1Corrosion Phenomenon
Evaluation of Mg Alloys Using
Surface Potential Difference
Measured by SKPFM

2BackgroundMg : Lightest industrial materials with Poor corrosion
resistance caused by galvanic potential
What causes corrosion ?
Salt
Water

Volumetricloss

AZ91D

Focusing on interface between -Mg and dispersoids
Interface between
-Mg anddispersoids

 phase

Al-Mn
AZ91D

AZ91D

3

Objectives
Galvaniccorrosion

Galvanic potential, V

Metal A

Metal A

Metal B

Metal BSKPFMScanning Kelvin Prove
Force Microscope

Surface Potential
Difference

VSPDdispersoid
-Mg

Mg alloy

Galvanic corrosion was quantitatively evaluated bysurface potential difference VSPD.

Corrosion phenomenon analysis
Measurement ofsurface potential
Surface potential map
5wt% Salt water immersion test
Corrosion time : 18 hour
Solution temperature : 30 C

Salt waterimmersion test

What phases are corroded ?
Topographicchanges atinterface4

Macro-scale corrosion analysis

5Salt water immersion test
Experimental condition• Solution concentration• Solution temperature• Testing time
• Rotation rate of stirrer: 5 wt%: 30 C: 18 hours: 420 rpm
AZ91D

Surface finish
1. Waterproof abrasive paper in tap water up to #4000
2. Mirror finish by buffing using diamond paste

Micro-scale corrosion analysis
Surface potential measurement by SKPFM
Cantilever coatedwith PtIr5Sample

Surface finish
i. Diamond paste polishingii.Ultrasonic cleaning with ethanol
Work function Surface potential : VSP

VCPD = ( PtIr5  sample )/e=Contact potential difference : VCPD

VSPConstantVariable

VCPD is dependent on sample
How about relationship between standardelectrode potential VSEP and work function  ?

6Micro-scale corrosion analysis

7Work function1) φ (eV)Standard electrode potential – Work function

 = 2.27 + 0.341)
6  ; Pauling’s electronegativity2)

Au1) J.K.Schaeffere, et al.,Microelectronic
Engineering,84(2007)2196-2200
2) A.L. Allred, Electronegativity values fromthermochemical data, J. Inorg. Nucl.
Chem. 17 (1961) 219

5Fe43

NiCuAg3) M.M. Avedesian, H. Baker, Magnesium
and Magnesium Alloys, ASM
International, OH, 1999, pp.194

ZnAlMg-2.5-2-1.5-1-0.5

00.5

11.5

Standard electrode potential3) (V)2

VSP = VCPD
= (PtIr5  sample)/eStandard electrode potentialsmall  large

Work function small  large

Surface potential VSP(=VCPD)

large  small

Micro-scale corrosion analysis
Surface potential difference∆VSPD

Standard electrodepotential difference∆VSEP∆VSPD = VMg  Vmetal= (PtIr5  Mg )/e  (PtIr5  metal )/e= (metal  Mg )/e

ΔVSPD (V)3

∆VSEP = SEPmetal  SEPMg

TheoreticalExperimental

2∆VSPD  ∆VSEPAuNi

10CuAgFeAl0Zn

12

VSEP (V)348

Identification of dispersoids 1/2
A) Difficult to identify dispersoids.
B) Markings effective to detect the same

position before and after corrosion test.
Optical microscope image
attached to SKPFM

Easy to identify dispersoids
B) Three indentations by Vickers hardness
tester.
A)

Cantilever

Marking by Vickers hardness

9Identification of dispersoids 2/2
SEM image

SKPFM image
blurredLock on Target !

300 m300 mclearｘ400 m

400 mPosition of dispersoids
can be identified bythree indentations.

Cantilever is set tothe target position.

10Macro-scale corrosion analysis result
SEM image before and after corrosion
by salt water immersion test phaseBefore corrosion
Al-Mn

After corrosion11

Micro-scale corrosion analysis result
 phaseHH
GG

150 nm

150 nm0 nm

0 nm

Difference of height (nm)Before corrosion

After corrosion+5 – +20-89.69

12

Micro-scale corrosion analysis result
Al-Mn

FE
Left

FE

RightLeft

750 nm

750 nm0 nm

0 nm

Difference ofheight (nm)RightBefore corrosion

After corrosion

LeftRightLeft

Right+183.58+158.30+517.18+435.0413

Micro-scale corrosion analysis result
Topographic change before and after corrosion test phase

0 nm
150 nm0 nm

750 nm0 nm
750 nm

0 nm

Al-Mn

Height compared to -Mg (nm)

150 nm phaseAl-Mn Left side
Al-Mn Right side
550

450

350

250+334, 277(nm)Before15050-50-150-102 (nm)After14Surface potential difference at  phase

15

Surface potential map
SEM image

Surface potential difference (V)Point

Surface potential

Low  HighStandard electrodepotential

High  Low

A-B

C-D

E-F

G-H

+0.06

+0.06

+0.04

+0.05

+0.05

+0.06

+0.06

+0.06

Average : 0.055 (V)Surface potential difference at Al6Mn

16Surface potential map
SEM image

Surface potential difference (V)Point

Surface potential

Low  HighStandard electrodepotential

High  Low

A-B

C-D

E-F

G-H

-0.46

-0.53

-0.46

-0.45

-0.50

-0.50

-0.53

-0.53

Average : 0.50 (V)17Effect of strain on surface potential
Pure Mg
Indentation

6V8 isn’t affectedby strain.

V878800m

60m

120m

3451Vave3 < V8 < Vave3220m

80m

160m

Vaverage 3 =1.17~1.50
 : standard deviation

Measurement
point

123

4

5678

Maximum (V)

1.38

1.37

1.36

1.39

1.40

1.39

1.37

1.40

Minimum (V)

1.31

1.25

1.29

1.31

1.27

1.29

1.28

1.25

Average (V)

1.35

1.31

1.33

1.35

1.34

1.34

1.33

1.33

Transport of corrosion
 phase-Mg

After 98 hours corrosion test

Galvaniccorrosion

Oxidefilm

 phase is higher than -Mg .

1819Conclusions
SKPFM available for quantitative evaluationof corrosion phenomenon.
Good correlation between surface potential
difference and corrosion loss.

-Mg   phase-Mg  Al6Mn

Right
Leftsurface
potentialdifference (V)before corrosion

after corrosion

0.0 - 0.1

5 - 2089.69

158.30517.18183.58435.04

0.4 - 0.6

Difference of height (nm)