A New Sacrificial Corrosion Protection Mechanism A New Sacrificial Corrosion Protection Mechanism

for High Performance Zinc/Aluminum Flake Coating for High Performance Zinc/Aluminum Flake Coating

Systems and ApplicationsSystems and Applications

Shusaku IshikawaShusaku Ishikawa

Senior Technical Manager, EngineeringSenior Technical Manager, Engineering

Yuken America Inc.Yuken America Inc.

1

0 500 1000 1500 2000

Zn/Al Flake coating

Zinc Nickel + Trivalent Cr

Zinc Nickel

Zinc plating + Trivalent Cr

Zinc plating

SST Time to Red Rust [Hr]

Introduction

Zinc/Al Flake Coating with High Corrosion Protection Performance

� Popular in Automotive and Construction Industries

2

Introduction

Important to Form Basic Zinc Chloride (Insulator)

Zn Zn2+, OH－

ZnO

Zn（OH）6（CO3）2

4Zn（OH）2・ZnCl26Zn（OH）2・ZnCl2

H2O

O2

βZn（OH）2

Protective, insoluble

Protective, insoluble

Less protective, insoluble

Protective, insoluble

Minimize ZnO formation！

Stop !Stop !Stop !Stop !

3

Introduction

Corrosion Product on Coating Film

Relationship with Corrosion Resistance

New & Current Product Comparison

2400

1800

1100

90

80

30

0 500 1000 1500 2000 2500 3000

New Type

（2C2B）

Current Type

(2C2B ＋YC-T)

Current Type

(2C2B)

Time to Red Rust [ hr ]

0 20 40 60 80 100 120

Cycles to Red Rust [cy]

SST

CCT

Investigation:

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Experiment (Sample)

■Test Samples

Sample

Base Coat Top Coat

Main

Components

Thickness

[ μ ]

Main

Components

Thickness

[ μ ]

B-1 Zn, Al, Si 8 - 10 No topcoat -

B-1 + T Zn, Al, Si 8 - 10 Si, O 1 – 2

B-2 Zn, Al, Si 8 - 10 No topcoat -

Zn-Ni Alloy Zn, Ni (17%) 8 - 10 No topcoat -

Table 1. Summary of Zinc/Aluminum Flake Coating Systems

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Experiment (Process)

■Coating Process of Base Coat

DryTop

Coat

212oF (100oC)

15 min

Degrease Blast BakeBase

Coat

First coat (1C1B)

Base

Coat

Second coat (2C2B)

Bake

500oF (260oC)

30 min

■Top Coat Process

500oF (260oC)

30 min

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Experiment (Evaluation)

■Corrosion Protection Evaluation

1) CCT:

Salt spray (5% NaCl, 50℃) 17 hrs → dry (70℃) 3 hrs → salt spray (5% NaCl,

50℃) 2 hrs → natural dry (25℃) 2 hrs

2) SST: 5% NaCl, 35℃

3) Salt water immersion test: 5% NaCl, 25℃, exposed to the atmosphere

■Corrosion Product Evaluation

X-ray diffractometry (XRD)

■Surface Condition Evaluation

SEM-EDS

■Time-Dependent Changes in Natural Potentials

Natural potentials measured in 5% NaCl solution

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Results and Discussion (Corrosion Resistance)

- CCT -

Cycles

Sample0cy. 20cy. 40cy. 80cy. 120cy.

B-1

B-1 + T

B-2

0cy. 10cy. 20cy.

Zn-Ni

Red Rust

Red Rust

Red Rust

Fig. 5. CCT results

Corrosion Resistance: B-2 > B-1+T > B-1 > Zn-Ni

8

Results and Discussion (Corrosion Resistance)

- SST -

Hours

Sample0h 1000h 1500h 2000h 2500h

B-1

B-1 + T

B-2

0h 250h 500h

Zn-Ni

Red Rust

Red Rust

Red Rust

Fig. 7. SST results

Corrosion Resistance: B-2 > B-1+T > B-1 > Zn-Ni

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Results and Discussion (Corrosion Resistance)

- NaCl Immersion Test -

Hours

Sample0h 24h 240h 480h 720h

B-1

B-1 + T

B-2

Zn-Ni

Fig. Salt water immersion test results

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Results and Discussion (XRD NaCl Immersion Test)

Fig. 2. XRD Pattern of Each Film Before and After 24-Hour NaCl Immersion Test

0

500

1000

1500

2000

2500

3000

Inte

nsi

ty [

counts

]

0

500

1000

1500

2000

2500

3000

Inte

nsi

ty [

coun

ts]

0

500

1000

1500

2000

2500

3000

Inte

nsi

ty [

coun

ts]

0

500

1000

1500

2000

2500

3000

3500

10 20 30 40 50 60 70 80 90

2θ [deg.]

Inte

nsi

ty [

cou

nts

]

a) B-1

b) B-1+T

c) B-2

d) Zn-Ni

Zn

ZnO

Zn5(OH)8Cl2・H2O

Zn5(OH)6(CO3)2

Before

After 24-Hour

Before

After 24-Hour

Basic Zinc Chloride Was Mainly Formed in B-1, B-2 and Zn-Ni.

Almost no change

Significant

decrease

■

■

■

■

▼ ▼

▼ ▼▼ ▼

▼ ▼

◆

◆

◆

◆

●

●

11

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 24 72 240 480 720

Immersion Time [H]

Inte

nsi

ty o

f Z

n5(O

H)8

Cl2・

H2O

or

Zn

5(O

H)6

(CO

3)2

[co

un

ts]

B-1

B-1+T

B-2

Zn-Ni

Zn5(OH)6(CO3)2

Zn5(OH)8Cl2・H2O

0

500

1000

1500

2000

2500

3000

3500

0 24 72 240 480 720

Immersion Time [H]

Inte

nsi

ty o

f Z

n [

co

un

ts]

B-1

B-1+T

B-2

Zn-Ni

Results and Discussion (XRD Intensity Changes)

- NaCl Immersion Test -

Fig. 4. XRD Intensity Changes of Corrosion Product and Zinc (NaCl Immersion Test)

ZnZn5(OH)8Cl2・H2O / Zn5(OH)6(CO3)2

Basic Zinc Chloride Intensity Change:

B-2 > B-1 > B-1+T > Zn-Ni

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0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 5 10 20 30 40

CCT [Cycle]

Inte

nsi

ty o

f Z

n5(O

H)8

Cl2・

H2O

[co

un

ts]

B-1

B-1+T

B-2

Zn-Ni

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 5 10 20 30 40

CCT [Cycle]

Inte

nsi

ty o

f Z

n [

counts

]

B-1

B-1+T

B-2

Zn-Ni

Results and Discussion (XRD Intensity Changes)

- CCT -

Fig. 6. XRD Intensity Changes of Corrosion Product and Zinc (CCT)

ZnZn5(OH)8Cl2・H2O

ConsistentConsistentConsistentConsistent

Basic Zinc Chloride Intensity Change: B-2 > B-1 > B-1+T > Zn-Ni

13

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 24 72 240 480 720 1000 1500 2000

SST [Hour]

Inte

nsi

ty o

f Z

n [

cou

nts

]

B-1

B-1+T

B-2

Zn-Ni

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 24 72 240 480 720 1000 1500 2000

SST [Hour]

Inte

nsi

ty o

f Z

n5(O

H)8

Cl2・ H

2O

[co

un

ts]

B-1

B-1+T

B-2

Zn-Ni

Results and Discussion (XRD Intensity Changes)

- SST -

Fig. 8. XRD Intensity Changes of Corrosion Product and Zinc (SST)

ZnZn5(OH)8Cl2・H2O

As consistent as As consistent as As consistent as As consistent as in CCTin CCTin CCTin CCT

XRD Trends in Salt Water Immersion Test

■ Basic Zinc Chloride Formation on Each Film in Immersion Test

Amount

Speed

BB--11

BB--1+T1+T

BB--22

ZnZn--NiNi

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Results and Discussion (SEM Image)

Fig. 9. SEM Image of Each Film After 240-Hour SST

B-2B-1 B-1 + T

Granular Crystals of Basic

Zinc Chloride

Granular Crystal Ratio: B-1 > B-1+T > B-2

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-1.10

-1.00

-0.90

-0.80

-0.70

-0.60

-0.50

-0.40

0 5 10 15 20 25 30 35 40

Immersion Time [ day ]

Pote

ntial

[

V ]

B-1

B-1+T

B-2

Zn-Ni

Zn plating

Results and Discussion (Natural Potential)

Fig. 10. Time-Dependent Change in Natural Potential of Each Film

B-2 formed basic zinc

chloride at early stage.

Potential increased with

formation of basic zinc chloride.

Potential was stable with basic

zinc chloride.

Characteristic in between

Zn-Ni & Zn plating

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Basic Zinc Chloride

Base Metal (Fe)

Na+ Cl-

H2O H2OH2O

Cl-Na+

Results and Discussion (Pattern Diagram)

Fig. 11. Corrosion Protection Mechanism of Zn/Al Flake

Coating

Corrosion Reaction Controlled by Basic Zinc Chloride

2H2O + 2e- � 2OH- + H2

Base Metal (Fe)

Zn � Zn2+ + 2e-

Zn2e-

Zn2+

Na+ Cl-Basic Zinc ChlorideBasic Zinc Chloride

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Summary

■ Important Factors for Achieving High Corrosion Resistance

Formation of Formation of Formation of Formation of basic zinc chloridebasic zinc chloridebasic zinc chloridebasic zinc chloride

Control excessive Control excessive Control excessive Control excessive zinc dissolutionzinc dissolutionzinc dissolutionzinc dissolution

Finer crystalsFiner crystalsFiner crystalsFiner crystalsFilm with high Film with high Film with high Film with high

corrosion resistancecorrosion resistancecorrosion resistancecorrosion resistance

Formation of basic Formation of basic Formation of basic Formation of basic zinc chloride during zinc chloride during zinc chloride during zinc chloride during

initial stage of corrosioninitial stage of corrosioninitial stage of corrosioninitial stage of corrosion

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Thank you forThank you for your attention!your attention!