3. damage mechanisms forms of corrosion v1

7/30/2019 3. Damage Mechanisms Forms of Corrosion V1

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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


(3) Galvanic corrosion

Group II(4) Velocity effects


(6) De-alloying attack

Group III(7) Cracking phenomena



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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


(3) Galvanic corrosionGroup II: May Require Supplementary Examination

(4) Velocity effects


(6) De-alloying attack

Group III: May Require Microscopic Examination

(7) Cracking phenomena


3. damage mechanisms forms of corrosion v1

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