CORROSION OF METALS AND ALLOYS Flashcards
Types Of Metallic
Corrosion
Uniform
Crevice
Galvanic
Pitting
Intergranular
Selective leaching
Erosion corrosion
Stress cracking corrosion
Hydrogen damage
Most common form of corrosion
uniform
Characterized by a chemical or
electrochemical reaction that
proceeds uniformly over the entire
exposed surface
uniform corrosion
Prevention of uniform corrosion includes
(1) proper materials, including coatings,
(2) inhibitors, or
(3) cathodic protection.
Metal deterioration that
happens in tight spaces where
a stagnant liquid gets trapped
crevice corrosion
Prevention of crevice corrosion
- Design considerations
- Corrosion-resistant materials
- Coatings
- Cleaning and maintenance
Exists between two dissimilar
metals with potential difference
in a conductive solution
galvanic corrosion
In galvanic corrosion, the rate of
attack on the anode is
______________, compared to the
rate when the metal is
uncoupled.
accelerated
serious threat because it accelerates the
deterioration of the less resistant metal.
galvanic corrosion
Prevention of galvanic corrosion
- Electrical insulation
- Use of compatible metals
- Sacrificial anodes
- Cathodic Protection
A highly localized form of attack that eats away at the metal,
creating small holes or cavities
pitting corrosion
One of the most destructive and insidious forms of corrosion. It can penetrate deep into the metal, leading to sudden and
unexpected failures even when overall material loss seems
___________
Pitting corrosion; minimal
Prevention of pitting corrosion
- Corrosion-resistant materials
- Protective coatings
- Environmental modification
- Cathodic protection
- Regular maintenance
List metals based on increasing pitting resistance
Type 304 stainless steel
Type 316 stainless steel
Hastelloy F, Nionel, or Durimet 20
Hastelloy C, Chlorimet 3
Titanium
Metals are made up of tiny
crystals called
grains
The boundaries where these grains meet are susceptible to
intergranular corrosion
Intergranular corrosion happens due to chemical composition difference at __________ from the ________
grain boundaries, bulk of the grain
Prevention of intergranular corrosion
- Material selection
- Heat treatment
- Corrosion inhibitors
- Design modifications
- Electrochemical techniques
This type of corrosion
targets alloys, which are
mixtures of different metals
Selective leaching
During selective leaching,
one specific component of
the alloy is preferentially
_______________ by the corrosive
environment
dissolved and removed from
the surface
Prevention of selective leaching
- Alloy selection
- Protective coatings
- Environmental modification
- Cathodic protection
- Regular maintenance
Occurs when a metal surface is repeatedly exposed to a fluid with abrasive particles
Erosion corrosion
The repeated impact of the particles can cause loss
of material that leads to
pits, grooves, or cavities.
Prevention of erosion corrosion
- Protective coatings
- Flow control
- Design modifications
- Regular maintenance
Occurs when a metal is
exposed to a corrosive
environment under tensile
stress
Stress cracking corrosion
Prevention of stress cracking corrosion
- Lowering Stress
- Changing Alloy
- Cathodic protection
- Inhibitors
- Coating
Degradation that occurs
when hydrogen penetrates
the surface of a metal
Hydrogen Damage
Can lead to various forms of
damage like embrittlement,
blistering, and cracking
hydrogen damage
Prevention of hydrogen damage
- Material selection
- Hydrogen removal
- Corrosion inhibitors
- Proper handling
Steel with ________________ is a corrosive combination
Concentrated Sulfuric acid
Stainless steel with ____________ is a corrosive combination
nitric acid
Occurs in the presence of a
liquid containing ions
(electrolyte)
* Electrochemical process.
* Follows thermodynamic
laws and electrochemistry
kinetics
Wet corrosion
High temperature corrosion occurs at elevated temperatures above __________ from gases, solid or molten salts, or molten metals.
400°C (750ºF)
Follows thermodynamic laws
and chemistry kinetics of
heterogeneous reactions.
High temperature corrosion
The kinetics of wet corrosion are influence by various
factors such as
temperature, pH,
and presence of impurities.
is commonly
used to describe corrosion kinetics,
relating the current density to
factors like activation energy and
overpotential
Tafel equation
This type is not primarily
electrochemical in nature, but
involves chemical attacks from
gases, solid or molten salts, or
molten metals at elevated
temperatures
High temperature corrosion
The kinetics of high temperature corrosion are influenced by
factors such as
The kinetics are influenced by
factors such as
Iron and its alloys
Cast
Gray cast
White cast
Malleable cast
Ductile Cast
High Silicon cast
Steels
Stainless steel
Martensitic SS
Austenitic SS
Ferritic SS
Precipitation hardened SS
generic term that
applies to high carbon-iron
alloys containing silicon.
Cast iron
Contain 2% to 4% carbon and
1% to 3% silicon.
* Considered as the least
expensive of the engineering
metals.
Gray cast iron
White cast irons have practically
all of the carbon in the form of
IRON CARBIDE
These are produced by high temperature heat treatment of
white iron of proper
composition
Malleable cast iron
In malleable cast iron, The graphite forms as __________ instead of flakes
and the material shows good
ductility
rosettes or clusters
The graphite is present as
nodules or spheroids as a
result of a special treatment of
the molten metal.
Ductile cast iron
The mechanical properties of
ductile irons can be altered by
______________similar to
ordinary steel.
heat treatment
due to
presence of chlorides
Crevice and pitting corrosion
For cast iron, intergranular corrosion occurs due to
presence of NH4NO3
that caused the
depletion of Cr
Selective leaching happens in cast iron due to
graphite corrosion
When the silicon content of gray cast
iron is increase to 14%, it becomes
extremely corrosion resistant to
many environment
high silcon cast iron
The excellent corrosion resistance of
high silicon irons is due to the
formation of a ____________surface
late
passive SiO2
contains about 14.5% silicon
and 0.95% carbon. It becomes _________ after the addition of 3% molybdenum
Duriron, durichlor
Composed of up to 2.14%
carbon
Steels
Two types of steels
Plain carbon, alloyed
Contained at least 11%
chromium
Stainless steel
Types of stainless steels
Martensitic, Ferritic, Austenitic, and
Precipitation-hardened
Can be hardened by heat
treatment similar to ordinary
steel.
* Heat treatment increases
tensile strength.
Martensitic
Stainless Steels
Essentially nonmagnetic and
cannot be hardened by heat
treatment. Can only be hardened by cold
working
Austenitic
Stainless Steels
Cannot be hardened by heat
treatment. Can only be hardened by cold
working, just like Austenitic
stainless steel
Ferritic
Stainless Steels
Hardened and strengthened by
solution-quenching, followed
by heating for substantial times
at temperature approximately in
the range of 800 to 1000 F.
Precipitation-hardened
Stainless Steels
different from
most other metals in that it
combines corrosion
resistance with high
electrical and head
conductivity, formability,
machinability, and
strength when alloyed
copper
alloys have better
physical properties than copper
alone, and they are also more
resistant to impingement
attack
Copper-zinc alloys
used in preference to copper for
condenser tubes
brass
factors due to high temperatures,
stagnant solutions, and porous
inorganic scale formation.
Dezincfication or Dealloying
Type of corrosion for Cu-Zn alloys due to ammonia
Stress-Corrosion Cracking
(Season Cracking)
Lightweight metal having good
corrosion resistance to the
atmosphere and most aqueous
media
Aluminum
Aluminum corrosion can occur at
extreme pH (>8, <4)
Has the lowest density of all
structural metals
Magnesium
Pure magnesium readily
forms a layer of
MgO2
Magnesium alloys are sought
after for its
high strength-toweight ratio
Magnesium is alloyed with
metals such as____________ to improve hardness,
ductility, corrosion resistance
etc. of the resulting alloy
Al, Zi, Mn and
Zn
Magnesium and magnesium alloys are resistant to
- Atmosphere
- Distilled water
- Alkalis
Magnesium and magnesium alloys are not resistant to:
- Hard water
- Seawater
- Organic acid, inorganic acid and acid salts
- Methanol
- Leaded gasoline mixtures
- Freon plus wate
ductile and
tough mainly because of its
face-centered cubic
structure
Pure nickel
Nickel metal is resistant to the following:
- Alkalis and fused alkalis
- Dilute nonoxiding inorganic and organic acids.
The resistance of nickel metal is improved if acids are _____
deaerated
Nickel metal is not resistant to the following:
- Oxidizing acids
- Oxidizing salts
- Aerated ammonium hydroxide
- Alkaline hypochlorites
- Seawater
- Sulfur or sulfur - containing reducing environments
Consists of 10-30% copper
and 70-90% nickel
Cupronickels
n seawater, ____________forms a stable
layer of Cu2O that provides
excellent resistance to
biofouling
cupronickels
Also known as Nichrome
Ni-Cr alloys
The mixture of formed
oxides (NiO and Cr2O3
)
combine to form nickel
chromite (NiCr2O4
) that
protects it from oxidation.
Ni-Cr alloys
FACTORS AFFECTING METAL CORROSION
Metal composition
Crystal structure
pH Level
Temperature
Aeration
Moisture
Stress
Microbial activity
In some environments,
microorganisms like
bacteria can accelerate
corrosion through
various mechanisms, a
phenomenon known as
microbiologically
influenced corrosion
(MIC)
can make it
more susceptible to
corrosion, especially
localized forms like
stress corrosion
cracking
Mechanical stresses on
the meta
essential
for most corrosion
processes to occur, as it
provides the necessary
conductive medium for
ionic movement.
presence of moisture
often plays a
crucial role in many
corrosion processes,
acting as an oxidizing
agent
oxygen
Generally, __________ temperatures
accelerate corrosion
reactions
higher
