2 - Thermodynamics and Kinetics of Corrosion Flashcards
usable energy in a thermodynamic system that can be used to perform work
Free energy
This is the maximum amount of work a system can perform at constant temperature and pressure.
Gibbs free energy
a property of the system that depends only on its current state and not on how it reached that state.
(temperature, pressure, composition)
state function
is a quantitative measure of a chemical reaction’s tendency to start, which includes the reaction of a metallic material with its environment (corrosion is an example).
Gibbs free energy change (∆G)
Indicates a spontaneous reaction, where the system releases usable energy and moves towards a more stable state (lower energy state).
Negative ΔG
Indicates a non-spontaneous reaction that requires an external energy input to occur.
Positive ΔG
It’s defined as the product of the charge moved (Q) and the potential difference (E) across which it moves.
electrical work (w)
Faraday’s constant
96,485 C/mol
Equation of free energy change (and electrical work)
w=∆G=-nFE
where:
∆G – free energy change
n – no. of electrons involved in the reaction
F – Faraday’s constant (96,485 C/mol)
E – cell potential
This refers to the potential difference (voltage) between the two electrodes in an electrochemical cell.
Cell Potential (E)
This is the change in free energy under specific standard conditions (1 atm pressure, 1 M concentration for solutions, and 25°C temperature).
Standard Free Energy Change (ΔG°)
a higher cell potential (E) corresponds to a more negative standard free energy change (ΔG° ) under standard conditions. This relationship is expressed by the equation:
ΔG° = -nF E°
ΔG°: Standard free energy change (J/mol)
n: Number of electrons transferred in the reaction (mol)
F: Faraday’s constant (conversion factor, 96485 C/mol)
E°: Standard cell potential (V)
A more (—–) value of ∆G results in a (—–) tendency to make the reaction proceed.
more, larger
When ∆G=0, it is said that the reaction is in an
equilibrium state
accounts for these non-idealities and quantifies how the concentration of a dissolved species deviates from its ideal behavior.
Activity coefficient (γ)
considers deviations from ideal gas behavior. It reflects how the actual “escaping tendency” of a gas molecule differs from a perfect gas at the same pressure.
fugacity coefficient (φ)
an instrument used to determine equilibrium potential
Danielle cell
Solids, Liquids, Gases, and Elements: Their standard state is the pure substance at a pressure and temperature of —–
1 atm (101.3 kPa) and a temperature of 25°C (298.15 K)
This implies the substance is in its most stable form @ these conditions
The standard concentration for solutes in solutions is
1M
The standard concentration for solutes in solutions is
E_cell=E_cathode-E_anode
a single electrode in contact with an electrolyte solution containing ions of the same element or compound as the electrode. It represents one half of a complete electrochemical reaction (oxidation or reduction).
half-cell
Nerst Equation
E = E° - (RT / nF) * ln (Q)
allows us to calculate the cell potential (E) of a galvanic cell under non-standard conditions, considering the actual concentrations/activities of reactants and products.
Nernst equation
The study of reaction rates at the interface between an electrode and a liquid
Electrode Kinetics
an electrode at which a net oxidation process occurs
Anode
an electrode at which a net reduction reaction occurs
Cathode
synonymous with oxidation reaction (loses electron)
Anodic reaction
synonymous with reduction reaction (gaining electron)
Cathodic reaction
the deviation from equilibrium potential
Polarization
measurement of magnitude of polarization with respect to the equilibrium potential of an electrode.
Overvoltage
Types of polarization
1.Activation polarization
2.Concentration polarization
- Refers to an electrochemical process that is controlled by the reaction sequence at the metal-electrolyte interface.
- Usually is the controlling factor during corrosion in media containing a high concentration of active species (e.g. concentrated solids)
- This type of polarization refers to the slowing down of the rate-determining step in an electrochemical reaction at the electrode surface. It arises due to the activation energy barrier that needs to be overcome for the reaction to proceed.
Activation polarization
- It refers to electrochemical reactions that are controlled by the diffusion in the electrolyte.
- This type of polarization occurs when the concentration of reactants near the electrode surface is depleted or the concentration of products accumulates due to the ongoing reaction.
Concentration polarization
polarization can lead to the formation of protective oxide layer on the metal’s surface which acts as a barrier, slowing down further corrosion.
Passivation
simply the amount of current (I) over a given area (A).
Current Density, i
Total charge (Q) through the cell when N moles of the metal M reacts
Q=zFn
- The current density at equilibrium
- When the system isn’t polarized/overpotential
Exchange Current Density, i¬0
A model for the current density of an electrode when the only significant limiting factor is activation.
Tafel equation
Tafel equation assumptions
- Reaction is slow, therefore: n_diffusion = n_ion resistance = 0
- Surface of a metal is at a potential E
- When no polarization
i_for=i_rev
- When potential is higher at anodic site
i_net=i_for-i_rev
simplified tafel equation
Butler-Volmer Equation
