Kutz M. Handbook of materials selection

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258 NICKEL AND ITS ALLOYS

Table 6 Registered Trademarks of Producer

Company

Trademark Owner

Duranickel Inco family of companies

Hastelloy Haynes International, Inc.

Incoloy Inco family of companies

Inconel Inco family of companies

MAR-M Martin Marietta Corp.

Monel Inco family of companies

Nilo Inco family of companies

Ni–Span–C Inco family of companies

Permanickel Inco family of companies

Pyromet Carpenter Technology Corp.

Rene General Electric Co.

Rene

⬘ 41 Allvac Metals Corp.

Udimet Special Metals Corp.

Waspaloy United Aircraft Corp.

REFERENCES

1. Joseph R. Boldt, Jr., The Winning of Nickel, Van Nostrand, New York, 1967.

2. Nickel and Its Alloys, NBS Monograph 106, May, 1968.

3. Kent’s Mechanical Engineer’s Handbook, 1950 edition, pp. 4–50 to 4–60.

4. Huntington Alloys, Inc., Alloy Handbook, and Bulletins.

5. Inco internal communication by A.J. Sedriks.

6. Alloy Digest, Engineering Alloy Digest, Inc., 1983.

7. Aerospace Structural Metals Handbook, 1983.

8. Materials and Processing Databook, 1983 Metals Progress.

9. J. S. Benjamin, Met. Trans. AIME 1, 2943 (1970).

10. J. P. Morse and J. S. Benjamin, J. Met. 29 (12), 9 (1977).

11. Wayne Z. Friend, Corrosion of Nickel and Nickel-Base Alloy, Wiley, New York, 1980.

12. F. L. LaQue and H. R. Copson, Corrosion Resistance of Metals and Alloys, 2nd ed., Reinhold,

New York, 1963.

13. J. Kolts et al., ‘‘Highly Alloyed Austenitic Materials for Corrosion Service,’’ Metal Prog., 25–36

(September, 1983).

14. High Temperature Corrosion in Reﬁnery and Petrochemical Service, Inco Publication, 1960.

259

CHAPTER 8

MAGNESIUM AND ITS ALLOYS

Robert S. Busk

Hilton Head, South Carolina

1 INTRODUCTION 259

2 USES 259

2.1 Nonstructural Applications 259

2.2 Structural Applications 260

3 ALLOYS AND PROPERTIES 261

3.1 Mechanical Properties of

Castings 261

3.2 Mechanical Properties of

Wrought Products 262

3.3 Physical Properties 262

4 FABRICATION 262

4.1 Machining 262

4.2 Joining 262

4.3 Forming 263

5 CORROSION AND FINISHING 264

5.1 Chemical-Conversion

Coatings 264

5.2 Anodic Coatings 264

5.3 Painting 264

5.4 Electroplating 265

REFERENCES 265

BIBLIOGRAPHY 265

1 INTRODUCTION

Magnesium, with a speciﬁc gravity of only 1.74, is the lowest-density metal

available for engineering use. It is produced either by electrolytic reduction of

MgCl

or by chemical reduction of MgO by Si in the form of ferrosilicon. MgCl

is obtained from seawater, brine deposits, or salt lakes. MgO is obtained prin-

cipally from seawater or dolomite. Because of the widespread, easy availability

of magnesium ores (e.g., from the ocean), the ore supply is, in human terms,

inexhaustible.

2 USES

Magnesium is used both as a structural, load-bearing material and in applications

that exploit its chemical and metallurgical properties.

2.1 Nonstructural Applications

Because of its high place in the electromotive series, magnesium is used as a

sacriﬁcial anode to protect steel from corrosion; some examples are the protec-

tion of buried pipelines and the prolongation of the life of household hot-water

tanks. Alloys used for this purpose are produced by permanent-mold castings

and by extrusion.

Reprinted from Mechanical Engineers’ Handbook, 2nd ed., Wiley, New York, 1998, by permission

of the publisher.

HandbookofMaterialsSelection.EditedbyMyerKutz

260 MAGNESIUM AND ITS ALLOYS

Table 1 Magnesium Alloys in Common Use

ASTM

Designation Ag Al

max Mn

max

Rare

Earth Si Zn Zr Forms

AM50A 4.9 0.004 0.32 0.002 0.22 DC

AM60B 6.0 0.005 0.42 0.002 0.22max DC

AS41B 4.2 0.0035 0.52 0.002 1.0 0.12 DC

AZ31B 3 0.005 0.6 0.005 1 S, P, F, E

AZ61A 6.5 0.005 0.33 0.005 0.9 F, E

AZ80A 8.5 0.005 0.31 0.005 0.5 F, E

AZ81A 7.6 0.24 0.7 SC, PM, IC

AZ91D 9 0.005 0.33 0.002 0.7 DC

AZ91E 9 0.005 0.26 0.0010 0.7 SC, PM

EZ33A 3.2 2.5 0.7 SC, PM

K1A 0.7 SC, PM

M1A 1.6 E

QE22A 2.5 2.2 0.7 S, PM, IC

WE43A 0.01 0.15 0.005 A 0.20 0.7 S, PM, IC

WE54A 0.15 0.005 B 0.7 S, PM, IC

ZE41A 0.15 1.2 4.2 0.7 S, PM, IC

ZE63A 2.6 5.8 0.7 S, PM, IC

ZK40A 4 0.7 E

ZK60A 5.5 0.7 F, E

A ⫽ 4 Yttrium; 3 RE

⫽ 5.1 Yttrium; 4 R.E.

⫽ die casting; E ⫽ extrusion; F ⫽ forging; IC ⫽ investment casting; P ⫽ plate; PM ⫽ permanent

mold; S

⫽ sheet; SC ⫽ sand casting

Magnesium in powder form is added to gray cast iron to produce ductile, or

nodular, iron, an alloy that has many of the producibility advantages of cast iron

but is ductile and strong.

A signiﬁcant use for magnesium powder is its addition to the iron tapped

from blast furnaces to remove sulfur prior to converting to steel, thereby in-

creasing the efﬁciency of the blast furnace and improving the toughness of the

steel.

Magnesium powder is also used to produce the Grignard reagent, an organic

intermediate used in turn to produce ﬁne chemicals and pharmaceuticals.

Magnesium sheet and extrusions are used to produce photoengravings.

Magnesium in ingot form is one of the principal alloying additions to alu-

minum, imparting improved strength and corrosion resistance to that metal.

2.2 Structural Applications

Magnesium structures are made from sand, permanent-mold, investment, and die

casting, and from sheet, plate, extrusions, and forgings. The base forms produced

in these ways are fabricated into ﬁnished products by machining, forming, and

joining. Finishing for protective or decorative purposes is by chemical-

conversion coatings, painting, or electroplating.

The most rapidly growing method of producing structural parts is die casting.

This method is frequently the most economical to produce a given part and is

especially effective in producing parts with very thin sections. A stimulus for

the recent very high growth rate has been the development of a high-purity

corrosion-resistant alloy that makes unnecessary the protective ﬁnishing of many

parts. See alloy AZ91D in Table 1. Die castings are produced by cold chamber,

3 ALLOYS AND PROPERTIES 261

by hot chamber, and by a recently developed method analogous to the injection

molding of plastic parts. The latter technique, known as Thixomolding,

⫺

uses

a machine that advances the alloy in a semisolid state by means of a screw and

then injects an accumulated amount into the die. The melting step is eliminated,

production rates are at least as high as for hot-chamber die casting, and metal

quality is superior to that produced by either cold- or hot-chamber die casting.

Two major ﬁelds dominate the die-casting markets: automotive (e.g., housings,

brake pedals, transmissions, instrument panels) and computers (e.g., housings,

disc readers).

Those properties mainly signiﬁcant for structural applications are density (au-

tomotive and aerospace vehicle parts; portable tools such as chain saws; con-

tainers such as for computers, cameras, briefcases; sports equipment such as

catcher’s masks, archery bows); high damping capacity (antivibration platforms

for electronic equipment; walls for sound attenuation); excellent machinability

(jigs and ﬁxtures for manufacturing processes); high corrosion-resistance in an

alkaline environment (cement tools).

3 ALLOYS AND PROPERTIES

Many alloys have been developed to provide a range of properties and charac-

teristics to meet the needs of a wide variety of applications. The most frequently

used are given in Table 1. There are two major classes—one containing alu-

minum as the principal alloying ingredient, the other containing zirconium.

Those containing aluminum are strong and ductile, and have excellent resistance

to atmospheric corrosion. Since zirconium is a potent grain reﬁner for magne-

sium alloys but is incompatible with the presence of aluminum in magnesium,

it is added to all alloys not containing aluminum. Within this class, those alloys

containing rare earth or yttrium are especially suited to applications at temper-

atures ranging to as high as 300

⬚C. Those not containing rare-earth or yttrium

have zinc as a principal alloying element and are strong, ductile, and tough.

Recently, the high-purity casting alloys, AZ91E for sand and permanent mold

castings and AZ91D, AM60B, AM50A, and AS41B for die castings, have been

developed. The high-purity die casting alloys are superior in corrosion resistance

to the commonly used aluminum die casting alloy. These alloys have been

largely responsible for the large expansion in magnesium automotive applica-

tions.

3.1 Mechanical Properties of Castings

Magnesium castings are produced in sand, permanent, investment, pressure die-

casting molds.

Castings produced in sand molds range in size from a few pounds to a few

thousand pounds and can be very simple to extremely complex in shape. If

production runs are large enough to justify higher tooling costs, then permanent

instead of sand molds are used. The use of low pressure to ﬁll a permanent

mold is a low-cost method that is also used. Investment casting is a specialized

technique that permits the casting of very thin and intricate sections with ex-

cellent surface and high mechanical properties. Die casting is a process for the

production of castings with good dimensional tolerances, good surface, and ac-

ceptable properties at quite low cost.

262 MAGNESIUM AND ITS ALLOYS

Table 2 Typical Mechanical Properties for Castings

Alloy Temper

Tensile Strength

(MPa)

Yield Strength

(MPa)

Elongation in 2 in.

(%)

Sand and Permanent Mold Castings

AZ81A T4 276 85 15

AZ91E F 165 95 3

T4 275 85 14

T6 275 195 6

EZ33A T5 160 105 3

K1A F 185 51 20

QE22A T6 275 205 4

WE43A T6 235 190 4

WE54A T6 270 195 4

ZE63A T6 295 190 7

Investment Castings

AZ81A T4 275 100 12

AZ91E F 165 100 2

T4 275 100 12

T5 180 100 3

T7 275 140 5

EZ33A T5 255 110 4

K1A F 175 60 20

QE22A T6 260 185 4

Die Castings

AM50A F 200 110 10

AM60B F 220 130 8

AS41B F 210 140 6

AZ91D F 230 160 3

Mechanical properties of cast alloys are given in Table 2.

3.2 Mechanical Properties of Wrought Products

Wrought products are produced as forgings, extrusions, sheet, and plate. Me-

chanical properties are given in Table 3.

3.3 Physical Properties

A selection of physical properties of pure magnesium is given in Table 4. Most

of these are insensitive to alloy addition, but melting point, density, and electrical

resistivity vary enough that these properties are listed for alloys in Table 5.

4 FABRICATION

4.1 Machining

Magnesium is the easiest of all metals to machine: it requires only low power

and produces clean, broken chips, resulting in good surfaces even with heavy

cuts.

4.2 Joining

All standard methods of joining can be used, including welding, riveting, braz-

ing, and adhesive bonding.

Welding is by inert-gas-shielded processes using either helium or argon, and

either MIG or TIG. Alloys containing more than 1.5% aluminum should be

stress-relieved after welding in order to prevent stress-corrosion cracking due to

4 FABRICATION 263

Table 3 Typical Mechanical Properties of Wrought Products

Alloy Temper

Tensile Strength

(MPa)

Yield Strength (MPa)

Tensile Compressive

Elongation in 2 in.

(%)

Sheet and Plate

AZ31B O 255 150 110 21

H24 290 220 180 15

Extrusions

AZ31B F 260 200 95 15

AZ61A F 310 230 130 16

AZ80A F 340 250 140 11

T5 380 275 240 7

M1A F 255 180 125 12

ZK40A T5 275 255 140 4

ZK60A F 340 250 185 14

T5 365 305 250 11

Forgings

AZ31B F 260 195 85 9

AZ61A F 195 180 115 12

AZ80A F 315 215 170 8

T5 345 235 195 6

T6 345 250 185 5

ZK60A T5 305 205 195 16

T6 325 270 170 11

Table 4 Physical Properties of Pure Magnesium

Density 1.718 g / cm

(Ref. 5)

Melting point 650

⬚C (Ref. 6)

Boiling point 1107

⬚C (Ref. 6)

Thermal expansion 25.2

⫻ 10

⫺

/ K (Ref. 7)

Speciﬁc heat 1.025 kJ / kg

䡠Kat20⬚C (Ref. 8)

Latent heat of fusion 360–377 kJ / kg (Ref. 8)

Latent heat of sublimation 6113–6238 kJ / kg Ref. 6)

Latent heat of vaporization 5150–5400 kJ / kg (Ref. 6)

Heat of combustion 25,020 kJ / kg (Ref. 10)

Electrical resistivity 4.45 ohm meter

⫻ 10

⫺

Crystal structure Close-packed hexagonal: a ⫹ 0.32087 nm; c ⫽ 0.5209 nm;

c / a

⫽ 1.6236 (Ref. 9)

Young’s modulus 45 Gpa

Modulus of rigidity 16.5 Gpa

Poisson’s ratio 0.35

residual stresses associated with the weld joint. Rivets for magnesium are of

aluminum rather than magnesium. Galvanic attack is minimized or eliminated

by using aluminum rivets made of an alloy high in magnesium, such as 5056.

Brazing is used, but not extensively, since it can be done only on alloys with a

high melting point, such as AZ31B or K1A. Adhesive bonding is straightfor-

ward, and no special problems related to magnesium are encountered.

4.3 Forming

Magnesium alloys are formed by all the usual techniques, such as deep drawing,

bending, spinning, rubber forming, stretch forming, and dimpling.

In general, it is preferable to form magnesium in the temperature range of

150–300

⬚C. While this requires more elaborate tooling, there is some compen-

264 MAGNESIUM AND ITS ALLOYS

Table 5 Physical Properties of Alloys

Alloy

Density

(g/cm

)

Melting Point (⬚C)

Liquidus Solidus

Electrical Resistivity

(ohm-metres ⫻ 10

⫺8

)

AM60B 1.79 615 540

AS41B 1.77 620 565 13.0

AZ31B 1.77 632 605 9.2

AZ61A 1.8 620 525 12.5

AZ80A 1.8 610 490 15.6

AZ81A 1.80 610 490 13.0

AZ91D 1.81 595 470 17.0

EZ33A 1.83 645 545 7.0

K1A 1.74 649 648 5.7

M1A 1.76 649 648 5.4

QE22A 1.81 645 545 6.8

ZK60A 1.83 635 520 5.7

sation in the ability to produce deeper draws (thus fewer tools) and in the elim-

ination or minimizing of springback. Hydraulic rather than mechanical presses

are preferred.

5 CORROSION AND FINISHING

Magnesium is highly resistant to alkalies and to chromic and hydroﬂuoric acids.

In these environments, no protection is usually necessary. On the other hand,

magnesium is less resistant to other acidic or salt-laden environments. While

most magnesium alloys can be exposed without protection to dry atmosphere,

it is generally desirable to provide a protective ﬁnish.

Magnesium is anodic to any other structural metal and will be preferentially

attacked in the presence of an electrolyte. Therefore, galvanic contact must be

avoided by separating magnesium from other metals by the use of ﬁlms and

tapes. These precautions do not apply in the case of 5056 aluminum alloy, since

the galvanic attack in this case is minimal.

Because magnesium is not resistant to acid attack, standing water (which will

become acidic by absorption of CO

from the atmosphere) must be avoided by

providing drain holes.

5.1 Chemical-Conversion Coatings

There are a large number of chemical-conversion processes based on chromates,

ﬂuorides, or phosphates. These are simple to apply and provide good protection

themselves, in addition to being a good paint base.

5.2 Anodic Coatings

There are a number of good anodic coatings that offer excellent corrosion pro-

tection and also provide a good paint base.

5.3 Painting

If a good chemical-conversion or anodic coating is present, any paint will pro-

vide protection. Best protection results from the use of baked, alkaline-resistant

paints.

BIBLIOGRAPHY 265

5.4 Electroplating

Once a zinc coating is deposited chemically, followed by a copper strike, stan-

dard electroplating procedures can be applied to magnesium to give decorative

and protective ﬁnishes.

REFERENCES

1. M. C. Flemings, ‘‘A History of the Development of Rheocasting,’’ in Proceedings of the Work

Shop on Rheocasting, Army Materials and Mechanics Research Center, Feb. 3–4, 1977, pp. 3–10.

2. S. C. Erickson, ‘‘A Process for the Thixotropic Casting of Magnesium Alloy Parts,’’ in Proceed-

ings of the International Magnesium Association, May 17–20, 1987, p. 39.

3. R. D. Carnahan, R. Kilbert and L. Pasternak, ‘‘Advances in Thixomolding,’’ in Proceedings of

the International Magnesium Association, May 17–18, 1994, p. 21.

4. K. Saito, ‘‘Thixomolding of Magnesium Alloys,’’ in Proceedings of the International Magnesium

Association, June 2–4, 1996.

5. R. S. Busk, Trans. AIME 194, 207 (1952).

6. D. R. Stull and G. C. Sinke, Thermodynamic Properties of the Elements, Vol. 18, Advances in

Chemistry, American Chemical Society, Washington, DC, 1956.

7. P. Hidnert and W. T. Sweeney, J. Res. Nat. Bur. St. 1, 771 (1955).

8. R. A. McDonald and D. R. Stull, J. Am. Chem. Soc. 77, 529 (1955).

9. R. S. Busk, Trans. AIME, 188, 1460 (1950).

10. J. W. Frederickson, ‘‘Pure Magnesium,’’ in Metals Handbook, 8th ed., American Society for

Metals, Metals Park, OH, 1961, Vol. 1.

11. Physical Properties of Magnesium and Magnesium Alloys, Dow Chemical Company, 1967.

BIBLIOGRAPHY

Bothwell, M. R., The Corrosion of Light Metals, Wiley, New York, 1967.

Busk, R. S., Magnesium Products Design, Marcel Dekker, New York, 1987.

Emley, E. F., Principles of Magnesium Technology, Pergamon Press, New York, 1966.

Fabricating with Magnesium, Dow Chemical Company.

Machining Magnesium, Dow Chemical Company.

‘‘Nonferrous Metal Products,’’ in Annual Book of ASTM Standards, 02.02, ASTM, 1995.

Operations in Magnesium Finishing, Dow Chemical Company.

‘‘Properties of Magnesium Alloys,’’ in Metals Handbook, 10th ed., American Society for Metals,

Metals Park, OH, 1990, Vol. 2.

Roberts, C. S., Magnesium and Its Alloys, Wiley, New York, 1960.

‘‘Selection and Application of Magnesium and Magnesium Alloys,’’ in Metals Handbook, 10th ed.,

American Society for Metals, Metals Park, OH, 1990, Vol. 2.

267

CHAPTER 9

CORROSION AND OXIDATION OF

MAGNESIUM ALLOYS

D. Eliezer

Department of Materials Engineering

Ben-Gurion University of the Negev

Beer-Sheeva, Israel

H. Alves

Department of Chemical Engineering

University of Dortmund

Dortmund, Germany

1 INTRODUCTION 267

2 OXIDATION 268

2.1 Ambient Temperature 268

2.2 High Temperature 270

3 CORROSION 274

3.1 Electrochemical Properties 274

3.2 Types of Corrosion 275

3.3 Environment and Surface Film 276

3.4 Improving Corrosion Resistance 278

4 OUTLOOK 288

REFERENCES 289

1 INTRODUCTION

The lightweight concept followed recently by the transportation industry made

Mg alloys a highly interesting choice for many structural applications. Apart

from the high strength-to-weight ratio, Mg alloys exhibit excellent die castability,

superior machinability, good ductility, and damping capacity. However, the use

of these alloys is far behind that of Al alloys, mainly due to their poor corrosion

resistance. Although in rural and some mild industrial environments Mg alloys

perform as good as Al and better than mild steel, in wet and salt-containing

environments they frequently suffer severe damage. Further detrimental factors

to the corrosion resistance of Mg alloys are galvanic coupling, high content of

impurities (e.g., heavy metals or ﬂuxes), surface contamination, inadequate sur-

face protection schemes, and poor design. Even in dry environments Mg alloys

oxidize easily at high temperatures of application.

The major alloying elements added to Mg in order to obtain the strong, light

alloys needed for structural applications are aluminum, manganese, zinc, silicon,

zirconium, rare earths (RE), and silver. Two main categories of alloys can be

distinguished according to whether they contain Al or Zr. The ﬁrst category

HandbookofMaterialsSelection.EditedbyMyerKutz

268 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

contains 2–10% of Al; high Al contents improve corrosion resistance, however,

ductility and fracture toughness are greater for lower Al contents. AM alloys

contain Al and small amounts of Mn, whereas AZ, AS, and AE alloys further

contain Zn, Si, and RE, respectively. Alloys from the second group contain no

Al but Zr combined with alloying elements such as RE, Zn, and Ag. These

alloys generally exhibit improved mechanical properties due to presence of Zr,

which has a powerful grain reﬁnement effect on magnesium. Mg alloys can be

processed into casting (die, sand, and mold) or used as wrought products (ex-

trusions, forgings, sheet, and plates); castings exceed wrought alloys and account

for 85–90% of the total outcome. Detailed information on alloys and processing

can be found in Ref. 1.

The prospects for the growing use of structural Mg alloys are particularly

attractive but must be supported by the development of more corrosion- and

heat-resistant alloys. Currently, the average amount of magnesium used in the

production of automobiles is 3 kg and is expected to rise to about 6 kg in 2005.

Another major area in which the use of Mg is expanding is the application in

electronic housings such as for computers and cell phones, where 100% recycl-

ability, compactness arising from thin walls, and electromagnetic shielding are

additional advantages. Applications for Mg alloys in automobiles are variegated

and include components such as instrument and door panels, seat frames, gear-

box housings, and wheels. Due to aggressive conditions such as salty road

splash, high corrosion resistance is required for exterior components. The use

of Mg, for example, in engine and transmission components requires additional

resistance to higher temperatures. A wider exploitation of the large potential of

structural Mg alloys will ultimately depend on a correct use of these alloys in

oxidative and corrosive environments.

2 OXIDATION

While aqueous corrosion is recognized to be a major problem of Mg alloys,

oxidation at elevated temperature is often ignored as a shortcoming for alloy

application and is not very widely studied. Magnesium has a high afﬁnity to

oxygen and in its presence it rapidly reacts according to:

–

Mg ⫹ O → MgO (1)

MgO has a face-centered-cubic (fcc) crystal structure with a ⫽ 0.42 nm. It is a

cubic close-packing of O ions with metal ions in all octahedral sites. The free

energy change that accompanies the formation of the oxide is

⌬G

⫽⫺1138 kJ/

mol O

at 25⬚C

; more negative than for most metals with only few exceptions

such as Be, Ca, and some rare earths. Therefore, the resistance of Mg and its

alloys to oxidation directly depends on the protective properties of the oxide

ﬁlm that forms ﬁrst.

2.1 Ambient Temperature

Despite the very negative free energy of formation of MgO, magnesium exhibits

good oxidation resistance in dry air at normal ambient temperature due to the

formation of a protective oxide ﬁlm. Unfortunately, the presence of water in the