CORROSION PRINCIPLES
Each of the physical degradation processes above can be assisted or
aggravated in the presence of an aqueous environment.
Corresponding to each of the degradation processes listed there are
environmentally assisted counterparts
If no external forces act on the system the system tends to transfer to its
lowest energy state.
An electrochemical reaction is defined as a chemical reaction involving the transfer
of electrons.
It is also a chemical reaction which involves oxidation and reduction.
Corrosion is the
simultaneous
transfer of mass
and charge across
a metal/solution
interface.
On a corroding metal surface, anodic and cathodic reactions occur in a coupled
manner at different places on the metal surface.
At certain sites on the iron surface, iron atoms pass into solution as Fe2- ions
The two electrons produced by this anodic half-cell reaction are consumed
elsewhere on the surface to reduce two hydrogen ions to one H2 molecule.
The reason that two different electrochemical half-cell reactions can occur on the
same metal surface lies in the heterogeneous nature of a metal surface.
Polycrystalline metal surfaces contain an array of site energies due to the
existence of various crystal faces (i.e., grains) and grain boundaries.
In addition, there can be other defects such as edges, steps, kink sites, screw
dislocations, and point defects.
Moreover, there can be surface contaminants due to the presence of impurity
metal atoms or to the adsorption of ions from solution so as to change the surface
energy of the underlying metal atoms around the adsorbate.
Metal atoms at the
highest energy sites
are most likely to
pass into solution.
four conditions which are necessary for corrosion to occur
(1) An anodic reaction
(2) A cathodic reaction
(3) A metallic path of contact between anodic and cathodic sites
(4) The presence of an electrolyte
Electrochemical reactions occurring
during the corrosion of zinc in air-free
hydrochloric acid.
In neutral waters the anodic corrosion of some
metals like aluminum, zinc, or magnesium
develops enough energy to split water directly
as illustrated in Fig
Electrochemical reactions occurring during the
corrosion of zinc in aerated hydrochloric acid.
both oxygen reduction and hydrogen evolution
are possible as cathodic reactions
corrosion of zinc by a solution containing
copper ions
The corrosion of metals can also occur in
fresh water,
seawater,
Salt solutions,
and alkaline or basic media.
In almost all of these environments, corrosion occurs importantly only if dissolved
oxygen is also present.
Water solutions rapidly dissolve oxygen from the air, and this is the source of the
oxygen required in the corrosion process.
rusting of iron when exposed to a moist atmosphere
ferric hydroxide dehydrates due to drying
• Electrochemical reactions proceed at a finite rate.
• In aqueous electrolyte solution the surface reaches a steady state potential
Ecorr.
• It depends on the ability and rate at which electrons can be exchanged by
the available anodic and cathodic reactions.
• If the surface potential changes the reaction rate changes accordingly
anodic or cathodic reaction increases.
Corrosion Control
Eliminating oxygen reduction by preventing air from contacting aqueous solution.
Coating surfaces with paint or other non-conducting films
Corrosion inhibitor
 organic compound that forms an impervious layer on the surface
 high mol. Wt amine that retard H2 evolution
Increase electrical resistance of electrolyte
 pure water
 pure acids
Speed is controlled by the slowest electrode reaction:
Hydrogen reduction:
Adsorption
Charge transfer
Reduction
Bubble of H2 gas
Concentration polarization is the polarization component caused by concentration
changes in the environment adjacent to the surface.
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 controlling.
Mass transport to a surface is governed by three forces, that is, diffusion, migration,
and convection.
In the absence of an electrical field the migration term is negligible since it only
affects charged ionic species while the convection force disappears in stagnant
conditions.
For purely diffusion controlled mass transport, the flux of a species O to a surface
from the bulk is described with Fick’s first law
Ohmic or resistance polarization
Resistance polarization is a part of electrode polarization arising from an
electric current through an ohmic resistance within the electrode or the
electrolyte. It results from the pure resistance elements along the current
path in electrochemical cells.
The effect of resistance polarization may be significant where this current
flows from the anode to the cathode in an electrolyte, depending on the
resistance of the electrolyte.
If the resistivity of the solution is high, a potential drop (IR) results from
the flow of the current in the resistive solution.
Increasing the solution resistance offers a good method of controlling
corrosion, since decreasing corrosion current means decreasing corrosion
rate.
Metals such as iron, nickel, chromium, and aluminum are all inherently reactive, as
evidenced by the fact that they occur in nature as their ores rather than in elemental
form.
These metals are used in industry because they react with water and/or oxygen to form
stable passive oxide films.
Transpassive region
Oxide films on metals are often (but not always) very thin and are not visible to the eye.
The transition metals (e.g., Fe, Cr, Co, Ni, Mo) and their alloys (e.g., the Fe−Cr stainless
steels) tend to have thin passive films, which are tens to hundreds of angstroms (Å) in
thickness.
The air-formed oxide film on titanium is 30–80 Å (3–8 nm) in thickness
The non-transition metals (e.g., Zn, Cd, Cu, Mg, Pb) tend to have much thicker passive
films, which can be thousands to tens of thousands of angstroms in thickness.
The passive film on copper piping in domestic water systems consists largely of Cu2O
and is typically about 5,000 Å (500 nm) in thickness
Theories of Passivity
There are three main theories of passivity. These are
(i) the adsorption theory
(ii) the oxide film theory and
(iii) the film sequence theory
The adsorption theory
A chemically adsorbed (chemisorbed) monolayer of oxygen (i.e., one molecular layer
in thickness) reduces the reactivity of surface metal atoms and thus provides
protection against further attack.
The adsorbed monomolecular film, of course, continues to grow in thickness.
oxide film theory
A thin three-dimensional oxide film separates the metal from its environment.
This oxide films acts as a barrier to the passage of the corrosive environment into the
film and the passage of metal cations from the substrate out through the film.
film sequence theory
Attempts to reconcile the differences between the adsorption theory and the oxide
film theory.
Hackerman noted that an adsorbed film can cause large potential changes associated
with passivity, but that it is unlikely that an adsorbed monolayer could provide longterm protection against an aggressive environment.
a passive film is formed in a sequence of steps, which involve:
(i) chemisorption of O2,
(ii) splitting of the adsorbed O2 molecule to form two adsorbed oxygen atoms Oads,
(iii) formation of a charged surface species Oads
(iv) intrusion of metal ions from the lattice into the adsorbed layer, and
(v) growth of a three-dimensional oxide.
In addition, the film must be able to regenerate itself when damaged so that steps (i)
through (v) are repeated when the oxide film is breached.
W is in gram.
Units for Corrosion Rates
Common units for the corrosion current density include microamperes per square
centimeter, milliamperes per square centimeter, and amperes per square meter.
Various units have been used when the mass loss is the experimentally observed
variable.
These include grams per square centimeter per day and mdd (milligrams per square
decimeter per day).
Sometimes the corrosion rate is given as a uniform penetration rate. Units include ipy
(inches per year), inches per month, and mpy (mils per year, where 1 mil = 0.001 in.).
1  normal
metals and
active-passive
metals in
active state
2 passive
state with
protective
layer
3 
transpassive
state
For activation
polarization velocity
has no effect on
corrosion rate
For concentration
polarization velocity
will increase corrosion
rate
Some metals show
corrosion resistance
due to formation of
bulk films (Pb).
These films are
damaged due to high
velocity due to
mechanical damage….
Corrosion rate
increases
A  exponential
rise in corrosion
B  negligible
corrosion rate ,
sudden increase
at very high
temperatures
In some cases
corrosion rate
decreases with
temperature
due to expulsion
of dissolved
gases
1  passive
state
2
transpassive
state
Curve B 
characteristic of
acids as
medium. With
increase in
conc. Initially
H+ conc.
increases.
At very high
conc. Ionization
is reduced
…corrosion rate
decreases
Zinc in
contact
with
platinum
immersed
in HCl.
Pt is inert,
provides
more
surface
for H2
evolution.
Corrosion
rate
increases
(a) body-centered cubic: V, Fe, Cr, Nb, and Mo
(b) face-centered cubic: Al, Ca, Ni, Cu, and Ag
(c) hexagonal close packed: Ti, Zn, Co, and Mg