Thermodynamics and Kinetics of Corrosion Flashcards
the term used to describe this
reduction, denoted as ΔG (G decreases along with the reaction
time)
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)
It is also a direct measure of the work capacity or the maximum electric
energy possible that is available from a system
Gibbs free energy change (ΔG)
the change in free energy can be classified as a ____________function and is__________ of the reaction path.
state; independent
can be defined as a product of charges moved (Q) and the
electrochemical potential (E) through which it is moved
electrical work
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
-nE
which are crucial part of the corrosion process, can be
used to express the tendency of a metal to corrode
emf
for any type of cell means there is also a higher
tendency for the overall chemical reaction of the cell to proceed, which is
applicable to any type of cell
higher value of E
the standard free energy of the cell reaction,
ΔG° is directly related to the standard cell potential difference across the
cell, wherein
G= -nFE
____________ value of ΔG results in a larger tendency to make the reaction proceed
more negative
__________value of ΔG° (the reactants and products
are in their standard states) signifies an obvious likelihood for magnesium
to react with water and oxygen
large significant negative
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 the
same (ΔG = 0) or when there is a negative driving voltage (ΔG > 0),
there is enough reason to conclude that a reaction might
not occur
when __________, it is said that the reaction is said to be in an equilibrium
state
ΔG = 0
When _____________, it is said that the system is more likely to change
in the other direction to that stated, unless the external energy of the
system is affected by external forces.
ΔG > 0
For dissolved species, ______________ which includes the effects of the system
’
s
environment
activity coefficient
In case of gaseous species, __________ includes the effects of the system’s environment
fugacity coefficient
Furthermore, the tendency of a material to corrode is ____________
of reaction rate
not a direct measure
It is not always true that a _____________ of ΔG will result in a higher corrosion rate
large negative value
a _______________
always indicates that a reaction will not proceed under certain
conditions
positive value of ΔG
The standard conditions for solid, liquid compounds, and even elements is
the pure compound or element
The standard conditions for gases is set at
P = 100kPa
C = 1M
The equilibrium electrodes are also termed
Half cells
If these electrodes are maintained at
equilibrium conditions, they are now called
standard half-cells
A potential difference of approximately ______________ V can be determined if a high-resistance voltmeter is connected between the Cu and Zn electrodes
1.10
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 is called
polarization
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 metalelectrolyte interface.
Activation Polarization
usually is
the controlling factor during
corrosion in media containing a
high concentration of active
species (eg. concentrated acids)
Activation Polarization
generally predominates when the
concentration of the reducible
species is small (eg. dilute acids,
aerated salt solutions).
Concentration Polarization
Two types of polarization
Activation
Concentration
It refers to electrochemical reactions that are controlled by
the diffusion in the electrolyte
Concentration Polarization
Depending on what kind of polarization is controlling the
reduction reaction, __________ produce
different effects
environmental variables
____________ the velocity or agitation of the corrosive
medium will increase rate only if the cathodic process is controlled by ____________
Increasing; concentration polarization.
increase diffusion rate will __________ the effects of concentration polarization and hence _________reaction rate
decrease, increase
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
Effect of polarization in corrosion
Passivation
Increased corrosion
Localized corrosion
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
Q= zFN
Current density , i and exchange current density, io, is estimated using ________
lab experiments
Losses are considered in estimating current densities, including:
anodic and cathodic loss
loss due to ion resistance
diffusion loss
a model for the current density of an
electrode when the only significant
limiting factor is activation
Tafel Equation
Assumptions of tafel equation
(1) reaction is slow, therefore: (n diffusion = n ion resistance = 0)
(2) surface of a metal is at a potential E
When Tafel equation is simplified, then:
Butler-Volmer Equation
___________if only the first term is significant n>50mV since ia»ic
Corrosion (anodic polarization)
Only the second term is significance if n<-50mV since ia«ic
Reduction reaction (cathodic polarization)
ifor=irev if ________
no polarization
inet=ifor-irev if _________
the potential is higher at anodic site
