01 Thermodynamics and Kinetics of Corrosion Flashcards
This determines whether the reaction is spontaneous, non-spontaneous or at equilibrium.
Gibbs free energy
In this process, G decreases along with the _________________.
reaction time
It can also be explained that during the reaction process, there is a driving voltage that is available. ________________ is the term used to describe this reduction, denoted as ΔG.
Reaction affinity
The _________________ is a quantitative measure of a chemical reaction ‘ s tendency to start, which includes the reaction of a metallic material with its environment.
change in Gibbs free energy (ΔG)
It is also a direct measure of the work capacity or the maximum electric energy possible that is available from a system.
change in Gibbs free energy (ΔG)
When the change in free energy that accompanies a chemical the change of a system from one state to another is ______________, free energy is said to be lost and a spontaneous reaction direction of the system is observed.
negative
Meanwhile, a _____________ value of Gibbs free energy change means that the transition indicates an energy increase, and it requires additional energy to be added to the system.
positive
The change in free energy can be classified as a _________________ and is independent of the reaction path.
state function
This can be defined as a product of charges moved (Q) and the electrochemical potential (E) through which it is moved.
electrical work (W)
Therefore, in the case of electrochemical cells and the charge is represented as 1 mol of reactant in which n mol of electrons are transferred, the electrical work done by the cell is quantified as ________.
-nE
w = ΔG = ________
-nFE
For electrochemical reactions, the _________________ of the corrosion cells, which are crucial part of the corrosion process, can be used to express the tendency of a metal to corrode.
electromotive force (emf)
A _____________ value of E for any type of cell means there is also a _____________ tendency for the overall chemical reaction of the cell to proceed, which is applicable to any type of cell.
higher; higher
Under standard conditions, the ________________ is directly related to the standard cell potential difference across the cell.
standard free energy of the cell reaction, ΔG°
ΔG° = ___________
-nFE°
A more ____________ value of ΔG results in a ____________ tendency to make the reaction proceed.
negative; larger
A _____________ ΔG° value refers to a reaction that has no tendency to start at all.
positive
For a reaction to take place, the driving voltage must be ______________.
positive
On the other hand, when the initial and final Gibbs free energy are __________, or when there is a ___________ driving voltage (ΔG > 0), there is enough reason to conclude that a reaction might not occur
same; negative
When ΔG = 0, it is said that the reaction is said to be in an _____________ state.
equilibrium
Meanwhile, when ΔG > 0, it is said that the system is more likely to change in the _____________ to that stated, unless the external energy of the system is affected by external forces.
other direction
In the case of dissolved species, _____________ includes the effects of the system’s environment.
activity coefficient (γ)
In the case of gaseous species, _____________ includes the effects of the system’s environment.
fugacity coefficient (Φ)
Furthermore, the tendency of a material to corrode is not a direct measure of reaction rate. It is not always true that a large ______________ value of ΔG will result in a higher corrosion rate. However, a ______________ value of ΔG always indicates that a reaction will not proceed under certain conditions.
negative; positive
The standard conditions for solid, liquid compounds, and even elements is the pure compound or element; in the case of gases, it is set at a pressure of _____________ and for solutes, the ideal concentration is _____________.
100 kPa; 1 M
To calculate for the cell potentials, electrode potentials can be combined using algebraic methods. For instance, in the case of galvanic cells like _____________, the equation shown can be used.
Danielle cells
In electrochemical or corrosion reactions, the accompanying change in free energy can be determined using knowledge regarding the _______________ of the reaction.
cell potential
In this set-up, it is required that the system be in equilibrium. Therefore, in Figs. 5 and 6, the rates of ________________ and ________________ must be equal.
metal dissolution; deposition
The equilibrium electrodes are also termed “____________”. If these electrodes are maintained at equilibrium conditions, they are now called “____________”.
half-cells; standard half-cells
The positive terminal is connected to the Cu electrode (___________) and the negative terminal is connected to the Zn electrode (___________).
cathode; anode
In this reaction, the change in Gibbs free energy, ΔG, is calculated from the difference between the ___________________ of products and reactants.
molal free energy
However, if all the activities of reactants and products are equal to __________, the logarithm term becomes zero, and ΔG = ΔG°.
unity
Since ΔG = −nFE , it follows that Δ G° = −nFE°, therefore, this expression shown is known as the _______________, which is used to determine the potential of a system wherein the reactants are not in unit activity.
Nernst equation
The study of reaction rates at the interface between an electrode and a liquid.
Electrode Kinetics
Galvanic Cell
reduction: _______________
oxidation _____________
Cathode
Anode
reaction of gaining electrons: _________________
anodic/oxidation reaction
The deviation from equilibrium potential is called ________________.
polarization
The measurement of magnitude of polarization with respect to the equilibrium potential of an electrode.
Overvoltage
It refers to an electrochemical process that is controlled by the reaction sequence at the metal-electrolyte interface.
Activation Polarization
This generally predominates when the concentration of the reducible species is small (eg. dilute acids, aerated salt solutions).
Concentration Polarization
___________________ usually is the controlling factor during corrosion in media containing a high concentration of active species (eg. concentrated acids)
Activation Polarization
It refers to electrochemical reactions that are controlled by the diffusion in the electrolyte.
Concentration Polarization
Increasing the velocity or agitation of the corrosive medium will increase rate only if the cathodic process is controlled by ________________ polarization.
concentration
Polarization can lead to the formation of a protective oxide layer on the metal’ s surface which acts as a barrier, slowing down further corrosion.
Passivation
Polarization can also increase corrosion rates by promoting more aggressive electrochemical reactions. This can happen if conditions favor the dissolution of the protective oxide layer.
Increased Corrosion
Polarization gradients within the metal surface can lead to the concentration of corrosive agents in specific areas, accelerating corrosion rates locally
Localized Corrosion
Simply the amount of current (I) over a given area (A).
current density (i)
Q = _________
zFN
i = ___/____
I/A
This is the current density at equilibrium, or when the system isn’t polarized/’ overpotential’
exchange current density, (io)
A model for the current density of an electrode when the only significant limiting factor is activation
Tafel Equation
n_diffusion = n_ion resistance = ____
zero
Where there’s no polarization, i_for is __________ to i_rev
equal
When potential is higher at anodic site, i_net is equal to:
i_for - i_rev
Simplifying the Tafel Equation gives the _____________.
Butler-Volmer Equation
Change in entropy is evaluated at constant _____________.
pressure
The process is facilitated by the presence of ______________, which allows for the flow of ions and electrons between the metal surface and the surrounding environment.
electrolyte
The overall corrosion reaction can be described in terms of ______________ involving oxidation and reduction.
half-reactions
The corrosion of metals occurs within a localized electrochemical cell known as a _______________.
corrosion cell
A _________________ involves the spatial separation of anodic and cathodic areas on the metal surface leading to formation of corrosion sites.
corrosion cell
The _____________ serves as a medium for ion transport.
electrolyte
The standard electrode potential (E°) is the potential difference measured under standard conditions (1 M concentration, 1 atm pressure, and a temperature of 25°C) for a half-cell reaction relative to the _____________.
Standard Hydrogen Electrode (SHE)
It is a historically important electrode. It is the halfcell in which hydrogen gas is bubbled over a platinum electrode immersed in a solution having a known concentration of hydrogen ions
Standard Hydrogen Electrode (SHE)
Hydrogen equilibrium bears _________ value.
zero
Those equilibria having higher standard potential than that of H+/H will be noble while the others having lower standard potential will be ___________ and corrode in acid solutions.
active
In any electrochemical reaction, the most negative or active half-cell tends to be ___________, and the most positive or noble half-cell tends to be ___________.
oxidized; reduced
Based on the value of Ecell, __________ in acid suffers corrosion.
Zn
Relations between the energy and the potential of a cell to the concentrations of participating ions and other chemical species.
Nernst Equation
This is a formula for:
Reaction Quotient
Potential-pH diagrams, also called ______________, outline regions of active corrosion, oxides/hydroxides formation, and thermodynamic immunity for a metal.
Pourbaix diagrams
If the oxides/hydroxides protect the underlying metal, then this region is called _______________.
passive region
The _______________, also known as polarization in the context of corrosion kinetics, refers to the deviation in the potential (voltage) of an electrode from its equilibrium or rest potential during an electrochemical reaction.
Overpotential
This is associated with the activation energy required to initiate the electrochemical reaction at the electrode surface. It represents the energy barrier that must be overcome for the reaction to proceed.
Activation Polarization
____________ activation overpotential can contribute to increased corrosion resistance by reducing the rate at which the metal reacts with the corrosive environment.
High
Activation polarization is due to ______________ that are an inherent part of the kinetics of all electrochemical reactions.
retarding factors
The __________________ is the single most important variable that explains the large differences in the rate of hydrogen production on metallic surfaces. It is influenced by factors such as the nature of the reactants, the specific electrochemical system, and the presence of catalysts.
exchange current density
________________ is often added to power cells such as the popular alkaline primary cells to stifle the thermodynamically favored production of gaseous hydrogen and prevent unpleasant incidents.
Mercury
______________ batteries (car batteries) can provide power in a highly acidic environment in a relatively safe manner unless excessive charging currents are used.
Lead acid
This type of overpotential arises from the variation in ion concentrations at the electrode interface.
Concentration polarization
When a chemical species participating in a corrosion process is in short supply, the mass transfer of that species to the corroding surface can become __________________.
rate controlling
Mass transport to a surface is governed by three forces, that is, _____________, _____________, and _____________.
diffusion, migration, and convection
In mass transport, the ______________ term is negligible since it only affects charged ionic species while the ______________ force disappears in stagnant conditions.
migration; convection
The flux of a species O to a surface from the bulk is described with _______________.
Fick’s first law
