Thermodynamics
The electrochemical reactions causing the physical corrosion of a material are spontaneous with no external driving forces, and thus, are driven only by nature’s tendency to seek lower energy states, as described by the Second Law of Thermodynamics. In other words, the electrochemical reactions occur to reduce the energy in the system. The surrounding system may, however, influence these reactions to occur at accelerated rates. For example, in an environment with an elevated temperature there is additional energy (from the heat) to drive the reactions at a faster rate. Thermodynamics, however, does not provide an indication of the rate of reaction, since it is independent of which path the reaction will take.
Thermodynamics is used primarily to determine, mathematically, the tendency for corrosion to occur, and can also be used to predict whether a metal will not experience corrosion. It cannot, however, be used to determine whether a metal will, in fact, experience corrosion or to what extent corrosion will occur.
Thermodynamics essentially quantifies the chemical stability of a system in terms of the Gibbs free energy. The amount of Gibbs free energy in a system represents the proximity of the system to equilibrium. That is, the lower the free energy, the closer the system is to equilibrium and conversely, the higher the free energy, the less stable the system is. (The free energy is at a minimum when the system is in equilibrium.) Gibbs free energy, G, at constant temperature, is given in terms of enthalpy, H, absolute temperature, T, and entropy, S as shown in Equation 18.
At equilibrium, when the free energy is at a minimum, the system has no tendency to undergo chemical change, and the free energy can be represented by Equation 19.
where
ΔGº – Gibbs free energy at standard state
R – gas constant
Keq – equilibrium constant
The equilibrium constant of a reaction can be determined for a range of conditions given the standard state free energy, which is commonly available or can be determined from the free energy of formations of the products.
The potential of an electrochemical cell can be given by Equation 20, if the system is thermodynamically reversible, and if the activities of the reactants and products remain approximately constant.
where
n – number of electrons/atom of the species involved in the reaction
F – Faraday Constant (electric charge of 1 mole of electrons)
Eº – electrochemical cell potential at standard state
The electrochemical cell potential (E) is derived from Equation 21. The greater the difference between the electrochemical potentials of the electrodes (anode and cathode) the greater is the driving force for the corrosion reaction.
where
Ec – electrochemical potential of the cathode
Ea – electrochemical potential of the anode
Combining Equation 19 and Equation 20 gives
Ultimately, Equation 22 provides the means to predict the potential of an electrochemical cell. The more negative the cell potential, the more reactive the material, and thus the material is more susceptible to corrosion. Conversely, if the cell potential is less negative or even positive, then the material is less susceptible to corrosion.
Congratulations, you have completed the knowledge section of the course.
You may now complete the course by successfully passing the course quiz.