The Influence of Cooling Rate on the Corrosion Properties of Mg

ZEK100 Alloy via Wedge Casting

W. Jeffrey Binns, Aaron Kirkey, D.W. ShoesmithNACE Student Symposium

July 6 2015

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

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 µm

50 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

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 Degrees585654525048464442403836343230

Cou

nts

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 Degrees585654525048464442403836343230

Cou

nts

4,0003,9003,8003,7003,6003,5003,4003,3003,2003,1003,0002,9002,8002,7002,6002,5002,4002,3002,2002,1002,0001,9001,8001,7001,6001,5001,4001,3001,2001,1001,000

900800700600500400300200100

Mg 91.39 %Mg17Al12 8.61 %

1_AM50_DieCast_full2hrs.raw

2Th Degrees585654525048464442403836343230

Cou

nts

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

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 - 12

3

4

5

Thick - 6

Thermocouple Placements

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 0.40D 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

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

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-Mg100 µm 100 µm 100 µm 100 µm

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 (V

vs

SC

E)

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

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

SEM Indicates Galvanic AttackA D

G M

Corrosion Product Domes

Zr-rich Intermetallic ParticlesRuptured Corrosion Product Domes

• Corroded in 1.6 wt% NaCl solution for 24 h

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

13

EDX Reveals Fe Enrichment Under Domes

Nd

Zn Fe Zr

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

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

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 (V

vs

SC

E)

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

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

18

Acknowledgements

• Dr. R. Matthew Asmussen

• Dr. Pelumb Jakupi

• Dr. Dave Shoesmith