Kutz M. Handbook of materials selection

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3 CORROSION 279

Fig. 8 Inﬂuence of Ni, Fe, and Cu impurities on the corrosion rate of die-cast AZ91 in the

standard salt spray test.

(Reprinted with permission from SAE paper number 850417 䉷 1985

Society of Automotive Engineers, Inc.)

hydrogen overvoltage. Noble impurities such as Fe, Cu, and Ni promote micro-

galvanic corrosion and show tolerance limits above which corrosion rate rapidly

increases

44,45

(see Fig. 8). Controlling the level of contamination below these

limits reduces the corrosion rate of Mg alloys containing Al by orders of mag-

nitude, making them very competitive. For example, high-purity AZ91 shows

lower salt spray corrosion rate than die cast 380 Al and cold-rolled steel

46,47

(see

Fig. 8). The individual tolerance limits depend on the speciﬁc alloy composition,

and a summary can be found in Ref. 27 for common high-purity alloys. For die-

cast high-purity AZ91 the ASTM speciﬁcation B94

recommends: Fe ⬍ 50

ppm, Ni

⬍ 20 ppm, Cu ⬍ 300 ppm. Mg alloys containing Zr can be considered

to be of high purity because the highly reactive nature of Zr ensures that im-

purities present in the melt precipitate as compounds before any Zr can dissolve

in the Mg matrix.

The tolerant limits are important for recycled alloys as well.

During recycling care has to be taken to avoid contamination and secondary Mg

alloys aiming structural application have to be brought within the ASTM spec-

iﬁcations.

The detrimental effect of noble metals decreases as follows: Ni

⬎ Fe ⬎ Cu.

Ni and Cu are usually not a problem because of their very low content in the

primary production. Fe is of most concern, especially due to the risk of Fe pick

up from carbon steel pots and casting molds. The solubility limit of Fe in Mg

is low, and it precipitates as Al

Fe, which acts as an effective cathode. The

detrimental effect of these precipitates is reduced by small additions of Mn (up

to 1%) to the melt (e.g., as MnCl

45,50

Manganese forms several Al–Mn–Fe

intermetallic particles, which can be removed after settling, and reduces the

effect of the remaining Fe containing particles compared to Al

Fe.

36,50,51

How-

ever, Mn should not be alloyed in excess of that necessary to reduce the Fe

content to the appropriate level because Fe-free Al–Mn particles also exhibit

280 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

some cathodic activity, particularly the Mn-rich ones. In addition to the maxi-

mum limit for Fe, speciﬁcations for high-purity alloys include requirements on

the Fe/Mn ratio (e.g., 0.032 and 0.021 for AZ91D and AM60B, respectively).

Poor corrosion resistance is also associated with the presence of nonmetallic

inclusions such as chlorides and oxides.

The development in the Mg foundry

practice, which includes the use of gas mixtures (e.g., SF

–air–CO

), to protect

the melt against excessive oxidation and the elimination of ﬂuxes reduced sub-

stantially this concern. For a number of alloys in both cast and wrought condi-

tions, the critical concentration of chloride for pit initiation was reported to range

between 2

⫻ 10

⫺

and 2 ⫻ 10

⫺

M NaCl.

Only if impurities are controlled, other approaches such as the beneﬁcial

presence of alloying elements and the use of favorable heat treatments or RS

become important.

Alloying

Keeping in mind the electrochemical properties of magnesium, it is clear that

measures to enhance corrosion resistance by alloying require the use of elements

with electrochemical potential close to that of magnesium (in order to avoid

microgalvanic processes) and ability to help formation of the passive ﬁlm. The

majority of the elements conventionally alloyed to magnesium do not improve

signiﬁcantly corrosion resistance, e.g., addition of Zn and Si. Mn and Zr con-

tribute signiﬁcantly by reducing the effects of the impurities. The most important

beneﬁt is provided by Al, which is simultaneously the most important alloying

element of Mg. RE and alkaline metals such as Ca are very attractive in regard

to the electrochemical potential, and efforts are being done to develop alloys or

to modify current alloys with small amounts of these elements.

Aluminum. Commercial Mg–Al alloys contain typically 2–10% of Al. In

chloride environments there is a signiﬁcant decrease in the corrosion rate, in-

dependent of the alloy type, as the Al is increased up to 4%. Further increase

in the alloyed Al results in a smoother improvement of the corrosion resistance

(see Fig. 9).

Alloying with Al results in the precipitation of Mg

(

␤

phase), in partic-

ular along grain boundaries (see Fig. 10a). In addition, there is some evidence

for an Al gradient in the matrix, i.e., a decrease from the vicinity of the

␤

phase

toward the bulk.

33,35

precipitates and Al-rich coring zones act as a

barrier against the extension of local corrosion, enhancing the corrosion resis-

tance of Al containing alloys.

The beneﬁcial effect of the

␤

phase results from two effects:

(1) It is elec-

trochemically nobler than the Mg matrix while having small cathodic activity

as compared to Al–Mn–Fe intermetallics. (2) It is corrosion resistant over a

wide pH range (4–14) by combining the passive properties of Mg in alkaline

solutions with those of Al in neutral and slightly acidic media. The polarization

curve of Mg

in 5% NaCl saturated with Mg(OH)

shows an anodic current

plateau

suggesting the formation of a partially protective ﬁlm. The preferential

corrosion of the anodically more active bulk matrix (see Fig. 10b) might be the

reason for the ‘‘honeycomb’’-type morphology observed on corroded Al con-

taining alloys.

50,51

3 CORROSION 281

Fig. 9 Inﬂuence of Al concentration on the corrosion rate of die-cast Mg alloys during immer-

sion in 5% NaCl.

(Reprinted with the permission from SAE paper number 930755

䉷 1993 Society of Automotive Engineers, Inc.)

Fig. 10 AZ91 ingots: (a) typical microstructure and (b) preferential dissolution of the matrix and

stability of the

␤

phase after 24 h immersion in 5% NaCl solution.

Aluminum additionally improves corrosion resistance by improving the prop-

erties of the surface ﬁlm. With increasing Al content all layers of the ﬁlm formed

on Mg–Al alloys become continuously enriched in Al

and dehydrated, and

layer thickness decreases.

12,13

Such stabilization of the ﬁlm is especially advan-

tageous for the inner layer, which is responsible for the passivity of the surface

in the presence of water. The beneﬁcial changes are signiﬁcant up to 4% Al

in the alloy and cause the sudden decrease in the corrosion rate observed in

Fig. 9. Further alloying results in only minor changes in the oxide properties as

well as in corrosion resistance. This transition relates with the alumina compo-

nent in the ﬁlm. Alloying up to 4% causes the oxide to enrich gradually in Al

282 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

and the ﬁlm properties to improve. At around 4% of alloyed Al a threshold of

35% Al in the oxide ﬁlm is achieved and further alloying does not enhance

corrosion resistance signiﬁcantly because the amount of Al in the oxide remains

unchanged. Improvements at this composition range have to be correlated with

other factors such as an increase of

␤

-phase. The Al threshold agrees well with

the composition of oxides grown on the same type of alloys in ultrahigh vacuum

and dry air shown in Fig. 1.

Alloying with Al is an effective way to develop corrosion-resistant Mg alloys

having surface ﬁlms with reduced propensity for hydration and ionic conductiv-

ity. However, alloying with at least 4% Al is necessary to obtain an oxide with

optimum corrosion properties. Only at this threshold the Al

component forms

a continuous passivating network, which could be a skeletal structure in the

amorphous mixture of aluminum and magnesium (hydr-)oxides.

12,55

It was also found that aluminum can have a detrimental effect on corrosion.

Al was claimed to decrease the tolerance limit for Fe in an almost linear way

37,56

and in small amounts (below 8%) to produce an anodically more active Mg solid

solution.

The negative trend in the

␣

phase, however, reverses for high Al

contents.

Rare Earth (RE) Elements. Further improvement in the corrosion resistance

of Mg–Al alloys can be achieved by alloying with RE elements. RE elements

further reduce the hydration of the oxide ﬁlm, being a possibility to diminish

the Al threshold for passivity.

The presence of RE promotes the enrichment of

the inner oxide layer uniformly with Al

also for Al contents in the alloy

below the critical value of 4%. The formation of an oxide that is more uniformly

enriched in Al agrees with the relatively small effect of RE on the oxide prop-

erties in the absence of Al.

An enrichment of trace amounts of RE in the oxide

ﬁlm was suggested by several authors but could not be deﬁnitely veriﬁed.

Another advantage of RE elements is their electrochemical potential, which

being very close to that of Mg does not promote galvanic processes. Although

RE show a small equilibrium solubility in the Mg matrix in the presence of Al

and precipitate as Al–RE particles, these phases are electrochemically passive.

Precipitation of Al

RE was even suggested to enhance corrosion resistance due

to the formation of Al coring along the grain boundaries.

RE elements also

assist in removing Fe from the melt by settling Al–Mn–Fe–RE phases and

reduce their cathodic activity.

55,58

Some RE-based intermetallics exhibit high

melting temperatures and enhance creep resistance at high temperatures.

Mg–Al–Zn alloys with RE and Mn additions exhibit attractive combination

of corrosion resistance, strength, and ductility. These alloys were successfully

developed by Allied-Signal’s Metals, both by conventionally casting and RS.

59,60

Another standard Mg alloy containing RE is the WE43 [approx. 4% Y, 3.3%

⫹ hydrogen reaction equilibrium (HRE)] developed by Magnesium Elektron

Ltd., which is a high-strength and corrosion-resistant alloy with long-term sta-

bility at higher temperatures. In chloride solutions AE alloys exhibit lower cor-

rosion rates than AS, AM, and AZ alloys with similar Al content (see Fig. 9).

Small additions of RE, typically 1–2%, were reported to reduce the dissolution

rate of conventional AM alloys by a factor of six.

3 CORROSION 283

Zinc. The presence of Zn improves corrosion behavior, although this effect

is not very remarkable. Zinc has an appreciable solubility in the Mg matrix and

the

␤

phase. It is reported to increase the tolerance limits, although to a lesser

extent than Mn,

32,44

and to make the Mg(Al) matrix more noble.

In alloys

containing Al it was suggested to improve the stability of the oxide in the same

manner as RE.

13,57

However, a related improvement in the corrosion rate of AZ

alloys is not observed probably because Zn also increases the cathodic reaction

rate.

Silicon. Silicon is added to AS alloys where it increases the strength due to

precipitation of Mg

Si particles. Mg

Si exhibits a corrosion potential close to

that of Mg being relatively innocuous to corrosion behavior.

A small fraction

of the alloyed Si is present in the form of Al–Mn–Fe–Si compounds and plays

a similar role to that of Mn in reducing their cathodic activity.

Zirconium. Mg alloys containing Zr generally exhibit good corrosion per-

formance and are relatively insensible to Fe and Ni. Zr combines with Fe to

form insoluble particles that precipitate before casting.

Since Zr also forms

stable compounds with Al or Mn and is removed from the melt, it cannot be

added to alloys containing these elements.

Calcium. The effect of Ca on the corrosion resistance of Mg is yet to be

understood. Regarding the electrochemical potential, addition of Ca should result

in low microgalvanic effects and reduce the anodically active area upon pre-

cipitation. This was reported for Mg–Ca–Cu splats where precipitation of

Ca neutralizes the galvanic effect from the Mg

Cu phase. In the case of

RS AZ91 alloy an addition of about 2% Ca improved the corrosion rate from

0.8 to 0.2 mm/yr.

However, a detrimental inﬂuence of Ca was reported for

both conventional and RS binary Mg alloys.

44,62

Alloying conventional AM

alloys with small amounts of Ca slightly increases the corrosion rate in NaCl.

Films formed on AM50⫹1%Ca are extremely ﬁne nanocrystalline [identiﬁed as

(OH)

䡠 4.5H

O and Mg(OH)

] and consist of at least two layers (see

Fig. 11): A hydrated inner layer, which under electron irradiation develops a

cellular structure and transforms into MgO, and a thin, more stable outer layer.

Similar morphology is observed in ﬁlms formed on Mg and Mg–Al alloys in

water-containing environments (see Fig. 7). Sometimes an additional crystalline

layer is observed at the interface alloy/inner layer.

Ca is very unstable in aqueous solutions and Ca(OH)

forms at pH even higher

than Mg(OH)

. The presence of Ca(OH)

is not necessarily expected to stabilize

the Mg ﬁlm due to its greater tendency to dissociate chemically in the presence

of water.

The question whether a combination of Ca and RE is a more prom-

ising approach is now under investigation. Mechanical properties and thermal

stability also beneﬁt from the presence of Ca and RE.

Processing

For a given alloy composition corrosion properties are further inﬂuenced by the

microstructure, which obviously depends on the production process. It is gen-

erally accepted that processes resulting in ﬁne-grained, pore-free, and heat-

284 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

Fig. 11 TEM cross section of the ﬁlm formed on AM50⫹1%Ca during immersion in NaCl

(after Ref. 24).

treatable microstructures enhance corrosion resistance. In this connection, high

cooling rates as achieved by new techniques such as RS are very useful.

Conventional Methods. Pressure die casting typically takes place at lower

temperatures and solidiﬁes in a fraction of time as compared to gravity casting,

thus leading to ﬁner microstructures. Die-cast alloys, e.g., AZ91, are generally

more corrosion resistant than their mold-cast counterparts.

If the matrix grains

are ﬁne, a more continuous

␤

phase forms along their boundaries, and the barrier

area per material volume is larger.

The casting process also affects the size

and distribution of cathodic phases formed when impurities are present. In AZ91

alloys the Ni tolerance was reported to be signiﬁcantly lower for the gravity-

cast application than for die-cast application.

The Fe tolerance was essentially

unaffected by the cast process, being in both cases determined by the amount

of Mn present. Heat treatments seem to have no inﬂuence on size and distri-

bution of primary phases in the Al–Mn–Fe series.

The corrosion resistance of Mg alloys can be further improved by heat treat-

ments which promote the precipitation of the

␤

phase.

33,54

In fact, the corrosion

resistance of AZ91 in the artiﬁcially aged condition (T6) is superior than in the

as-cast (F) or homogenized (T4) states. A reduction on the maximum pit depth

is also observed in artiﬁcially aged specimens. Aging causes precipitation of

secondary

␤

phase along the grain boundaries and improves the barrier effect

against corrosion. On the other hand, solution heat treatment dissolves the

␤

phase and often leads to slightly increased corrosion rates relative to the as-cast

condition.

Thixomolding is the high-speed injection molding of thixotropic, semisolid

alloys. In the recent years the thixomolding process is becoming popular for

production of Mg parts. In this technique ﬂuxes are eliminated because no ex-

ternal melting is required and oxide formation is precluded by the use of an

argon atmosphere. Thixomolded materials exhibit less porosity and nonmetallic

inclusions. The corrosion rate of thixotropic-molded Mg alloys containing Al is

3 CORROSION 285

reduced by 50% or more as compared to die-cast counterparts.

This reduction

is attributed to an Al enrichment at the surface, due to shear thinning that results

in few if any

␣

particles at the surface, and a reduction in pinhole porosity. Die

castings are more prone to porosity (due to the high velocity turbulence of

molten metal entering the die) and oxide and ﬂux inclusions.

Micropores should have a detrimental effect on the corrosion resistance of

Mg parts due to the following reasons:

(1) Autocatalytic corrosion cells can

form when electrochemical reactions inside the pore are obstructed by the cor-

rosion products. (2) Micropores result normally from defects in the alloy, which

are more active points for corrosion. (3) By increasing the real exposed surface,

they increase the corrosion rate (i.e., amount of corrosion per unit area of ap-

parent surface).

Rapid Solidiﬁcation (RS). Rapid solidiﬁcation introduces important micro-

structural changes, e.g., signiﬁcant reﬁnement of matrix and intermetallics, more

homogeneous distribution of impurities, metastable extension of the solid solu-

bility, and formation of amorphous phases, which contribute toward the forma-

tion of more protective ﬁlms and the elimination of microgalvanic effects. RS

can be successfully applied to Mg alloys.

However, one should keep in mind

that the industrial feasibility of RS Mg alloys is restricted due to the limited

geometry of these materials necessary to achieve high cooling rates. A common

method to achieve high cooling rates during solidiﬁcation is melt spinning. The

melt-spun ribbons can be mechanically grounded to powders, sealed in cans,

and extruded to produce bars. Another way to overcome the problem of ge-

ometry might be the application of the corrosion-resistant RS material at the

surface, e.g., by laser melting/alloying.

As in conventional alloys, Al suppresses the corrosion rate of binary RS Mg

alloys, in particular for large additions; however, addition of Zn, Si, and Ca

enhances it.

62,64

Zinc shows a somewhat anomalous effect, causing the corrosion

rate of RS magnesium to increase to a maximum at 18.6% Zn and then decrease

with further additions.

RS intensiﬁes the beneﬁts of Al and RE in the stabilization of the passive

ﬁlm in two ways: It increases their solid solubility limits and provides a more

homogeneous substrate where it is easier to form the ﬁlm. The maximum solid

solubility of Al in Mg was extended up to 24.5 or 23.4% by splat cooling or

melt spinning, respectively.

65,66

In 0.001 M NaCl an anodic plateau develops at

low currents, the pitting potential shifts to more noble values, and the corrosion

rate decreases by two orders of magnitude when the Al in solid solution is

extended from 9.6 to 23.4%.

Homogeneous Mg(Al) solid solutions are advan-

tageous because of the formation of Al-stabilized ﬁlm over the entire surface.

Improvements in the passivation behavior in 0.01 M NaCl were also observed

in several RS alloys containing RE, e.g., Mg–Zn–Zr–RE (RE

⫽ Nd, Ce, or Y),

Mg–Nd, and Mg–Y.

67,68

As mentioned above, it is not clear whether RE elements

become enriched in the surface ﬁlm; in RS alloys both enrichment

68,69

and de-

pletion

were reported.

If the ﬁlms become more stable along with a more homogeneous alloy mi-

crostructure, susceptibility to pitting should be reduced. This was observed for

AZ61, where RS increases the breakdown potential by about 200 mV in Cl

⫺

286 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

Fig. 12 Corrosion rate of light alloys obtained by weight loss measurements in 5% NaCl

solution. * Data from electrochemical impedance spectroscopy. (Reprinted, by permission

containing solutions.

Since pitting is assumed as a necessary precursor to SCC,

RS might help to produce SCC-resistant Mg alloys.

RS was also suggested to increase the tolerance limit for Fe.

The ennoble-

ment of the matrix with Al and the reduced rate of proton discharge around the

reﬁned Fe inclusions were considered to be the origin of this improvement. RS

does not necessarily eliminate microgalvanic reaction between Mg matrix and

heavy metal precipitates as shown by the higher corrosion rates of RS Mg–Mn

alloys containing Ni in comparison with RS pure Mg.

However, if RS leads

to a sufﬁciently ﬁne partially crystallized or fully amorphous structure (where

heavy metals and Mn are dispersed or even solved), the microgalvanic reaction

is suppressed and the corrosion moves to a more general kind.

73,74

RS Mg–Al–Zn alloys (around 9–21% Al and 3–7% Zn) exhibit up to one

order of magnitude lower corrosion rates and better anodic polarization behavior

in NaCl than the conventionally cast counterparts.

A several years development

program on extruded RS Mg–Al–Zn alloys with, e.g., Mn and RE additions,

resulted in the evaluation of alloys, e.g., EA55RS (4.8–5.2% Al

⫹ 4.7–5.0% Zn

⫹ 5.0–5.5% Nd), with better performance than the corrosion resistant AZ91.

Figure 12 shows that corrosion rates of RS alloys in NaCl are approaching to

those of Al alloys.

Amorphous materials are well known for their improved corrosion resistance,

arising from the chemically homogeneous single-phase nature without compo-

sitional ﬂuctuations and crystal defects. Glass-forming ability can be found in

many Mg systems, e.g., in Mg–Ni, Mg–Cu, Mg–Ln (Ln

⫽ lanthanide metal),

Mg–Y and Mg–Ca systems often with ternary additions

but allows only thin

glassy ribbons. Many of these new partially or fully amorphous alloys are at-

tractive, since they attain high corrosion resistance. For example, amorphous

and/or nanocrystalline Mg–Y (20–26 at.% Y) ﬁlms produced by magnetron

cosputter deposition perform better in 0.1 M NaCl than pure Mg and even the

3 CORROSION 287

WE43 alloy.

Recently, new perspectives were open with bulk amorphous alloys

in the system Mg–TM–Ln (where TM is Ni or Cu and Ln is a lanthanide

metal),

77–79

which could be used as structural materials. These glasses display

an exceptionally large glass-forming ability and high thermal stability. The

supercooled liquid region before crystallization is very wide (

⬎60⬚C for

and ⬎40⬚C for Mg

) and the temperature of crystallization

increases with the solute content.

77,80

The most promising bulk amorphous alloy

is Mg

10,

which can be produced with a thickness of 4 mm by metallic

mold casting and 7 mm by high-pressure die casting

77,78

and exhibits good cor-

rosion resistance.

81,82

Bulk amorphous Mg–Ni–Nd alloys were obtained with a

thickness of 3.5 mm and show higher corrosion resistance than AZ91E and

EA55RS alloys in 3.5% NaCl solutions.

Surface Protection

As already pointed out, the ﬁlms that form on Mg, although quite protective in

the absence of humidity and contaminants, do not serve perfectly in all environ-

ments. Coatings are used to supplement or replace the natural ﬁlm, or as better

foundation for subsequently applied paint. Direct application of a paint on Mg

alloys is difﬁcult due to the basic character of the naturally formed ﬁlm, typically

with a pH of 10.5. Coating can be done by chemical conversion, metal plating,

anodization, or application of an organic coat.

Effective protection against cor-

rosion usually requires ﬁnishing schemes that combine several of these tech-

niques.

Traditional chemical treatments for Mg are generally based on chromate so-

lutions, which are under increasing environmental regulation. With high-purity

alloys, selected phosphate treatments can be as effective as chromates. Chemi-

cal treatments increase paintability and retard the natural alkali that forms

at any point of damage on a painted Mg surface. Used alone they do not

provide enough corrosion protection (among other reasons due to their solubility

in water).

Any metal that can be electrodeposited can be applied successfully to Mg,

but caution should be taken when using electroplating to protect components

exposed to corrosive environments. This process is used to attain bright, tarnish-

resistant surfaces and to improve wear resistance. In general, plating of Mg

consists of a zinc immersion plating and a copper strike, followed by electro-

plating in standard plating baths. Copper–nickel–chromium plating systems

provide good protection, even though galvanic corrosion problems would be

expected when the plate is damaged.

Anodic coatings combine reasonable corrosion resistance with excellent paint

base qualities and abrasion resistance. These hard ceramiclike coatings exhibit

different degrees of inherent porosity. The pores must be sealed to enhance

corrosion resistance. The coatings can be ﬁlled with polymers such as polytet-

raﬂuoroethylene (PTFE) to obtain special properties, e.g., lubricity. Typical an-

odic coating processes are HAE and Dow 17.

Both processes give rise to

coatings with thickness in the range of 5–30

␮

m, consisting mainly of mixed

oxides. Anodic coatings represent a special type of chemical conversion coating

because some Mg is used to build up the protecting ﬁlm. The Dow 17 process

utilizes a chromate-based formulation. Permanganate is the key ingredient in the

288 CORROSION AND OXIDATION OF MAGNESIUM ALLOYS

HAE process, however, a chromate sealant is necessary to obtain acceptable

corrosion resistance.

Tagnite

and Magoxid-Coat

are anodic spark processes carried out at higher

voltages (ﬁnal voltage up to about 400 V AC). These anodic coatings are more

protective and not based on chromates. The Tagnite process uses a hydroxide–

silicate–ﬂuorid electrolyte and the thickness of the coating can be varied be-

tween 2 and 30

␮

m depending on current density and time. Magoxid-Coat is

formed in a slightly alkaline electrolyte and results in a partially amorphous

oxidic layer based on Mg, Al, and P. The coating consists of a thin barrier layer,

a virtually nonporous intermediate ceramic layer responsible for the good cor-

rosion resistance and a very porous outermost ceramic layer. The coating builds

up to an optimal thickness of 15–25

␮

m. Probably, the amorphous nature of the

coating contributes toward its improved corrosion behavior. Codeposition of

transition metal oxides and organic materials is used to produce additional char-

acteristics, e.g., colored coatings.

Painting of Mg parts may be used to apply a decorative ﬁnish or as a mean

of protection against corrosion and tarnish. Organic coating can vary from simple

oils or waxes to multipaint coatings. If the environment is severe, primers should

be based in alkali-resistant vehicles as, e.g., vinyl, epoxy, polyvinyl butyral,

acrylic resins, or baked phenolic resins, and contain inhibitive pigments with a

slightly soluble chromate.

Following the recent developments of surface engineering, a variety of new

surface modiﬁcation technologies, including ion implantation, chemical vapor

deposition/physical vapor deposition (CVD/PVD), and laser surface alloying/

melting, can be used to develop more effective and environmental friendly coat-

ings.

Hydride coatings were recently proposed as an alternative to chromate

coatings.

A magnesium hydride or hydrogen-rich layer with thickness of 1–2

␮

m is created on the Mg surface by cathodic electric charging in aqueous so-

lution. The hydride coating promotes pseudo-passivation of Mg. It can be used

either as a stand-alone protective coating or as an effective paint base. Implan-

tation of N

⫹

ions on AZ91D surfaces enhances corrosion resistance; an optimal

dose of 5

⫻ 10

ions/cm

results in a corrosion rate approximately 15% of that

for the unimplanted alloy.

4 OUTLOOK

Whereas oxidation resistance of Mg alloys is generally considered to be sufﬁ-

cient, at least in the temperature range of typical application, corrosion can still

be a problem. Further trends to improve corrosion properties of Mg alloys in-

clude: (1) Control of heavy metal content, (2) alloying with elements that sta-

bilize the surface oxide, e.g., RE, and with Mn that improves corrosion behavior

due to iron control effect, (3) use of RS techniques, and (4) coating and painting.

Heavy metal impurity control is the key action in the design of highly cor-

rosion resistant Mg alloys. These impurities enhance corrosion by orders of

magnitude and only their accurate control gives the indispensable basis for a

successful application of all other corrosion protection measures.

In marked contrast to Al and its alloys, Mg does not form a dense protective

oxide. The ﬁlms naturally formed on Mg, although stable in dry environments,

are rapidly undermined in the presence of water and contaminants. The useful-

ness of Mg alloys can be successfully increased by improving the oxide layer