Department of Mechanical & Industrial Engineering

Graduate Program in Mechanical Engineering

Ryerson University

Prepared For: Prof Dr. Ravi Ravindran

Prepared By: Md Matiur Rahman

Student No.: 500323811

Literature Review On Corrosion of Magnesium Alloys

ME8109: Casting and Solidification of Materials

Literature Review On Corrosion of Magnesium Alloys

Project Presentation Outlines

1. Introduction

2. Objectives

3. Type of Corrosion

4. Influencing Factors of Corrosion

5. Application and Use

6. Recommendations

7. Conclusion

Literature Review On Corrosion of Magnesium Alloys

• Introduction • Types of Corrosion• Influencing Factors• Conclusion

1. Introduction

Magnesium is the 6th most abundant element on the Earth’s surface,

Mg is also the 3rd most plentiful element dissolved in seawater,

Good electromagnetic interference shielding,

Excellent sound damping capabilities,

Have excellent specific strength,

An extremely light metal,

Good castability,

Hot formability,

Excellent machinability,

Recyclable

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

2. Objectives

To review and analysis the different type of corrosion on Magnesium and Mg Alloys,

Review and analysis of influencing factors that affect of corrosion on Mg Alloys,

To review the application and use of Mg alloys,

Recommendations how to protect Mg alloys from corrosion.

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Properties of Pure Magnesium

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Source: K.U. Kainer (2003), Magnesium Alloys and Technology, Wiley-VCH Verlag GmbH & Co, Germany.

Light Weight

30% lighter than aluminum (Al),

75% lighter than zinc (Zn),

70% lighter than steel (Fe),

Magnesium has the highest strength-to-weight ratio [σ/ρ]

Mmagnesium alloys used commercially for die castings:

Magnesium Series and Alloying Elements

1. AZ series : Mg–Al–Zn–Mn,

2. AM series: Mg–Al–Mn,

3. AS series: Mg–Al–Si,

4. AE series: Mg–Alrare earth,

5. QE Series: Ag, Rare earths

6. WE Series: Y, Rare earths

Abbreviations :

Al=Aluminum,

Cu=Copper,

ER=Rare Earths,

T=Thorium,

Zr=Zirconium,

Li=Lithium,

Mg=Magnesium

Mn=Manganese,

S=Silver,

Si=Silicon,

Tn=Tin,

W=Tttrium,

Zn=Zinc

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Table 1:General effect of Alloying Elements in Mg alloys (Werner and Hen, 2005).

Table 2: Nominal composition of selected Cast Mg alloys (Werner and Hen, 2005).

Table 3: Nominal composition of selected Wrought Mg Alloys (Werner and Hen, 2005).

AZ91D

Al Zn Mn Si Ni Cu Fe Mg

AZ91 9 1 Remainder

AZ91D 9.0 0.79 0.23 0.02 0.0007 0.0027 0.0014 Remainder

AM50 5.0 0.013 0.28 0.016 0.004 0.0016 0.0008 Remainder

WE43 : 4% Yttrium, 3.3% RE (Rare earth), 0.55 Zirconium, Remainder Magnesium

AE41: 4% Zn, 1.7% Rare Earth, 0.6% Zirconium, Remainder Magnesium

Table 4: Composition of magnesium alloys for die-cast magnesium alloys,

(Martin Jonsson, 2007).

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure1: Microstructure of as-cast AZ91 magnesium alloy.

(Jianqiu Wang et al., 2008).

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

1. Galvanic Corrosion

Galvanic corrosion is an electro-mechanical procedure

One metal Magnesium (Mg) corrodes preferentially to another Iron (Fe)

Mg and Fe are in electrical contact and submerged in an electrolyte.

Figure 2: The

Galvanic

Circuit (David

Tawil, 2004)

3. Type Of Corrosion

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure 3: Example of the Galvanic Corrosion of Mg Alloy (David Tawil, 2004)

Magnesium alloys are highly at risk to galvanic corrosion

Ni, Fe and Cu, compose efficient cathodes for magnesium and cause rigorous galvanic corrosion

1. Galvanic Corrosion

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure 4 : The Galvanic Corrosion test cylinder after

ASTMB117 salt fog testing (David Tawil, 2004)

1. Galvanic Corrosion

Corrosion test cylinder after ASTMB117 salt fog testing

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

3. Type Of Corrosion 2. Pitting Corrosion Pitting corrosion is localized corrosion that creates a small holes in the metal.

Magnesium is a naturally passive metal.

Pitting corrosion will occur at free corrosion potential of magnesium, when

exposed to chloride ions in a non-oxidizing medium.

For example, the as-extruded magnesium alloy AM60 was immersed in natural

3.5% NaCl solution, and the corrosion pits occurred on the surrounding of

AlMn particles

Anodic reaction:

Mg → Mg2++2e

Cathodic reaction:

2 H2O+2e → 2 H2↑+2 OH-

Total reaction:

Mg2++2 H2O =Mg(OH)2+2H2 ↑

Figure 5: Scheme of pitting corrosion

mechanism for magnesium alloy AM60

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure 6: Pitting corrosion, small hole on surface of Mg Alloys

(ASM Handbook, 1987)

3. Type of Corrosion

2. Pitting Corrosion Pitting is a localized form of corrosive attack. Pitting corrosion is typified

by the formation of holes or pits on the metal surface

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

3. Type Of Corrosion 3. Intergranular Corrosion

•Localized attack at grain boundaries, with relatively little corrosion of the

grains. As corrosion proceeds, the grains fall out and the metal or alloy

disintegrates.

•Intergranular corrosion is a selective attack of a metal at or adjacent to grain

boundaries.

•Inter-granular corrosion (IGC) take places at the grain boundaries due to the

precipitation of secondary phase.

•Recent studies, demonstrate that inter-granular corrosion can occur on

magnesium alloys.

Figure 7: Intergranular corrosion morphology of AZ80-T5 in 3.5%NaCI aqueous solution after 1

hour (Don et al., 1997)

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

3. Type Of Corrosion 4. Filiform Corrosion

•Filiform Corrosion. This type of corrosion occurs under painted or plated

surfaces when moisture permeates the coating.

•Filiform corrosion is caused by active galvanic cells across the metal surface.

•Filiform corrosion often occurs on the metals surface such as steel, Al alloys

and Mg alloys.

•It is typically associated with metal surfaces having an applied protective

coating. It does not occur on bare pure Mg

Figure 8 : Filiform Corrosion (WebCorr Corrosion Consulting Service, 2012)

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

3. Type of Corrosion

5. Crevice Corrosion

•Crevice corrosion is the localized attack on a metal surface at, or immediately

adjacent to, the gap or crevice between two joining surface. It is corrosion of

metals in the metal-to-metal joint or metal-to--Non-metallic material.

•This type of attack is usually associated with small volumes of stagnant

solution caused by holes, gasket surfaces, lap joints, surface deposits, and

crevices under bolt and rivet heads.

•The major factors influencing crevice corrosion are : Crevice type, Crevice

geometry, materials and environment (pH, Temp, halide ions & oxygen)

•The formation of Mg hydroxide should influence the properties of the

interface between the Mg and the solution in the crevice. Figure 9 :

Crevice

corrosion

(Corrosion

Clinic, 2012,

David

Pascoe, 2011

& Insa Lyon )

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure 10: Corrosion rates (1 mpy = 25 μm/year) of rapidly solidified magnesium alloys tested in 3%

NaCl at 21 ˚C compared with some commercial cast alloys (Edward Ghali, 2010).

Figure 11: Schematic illustration of corrosion process at the surface of AXJ530 ingot

specimen: (a) before corrosion attack and (b) after corrosion attck (Edward Ghali, 2010).

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Corrosion Comparison of some Mg-Alloys and Al-Alloys

Figure 12: Corrosion Comparison of some Mg-Alloys and Al-Alloys (David Tawil, 2004)

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure 13: Salt Fog Corrosion Improvement AZ91C vs. AZ91E , Casting approx.

24”X16”X4”, Tested 10 days to ASTM B117 (Wellman Dynamics & David Tawil, 2004)

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Corrosion Fatigue

•Corrosion Fatigue : It is sequential stages of metal damage that evolve accumulated load cycling up to plastic deformation of metals and alloys.

•Corrosion fatigue, is the mechanical degradation of a material under the joint action of corrosion and cyclic loading.

•Failure of a part when it is exposed simultaneously to corrosive attack and cyclic stresses is called corrosion fatigue.

•This type corrosion seems to begin at localized areas on the metal surface when the protective film on the part is reftured by the push - pull bending induced by cyclic stresses.

•The corrosive agent attacks the vulnerable localized area, causing a fissure ( corrosion fit ) to form. The pit continues to deepen until the part is so weakened that ultimate in cracking occurs the part breaks.

Figure 14: Richard P.

Gangloff, Envirionmental

Cracking- Corrosion Fatigue

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Figure: Multiple array of corrosion fatigue cracks

Figure 16: Multiple array of corrosion fatigue cracks

(Barry Dooley et al., 2009),

Figure 15 : Effect of a corrosion

environment on stress vs. cycles to

fatigue failure.

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

4. Influencing Factors (A) Metallurgical Influence

1. Alloying Elements • The major alloying elements are Al, Zn, Mn and so on. Fe, Co, Ni and

Cu are detrimental for the corrosion of magnesium alloys.

• Corrosion resistance improves with the Al content. For example, the corrosion rate of AZ91, AZ61 and AZ31 in 5%NaCl solution increased with the decrease of Al content.

• Mn can improve the corrosion resistance of magnesium alloys; but this is not always the case. The corrosion rate of magnesium alloys is related to iron content and Fe/Mn ratio.

2. Microstructure and Grain Size • The rapid solidification process can refine the microstructure which

beneficial to the corrosion properties.

• It can change the mechanism of corrosion; turning pitting corrosion of Mg-A1 magnesium alloys into overall corrosion.

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

4. Influencing Factors

(B) Effect of Post Processing

(1). Heat Treatment Heat treatment can change the microstructure of magnesium alloys.

Heat treated Mg alloy exhibited high corrosion resistance, but their corrosion rates increased with increasing the heat treatment temperature.

(2). Effect of Welding The high welding speed and fast cooling rate of the welds could improve the

corrosion resistance of the weld zone connected with the same material because of its fine grain sizes and of solid solutions with higher aluminum content.

The corrosion tendency of rapidly solidified welds, as in the case of high power laser welding, is relatively low.

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

4. Influencing Factors (C ) Environmental Influence

Environmental Temperature and Humidity Corrosion of magnesium alloys increases with the increase of relative

humidity (RH). At 9.5% RH, neither pure magnesium nor any of its alloys exhibit evidence of

surface corrosion after 18 months. At 30% humidity, only minor corrosion may occur. At 80% humidity, the surface may exhibit considerable corrosion. In marine atmospheres heavily loaded with salt spray, magnesium alloys

require protection for prolonged survival. SCC (Stress Corrosion Cracking) is affected by elevated environmental

temperature and relative humidity. The' susceptibility of SCC increases with the raised temperature. Creep deformation can improve SCC resistance. High humidity accelerates

SCC during atmospheric exposure. The fatigue life is reduced, and the FCP (Fatigue Crack Propagation) rate is

enhanced by the increasing environmental temperature and relative humidity (RH). The FCP rates of Mg alloys increased remarkably with the increase of RH in air.

Results demonstrated very high sensitivity of magnesium alloys to humidity.

5. Application and Use

•Aerospace

• Aircraft

• Automobiles

• Electronics, Computer Hardware and IT

• Employed in Nuclear Energy

• Employed in Industrial Equipments

• etc.

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Reduction gears on Pratt & Whitney Canada PT6 gas turbine engine (Pratt &

Whitney, 2010)

Power Tools

Aircraft components Sony video camera

iPhone

Motorcycle wheels

Milatary equipment (http://www.rccomponents.com)

Application and Use

Computer Part

Sand Blasting and coating of Magnesium Alloys as well as all metal surfaces as

soon as possible

Taking necessary steps to get existing Cathodic Protection systems back in full

operation

Adding supplemental Cathodic Protection for quoin area protection

Replacing corroded gates (not repair) as soon as possible

Coating gates—installation and maintenance of new Cathodic Protection systems

Consequence of exceeding impurity limits: Corrosion resistance decreases with

increasing Fe, Cu, or Ni content.

6. Recommendations to Avoid Corrosion

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

7. Conclusion

1. It is noted that coating and cathodic protection will stop further corrosion, but it

will not repair existing corrosion. So existing corrosion must removed by

sandblast and other physical cleaning/removing of corrosions.

2. Combination of Mg Alloy-forming elements and contaminations of Magnesium

alloys have to be restricted according to the staying power (stamina) limit.

3. The manufacturers of Magnesium Alloys generally add more manganese to

reduce the iron content in the melting point. Much more manganese, however,

is also negative to the corrosion of Mg alloys.

4. It is critical for the coating which should have sufficient bond on Magnesium,

high hardness and mechanical strength, good toughness, environmental

friendliness, excellent corrosion resistance, and even better fatigue and wear

resistance. But, as a matter of fact there are no such coatings that can satisfy

all of these demands.

5. The corrosion mechanisms of magnesium alloys still need further more

research.

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

References

1. Springer HandBook, Werner Martiessen and Hen Warlimont, 2005.

2. Edward Ghali (2010), Corrosion Resistance of Aluminum and

Magnesium Alloys, John Wiley & Sons Inc., New Jersey

3. ASM Handbook, Volume 13, "Corrosion“, ISBN 0-87170-007-7,

ASM International, 1987.

4. Kelvii Wei Guo, (2010), A Review of Magnesium/Magnesium Alloys

Corrosion and its Protection, City University of Hong Kong.

5. Perry's Chemical Engineers' Handbook, by Don W. Green and

James O. Maloney. 7th ed., 1997

6. R. Barry Dooley and Albert Bursik, (2009), Corrosion Fatigue, Boiler

and HRSG Tube Failure, Power Plant Chemistry, 2009, 11 (10)

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Questions

And

Answers

Thank You

• Introduction • Types of Corrosion • Influencing Factors • Conclusion

Table: Composition of AM60A, AM60B, AZ91A, AZ91B, AZ91C, AZ91D, AZ91E (ASM Handbook, 2012)

AZ91 mean magnesium alloy where is roughly 9 % weight percent

Aluminum (Al) and 1% weight percent Zinc (Zn).

WE43: Magnesium alloy, WE43 is a high-strength magnesium alloy

characterized by good mechanical properties both at an ambient and

elevated temperature (up to 300°C). It contains mainly yttrium and

neodymium. WE43 magnesium alloy after casting.

WE43 magnesium alloy is used in the aircraft industry, for wheels, engine

casings, gear box casings and rotor heads in helicopters.

WE43 : 4% Yttrium, 3.3% RE (Rare earth), 0.55 Zirconium, Remainder

Magnesium

ZE41: 4% Zn, 1.7% Rare Earth, 0.6% Zirconium, Remainder Magnesium

ZE41: Magnesium alloy, ZE41 [Mg−Zn−Rare Earth (RE)-Zr, nominal

composition 4 wt % Zn, 1.7 wt % RE, 0.6 wt % Zr, remaining balance, Mg],

Aerospace, automobile, military, electronics applications.

Notes

• Introduction • Types of Corrosion • Influencing Factors • Conclusion