1
All three equations are independent of E, so they are parallel to the y-axis.
1.Zn(OH)2 + 2H+ = Zn2+ + 2H2O
pH = 8.5
2. Zn(OH)2 = HZnO2− + H+
pH = 10.7
3. HZnO2− = ZnO22− + H+
pH = 13.1
All three equations are independent of E, so they are parallel to the y-axis.
6. Metal Behaviour
The behaviour of the metal is determined by the nature
of the stable oxidation product. For example, in Zinc's system:
Zn2+ , HZnO2− and ZnO22− are soluble in aqueous solution, therefore corrosion occurs in these regions.
Zn(OH)2 is not soluble in water, instead it forms an oxide film on the zinc which causes it to passivate, preventing any further corrosion.
CORROSION
PASSIVITY
IMMUNITY
Zn2+ , HZnO2− and ZnO22− are soluble in aqueous solution, therefore corrosion occurs in these regions.
Zn(OH)2 is not soluble in water, instead it forms an oxide film on the zinc which causes it to passivate, preventing any further corrosion.
CORROSION
PASSIVITY
IMMUNITY