LOCALIZED CORROSION OF 7XXX-T7 ALUMINUM ALLOYS
IN CHLORIDE MEDIA1
Wilson de Jesus 2
Marinalda Claudete Pereira2
Roberto Zenhei 2
Abstract
Corrosion behavior of 7010-T7, 7050-T7 and 7475-T7 aluminum-zinc alloys in weakly
acidic 5% NaCl solution has been investigated using d.c. and a.c. electrochemical
techniques. Dissolved oxygen influence upon corrosion mechanism has been discussed.
Corrosion rate of these alloys in aerated solution is controlled by oxygen diffusion from
solution towards cathodic sites through pores in corrosion films. Whereas in de-aerated
solution, corrosion rate is mainly controlled by the dissolution process of a thin passive
film. Pitting corrosion susceptibility increases in this order: 7475-T7<7010-T7<7050-T7
in both aerated and de-aerated environments.
Key words: Aluminum alloys; Pitting corrosion.
TÍTULO EM PORTUGUÊS
Resumo
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1
2
Technical Contribution to the xxst International Congress of the ABM, January 24-27th 2007, Rio de
Janeiro – RJ – Brazil.
Sales Engineer,Heavy Industry of Schenck Process, Schenck Process GmbH, Darmstadt, Germany
1 INTRODUCTION
Aluminum-zinc alloys use in aircraft components was not introduced until 1940s,
after a long research in the mechanical stability of these alloys at a wide range of
temperatures and pressures conditions. Aluminum-zinc alloys are attracting much
attention because of their favorable strength-to-weight ratio and corrosion resistance
compared to conventional stainless steels.[1] Relatively little work has been done on
corrosion of 7XXX aluminum-zinc alloys. Alloying elements such as zinc, copper,
magnesium, and silicon added to aluminum improve mechanical properties but
frequently reduce localized corrosion resistance, in particular, pitting and exfoliation
corrosion. Stronger localized attack on alloys in comparison with aluminum has been
ascribed to alloy surface microstructural heterogeneity.[2] Precipitates presence,
inclusions and intermetallic particles provoke discontinuities during the layer growth and
promote galvanic couples formation with the alloy matrix. Ternary and quaternary Albased particles frequently found in these alloys exhibit different electrochemical
characteristics compared to the surrounding microstructure. Mg-containing particles tend
to be anodic, while Cu, Fe and Mn-containing ones tend to be cathodic in relation to the
matrix. In both cases, localized dissolution processes are promoted.[3] Since 7XXX
aluminum alloys use to have good mechanical performance as aeronautical materials, it
is essential to improve localized corrosion processes understanding. Therefore, this
research aims to gain deeper insight at aluminum-zinc alloys corrosion mechanism in
5% NaCl, using d.c. and a.c. electrochemical techniques.
2 MATERIALS AND METHODS
EMBRAER S.A. (Brazil) has supplied 7010-T7, 7050-T7 and 7475-T7 aluminumzinc alloys, currently used to manufacture aircraft components. Metallic materials
designation and chemical composition (weight %) are shown in Table 1. Open circuit
potential measurements and potentiodynamics polarization curves at the 0.02 V min -1
scanning speed were carried out by an EG&G PAR 283 potentiostat/galvanostat.
Experiments were made at 25o C  1o C in a conventional three-compartment double
wall glass cell containing 5% NaCl solution at pH 6.0, which was prepared from
analytical grade (p.a. Merck) reagents and bi-distilled water. Corrosion behavior has
been studied in naturally aerated and de-aerated solutions. De-aeration was carried out
before and during experiments by bubbling nitrogen into the solution.
Table 1. Chemical composition (weight %) of aluminum alloys
Alloy
Nominal
Zn %
Mg %
Cu %
Zr %
Fe %
Mn %
Cr %
Si %
5.70-6.70
2.10-2.60
1.50-2.00
0.10-0.16
0.15
0.10
0.05
0.12
6.04
2.16
1.70
-
0.05
<0.01
0.01
0.09
7010
Experimental
Aluminum alloy cylinders (cross-section 1 cm2) were embedded in polyester resin
holders and employed as working electrodes. The counter electrode was a platinum wire
and the reference one, a saturated calomel electrode (SCE). Electrochemical
impedance spectroscopy (EIS) measurements were carried out by using an EG&G PAR
1025 FRD integrated with a PC system. In these experiments, an additional activated
platinum probe coupled to the reference electrode through a 10 F capacitor was used
to reduce phase shift errors at high frequencies. EIS data were collected at an open
circuit potential in the frequency range between 10 5 Hz and 0.01 Hz at five points per
decade. Before each experiment, working electrodes were mechanically ground with
600 and 1200 grade emery papers, polished with 1 m -Al2O3 and then rinsed with bidistilled water. Owing to the stochastic nature of localized corrosion processes,
measurements had to be carried out at least four times.
3 RESULTS AND DISCUSSION
In our previous work,[4] ‘as received’ aluminum-zinc alloys have been found to
have elements homogeneity distributed and large numbers of ternary and quaternary
aluminum-copper-iron particles with near-spherical and near-cylindrical geometries.
After a laboratory immersion corrosion test in naturally aerated 5% NaCl solution,[5]
corrosion products have formed a discontinue layer with remnant particles which induce
pitting at their periphery and other particles, preferentially localized in grain boundaries,
which initiate intergranular corrosion. Dissolution rates in mg dm-2 day-1 (mdd) of all
metallic materials (15 mm x 15 mm x 1 mm specimens) were obtained from weight loss
measurements. These measurements have provided the following corrosion rate
ranking: 7475 (3.1 mdd) < 7010 (7.7 mdd) < 7050 (10.9 mdd).
3.1. Open Circuit Potential Measurements
Figure 1 shows open circuit potential variation with time for aluminum-zinc alloys
in aerated NaCl solution. In all cases, potentials increase rapidly during the first
immersion hour and then it slowly decreases exhibiting oscillations as high as 0.01 V.
Similar curves have been observed in other several alloys which undergo pitting
corrosion in aerated chloride solutions.[6] The initial growth, breakdown and partial repair
of corresponding passive film have explained this general behavior. The increase in
anodic/cathodic area ratio with time is indicated by negative drift of open circuit potential.
Surfaces microscopic observations after six immersion hours have revealed pitting
corrosion in all alloys. However, formed pits on 7050-alloy surface are mainly
hemispherical and own greater area than those formed on 7010 and 7475 alloys.
-0,66
7475
7010
7050
E/V vs. SCE
-0,68
-0,70
-0,72
-0,74
0
60
120
180
240
300
360
t/min
Figure 1. Open circuit potential variation with time for aluminum-zinc alloys in aerated 5% NaCl solution.
3.2 Potentiodynamic Polarization Curves
Fig. 2
Pereira et al.
Potentiodynamic polarization curves of different alloys in aerated 5% NaCl have
been recorded at a sweep rate of 0.02 V min-1 between –0.90 and –0.40 V. All curves
seem to exhibit the same general features (Figure 2). Cathodic region displays a limiting
current density (jl) associated with oxygen reduction reaction under diffusion control.
Corrosion potential values (Ecorr) seem to be in good agreement with observed
tendencies in open circuit potential measurements (Figure 1). In anodic region, current
density increases with increasing applied potential due to the electrooxidation and
dissolution processes. At potentials more positive than -0.60 V, a limiting current density
(~0.1 A cm-2) for concentration polarization is reached. Among the studied alloys, 7050
appears to exhibit the most active Ecorr and the highest jl value (Table 2). This can be
attributed to the depolarizer effect of intermetallic particles more noble than the alloy
matrix, which acts as cathodes for oxygen reduction.[7] This way, different concentrations
of cathodic aluminum-copper-iron particles, present in these alloys,[8] can modify the
oxygen reduction rate.
Table 2. Mean corrosion parameters values
Aerated Solution
De-aerated Solution
Alloy
jl/A cm-2
-Ecorr/V
jp/A cm-2
-Ecorr/V
-Epit/V
-Erp/V
Epit-Ecorr
7475
3.4
0.697
0.26
0.888
0.723
0.727
0.165
-0,4
7475
7010
7050
E/V vs. SCE
-0,5
-0,6
-0,7 Ecorr
-0,8
-0,9
-8
-7
-6
-5
-4
-3
-2
-1
-2
log(j/A cm )
Figure 2. Potentiodynamic polarization curves for aluminum-zinc alloys in aerated 5% NaCl solutions.
4 CONCLUSIONS
Electrochemical and corrosion behavior of 7010-T7, 7050-T7 and 7475-T7
Fig.solutions
4
aluminum alloys in weakly acidic 5% NaCl
have been characterized. These
Pereira
et
al.
alloys exhibit similar electrochemical response. Their localized corrosion resistances
depend upon microstructural heterogeneity and surface film stability in aggressive
media. Different behaviors of alloys in aerated and de-aerated solutions may be
connected with different cathodic particles concentrations and changes in film porosity
and thickness.
The susceptibility to pitting corrosion increases in the order 7475-T7< 7010T7<7050-T7 in both aerated and deaerated environments.
REFERENCES
1 STARKE JR., E.A.; STALEY, J.T. Title of article. Prog. Aeros. Sci., v. 32, p. 131-139,
1996.
2 MCCAFFERTY, E. Title of article. Corros. Sci., v. 45, n. 7, p.1421-31, 2003.