1

L. ARURAULT

IHAA 2016 – 16th Technical Symposium

Düsseldorf, October 5-7, 2016

Innovative Cr-free

anodizing and sealing process

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Toulouse

France

University of Toulouse III – Paul Sabatier

CIRIMAT Laboratory

Preparation and Characterizations of Advanced Materials

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3

1. Introduction

2. Alternative anodizing processes

3. Alternative sealing processes

4. Conclusion and prospects

Innovative Cr-free

anodizing and sealing process

Outline

4

1. Introduction

Anodizings

H2O and chromate sealings

2. Alternative anodizing processes

3. Alternative sealing processes

4. Conclusion and prospects

Innovative Cr-free

anodizing and sealing process

Outline

5

1. Introduction

• Anodizing

• Sealing(unpainted parts)

sulfuric anodizing (types II, III)alternative anodizings

• Pretreatments

soft acid/alcaline etching

hot H2O sealingalternative sealings

with green additives

organic solvant

sulfo-chromic etching

chromic anodizing(type I)

hot H2O sealinghexavalent chromium

Past Future

DegreasingEtching

Surface treatment line

Registration, Evaluation, Authorisation

and Restriction of Chemicals (REACH)

• paintingorganic solventhexavalent chromium

aqueous solventgreen inhibitors

aqueous degreasing

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1. Introduction - anodizing

Anodizing of aluminium alloys

Well-established process

discovered in 1855 by H. Buff

developed in 1911 by De Saint Martin

first CAA and OAA industrial use (1923) ; first SAA patent (1927)

At the anode

Al Al3+ + 3e−

2Al3+ + 3O2- Al2O3

2Al + 3O2- 6e− + Al2O3 Eq. 1global mechanism, still open for discussion

Chromic acid anodizing (type I)low thickness (1 - 8µm)

Incorporation of chromate ions healing power

Interfacial charges, oxygen vacancies, cathodic sites

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1. Introduction – H20 Sealing

Hydrothermal sealing Al2O3 + H2O → 2AlO(OH) Eq. 2

A. Pernot-Géhin, PhD, 2007

Typical « sand roses » top aspect

Usually superficial sealing (1-3 µm)

K. Giffard, PhD, 2015

AA 2024T3, SAA, H2O sealing AA 7175, SAA, H2O sealing

100nm

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1. Introduction – chromate sealing

(di)chromate sealing K2Cr2O7 additive

hexavalent chromium : corrosion inhibitor → absorption in the film

2 CrO42− + 2 H+ ⇌ Cr2O7

2− + H2O

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1. Introduction – chromate sealing

(di)chromate sealing

Al2O3 + H2O → 2AlO(OH) Eq. 2

Al2O3 + 2HCrO4- + H2O → 2Al(OH)CrO4 + 2OH- Eq. 3

Al2O3+ HCrO4- → (AlO)2CrO4 + OH- Eq. 4

Competition between reactions 2, 3, 4

at low chromate content (20-200mg/L), higher boehmite content

at high chromate content (50-100g/L), lower boehmite content

at low pH, more hydroxychromate Al(OH)CrO4

at high pH, more oxychromate (AlO)2CrO4

Cr2O72− + 14 H+ + 6 e− → 2 Cr3+ + 7 H2O Eq. 5

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1. Introduction – chromate sealing

(di)chromate sealing

Atypical top aspect

Similar to the raw porous anodic film

High resistance against corrosion

Incorporation of chromate ions

healing power

A. Pernot-Géhin, PhD, 2007

AA 7175, SAA, K2Cr2O7 sealing

100nm100nm

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1. Introduction – Cr (VI) process

2. Alternative anodizing processes

Substrate

Electrolyte

Input electric signal

3. Alternative sealing processes

4. Conclusion and prospects

Outline

12

The characteristics of the porous anodic films depend on :

Operationnal electrical parameters

(current, voltage, time)

Electrolyte(composition, temperature)

Aluminium substrate(composition, alloying elements, surface)

2. Alternative anodizings

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2. Alternative anodizings – Substrate

AA 2XXX : Matrix and dispersed intermetallic phasesHigh copper constituents (Al-Cu, Al-Cu-Fe, Al-Cu-Mg)

→ high mechanical properties→ galvanic corrosive attacks

→ critical disturbance during the anodizing

→ oxygen evolution ; entrapment of unoxidized metal particles→ influence on porosity

• on AA 1XXX : straight pores, perpendicular to the substrate• on AA 2XXX : spongious porosity

AA 1050, SAA AA 2024T3, SAA

K. Giffard, PhD, 2015

AA

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14

F. Snogan et al, Surf & Coat Techno, 2002

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2. Alternative anodizings – Substrate

Rolled AA 2024

Limits : - Image resolution

- pore identification- porosity representativeness

Rolled AA 1050

AA 1050, SAA AA 2024T3, SAA

Average pore diameter (nm) 10 ± 4 8 ± 2

Pore density (cm-2) 1.8 1011 2.5 1011

Porosity (%) 15 ± 3 15 ± 3

0

5

10

15

20

25

6 7 8 9 10 11 12 13 14 15 16 17 18

Fré

qu

en

ce

(%

)

Diamètre de pore (nm)

0

5

10

15

20

25

4 5 6 7 8 9 10 11 12 13 14

Fré

qu

en

ce

(%

)

Diamètre de pore (nm)

Porosity :- Influenced by the substrate

- Difficult evaluation using FEG-SEM

Pore diameter (nm) Pore diameter (nm)

Fre

qu

en

cy

(%)

Fre

qu

en

cy

(%)

K. Giffard, PhD, 2015

AA 1050, SAA AA 2024T3, SAA

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2. Alternative anodizings – Substrate

AA 1050, SAA AA 2024T3, SAA

(S1 ± 0,02).10-2 (m2.cm-2) 2.74 4.06

Pore density (/cm2) 1.8 1011 2.5 1011

SFEG .10-2 (m2.cm-2) 2.86 2.83

Tortuosity (L/L°) 0.96 1.43

10502024

Tortuosity

→ influenced by the susbtrate

→ influence on the sealing

K. Giffard, PhD, 2015

L°

L

L

L°

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2. Alternative anodizings – Electrolyte

Usual electrolytes

Conventional sulfuric acid (type II)

TFSAA (type IIB)

Hard anodizing (type III)

Boric/sulfuric acids (Boeing)

Tartaric/sulfuric acids (TSA-Airbus)

Phosphoric acid (bonding applications)

C. Boisier et al, Surf & Coat Techno, 2009

AA 2024T3, TSA

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2. Alternative anodizings – Electrolyte

Unusual electrolytesOrganic electrolytes → different acid dissociation constants (pKa)

Cyclic oxocarbonic acids (squaric, croconic, rhodizonic acids)

Carboxylic acids (oxalic, malonic, citric, malic, acetlenedicarboxylic, tartaric,

tartronic, glycolic, formic acids) → Hydrophobicity

D. Nakajima et al , Applied Surf Sc, 2014

S. Akiya et al, Electrochem Acta, 2016

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Direct input electrical signals- Direct voltage

- Direct current (DC)

with or without initial ramp

2. Alternative anodizings – Current

Other input electric signals- Multistep direct current (MSDC)

- Pulsed current (PC)

- Combinations (e.g. DC & PC)

Recovery effect

→ better heat dissipation

→ restoring the electrical double-layer

→ to slow down and homogenize the oxide growth

→ removal of the oxygen developed on some microprecipitates

→ Improvement of the roughness and the mechanical properties

Cu

rren

t

Time

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2. Alternative anodizings – Current

Hard anodizingH2SO4 (190g/L), Al3+ (8g/L)

T = -2°C2.35 A/dm2

a) Direct current (DC)

b) Multistep direct current (MSDC)

c) Pulsed current (PC)

d) DC & PC combination

M. Bononi et al, Surf & Coat Techno, 2016

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2. Alternative anodizings – Current

- Direct current (DC)

→ good hardness (~ 330Hv)

→ defects (adhesion and corrosion resistance)

- Multistep direct current (MSDC)

→ significant but low improvement

- Pulsed current (PC)

→ hardness decay (~ 280Hv)

- DC & PC combination

→ good hardness (~ 340Hv)

→ defect free interfaces

M. Bononi et al, Surf & Coat Techno, 2016

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1. Introduction – Cr (VI) process

2. Alternative anodizing processes

3. Alternative sealing processes

Hot H2O sealing

Hot and cold H2O sealings with additives

Innovative additives

Combined sealings

4. Conclusion and prospects

Outline

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• Morphology : porosity and tortuosity

• Chemical composition

• Surface charges

• wettability-capillarity

1. Understanding the reactionmechanisms tuning :

- the solvent penetration

- the additives interactions and

incorporation

Characteristics of

the raw porous anodic film

2. Understanding the reactionmechanisms

Characteristics of

the sealed anodic film

3. Explaining the properties

Final properties in use

• Morphology

• Chemical composition

• Surface charges

• Corrosion

• Fatigue

• Fatigue-corrosion

+

+

++

+

+

+

+++

+

+

+

+

+

+

+

+

+++

++ +

+

+++

+

-

-

-

--

-

--

-

--

-

-

-

-

-

-

--

-

-

H2OCations

++ + +

+

+

+

+

+

+

+

++

+

Cl-

3. Alternative sealings

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3. Alternative sealings – hot H2O sealing

Main critical parameters :

Temperature

T > 80°C Al2O3 + H2O → 2AlO(OH) Eq. 2

T < 80°C Al2O3 + 3H2O → 2Al(OH)3 Eq. 6

Water purity

High quality water

without Ca2+ → powdering

Cl- → corrosion

Cu2+ (10ppm), Fe2+ (< 10ppm),

SiO3- (<10ppm), PO4

3- (< 5ppm)

→ sealing impenders

Bath pH

4 < pH < 8

(boehmite insolubility zone)

AlOOH

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3. Alternative sealings – hot H2O sealing

Main critical parameters :

Chemical composition of the anodic film

Inclusion of the anions (SO42-, PO4

3-) from the anodizing bath

Different superficial charges (mainly positives)

Different hydration levels

< 3wt% after phosphoric anodizing

10-20wt% after sulfuric anodizing

→ very difficult sealing of phoshoric anodic films

→ self-sealing of unsealed sulfuric anodic films

Al2O3 + H2O → 2AlO(OH) Eq. 2

Al2(SO4)3 + 6H2O → 2Al(OH)3 + 3H2SO4 Eq. 7

Al-O-SO3H + H2O → Al-OH + H2SO4 Eq. 8

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3. Alternative sealings – hot H2O sealing

unsealed sealed

Sulfu

ric

film

on

202

4T3

Sulfu

ric

film

on

105

0

Main critical parameters :

Porosity of the anodic film

- « sand roses »

superficial aspect

- Whole sealing on 1050

- Superficial sealing (1µm)

on 2024T3

Sealing depth depends on :

- the porosity/tortuosity

- the chemical composition and superficial charges

K. Giffard, PhD, 2015 A. Bautista et al, Surf & Coat Techno, 2002

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3. Alternative sealings – with additives

hot H2O sealing with additivesAl2O3 + H2O → 2AlO(OH) Eq. 2

interactions with the film → formation of boehmite

Ni2+ + 2OH- → Ni(OH)2 Eq. 9

additional precipitation

Ni(OH)2 catalyzes the boehmite formation

- « sand roses »superficial aspect

- superficial sealing (1µm)

- anion influence(acetate, sulfate, nitrate…)

F. Snogan et al, Surf & Coat Techno, 2002

AA 2214, SAA, Ni & Co sealing

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3. Alternative sealings – with additives

cold H2O sealing with additivesusually 30-40°C ; 5.5 < pH < 6.5

Al2O3 + H2O → 2AlO(OH) Eq. 2

Al2O3 + 3H2O → 2Al(OH)3 Eq. 6

Al2O3 + 6F- + 3H2O → 2AlF3 + 6OH- Eq. 10

additional precipitation of fluorided compounds

Innovative sealings with organic additivestriethanolamine (TEA)

→ catalyst for boehmite formation

carboxylic acids CH3(CH2)n-2COOH (fatty acids)

organic compounds (e.g. esters of polyethylene glycols)

→ hydrophobic superficial film

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3. Alternative sealings – with additives

Innovative sealings with mineral additives- Trivalent chromium

- Mo, W ions

- Li cations

- Light rare earth salts (e.g. Y, La, Ce, Sm)

→ additional precipitation, inhibitor behavior

→ usually superficial sealings

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3. Alternative sealings – Combined

Supplementary additives (in anodizing bath or sealing bath)

H2O2, permanganate → additional oxidation

Fluoride → additional precipitation

Innovative combined sealingsMultistep sealing

first step : impregnation → all along the pores

second step : final precipitation

compact top-coating

control of the surface charges

B. Priet et al., Surf & Coat Techno, 2016

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4. Conclusion and prospects

Alternative anodizing processes

Anodizing electrolytesSulfuric acid baths

Mixed acid electrolytes

→ multiproperties

Organic electrolytes

→ multiple dissociation constants

Input electric signals- multistep direct current (MSDC)

- pulsed current (PC)

- combinations (e.g. DC & PC)

→ recovery effect

sulfuric acid baths

bath stability and control

incorporation of organic ions

coating thermal stability

more difficult to perform

less defects

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4. Conclusion and prospects

hot H2O sealing

Typical « sand roses » top aspect

Usually superficial sealing

(di)chromate sealing

atypical top aspect

high resistance against corrosion healing power

Strategies for alternative sealing processesuse of organic additives

→ hydrophobic superficial film

use of oxidizing compounds

→ additional oxidation

use of fluoride ions

→ additional precipitation

combined mutistep sealing

→ all along the pores

coating thermal stability

bath stability and control

additional step

promising properties

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K. Giffard, Ch. Blanc, G. Odemer, V. Turq

Thank you for your attention

Laurent Arurault

CIRIMAT – Carnot Institute

University of Toulouse III - UPS

Bâtiment CIRIMAT, 118 route de Narbonne,

31062 Toulouse Cedex 9, France

Phone : 33.561.556.148.

Fax : 33.561.556.163.

arurault@chimie.ups-tlse.fr

K. Giffard, L. Arurault, C. Blanc, Dynamic measurements and wettability phenomena in mesoporous anodic films prepared on 1050 and 2024T3 aluminium alloys, Microporous and Mesoporous Materials, 235 (2016) 32 -41.

B. Priet, G. Odemer, C. Blanc, K. Giffard, L. Arurault, Effect of new sealing treatments on corrosion fatigue lifetime of anodized 2024 aluminium alloy, Surface & Coatings Technology, 307 (2016) 206-219.