cr(vi)-free pre-treatments for adhesive bonding of aerospace aluminum ... symposiu… · for...

10/26/2016

1

1

Cr(VI)-free pre-treatments

for adhesive bonding of

aerospace aluminum alloys

S.T. Abrahami, V.C. Gudla, T. Hauffman, J.M.M. de Kok, R. Ambat,

J.M.C Mol and H. Terryn

IHAA, Oct. 5-7th 2016, Düsseldorf, Germany

2

Porous

anodic

oxide

adhesive

Al/ Al alloy

adhesive

primer

AA2024-T3, AA7075-T6 clad/bare

Introduction - Experimental - Results & Discussion - Conclusions

Structural adhesive bonding in aerospace

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3

cleaning de-oxidizing anodizing primer

Cr(VI)-free alternatives:

• H2SO4 (SAA)

• H3PO4, (PAA)

• H3PO4 - H2SO4 (PSA)

Aluminium Pre-treatment in Aerospace


CrO3 (CAA)

4

1. Chemistryanions incorporation into the Al2O3

matrix

Al

oxide

electrolyte

2Al3+ +3O2−→ Al2O3(s)

Al→ Al3++3e−

O2−OH

−

2Al3+ +3O2−→ Al2O3(s)

,SO42−,PO4

3−

2. Morphology

pore

cell

barrier

layer

(Zhou 2011)

film thickness:

PAA < CAA < SAA

pore diameter:

SAA < CAA < PAA

Research question

How will changing the electrolyte affect adhesion

with epoxy?


(Aerts 2009)

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3

5

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14

I II III IV

time (s)

oxide growth stages

III

IVIII

Sulka, Nanostructured Materials in Electrochemistry (2008)

voltage (

V)

Sample Preparation

Type I

Barrier AAOType IV

Porous

AAO

............. //

...


6

1. Barrier AAO

Aluminium

Oxide

AES, XPS surface analysis

Aluminium

Oxide

Adhesive

Oxide

Aluminium

Floating roller peel test

FM 73 epoxy

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7Introduction - Experimental - Results & Discussion - Conclusions

(Phi.com)

Auger Electron Spectroscopy (AES)

Auger electron is emitted from the top-

most atomic layers of the martial


Auger Electron Spectroscopy (AES)

Co

nc.

Time (� depth)

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9

AES Depth Profiles


0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Co

nce

ntr

ao

n (

at.

%)

Spu er me (min.)

P (×10)

O

C

Al (metal) oxide film

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9

Co

nce

ntr

ao

n (

at.

%)

Spu er me (min.)

S (×10) C

O

Al oxide film

element Barrier layer

This Study

(at.%)

Porous oxides

Literature

(at.%)

P 2 - 4 6 - 8

S 1 - 2 10 - 13

Cr 0.0 – 0.2 0.1 – 0.3

Oxide film

Oxide film

PAA

SAA

Sulka, Nanostructured Materials in Electrochemistry (2008)

10

AES Depth Profiles


PSA ≠ SAA + PAA

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9

Co

nce

ntr

ao

n (

at.

%)

Spu er me (min.)

oxide film oxide film

O

S (×10)

P (×10)

Al (metal)

PSA

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11

X-Ray Photoelectron Spectroscopy (XPS)

photoelectronshν

X-ray source

Electron energy

analyzer KE = hν - BE

Survey spectra

12

527 528 529 530 531 532 533 534 535 536 537

Binding energy (eV)

O1s

O2-

OH-,

C-O, O=C-O,

P-O and S-O

H2O and

O=C-O

280 282 284 286 288 290 292 294

Binding energy (eV)

C1s

C-C/C-H

C-O O=C-O

XPS Analysis: Curve Fitting


Curve fitting and model based on McCafferty & Wightman

(1998), Surf. Interface Anal.

0

5000

10000

15000

20000

25000

30000

0 100 200 300 400 500 600

Inte

nsi

ty

Binding Energy (eV)

C1s

Al2s

Al2p

O2s

O1s

S2s P2s

S2P P2p

Survey spectrum

High-resolution photopeaks

166 168 170 172 174

Binding energy (eV)

S2p

SO42-

186 188 190 192 194 196 198

Binding energy (eV)

P2s

PO43-

Abrahami et al., (2015), J. Phys. Chem. C

OH, C-O,

O=C-C

IO1s

Al metal

oxide

contamination layer IC1s

IAl2p, metal

emitted photoelectron

θ

tcont

tox

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13

XPS Analysis: Extended Model

Binding energy (eV)

527 529 531 533 535 537

O2- OH-,

C-O, O=C-O,

P-O and S-O

H2O and

O=C-O

O2- OH-,

C-O, O=C-O,

P-O and S-O

H2O and

O=C-O


IO1s, IP2s, IS2p

Al metal

oxide

contamination layer IC1s

IAl2p, metal

emitted photoelectron

θ

tcont.

tox.

tcont. ≈

2nm

14

Barrier AAO: Surface chemistry


O2-

OH-

PO43-

SO42-

• The type of electrolyte is highly affecting the surface chemistry!

• Chemistry is slightly varying with the anodizing temperature.

• General trend:

hydroxyl fraction decreases:

PAA < PSA, HcPSA < SAA < CAA

0

20

40

60

80

100

PAA PSA HcPSA SAA CAA

Re

lave

am

ou

nt

(%)

16 6

15

13

41

60 59

16

25 39

11

11

60 68 61

θ=15°T = RT (22°C)

0

20

40

60

80

100


Re

lav

e a

mo

un

t (%

)

33

5

7

21

8

10

15

13

14

17

28

69 72 63 64 63

θ=15°

0

20

40

60

80

100


Re

lave

am

ou

nt

(%)

20

7

9

15

9

10

14

10

16

14

28

72 71 68 70 72

θ=15°T = 35°C T = 50°C

Abrahami et al., J. Phys. Chem. C, 2015

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ASTM D3167 - 03a

Floating roller (Bell) peel test configuration

16

Dry peel strength

0

100

200

300

400

0 1 2 3 4 5 6

ave

rag

e d

ry p

ee

l st

ren

gth

[N

]

20°C

35°C

50°C

pure Al

Dry peel is independent of:

• pre-treatment

• anodizing Temp.

Barrier AAO: Floating roller peel tests

PAA PSA HcPSA SAA CAA PAA PSA HcPSA SAA CAA0

100

200

300

400

ave

rag

e w

et

pe

el

stre

ng

th

[N]

20°C

35°C

50°C

pure Al

Wet peel strength

Wet peel strength

• increase with the anodizing

Temp.

• influenced by the type of

electrolyteWhy?

Abrahami et al.., J. Phys. Chem. C, 2016.


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Barrier AAO – Wet peel strength


• Wet peel increases:

PAA < PSA, HcPSA < SAA < CAA

• Wet peel increase with OH- (%)

• PO43- and SO4

2- do NOT contribute

T= RT (22°C)

0

50

100

150

200

0 10 20 30 40 50

0

50

100

150

200

0 5 10 15 20 25 30

0

50

100

150

200

0 5 10 15 20 25 30

T= 35°C T= 50°C

OH- (%) OH- (%)

OH- (%)

We

t p

ee

l st

ren

gth

(N

)

We

t p

ee

l st

ren

gth

(N

)W

et

pe

el

stre

ngth

(N

)Abrahami et al.., J. Phys. Chem. C, 2016.

18

Wet peel – barrier AAODry peel

Cohesive failure

Wet peel

Interface failure

Wet peel increases with OH- (%), yet interface

failure observed in all panels ( 9 N � 143 N)

Interfacial adhesion with epoxy proceeds through

the surface hydroxyls!!

Higher density � larger strength, but bonds are

not stable in water!!


Abrahami et al.., J. Phys. Chem. C, 2016.

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19

2. Porous AAOSEM, TEM Floating roller peel test

AluminiumAluminium

Aluminium

AdhesiveAF 163 epoxy

20

Porous AAO – SAA (10 g/l H2SO4,19V, 30 min.)

TE

M

20°

C


�

500 nm

�

500 nm

SE

M

50°

C

Pore

diameter

8 nm

13 nm

22 nm

8 nm

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21

Porous AAO – SAA (10 g/l H2SO4,19V, 30 min.)

TEM-

EDS

Dry

peel

153 N20°

C

50°

C

Dry

peel

324 N


22

SAA 20°C

0

100

200

300

400

0 20 40 60

ave

rag

e d

ry p

ee

l st

ren

gth

[N

]

typical pore size [nm]

20°C

35°C

50°C

AA7075T6

alclad

Porous AAO


Dry peel – general trend (SAA, PSA)

• Dry peel depends on the pore size:

• Good adhesion at d > 15 nm

• There is an upper limit: approx.

constant peel at d > 25 nm

Barrier AAO

0

100

200

300

400

0 1 2 3 4 5 6

ave

rag

e d

ry p

ee

l st

ren

gth

[N

]

20°C

35°C

50°C

pure Al

Dry peel is independent of:

• pre-treatment

• anodizing Temp.


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Adhesion mechanism – dry peel

morphology chemistry

Dry peel Yes (pore size) no

24

PSA 20°C

SE

M


Porous AAO – PSA (varying conditions)

PSA 20°C PSA 35°C PSA 50°C

Temp.

H3PO4 conc.

Both micro- and

nano- surface

roughness increases

“Bird’s nest” structure

2 μm�

2 μm

�

500 nm

�

10 μm

2 μm�

2 μm

�

500 nm

�

10 μm

2 μm�

2 μm

�

500

nm

�

10 μm

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25

PSA 20°C

PSA 35°C

PSA 50°C

Porous AAO – PSA (varying conditions)

TEM-

EDS

TE

M

Dry: 147

N

Wet: 46

N

Dry: 333

N

Wet: 335

N

Dry: 326 N

Wet: 338

N


26

0

100

200

300

400

0 20 40 60

ave

rag

e w

et

pe

el

stre

ng

th

[N]

typical pore size [nm]

AA7075T6

alclad

Porous AAOPeel

test


Wet peel – general trend (SAA, PSA)

• No correlation to the pores size

• Higher wet peel is related to Temp. and H3PO4 conc. �

surface roughness!

PSA 20°C

PSA 50°C

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27

No correlation between oxide thickness and

mechanical performance!


0

100

200

300

400

0 1 2 3 4 5 6

ave

rag

e w

et

pe

el

stre

ngth

[N

]

Average oxide thickness (μm)

20°C

35°C

50°C

0

100

200

300

400

0 1 2 3 4 5 6

ave

rag

e d

ry p

ee

l st

ren

gth

[N

]

Average oxide thickness [μm]

20°C

35°C

50°C

Porous AAO

Peel test

• No need to mimic the 3 µm CAA for good adhesion!

• The contact area is less important than the fact that a fully

cohesive interphase is created.

28

morphology chemistry

Dry peel Yes (pore size) no

Wet peel Yes (surface

roughness)

yes

Adhesion mechanism – wet peel

�

500 nm

0.2 μm


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29

25

mm

0

100

200

300

400

0 20 40 60 80 100

ave

rag

e p

ee

l st

ren

gth

aft

er

18

0 d

ays

[N

]

bondline corrosion after 180 days [%]

20°C

35°C

50°C


Bondline corrosion – porous AAO

Peel

test

SS

T

More corrosion for panels prepared at 35 °C and 50 °C and

relatively lower anodizing voltages � related to barrier layer

thickness?!

+

30

Conclusions

Research question: How will changes in the chemical and

morphological aspects changes affect adhesion with epoxy?

Finding:

1) The different surface chemistries did not change the

type of bonding with epoxy, only the bonding density.

Incorporation of anions has a negative effect on

adhesion due to a reduced amount of surface

hydroxyls.

1) Morphology is important:

• pore size

• surface roughness (bird’s nest)


Can be controlled by the

anodizing conditions

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31

Thank you for your attention !

Questions?

cr(vi)-free pre-treatments for adhesive bonding of aerospace aluminum ... symposiu… · for...

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