Corrosion 6 Flashcards
is also
known as general corrosion. A
type of corrosion where the
corroded areas occur in a
manner evenly distributed
across the material being
attacked.
Uniform corrosion
is a primary cause of
uniform corrosion in steels,
alloys, and other metals in
natural environments.
Oxygen
is
also known as
intergranular attack (IGA).
This is a specialized type of
attack that takes place at
the grain boundaries of a
metal.
Intergranular corrosion
This form of corrosion is
often caused by impurities
in the metal, leading to
chemical or
electrochemical attack on
the grain boundaries.
INTERGRANULAR CORROSION
is also known
as dissimilar metal corrosion or
bimetallic corrosion. When two
different metallic
materials are electrically
connected and placed in a
conductive solution (electrolyte),
an electric potential exists.
Galvanic corrosion
is a
localized type of corrosion
occurring within or adjacent
to narrow gaps.
Crevice corrosion
is
characterized by a highly
localized loss of metal. The
initiation of a pit is associated
with the breakdown of the
protective film on the metal
surface
Pitting corrosion
is
usually caused by an
aqueous or gaseous
corrodent flowing over
the metal surface or
impinging upon it.
Erosion corrosion
is a phenomenon where cracks
form and propagate in a
material due to the combined
effects of sustained tensile
stress and exposure to a
corrosive environment.
Stress corrosion cracking
refers to the
degradation of metals caused
by the activity of living
organisms. This is also referred
to as microbial-induced
corrosion (MIC).
Biologically influenced
corrosion
is the term to
describe the leaching of zinc
from brass.
Dezincification
is the term
used to describe the selective
leaching of iron from gray cast
iron.
Graphite corrosion
When intergranular corrosion
takes place in a metal with a
highly directional grain structure,
it propagates internally, parallel
to the surface of the metal.
EXFOLIATION
is the cracking of a
metal or alloy under the combined
action of a corrosive environment and
repeated or fluctuating stress.
Corrosion fatigue
is when metals and
alloys fail by cracking when subjected
to cyclic or repetitive stress
Fatigue failure
The growth formation is explained by the
formation of a differential aeration cell
FILIFORM CORROSION
the process
of a metal returning to
the material’s
thermodynamic stat
Corrosion
The measure of the potential
difference between two half
cells in an electrochemical cell
Cell Potential
may be formed when two
dissimilar metals are in
contact or due to the
heterogeneity of the
same metal surface
Dissimilar electrode cells
are
formed when the
electrodes are identical
but are in contact with
solutions of differing
composition
Concentration cells
Formed when electrodes
of the same metal, each of
which is at a different
temperature, are
immersed in an
electrolyte of the same
initial composition
DIFFERENTIAL TEMPERATURE CELL
This situation arises when
a significant difference in
metal ion concentration
exists over ametal
surface
METAL ION CONCENTRATION CELLS
Also knownas Pourbaix Diagram
Potential-Ph Diagrams
Constructed from calculations
based on the Nernst equation and
the solubility data for various
metal compounds
Potential-Ph Diagrams
Under these
condition of potential and pH, the
metal remains in metallic form
and is free from corrosion
Immunity Zone
Under these
condition of potential and pH, the
metal corrodes
Corrosion Zone
Under these
condition of potential and pH,
protective layer of the metal’s
hydroxide or oxide form on iron
and further corrosion of metal
doesn’t take place
Passive zone
The potential change caused by net
current to or from an electrode,
measured in volts
POLARIZATION
is a manifestation of
the relative changes in the activation
energies for dissolution and deposition,
when equilibrium is disturbed.
Activation polarization
Polarization of an electrolytic cell resulting
from changes in the electrolyte
concentration due to the passage of current
through the electrode/solution interface
Concentration Polarization
The change in voltage associated with
effects of the environment and the circuit
between the anode and cathode sites. It
includes the effects of the resistivity of the
media, surface films, corrosion products,
etc.
Resistance Polarization
is a
complicated electrochemical
process taking place in
corrosion cells consisting of
base metal, metallic corrosion
products, surface electrolytes,
and the atmosphere
Atmospheric corrosion
is negligible in climates where the air temperature
is below the freezing point of water because ice is a poor
conductor and does not function effectively as an
electrolyte
Rusting
refers to the length of
time during which
the metal surface is
covered by a film of
water
Time of wetness
The amount of water adsorbed
on a metal surface depends on
the relative humidity of the
atmosphere and on the chemical
and physical properties of the
corrosion products.
Adsorption Layers
Occurs when the temperature of the metal
surface is below the dew point of the
atmosphere
Dew
creates even thicker layers of electrolyte on
the surface than dew. It affects corrosion by adding a phase layer of
moisture and corrosion stimulators.
Rain
high acidity and high concentrations of sulfates
and nitrate can be found in fog droplets in areas
of high air pollution
Fog
can promote corrosion because of their
hygroscopic nature
Dust
on the surface will contain
various species deposited from the atmosphere or
originating from the corroding metal.
electrolyte film
The metal is covered with a spontaneously formed
oxide and aqueous layer
INDUCTION PERIOD
The oxide layer transforms into a fully developed
layer of corrosion products
TRANSITION PERIOD
The surface being fully covered by corrosion
product and reach constant corrosion rate
STATIONARY PERIOD
defined as the ratio of deposition rate of
any gaseous compound and the concentration of
that compound in the atmosphere
dry deposition velocity
When small amounts of copper,
chromium, nickel, etc. are
added to carbon steel, low-alloy
carbon steel results. These
steels are known as
weathering
steels
