Stress Corrosion Cracking and Fatigue:

mechanical load (tension)+

Corrosive environment

1

Material

EnvironmentStresses - Aggressive media- Temperature- Potential /Current- Microfluidics

- Design- Mechanical bulk stresses- Internal stresses Production

When does Stress Corrosion Cracking (SCC) occur ?

° Simultaneous influence of tensile stresses and aggressiveenvironment

° Only one of these parameter does not induce crack growth

SCC is a system not a materialproblem Fatigue Corrosion

SCC

- Composition- Heat treatment- Microstructure- Surface condition

2

Stress Corrosion Cracking: process steps

Definition: Stress Corrosion Cracking is the initiation and slow growth of cracks under the simultaneous influence of tensile stresses and aggressive environment

SCC processes is divided in three phases:

1. Incubation2. Crack growth 3. Breaking

° Incubation time is the most important aspect for the life time of a component

° SCC is from all the corrosion attack, the one resulting in the fastest damage of materials 3

Type of crack propagation

Intergranular: Transgranular:

Attack at the grain attack through theboundary grains

4

SCC: important features

° Materials breakdown happens at macroscopic scale, without deformation and perpendicular to the stress direction

° No measurable material removal

° No visible corrosion products

° In most of the SCC failure, corrosion initiation is difficult to detect

5

Types of SCC corrosion attack° There is a whole range of attack ranging from: at one end the purely intergranular attack and on the other end the brittle fracture

Inte

rgra

nula

r cor

rosi

on

Brit

tle fr

actu

re

Corrosion Tension

Stress – deformation inducedMetal dissolution

Adsorption induced brittle fracture

Steel

NO-3

Al-Zn-Mg

Cl-Brass

NH3

18/8 CrNi

Cl-

Mg-Al

Cr2-4/ Cl-

Ti

CH3OH

SteelHigh strength

H2O

small crack propagation rate large

Defined Crack direction

Crack evolutionControlled by deformation

6

SCC: 2 main mechanisms° For small until medium crack propagation rates:

Type 1: anodic metal dissolution accelerated by stress

It is an electrochemically controlled processes with following model:

crackpropagation

metalactivecrack tip

passivecrack wall

Passive oxide

elec

trol

yte

Cathodic partial reaction

Plasticdeformation

Crack electrolyte

diffusion, convection

7

Type 2: adsorption induced brittle fracture° For fast crack propagation rates, the cathodic reactions plays a larger role (anodic dissolution is too slow)

°At the crack tip, adsorbed or speciesdiffused in the metalweaken the metallicbinding forces

° the most dangerousspecies: hydrogen

causes embritlementHIC (hydrogen inducedcracking)

electrolyte

metal

Adsorbed anions

shear plane

crevice plane

From adsorption weakened metal bounds

8

SCC: summary of mechanisms

Anodic SCC

° Dominated by anodic metal dissolution

° Crack propagates through accelerated dissolution due to applied stress

Stainless Steel

° Cracks are propagating from the surface

Cathodic SCC

° Dominated by H2 production (also for example from cathodic deposition of protecting layers)

°Crack propagates because of hydrogen embritlement due to hydrogen diffusion

High strength steel

°Crack are also generated inside the material

9

° Not a single clearly defined crack !Near the surface:

Secondary crack with ramification

ca. 2 mm

Ramified crack propagation observed on the metallographic cross section

ca. 50 µm

SCC: typical crack evolution

10

- Sem investigation of the fracture surface

Empa unterwegs – Sion, 09. 11. 2006

Example: fractographic analysis of broken cable

Ductile

Brittle (fragile)

Intergranular

11

Damaged surface eloxal layer

Intergranular attack

SCC: metallographic investigation° Example of SCC on coated (thick oxide) aluminum

- Presence of intergranular attack near the main crack- Detection of Cl- in the material by EDX

EDX analysis of internal

12

Model experiments: aspects of fracture mechanics

° Fracture mechanics can give useful information on crack propagation rates in technical application

° It is based on consideration of macroscopic parameterslike crack length, applied tensile stress (S), sample geometry

° Assumption: test are alwaysperformed on notched(depth:d) specimens (width:w)

° Initial crack geometry + environment

critical applied stress

SS

S

W

Wd

d

13

Experimental characterization of SCC° Crack propagation rate as a function of applied stress intensity

Domain 1:When the critical load for SCCis reached (KIscc) , a fast increaseof the crack propagation rateIs observed

Domain 2:Constant SCC propagation rate typical for the influence of electrochemical control

Domain 3 and 4:If the KIc is reached, the standard brittle fracture is taking place as in absence of aggressive environmentalinfluence Applied stress K

Log

(cra

ck p

ropa

gatio

n ra

te

d /

t)

Schematic evolution of crack propagation for SCC

14

Important parameters for the evolution of SCC° Incubation (crack formation)

Chemical or mechanical damage of the passive layer

Critical factors are electrochemical nature (temperature, electrochemical potential, aggressive ion concentration)and the influence of stresses on the passive film

° Crack propagation

Metal dissolution followed by repassivation

Critical factors is the ratio of the tensile deformation and the repassivation rate at the crack tip

15

SCC: influencing parameters1) Materials

° Susceptibility can be controlled

inadequate heat treatment resulting in sensitization (chrome depletion at grain boundaries) for example is extremely detrimental

° Alloying of nickel is beneficial(around 20 %)

Example:

CrNi wires in boiling MgCl2 (154°C)

Austenitic structure is moreresistant then ferritic

Tim

e to

failu

re (

h)Weight % Nickel 16

SCC: influencing parameters II° Molybdenum decrease the SCC susceptibility in the critical Ni concentration domain mainly in increasing the critical KISCCthreshold

Example:

CrNiMo Steel

With 15.5 – 21% Ni22 %Cr

In aerated NaCl solution (105°C) Weight % molybdenum

Crit

ical

KIS

CC

(MN

/ m

1.5 )

17

SCC: influencing parameters III° TemperatureAt room temperature, SCC is usually not observed in chloride containing environment (seawater or similar environment )

Be careful with acidic environments (crevice condition)where SCC is occurring also at room temperature

° Aggressive ions

CrNi SteelsChloride anions

Cl- concentration in aqueous electrolyte (ppm)

Tem

pera

ture

(°C

)SCC (initiated at pits)

No SCC after 10’000 hours exposure

18

Measures to avoid SCC

° Avoid stresses on the material

- Internal stresses can be reduced with adequate heat treatment

- External stresses are often decreased just by design consideration (avoid having applied stresses on welds !)

° Remove aggressive environment (special care have to be taken to crevice conditions)

To avoid right design

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Corrosion - Fatigue: cyclic loading

° Special case of Stress Corrosion Cracking (SCC)

° Applied stress is not constantbut experiences cyclic variations

aggravated SCC attack

° Much more materials and environments are concerned

Transgranular crack propagation

Metallographic

cross section

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Corrosion – Fatigue: mechanisms

° Factors influencing corrosion-fatigue processes are similar toSCC

- The cyclic loading caninduce constantdepassivation

- Interaction between glidingplanes and electrolyte playsa key role

- At the induced micro notches,additional gliding and acceleratedcorrosion is induced

Gliding plane

Corrosion susceptible area

Passive layer

Initiation sites for corrosion fatigue

21

Wöhler curves in the case of corrosion - fatigue° Corrosion rate is dependent on the cycle number and amplitude

This has to be taken intoaccount for fatigue-corrosion

The Wöhler curves display the failure time in relation tocycle number and stress amplitude

Important to note:surface defects (notches) plays a tremendous role in the life of a componentexposed to fatigue condition

Cycle number

Stre

ss a

mpl

itude

1. smooth surfaceAir, RT2. notched surfaceAir, RT3. smooth surfaceConc. NaCl4. notched surfaceConc. NaCl

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Fatigue: experimental setups° The specimens used were CT-specimens (CT: compact tension)° The geometry of the specimen is given as relative dimensions of the width W

° The specimen made from 7075-T651 had a width of 60 mm and a thickness of 10 mm ( with W = 40 mm and B = 3.63mm). The initial notch depth a0 was 17 mm or 32 mm.

° The specimen were loaded by a pair of pin loads at x=0, y=0, z=±0.225W in the z direction. The loading was a sinusoidal constant amplitude history with a frequency of 83 or 54 Hz.

° The CT-specimen was equipped with a clip gauge at the mouth of the notch. The crack length was monitored optically by two traveling microscopes, fixed to the test bed and allowing to measure both surface crack lengths, on the front and on the back face. 23

a crack length, crack depth, crack sizea0 original crack size, initial crack length

Keff effective stress intensity factor range

da/dN fatigue-crack-growth rate measured in constant amplitude tests

R load ratio := Pmin/Pmax or Kmin/Kmax

Pmax maximum load on the C(T) specimenPmin minimum load on the C(T) specimen

Kmax maximum stress-intensity factorKmin minimum stress-intensity factor

Some important parameters and relations

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• The T651 temper is a solution heat-treated, quenched, 1.5 to 3% controlled stretched, and artificially aged condition with a maximum static strength.

• Second phase particles in these alloys are usually categorized into three groups:- Large (approx. 1 to 30 m) intermetallic particles formed during solidification by combination of impurities (Fe, Si) and solute elements: Al12(Fe,Mn)3 Si and Al7Cu2Fe- Smaller (approx. 0.3 m) dispersoid particles formed by solid state precipitation of Cr and Mn at temperatures above 425°C: Al20Cu2Mn3

- Fine (0.5 nm - 10 nm) precipitates, containing solute elements. Formed during quenching or aging: Al2CuMg

Measurementsin weight %

Specificationin weight-%

Remarks

Element A B averageSi Silicon 0.06 0.06 0.06 0.40Fe Iron 0.30 0.30 0.30 0.50Cu Copper 1.70 1.70 1.70 1.2 - 2.0Mn Manganese 0.03 0.03 0.03 0.30Mg Magnesium 3.10 3.10 3.10 2.1 - 2.9 *Cr Chromium 0.18 0.18 0.18 0.18 - 0.28Zn Zinc 5.60 5.60 5.60 5.1 - 6.1Ti Titanium 0.03 0.03 0.03 0.20Ni Nickel <0.01 <0.01 <0.01 0.05Others, each <0.01 <0.01 <0.01 0.05Others, total 0.15Al Aluminum Balance Balance Balance Balance

Used aluminum alloy: 7075

25

26

- Oxide film growth will occur very rapidly (formation of the 5-6 nm passive film in a few millisecond in air) in air with a slight humidity related acceleration

- Even in Nitrogen atmosphere, a nm-thick oxide will form in millisecond- In fine vacuum (high vacuum), the formation is then obviously hindered and

only monolayers of oxygen will be chemisorbed on the surface

Al oxidation process as function of atmosphere

7075-T651: crack propagation in vacuum

27

crack growth rate in 7075-T651 in fine vacuum

0.001

0.010

0.100

1.000

10.000

100.000

1'000.000

10'000.000

1 10 100Delta K eff [MPam^0.5]

da/d

N [n

m/c

ycl

SCVA, R = 0.1, Delta K upSCVF, R = 0.1, Delta K upSCVA, R = 0.1, Delta K dwSCVB, R = 0.1, Delta K dwSCVB, R = 0.3, Delta K upSCVD, R = 0.3, Delta K upSCVB, R = 0.3, Delta K dwSCVC, R = 0.5, Delta K dwSCVC, R = 0.5, Delta K dwSCVC, R = 0.5, Delta K upSCVC, R = 0.5, Delta K upSCVE, R = 0.5, Delta K dwSCVE, R = 0.5, Delta K upmodel with slip

The experimentally measured results (with the fitted parameters) for 7075-T651 are shown below with the propagation rates (da/dN) and the onset ofunder critical crack growth

7075-T651: crack propagation in nitrogen

28

crack growth rate in 7075-T651 in purified nitrogen

0.001

0.010

0.100

1.000

10.000

100.000

1'000.000

10'000.000

1 10 100Delta K eff [MPam^0.5]

da/d

N [n

m/c

ycl

SCNC, R=0.1, f = 54 Hz

SCND, R=0.3, f = 54 Hz

SCNE, R=0.3, f = 83 Hz

SCNF, R=0.5, f = 54 Hz

SCNG, R=0.5, f = 54 Hz

SCNF, R=0.5, f = 83 Hz

model with d_ox

- In nitrogen, the critical stress intensity Keff to initiate fatigue crack growth is decreased and the propagation rates in the undercritical domain is also faster

- There is clearly a material-environment combination in this process

7075-T651: crack propagation in air

29

crack growth rate in 7075-T651 in humid air

0.001

0.010

0.100

1.000

10.000

100.000

1'000.000

10'000.000

1 10 100Delta K eff [MPam^0.5]

da/d

N [n

m/c

ycl

S0LA, R=0.1, Delta K upSCVA, R=0.15, Delta K dwSCLB, R=0.3, Delta K upSCLB, R=0.3, DeltaK dwSCLC, R=0.5, Delta K upSCLC, R=0.5, Delta K dwnew model with d_oxnew model without d_ox

- In air, the critical stress intensity Keff to initiate fatigue crack growth is similar to nitrogen but the propagation rate is then slightly higher

- The environmental component is related to the presence and thickness of passive film that is constantly broken at every cycle and reforms

Summary: role of passive oxide film on fatigue

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Crack Growth in 7075-T651 in Different Environment

0.001

0.010

0.100

1.000

10.000

100.000

1'000.000

10'000.000

1 10 100Delta K_eff [MPam^0.5]

da/d

N [n

m/c

ycle Vacuum

Purified Nitrogen

Laboratory Air

- The fatigue crack propagation rate increase as function of stronger oxidizing conditions can be seen by overlapping the curves measured in different environments

Passivation process and stress distribution

31

• The empirically found crack growth rates are understood as the superposition of these mechanisms:

- In vacuum near threshold a partly non-reversible cyclic slip mechanism results in crack propagation. For higher loads after this cyclic slip mechanism the crack tip begins to blunt and after each fatigue cycle a fatigue striation is left on the crack surface

- In air and nitrogen, near threshold, the crack growth increment is given by the oxide film thickness build-up after each half cycle. For higher loads the crack tip is blunted and fatigue striations occur o the crack surface

- The main outcome of these experiments was to proof the hypothesis that the crack growth increment near threshold in air and nitrogen is the oxide film thickness. Corrosive liquids would result in further acceleration of the processes

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Short summary: Al Fatigue-corrosion

How to avoid Corrosion Fatigue° Same measures than for Stress Corrosion Cracking

Further indicated is:

° Decrease of the stress amplitude under a critical value (look at the Wöhler’s curve parameters)

° Try to have a smooth surface (avoid notches or localized corrosion attack) in the areas where stress is expected

° Try to avoid resonance frequencies of the structure (design consideration)

° Improve the passivation of the surface. Brittle coating do not help, organic coating is better in this case 33