Corrosion Flashcards
determines whether the reaction is
spontaneous, non-spontaneous or at equilibrium .
Gibbs free energy
quantifies the tendency of any
metal’
s to react with its environment.
CHANGE OF GIBBS FREE
ENERGY
a simple
“chemical” reaction that can be used to understand how
electrochemical concepts can be applied to predict
corrosion tendency of a metal.
Corrosion of iron in an acid
metal atoms lose
electrons and form metal ions
Oxidation
non-metallic
substances such as oxygen gain
electron
Reduction
overall corrosion reaction can be described in terms of
half-reactions
corrosion of metals occurs
within a localized electrochemical
cell
corrosion cell.
involves the spatial
separation of anodic and cathodic
areas on the metal surface leading
to formation of corrosion sites.
corrosion cell
where oxidation occurs,
leading to the release of metal ions.
anode
Reduction reactions take place at
the
cathode
serves as a medium
for ion transport
electrolyte
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 SHE.
standard electrode potential
quantifies the tendency of a metal to undergo
oxidation or reduction relative to a standard reference electrode
standard electrode potential
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
On coupling
one of them turns into anodic and the other cathodic such that
the cell potential becomes positive.
ELECTRO MOTIVE
FORCE (EMF) SERIES
Hydrogen equilibrium bears
zero value
most negative or
active half-cell tends to be
oxidized
most
positive or noble half-cell tends to be
reduced
Relations between the energy and the potential of a cell
NERNST EQUATION
outline regions of
active corrosion, oxides/hydroxides
formation, and thermodynamic immunity
for a metal
Potential-pH diagrams/Pourbaix diagrams
If the oxides/hydroxides protect the underlying metal, then
this region
passive region
Thermodynamically immune region ensures complete
prevention of metal from corrosio
anodic protection and cathodic protection
control the cathodic and
anodic currents that are integral components of
corrosion processes
polarization effects
deviation in
the potential (voltage) of an electrode from its
equilibrium
Overpotential
quired to
initiate the electrochemical reaction at the electrode
surface.
activation energy
Activation polarization is due to
retarding factors
most important
variable that explains the large differences in the rate of
hydrogen production on metallic surfaces.
exchange current density
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
provide power in a highly
acidic environment in a relatively safe
manner unless excessive charging
currents are used.
lead acid battery
arises from the variation in ion
concentrations at the electrode interface
CONCENTRATION POLARIZATION
related to the mass transport limitations of reactants or
products to or from the electrode surface.
CONCENTRATION POLARIZATION
When a chemical species participating in a corrosion process is
in short supply, the mass transport of that species to the
corroding surface can
become rate limiting
Mass transport to a surface is
governed by three forces, that is
diffusion, migration, and
convection.
The flux of a species O to a surface from the bulk is described with
Fick’s first law.
