Environmentally-assisted Cracking Corrosion Fatigue and ...Specific Forms of Corrosion 1. General or...

Environmentally-assisted Cracking

Corrosion and Corrosion Control in LWRs© 2011 by SIA, Inc. All rights reserved.

Corrosion Fatigue andIntroduction to SCC

Note: While this course does not contain proprietary BWRVIP or MRP information, per se, it does contain open literature information that was used in the creation of BWRVIP or MRP documents or

BWRVIP or MRP information that was subsequently made non-proprietary via publication, etc.

Corrosion Fatigue and SCCLearning Objectives

• Understand the mechanism of corrosion fatigue vs. mechanical fatigue

♦ BWR feedwater nozzle

♦ BWR steam dryer brackets

♦ PWR steam generator feedwater nozzle cracking

Corrosion and Corrosion Control in LWRs© 2011 by SIA, Inc. All rights reserved. PRS-11-037 F BMG/ 2

♦ PWR steam generator feedwater nozzle cracking

• Understand the fundamental SCC mechanism♦ Susceptible material, corrosive environment

and tensile stress

♦ SCC testing

♦ SCC initiation vs. SCC propagation

Specific Forms of Corrosion

1. General or uniform corrosion

2. Galvanic corrosion

3. De-alloying corrosion

4. Velocity phenomena - erosion corrosion,

cavitation, impingement, fretting and FAC

MacroLocalized Corrosion


cavitation, impingement, fretting and FAC

5. Crevice corrosion

6. Pitting corrosion

7. Intergranular corrosion

8. Corrosion fatigue

9. Stress corrosion cracking

Micro Localized Corrosion

Microbiological activity can affect all of the above

MechanicalEffects

Environmentally-assisted Cracking

• EAC♦ Corrosion fatigue♦ Stress corrosion cracking (SCC)

° Intergranular stress corrosion cracking (IGSCC)– BWR IGSCC– Primary water stress corrosion cracking (PWSCC)

Irradiation assisted stress corrosion cracking (IASCC)


– Irradiation assisted stress corrosion cracking (IASCC) – Interdendritic stress corrosion cracking (IDSCC)– Low potential stress corrosion cracking (LPSCC)

° Transgranular stress corrosion cracking (TGSCC)• Criterion for EAC propagation:

♦ There is a mechanism to protect the crack sides♦ If this is not met, then the incipient sharp crack will

devolve into a blunt notch and will arrest♦ Criterion is readily met in passive alloys

PRS-11-037 F BMG/ 4

Fatigue


Mechanical Fatigueand Corrosion Fatigue

• Fatigue #1 cause of metallic failures

• Fatigue is the tendency of a metal to fracture under repeated cyclic loading♦ SCC is due to static loading

Corrosion fatigue is mechanical fatigue


• Corrosion fatigue is mechanical fatigue aggravated by corrosion reactions

• All environments will reduce the fatigue life of a component and can eliminate any fatigue/endurance limit

• CF is a non-environment specific cracking phenomenon (unlike SCC!)

Example of Mechanical Fatigue


Open University

Where better to show fatigue due to cyclic stresses than a bicycle!

Mechanical Fatigueand Corrosion Fatigue

• Cracks formed by CF are usually wider and less uniform in appearance than mechanical fatigue because of the metal removed by corrosion

• Initiation may occur at multiple sites

• Microscopically, the cracks are typically transgranular following the load usually oxide filled


transgranular following the load, usually oxide-filled and blunt-tipped, with irregular crack profiles and signs of discontinuous propagation

• Similar to purely fatigue-driven cracks, orientation is generally normal to the predominant stress field

• However, CF cracks are more likely to form branches that follow grain boundaries in the metal

Corrosion Fatigue in LWRs

• Fatigue is a major consideration where the components are subjected to a very large number of cycles (e.g., high–cycle fatigue) and the primary concern is the endurance limit, i.e., the stress that can be applied an infinite number of times without failure

• Cyclic loadings on a LWR component occurs because of


changes in mechanical and thermal loadings♦ Number of cycles applied during the design life of an LWR

seldom exceeds 105 and is low–cycle fatigue

• Main difference between high–cycle and low–cycle fatigue is that the former involves little or no plastic strain, whereas the latter involves strains in excess of the yield strain

♦ Design curves for low–cycle fatigue are based on tests in which strain rather than stress is the controlled variable

NUREG-6909, 2/07

Corrosion Fatigue


Fatigue Mechanism

Corrosion Fatigue


• Mechanism• LWR Case Study Examples

♦ BWR feedwater nozzle♦ BWR steam dryer support bracket♦ Hamaoka 5 ABWR condenser recirc cap♦ PWR RCP upper seal cavity pressure sensing

line socket weld

Some NRC Corrosion Fatigue Documents

• BL-88-02 - Rapidly Propagating Fatigue Cracks in Steam Generator Tubes

• BL-79-13 - Cracking in Feedwater System Piping

• BL-79-13 Rev. 2 - Cracking in Feedwater System Piping

• IN98045 - Cavitation Erosion of Letdown Line Orifices Resulting in Fatigue Cracking of Pipe Welds


• IN93020 - Thermal Fatigue Cracking of Feedwater Piping to Steam Generators

PRS-11-037 F BMG/ 12

Some Corrosion Fatigue History

• World War I – failure of steel wire towing ropes attached to paravane equipment (torpedo-shaped underwater devices with serrated teeth) designed to sever mine moorings

• Tried higher tensile strength wire – no improvement• Galvanized original tensile strength wire – worked• This was a corrosion problem, not a strength problem


Cable failures

Recommended Corrosion Fatigue Film

No Highwayin the Sky

1951 - Jimmy Stewart and Marlene Dietrich


• Based on Nevil Shute’s novel No Highway

• 1st aircraft disaster film!

• 1953 – Crash of world's first passenger jet (de Havilland Comet) due to corrosion fatigue

World Wide Worries OverCorrosion Fatigue!


D. H. Comet 106


D. H. Comet 106 Corrosion Fatigue

• de Havilland developed/flew the 1st commercial jet aircraft, D. H. Comet 106, in 1949

♦ Several years ahead of rival Boeing

♦ Commercial operations in early 1952

• Comet crashed shortly after takeoff on May 2, 1953


♦ 2 crashes in early 1954 forced British authorities to ground the entire fleet

• Tested fuselage submerged in a tank of water and repeatedly pressurized and depressurized to represent repeated flight cycles

♦ >>1000s cycles, CF cracks were found to be spreading from the square edges of the windows in the passenger cabin

PRS-11-037 F BMG/ 17

Relationships among SCC, CF and Fatigue

SCC CF Fatigue

Cyclic Stress


Environment

Corrosion Fatigue

• Reduction of fatigue resistance due to the presence of a corrosive medium

• Mechanical fatigue resistance values are nearly independent of stress-cycle frequency – allows accelerated testing

• Corrosion fatigue is greatly dependent by stress-cycle


g g y p y yfrequency – need time for corrosion reactions –disallows accelerated testing

♦ High cycle fatigue crack growth is not strongly influenced by environmental effects

♦ Low cycle fatigue crack growth is a strong function of environment

• Corrosion fatigue cracks are typically transgranular (through the grains) not along gbs. Fracture surfaces may have striations and oxide particles.

Corrosion Fatigue

Fatigue crack advance occurs by a reversed slip mechanism and the resulting beach marks and striations are often visible

Fatigue data (initiation and growth) is cycles-based, but environmental effects

Beach Marks


but environmental effects are time-dependent.Thus, problems often occur at low frequency or strain rate.

Cracks are often TG, butif cycling is “gentle,” can be IG

P. Andresen, NRC, 7/06

Corrosion Fatigue Striations with Corrosion Products


Corrosion Fatigue - Spatula


Corrosion Fatigue

Corrosion Fatigue - MGB Door


Nice paint!

Fatigue and Corrosion Fatigue S-N Curves

sFatigue or

Endurance Limit Mechanical Fatigue

What would the results of a fatigue test in a vacuum look like?

Initiation Data


104 105 106 107

Str

es

Number of Cycles to Failure

Corrosion Fatigue

Sample Mechanical TheoreticalFatigue S-N Diagram

70

80

90

100si

le S

tres

s


30

40

50

60

1.0.E+00 1.0.E+01 1.0.E+02 1.0.E+03 1.0.E+04 1.0.E+05 1.0.E+06 1.0.E+07 1.0.E+08

% U

ltim

ate

Tes

Cycles

Endurance limit

Fatigue and Corrosion Fatigue of Steel

800

1000

1200

mit

, MP

a

Theoretical 50%

Polished

Notched

Corrosion

UTS = ultimate tensile strength


0

200

400

600

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

En

du

ran

ce L

im

UTS, MPa

S-N Fatigue Curve for Type 304 SS


ASME

εa- N HWC CF of Type 304 SS


NUREG-6909, 2/07

Fatigue Monitoring

Pressurizer

SG #1

Pump Pump

Pump No. 1B


Pump No. 2B

Reactor Pressure Vessel

SG #2No. 1A No. 2A

FatiguePro®

• EPRI licensed software

• Tracks transients (cycles) and fatigue usage for Class 1 components

♦ Automated cycle counting (ACC) = counts and categorizes plant transients into design basis transients based on raw plant instrument data


♦ Computes fatigue usage

° Stress-based fatigue (SBF) = computes CF based on stress histories determined from raw plant instrument data

° Cycle-based fatigue (CBF) = computes CF based on design stress report algorithms and counted plant cycles

• Rapidly evaluates plant transients

PRS-11-037 F BMG/ 30

BWR Feedwater Nozzle Cracking


Nozzle Cracking

BWR Feedwater Nozzle Cracking

Late 1970s – early 1980s


NRC as Generic Activity A-10, “BWR Nozzle Cracking.” Interim guidance was provided in NUREG-0312, which specified augmented inspections for the nozzle assembly.

Cross Section of BWR Feedwater Nozzle


Cause of BWR Feedwater NozzleCorrosion Fatigue Cracking

• High frequency thermal cycling occurring along the nozzle bore and inner blend radius regions

• Source of the thermal cycling was the mixing of relatively cold feedwater (177-204°C [350-400°F]) with hot reactor water (288°C [550°F]) on the surface of the nozzle inner bore


• The mixing resulted from leakage of the feedwater into the annulus region through the gap existing between the loose-fitting end of the thermal sleeve and the nozzle safe end

• Cracks initiated in cladding by high cycle CF• Propagation into LAS by low cycle CF - pressure and

thermal cycles from start-up/shutdowns and feedwater on-off transients

Corrosion Fatigue of BWR RPV Feedwater Nozzle Blend Radius


Photomacrograph Photomicrograph Photomicrograph Unetched Etched

Corrosion Fatigue Striationsin a Feedwater Nozzle


Mitigation of BWR RPV Feedwater Nozzle Cracking

• Remove nozzle stainless steel cladding

• Installation of a modified sparger/thermal sleeve arrangement that eliminated the loose fit at the thermal sleeve/safe end interface


interface

• Changes to operating procedures and/or feedwater system modifications

BWR Triple Thermal SleeveFeedwater Nozzle Design

(Low Alloy Steel)

(Carbon Steel)


(Alloy 600)Alloy 600

Temperature variations w/woBypass Leakage


Steam Dryer Support Bracket

C


Cracking

Susquehanna 1 Steam Dryer Support Bracket

• BWR 4 – February 1985 – 1st refueling outage –184º SDSB

• Cracks located through the entire Alloy 600 bracket

• Cause of cracking was CF. Severe wear found


Cause o c ac g as C Se e e ea ou don other brackets.

• Bracket was replaced in 1985, using the same design as the original

• 1985 analysis and subsequent vibration testing failed to identify source of the fatigue loading that caused the CF and severe wear


Alloy 600



Alloy 182 Weld Pad

Low RPV

Type 308/309 cladding


Alloy 600

Low Alloy Steel RPV

CF Crack

Fillet Weld

Full penetration double bevel weld

RPV

Alloy 182

Type 308/309 cladding

Corrosion Fatigue of Susquehanna 1 Steam Dryer Support Bracket


Photomicrograph - Transgranular Cracking - Unetched

Corrosion Fatigue of Susquehanna 1Steam Dryer Support Bracket


Transgranular crackingNo gross plastic deformation

Corrosion Fatigue of Susquehanna 1Steam Dryer Support Bracket Metallography


SEM of fracture face near crack tipTransgranular with fatigue striationsSpacing 1 to 4 μm

300x 1000x

Cofrentes Steam Dryer Support Bracket

• BWR 6 – November 1991 - 146º SDSB

• Crack was located on the inboard top corner of the Type 304 SS bracket

• Most probable cause of cracking was CF from an unknown alternating load, no evidence of SCC


• Removed segment (to eliminate loose part concern) showed no evidence of gross plastic deformation

• Planar crack surface apparently resulting from propagation of a crack from a single initiation site

• Plant operators elected to continue operation without any further repair or modification

Cofrentes Steam Dryer Support Brackets


Kuo Sheng 1 Steam Dryer Support Bracket

• BWR 6 – October 2001 - 34 and 214º SDSB

• Cracks located on the inboard corner of the Type 304 SS bracket where there were signs of contact with the bottom of the steam dryer support ring


• Most probable cause of cracking was CF

• Cracked sections removed to prevent loose parts concern

• Based on good Cofrentes experience, plant operators elected to continue operation without any further repair or modification

Kuo Sheng 1 Steam Dryer Support Bracket


Nine Mile Point 1 Steam DryerSupport Bracket

• BWR 2 – March 2011 - 50º, 131º and 230º SDSB• 50° 1-587A Type 304 SS

♦ Left side bottom 25.8 mm (1.015”) raps around the bottom left corner of bracket for 52.8 mm (2.081”)

♦ Center bottom 22.2 mm (0.875”)

• 130° 1-587B Type 304 SS


yp♦ 8.9 mm (0.35”) raps around top right corner of bracket

extending down right side for 82 mm (3.23”)♦ 63.5 mm (2.5“) bottom indication in the base material

across the entire lug

• 230º 1-587C Type 304 SS♦ Right side/bracket top 83 mm (3.25”) and transverses

around the top right corner, continues 24 mm (0.93”)

• Most probable cause of cracking is EAC

Schematic of NMP-1 Steam Dryer Support Bracket Configuration

63.5 mm


127 mm50.8 mm

mm

203 mm

12.7 mm

63.5 mm

(Collar)

Nine Mile Point 1 Steam DryerSupport Bracket Design


(COLLAR)

NMP- 1 Steam Dryer Support Bracket50º (1-587A) – Left Side Bottom

63.5 mm

Type 304

Type 304


Alloy 182

Saddle

Alloy 182

52.8 mm

NMP- 1 Steam Dryer Support Bracket50º (1-587A) – Bracket Center Bottom

22.2 mm 1.8 mmType 304


Bottom of Alloy 182 Weld

SaddleSaddle

NMP- 1 Steam Dryer Support Bracket 231º (1-587C) – Right Side

Type 304


Bracket Side

83 mm

83 mm

Bracket Top

Type 304

Type 304

NMP- 1 Steam Dryer Support Bracket 231º (1-587C) – Bracket Top

Type 304


24 mm

127 mm

Type 304

Alloy 182

Alloy 182

Alloy 182

Type 304

Hamaoka 5 ABWR CF

• Damage to 43 Ti condenser tubes in a turbine steam condenser

• May 14, 2011 ~5 tons of seawater entered reactor following the discovery of ~400 tons of seawater in the condenser, which cools steam


seawater in the condenser, which cools steam from the turbine♦ All LPRMs fail a la Millstone 1 in 1972

• 530 tons/h of seawater were flowing through the recirculation pipe

• Corrosion fatigue of weld on recirc pipe cap?

PRS-11-037 F BMG/ 58

Mainichi Daily News, 6/28/11

Hamaoka 5 ABWR Damage

21,000 Ti tubes

3 cm Ø


20 cm3.5 kg

Susquehanna 2 New Steam Dryer

• 15th refuel and inspection outage

• First cycle of 100%inspection for NEW dryer♦ New dryer was installed to “do the right

thing,” i.e., old dryer was OK

• Major indications:


Major indications:♦ Dryer seismic ring lug A - through-wall

indication in skirt panel ~ 115 mm (4.5”)

♦ Dryer lug LL-B 140⁰ - Crack-like indication on dryer lower bracket to lifting lug weld 13 mm (~0.5”)

PRS-11-037 F BMG/ 60

E. Camacho, BWRVIP, 6/11


Indication running through the dryer skirt panel to mid ring 0⁰ weld (th h ll)


(through-wall)





Multiple IGSCC indications (4 areas) in dryer skirt weld 135⁰Most prominent cracks are 13 mm (0.5”)- shallow depth

4 indication areas

RCP Upper Seal Cavity Pressure

Sensing Line Socket


Sensing Line Socket Weld Failure

RCP Socket Weld Failure

• RCS pressure boundary leakage

• 19 mm (¾”) flange to pipe socket weld

• Schedule 160, Type 304 SS pressure


304 SS pressure sensing line

• First field flange connection off seal package

• NOT = 54ºC (130ºF)• NOP = 5.5 MPa (800

psig)

Close Up of Leak

Environment:

• 837 ppb Cl!

• 626 ppb SO4!

• 27 ppb F


• Nominal values all <50 ppb


• At OD surface, crack located entirely within weld metal

• Crack extended ~120o around


~120o around circumference

• No gross plastic deformation evident

120º


Circumferential crack



• Radial section opposite fracture

• Good quality weld

• Weld dimensions exceeded Code requirements

Pipe

Weld


requirements

• No evidence of inadequate pull-back

Socket base metal Close up next slide


• Multiple, minimally branched cracks in weld metal and pipe

• Transgranular crack path in Type 304 SS pipe base material

Weld


pipe base material• No evidence of

sensitization• No cracking observed

in Type 316 SS flange base metal

Pipe

Flange

Stress concentration



Striations evident on portions of leak path fracture surface


• Observations suggest:♦ Environmentally assisted cracking

♦ Causative loads are cyclic in nature

♦ Stagnant, oxygenated RCS likely a key factor

S i lif t f il h i i


♦ Service life suggest failure mechanism is age related

Introduction to Stress Corrosion


Cracking

The Poetry of SCC

The image of stress corrosion I seeIs that of a huge unwanted tree,Against whose trunk we chop and chop,But which outgrows the chips that drop;

At intervals researches gather,And on mechanisms all palaver;Each to his own work will refer,Ignoring those who don’t concur.

On Stress Corrosion (Abridged Version)

S. P. Rideout, Savannah River Laboratory


And from each gash made in its barkA new branch grows to make more darkThe shade of ignorance around its base,Where scientists toil with puzzled face.

Chemists and metallographers,Technicians and philosophers,Through struggling individuallyTheir common goal: to fell the tree

But as we speculate and ponder,Those who run the mills out yonderTo us with anxious voices wail,“Please help us lengthen ‘time to fail!’”

Stress Corrosion Cracking

• Ultimate in localized corrosion!• Cracks that create the impression of inherent

brittleness, but it is corrosion!• Bulk alloy retains its typical ductility values• All alloys are susceptible to SCC in at least one

environment Pure metals are very resistant


environment. Pure metals are very resistant.• SCC does not occur in all environments• An environment that induces SCC in one alloy

does not necessarily induce SCC in another alloy• Environmentally specific cracking (unlike most

corrosion fatigue)• Complex since SCC involves many technical

disciplines, variables and interdependencies

BWR Example of SCC Mitigationwith Pure Metals

• Pellet cladding interaction (PCI)

• SCC due to corrosive fission products and cladding tensile stress from pellet expansion

• Solution: pure Zr barrier interior cladding by co-extrusion with Zircaloy-2


Zircaloy-2

Fuel Pellet

Fuel Pellet

Zr

SCC in LWRs

• SCC is the most virulent of degradation processes in LWRs and is likely to continue

• SCC is not easily detected

• SCC affects the integrity of the most critical components


components

• SCC is a precursor to subsequently dangerous although low probability phenomena♦ Break before leak (BBL), not LBB

♦ Loss of coolant accident (LOCA)

PRS-11-037 F BMG/ 76

R. Staehle, QMN, 6/10

SCC Mechanism



• There are three necessary fundamental parameters that must be simultaneouslypresent for SCC:

♦ Tensile stress (total of all stresses, i.e., applied, residual etc )


residual, etc.)♦ “Corrosive” environment♦ Susceptible material

• If anyone of these three necessary fundamental parameters is absent or reduced below some “threshold” value, SCC cannot occur

Tensile Test

• Most fundamental type of mechanical properties test

• Tensile tests determine how the material will react to forces being applied in tension, i.e., its strength and elongation (ductility) via a stress strain plot

♦ Initially the relationship between the applied load and the elongation is linear


and the elongation is linear° “Hooke's Law" where the ratio of stress (σ) to strain (ε) is

a constant E (Young’s Modulus), σ/ε = E

♦ Yield strength (YS) = stress applied to the material at which plastic deformation starts to occur

♦ Since departure from the linear elastic region is not easily identified, the ASTM E8 0.2% offset method is used to determine the offset YS (proof stress)

PRS-11-037 F BMG/ 79

Engineering Stress Strain CurveS

tres

s (σ

)

0.2% Offset Line Ultimate Tensile

Strength Fracture StrengthStrain

Hardening Necking


En

gin

eeri

ng

S

Engineering Strain (ε)

Offset Yield Strength

Uniform Elastic Deformation

Uniform Plastic Deformation

Non-uniform Plastic

Deformation (Necking)

Fracture

Tensile Specimens

Proportionality limit

0.2%

Engineering Stress Strain CurveClose Up

Proportionality limit

Offset Yield Strength


Young’s Modulus E = σ/ε

Engineering vs. “True”Stress Strain Curves

s (σ

) Fracture Strength

Engineeringσe = P/A0

εe = (L-L0)/L0

“True”

“True” Stress Strain


Str

ess

Strain (ε)

σt = P/Aεt =

= ln(L/L0)

Engineering Stress Strain /

Lf

LodL L∫

Engineering stress and engineering strain are calculated based on the original dimensions of the specimen and not the instantaneous values as is the case for “true” stress and strain

Tensile Testing Equipment


Resistance heated

Example of Tensile Test Results

As-received copper specimen


Post-tensile test copper specimen

Necking

Cup Cone Fracture Surface - Aluminum


Cup Cone

Ductile vs. SCC Failures

Ductile Failures

• High % reduction in area (necking)

• High % elongation

• Dimpled fracture surface

A0

A


SCC

• ~0% reduction in area

• ~0% elongation

• IG/TG fracture surface

A0

Ductile vs. SCC Failures


NUREG-6892, 1/06

Wow!

SCC Fundamental Parameters

Tensile Stress

Susceptible Material

SCC


“Corrosive” Environment

Venn Diagram

Some SCC History – “Caustic Embrittlement”

• 19th and 20th

century riveted carbon steel steam boilers explode

• Cracks around rivet holes

ca 1850


rivet holes

• Susceptible areas cold worked by riveting

• NaOH from chemical treatment of boiler water – Caustic SCC

• Due to local deposition of concentrated OH- at 200 to 250oC

Some SCC History – “Remember the Maine!”

1898 Remember the SCC!


1st war started by SCC!

SCC Parameters and Mitigationfor “Caustic Embrittlement”

High residualtensile stress from riveting

Carbon steel

SCC

Change the alloy

Anneal out residual

stress/cold work after

riveting


High temperature water with NaOH

Replace NaOH in coolant or add Na2HPO4

Some SCC History – “Season Cracking”

• British colonial times in India (1920s)

• Small arms brass cartridges developed small cracks

• Shells would misfire, explode


• Cracking occurred only during the monsoon season – high RH, high ambient temperatures

• Ammunition stored in horse stables

SCC

SCC Parameters and Mitigationfor Brass Cartridges in India

High residualtensile

stress from drawing

Cartridge Brass

SCC

Change the alloy (add 1% Si)

Anneal out residual

stress/cold work after drawing


Warm, high humidity and horse urine!

(NH3)

Remove the ammo from stables

A Few Examples of Alloy-Environmental Systems Exhibiting SCC

Alloy EnvironmentCarbon Steel Hot nitrite, hydroxide and carbonate

/bicarbonate solutions

High strength steels Water with H2S

Austenitic stainless steels

Chloride solutions; chloride -contaminated steam oxygenated high


steels contaminated steam, oxygenated high temperature water

High nickel alloys High purity steam, LWR environments

α Brass Ammonia solutions

Aluminum alloys Aqueous Cl-, Br - and I- solutions

Titanium alloys Aqueous Cl-, Br - and I- solutions; organic liquids; N2O4 (dinitrogentetroxide)

This list never gets smaller!

Anodic Polarization Curvewith SCC Zones

Noble

Passive

Pitting

SCC Potential Zones

Transpassive


ActiveCurrent Density log | i |, A/cm2

Passive

Active

Zones

Po

ten

tial


• SCC propagates in two modes:♦ Intergranular – IGSCC – between the grains (e.g., BWR

IGSCC, IASCC, PWSCC)

♦ Transgranular – TGSCC – across the grains (e.g, Cl SCC)

♦ Can be mixed mode, i.e., IGSCC + TGSCC

C ki i t i ll di l t th t il t


• Cracking is typically perpendicular to the tensile stress

• Increasing the tensile stress decreases the time to crack initiation and increases the crack propagation rate

• Tensile stresses include applied, residual, thermal and even corrosion product

• Higher the yield stress, the higher the crack growth rate

• Total t failure = t initiation + t propagation

Microstructures of IGSCC,TGSCC and IDSCC

TGSCC IGSCC IDSCC


Effect of Tensile Stress on SCCInitiation and Propagation


Alloy 600 Alloy 82

Effect of Yield Stress/Cold Work on Crack Growth Rate of Stainless Steel


T. Kamada, Kansai, 7/07

1 N/mm2 = 1 MPa =0.145 ksi

Perspective on SCC Initiation

• Initiation data is essential to large systems like SG tubing SCC

• Large, complex components, long operation, welding defects, surface inclusions and damage, etc. “conspire” to make the assumption that initiation is the limiting factor


assumption that initiation is the limiting factor uncertain and non-conservative

• Few initiation tests are well-designed due to a lack of:

♦ Sufficient number of specimens♦ Continuous monitoring♦ Surface area and representative conditions

PRS-11-037 F BMG/ 100

P. Andresen, Beaune SCC Initiation Conf., 9/08

Perspective on SCC Initiation (continued)

• Initiation is typically single-point data:♦ 1 data point per initiation test vs. ~106 data

points per crack growth test

• On-the-fly environmental/stress changes are not possible


p

• Often use arbitrary/severe conditions with reverse bending, high stresses/strains, graphite wool crevices, severe surface treatments

PRS-11-037 F BMG/ 101


Definition of Crack Initiation

• What are the crack dimensions or characteristics associated with “crack initiation”?

♦ Complete failure? (Includes crack growth!)

♦ Evidence of SCC? (All crack sizes often treated equally)

♦ Post-test destructive exam by metallography or SEM?

♦ Video photo or replicate detectability of cracking?


♦ Video, photo or replicate detectability of cracking?

♦ In-situ monitoring detectability crack depth vs. time?

♦ Electrochemical noise monitoring for incipient cracking?

♦ Eddy current, UT, acoustic emission indications?

♦ Development of “long” crack mechanics or chemistry?

♦ Development of crack of arbitrary dimension (e.g., grain size)?

PRS-11-037 F BMG/ 102


Origins of Crack Initiation

• Phenomena responsible for crack initiation can be many:

♦ Purely mechanical cracking from straining of hardened layers

♦ Intergranular corrosion


♦ Selective oxidation of grain boundaries during heat treatment

♦ Pitting corrosion

PRS-11-037 F BMG/ 103


Mature chemistry forms in crack of depth ~50 μm (2 mils)Early growth controlled by crack coalescence

Crack Growth and Crack Initiation

• Continuum in crack initiation and growth:♦ Most factors that accelerate CGR, accelerate initiation

(e.g., CW, temperature, potential, sensitization, stress)

♦ When growth rates decreases to ~10-8 mm/s (12 mpy), initiation is rare

♦ Exceptions may be particularly noteworthy (e g initiation


♦ Exceptions may be particularly noteworthy (e.g., initiation is much easier in sensitized SS than CW SS in BWRs)

• Advantages of CGR measurements♦ ~5000 data/hr, 3000-9000 hrs/test vs. 1 initiation data point

♦ On-the-fly evaluation of impurities, potential, temp, K, pH

♦ Precision measurement vs. absence of any on-line detection of “initiation” in most experiments

PRS-11-037 F BMG/ 104


SCC Initiation vs. Growth

• Calculations show that for a typical 20 mm thick component:

♦ 50% of life is used in growing to 0.01% depth (0.02 mm)

30% lif i d t t 1% d th (0 2 )


♦ 30% more life is used to grow to 1% depth (0.2 mm)

♦ Depending on detectability, 10 - 20% of life is actual “growth”

P. Andresen, RIP, Corrosion 2001

Overall Initiation andPropagation Process


R. Staehle, QMN, 6/10

Examples of SCC


SCC of an Aluminum Aircraft Component


Corrosion Doctors

IGSCC of Type 416 Stainless Steel


Palisades service water pump shaft coupling IGSCC due to poor heat treatment, i.e., hardness exceeded limit

TGSCC in Annealed BWR CRD Line

19 mm (¾ in.) annealed Type 304 SS CRD withdraw lines with surface Cl- contamination operating at 138-288°C (280-550°F)


Caused by PVC-containing Dymo™ tape crevice (1998)

St. Lucie Air Receiver Tank Cracking

Th h d f thi


The head of this tank was formed from a flat plate! No annealing after cold deformation!Marine atmosphere SCC.

Testing for SCC Initiation and


Growth

Constant Displacement Specimens

• Bent beams (BBs), creviced bent beam (CBB), U-bends, reverse U-bends (RUBs), etc.

• Stresses and strains are typically unrealistically high

• Crack initiation process is sensitive to:


Crack initiation process is sensitive to:♦ Surface oxides

♦ Surface finish

• Suffer load loss upon heating due to modulus changes ♦ Alloy 600 stress relation of >75% from thermal

creep can occur

EPRI, 1011202, 10/04

Constant Displacement SCCInitiation Test Specimens

C Ring


C Ring

Bent Beam

U-bend

Double U-bend

U-bend

Creviced Bent Beam Test

Graphite Wool

Loading Bolts

Graphite wool may have >ppm levels of impuritiesvs. tests with SS-foil crevices with rare failures


Test Specimen Retaining clips

and screws

H. Offer, et al., 13th Env. Deg., 2007

Considered an “extreme” initiation test

Westinghouse Bent Beams


Areva Four Point Bent Beams

Three springs to maintain the stresses during test


P. Scott, Cold Work Workshop, 6/07

41

2 3

Reverse U-bend Specimen


Rack of RUB Specimens

Pre-test


Pre-test

Post-test

O-ring Specimen

T C C TT

CT = tensionC = compression


p

1st crack initiation

2nd crack initiation

Actively Loaded Specimen Tests

• Pressurized tube

• Constant load

• Constant extension rate test (CERT)/slow strain rate test (SSRT)

C t t i (CT) d l d


• Compact tension (CT) or wedge open load (WOL) fracture mechanics specimens

• Double cantilever beam (DCB) fracture mechanics specimens

In-reactor Biaxial TestingPressurized Tube Specimen

Pinch-off pressure seal


Test Material

Possible Crevice

Laboratory Constant Load Testing

System uses internal pressure to actively load specimens - on failure, indicator ball drops and is detected

Regenerative

Gas In

Gas Out/Bleed

Rupture DiscAssembly

PressureRecorder


Manifolds

Type 347Test Vessel

Thermocouple

SectionAA

RegenerativeHeat ExchangerA A

Constant LoadTest Modules

Solution make-upBall Processing

Devices

Insulated Jacket

StripHeaters

CERT/SSRT

Identical Tests

CERT = constant extension rate test

TensileSpecimen


SSRT = slow strain rate test

Alloy 600 and 718 Tensile Specimens

N. Totsuka, et al., Scripta Met., Vol. 20, No. 7, 1986


J. Deleume, et al., Env. Deg., 2007

All dimensions in mm

Welded Pipe Tensile Specimen


Effect of Strain Rate on SCC

Alloy 600 360ºC500 ppm B + 2 ppm Li

30.8 cc/kg (2.75 ppm) H2

1 x 10-7 /s


5 x 10-7 /s

The higher the strain rate, the less SCC!!

N. Totsuka, paper 04679, Corrosion 2004

Relationship between Strain Rate and PWSCC


N. Totsuka, et al., Corrosion, Vol. 61, No.3, 2005

SCC Crack Growth Test Specimens

12.78 mm12.70 mm

Dia. (2 holes)

30o

<0.076 mm R <0.003 inch R

60.

96 m

m

2.40

inch

13.

97

mm

0.55

inch

3.175 mm0.125 inch

0.503 inch0.500 inch

Fatigue precrack, then intergranular

SCC precrack

Compact Tension (CT)

or

Wedge–Open-Load (WOL) Specimen


9.65 mm

0.38 inch50.8 mm

2.00 inch

63.5 mm

2.50 inch

30.

48

mm

1.20

inch

double cantileverbeam specimen

Old WOL Compliance Technique

Specimen transducer mounted on the WOL that spans the crack


Compliance opening displacement

CERT/SSRT vs. DCPD

CERT DCPD

X

X = deepest IG crack


Machined slot

Fatigue pre-crack

IGSCC pre-crack Test

IGSCC

X

X/t = CGR!Remaining ligament

Good and Bad CT Crack Fronts


Bad Bad

Good

Crack Growth Measurement Techniques

• Crack growth rate is commonly obtained by two methods:

♦ Reversing DC potential drop (DCPD) technique using compact tension (CT), wedge-open-load (WOL) or double cantilever beam (DCB) fracture mechanics specimens


♦ Constant extension rate test (CERT) (a.k.a. slow strain rate test [SSRT]) using simple tensile specimens

• DCPD technique is the only legitimate CGR technique!

• CERT CRG tests are quick, cheap and very inaccurate!

• CERT is OK for crack initiation, but not for CGR

• Beware of CERT/SSRT generated CGRs!

Common CT Specimen Orientations

Loading Direction


L = longitudinalS = shortT = transverse

Crack Growth

Direction

Examples of CT Specimen Orientations

Areva Heat 93510


Various CT Orientations Mid-Wall CT Specimen C-L Orientation

Example of Composite CT Specimens


Specimen designed to evaluate SCC CGR that initiated inAlloy 182 and propagated into A533 low alloy steel

K. Kumagai, et al., 14th Env. Deg, 2009

EB = electron

beam

Reversing DC Potential Drop Technique


In-reactor Reversing DCPD CGR


W. Wiesenack, PSI NES, 1/07

Example of Reversing DCPD CGR

SCC#2a - c280 - 690, 41%RA, WN415 CRDM

10.99

10.995

11

11.005

mm

0

0.2

0.4

ote

nti

al,

Vsh

e

Outlet conductivity x0.01

1108h

0.0

01 H

z@

581h

3 x 10-9

mm/s

.7,

0.0

01 H

zold

@ 1

854h

9

0.0

01 H

zd @

3320h

~0 mm/s

Alloy 690 P. Andresen, et al. 13th Env. Deg., 2007


10.965

10.97

10.975

10.98

10.985

500 1000 1500 2000 2500 3000 3500 4000

Test Time, hours

Cra

ck le

ng

th, m

-1

-0.8

-0.6

-0.4

-0.2

Co

nd

uc

tiv

ity

, μS

/cm

or

Po

CT potentialPt potential

Con

stan

t K

@

c280 - 0.5TCT of 690 + 41%RA, 340C25 ksi√in, 550 B / 1.1 Li, 18 cc/kg H2

To R

=0.7

, 0

+ 9

000s

hold

~5 x 10-10 mm/s

At 340C, pH = 7.60. At 300C, pH = 6.93 and potential would be ~155 mV higher

6.2 x 10-9

mm/s

To R

=0

+ 9

000s

ho

4.3 x 10-9

mm/s

To R

=0.7

, + 8

5,4

00s

hold

1 x 10-9 mm/s = 1.24 mpy

Stress vs. Stress Intensity Tutorial

• Stress, σ, = load (P)/cross sectional area (A) = P/A• Designers usually use load and stress as design

parameters for comparison with yield strength, σy

• Load is also used in fracture mechanics evaluations, but the parameter is stress intensity factor, K1, rather than σ

• K1 (Mode 1) relates the magnitude of stress at the tip of a


1 ( ode ) e ates t e ag tude o st ess at t e t p o acrack or flaw to the global stress state of a component

K1 = Gσ √(πa)where G = geometry factor

a = crack length/depth

• All legitimate LWR CGR data is related to K1

• Units of K1 are ksi√in or MPa√m (1 ksi√in ≈ 1.1 MPa√m)• Reference: “Fracture Mechanics in the Nuclear Power

Industry,” EPRI NP-5792-SR, Rev 1. May 1991

Stress Intensity Factor, K1

• K is used in fracture mechanics to more accurately predict the stress state (“stress intensity”) near the tip of a crack caused by a remote load or residual stresses

• When this stress state becomes critical a small crack grows (“extends”) and the material can fail

• The load at which this failure occurs is referred to as the


• The load at which this failure occurs is referred to as the fracture strength

• Experimental fracture strength of solid materials is 10 to 1000 x lower than the theoretical strength values since tiny internal or external surface cracks create higher stresses near these cracks

Stress Intensity Factor, K1 (continued)

• Exposure time needed to cause SCC failure depends on the stress intensity at any pre-existing or developing crack tip

• Stress concentration at the crack tip or flaw can be quantified in terms of K1

• It determines the crack growth rate of SCC for a specific


• It determines the crack growth rate of SCC for a specific alloy/environment combination

• Catastrophic failure of a component will occur when K1

reaches a critical value, i.e., the fracture toughness of the material, K1C

• Enables the determination of allowable defect size in design to avoid failure under given loading conditions

Three Basic Modes of Crack Surface Displacements

Y

Y

X

X

Z

Mode 1


Y

X

X

Z

Z

Mode 2

Mode 3

Stress Intensity Factor, K1 (continued)

• Below a “threshold” value of K1, i.e., K1SCC, SCC crack growth is not expected

• Stage 1 - Above this value the initial SCC growth rate increases with increasing K1

• Stage 2 - crack growth rate is independent of K1 and depends instead on the environment and temperature


p p♦ During Stage 2 growth, K1 continues to increase and leads

to the rapid acceleration of the crack, i.e., Stage 3

• Final fast fracture occurs when K1 reaches K1C, which is the fracture toughness of the material

• The higher the value of K1SCC under given conditions, the greater the SCC resistance

• However, many alloys do not appear to have a “threshold” K1SCC

Growth Rate of SCC Cracks

Stage 1

h R

ate

Stage 2 Stage 3


Stress Intensity, K1

Cra

ck G

row

t

SCC growth rate depends on environment Fracture

Toughness

K1SCC K1C

Effect of K on Crack Velocity in Zn and Zn-free PWR Environments

Corrosion and Corrosion Control in LWRs© 2011 by SIA, Inc. All rights reserved. PRS-11-037 H BMG/ 146

H. Kawamura, paper 141, Corrosion 98

IGSCC in LWRs


IGSCC in BWRs

PWSCC in PWRs

IASCC in LWRs

follow in the next three class segments

BWRs and PWRs Have a Lot in Common

• Increasingly sophisticated measurement techniques and patient observation have eroded the historical concepts of immunity and thresholds to SCC in high temperature water

• Tendency to compartmentalize LWR SCC into unique modes with individualized mechanisms and d d i H


dependencies. However: ♦ CGR data reveal that all SSs and Ni alloys are SCC

susceptible in LWR water whether the water has:° High or low DO or DH° High purity or buffered or contaminated° Higher or lower temperatures

♦ Crack tips are deaerated and at low corrosion potential in both BWRs and PWRs, thus, crack advances under similar conditions

PRS-11-037 F BMG/ 149

P. Andresen and G. Was, ICC, 10/08

BWR and PWRWater Chemistry Differences

• Coordinated changes vs. time in the B and Li level in PWR primary water that increases the pH at temperature in pure water from 5.62 (BWR) to ~7.2 (PWR)

• H2 concentration from ~40 to 3000 ppb H2


• Temperature (274°C BWR vs. 288/323/343°C for the PWR core inlet, core outlet and pressurizer)

• Of these differentiating factors, temperature is the most important in stainless steels, whereas for Ni alloys, both temperature and DH are important

PRS-11-037 F BMG/ 150

P. Andresen and G. Was, ICC, 10/08

IGSCC Dominates FailureProcess in LWR Components


• Stainless steels and Ni-base alloys have shown increasing numbers of failures in both BWRs and PWRs due to IGSCC

• Most pressing concerns are currently PWR vessel penetrations (nozzles and weldments) and BWR/PWR core internals

S. Bruemmer, 2005

CF and SCC Introduction Summary

• Corrosion fatigue is environmentally-assisted cracking (EAC) due to cyclic stresses♦ Not environment specific

♦ Mitigated by design, materials and environmental changes


g

• SCC is EAC due to nearly constant stress♦ Environment specific

♦ Mitigated by addressing the 3 fundamental parameters for SCC

♦ Dominant corrosion concern in LWRs!

PRS-11-037 F BMG/ 152

Environmentally-assisted Cracking Corrosion Fatigue and ...Specific Forms of Corrosion 1. General or...

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