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API 571 Damage Mechanisms
GENERAL MECHANICALAND METALLUGICAL
FAILURE MECHANISMS
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Graphitization Strain Aging Brittle Fracture
A change in the carbide phase of C/S
and 0.5 Mo steels after long term
exposure to 800 F to 1100F
temperatures causing decomposition
into graphite nodules.
The deformation and aging at an
intermediate temperature of older
C/S and C-0-5 Mo low alloy steels.
Rapid fracture under stress with little
evidence of plastic deformation.
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Short Term
OverheatingThermal Shock
Softening
(Spheroidization)
Localized
overheating
causingdeformation
and/or rupture at
low stress levels.
Occurs when high
and non-uniform
thermal stressesdevelop over a
short period of
time. If restrained,
stresses above the
yield strength canoccur.
A change in the
microstructure of
steels where thecarbide phase
change from
normal plate like
forms to
spheroidal in thetemperature range
of 850F to 1400F
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885
Embrittlement
Creep/ Stress
RuptureSteam Blanketing
A loss of
toughness in alloys
containing a ferritephase (400 series
SS, duplex SS,
wrought and cast
SS, welds &
overlay) due toexposure to 600F
to 1000F.
At high
temperatures
metals deformunder load below
the yield stress.
A steam blanket
inside a tube
caused by a"departure from
nucleate boiling"
that causes
localized
overheating anddeformation
and/or rupture.
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Erosion/ Erosion
Corrosion
Temper
Embrittlement
Sigma Phase
Embrittlement
The accelerated
removal of
material from
impacts of solids,liquids or vapors.
The erosion can be
increased when
corrosion removesprotective films or
scales.
A reduction in
toughness in low
alloy steel due to
long termexposure to 650F
to 1100F.
Equipment may
fail during startupor shutdown.
Brittle phase in SS
due to high temp
exposure of
1000F to 1750F.Increased
likelihood due to
higher ferrite,
chromium, andmolybdenum
content.
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Thermal
Fatigue
Dissimilar
Metal CrackingCavitation
Thermal cycling
resulting in
cracking from
high stresses at
restrained areas
of equipment.
Cracking in the
ferritic side of a
weld between a
300 series SS
and a ferritic
material
operating athigh
temperature.
Localized
impact forces of
collapsing vapor
bubbles causing
erosion, usually
in pumps and
downstream oforifices or
control valves.
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Mechanical
Fatigue
Vibration-Induced
Fatigue
Refractory
Degradation
Cracking from
cyclical stresses
resulting from
mechanicalloading or thermal
cycling.
Mechanical fatigue
from dynamic
loading due to
vibration, waterhammer, or
unstable fluid flow
initiating at stress
risers or notches.
Mechanical
damage and
corrosion to
refractory due tothermal shock,
expansion, and
oxidation,
sulfidation, andhigh temperature
mechanisms.
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Reheat CrackingGaseous Oxygen-Enhanced
Ignition and Combustion
Cracking most often observed inheavy wall sections due to stress
relaxation from PWHT and service
at elevated temperatures.
Many metals are flammable inoxygen and enriched air (>25%
oxygen) services even at low
pressures, whereas they are non-
flammable in air. The spontaneous
ignition or combustion of metallicand non-metallic components can
result in fires and explosions in
certain oxygen-enriched gaseous
environments if not properly
designed, operated andmaintained. Once ignited, metals
and non-metals burn more
vigorously with higher oxygen
purity, pressure and temperature.
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API 571 Damage Mechanisms
UNIFORM OR
LOCALIZED LOSS OFTHICKNESS
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Galvanic
Corrosion
Atmospheric
Corrosion
Corrosion Under
Insulation
Electrochemicalinduced metal loss
of dissimilar
metals when
oined together ina suitable
electrolyte such as
a moist or
aqueous
environment or
moist soil.
Corrosion frommoist atmospheric
conditions, more
severe in marine
and industrialenvironments.
Corrosion fromwater trapped
under insulation
or fireproofing.
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Cooling Water
Corrosion
Boiler Water
Condensate
Corrosion
CO2 Corrosion
General or
localized corrosion
of C/S and other
metals caused bydissolved salts,
gases, organic
compounds or
microbiological
activity.
General corrosion
and pitting in
boilers and
condensate returnpiping from
dissolved oxygen
and CO2.
Carbonic acid from
CO2 in water
causing general or
pitting corrosionof C/S.
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Flue Gas Dew-
Point Corrosion
Microbiologically
Induced CorrosionSoil Corrosion
Sulfur and chlorinespecies in fuel gas
with water vapor
condense and
form sulfurousacid, sulfuric acid,
and hydrochloric
acid, leading to
corrosion.
Corrosion frombacteria, algae, or
fungi in aqueous
environments
especially instagnant or low
flow conditions.
The deteriorationof metals exposed
to soils related to
temperature,
moisture, andoxygen availability
and other
variables.
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Caustic
CorrosionDealloying
Graphite
Corrosion
Corrosion either
local or general
caused by
caustic or
alkaline salts,
usually in high
heat transfer
conditions or
high solution
strengths.
Preferential
attack on one
or more alloy
constituents
leaving a
dealloyed often
porous
structure.
Corrosion of the
cast iron matrix
of cast iron
leaving
corrosion
products and
porous
graphite.
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HIGH TEMPERATURECORROSION (400F)
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Oxidation Sulfidation Carburization
Oxygen
combined with
C/S and other
alloys at hightemperature
creating oxide
scales.
Carbon
absorbed into a
material at
elevatedtemperature
while in contact
with a
carbonaceousmaterial or
carburizing
environment.
Carbon
absorbed into a
material at
elevatedtemperature
while in contact
with a
carbonaceousmaterial or
carburizing
environment.
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Decarburization Metal DustingCorrosion
Fatigue
The removal of
carbon from
mainly carbon
steel at hightemperatures
resulting in low
strength.
Carburization
resulting in
accelerated
localized pittingoccurring from
carburizing
gasses and
streams
containing
carbon and
hydrogen.
Fatigue cracking
from cyclic
loading and
corrosioninitiating from
stress risers.
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Chloride Stress
Corrosion
Cracking
Ethanol Stress
Corrosion
Cracking
Sulfate Stress
Corrosion
Cracking
Surface cracks of
300 SS and some
nickel alloys from
tensile stress,
temperature, and
an aqueous
chloride
environment.
Surface-initiated
cracks caused by
environmental
cracking of carbon
steel under the
combined action
of tensile stress
and a fuel gradeethanol
Surface initiated
cracks caused by
environmental
cracking of copper
alloys in sulfate
solutions over
many years. Most
commonly foundin heat exchanger
tubes, primarily in
cooling water
services.
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Ammonia Stress
Corrosion
Cracking
Liquid Metal
Embrittlement
Hydrogen
Embrittlement
Aqueous ammonia
streams cause
cracking in some
copper alloys. C/Scracks in
anhydrous
ammonia.
Cracking when
certain liquid
metal contacts
specific alloys.
Hydrogen charging
of metals leading
to brittle cracking.
Charging can comefrom
manufacturing,
welding, or service
environment.
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REFINING INDUSTRY UNIFORM
OR LOCALIZED LOSS ON
THICKNESS PHENOMENA
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Amine Corrosion
Ammonium
Bisulfide Corr.
(Alkaline Sour
Water)
Hydrofluoric Acid
Corrosion
General or
localized corrosion
principally on C/Sin amine treating
processes.
Alkaline sour
water corrosion in
hydro processingreactor effluent
streams and in
alkaline sour water
streams.
HF acid causes
high rates of
general orlocalized corrosion
with hydrogen
cracking,
blistering, and/orHIC/SOHIC.
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Naphthenic
Acid Corrosion
Ammonium
Chloride
Corrosion
Hydrochloric
Acid (HCI)
High
temperature
corrosion fromnaphthenic acid
content,
temperature,
sulfur content,velocity and
alloy
composition.
General or
localized
corrosionoccurring under
ammonium
chloride or
amine saltdeposits, often
without free
water.
Aqueous HCL
causing both
general andlocalized
corrosion
aggressively
affects mostmaterials.
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High Temp
H2/H2S Corrosion
Sulfuric Acid
Corrosion
Aqueous Organic
Acid Corrosion
Hydrogen in H2Sstreams increases
high temperature
sulfide corrosion
above 500F withuniform loss in
thickness in hot
hydro processing
circuits
Sulfuric acidcorrodes CS both
generally and
locally in HAZ's
especially. Verysensitive to flow
rates and water
concentration.
Organiccompounds
present in some
crude oils
decompose in thecrude furnace to
form low
molecular
weight organic
acids which
condense in
distillation tower
overhead systems
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Phenol (Carbonic
Acid) Corrosion
Phosphoric Acid
Corrosion
Sour Water
Corrosion
Acid solventcorrodes C/S in
phenol extraction
of aromatics in
lube oil feed
stocks.
Phosphoric acidcan cause pitting
and localized
corrosion of C/S
depending on acid
concentration,
temperature, and
contaminants (free
water content).
Corrosion of steeldue to acidic sour
water (H2S)
between 4.5 and
7.0 ph.
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Environment-Assisted Cracking
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Amine Stress
Corrosion
Cracking
Wet H2S Damage
(Blistering)
Hydrogen Stress
Cracking-HF
Cracking most
often found at non
PWHT'ed carbon
steel weldments inaqueous
alkanolamine
service.
Hydrogen
blistering,
Hydrogen induced
cracking, Stressoriented hydrogen
induced cracking,
and sulfide stress
corrosion cracking
from hydrogen
permeation of
steel and low alloy
steel.
Cracking of C/S
and low alloy
steels in weld
metal and HAZ'sfrom exposure to
aqueous HF acid
environments.
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Carbonate StressCorrosion Cracking
Polythlonic Acid
Stress Corrosion
Cracking
Cracking adjacent
to C/S welds from
alkaline corrosion
and tensile stress.
Cracking due to
sulfide scale, air,
and moisture
acting on sensitized
austenitic SS.
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Other Damage Mechanisms
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High Temp Hydrogen
Attack (HTHA)Titanium Hydriding
Hydrogen at high
temperatures reacts with
carbides to form methane
which cannot diffusethrough the steel and also
cause a loss of strength.
Hydrogen diffusing into
titanium creates a brittle
phase.
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