Replacements for Chromic Acid Anodising in Bonded

Structures

G.W.Critchlow1, K.A.Yendall1, I.A.Ashcroft2, T.Cartwright1, V.Shenoy2

and R.H.Dahm1

1Department of Materials2Wolfson School

Loughborough University, UK

Summary of Presentation

1. Introduction to structural metal bonding2.

Pretreatment

options –

importance of CAA

3.

Alternatives:

4.

Concluding remarks

Grit-blast plus hot water immersion plus silane

DC phosphoric-sulphuric acid anodising

DC boric-sulphuric acid anodising

EPAD plus SAAACDC anodising –

the state-of-the-art -

adhesion plus fatigue

1. Introduction

Durability or permanance

of the bonded structure is a function of:

Alloy

Adhesive/Primer

Surface condition

2. Pretreatment

options

Mechanical (degrease, grit-blast, laser texturing, cryoblasting)

Chemical (acid etch, conversion coating)

Electrochemical (anodic oxidation)

Standard Processes –

40/50V CAA

Barrier layer

Pore diameter25nm

Cell wallthickness10nm

Base Metal

Oxide thickness4μm

VapourDegrease

Desmut

Deoxidise

Anodise

Seal(if required)

Summary of DC Anodising ProcessSummary of DC Anodising Process

CAA 40/50V Anodising Process

Vapour Degrease

Optional Seal

Rinse- DI Water

Acid Etch

Rinse- DI Water

Alkaline Clean

Rinse- DI Water

Anodise Optional Rinse

Hot Air Dry

IsoprepIsoprep 4444 FPL EtchFPL Etch CAACAA SealSeal

Solvent Wipe

MEK

60g/l Concentration (pH 9.3 –10.45)

10 minute Submersion

60°C

60g/l – Na2 Cr2 O7 166ml/l – H2 SO4 1.5g/l – Al

30 minute Submersion

60°C

30.5 – 50.0g/l

35 – 45 minute Anodising

40°C

DI Water

15 minute Submersion

Min 96°C

J.L. Cotter and R. Kolhler, Int.J.Adhesion & Adhesives, 1981

Disadvantages of CAA

There are health & safety and environmental issues associated with the use of hexavalent chromium.

Process is highly complex and time consuming to completion – costly.

3. ALTERNATIVES

3.1 Grit-blast plus hot water immersion plus silane

Degreasein Alconox

Grit –blast50 μm alumina

Water immersion40ºC or 50ºC

1% solutionof silane

Dry

Summary of Grit-blast plus Hot Water Immersion plus Silane

Process

eg. Underhill, Rider and DuQuesnay, Proc.Euradh

2002, Glasgow, UK

3.2 DC Phosphoric-sulphuric Acid Anodising (PSA)

PSA

Developed by Airbus (Germany)

Presented at 1996 AGARD Symposium by Matz et al

3.3 Modified Boric-sulphuric acid anodising (BSAA)

Standard BSAA processStandard BSAA process

B arrier layer

P ore d iam eter10 nm

C ell w allth ickness10nm

B ase M etal

O xide th ickness3 μm

BSAA 2024-T3 clad alloy

200 nm

BSAA 2024-T3 clad alloy

200 nm

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160 180 200

Exopsure Time (hours)

Crac

k ex

tens

ion

(mm

)

Wedge test results for BSAA exposed to 60Wedge test results for BSAA exposed to 60°°C and 100% RH using 2024C and 100% RH using 2024--T3 clad alloy.T3 clad alloy.

Varied ParametersVaried Parameters

Deoxidise Bath Temp. Anodising Voltage

Post- Treatments

ElectrolyteConcentration

Number of Deoxidisers tried

Electrolytic Phosphoric Acid Deoxidiser

Varied between 25 – 40 O C

Bath Temp. 35O C

Varied between 15 – 30V

Higher voltages show limited improvement

Treatment after Anodising

Phosphoric acid dip

Boric Acid 2.5 – 10 g/l Sulphuric Acid 20 – 60g/l

No improvement compared to standard BSAA

200 nm

STEM micrograph of Electrolytic Phosphoric acid deoxidised on clad 2024 alloy

200 nm

STEM micrograph of Electrolytic Phosphoric acid deoxidised followed by BSAA on clad 2024 alloy

Variations In Electrolytic Bath Temperature

Standard temperature

200 nm200 nm

350 C.

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

CAA 40 /50V Tri-acid + BSAA Pyrene 7-77 @ 2 5 C + BSAAPyrene 7-77 @ 50 C + BSAA Chemcid 2 218 + BSAA Pyrene 14 -73 + BSAA Pho spho ric Acid Electro lytic Deoxid ise + BSAA Phospho ric Acid Electro lytic Deoxid ise + BSAA + PAD Tri-acid + BSAA + PADTri-acid + BSAA @3 5 C Tri-acid + BSAA @20 V Tri-acid + BSAA @25VBSAA (25g / l Sulp huric, 10 g /l Bo ric) BSAA p lus g rit -b las ted

BSAA Summary

Standard BSAA treatment proved to be inadequate as a pretreatment for bonding in comparison to the CAA control.

A number of modified BSAA alternatives are available which give the required surface topography and equivalent mechanical performance relative to that of the CAA process

However, processes are complex and times are slow

3.4 EPAD plus SAA process

32

Anodic Oxide Morphology Anodic Oxide Morphology

EPAD –

SAA Anodised

EPAD –

SAA Anodised –

PAD

34

Wedge Test ResultsWedge Test Results

0

10

2 0

3 0

4 0

5 0

6 0

7 0

CAA 4 0 / 5 0 V ( Na OH) + SAA 4 0 g / l2 6 °C

( E P AD) + SAA 4 0 g / l2 6 °C

( E P AD) + SAA 4 0 g / l3 5 °C

( E P AD) + SAA 4 0 g / l3 5 °C + ( P AD)

( E P AD) + SAA 1 8 0 g / l2 0 °C

( E P AD) + SAA 1 8 0 g / l3 5 °C

100 hours24 hours5 hoursIo

35

EPAD plus SAA Summary EPAD plus SAA Summary

EPAD Serves Two FunctionsClean/deoxidiseModify the surface aid adhesion

Duplex oxide formed which aids primer penetration and acts as barrier oxide to improve corrosion resistance

Comparable initial joint strengths and durability have been demonstrated compared with CAA

3.5 ACDC process

AC offers:Degrease only requiredOpen porous structureRapid processing

DC offers:Thick, compact film

Objective

To produce an AC/DC film in a single electrolyte at a constant temperature with approx 300

nm of outer porous oxide and >1 micrometre of inner dense oxide.

Variable parameters

Parameter AC DC

Electrolyte mixed sulphuric-phosphoric

Voltage (V) 50Hz, 10-20 <25

Time (s) 120-240 600

Temperature (°C)

RT-50 RT-50

Surface after AC anodising

5s AC anodising 30s AC anodising

degreased-only

120s AC anodising 240s AC anodising

surface atom % for the main element after different anodising periods

-5

5

15

25

35

45

55

0 50 100 150 200 250

anodising periods (s)

atom

% Al(o)

C

O

surface atom% for second element after differents anodising periods

-0.5

1.5

3.5

5.5

7.5

9.5

11.5

13.5

15.5

17.5

19.5

0 50 100 150 200 250

anodising periods (s)

atom

%

P

S

Cl

K

Mg(o)

AlPO4 Al2

O3 AlOOH Al(OH)3

Slow dissolution

Influence of DC voltage

Film thickness Vs Voltage (10m)

0

0.5

1

1.5

2

0 5 10 15 20 25 30

Voltage / V

Th

ickn

ess

/ m

icro

ns

10v AC, 120 s: 20v DC, 600s @35°C

Crack extension as a function of time

0

1

2

3

4

5

6

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

exposure time / hours

crac

k ex

tens

ion

/ mm

0

0.5

1

1.5

2

2.5

3

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

Exposure time / hours

Gc

/ KJm

-2

10 120 35

10 120 50

10 240 35

10 240 50

15 120 35

15 240 35

CAA

Results -

FEGSEM Images

(a) AC/DC plan(b) SAA plan(c) AC/DC plan higher magnification(d) SAA plan higher magnification(e) AC/DC cross section(f) SAA cross section

(a) (b)

(e)(d) (f)

(c)

Oxide Thickness~ 1.8μm Oxide

Thickness~ 8μm

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Ultimate Failure Load (N)

Degrease Grit Blast SAA AC/DC

Pretreatment Initial Bond Strength 1 Week Exposure 1 Month Exposure2 Month Exposure 3 Month Exposure

ACDC Summary

A two-stage anodisation process has been developed to facilitate interphase formation without compromising corrosion protection.

The process is simple, fast and robust.

Initial results show promising adhesion and corrosion performance.

Additional studies are underway to further validate this process.

Fatigue

Bare metal

Bonded Joints

Basquin Comparison

y = -7.6162x + 23.797R2 = 0.9399

y = -8.1666x + 25.177R2 = 0.9674

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70

Log(Stress)

Log(

Cyc

les)

ControlEPADLinear (Control)Linear (EPAD)

≈ 1.5 % difference at 106 cycles

Basquin Comparison

y = -6.9664x + 22.235R2 = 0.855

y = -7.6162x + 23.797R2 = 0.9399

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70

Log(Stress)

Log(

Cyc

les)

ControlACDCLinear (ACDC)Linear (Control)

≈ 2 % difference at 106 cycles

Types of load interaction effects

Mean load change

Load ratio change

Mixed effects

Complex loading

SEM damage characterisation

Damage in the glue-line

1µm

Adherend

Imaginary boundary between fillet and glue-line

Damage in the fillet

Damage in the glue-line

Damaged adhesiveUndamaged adhesive

Fractured surface

Damage mechanics

Fatigue lifetime

2mp )(1mdN/dD υ=

FEA and Fatigue damage calculation.

Geometry

Material property

Damage evolution law

Variable amplitude fatigue

Type of load interactionExperimental results (No. of cycles)

Miner rule prediction (No. of cycles)

LCM approach (No. of cycles)

Vertical jump approach(No. of cycles)

L1 = 6.5kN; L2 = 8kNn1=10; n2=5 1706 5020 1850 1707

L1 = 6.5kN; L2 = 8kNn1=1000; n2=5 18085 32127 21174 17757

L1 = 8kN; L2 = 9.5kNn1=10; n2=5 976 845 1869 1022

L1 = 8kN; L2 = 6.5kNn1 = 10; n2=5 12966 2704 19925 9518

Results and discussion

AC DC against CAA surface pre-treatment

Static strength

Cycles to failure for max. fatigue load of 60% of static strength

0

2

4

6

8

10

12

14

0 500 1000 1500

Conditioning [ no. of hours]

Stat

ic s

treng

th [k

N] v

AC DCCAA

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

0 500 1000

Cyc

les

to fa

ilure

v

Conditioning [ no. of hours]

CAA

AC DC

4. CONCLUSIONS

Hexavalent chromium chemistry is widely used in Al processing within the aerospace, defence, automotive sectors.A number of potential drop-in replacement technologies have been studied.These may offer additional advantages eg. simplicity or increased processing speeds.Additional studies are required for full validation.