What is Faradays law (of electrolysis) and how does the corrosion rate result from that?
W transferred weight
Aw atomic weight
a surface
i current density (I/a)
What is a faradic surface interaction compared to a non faradic?
in Faradic surface interaction a charge is transferred in a non faradic, the charge is sotred by adhearance
Name three ways to measure the corrosion rate.
Wheigt loss
Penetration
Corrosion current density
What is described by the overvoltage or overpotential?
Generally E:Voltage values
E the applied voltage for current to flow
E0 the base potential for 0 current flow (not equal to the standard potential)
What is understood under anodic polarization and what are the effects on the Voltage?
The Voltage (E) becomes more positive
<=> more potential “sucking” the electrons away
What is understood under cathiodic polarization and what are the effects on the Voltage?
The Voltage (E) becomes more negative
<=> more potential leading electrons to the electrode
What are the three forms of electrochemical polarization and their causes?
activation polarization
caused by slow electrode reaction
concentration polarization
caused by concentration change near electrode
Ohmic polarization
caused by resistance drops in solution or film
How can cathodic polarization occur?
Faster electron supply than reaction at cathode => excess of electrons and decrease in potential
How can anodic polarization occur?
The electrons leave the anode faster then teh ions go into solution.
How is the overpotential related to the flow of electic current?
What is i0?
The equilibrium current of each reduction and oxidation of one electrode. This results in a net 0 current.
What is the Butler-Volmer equation and what are the two parts?
anodic + cathodic current
How is the display of the Butler Volmer equation changed for application?
The axes are switched and the current is displayed logarithmically.
Give the tafel law.
How is the Tafel law derived?
Butler Volmer equation as origin
neglecting of reverse reaction(red)
appliation of dec. log
Why is corrosion not reversible? (The dissolved ions are redeposited)
solutions dont initially contain ions of the metal
the solution contains foreign ions
What is the cathodic, what the anodic branch?
upper anodic, lower cathodic
Why is there corrosion happening at the equilibrium point?
Because the flow is net 0 but the reaction is irreversible, which leads to ions dissolving into the solution.
What is the corrosion potential?
The potential in a freely coroding state of two (or more) combined half cells
Give the Butler volmer equation for the mixed potential theory.
What is the reason for concentration polarization and how is it described mathematically?
iL limited current by diffusion
n number of electrons
C electrolyte concentration in bulk
delta diffusion layer thickness
F faraday constant
How is the concentration polarization incorporated into the Evans diagramm?
Why does the corrosion rate increase again at lower potentials?
Change in electron evolution mechanism.
Hydrogen becomes dominant.
When can the Tafel extrapolation method be applied?
anodic and cathodic processes under activation control
well defined anodic and cathodic tafel regions
no additional electrochem. reactions
uniform corrosion
What is the Tafel extrapolation method?
assuming a linear increase in corrosion rate
What is the meaning of activation controlled reactions?
A Metal atom needs to pass the energy barrier (of ionization) to go into the solution
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