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DAMAGE MECHANISMS
Forms of Corrosion
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Corrosion Damage Mechanisms:
AS/NZS 3788 Appendix MGroup I
(1) General (uniform) corrosion
(2) Localised corrosion
(3) Galvanic corrosionGroup II
(4) Velocity effects
(5) Intergranular attack
(6) De-alloying attackGroup III
(7) Cracking phenomena
(8) High temperature corrosion
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Material Properties
Composition
Metallurgy
Hardness
Strength
Fabrication
Toughness (Impact Properties)
Fatigue Strength
Expansion
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Time of Corrosion and Damage
On-Line
Off-Line
Start-Up and Shut-down
Standby
Upset (Operational Excursion)
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Chemical Conditions
CO2 / H2S / NH3 / H2 / O2 / Cl-
Calculated Total H2S, CO2, NH3 and pH
Passive Film Formation
Aeration
Microbiological Activity
Scale Formers and Inhibitors
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Physical Conditions
Pressure
Temperature
Phases: Steam / Water / Condensate
Flowrates and Geometric Effects
Constraints and Supports
Thermal and Mechanical Stresses
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Forms of Corrosion: AS/NZS 3788
Group I(1) General (uniform) corrosion
(2) Localised corrosion
(3) Galvanic corrosion
Group II(4) Velocity effects
(5) Intergranular attack
(6) De-alloying attack
Group III(7) Cracking phenomena
(8) High temperature corrosion
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Group I: Readily Observed byVisually Examination
(1) General Corrosion
(2) Localised Corrosion
(i) General
(ii) Pitting
(iii) Crevice(3) Galvanic
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Group II: Additional Inspection
Techniques May Be Required(4) Velocity phenomena
(i) Erosion
(ii) Cavitation(iii) Fretting
(5) Intergranular Corrosion
(i) Weld Decay
(ii) Exfoliation
(iii) De-alloying (Plug and Layer)
(iv) Hydrogen blistering
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Group III: Microscopic ExaminationRequired
(6) Cracking Phenomena
(i) Environmental Cracking
(Sulfide, Chloride, Mixed and Hydrogen)
(ii) Corrosion Fatigue
(7) High Temperature Corrosion(i) Scaling
(ii) Internal Attack and Fissuring
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Corrodible Plant
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Corrodible Plant
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Corrodible Plant
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Group I Appearance Can often be identifiedupon visual examination
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Group I Corrosion Examples(1) General Corrosion
even, regular loss of metal planar surface
Steam, atmospheric rusting, dissolution of zinc by dilute acid
Boiler Water Aerated Vent Steam
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Group I Corrosion Examples(2) Localized Corrosion attack occurs at discrete areas or sites, large and
shallow to small and narrow pitting and crevice Cooling waters, Contaminated surfaces, unwashed areas
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Corrosion Kinetics Corrosion Products and Scales precipitated from
corrosive solutions may be poorly formed (non-protective) with LINEAR REACTION RATE or denseand adherent (protective layers) with PARABOLICREACTION RATE, ie slowing with time)
time
Extent of
corrosion
non-protective layer
time
Extent of
corrosion
protective layer
Linear Kinetics Parabolic or Logarithmic
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Carbon Steel in Geothermal Steam
Illustration of localised corrosion on shutdown/startup
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Material Loss Rate Equations
Material Loss Constant * timen
Linear Kinetics: ML = Constant * t and n = 1
ln ML = ln Constant + n ln t and Slope = 1
Parabolic Kinetics: ML = Constant * t0.5 and n = 0.5
ln ML = ln Constant + 0.5 ln t and Slope = 0.5
Logarithmic Kinetics: ML = Constant * Log (Constant*t + Constant)ln ML = ln A + ln (ln(Bt+C)) and no single Slope
If Constants A=B=C the Slope tends to 0.06
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Linear Plotting
ML t
Linear Kinetics
ML t0.5
Parabolic Kinetics
ML ln (Bt + C)Parabolic Kinetics
ML t10.5 + t2
0.5
Breakaway
Kinetics
ML
time
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Logarithmic Plotting
n = 1
n = 0.5
n = 0.06
time = 1, 3 and 12 Mo for Doubling of ML
Change of
Slope =
Change of
Mechanism
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Group I Corrosion Examples(3) Galvanic Corrosion bimetallic corrosion, from electrical contact between
dissimilar metals Preferential attack on more anodic metal
Al Conductor
Steel Reinforced
Galvanising Corrodes
To Protect Al
Al Corrodes
To Protect Steel
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Group II Appearance May require supplementary means of examination
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Group II Examples(4) Velocity effects
Erosion-corrosion
- caused by relative
movement betweencorrosive fluid and metalsurface
- accelerated by highvelocity flow
- mechanical effects
- corrosion-related effectssuch as pH and oxygencontent
Corrosion Rate at 55C
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
3 4 5 6 7 8 9 10 11 12
pH
CorrosionRate[mm/yea
r] Dissolved O2=1
Dissolved O2=2
Dissolved O2=3
Dissolved O2=4
Dissolved O2=5
Dissolved O2=6
Dissolved O2=7
Dissolved O2=8
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Group II Examples(4) Velocity effects Cavitation
- caused by bubbles formed where the local pressure is belowthe vapour pressure
Fretting- damage (often electromechanical) associated with motion
between mating surfaces under load and vibration
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Group II Examples(5) Intergranular attack At grain boundaries
Grains fall out (sugaringor grain dropping)
Improperly heat-treatedHAZ of welds
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Group II Examples
(6) De-alloying corrosion
Selective removal of onemetallic constituent of analloy
De-zincification of yellowbrass
Graphitic corrosion of greycast iron
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Group III Appearance May see surface breaking features or fracture but must be
verified by microscopy, SEM
Stress Corrosion Cracking
High-temperature attack
Internal attackCorrosion fatigue
Dynamicstress Fissures
Scale
Cracking phenomena
Scaling
Staticstress
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Group III Examples(7) Corrosion related cracking phenomena
Corrosion-related environment cracking
- Stress corrosion cracking (SCC)
- Hydrogen-assisted cracking (HAC)
- Liquid metal cracking (LMC)
Mechanical-electrochemical fatigue- corrosion fatigue
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Transgranular/Intergranular SCC
Simple slip system
Formation of coarseslip steps
Produce discontinuities
Initiating SCC
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Transgranular/Intergranular SCC
Corrosive environment
Pit initiation
Crack initiation
Propagation SCC
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SCC of Corrosion Resistant Alloys
SCC Cracks May Propagate byCorrosion Fatigue
SCC CONDITIONS+ Aeration (oxygen)+ Corrosive Species+ Evapourative Concentration+ Moisture or wetness
+ Tensile Stress (residual)+ T > 60C+ Material Susceptibility
Alloy 2RK6563 weeks at 100C
Drip solution of : steam condensate with H2S 30 mg/kg chloride added
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SCC and Corrosion Fatigue
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Corrosion Fatigue Alternating loading + corrosion process
formation of extrusion & intrusions
protective surface layer destroyed
local electrochemical attack
the lower the frequency of loading cycles, the moreimpact of corrosion because of time-dependency ofcorrosion processes
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Hydrogen Embrittlement / SCCSour environment
Corrosion and hydrogen
charging
Tensile stress
Propagation HE
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Low Strength Steels Resist Sulfide SCCHydrogen readily diffuses into steels and high strength alloys sufferSulfide Stress Corrosion Cracking or Hydrogen Induced Cracking
NACE MR0175 (1975 to 2001) Standard Material Requirement forSulfide Stress Corrosion Cracking Resistant Metallic Materials for Oilfield
Equipment Sour Water and Sour Gas Systems Definitions
Low H2S Systems May be Classified as Sweet
Hardness and Cold Work Limits for Accepted Alloys Might AppearConservative BUT Based on Experience
Use as low a Strength as can be tolerated by the Design Heat Treatment Processes Specified
Materials for Specific Facilities Identified
Example Vessel
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182231242
194 193 198 197
201212210 180
201 198 218 212
172 196
197
230 227 212
218
181
169 215189
193192195
188192
209215216
202
Example Vessel
Meets hardness criteria
of NACE Standard
Thickness at limit for
heat treatment (ASME)
Welded with limited
number of passes high
heat input
High Residual StressHIC or SCC?
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Constraints On the StandardNACE MR0175/ISO 15156-1:2001(E), -2:2003(E) and -3:2003(E)
Guideline only
Basis for Agreement to Supply
Limited to Sulfide SCC and HIC Does not include synergistic effect of Chloride (NACE Test)
Hardness of sub-surface areas can not be measured
Must do test pieces and cut up beforehand
Hardness conversions often debated
Hardness relationship to strength is material dependent
Metallurgical variations, new alloys or fabrication processes maynot be classified
Testing can be expensive
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(8) High Temperature Corrosion Kinetics
High Temperature Oxide films may be porous (non-protective layers) with LINEAR REACTION RATE orcontinuous and adherent (protective layers) withPARABOLIC REACTION RATE i.e. slowing with time.
time
Extent of
corrosion
non-protective layer
time
Extent of
corrosion
protective layer
Linear Kinetics Parabolic or Logarithmic
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Chlorination Corrosion in flue gas derived fromlandfill gas engines.
HCl Corrosion
Fe2O3 + HCl FeOCl
Cl-
-FeO(OH) Fe2O3 + H2O
Cl-
Fe3O4 + HCl FeCl2.xH2O
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Oxidation
Oxidation of turbine alloy throughto catastrophic stages
High Temperature Oxidation of alloy 800H
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Stability Diagram for Fe-SO2-O2 at 230oC
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Metal Dusting
High temperatures (400 to700oC)
Localised corrosion oftencatastrophic failures
In processes with highcarbon activity in the gasphase
Components disintegrate tofine dust of alloycomponents and carbon
Inhibited by presence ofsulphur compounds
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Metal Dusting Industry Problem
Reformer plants Syngas plants
Methanol producers
HBI plants
Ammonia synthesis
MTBE Although sulphur helps
inhibit the problem it alsopoisons the catalyst
New technologies arebeing researched toovercome the problem
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Corrosion Damage Mechanisms:
AS/NZS 3788 Appendix MGroup I: Readily Observed by Visual Examination
(1) General (uniform) corrosion
(2) Localised corrosion
(3) Galvanic corrosionGroup II: May Require Supplementary Examination
(4) Velocity effects
(5) Intergranular attack
(6) De-alloying attack
Group III: May Require Microscopic Examination
(7) Cracking phenomena
(8) High temperature corrosion