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The effect of T4 heat treatment on the microstructure and corrosion behaviour

of Zn27Al1.5Cu0.02Mg alloy

Biljana Bobic a,⇑, Jelena Bajat b, Zagorka Acimovic-Pavlovic b, Marko Rakin b, Ilija Bobic c

a IHIS R&D Center, Batajnicki put 23, 11080 Zemun, Serbiab Faculty of Technology and Metallurgy, Karnegijeva 4, 11120 Belgrade, Serbiac INN Vinca, University of Belgrade, Mike Petrovica Alasa 12-14, 11001 Belgrade, Serbia

a r t i c l e i n f o

Article history:

Received 3 March 2010

Accepted 22 September 2010

Available online 29 September 2010

Keywords:

A. Zinc

A. Alloy

B. Weight loss

B. Polarization

B. SEM

a b s t r a c t

The effect of heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy

was examined. The alloy was prepared by melting and casting route and then thermally processed (T4

regime). Corrosion behaviour of the as-cast and heat treated alloy was studied in 3.5 wt.% NaCl solution

using immersion method and electrochemical polarization measurements. The applied heat treatment

affected the alloy microstructure and resulted in increased ductility and higher corrosion resistance of

the heat treated alloy. Electrochemical measurements of the corrosion rate at the free corrosion potential

are in agreement with the results obtained using the weight loss method.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Zn27Al1.5Cu0.02Mg alloy (ZA27 alloy in the further text) be-

longs to a group of zinc alloys with increased content of aluminium

(ZA alloys). The alloy has been of used in technological applications

for several decades. ZA27 alloy with a nominal aluminium content

of 27 wt.% has the highest strength and the lowest density of the

ZA alloys [1]. The alloy has been shown to possess favorable com-

bination of physical, mechanical and technological characteristics

(low melting point, high strength, exceptional castability, easy

machinability, high corrosion resistance, as well as excellent bear-

ing and wear resistance properties) [1,2]. ZA27 alloy has been used

for pressure die castings and gravity castings wherever very high

strength is required: in automobile engine mounts and drive trains,

general hardware, agricultural equipment, domestic and garden

appliances and heavy duty hand and work tools [3,4]. The alloy

has been also used in bearings and bushing applications as a

replacement for bronze bearings because of its lower cost and

equivalent or superior bearing performances [5].

During past two decades a few different approaches have been

taken in order to improve physical, mechanical, tribological and

corrosion properties of ZA27 alloy at room temperature: (a) addi-

tion of elements like Ni, Ti and Sr [6,7], Mn [8] or Mg and rare

earths [9]; (b) using different heat treatment regimes [6,10–12]

and thermomechanical treatments [13,14]; (c) improvements in

the alloy manufacturing techniques e.g. the use of thixoforming

[15,16] or unidirectional solidification [17–19]; (d) production of

composites with Al2O3 [20], SiC [21], ZrO2 [22] and graphite parti-

cles [23] or glass fibres [24].

Mechanical properties of ZA27 alloy can be influenced by ther-

mal processing. It was reported [12] that ductility and structural

stability of Zn25Al3Cu alloy were markedly improved by applying

T4 heat treatment. It was also shown that T4 regime had a benefi-

cial effect on the tribological characteristics of the commercial

ZA27 alloy [11], although it resulted in a minor reduction in hard-

ness and tensile strength. In addition, T4 heat treatment is rela-

tively cheap and easy to perform, thus providing time and energy

savings. Upon exposure to the corrosive environment many ther-

mally processed alloys are subjected to drastic changes. Possible

effects of used heat treatments on the alloy performance in a cor-

rosive medium are essential for a complete understanding of the

alloy corrosion behavior [25]. The influence of metallic microstruc-

ture on the corrosion performance of zinc and zinc alloys has been

recently evaluated in dependence on the applied thermal treat-

ment [26]. Determination of mass loss during field trials and

immersion tests and anodic polarization studies were used for cor-

rosion behaviour assessment [26].

Corrosion characteristics of the as-cast ZA27 alloy have been

previously studied and reported in [27,28]. Aluminium presence

in the alloy has a favourable effect on its corrosion behaviour

[27,28]. The alloy exhibits high corrosion resistance in the atmo-

spheric conditions, natural waters, soil etc. because of zinc ability

to form a protective layer of corrosion products at the surface

0010-938X/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2010.09.051

⇑ Corresponding author. Tel.: +381 11 316 8154; fax: +381 11 194 991.

E-mail address: [email protected] (B. Bobic).

Corrosion Science 53 (2011) 409–417

Contents lists available at ScienceDirect

Corrosion Science

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After each LPR test Tafel plots were obtained by starting the po-

tential scan from a cathodic potential and increasing the potential

towards the anodic side [35,44,45] at a scan rate of 0.2 mVs1.

Each electrode was potentiodynamically polarized in the potential

range ±0.250 V over the respective OCP .

2.3. Microstructure and surface morphology characterization

Microstructure and surface morphology of the as-cast and heat

treated samples of ZA27 alloy were examined before the immer-

sion test and after 1 month of exposure in the test solution. The

samples were analyzed by optical microscopy (OM) and scanning

electron microscopy (SEM) combined with energy dispersive spec-

trometry (EDS). SEM/EDS and X-ray diffraction method (XRD) were

used to characterize corrosion products of both ZA27 alloys. Carl

Zeiss optical microscope and JEOL JSM – 5800 scanning electron

microscope coupled with Oxford Link ISIS energy dispersive spec-

trometer were used. XRD patterns were obtained using Siemens

model D500 X-ray diffractometer.

The samples for microstructure studies were rinsed with ace-

tone and dried in the air before their exposure to test solution

(3.5 wt.% NaCl). After exposure, surface of the samples was ground

and polished. Wet grinding was performed on progressively finer

abrasive paper (240, 360, 600 and 800 grit SiC), while polishing

was done using polishing cloth and diamond paste (up to 2 lm

particles size). After washing in distilled water and drying with

warm flowing air, the samples were etched in 9% v/v nitric acid

to reveal the microstructure, while polished samples were sub-

jected to SEM and SEM/EDS analysis.

3. Results and discussion

3.1. Corrosion studies

3.1.1. Immersion test

After 1 month of exposure to 3.5% NaCl solution corrosion prod-

ucts were removed from the surface of the samples. It could be no-

ticed that corrosion had occured uniformly over the surface of the

exposed as-cast and heat treated samples. The average corrosion

rate C R, in mm year1, was calculated on the basis of the samples’

mass loss during immersion test [50]:

C R ¼ K W

A T D ð1Þ

where K is a constant [49], W is sample mass loss in grams, A is sam-

ple area in cm2, T is time of exposure in hours and D is density of

ZA27 alloy in g cm3.

The calculated values of average penetration rate were

0.118 mm year1 for the as-cast and 0.095 mm year1 for the heat

treated ZA27 alloy. The result obtained for the as-cast alloy is in

good agreement with the result reported in [52].

3.1.2. Electrochemical polarization measurements

3.1.2.1. LPR test. Polarization curves in a small potential range near

to OCP were obtained in the LPR test for both as-cast and heat trea-

ted ZA27 alloy. Polarization resistance Rp was determined from the

slope of iR corrected experimental curve (dE /d j) at the corrosion

potential E corr (Fig. 1a and b).

It can be seen that applied heat treatment resulted in increased

value of R p. Polarization resistance can be converted to corrosion

current density jcorr using the Stern–Geary equation [38]:

jcorr ¼ B

Rpð2Þ

where B is a parameter dependent on the values of anodic ba andcathodic Tafel slope bc :

B ¼ ba bc

2:303 ðba þ bc Þ ð3Þ

Accordingly

jcorr ¼ ba bc

2:303 ðba þ bc Þ Rpð4Þ

This expression was derived on the assumption that both ano-

dic and cathodic reactions were charge-transfer controlled and

that ohmic drop iR was negligible [25]. For a process that is con-

trolled by diffusion of the cathode reactant (transport control)

and in which the anodic process is under activation control the

modified Stern–Geary equation applies [25]:

jcorr ¼ ba2:303 Rp

ð5Þ

It can be noticed from Tafel plots in Fig. 2a and b that anodic

reaction is activation controlled alloy (ZA27) dissolution with ano-

dic Tafel slope ba = 40 mV dec1, while the cathodic reaction is un-

der diffusion control of oxygen reduction. Accordingly, the Eq. (5)

was used to calculate corrosion current density. The calculated val-

ues were 8 lA cm2 for the as-cast and 7.15 lA cm2 for the heat

treated ZA27 alloy, which is in a very good agreement with the

results reported [7].

3.1.2.2. Tafel plots. Polarization curves for the as-cast and heat trea-

ted ZA27 alloys were recorded in the potential range ±0.250 V with

respect to OCP . The curves were corrected for the ohmic drop iR.Corrected polarization curves are presented in Fig. 2a and b.

Fig. 1. LPR plots of ZA27 alloy in 3.5 wt.% NaCl. (a) ZA27 as-cast, (b) ZA27 heat

treated.

B. Bobic et al. / Corrosion Science 53 (2011) 409–417 411



The anodic polarization curves show a considerable increase in

current density for a small increase in polarization indicating ac-tive alloy dissolution during anodic polarization. Such behaviour

is typical of Zn dissolution in natrium chloride solution [35]. The

curves exhibit Tafel behaviour over about 1.5 current decade which

means that the anodic reaction for both ZA27 alloys is activation

controlled with anodic Tafel slope ba = 40mVdec1. This value cor-

responds closely to values reported in literature for the anodic dis-

solution of zinc [34,52]. The anodic Tafel slope ba = 40 mV dec1

was used to calculate corrosion current density in the LPR test

according to the Eq. (5) (Section 3.1.2.1).

The cathodic polarization curves show highly polarized behav-

iour, i.e., only a small increase in current density was obtained

for a significant increase in polarization. This is indicative of catho-

dic reaction under diffusion control [25,39,53]. Low solubility of

oxygen (about 103

mol dm3

) [25] in the test solution limits thetransport of oxygen to the electrode surface and the cathodic reac-

tion is under dominant diffusion control of oxygen reduction.

When the reaction rate is entirely controlled by the rate of mass

transport it no longer depends on potential [36,53]. Accordingly,

the value of the cathodic Tafel slope bc ?1 [29,53].

Considering that the cathodic reaction is under diffusion con-

trol. The corrosion current densities for the as-cast and heat trea-

ted ZA27 alloys were determined by extrapolation of the anodic

Tafel lines to the corrosion potential E corr (Fig. 2a and b). The value

of corrosion current density for the as-cast ZA27 alloy as

14lA cm2 whereas T4 heat treatment resulted in the alloy with

somewhat lower corrosion current density (6.6 lA cm2), pointing

to its higher corrosion resistance.

The results obtained by using Tafel plots are in accordance withthe results obtained in LPR test, where it was also shown that

applied heat treatment of ZA27 alloy resulted in the alloy of greater

corrosion stability.

Theweightloss methodhas beenconsidered asthe most accurate

method in determining corrosion rate of metal materials liable to

general (uniform) corrosion andhas been widely used as a criterion

to investigate the accuracy of electrochemical polarization tests

[26,41,45]. With regardto this values of jcorr obtainedin the LPR test

(Section3.1.2.1.) andby usingTafel plots(Section 3.1.2.2.) werecon-

verted into penetration rate C R and compared with the results of

immersion test. Corrosion current density jcorr , inlA cm2, and cor-

rosion rate C R expressed in mm year1 are related by the following

equation [39]:

C R ¼ K i jcorr D

E w ð6Þ

where K i is a constant [39], E W is equivalent weight of ZA27 alloy

and D is as in Eq. (1).

Corrosion rates obtained by the weight loss method and by

electrochemical polarization measurements are expressed as pen-

etration rates and presented in Fig. 3.

It can be seen that lower values of corrosion rate were obtained

for the heat treated alloy in relation to the as-cast alloy whichmeans that applied heat treatment resulted in increased corrosion

stability of ZA27 alloy. In addition, the results of polarization mea-

surements are in a very good agreement with the results of weight

loss measurements, excluding the result obtained by using Tafel

plot (ZA27 alloy as-cast). This could be possibly explained by the

irreversible changes of the electrode surface at higher polarizations

[54] which confirms that corrosion rate estimations based on Tafel

extrapolation should be compared to weight loss measurements

whenever possible [45].

The value of corrosion rate for each ZA27 alloy can be expressed

as an average of the values obtained by the weight loss method and

by electrochemical measurements. Comparing these average val-

ues of the corrosion rate one can reveal that corrosion rate of the

heat treated alloy is about 30% lower than that of the as-cast alloy.Although this effect is not remarkable it should not be neglected.

3.2. Microstructures

Corrosion behaviour of ZA27 alloy is determined by the alloy

microstructure, that is by chemical composition and distribution

of the alloy constituents (phases) [7,17,19].

Fig. 3. Corrosion rates of ZA27 alloy. (A) weight loss method, (B) linear polarizationresistance test, (C) Tafel plots.

Fig. 2. Tafel plots of ZA27 alloy correctedfor iR drop. (a) ZA27 as-cast, (b)ZA27 heat

treated.

412 B. Bobic et al. / Corrosion Science 53 (2011) 409–417



3.2.1. ZA27 alloy, as-cast

The alloy was casted in metal mold (Section 2.1.) and hence

subjected to rapid cooling. Solidification of the alloy in these con-

ditions resulted in the alloy with dendritic microstructure. Micro-

structure of the as-cast ZA27 alloy is shown in Fig. 4a–c. Inclusions

can be noticed in the alloy sample (Fig. 4a) while porosity was not

observed at this level of examination.

Non-metallic inclusions in ZA27 alloy might be oxides of Al, Zn,

Cu, Mg or intermetallic compounds like CuZn4, AlCu, Al4Cu, AlMg,

Mg2Zn11 [6,55] and FeAl3 [55]. The inclusions arise because of var-

ious physical–chemical effects that occur during melting and solid-

ification of an alloy [55].

General appearance of the alloy microstructure after etching is

shown in Fig. 4b. Dendrites are complex (Fig. 4b and c), consisting

of a core (a phase) and a periphery (a mixture of a phase and hex-

agonal g phase). g phase is located into interdendritic regions.

According to the aluminium–zinc phase diagram [1] solidifica-

tion of ZA27 alloy in the equilibrium conditions starts with the

appearance of a phase particles at 493 C. At 443 C the peritectic

reaction occurs: L + a = b. This b phase is unstable at lower temper-

atures and at about 320 C a phase emerges. The rest of b phase

decomposes at 275 C (eutectoid temperature) according to the

following relation:b = a + g (phase mixture). Considering that mu-

tual solubility of zinc and aluminium is insignificant, it could be ex-

pected that a fine mixture of aluminium and zinc would be created

during equilibrium solidification. However, in a real casting pro-

cess regardless of the procedure applied, the solidification rate of

ZA27 alloy is much higher and the time for peritectic reaction to

occur is significantly reduced. ZA27 alloy solidifies dendritically

therefore. Particles of a phase react partially with the melt creating

high-temperature b phase that solidifies on the surface of a parti-

cles. Upon further cooling b phase transforms at eutectoid temper-

ature into a phase mixture (a + g), which makes the periphery of

dendrites (Fig. 4b and c).

Variations in chemical composition of microconstituents in the

as-cast sample of ZA27 alloy areshown in Fig. 4d. SEM/EDS analysis

was performed along the L line (Fig. 4c). The line of analysis startsfromthe core of a dendrite goes through its periphery and interden-

dritic phase, passes the periphery of a second dendrite, and ends in

thecoreof theseconddendrite. It canbe seen (Fig.4d) that dendritic

cores are rich in aluminium; interdendritic phase is rich in zinc,

while the composition of the dendritic periphery is approximately

equal to the chemical composition of ZA27 alloy. Variations in cop-

per concentration (low in the dendritic core and very high in the

interdendritic phase) indicate presence of intermetallic compound

CuZn4 (e-phase) in the interdendritic regions. It was reported that

presence of e phase had a beneficial effect on mechanical and wear

properties of Zn25Al3Cu alloy, particularly after certain heat treat-

ment regimes [11]. Oxygen presence points to the existence of an

oxide/hydroxide film at the alloy surface [30,31,42].

The effect of corrosion on the microstructure of the as-cast sam-ple after one month exposure in the test solution is shown in

Fig. 5a–d. It can be seen that corrosion has started on the edge of

the sample and around inclusions (Fig. 5a), like it was reported ear-

lier [56]. It was noticed [25] that corrosion or zinc andzinc alloys in

salt solutions usually started at places where defects (scratches,

abrasion) or impurities were present. Chemical composition of

inclusions and their chemical and physical characteristics, as well

as their distribution and quantity are dependent on the chemical

composition of an alloy, smelting method and solidification regime

[55]. Accordingly, an investigation of the inclusions’ effect on the

alloy corrosion behaviour is rather complex and has to be treated

from the standpoint of metal solidification theory related to the

method of the alloy processing (in liquid state), but that was be-

yond the scope of this work. Corrosion behaviour of the as-castand heat treated ZA27 alloy was examined assuming that chemical

composition of inclusions and their amount were unaffected by theapplied thermal treatment. Chemical composition of ZA27 alloy

Fig. 4. Microstructure of as-cast ZA27 alloy. (a) OM, polished, (b) OM, etched, (c)

SEM, polished; DC, dendritic core; DP, dendritic periphery; IDS, interdendritic

space; (d) EDS, variations of chemical composition along the L line (Fig. 4c).




has remained identical after thermal processing although a signif-icant morphological change took place.

Corrosion processes around inclusions in the as-cast sample

mainly take place laterally over the sample surface, which can be

also seen on the etched sample (Fig. 5b). Destruction of g phase

and a + g phase mixture regions occur during the corrosion pro-

gress (Fig. 5b). Microcracks can be observed (B) at higher magnifi-

cation, besides the corrosion products on the sample surface and

around some inclusions (A) (Fig. 5c). The microcracks are sites

of more intensive corrosion [56]. Besides the microcracks

propagation along the a/a + g phase boundary, transcrystalline

microcracks can be also seen (Fig. 5d) probably as a result of

machining process during the sample preparation.

3.2.2. ZA27 alloy, heat treated

Microstructures of the heat treated ZA27 alloy are shown in

Fig. 6a–c.

After thermal processing the microstructure of ZA27 alloy has

remained dendritic although a significant morphological change

Fig. 5. Microstructure of as-cast ZA27 alloy after exposure in 3.5 wt.% NaCl. (a) OM,

polished, b) OM, etched, (c) SEM, polished; A, inclusions, B, microcrack, (d) SEM,

polished.

Fig. 6. Microstructure of heat treated ZA27 alloy. (a) OM, polished, (b) OM, etched,(c) SEM, polished.




took place (Fig. 6b and c) as a result of T4 heat treatment. Solution-

izing time (3 h) was not enough for a complete homogenization of

the alloy to take place i.e., for a complete destruction of dendritic

cores and interdendritic g phase. The regions of a + g phase mix-

ture were extended while dendritic cores (a phase region) and

interdendritic regions (g phase) were reduced (Fig. 6c). A decrease

in size of dendritic cores (a phase) and rounding off their edges

happened, as well as the separation of individual dendritic cores

into several smaller segments. Smaller dendritic cores were trans-

formed into a + g phase mixture. During heating at 370 C for 3 h,

there was an expansion of b phase at the expense of supersaturated

a andg phases. After cooling, the newly createdb phase was trans-

formed into a + g phase mixture. According to our results [12] the

lattice parameter of a phase in the heat treated alloy was reduced

comparing to the lattice parameter of a phase in the as-cast alloy

as a consequence of zinc diffusion from the metastablea phase. Be-

sides, it was shown by quantitative metallographic analysis [47]

that volume fraction of a + g phase mixture was increased while

both volume fractions of a and g phase were reduced in the heat

treated ZA27 alloy in relation to the as-cast alloy.

T4 heat treatment that was applied within this work differs

from the heat treatment regime prescribed by standard where fur-

nace cooling instead of water quenching was prescribed [46]. It

was shown [12] that structure coarsening and appearance of T0

phase (Al4Cu3Zn2) took place during furnace cooling of ZA27 alloy.

The presence of T0 phase in the alloy structure was also confirmed

in [57]. This phase is brittle and thus has a bad effect on the alloy

ductility. The appearance of T0 phase in the heat treated samples of

ZA27 alloy was avoided within this work by using T4 regime,

which resulted in increased ductility of the heat treated alloy as

was reported in [12].

The effect of corrosion on the microstructure of the heat treated

sample is shown in Fig. 7a–d. Corrosion attack is observed on the

sample edge (area of mechanical damage) and around some inclu-

sions (Fig. 7a). The corrosion has taken place in the region of a + gphase mixture and in the interdendriticg phase (Fig. 7b). Bright is-

lands of a phase are surrounded by dark corrosion products. Cor-roded areas are shown in Fig. 7c and d. The arrows in Fig. 7c

indicate corrosion progress through the region of a + g phase mix-

ture as well as through the interdendritic g phase. Zinc-rich corro-

sion products are mainly needle-like and rosette-shaped crystals

(Fig. 7d). The appearance of microcracks in the heat treated sam-

ples was not observed after 1 month of exposure in corrosion envi-

ronment which indicates increased ductility and thus, greater

corrosion stability of the heat treated alloy.

3.3. Surface appearance of test samples after electrochemical tests

Surface appearance of the as-cast and heat treated sample of

ZA27 alloy after electrochemical polarization measurements is pre-

sented in Fig. 8a and b. It is noticeable that the layer of corrosionproducts on the sample surface is quite thin (A), so that the alloy

microconstituents can be observed through this layer, e.g., white

traces of g phase are visible.

According to the results of EDS analysis ‘‘in point” (surface area

A and B, Fig. 8a and b) an increase in oxygen amount can be ob-

served, which indicates formation of oxides and hydroxides during

corrosion process. This effect was more pronounced in the heat

treated sample (Fig. 8c).

3.4. Corrosion products

After 1 month exposure to quiescent NaCl solution the surface

of test samples was covered with white powdered corrosion

products. A part of the corrosion products that had been detachedfrom the samples’ surface was precipitated in the test solution.

Corrosion products were collected by filtration of the test solutionand dried before subjected to XRD analysis. Characteristic XRD pat-

Fig. 7. Microstructure of heat treated ZA27 alloy after exposure in 3.5 wt.% NaCl. (a)

OM, polished, (b) OM, etched, (c and d) SEM, polished.




terns are presented in Fig. 9a and b. It can be seen that corrosion

products of ZA27 alloy are composed mainly of Zn(OH)2 and ZnO,

as it was reported in [31]. According to the intensities of reflexion

it can be concluded that corrosion products of the as-cast alloy

contain a larger amount of Zn(OH)2 and a smaller amount of ZnO

compared to the heat treated ZA27 alloy.

Corrosion products of aluminium were not detected by XRD

analysis after 1 month exposure of the as-cast and heat treated

ZA27 alloys to NaCl solution, possibly because the time of exposure

was too short for crystallized corrosion products to be formed. It

was reported that in the initial stage of seawater corrosion of

Zn–Al alloy coating (22–30 wt.% Al) the gelatinous deposit of

Al(OH)3 was first produced on the coating surface and that with

lengthening of immersion time corrosion products tended to in-crease in crystallinity and grain size [58]. After 3, 6, 12 and

18 months exposure to seawater corrosion products were typical

nanometer microcrystals, containing mainly Zn4CO3(OH)6H2O,

Zn5(OH)8Cl2 and Zn6Al2CO3(OH)164H2O [58].

Based on all the results presented it could be concluded that ap-

plied heat treatment (T4) affected the ZA27 alloy microstructure,

as well as its corrosion stability. The alloy morphology was chan-

ged so that the regions of a + g phase mixture were extended

while dendritic cores (a phase region) and interdendritic regions

(g phase) were reduced (Fig. 6c). After T4 heat treatment micro-

cracks were not observed in thermally processed ZA27 alloy

(Fig. 6a). Also, the appearance of microcracks in the heat treated

samples was not noticed after one month exposure to NaCl solu-tion (Fig. 7). This indicates an increase in ductility and thus, greater

corrosion stability of the heat treated alloy. Corrosion products of

the heat treated ZA27 alloy contain a larger amount of ZnO com-

pared to the as-cast alloy like it was shown by XRD analysis

(Fig. 9a and b). All these resulted in greater Rp values (Fig. 1a and

b) and lower jcorr values (Figs. 2 and 3) of the heat treated ZA27 al-

loy, namely in reduced corrosion rate. These results allow us to

conclude that T4 heat treatment of ZA27 alloy has a small benefi-

cial effect on its corrosion stability and that was the aim of this

work.

4. Conclusions

On the basis of the results presented the following conclusionscan be made:

Fig. 8. Surface appearance of test samples after electrochemicaltests. (a)SEM, ZA27

as-cast, (b)SEM, ZA27 heat treated, (c)oxygen amount in thesamples of ZA27 alloy;

A, corroded area; B, non corroded area.

Fig. 9. XRD patterns of ZA27 alloy corrosion products. (a) ZA27 as-cast, (b) ZA27

heat treated.1 – Zn(OH)2, 2 – Mg(OH)2, 3 – ZnO.




1. T4 heat treatment affected the microstructure and corrosion

resistance of ZA27 alloy.

2. The alloy structure remained dendritic after T4 heat treatment

and corrosion process takes place through g phase and a + gphase mixture.

3. T4 heat treatment has a small beneficial effect on the corrosion

resistance of ZA27 alloy.

4. Increased ductility and favourable corrosion properties of the

heat treated ZA27 alloy indicate its potential use in manufactur-

ing machine parts like gears and worm gears.

5. Electrochemical measurements of corrosion rate, based on the

corrosion current at free corrosion potential, are in good agree-

ment with the results obtained by the weight loss method.

Acknowledgements

This work was financially supported by the Ministry of Sciences

and Environmental Protection of the Republic of Serbia through the

projects TR 19061 and TR 14005B. The authors are gratefully

acknowledged to RAR foundry Batajnica, for providing the master

alloy for performance of the research.

References

[1] E. Gervais, R.J. Barnhurst, C.A. Loong, An analysis of selected properties of ZA

alloys, JOM 11 (1985) 43–47.

[2] E.J. Kubel Jr., Expanding horizons for ZA alloys, Adv. Metal Prog. 7 (1987) 51–

57.

[3] H. Chendler, Heat Treater’s Guide: practices and procedures for nonferrous

alloys, ASM International, 1996.

[4] ASM Handbook, Properties and selection: nonferrous alloys and special-

purpose materials, vol. 02, ASM International 1990.

[5] P.P. Lee, T. Savasakan, E. Laufer, Wear resistance and microstructure of Zn–Al–

Si and Zn–Al–Cu alloys, Wear 117 (1987) 79–89.

[6] P. Choudhury, K. Das, S. Das, Evolution of as-cast and heat-treated

microstructure of a commercial bearing alloy, Mater. Sci. Eng., A 398 (2005)

332–343.

[7] P. Choudhury, S. Das, Effect of microstructure on the corrosion behavior of a

zinc–aluminium alloy, J. Mater. Sci. 40 (2005) 805–807.

[8] C. Dominguez, M.V. Moreno Lopez, D. Rios-Jara, The influence of manganese on

the microstructure and the strength of a ZA27 alloy, J. Mater. Sci. 37 (2002)5123–5127.

[9] T.J. Chen, Y. Hao, Y. Sun, Y.D. Li, Effects of Mg and RE additions on the semi-

solid microstructure of a zinc alloy ZA27, Sci. Technol. Adv. Mater. 4 (2003)

495–502.

[10] M.T. Jovanovic, I. Bobic, B. Djuric, N. Grahovac, N. Ilic, Microstructural and

sliding wear behavior of a heat-treated zinc-based alloy, Tribol. Lett. 3 (2007)

173–184.

[11] M. Babic, A. Vencl, S. Mitrovic, I. Bobic, Influence of T4 heat treatment on

tribological behavior of ZA27 alloy under lubricated sliding condition, Tribol.

Lett. 2 (2009) 125–134.

[12] I. Bobic, B. Djuric, M.T. Jovanovic, S. Zec, Improvement of ductility of a cast Zn–

25Al–3Cu alloy, Mater. Charact. 29 (1992) 277–283.

[13] H. Aashuri, Globular structure of ZA27 alloy by thermomechanical and semi-

solid treatment, Mater. Sci. Eng., A 391 (2005) 77–85.

[14] T.J. Chen, Y. Hao, J. Sun, Microstructural evolution of previously deformed

ZA27 alloy during partial remelting, Mater. Sci. Eng., A 337 (2002) 73–81.

[15] T.J. Chen, Y. Hao, Y.D. Li, Effects of processing parameters on microstructure of

thixoformed ZA27 alloy, Mater. Des. 28 (2007) 1279–1287.

[16] T.J. Chen, Y. Hao, J. Sun, Y.D. Li, Phenomenological observations on

thixoformability of a zinc alloy ZA27 and the resulting microstructures,

Mater. Sci. Eng., A 396 (2005) 213–222.

[17] W.R. Osorio, C.M. Freire, A. Garcia, Theeffectof thedendritic microstructure on

the corrosion resistance of Zn–Al alloys, J. Alloys Compd. 397 (2005) 179–191.

[18] G.A. Santos, C.M. Neto, W.R. Osorio, A. Garcia, Design of mechanical properties

of a ZA27 alloy based on microstructure dendriticarray spacing, Mater. Des. 28

(2007) 2425–2430.

[19] A.E. Ares, L.M. Grassa, S.F. Gueijman, C.E. Schvezov, Correlation between

thermal parameters, structures, dendritic spacing and corrosion behavior of

Zn–Al alloyswith columnar to equiaxed transition, J. Cryst. Growth310 (2008)

1355–1361.

[20] I. Bobic, M.T. Jovanovic, N. Ilic, Microstructure and strength of ZA27 based

composites reinforced with Al2O3 particles, Mater.Lett. 57 (2003) 1683–1688.

[21] Z.Q. Li, B.Y. Wu, S.Y. Zhang, Pretreatment process of SiC particles and

fabrication technology of SiC particulate reinforced Zn–Al alloy matrix

composite, Mater. Sci. Technol. 17 (2001) 954–960.

[22] S.C. Sharma, D.R. Somashekar, B.M. Satish, A note on the corrosion

characterization of ZA-27/zircon particulate composites in acidic medium, J.

Mater. Process. Technol. 118 (2001) 62–64.

[23] S.C. Sharma, B.M. Girish, R. Kamath, B.M. Satish, Graphite particles reinforced

ZA-27 alloy composite materials for journal bearing applications, Wear 219

(1998) 162–168.

[24] S.C. Sharma, B.M. Girish, B.M. Satish, R. Kamath, Mechanical properties of as-

cast and heat-treated ZA-27 alloy/short glass fiber composites, J. Mater. Eng.

Perform. 7 (1998) 93–99.

[25] L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, third ed., Butterworth,

Heinemann, Oxford, 2000.[26] T.H. Muster, W.D. Ganther, I.S. Cole, The influence of microstructure on surface

phenomena: rolled zinc, Corros. Sci. 49 (2007) 2037–2058.

[27] F.C. Porter, Corrosion Resistance of Zinc and Zinc Alloys, in: P.A. Schweitzer

(Ed.), Marcell Dekker, New York, 1994.

[28] R.J. Barnhurst, S. Belisle, Corrosion properties of Zamak and ZA alloys, Noranda

Report, 1992.

[29] V. Barranco, S. Feliu Jr., S. Feliu, EIS study of the corrosion behaviour of zinc-

based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed

coatings, Corros. Sci. 46 (2004) 2203–2220.

[30] Y. Hamlaoui, F. Pedraza, L. Tifouti, Comparative study by electrochemical

impedance spectroscopy (EIS) on the corrosion resistance of industrial and

laboratory zinc coatings, Am. J. Appl. Sci. 4 (2007) 430–438.

[31] T.H. Muster, I.S. Cole, The protective nature of passivation films on zinc:

surface charge, Corros.Sci. 46 (2004) 2319–2333.

[32] H. Dafydd, D.A. Worsley, H.N. McMurray, The kinetics and mechanism of

cathodic oxygen reduction on zinc and zinc–aluminium alloy galvanized

coating, Corros. Sci. 47 (2005) 3006–3018.

[33] K. Aramaki, The inhibition effects of cation inhibitors on corrosion of zinc in

aerated 0.5 M NaCl, Corros. Sci. 51 (2009) 1043–1054.

[34] C. van den Bos, H.C. Schnitger, X. Zhang, A. Hovestad, H. Terryn, J.H.W. de Wit,

Influence of alloying elements on the corrosion resistance of rolled zinc sheet,

Corros. Sci. 48 (2006) 1483–1499.

[35] S.T. Vagge, V.S. Raja, R.G. Narayanan, Effect of deformation on the

electrochemical behavior of hot-dip galvanized steel sheets, Appl. Surf. Sci.

253 (2007) 8415–8421.

[36] P.A. Schweitzer, Fundamentals of corrosion, Mechanisms, Causes and

Preventive Methods, CRC Press, Taylor & Francis Group, Boca Raton, 2010.

[37] P. Marcus, F. Mansfeld, Analytical Methods in Corrosion Science and

Engineering, CRC Press, Taylor & Francis Group, Boca Raton, 2006.

[38] M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56.

[39] R.G. Kelly, J.R. Scully, D.W. Shoesmith, R.G. Buchheit, Electrochemical

Techniques in Corrosion Science and Engineering, Marcell Dekker, Inc., 2002.

[40] F. Mansfeld, Tafel slopesand corrosion rates obtained in the pre-Tafel regionof

polarization curves, Corros. Sci. 47 (2005) 3178–3186.

[41] E. McCafferty, Validation of corrosion rates measured by the Tafel

extrapolation method, Corros. Sci. 47 (2005) 3202–3215.

[42] H.J. Flitt, D.P. Schweinsberg, Synthesis, matching and deconstruction of polarization curves for the active corrosion of zinc in aerated near-neutral

NaCl solutions, Corros. Sci. 52 (2010) 1905–1914.

[43] H.J. Flitt, D.P. Schweinsberg, Evaluation of corrosion rate from polarisation

curves not exhibiting a Tafel region, Corros. Sci. 47 (2005) 3034–3052.

[44] M.A. Amin, K.F. Khaled, S.A. Fadl-Allah, Testing validity of the Tafel

extrapolation method for monitoring corrosion of cold rolled steel in HCl

solutions – Experimental and theoretical studies, Corros. Sci. 52 (2010) 140–

151.

[45] E. Poorqasemi, O. Abootalebi, M. Peikari, F. Haqdar, Investigating accuracy of

the Tafel extrapolation method in HCl solutions, Corros. Sci. 51 (2009) 1043–

1054.

[46] BS EN 12844:1999 Zinc and zinc alloys. Castings. Specifications.

[47] I. Bobic, PhD Thesis, Belgrade, 2003.

[48] ASTM E10–08StandardTest Method for BrinellHardness of Metallic Materials.

[49] ASTM G31–72 (2004) Standard Practice for Laboratory Immersion Corrosion

Testing of Metals.

[50] ASTM G1–03 Standard Practice for Preparing, Cleaning and Evaluating

Corrosion Test Specimens.

[51] K.R. Baldwin, M.J. Robinson, C.J.E. Smith, Corrosion rate measurements of electrodeposited zinc–nickel alloy coatings, Corros. Sci. 36 (1994) 1115–

1131.

[52] X.G. Zhang, Corrosion and Electrochemistry of Zinc, Plenum Press, New York,

1996.

[53] D. Landolt, Corrosion and Surface Chemistry of Metals, first ed., EPFL Press,

2007.

[54] G. Rocchini, The determination of Tafel slopes by an integral method, Corros.

Sci. 36 (1994) 113–125.

[55] ASM Handbook, Metallography and Microstructures, vol. 9, ASM International,

2004.

[56] R.J.H. Wanhill, T. Hattenberg, Corrosion-Induced Cracking of Model Train Zinc–

Aluminium Die Castings, NLR-TP-2005-205, (2005).

[57] S. Murphy, T. Savaskan, Metallography of zinc-25% Al based alloys in the as–

cast and aged conditions, Prakt. Metallogr. 24 (1987) 204–221.

[58] Y. Li, Formation of nano-crystalline corrosion products on Zn–Al alloy coating

exposed to seawater, Corros. Sci. 43 (2001) 1793–1800.


the effect of t4 heat treatment on the microstructure and corrosion behaviour of zn27al1.5cu0.02mg...

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