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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
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o r s c i
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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