Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5

AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
© 2011, Science Huβ, http://www.scihub.org/AJSIR
ISSN: 2153-649X doi:10.5251/ajsir.2011.2.2.283.296
Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5% NaCl
solution with range of temperature (100-500)°C
1
2
Abdulkareem Mohammed Ali Alsamuraee , Hani Aziz Ameen and Sami Ibrahim Jafer
3
Al-Rubaiey
1
2
3
University of Baghdad-Col. of Science-Chemistry Dept - Baghdad, Iraq,
E-mail:[email protected]
Technical College – Baghdad – Iraq, E-mail:[email protected]
University of Technology – Baghdad-Iraq, E-mail: [email protected]
ABSTRACT
The effect of homogenous heating variables (temperature and time holding) on the tendency of
high strength Al-alloys type 7020 is investigated. The experiments were carried out by heating the
specimens at different temperatures (100, 150, 200, 250, 300, 350, 400, 450 and 500) °C for two
periods of holding time (one hour and two hours). Microstructure of an alloy was investigated by
the optical microscope before and after heating at each temperature and precipitation phase and
clearing revealed MgZn2. An immersion test was applied on specimens after heating to show the
reduction in weight. Polarization technique was used to study the effects of microstructures on
their electrochemical behavior in 3.5% NaCl solution. Microscopic and X-ray examination were
used to reveal pitting corrosion on the microstructure of non-heated and heated 7020 Al-alloys.
The results show that increasing the temperatures of heating from 100°C to 500°C caused the
appearance of pitting corrosion on the precipitated phases and increase of pitting corrosion
tendency with increase of temperature of heating; increasing time holding of heating from 1 hr. to
2 hrs. increases the pitting corrosion.
Keywords : Pitting corrosion , Heat treatment, Aluminum alloys
INTRODUCTION
Many metals suffer from corrosion; aluminum alloys
are one of them. They are extensively employed as
structural components in industrial practice. Some
surface treatments (e.g. anodizing, chromating,
painting, coating and cathodic production) are used
to improve their corrosion resistance. Very few
studies have been devoted to the case of pitting
corrosion. Zhao et al, 1998 study of the morphology
of pits formed by corrosion of thin films of aluminum
using an in-plane light scattering technique. It can be
shown that the corrosion front of the thin aluminum
film can be treated as a quasi-two-level random
rough surface. Zuhair and Amro, 2002. The
electrochemical behavior of aluminum alloy
6061/Al2O3 metal matrix composite was investigated
in a 3.5% NaCl aqueous solution; cyclic polarization
tests were carried out to determine pitting potentials
and repassivation potentials in 3.5% NaCl solution.
This work included an investigation of pitting
corrosion of aluminum alloy 7020 is predicted, thus
the prediction of the pitting corrosion is a challenging
task. This paper has a novel discussion of the pitting
corrosion behavior.
Experimental Work
Materials and Equipments : A plate of aluminum
alloy (7020) having dimension (300x80)mm of 10mm
a thickness was prepared. Table 1 illustrates the
comparison of the chemical composition of the alloy
with the standard condition of 7020 aluminum alloys.
An analysis of the chemical composition was carried
out by using Arun Technology Metal Scan Desktop
Metals Analysis (2500 series – England, 2004).
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Table 1: The Chemical Composition of 7020 Alloy
Elements
Standard value %
(Aluminum manual,2007)
Si
< = 0.35
Fe
< = 0.40
Cu
< = 0.20
Mn
0.05 - 0.5
Mg
1 -1.5
Cr
0.10 – 0.35
Zn
4–5
Ti
0.08
Al
Balance
The Preparation of the Specimen: Specimens of
(25x30) cm were cut from the metal plate. The
dimension of the surface to be tested is 25cm x
25cm, a 0.5cm upper edge was left in order to
suspend the specimen in the solution from a 0.2cm
diameter hole located in the center upper edge of the
0.5cm.
The Preparation of the Surface Specimen: The
surface specimen is grounded with abrasive paper of
grades 400, 800, 1000 and 1200 respectively .The
grinding process was continued until the clay layer
was removed. This process was followed by two
stages of polishing. In the first stage alumina paste
having particles of 5µ diameter was used. In the
second stage 0.5µ diameter of alumina paste was
used in order to obtain good polished surface. The
specimen was then cleaned with acetone to remove
any dirt or grease from the polished surface.
Measured value%
0.121
0.290
0.200
0.0764
1.25
0.228
4.56
0.0319
Balance
8. Heating No.8: homogenization treatment at 450°C,
for 1hr and 2hrs; each follow by cooling in furnace.
9. Heating No.9: homogenization treatment at 500°C ,
for 1hr and 2hrs; each follow by cooling in furnace.
Classification of specimens: The specimens were
classified into three groups, as shown in Table 2.
Program Procedures: Program procedures are
summarized in Figure 1 .
Heating: Nine heat treatments were performed in the
electrical furnace type GARBOLITE – SHEFFIELDENGLAND +15°C with maximum of 1200°C in the
following manner:
1. Heating No.1: homogenization treatment at 100°C,
for 1hr and 2hrs; each follow by cooling in furnace.
2. Heating No.2: homogenization treatment at 150°C,
for 1hr and 2hrs; each follow by cooling in furnace.
3. Heating No. 3: homogenization treatment at 200°C,
for 1hr and 2hrs; each follow by cooling in furnace.
4. Heating No.4: homogenization treatment at 250°C,
for 1hr and 2hrs; each follow by cooling in furnace.
5. Heating No.5: homogenization treatment at 300°C,
for 1hr and 2hrs; each follow by cooling in furnace.
6. Heating No.6: homogenization treatment at 350°C,
for 1hr and 2hrs; each follow by cooling in furnace.
7. Heating No.7: homogenization treatment at 400°C,
for 1hr and 2hrs; each follow by cooling in furnace.
Fig 1: Flowchart of the Program Procedure
284
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Environment “Electrolyte Preparation”: In order to
study the pitting corrosion of 7020 aluminum alloy; a
solution of a 3.5%NaCl and de-ammonized distilled
water for the solution was prepared and measured by
a pH meter. It was found to be 6.9 .
Corrosion test: The pitting corrosion behavior of the
specimen in Table 2 was carried out by two
examination methods:
Table 2: Classification of Specimens
Symbol
A
B
C
State
As received
Heat (100 – 150 – 200 – 250 – 300 – 350 – 400 – 450– 500°C) for 1 hr and furnace
cooling
Heat (100 – 150 – 200 – 250 – 300 – 350 – 400 – 450 – 500°C) for 2 hr and furnace
cooling.
Immersion tests: Immersion test was applied on the
specimens of Table 2 as follows: Sea water was
prepared containing a 3.5%NaCl solution and one
liter of distilled water. Specimens were measured
before immersion for seven days in cups containing
sea water.
After surface preparation, the surface areas of the
specimens of Table 2 were carefully measured. Since
area enters into the formula for calculating the
corrosion rate, the original areas 25mm x 25mm were
used to calculate the corrosion rate. Specimens are
electrically insulated and isolated from contact with
another metal to avoid any galvanic effects. All
original the areas of 25mm x 25mm are completely
immersed in the tested solution.
Electrochemical tests: The prepared specimen of
Table 2 was fixed in the holder shown in Figure 2.
The reference electrode was fixed about 1mm away
from the surface of the specimen to be tested. The
reference electrode used in this study was Saturated
Calomel Electrode (SCE). The auxiliary electrode
used in the electrochemical cell was platinum type.
The specimen holder (working electrode), together
with the reference and auxiliary electrodes were
inserted in their respective positions in the
electrochemical cell used for this purpose so that
they fit all these electrodes as shown in Figure 3. The
cell used was made of glass. Constant potentials
(anodic or cathodic) can be impressed on the
specimen, by using the potentiostat (Mlab200 of
Bank Eleck Germany). This potentiostat is able to
induce a constant potential ranging from –1V to +1V.
They are the potentials of the standard reference
electrode used in this study. The potential difference
between the working electrode (WE) and the
reference electrode (RE) and any current passing in
the circuit of the working electrode where the
auxiliary electrode can be measured by using the SCI
Computer Software. Any potential difference between
the working and reference electrodes as well as any
current in the working electrode circuit were be
automatically recorded. The results and plots were
recorded using Window XP. The scan rate can be
selected also. Polarization resistance tests were used
to obtain the micro-cell corrosion rates. In the tests,
cell current reading was taken during a short, slow
sweep of the potential. The sweep was taken from –
100mV to +100mV relative to OPC. The scan rate
defines the speed of potential sweep in mV/sec. In
this range, the curve of current density versus voltage
is almost nearly linear. A linear data fitting of the
standard model gives an estimate of the polarization
resistance in order to calculate the corrosion current
density (Icorr) and corrosion rate. The tests were
performed by using a WENKING MLab multichannels and SCI-Mlab corrosion measuring system
from Bank Electronics- Intelligent controls GmbH,
Germany 2007, as shown in Figure 4.
Microstructure Inspection: The microstructure
evolution was investigated by using computerized
optical microscopy as shown in Figure 2. The
specimens were prepared for this purpose, the figure
in Table 2 were taken for all specimens .
1.
By using electro-etching in solution [2.5%
NaCl + 8.7% NaSo3 + 11.92% NaCo3] with
distilled water .
2.
The photographs were investigated as shown
in Figure 5.
RESULTS AND DISCUSSIONS
This investigation includes the results and
discussions of the effects of temperatures (100,150
,200, 250, 300, 350, 400, 450 and 500 ) °C, and
holding time (1 hr. and 2 hrs.) of heat treatment on a
microstructure and pitting corrosion of Al-alloys type
285
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
7020 in 3.5% NaCl.
three categories:-
The results were divided into
1. Effects of heating temperature.
2. Effect of holding time of heating.
3. Study the Morphologhy of pits.
6 in the aggressive chloride ions. Pits are initiated at
weak sites in the oxide film by chloride attack. Pits
propagate according to the following anodic
reactions:Al
Effect of Heating Temperature: Figure 6 shows the
effect of heating temperature at 1 hr. holding time on
the electrochemical behavior of 7020 Al-alloys in
3.5% NaCl solution. They indicate that increasing the
heating temperature (100, 150, 200, 250, 300, 400,
450 and 500) C° led to a decrease in the passive
region of the alloy and shift the Ecorr to more negative
direction (-745,-830,-805,-790,-760,-595,-621,-705,695,-680) respectively. From this figure, it can be
concluded that the pitting resistance of 7020 Al-alloys
decreases with the increase of the heating
temperatures. The microstructure of an alloy consists
of solid solution and inter-metallic particles (IMCs)
remain non-dissolved, when heating at low
temperatures. Inter metallic particles can be grouped
into coarse inter metallic particles and fine
precipitates in aluminum alloys (Rollason,1973).
Coarse inter metallic particles form during the
solidification process, while fine precipitates
(including hardening precipitates in the matrix and
grain boundary precipitates) form during ageing
process (Rollason ,1973). As mentioned earlier, intermetallic particles play a crucial role in localized
corrosion of aluminum alloys. The coarse intermetallic particles can be divided into two groups.
Active particles and Noble particles relative to the Almatrix. Therefore, the inter-metallic particles exhibit
electrochemical properties different from the Matrix.
Mg containing phases are active to the matrix and act
as anode. They are susceptible to active dissolution
or (Mg) de-alloying when immersed in chloride
solution. However, the higher is the level of
precipitation hardening which occurs at high heating
temperatures the higher is the susceptibility to
corrosion. Active components of the matrix alloy and
the inter-metallic phases may corrode selectively,
resulting in changed corrosion properties. Pitting is a
highly localized type of corrosion as shown in Figure
- Al+3 + 3eَ
Al+3 + 3H2O
(1)
Al(OH)3 + 3H+ (2)
Hydrogen evolution and oxygen reduction are the
important reduction processes at the inter metallic
cathodes as
2H+ +2eَ
O2 + 2H2O + 4eَ
H2
(3)
4OHَ
(4)
According to reaction 2, the pH will decrease as
pitting propagates. Therefore, the environment inside
the pit (anode) changes. To balance the positive
charge produced by reactions 1and 2, chloride ions
will migrate into the pit. The resulting HCl formation
inside the pit causes accelerated pit propagation. It is
postulated that, at the critical pitting potential,
breakdown occurs by a process of field assisted Cl
adsorption on the hydrated oxide surface and
formation of a soluble basic chloride salt which
readily goes into solution. This salt was considered to
be AlCl3 or Al(OH)2Cl. Figure 6. Table 3 shows also
the pitting current density. The pitting current density
(as shown in this Figure and Table) decreases
sharply when the oxide film of Al2O3 gives good
protection against environment attack. The pitting
current density have 50.1 μA/cm2 when the alloys
7020 have heated to 100 °C and remain 1hr then
cooling in furnaces. This value indicates low pitting
corrosion. The oxide Al2O3 can be used as a
protective film. When the heating temperatures
increases, the current density of pitting corrosion
increases. This means that the oxide film fails to
protect the surface alloy from corrosion attack. From
Figure 6 the maximum value of current pitting
corrosion take place at heating temperatures as
follows: 100> 150> 300> 400> 200> 350> 450>
250> 500°C. These results are not only attributed to
the change in the oxide film nature but also in the
characteristics of the inter-metallic particles and Almatrix.
286
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Table 3: Values of; Ecorr , Icorr , Erp , and Eb Estimated from Tafel and Cyclic Polarization Curves of Al7020 at Different Heat Treatment Temperatures with Time.
Specimens
Heat treatment
Ecorr / mV vs
Icorr /
Eb / mV vs
Erp / mV vs
Al 7020
temperature ºC
SCE
µA/cm2
SCE
SCE
1
With out(R.T.)
-745
14.3
-306
55
2
100/1h
-830
50.1
-305
48
3
100/2h
-870
53.5
-300
55
4
150/1h
-805
42.7
-330
50
5
150/2h
-819
47.0
-338
53
6
200/1h
-790
22.5
-350
0
7
200/2h
-822
25.9
-367
11
8
250/1h
-760
15.5
-358
14
9
250/2h
-810
18.6
-364
16
10
300/1h
-595
31.9
-197
23
11
300/2h
-630
33.8
-207
26
12
350/1h
-621
22.2
-211
24
13
350/2h
-609
17.9
-246
28
14
400/1h
-705
24.3
-198
82
15
400/2h
-700
19.9
-200
89
16
450/1h
-695
18.1
-166
110
17
450/2h
-691
15.8
-176
118
18
500/1h
-680
12.4
-198
145
19
500/2h
-663
11.6
-220
153
As there are differences in the composition of the
alloy, the first step in the precipitation is the
Effect of Holding Time: Figures 7, 8, 9, 10 and 11
segregation of the atoms of the alloy element into
show the effect of holding time on the
clusters or platelets with a few angstroms in
electrochemical behavior of 7020 Al- alloys in 3.5%
thickness and a hundred angstroms in diameter; it is
NaCl solution at various heating temperatures (100,
still part of the parent lattice. These platelets are
150, 200, 250, 300, 350, 400, 450 and 500) °C. They
often called Gainure- Preston Zones or GP1 Zones.
indicate that the increase of the holding time from
In the next step, the atoms of the alloy element
one hour to two hours led to an increase of the
diffuse to the GP1 Zone to form a larger one called
potential of the pitting corrosion Epit to a more
GP2 Zone. A new transition phase which is coherent
negative value. This indicates that the pitting
with the matrix is precipitated. With increasing time
resistance of 7020 Al-alloys decreases with an
and temperature of heating, the coherent transition
increase in the holding time of the heating. This is
phases convert into the equilibrium precipitate
due to precipitation of phases and intermetallic
without coherency. The tendency to pitting corrosion
compounds. It was observed that when the holding
is associated with these intermediate forms of
time of the heating temperature increases, the
precipitates. Wojcirch,1991 had observed that the
precipitation process would have sufficient time to
optimum strength was obtained by heat treatment at
occur. The kinetics of phase precipitations are usually
165°C for ten hours, after which, if the treatment time
affected by the variations in the holding time of the
is prolonged, a rapid precipitation of non coherent
heating temperature. This behavior is attributed to the
particles would take place. The solid solution (Matrix)
difference in the micro- structure of the alloy. The
rejected the non- coherent phase. This process in
study of Wojcirch,1991, evidenced that time and
this structure due to faulty heating is generally termed
temperature of heating must be chosen to give the
“reversion”. It is seen from Figure 7, that corrosion
proper precipitate size; if temperature is too low, the
current density decreases with increasing heating
critical dispersion was never be achieved, although
temperature from 100, 150, 200 and 250 °C. This can
the hardness grows with time as the precipitate size
be attributed to intermediate forms of the precipitate.
increases. If the temperature is too high, the
It is seen also that, at 2 hrs. holding time the current
precipitate is too coarse. Thus, the system’s free
density is always higher than at 1 hr. holding time. At
energy is lowered in steps by passing through a
300°C temperature the specimens were protected by
series of metastable state on the road to equilibrium.
287
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
very adhesive oxide layer which acted as a barrier
between the environment and the surface specimen.
Therefore, the current corrosion increases. The
tendency of different heated specimens to corrosion
is illustrated in Figure 8. It is seen that when heating
temperature increases, the Ecorr shifts to a more
negative value. This is because of the particles
precipitation. Figure 9 illustrates that the breakdown
potentials are constant at heating temperature of 100
and 150°C, at temperature of 400, 450 and 500 °C,
the Eb increases due to particle precipitation. The
behavior of 2 hrs. holding time is the same at that of
the 1 hr. holding time. Figure 10 illustrates the
dependence of Erp on the heating temperature. At
temperatures of 100, 150 and 200 °C the Erp
decreases; at temperature higher than 250°C, the Erp
increases and the mean value of Erp = -300 ≈ 320
mV.
Morphology of pits : The microstructure for
aluminum alloy 7020 which presented the specimen
classification A,B and C for Table 2 were shown in
Figure 11. It is observed that variation in grain size of
the alloy 7020 depend mainly on the proportion of the
zinc and magnesium elements, their values and the
presence of other alloying elements causing the
formation of different soluble or insoluble compounds
in the solid state (Rao ,2004). The morphology of pits
depends mainly on the microstructure of 7020 Alalloys. Heating at low temperatures results in a fine
distribution of (G-P) zones. Therefore, these zones
suffered pitting corrosion. The pit growth usually also
causes a fine distribution of shapes. As heating
temperature increases, (G.P) zones grow into indirect
phases of (MgxZny) or (Al2Mg3Zn3). Therefore, the
alloy
hardness
increases
with
increasing
temperature. In this condition, uniform pitting attack is
promoted and coherent precipitates are most
desirable for pitting corrosion. This is particularly
remarkable for the shape of precipitates as shown in
Figures 11 and 12. At high heating temperatures,
dark portions are formed. The portions consist of a
mixture of α-Al and precipitates. These results are
good evidences that the heating temperature and
holding time have an important and similar effect on
the microstructure of the alloy and its electrochemical
properties. It is evidenced from the figures that with
increasing heating temperature the corrosion
resistance decreases. This is an important case
related to the amount of the precipitated phases.
However, the higher is the level of precipitation, the
higher is the susceptibility to corrosion. The degree of
both precipitation and corrosion susceptibility is a
function of heated temperature and holding time. Pits
have the shape of the active phase, which may
corrode preferentially. Some phases may serve as a
critical anode and provide cathodic protection to the
surrounding matrix. Moreover, pit nucleation and
propagation occur in the zones where MgZn2 is
accumulated which take place at high heating
temperature (400-500)°C. At these temperatures, a
double pitting behavior is observed; the first one is in
the Mg and Zn enriched region and the second is in
the matrix. Tables 4 and 5 show the XRD results after
and before immersion. \
The intermetallic precipitate phase were formed after
heating at different temperatures. The diffusion of
element alloy (zinc, magnesium) was high but the
solubility at normal temperature was low. Figure 11
shows an increase of grain size if the grain size is
compared with specimen classification A in Table 2
and specimen classification B in the same Table.
After heating at different temperatures for one hour; it
can be seen that the grain size for specimen in
classification B was large; this shows the effect of
heating on grain size. The grain size of specimen
classification C heated at different temperatures for
two hours shows increasing the solubility of element.
Results of Immersion Test : The results of the
weight of specimens at different temperature before
immersion and after immersion are shown in Table 6.
288
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Table 4: Analysis of X – Ray Diffraction
Sample
A
Temperature
°C
Without heating
B
100
B
450
C
400
C
350
2θ
D(A°) measured
D(A°) standard
Phases
38.4
44.4
44.8
41.5
40.5
41.4
45.4
38.6
44.8
37.2
45.0807
40.55
38.06
41.5
42.01
44.85
41
44.5
40.5
37.5
41.4
40.5
2.34
2.03
2.01
2.174
2.225
2.179
2.330
2.02
2.35
2.03
2.01
2.18
2.23
2.182
0.02
AlMg4Zn11
Al
Al
MgZn2
MgZn
MgZn2
Al
Al
2.41
2.009
2.222
2.362
2.174
2.251
2.018
2.199
2.03
2.225
2.39
2.179
2.225
2.40
2.02
2.22
2.36
2.18
2.25
2.01
2.182
2.02
2.22
2.40
2.18
2.22
Al3Mg2
Al
Mg4Zn7
MgZn
MgZn2
MgZn2
MgZn2
MgZn2
Al
Mg4Zn7
Al2Mg3
MgZn2
Mg4Zn7
Table 5: X – Ray Diffraction Analysis of 7020 at 400°C after Immersion
θ
D(A)
D(A) standard
Phases
400 °C
1 hr.
38.8
40.44
45
2.319
2.179
2.01
2.30
2.182
2.02
Mg7Zn3
MgZn2
Al
400 °C
2 hrs.
40.6
38.45
45
2.220
2.319
2.01
2.22
2.30
2.02
Mg4Zn7
Mg7Zn3
Al
2
Table 6: Results of Weight of Specimens during the Immersion Test
Temperature
Weight of specimen before
immersion (mg)
Weight of specimen after
immersion (mg)
100
150
250
300
350
400
450
500
2.8941
4.7258
4.0490
4.1954
5.2502
4.1816
5.7875
5.5740
2.8928
4.7248
4.0487
4.1946
5.2498
4.1805
5.7856
5.5710
Corrosion Rate
C.R=(534* Δ w)/ ( ρ *A*t)
(mpy)
0.0003
0.00015
0.000058
0.00014
0.000068
0.00018
0.00026
0.0004
289
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Figure: 2
Figure: 3
Figure: 4
Without Heat Treating (R.T.)
Heat Treated At 150 °C /1hr.
Figure:5
Heat Treated At 100 °C /1hr.
Heat Treated At 200 °C /1hr.
290
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Heat Treated At 250 °C /1hr.
Heat Treated At 300 °C /1hr.
Heat Treated At 350 °C /1hr.
Heat Treated At 400 °C /1hr.
291
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Heat Treated At 450 °C /1hr.
Heat Treated At 500 °C /1hr.
Fig 6: Tafel and Cyclic Polarization Curve of Aluminum alloy 7020
292
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Figure 7: Icorr of Different Specimen’s conditions
Fig 9: Eb at Different Specimen’s Conditions
Figure 8: Ecorr at Different Specimen’s Conditions
Fig 10: Erp at Different Specimen’s Conditions
293
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Fig 11: Comparison Pit Shape with the Predicated Particles Shape at Different Heating Temperatures for
1 Hour Holding Time
294
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Fig 12: Comparison Pit Shape with the Predicated Particles Shape at Different Heating Temperatures for 2
Hours Holding Time
295
Am. J. Sci. Ind. Res., 2011, 2(2): 283-296
Films by Light Scattering", Applied Physics Letters,
Volume 73, Number 17, October 1998.
CONCLUSIONS:
The results of the study evidenced the following
conclusions:
1- Heating of the alloys has a great significant
on the corrosion resistance of 7020 Al-alloy.
2- As temperatures of heating increase from
100-500°C, pitting corrosion appears on the
precipitated phases.
3- Pitting corrosion tendency increases with
increasing the temperature of heating.
4- Increasing the time holding of heating from 1
hr. to 2 hrs increases the pitting corrosion.
REFERENCES
[1] Y.P. Zhao, C.F. Cheng, G.C. Wang, and T.M. Lu
"Characterization of Pitting Corrosion in Aluminum
[2] M. G. Zuhair and M. Amro "Corrosion Behavior of
Powder Metallurgy Aluminum Alloy 6061/Al2O3 Metal
Matrix Composites", The 6th Saudi Engineering
Conference, KFUPM, Dhahran, December 2002.
[3] Mat web, The Online Materials Database, “Aluminum
7020, 7000 series Aluminum Alloy, Aluminum
Association, 2007.
[4] E.C. Rollason "Metallurgy for Engineering”, Edward
Arnold Publishers, 4th Edition, 1973.
[5] J. Wojcirch " Faculty of Mechanism and Electrics
Influence of the Heat Treatment in the Electrochemical
Corrosion of Al-Zn-Mg alloys", revised 2, pp(81-103),
Glynia Poland, October 1991.
[6] K. S. Rao "Pitting Corrosion of Heat Treatable
Aluminum Alloys and Weld", Trans. India in St. Met,
Vol. 57, No. 6, pp (503-610), December 2004.
296