MAGNESIUM ALLOYS

22.3 MAGNESIUM ALLOYS

Because unalloyed magnesium is not used extensively for structural applications, it is the corrosion resistance of magnesium alloys that is of primary interest. To enhance strength and resistance to corrosion, magnesium is alloyed with alumi- num, lithium, zinc, rhenium, thorium, and silver, with minor additions of cerium, manganese, and zirconium sometimes being used as well.

There are two major alloying systems:

1. Alloys containing 2 – 10% aluminum with minor additions of zinc and manganese; these alloys are used up to 95 – 120 ° C (200 – 250 ° F), above

MAGNESIUM ALLOYS

Figure 22.1. Corrosion of magnesium in 3% NaCl, alternate immersion, 16 weeks, showing tolerance limit for iron and benefi cial effect of alloyed zinc and manganese [ Figure 4 from J. Hanawalt, C. Nelson, and J. Peloubet, Corrosion studies of magnesium and its alloys, Trans. AIME, Inst. Metals Div . 147 , 281 (1942) ].

which temperature range both corrosion resistance and strength deteriorate.

2. Alloys containing various elements (rare earths, zinc, thorium, and silver), but not aluminum, and containing zirconium, which provides grain refi ne- ment and improved mechanical properties; these alloys provide improved elevated temperature properties compared to those in the fi rst group.

402 MAGNESIUM AND MAGNESIUM ALLOYS

Some alloys in both systems that are commonly available are listed in Table 22.1 . In the ASTM system for designating magnesium alloys, the fi rst two letters indi- cate the principal alloying elements: A, aluminum; E, rare earths; H, thorium; K, zirconium; M, manganese; Q, silver; Z, zinc. The letter corresponding to the element present in greater concentration is listed fi rst; if the elements are in equal concentration, they are listed alphabetically. The letters are followed by numbers that indicate the nominal compositions of these alloying elements, rounded off to the nearest whole number; for example, AZ91 indicates the alloy Mg – 9Al –

1Zn. In addition, letters that are sometimes appended to the alloy designation are assigned chronologically and usually indicate alloy improvements in purity [5] . In saline solutions, the corrosion rate is controlled by the concentration and distribution of the critical elements, iron, nickel, and copper, which create cathodic sites of low hydrogen overpotential. For this reason, these elements are usually controlled at low impurity levels.

Alloy purity, however, does not prevent galvanic corrosion that occurs if the magnesium alloy is coupled, for example, to a steel bolt. Design to prevent gal- vanic corrosion is, therefore, essential. In addition to the anode/cathode area ratio, the conductivity and composition of the medium in which a couple is immersed are controlling factors in the rate of galvanic corrosion. All commonly used metals cause galvanic corrosion of magnesium in saline solutions, but zinc plating the cathode (e.g., iron or steel) has been found to reduce the galvanic corrosion of the magnesium to one - tenth the rate. In addition, a reduction in the conductivity (e.g., changing from 3% NaCl to tap water), results in an even greater reduction in the galvanic corrosion rate.

22.3.1 Stress-Corrosion Cracking

The magnesium alloys with greatest susceptibility to stress - corrosion cracking (S.C.C.) are the Mg – Al alloys, and susceptibility increases with Al concentration. Magnesium – zinc alloys have intermediate susceptibility, and alloys that contain neither aluminum nor zinc are the most resistant [7] .

Stress - corrosion cracking of magnesium alloys occurs in many environments, including distilled water, seawater, and atmospheric environments in rural, urban, industrial, and coastal areas. In water, dissolved oxygen accelerates S.C.C. and deaeration retards it. S.C.C. in magnesium alloys is usually transgranular with secondary cracks, or branching, and initiation usually occurs at pits. Although several mechanisms for S.C.C. of magnesium alloys have been proposed, hydro- gen embrittlement is most likely a factor [8] .

Intergranular S.C.C. of magnesium alloys has also been reported, attributed to a grain - boundary phase of Mg 17 Al 12 that causes galvanic attack of the adjacent matrix [8] . As in most systems in which S.C.C. occurs, both applied and residual stresses are important. Welded components of magnesium alloys are normally stress - relieved after welding to reduce the residual stresses that develop during welding.

MAGNESIUM ALLOYS

T A B L E 22.1. Nominal Compositions of Some Magnesium Alloys a Alloy Number

Element (%)

Product Form b

Th Re M1

ASTM UNS

— — W AM50

M15100

— — C AM60

M10500

— — C AZ31

M10600

— — W AZ61

M11310

— — W AZ63

M11610

— — C AZ80

M11630

— — C, W AZ91

M11800

— — C EZ33

M11910

0.5 — 2.5 C ZM21

M12331

— — W HK31

0.5 3 — C, W HZ32

M13310

0.5 3 — C QE22

M13320

2.5 0.5 — 2 C QH21

M18220

2.5 0.5 1 1 C ZE41

M18210

0.5 — 1.5 C ZE63

M16410

0.5 — 2.5 C ZK40

M16630

0.5 — — C, W ZK60

M16400

0.5 — — C, W a Adapted from Ref. 6 .

M16600

b C, cast alloy; W, wrought alloy.

Resistance to S.C.C. may also be increased by using shot peening and other pro- cesses that produce compressive residual stresses at the surface [8] .

22.3.2 Coatings

Coatings provide an important strategy for protecting magnesium alloys from corrosion. In light - truck applications, heads of bolts made from AZ91D were protected from galvanic corrosion by using a nylon coating that was electrostati- cally applied. Molded plastic caps were also found to be effective. Zinc alloy coatings (e.g., 80Sn – 20Zn) electroplated on magnesium and given a chromate treatment were found to decrease the corrosion rate of magnesium by more than 90% in salt spray testing [9, 10] .

Various protective anodized coatings are available as produced in electro- lytes composed mainly of fl uorides, phosphates, or chromates [11] . Anodizing treatments result in porous coatings that provide excellent bases for subsequent painting. Anodized magnesium castings are used successfully, for example, in helicopter drive system components. In general, in aerospace applications where extended component life and low maintenance costs are required, all surfaces of magnesium alloys exposed to corrosive environments must be coated.

404 MAGNESIUM AND MAGNESIUM ALLOYS

Driven by the need to reduce both gasoline consumption and greenhouse gas production, the automotive industry is reducing vehicle weight to the extent possible, and one approach being used is increasing the magnesium content of automobiles. Large heavy automobile parts are particularly attractive candidates for replacement by magnesium. The extent of corrosion protection required depends on the severity of the environment; for example, for underbody and wheel applications, comprehensive protection schemes are essential, including protection against galvanic corrosion. Underbody wax - type coatings may provide additional protection. One of the more recent developments in the automotive industry is the application of Alloy AM50 for the front - end support assembly for light - duty Ford trucks [12] . Magnesium alloys are also being used for air cleaner covers, engine compartment grills, retractable headlight assemblies, clutch and brake pedal supports, and clutch and transmission housings. Fasteners that are specially designed using nylon or plastic washers, sleeves, and so on, are used to help control galvanic corrosion of magnesium [9] .