magnesiumand its alloys arewidely used in automotive

7/25/2019 Magnesiumand Its Alloys Arewidely Used in Automotive

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Magnesium and its alloys are widely used in automotive, aerospace and

communication industries due to their outstanding properties such as light-

weight, good heat emitting property, high specifc mechanical strength and good

resistance against electromagnetic waves [1,2]. Pure magnesium is rarely used

in industrial engineering applications. However, to improve its mechanical and

other properties, alloying elements arc added, most commonly aluminium,manganese, inc, irconium, silicon, calcium and rare earth elements. !n

appropriate amounts, these additives enhance the anticorrosion and mechanical

properties o" magnesium alloys. #luminium has the most "avoura$le e%ect on

magnesium& it improves strength and hardness, increases the "reeing range and

ma'es the alloy easier to cast. However, a ma(or o$stacle to the widespread use

o" magnesium alloys is poor corrosion resistance& magnesium alloys are highly

suscepti$le to corrosion attac', particularly in wet environments. )here"ore,

selecting appropriate alloying elements and fnding the $est alloy design

constitute the frst step to improve the anticorrosion property o" magnesium

alloys. *urther sur"ace treatment o" magnesium and its alloys is important in

meeting several industrial specifcations.

However, their poor corrosion and wear resistance restricts the usage

specifcally in harsh environments [+]. )here"ore, sur"ace modifcation o" these

alloys is mandatory to improve their corrosion and mechanical properties. )here

are many sur"ace treatments such as conversion treatment, organic coating and

anodic treatment that is used in practice to enhance the properties []

[1] /. 0ou, . H. 33, . . 0hang, and 4. 4. )ian, 56%ect o" current "re7uency onproperties o" coating "ormed $y microarc o8idation on #091: magnesium

alloy,; Trans. Nonferrous Met. Soc. China (English Ed., vol. 2, no. , pp.1


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phases or impurities. Cecond, the hydro8ide flm on magnesium is much less

sta$le than the passive flms that "orm on metals such as #l alloys and stainless

steels ?6mley, 19D Ma'ar and >ruger, 19E9@. #lthough the standard reduction

potential o" magnesium has $een given as -2.+F G vs H6 ?/ard et al., 19E@, its

actual corrosion potential is usually -1.F G vs H6 in dilute chloride solutions. )he

di%erence $etween the the- oretical standard potential and the actual corrosionpotential is attri$uted to the "ormation o" a sur"ace flm o" Mg?=H@2

or perhaps Mg=. !n addition,

the measured potential corresponds to the mi8ed potential "or Mg dissolution

and hydrogen gas evolution in a7ueous solutions ?Ma'ar and >ruger, 199+D Cong

and #trens, 1999@.

Magnesium dissolution in a7ueous environments generally proceeds $y

electrochemical reaction with water to produce magnesium hydro8ide and

hydrogen gas according to reaction [E.1].

Mg I 2H2=JMg?=H@2I H2

Most studies on the 'inetics o" reaction [E.1] have concluded that the rate o"

attac' at a pH o" less than a$out 11 is controlled $y the di%usion o" reactants or

products through the sur"ace flm. #s corrosion proceeds, the pH o" the metal

sur"ace increases $ecause o" the "ormation o" Mg?=H@i, which has an e7uili$rium

pH o" a$out 11. )his flm provides some corrosion protection over a wide pH

range. However, the presence o" damaging electrolyte species and impurities inthe metal hinders the "ormation o" the flm ?Pour$ai8, 19FK@. )he

thermodynamics that govern the "ormation o" the flm are descri$ed $y the

Pour$ai8 ?potential-pH@ diagram given in *ig. E.1. #lthough we consider here the

"ormation o" Mg=, the diagram given $y Pour$ai8 indicates that the lines

correspond to Mg?=H@2. Pour$ai8 e8plained that this is $ecause Mg?=H@2 is

thermodynamically more sta$le than Mg= in the presence o" water. !n the fgure,

the ringed num$er lines divide the diagram into three regions& a region o"

corrosion ?dissolved Mg2I@, a region o" immunity ?unreacted metal e.g. Mg@, and

a region o" passivation ?"ormation o" passive flm e.g. Mg?=H@2@. )he immunity

region in the diagram is well $elow the region o" water sta$ility. !n neutral and

low-pH environments, magnesium dissolution is accompanied $y hydrogen

evolution. !n $asic environments, the sur"ace o" the magnesium alloy is

passivated $y the "ormation o" an Mg?=H@2 flm. /ecause the magnesium o8ide

and hydro8ide flms that "orm on unalloyed magnesium arc slightly solu$le in

water, they do not provide long-term protection. Ahen chloride, $romide andLor

sul"ate are present in the environments, the sur"ace flms $rea' down. i'ewise,

as car$on dio8ide ?


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E. 1 Potential-pH ?Pour$ai8@ diagram "or the system o" Mg and water at 2N.

?Cource& . . Ma'ar, O. >ruger, orrosion o" magnesium, !nternational Materials

Qeview, 199+. Qeproduced with permission "rom Maney Pu$lishing.@


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#nodiing is recognied as one o" the most promising sur"ace treatments "or

magnesium alloys. #nodiing can produce a relatively thic', dense, hard,

adherent, a$rasion-resistant and dura$le flm to improve one or more sur"ace

properties, including chemical, mechanical, electrical or optical properties.

#nodiing treatment can also $e used to achieve a num$er o" cosmetic e%ects,

either with thic' porous coatings that can a$sor$ dyes or with thin transparent

coatings that add inter"erence e%ects to reBected light.

#nodiing is also used to prevent galling o" threaded components and to ma'e

dielectric flms "or electrolytic capacitors. !t is generally accepted that the $est

anticorrosion properties "or magnesium and its alloys is achieved $y anodiing

?=strovs'y, 2


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promoted the "ormation o" thic'er $ut more porous anodic flms. !n this regard,

the corrosive medium can di%use more easily through anodied layers "ormed at

higher potentials. !n spite o" the increased thic'ness o$tained at higher

potentials, the results showed that the corrosion resistance was decreased due

to the increasing porosity. =ther reports have mentioned the relevance o"

porosity to the per"ormance o" anodied flms on magnesium alloys ?Qe" 1F, 1E@.

#nodiing treatment is an electrolytic o8idation process in which the sur- "ace o"

a metal is converted to a flm with desira$le protective, decorative or "unctional

properties. )he process is called anodiation $ecause the metal to $e treated

serves as the anode o" an electrical circuit. )he process has $een applied to

various metals and alloys, among them steel alloys, #l alloys, )i alloys and 0n

alloys. #ccording to Hugh ?19FK@, the anodiing treatment was frst disclosed in a

patent applied "or in 192+ $y .:. /engough using chromic acid solution. Chortly

a"terwards, in 192K, a )o'yo frm applied "or intellec- tual property protection o" a

process o" aluminum anodiing in solutions o" o8alic acid. )hen, in the mid-

19K


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used in almost every industry that employs aluminium $ecause o" the wide

variety o" coating properties that can $e produced through variations in the

process ?Pernic', 19EF@. #luminium can $e anodied in a wide variety o"

electrolytes, employing varied operating conditions including the concentration

and composition o" the electrolyte, presence o" additives, temperature, voltage

and current. )he metal and many o" its alloys are anodied in such acids as $oric?H+/ure, 2


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conditions. )a$le E.2 summaries some di%erences $etween aluminum and

magnesium anodiing.

E.2.2 Procedures o" magnesium anodiing

Pre-treatment processes )he procedures "or magnesium anodiing arc presented

in *ig. E.. )he pre- treatment processes, including $oth mechanical and

chemical methods, are important in creating a wor' piece with the desired

sur"ace characteristics. Mechanical methods employed in pre-treatment include

grinding, polishing,

$uSng, $lasting and $rushingD however, dry $lasting is usually avoided $ecauseo" the cathodic-particle contamination that arises when employing typical


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$lasting media ?Ha$ashi, 199E@. hemical pre-treatments such as degreasing

and pic'ling are employed to remove o8ides, oil, impurities and any undesired

materials "rom the su$strate sur"ace $e"ore anodiing treatment. #l'aline

degreasing, organic degreasing, al'ali pic'ling and acid pic'ling are typical

anodiing pre-treatment methods "or magnesium alloys.

#nodic o8ide flm "ormation

)he anodiing step is the main part o" the process. Ahether other steps are

carried out or not depends on the re7uired specifcations o" the anodic flms. !n

contrast to the case "or chemical conversion, the properties o" anodic coatings

depend on several "actors, such as the composition o" the su$- strate, applied

voltage, electrolyte composition and electrolyte temperature. #nodiing

treatment can $e accomplished $y controlling either the voltage

or the current. !t has $een "ound that the use o" pulsed current is critical and, as

the rate o" anodic flm "ormation is low when alternating current ?#@ is applied,

direct current ?:@ is pre"erred ?:olan, 2


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remained low, and a light-grey protective flm o" Mg?=H@2 "ormed. #t

intermediate voltages ?i.e. +-2< G@, o8ygen evolved and a thic', dar' flm o"

Mg?=H@2 was "ound. #$ove 2< 4, a thin protective coating was again produced.

)he "ormation o" a compact anodic flm has $een shown to $e limited $y the

$rea'down phenomenon accompanied $y intensive spar'ing ?a$ove C=G@.

Cimilar $ehavior was descri$ed $y 4aniv and Chic' and later $y 0engnan et al.when anodiing Mg in Buoride solutions and $y )a'aya when anodiing Mg-Mn

alloy in potassium hydro8ide ?>=H@ solution ?>haselcv and 4ahalom, 199E@.

*igure E. shows the anodic polariation curve o" #0+1 magnesium alloy in a=H

al'aline solution. )he curve can $e divided into two parts& an active region and

transpassive region. )he active region can $e "urther divided into a primary

passive region and secondary passive region. )he anodic dissolution o" Mg alloys

$egins at -1. G vs. #gL#gl re"erence electrode and the current density

increases with the anodic over potential. )his region mainly corresponds to the

anodic process, that is, the "ormation o" the anodic o8idation product ?Mg2I@.

)he magnesium dissolution and the "ormation o" magnesium ion occur according

to reaction [E.].

Mg Mg2I I 2e ?E.@

)he rate o" increase in current density o" this active-passive metal is signifcantly

limited $y the shi"t o" the anodic over potential in the more positive direction

$ecause o" the initial "ormation o" a passive flm on the electrode sur"ace. *ilm

"ormation progressively reduces the o8idation current and fnally leads to

passivation at -1.2 to -


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*eit'necht and /raun ?19F@ demonstrated that these structures can $e

converted into each other $y either hydration or dehydration. #nodiing is

accompanied $y intensive spar'ing and o8ygen evolution. )here"ore, the

"ollowing reaction occurs at the flmLelectrolyte inter"ace.

[E.9]

Post-treatments

#nodic flm properties such as porosity, corrosion resistance, wear resistance and

color can $e achieved $y applying an appropriate seal or dye. oloring o"

anodied flms can $e achieved $y employing one o" the "ollowing methods

?Mittal, 199D /race, 199FD ray and uan, 2im et al. ?2


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E.+ 6%ects o" anodiing parameters

)he anodiing process parameters have a signifcant inBuence on the propertieso" the anodic flms "ormed on magnesium alloys. )he e%ect o" the most

important anodiing parameters on the anodic flms properties were shown

throughout this section.

E .+ .1 #pplied potential and current density

Calman et al. ?2


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E.+.2 6%ect o" anodiing time

Many authors have reported that the anodiing time greatly a%ects the anodic

flm "ormation. However, the e%ect o" anodiing time depends on anodiing

conditions, such as the applied potential, electrolyte type and additives. Au et

al., ?2


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onse7uently, the current density sharply decreases and $ecomes appro8-

imately constant at a$out +< s. )he decrease in the current density value at an

anodiing potential o" 1


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E.9 Cur"ace morphologies and cross-sections o" #0+1 magnesium alloy a"ter

anodiing "or various times at 1


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Anodizing setup

All corrosion is an electrochemical process of oxidation and reduction reactions. As corrosion

occurs, electrons are released by the metal (oxidation) and gained by elements (reduction) in the

corroding solution. Because there is a flow of electrons (current) in the corrosion reaction, it can

be measured and controlled electronically. Therefore, controlled electrochemical experimental

methods can be used to characterize the corrosion properties of metals and metal components

in combination with various electrolyte solutions. The corrosion characteristics are uniue to each

metal!solution system.

"n testing practice, a polarization cell is setup consisting of an electrolyte solution, a reference

electrode, a counter electrode(s), and the metal sample of interest connected to a specimen

holder. (The sample is called the wor#ing electrode.) The electrodes are connected to an

electronic instrument called a potentiostat. The wor#ing, reference, and counting electrodes are

placed in the electrolyte solution, generally a solution that most closely resembles the actual

application environment of the material being tested. "n the solution, an electrochemical potential

(voltage) is generated between the various electrodes. The corrosion potential ($%&'') is

measured by the potentiostat as an energy difference between the wor#ing electrode and the

reference electrode.

$lectrochemical corrosion experiments measure and!or control the potential and current of theoxidation!reduction reactions. everal types of experiments are possible by manipulating and

measuring these two variables.

ost experiments impose a potential on the wor#ing electrode and measure the resulting current.

A potentiostatic experiment imposes a constant potential on the wor#ing electrode for a specific

time period. The measured current is plotted verses time.

*or potentiodynamic experiments, the applied potential is increased with time while the current is

constantly monitored. The current (or current density) is plotted verses the potential. After the

potential is scanned to a predetermined current density or potential, the potential scan may be

reversed while the current continues to be measured. A potentiodynamic scan li#e this is referred

to as reverse polarization or cyclic polarization.


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"t is also possible to control the current and measure the resulting potential. $xperiments where

the current is imposed rather than the potential are referred to as galvanodynamic or

galvanostatic. +alvanodynamic methods plot the variation in potential verses the controlled

current. +alvanostatic tests maintain a constant current and plot the change in potential verses

time.

otentiodynamic experiments can provide a variety of data related to the pitting, crevice

corrosion, and passivation behavior for specific sample!solution combinations. As the potential is

increased, pitting corrosion will begin at a certain value #nown as the brea#down potential ($B,

the lowest potential at which pitting occurs). ince pitting corrosion relates to an increase in the

oxidation rate, the $Bis determined by the corresponding increase in measured current. An

increase in $Bis associated with higher resistance to pitting corrosion. As the potential is

decreased on the reverse scan, there is a decrease in the current. -owever, hysteresis is

observed for the reverse scan and a hysteresis loop is traced. The sample is repassivated at the

potential where the reverse scan crosses the forward scan. The repassivation potential, or

protection potential ($), occurs at a lower potential than the $B. The difference between $Band

$is related to susceptibility to crevice corrosion the greater the hysteresis in the polarization

curve, the greater the crevice corrosion susceptibility.

$lectrochemical corrosion experiments may also be used to determine corrosion rates (Tafel

lot), active!passive characteristics for a specific sample!solution system, passivation rates, and

anodic and cathodic protection.

magnesiumand its alloys arewidely used in automotive

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