ELECTROCHEMISTRY AND CORROSION Flashcards
USUALLY DEFINED
AS THE DEGRADATION OF METALS
DUE TO AN ELECTROCHEMICAL
PROCESS
corrosion
One in which electricity is
produced by a chemical reaction.
electrochemical process
a chemical reaction
involving the transfer of electrons
between two electrodes in an
electrochemical cell
electrochemical process
These are also called Galvanic cells or simply
Voltaic cells
electrochemical cell
s a device that converts
chemical energy produced in a redox reaction
into electrical energy
electrochemical cell
Electrochemical cells are a combination of two electrical conductors known as _________ immersed in an __________
electrode, electrolyte
he galvanic cell is named after
Luigi Galvani
EChemical cell is composed of two compartments or
half cells
On _______________, positive current flows through the metallic path
from positive electrode to negative electrode.
short-circuiting
the gain of electrons or a
decrease in oxidation state by a
molecule, atom, or ion
reduction
the loss of electrons or an
increase in oxidation state by a
molecule, atom, or ion
oxidation
device used in an
electrochemical cell for connecting
its oxidation and reduction half
cells wherein an inert electrolyte is
used.
salt bridge
junction that connects the
anodic and cathodic
compartments in a cell or
electrolytic solution
salt bridge
The more _________ the reduction potential, the more these elements
want to attract electrons
towards them.
positive
The energy conversion is
achieved by __________ redox reactions, producing a
flow of electrons in electrochemical cells.
spontaneous (ΔG <0)
In __________, electrical energy is used to drive
a non-spontaneous (ΔG ≥ 0)
chemical reaction (redox
reactions)
electrolytic cell
Types of electrochemical cells
Dissimilar Electrode Cell
Concentration Cell
Differential Temperature Cell
These cells involve
different types of metals
or metal compositions
coming into contact,
leading to electron
transfer between them.
Dissimilar Electrode Cell
Types of concentration cells
Salt concentration
Differential aeration
These are cells with two identical
electrodes, each in contact with a
solution of different composition.
Salt concentration cell
Occurs due to differences in potential
between differently Aerated areas
Part of the metal exposed to air is more
oxygenated and acts as a Cathode
Part of the metal immersed in the
electrolyte is poorly oxygenated & acts
as an anode
Differential aeration cells
This is the type of cell when
two identical electrodes are
immersed in same
electrolyte, but the electrodes
are immersed into solution of
two different
temperatures
Differential temperature cell
The _____________ series s has been established by measuring
the potential of various electrodes versus ____________
Electrochemical, standard hydrogen electrode (SHE)
orderly arrangement of the
standard potentials for all metals.
Emf Series
The more ________ values correspond to the more reactive metals
negative
It arises when two metals with
dissimilar compositions come into
contact in the presence of an
electrolyte.
Galvanic cell
Determines the
electrochemical potential and
nobility of metals (and metal
alloys)
Galvanic series
Ranking of metals and alloys
according to their free
corrosion potential in a given
electrolyte.
Galvanic series
THE CLOSER A METAL
OR AN ALLOY IS IN
THE GALVANIC
SERIES, THE __________ ARE
THE EFFECTS OF
GALVANIC
CORROSION
COMPARED TO THOSE
METALS FAR APART.
LESS
Conditions for Galvanic Corrosion
- Metals involved must establish electrical connectivity
- Metals must maintain ionic connectivity
Current is produced when electrons flow (through
anode to cathode)
Electrode potentials
Types of reference electrode potentials
Standard Hydrogen Electrode
Calomel Reference Electrode
Silver-Silver Chloride Reference Electrode
Saturated Copper-Copper Sulfate Reference Electrode
Standard Electrode
Potential (E = 0) at
temperature of ______
278K
The half cell reaction of SHE can be written as follows:
2H+ + 2e- <-> H2
The reaction for SHE generally takes place on a
platinum electrode
The pressure of the hydrogen gas present in this half cell for SHE is
1 bar
Why Platinum is Used?
- Relatively inert
- . Has catalytic qualities
- Surface can be platinized platinum electrode
- Improve kinetics
Reference electrode used in laboratory
Calomel reference electrode
For CRE, _________ is placed at the
bottom of the tube, which is
covered with a paste of ____________
Pure mercury, mercury-mercurous chloride
The calomel is only ___________ in potassium chloride solution
slightly soluble
The half cell reaction of CRE can be written as
Hg2Cl2 + 2e- -> 2Hg + 2Cl-
The activity of Hg22+,
depends on the _________
concentration of KCl
Composed of a silver
wire, sometimes
coated with a layer of
solid silver chloride,
immersed in a solution
that is saturated with
potassium chloride
and silver chloride.
Silver-Silver Chloride
Reference Electrode
When the silver – silver chloride electrode is immersed in a
chloride solution, the following equilibrium is established
AgCl + e- -> Ag + Cl-
Like the calomel electrode, the potential in Silver-AgCl RE is more active the _____
HIGHER the KCl concentration
In 0.1 N KCl for Ag-AgCl, the value is ________, and the temperature coefficient is__________
0.288 V; -4.3 × 10− 4 V/ ° C.
Consists of metallic copper immersed in
saturated copper sulfate.
Saturated Copper-Copper
Sulfate Reference Electrode
It is used primarily in field measurements where
the electrode must be resistant to shock and where
its usual large size minimizes polarization errors.
Saturated Copper-Copper
Sulfate Reference Electrode
The electrode’s
precision, though
slightly lower than
that of calomel or
silver chloride
electrodes, is still
sufficient for most
corrosion studies
Saturated Copper-Copper
Sulfate Reference Electrode
Half cell reaction for Cu-CuSO4
Cu2+ + 2e- -> Cu (0.316V, 7E-4)
relate the current of an
electrochemical reaction to the
number of moles of the element
being reacted and the number of
moles of electrons involved.
Faraday’s empirical laws of
electrolysis
The charge carried by 1 mol of electrons
is known as
1 faraday (F) = 96,485 C/mol
It is the current produced in
an electrochemical cell
during corrosion. It is the
dissolution current at the
corrosion potential.
CORROSION
CURRENT
It is the speed at which any
given metal deteriorates or
corrodes in a specific
environment
CORROSION
RATE
provides a way to calculate corrosion
rate.
Corrosion current
However, corrosion current
alone doesn’t directly tell
you the____________
rate of material loss
A reduction current that occurs when
electrons flow from the electrode
surface to a species in solution. For
example, oxygen reduction in a fuel cell
CATHODIC
CURRENT
An oxidation current that occurs when
electrons flow into the electrode from a
species in solution. For example,
hydrogen oxidation in a fuel cell
ANODIC
CURRENT
The absolute equality between the anodic and cathodic currents
expressed below does not mean that the current densities for these
currents are equal
SURFACE AREA EFFECT
the effect of a certain amount of anodic current concentrated on a small area of
metal surface will be __________ than when the effect of the same amount of
current is dissipated over a much larger area
much greater
If Sc/Sa»_space; 1, then __________. If Sc/Sa «_space;1, then ________
amplifying factor, stifling factor
When polarization occurs
mostly at the cathode, the
corrosion rate is said to be
cathodically controlled
occurs when the electrolyte
resistance is so high that the
resultant current is not suffi
cient to appreciably polarize
anodes or cathodes
Resistance control
can be
calculated from the
corrosion potential and the
thermodynamic potential
Corrosion current
impressed current cathodic
protection (ICCP) or by another
metal that corrodes more readily
than the metal being protected and
therefore is sacrificed is the process
THEORY OF
CATHODIC
PROTECTION
If the metal is polarized slightly beyond the
open -circuit potential, φA , of the anode, the
corrosion rate remain
zero
Should the applied current fall
below that required for
complete protection, some
____________________
occurs
degree of protection
