Lesson 2 Thermodynamics, Kinetics Of Corrosion Flashcards
-
Butler-Volmer equation is used for the determination of the exchange current density of
a material. Which of the following is NOT related or included in the formula?
Viscosity
- In corrosion kinetics, mass transfer is governed by three forces, but two of these are
negligible under some specific conditions. What is the remaining mass transfer force
considered in the corrosion process?
Diffusion
- For the approximation of the diffusion coefficient, tables are available in different handbooks. In case there is none, an equation under Fick’s First Law is utilized. Based on that equation, what is the relationship of the diffusion coefficient with temperature and
viscosity?
Temperature is directly proportional to the diffusion coefficient, while it is the opposite with viscosity
What does a high activation overpotential indicate?
The metal has increased corrosion
resistance
The following variables or properties are included in the computation of flux under Fick’s
first law, except:
Faraday Constant
Calculate the ΔG of this reaction.
Mg + H2O + 1/2 O2 —> Mg(OH)2
-596600
It is useful relations between the energy and the potential of a cell to the concentrations of
participating ions and other chemical species.
Nernst Equation
Nernst Equation is named after _____?
Walther Nernst
A large negative ΔG means a _____?
It can’t be determined
The more negative the ΔG indicate a______?
Greater tendency of metal to react with its environment.
This equation describes the relationship between the current density (i) at an electrode and
the overpotential (η) applied to it.
Tafel equation
Based on the anodic polarization curve, decreasing the Ip will result to?
High resistance to
uniform, galvanic, erosion, stress corrosion cracking
Lowering the Epit in anodic polarization curve will result lower resistance to pitting.
False
We plot the Tafel equation with the electric potential on the X-axis and the logarithm of current density on the Y-axis.
False
Which of the following could result to low passivity?
Low Ep
In deaerated acid, which of the following metals will suffer corrosion?
Co
The Pourbaix diagram outline regions of active corrosion, oxides/hydroxides formation, and
thermodynamic immunity for a metal. They are essentially phase diagrams that map the conditions of _______ as a function of ______.
Potential
pH
It is the half-cell in which hydrogen gas is bubbled over a platinum electrode immersed in a
solution having a known concentration of hydrogen ions.
Standard Hydrogen Electrode
The Pourbaix diagram can be useful in predicting localized corrosion such as pitting and
stress corrosion cracking.
True
What will happen when Al3+/Al* and H+/H equilibria are allowed to couple?
The resulting electrochemical cell will give rise to a positive potential only when Al3+/Al equilibrium turns into anodic and H+/H cathodic
In process of corrosion, a state function called ___ or ___ can possibly be associated
free energy or Gibbs free
energy, G
In gibbs free energy, G ___ along with the reaction time. It can also be explained that during the reaction
process, there is a driving voltage that is available
decreases
is the term used to describe this reduction, denoted as ΔG.
Reaction affinity
is a quantitative measure of a chemical reaction’s tendency to start, which includes the reaction of a metallic material with its environment.
Gibbs free energy change (ΔG)
a direct measure of the work capacity or the maximum electric
energy possible that is available from a system
Gibbs free energy change (ΔG)
When the change in free
energy that accompanies a chemical the change of a system from one
state to another is negative, free energy is said to be ___ and a
____ direction of the system is observed.
lost
spontaneous reaction
a positive value of Gibbs free energy change means that the transition indicates an energy ___, and it requires additional energy to be added to the system.
increase
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)
1 cal = ___ absolute joules.
4.184
In summary, the change in free energy can be classified as a ___ and is ___ of the reaction path.
state function
independent
With regards to electrical and electrochemical processes, ____ can be defined as a product of charges moved (Q) and the electrochemical potential (E) through which it is moved.
electrical work (w)
A higher value of E for any type of cell means there is also a ___ for the overall chemical reaction of the cell to proceed, which is applicable to any type of cell.
higher
tendency
A more negative value of ΔG results in a ____ to make the
reaction proceed
larger tendency
a positive ΔG° value refers to a reaction that has _____.
no tendency to start at all
gold (Au) is an element that does not undergo corrosion in ____ to produce Au(OH)3.
aqueous medium
For a reaction to take place, the _____ must be positive (-ΔG> 0, then ΔG > 0).
driving voltage
When ΔG = 0, it is said that the reaction is said to be in an ______
equilibrium state.
On the other hand, when the initial and final Gibbs free energy are the
same (ΔG = 0) or when there is a _____driving voltage (ΔG > 0),
there is enough reason to conclude that a reaction might not occur.
negative
Meanwhile, when ΔG > 0, it is said that the system is more likely to ____ to that stated, unless the external energy of the system is affected by external forces.
change in the other direction
It is not ___ that a large negative value of ΔG will result in a higher corrosion rate.
always true
a positive value of ΔG _____ indicates that a reaction will not proceed under certain conditions.
always
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 (mol/L).
the case of galvanic cells like _____, the equation below can be used:
Ecell=Ecathode - Eanode
Danielle cells
In _____, the accompanying change in free energy can be determined using knowledge regarding the cell
potential of the reaction.
electrochemical or corrosion reactions
The equilibrium electrodes are also termed “______”.
half-cells
If these electrodes are maintained at equilibrium conditions, they are now called “_____”.
standard half-cells
The positive terminal is connected to the _____ and the negative terminal is connected to the _____.
Cu electrode (cathode)
Zn electrode (anode).
If all the activities of reactants and products are equal to unity, the ____ becomes zero (ln 1 = 0), and ΔG = ΔG°.
logarithm term
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
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
2 types of polarization
Activation
Concentration
____ usually is the controlling factor during corrosion in media containing a high concentration of active species (eg. concentrated acids)
Activation polarization
It refers to an electrochemical
process that is controlled by the
reaction sequence at the metal-
electrolyte interface.
Activation polarization
hydrogen reduction on zinc surface.
Activation polarization
generally predominates when the
concentration of the reducible
species is small (eg. dilute acids,
aerated salt solutions).
Concentration polarization
It refers to electrochemical reactions that are controlled by the diffusion in the electrolyte.
Concentration polarization
any changes in the system that ___ diffusion rate will ___ the effects of concentration polarization and hence increase reaction rate.
increase
decrease
Increasing the velocity or agitation of the corrosive medium will increase rate only if the _____ is controlled by concentration polarization.
cathodic process
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
Simply the amount of current (I) over a given area (A).
Current density
The total charge that passes through the cell when N moles of the metal M react is Q
Current density, i
current density at equilibrium
EXCHANGE CURRENT DENSITY, i0
when the system isn’t polarized/’overpotential’
EXCHANGE CURRENT DENSITY, i0
a model for the current density of an electrode when the only significant limiting factor is activation
Tafel equation
Tafel equation when simplified
Butler-Volmer Equation
Gibbs free energy determines whether the reaction is ____
spontaneous, non-spontaneous or at equilibrium
quantifies the tendency of any
metal’s to react with its environment.
CHANGE OF GIBBS FREE ENERGY
Corrosion typically involves two
electrochemical reactions:
Oxidation
Reduction
metal atoms lose
electrons and form metal ions
Oxidation
non-metallic
substances such as oxygen gain
electrons
Reduction
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.
The ___ serves as a medium for ion transport.
electrolyte
The ____ 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
standard electrode potential (E°)
This parameter quantifies the tendency of a metal to undergo
oxidation or reduction relative to a standard reference electrode.
standard electrode potential (E°)
It is a historically important electrode. It is the half-cell in which hydrogen gas is bubbled over a
platinum electrode immersed in a solution having a known concentration of hydrogen ions.
STANDARD HYDROGEN ELECTRODE
Hydrogen equilibrium bears __ value.
zero
Those equilibria having higher standard potential than that of H+/H will
be ___ while the others having lower standard potential will be ___ and corrode in acid solutions.
noble
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
Relations between the energy and the potential of a cell to the
concentrations of participating ions and other chemical species
Nernst equation
___, also called ____, outline regions of active corrosion, oxides/hydroxides formation, and thermodynamic immunity for a metal.
Potential-pH diagrams
Pourbaix diagrams
In Pourbaix diagrams, if the oxides/hydroxides protect the underlying metal, then this region is called ___
passive region.
__, also known as __ 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
polarization
It represents the energy barrier that must be overcome for the reaction to proceed.
ACTIVATION ENERGY
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.
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
This type of overpotential arises from the variation in ion
concentrations at the electrode interface.
CONCENTRATION POLARIZATION
is related to the mass transport limitations of reactants or
products to or from the electrode surface.
CONCENTRATION POLARIZATION
In conc polarization, 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.
