1. The Influence of Cooling Rate on
the Corrosion Properties of Mg
ZEK100 Alloy via Wedge Casting
W. Jeffrey Binns, Aaron Kirkey, D.W. Shoesmith
NACE Student Symposium
July 6 2015
2. 2
ZEK100 Mg Alloys Have Useful Properties
Pros:
Cons:
• Excellent strength-to-weight ratio
• Good low temperature castability
due to rare earth additions
• Alloying elements create multiple
phases
• Mg alloys experience galvanic
coupling which accelerates
corrosion
3. 3
Effect of Casting Procedure on Corrosion
• Al distribution was most uniform in the
order: die cast > graphite cast > sand cast
• Al network collects corrosion product while
Mg matrix experiences dissolution
• Average corrosion penetration depth
increases in the order: die cast < graphite
cast < sand cast.
20 µm
50 µm
Sand Cast
Graphite Cast
Die Cast Al
50 µm50 µm
50 µm 50 µm 50 µm
20 µm 20 µm
SEM - Polished Al Elemental Map Depth Profile
RM Asmussen; WJ Binns; P Jakupi; D Shoesmith, JES, (2014) 161 (10), C501-C508.
Slow Cooling Rate
Fast Cooling Rate
4. 4
Corrosion Depends on Microstructure Distribution
• Volume fraction of α-Mg and β-Mg17Al12 is the same
independent of casting method
• Corrosion processes are controlled by more than just elemental
composition
• Size, shape and distribution of secondary phases is also
important
Volume Fraction of AM50 Phases
3_AM50_SandCast_full2hrs.raw_1
2Th Degrees
585654525048464442403836343230
Counts
4,600
4,400
4,200
4,000
3,800
3,600
3,400
3,200
3,000
2,800
2,600
2,400
2,200
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
0
Mg 91.67 %
Mg17Al12 8.33 %
2_AM50_GraphiteCast_full2hrs.raw_1
2Th Degrees
585654525048464442403836343230
Counts
4,000
3,900
3,800
3,700
3,600
3,500
3,400
3,300
3,200
3,100
3,000
2,900
2,800
2,700
2,600
2,500
2,400
2,300
2,200
2,100
2,000
1,900
1,800
1,700
1,600
1,500
1,400
1,300
1,200
1,100
1,000
900
800
700
600
500
400
300
200
100
Mg 91.39 %
Mg17Al12 8.61 %
1_AM50_DieCast_full2hrs.raw
2Th Degrees
585654525048464442403836343230
Counts
4,400
4,200
4,000
3,800
3,600
3,400
3,200
3,000
2,800
2,600
2,400
2,200
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
0
Mg 91.15 %
Mg17Al12 8.85 %
Sand cast
Graphite cast
Die cast
Danaie, Mohsen; The Cathodic Behaviour of AlMn Intermetallic Particles in Mg AM50 Alloy, Montreal, QC, (2013).
5. 5
Wedge Casting to Tailor Microstructure Size & Shape
Experimental
• 60 kg bricks were melted in a resistance furnace using a mild steel crucible under the protection of CO2 + 0.5% SF6 gas mixture.
• Time-temperature cycle was the same for all melts.
• After removal of any oxide films the castings were poured at 730 °C in a water-cooled copper mold. To generate microstructures
under different solidification rates (with varying section thickness).
• The cooling rate during solidification was measured using five or six thermocouples, positioned along the centerline of the wedge
casts
3.5”
7.0”
¼”
½”
1”
2”
Thin - 1
2
3
4
5
Thick - 6
Thermocouple Placements
6. 6
Materials Preparation
• test our findings i.e., microstructure distribution effect, on an individual wedge cast
• determine which processing (i.e., cooling, crystallization) rate parameters optimize corrosion resistance
0.5 cm
1.0 cm
Section - A
Section - M
Wedge
Section
%Al %Zn %Mn %Fe %Nd %Zr
A 0.02 1.31 0.02 < 0.01 0.17 0.40
D 0.02 1.38 0.02 < 0.01 0.19 0.37
F 0.01 1.32 0.02 < 0.01 0.15 0.36
L 0.02 1.20 0.02 < 0.01 0.15 0.40
7. 7
Computer Modelling Confirms Gradient in Cooling Rate
t = 2.5 s t = 5.0 s t = 7.5 s
t = 10.0 s t = 20.0 st = 15.0 s
• Modelling constructed using data from 6
thermocouples placed along the center line
• Columnar dendritic crystallization
properties are influenced by cooling rate
M. Farrokhnejad Numerical Modeling of Solidification Process and Prediction of Mechanical Properties in Magnesium Alloys. PhD. Thesis, University of Western Ontarion, London, ON, 2013
8. 8
Cooling Rate Effects Microstructure Size and Distribution
A D F M
Zn Zn Zn Zn
Zr Zr Zr Zr
• Distinct segregation in alloying elements for different solidification rates; i.e., Zr vs. Nd
• Larger secondary phases produced at slower solidification rates
Mg7Zn3RE
T-phase
Zr IMP
α-Mg
100 µm 100 µm 100 µm 100 µm
9. 9
ECORR is Similar for Different Cooling Rates
0 2 4 6 8 10 12 14 16 18 20 22 24
-1.95
-1.90
-1.85
-1.80
-1.75
-1.70
-1.65
-1.60
-1.55
-1.50
E(VvsSCE)
Time (h)
ZEK100 WC A
ZEK100 WC D
ZEK100 WC G
ZEK100 WC M
• 1.6 wt% NaCl solution
• Length of time required to reach a steady state ECORR
differs depending on cooling rate
• Steady state ECORR is consistent independent of cooling
rate
10. 10
Corrosion Morphology is Similar for Different Cooling Rates
100 µm
1000 µm
A D G M
A D G M
1000 µm1000 µm1000 µm
100 µm100 µm100 µm
• Corroded in 1.6 wt% NaCl solution for 24 h
11. 11
SEM Indicates Galvanic Attack
A D
G M
Corrosion Product Domes
Zr-rich Intermetallic Particles
Ruptured Corrosion Product Domes
• Corroded in 1.6 wt% NaCl solution
for 24 h
12. 12
Multiple Damage Morphologies Exist
With Corrosion Product Without Corrosion Product
• Corrosion product removed with 0.2 M chromic acid solution
• Damage appears in pit-like formations within grain body
14. 14
Damage Occurs Adjacent to T-phase
• FIB cut on section A following immersion for 24 h in 1.6 wt% NaCl solution
• Damage occurs next to T-phase particles which segregate in grain boundaries
15. 15
Possible Mechanism of Corrosion
W.C. Neil; M. Forsyth; P.C. Howlett; C.R. Hutchinson; B.R.W. Hinton, Corros. Sci., (2009) 51, 387-394.
16. 16
What About the Tracking Corrosion?
0 4 8 12 16 20 24
-2.0
-1.9
-1.8
-1.7
-1.6
-1.5
-1.4
E(VvsSCE)
Time (h)
MgZEK100 WC A epoxy
MgZEK100 WC D epoxy
MgZEK100 WC F epoxy
MgZEK100 WC B no epoxy
MgZEK100 WC M no epoxy
• Corroded in 0.16 wt% NaCl
ECORR of Wedge Cast MgZEK100 Alloy No Epoxy Epoxy
B
FM
D
17. 17
Summary and Conclusions
• Wedge casting produced an Mg ZEK100 alloy with large differences in grain
size as well as the size and distribution of secondary T-phase particles
• Size and distribution of Zr-rich intermetallic particles was unaffected by the
gradient in the cooling rate of the wedge
• Corrosion damage occurred next to T-phase particles which segregate into
grain boundaries as well as in pit-like features within the grain body.
• Deep, tracking corrosion was only observed in samples cast in epoxy and
may be the result of crevice formation between the alloy and the epoxy
