l i h h i f S C i C ki ( )Exploring the Mechanisms of Stress Corrosion Cracking (Part II)Exploring the Mechanisms of Stress Corrosion Cracking (Part II)Exploring the Mechanisms of Stress Corrosion Cracking (Part II)Exploring the Mechanisms of Stress Corrosion Cracking (Part II)p g g ( )Jie Gao and David J Quesnel University of RochesterJie Gao and David J Quesnel University of RochesterJie Gao and David J. Quesnel, University of RochesterQ , y

Effects due to Environments Effect due to Mechanical Loading DiscussionEffects due to Environments Effect due to Mechanical Loading DiscussionEffects due to Environments Effect due to Mechanical Loading Discussiongff f Cl C i ff f S i D i h i A di di l iEffect of NaCl Concentration Effect of Stress Intensity Dominant mechanism: Anodic dissolutionEffect of NaCl Concentration Effect of Stress Intensity Dominant mechanism: Anodic dissolution   y

0 20 0 8 1 5 0.254 0

0.20 0.8 1.51 l/LN Cl The intergranular separation by4.0

0 7) 1 mol/L NaCl The intergranular separation by 3.5

0 16e 0.7

ch)

0 2e e- e- e-anodic dissolution of β‐phase

r)

0.16

ate

0 6inc

1.2) 3 mol/L NaCl 0.2ate e e eanodic dissolution of β phase 

b d3.0our

Ra 0.6

h (i ur)

R grain boundary precipitates is 

ho h R

0 5wth ho

u

4 5 mol/LNaCl th

Al3+ H+grain boundary precipitates is th d i t h i f2.5e (

h

0.12wt 0.5

ow 0 9

(h 4.5 mol/L NaCl0 15w

tr) M 2+the dominant mechanism for 

2 0me

row

r) 0 4Gro 0.9

me 0.15

ro our Mg2+

+++ +→+ HMg(OH)OHMg2SCC in sensitized AA5083 alloy2.0Tim

Gr /h 0.4

k G

Tim G

rho β-phase

+→+ HMg(OH)OHMg 2SCC in sensitized AA5083 alloy.1 5on

T

0 08k G

ch 0 3ck n T ck

h/h β phase

(Mg Al )A greater amount of β phase1.5tio

0.08

ac inc 0.3

rac

0 6on 0 1ac nch (Mg2Al3)+++ +→+ HAl(OH)OHAl 23A greater amount of β‐phase 

1 0bat

Cra (i

0 2Cr 0.6

atio 0.1

Cr (in Al3+

+→+ HAl(OH)OHAl 2on grain boundaries will lead to1.0

cub l C 0.2

al C

ba al C Alon grain boundaries will lead to 

0 5nc 0.04tia 0 1ota

cub

tia NaCl Solution Al3+ H+a higher crack growth rate, as0.5In nit 0.1To 0.3Inc

0.05nit 1 mol/L NaCl NaCl Solution Al3+ H+a higher crack growth rate, as 

ll hi h0.0 In

0 00.3I 0.05In 3 mol/LNaCl well as a higher average 0.0

0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 0.000.0 3 mol/L NaCl e-e-e-

g gdissolution rate per unit area0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.00

0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.5 mol/L NaCl dissolution rate per unit area, Concentration of NaCl (mol/L) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

C i f C ( / ) Concentrationof NaCl (mol/L)0 0 which eventually leads to a( )

Concentration of NaCl (mol/L) Concentration of NaCl (mol/L) 5 10 15 20 25 5 10 15 20 25which eventually leads to a 5 10 15 20 25

St ti St I t it (k i√i )5 10 15 20 25

√ shorter incubation timeSCC behaviors of specimens (sensitized for 240 hours) indicate a fully developed trend as the NaCl concentrations Starting Stress Intensity (ksi√in) Starting Stress Intensity (ksi√in) shorter incubation time.SCC behaviors of specimens (sensitized for 240 hours) indicate a fully developed trend as the NaCl concentrations  g y ( )

increases The average incubation time decreases from more than 200 hours (no SCC occurs during the entire I b ti ti i t d d t H i iti l k th t i t lincreases.  The average incubation time decreases from more than 200 hours (no SCC occurs during the entire  Incubation time is not dependent on  Mechanical and Chemical Driving ForceHowever, initial crack growth rate is strongly testing period) to less than half an hour as the NaCl concentration increases from 0 01 mol/L to 3 mol/L The initial

pt ti t i t it l b i

Mechanical and Chemical Driving Force, g g yd d t th t ti t i t ittesting period) to less than half an hour as the NaCl concentration increases from 0.01 mol/L to 3 mol/L.  The initial  starting stress intensity on a regular basis.   1718dependent on the starting stress intensity 

crack growth rate and total amount of crack growth are also found to be quite sensitive to NaCl concentration,g y g

Th i l t d f h th t ti1718p g y

d i ith hi h t ti tcrack growth rate and total amount of crack growth are also found to be quite sensitive to NaCl concentration, f

There is no clear trend of how the starting  16 K t t = 15 ksi √in16 √and increases with higher starting stress 

both of which increase monotonically with increasing concentration. The total crack growth appears to be a linearg

stress intensit ill infl en e the 15Kstart 15 ksi √in16 Kstart = 15 ksi √in

g gintensit s estin that the me hanismsboth of which increase monotonically with increasing concentration.  The total crack growth appears to be a linear 

f f h h l h l k h b l d b hstress intensity will influence the 

14intensity, suggesting that the mechanisms 

function of the concentration while the initial crack growth rate suggests a sub‐linear concave down behavior.y

incubation time14

n) KEXCESS14

n) SCC Region

y, gg gdetermining these factors are differentfunction of the concentration while the initial crack growth rate suggests a sub linear concave down behavior. incubation time.  13√

in EXCESS

√ in SCC Regiondetermining these factors are different.

12ksi√ KISCC12i√

g

Th i il it i th12

C(k(ks

The similarity in the  11

SCC

10CC

(yf t h t l t 10K

I10

KIS

C

fractography strongly suggests 98Kg p y g y gg

that the SCC mechanism98

that the SCC mechanism  8AA5083 S iti d f 240 H6 No SCC Region

associated with the AA5083 7AA5083, Sensitized for 240 Hours6 No SCC Region

associated with the AA5083 6

7

alloy under study is6

0 0 0 1 0 1 2 0 2 3 0 34

alloy under study is  0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

independent of the NaCl Concentration of NaCl (mol/L)Concentration of NaCl (mol/L)independent of the NaCl  Concentration of NaCl (mol/L)

solution concentration even • For each starting K as the25solution concentration, even  For each starting K, as the Kstart = 10 ksi root in

though the specific SCC concentration of NaCl solutionK t t 15 k i t ithough the specific SCC  concentration of NaCl solution i K d

Kstart = 15 ksi root in

behaviors are very sensitive to increases, KISCC decreases;20 Kstart = 20 ksi root inbehaviors are very sensitive to  , ISCC ;• K depends on the starting stressn)

Kstart 20 ksi root in

the NaCl concentration • KISCC depends on the starting stress √ inthe NaCl concentration.  ISCC

intensity K ;si√

0 6 mol/L NaCl 3 mol/L NaClintensity, Kstart; 15(k

s

0.6 mol/L NaCl 3 mol/L NaCl K 10 ksi√in K 15 ksi√in • 3 mol/L is critical concentration:CC

(/ / Kstart = 10 ksi√in  Kstart = 15 ksi√in  3 mol/L is critical concentration: 

ISC

ff fstart( l/ l)

start( l/ l)

below it, KISCC is determined byKI

Effect of pH (3 mol/L NaCl) (3 mol/L NaCl)below it, KISCC is determined by b th h i l d h i l

10Effect of pH (3 mol/L NaCl) (3 mol/L NaCl) both mechanical and chemical pparameters; above it chemistry is

1 4 0 14 B h i h h i il f f h i l i h h bparameters; above it, chemistry is 

1.4 0.140.18 Both micrographs show similar features of the intergranular separation that has been  the determining factor5g p g p

di d i l fl t d ltil l k ti Thi l fi th tthe determining factor.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.01.2) 0.12

ate 0.16h) discussed previously :  flatness and multilevel crack propagation.  This also confirms that  0.0 0.5 .0 .5 .0 .5 3.0 3.5 .0 .5 5.0

Concentration of NaCl (mol/L)

ur)

Ra

0 14nch p y p p g

th SCC i th i d t d i l d b di di l ti b dConcentration of NaCl (mol/L)

1.0hou

0.10h R 0.14(in the SCC in the specimens under study is also caused by anodic dissolution based  Based on the observations above the interactions between mechanical and1.0

(h

0. 0

wth ) 0.12th p y y

i t l ti d t th t th SCC h i i AA5083 ll iBased on the observations above, the interactions between mechanical and 

0 8me

0 08row

ur) 0.12

wt intergranular separation and suggests that the SCC mechanism in AA5083 alloy is  chemical driving forces as well as the SCC process itself can be generalized as0.8

Tim 0.08

Gr

hou

0.10ro

g p gg yindependent of the starting stress intensity although the specific cracking velocity can be

chemical driving forces, as well as the SCC process itself, can be generalized as 

n T k G

h/h

0 08Gr independent of the starting stress intensity, although the specific cracking velocity can be  a type of subcritical cracking that is dominated by chemical driving force and0.6on 0.06ac

knc

h 0.08

ck p g y g p g y

affected by the mechanical driving forcea type of subcritical cracking that is dominated by chemical driving force and 

i d b h i l d i i fatio

Cra (in 0 06rac affected by the mechanical driving force. assisted by mechanical driving force. 

0.4ba 0.04l C( 0.06

Cr y g

0.4

cub 0.04

tia 0.04al C

0 2Inc

0 02nit

ota

0.2I 0.02In 0.02To

0 00

Summary0.0 0.00 0.00

Future Directions & Open Issues Summary3 4 5 6 7 8 9 10 3 4 5 6 7 8 9 10 3 4 5 6 7 8 9 10 Future Directions & Open Issues Summary3 4 5 6 7 8 9 10

f 1 / C S i3 4 5 6 7 8 9 10

H f 1 l/LN Cl S l ti pH of 1 mol/LNaCl Solution Future Directions & Open Issues Su a ypH of 1 mol/L NaCl Solution pH of 1 mol/L NaCl Solution pH of 1 mol/L NaCl Solution utu e ect o s & Ope ssuesp

The trend of the dependence on the pH value is similar to that on the NaCl concentrations but in the oppositeThe trend of the dependence on the pH value is similar to that on the NaCl concentrations, but in the opposite Anodic dissolution is the dominantF th i ti ti b t th t ti l ff t th SCC b h i fsense Lower pH implying a more acidic condition seems to act in the same way as increasing the ionic strength • Anodic dissolution is the dominantFurther investigations about the potential effects on the SCC behavior ofsense. Lower pH, implying a more acidic condition, seems to act in the same way as increasing the ionic strength  Anodic dissolution is the dominant Further investigations about the potential effects on the SCC behavior of 

dof the salt solution The similarity between hydrogen ion concentration as evidenced by lower pH and NaCl mechanism for SCC in AA5083 alloyAA5083 is under wayof the salt solution.  The similarity, between hydrogen ion concentration as evidenced by lower pH, and NaCl  mechanism for SCC in AA5083 alloy.AA5083 is under way.  concentration, in their influence on the SCC behaviors of these materials suggests that both of them are related to

mechanism for SCC in AA5083 alloy.concentration, in their influence on the SCC behaviors of these materials suggests that both of them are related to h h l d f f There are interactions between mechanicalAlso due to the difficulties in observing the complex geometry of the crackthe chemical driving force for SCC. • There are interactions between mechanicalAlso, due to the difficulties in observing the complex geometry of the crack the chemical driving force for SCC.  There are interactions between mechanical 

tip creep behavior is studied as an alternative way to better understandand chemical dri ing forces for SCC

tip, creep behavior is studied as an alternative way to better understand The similarities in fractography and chemical driving forces for SCC.how environmental factors can modify the mechanical response of aThe similarities in fractography and chemical driving forces for SCC.how environmental factors can modify the mechanical response of a between specimens tested at Ch i l d i i f i i t t

y pd f i tbetween specimens tested at  • Chemical driving force is more importantdeforming system.  

different pH values and between Chemical driving force is more important deforming system.different pH values and between h d d ff l th h i l d i i f i SCCO i i l d b li i dthose tested at different NaCl than mechanical driving force in SCCOpen issues include but are not limited to:those tested at different NaCl 

i fithan mechanical driving force in SCC.Open issues include but are not limited to:

concentrations confirm anconcentrations confirm an i l b • Dislocation phenomenon at surfaces

Acknowledgementsapparent equivalence between  Dislocation phenomenon at surfacesAcknowledgements pp q

d d H (hi h h d • Vacancy injection by chemical means gb h ll d

reduced pH (higher hydrogen  • Vacancy injection by chemical meansSteve Robinson John Miller and Brian McIntyre

p ( g y gi t ti ) d N Cl • Hydrogen enhanced localized plasticity Steve Robinson, John Miller, and Brian McIntyreion concentration) and NaCl  • Hydrogen enhanced localized plasticity

Thi k t d b ONR D A K V d)

t ti h i ly g p y

R l f b id i li t t d t i bilit i th t This work supported by ONR Dr A K Vasudevanconcentration as chemical  • Role of bridging ligaments to produce apparent variability in growth rates This work supported by ONR, Dr. A.K. Vasudevan, driving forces for SCC

o e o b dg g ga e ts to p oduce appa e t a ab ty g o t ates

S i tifi Offi/ /driving forces for SCC.  

Scientific Officer.1 mol/L NaCl pH = 4 1 mol/L NaCl pH = 9g

Scientific Officer.1 mol/L NaCl, pH = 4 1 mol/L NaCl, pH = 9