The stress corrosion cracking behaviour of 7075 (Al–Zn–Mg–Cu) alloy have been studied in a salt spray fog chamber with two vapourised aqueous solutions (0 and 5% NaCl). The paper analyses the stress corrosion resistance of 7075 aluminium alloy with several precipitation-ageing heat treatments. The results are compared with that obtained in 3.5% NaCl aqueous solution at 20 °C. The salt spray fog testing has permitted a good evaluation of SCC susceptibility in 7075 alloy. All temper conditions studied were susceptible to SCC in the different environments tested. 7075-T6 temper was the most susceptible, while in all the cases studied 7075-T73 temper was the least susceptible. Compared to 7075-T6, 7075-RRA temper improved the resistance against the SCC process, but the mechanical properties obtained were lower

Oñoro Lopez, Javier

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The stress corrosión cracking behaviour of heat-treated 
Al-Zn-Mg-Cu alloy in modified salt spray fog testing 
J. Oñoro* 
The stress corrosión cracking behaviour of 7075 (Al-Zn-Mg-Cu) alloy have been 
studied in a salt spray fog chamber wi th two vapourised aqueous solutions 
(0 and 5% NaCl). The paper analyses the stress corrosión resistance of 7075 
aluminium alloy wi th several precipitation-ageing heat treatments. The results 
are compared wi th that obtained in 3.5% NaCl aqueous solution at 20 °C. The 
salt spray fog testing has permitted a good evaluation of SCC susceptibility in 
7075 alloy. All temper conditions studied were susceptible to SCC in the different 
environments tested. 7075-T6 temper was the most susceptible, while in all the 
cases studied 7075-T73 temper was the least susceptible. Compared to 7075-T6, 
7075-RRA temper improved the resistance against the SCC process, but the 
mechanical properties obtained were lower. 
1 Introduction 
Stress corrosión cracking (SCC) occurs when a susceptible 
material is subjected to the combined influence of tensile stress, 
as well as to an aggressive environment. Crack initiation and 
propagation thus take place in a great variety of metáis and alloys. 
Among the high-strength aluminium alloys, 7075 belongs to the 
Al-Zn-Mg-Cu group and is widely used in structural elements 
made of light components, such as in the aeronautical industry. 
Despite the favourable mechanical behaviour of this alloy, SCC 
has been detected in different environmental conditions, for 
example, where the environment is not very aggressive, such as in 
the case of atmospheric humidity at room temperature [1, 2], 
Cracking is generally initiated on the surface of the component 
when small surface imperfections genérate a fissure that will 
propágate through the component. 
SCC in aluminium alloys is complex and a great number of 
parameters are involved. Once the crack appears, its growth is 
influenced by a combination of different factors: the micro-
structure of the material (heat treatment, level of impurities, 
manufacturing background, etc.), the stress level, and the 
aggressive conditions in the environment. The residual stresses 
associated to welding or fastening have a significant impact on 
material susceptibility [2, 3]. The SCC crack growth mechanisms 
fall into two generic categories: in one, crack propagation occurs 
via anodic dissolution, and in the other, it is via hydrogen 
embrittlement. The micro-structural state of the material has 
been the parameter most studied to find a microstructure 
J. Oñoro 
Ingeniería y Ciencia de los Materiales, Universidad Politécnica de 
Madrid, ETS Ingenieros Industriales. c/José Gutiérrez Abascal, 2. 28006 
Madrid, (Spain) 
E-mail: javier.onoro@upm.es 
resistant to SCC and simultaneously high mechanical properties 
[2, 4], grain-boundary precipítate size and spacing, precipítate free 
zone width, slip characteristics and grain-boundary segregation 
has been extensively studied [2]. The precipitation hardening 
process is directly responsible for SCC susceptibility in high-
strength aluminium alloys [5, 6]. These materials are immune to 
SCC in the as-quenched condition where grain-boundary 
precipitation is prevented [7], but they usually become susceptible 
with increasing precipitation hardening reaching a máximum 
before peak strength [4]. Over-aged heat treatments decrease SCC 
susceptibility and henee reduces the strength. A heat treatment 
known as retrogression and re-ageing (RRA) has been employed 
with Al-Zn-Mg-Cu alloys to obtain SCC resistance equivalent to 
the over-aged temper (T73) together with the máximum strength 
level (T6) [8-11], beneficial effeets of RRA upon SCC suscept-
ibility with respect to T6 have been attributed to increase in grain-
boundary precipítate size and spacing. Despite all the work done 
in this field, SCC continúes to be a typical mechanism of failure in 
these types of alloys. 
In the SCC process of aluminium alloys, it is possible to 
identify two different cracking performances that correspond to 
the so-called propagation región I and II [12], Fig. 1. Región I is 
basically dependent on the stress level at the crack tip. Región II, 
however, does not depend on the level of stress and has been 
defined as a material corrosión cracking process controlled by a 
physical-chemical mechanism. This mechanism is identified as 
an attack of the aggressive elements at the crack tip, which is 
limited by their diffusion speed from the bulk to the said crack tip. 
Región I starts at the stress-intensity factor threshold (íCiscc)> 
under whose limit no crack growth exists. Región II ends at the 
critical stress-intensity factor (ÍCIC) beyond which crack propaga-
tion is produced by a mechanical action that does not depend on 
environmental factors. Crack velocity has been studied by 
assuming the existence of a principal mechanism that controls 
da/dt Table 2. Precipitaron heat treatments applied 
Figure 1. SCC regions I and II 
crack growth [4], or by developing macroscopic-empirical 
parameters that globally describe the phenomenon [13]. On 
the other hand, the forces generated by corrosión producís 
modify the KISCC valúes, which is improved with over-aged heat 
treatments [14]. An effective SCC evaluation requires a thorough 
understanding of the mechanisms that come into play. 
The objective of this paper is to analyse the SCC 
susceptibility of the 7075 (Al-Zn-Mg-Cu) aluminium alloy at 
different temper conditions exposed in a fog chamber with two 
aggressive environments. 
2 Experimental 
The SCC tests were carried out with a 25 mm thick 7075 alloy 
aluminium píate, Table 1 shows the composition in wt%. The 
material was machined into specimens measuring 200 x 25 x 
25 mm3 and subjected to heat treatment. Table 2 shows the 
different temper heat treatments studied. The heat-treated 
specimens were mechanically notched and pre-cracked. Crack 
propagation testing was carried out with bolt-loaded double 
cantilever beam (DCB) specimens. The stress load was applied 
with two bolts placed at the crack front that produced a constant 
displacement (mode I) during the whole testing process. The 
crack was machined for short-transverse propagation since this 
direction is the most susceptible to SCC [1]. 
Table 1. AA-7075 aluminium alloy composition (wt%) 
Al Zn Mg Cu Cr Si Fe 
90.17 5.5 2.3 1.6 0.21 0.06 0.11 
Mn 
0.03 
Ti 
0.02 
Temper Precipitation heat treatment 
7075-T6 
7075-RRA 
7075-T73 
1 h at 495 °C, quenched in 
water + aged 24 h at 120 °C 
T6 + 10 min at 200 °C, quenched in 
water + re-aged 24 h at 120 °C 
1 h at 495 °C, quenched in 
water + aged 1 h at 107 °C and 18 h at 168 °C 
The SCC study was carried out with the samples exposed to 
the following aggressive environments: 
i. 3.5% NaCl aqueous solution at 20 °C. 
ii. Fog chamber with 0% NaCl at 30 °C. 
iii. Fog chamber with 5% NaCl at 30 °C. 
The samples tested in an aqueous solution were continu-
ously immersed in an aerated thermostatically controlled bath. 
The test was used as a reference for SCC behaviour of alloy. 
The samples tested in the fog chamber were constantly 
vapourised with an aggressive solution inside a standard cabinet 
according to ASTM G 85 [15]. The environments described as fog 
at 0 or 5% NaCl were the compositions of aqueous solution 
spraying in the fog chamber. Three samples were tested in the 
same heat-treated conditions for each environment for the 
purposes of reproducibility of the results. 
The cracking process was quantified by measuring the 
fissure on both sides of the sample with a binocular magnifying 
glass. The cracks were initially measured every 3 h, but when the 
crack growth was slower, they were then checked every 24 h. The 
samples remained under test until there was no change in 
the size of the crack. 
Since the geometry of the crack was known, the crack growth 
rate (v) was obtained as a function of the mode I stress-intensity 
factor (Ki), v—Ki curves, with the following equation: 
KT 
8Eh[3h(a + 0.6h)2 + h 
4[(a + 0.6h)3 + h2a] 
3ll/2 
(1) 
where E represents the elastic module, h the half specimen 
height, a the crack length and 8 the constant displacement at the 
load point. 
DCB samples were used in this study since their geometry 
makes it possible to obtain different stress-intensity factor valúes 
as the crack increases. In the end, either the cracking process is 
arrested or the sample is completely fractured. 
Once the SCC tests were concluded, the cracked specimens 
were studied. The crack morphology was analysed by scanning 
electrón microscopy (SEM). The influence of the microstructure 
on crack development was studied by metallographic techniques. 
Together with the specimens used for the SCC tests, 
reference samples were heat-treated in order to characterise the 
material mechanically. The samples were tested in a universal 
testing machine. The hardness valúes were additionally deter-
mined for every specimen. 
1,E-05 £ 
1,E-06 
^ 1,E-07 
E 
^T 1,E-08 
75 
•o 
1,E-09 
1,E-10 
1,E-11 
E Al terna ti ve i nmersion 
t 3.5%NaCI aqueussolution20QC I 
o o o o 
QO°° ° 
i 5 • - • * 
8 - • • 
C o * 0 
k 
i 
• 
• 
• 
• 
[ O 7075-T6 
I • 7075-RRA 
E •7075-T73 
I ' 
o 
• • • 
1 
i 
0 5 10 15 20 25 30 
K, (MPa m1/2) 
^^ 
(/) 
E 
+•» 
"O 
Cü 
• a 
i, c-yjo 
1,E-06 
1,E-07 
1,E-08 
1,E-09 
1,E-10 
1 C_H 
Fog Chamber 
5% NaCI aque 
: o o °°° 
o° .%•••• 
: o § -
: © • 
r 
r 
] O 7075-T6 
• 7075-RRA 
• 7075-T73 
us solution 30gC 
0 O 
• 
¿+ t 4 
I 
0 5 10 15 20 25 30 
K, (MPam1/2) 
Figure 2. SCC growth rate of AA-7075 aluminium alloys in a 3.5% NaCI Figure 4. SCC growth rate of AA-7075 aluminium alloys in a 5% NaCI 
aqueous solution fog chamber 
3 Results 
The results obtained in the SCC tests are shown in Figs. 2-4. The 
influence of the stress-intensity factor on crack velocity for the 
DCB specimens can be observed in these figures. Each figure 
represents the behaviour of the 7075-type alloy in a specific 
aggressive environment, subjected to different temper condi-
tions. 7075 aluminium alloy was susceptible to SCC in all the 
temper conditions studied. The susceptibility of every heat 
treatment was very different, as is shown in Figs. 3 and 4. The 
1,E-05 r 
1,E-06 
1,E-07 
E 
(0 
•o 1,E-09 
1,E-10 k 
1,E-11 
Fog Chamber 
0% NaCI aqueus solution 30gC 
Dcooooo PS 
••• 
i 
r 
1 O 7075-T6 
• 7075-RRA 
• 7075-T73 
11 11 11 11 11 11 1 1 1 1 1 11 11 11 1 1 11 11 
0 5 10 15 20 25 30 
K, (MPa m12) 
Figure 3. SCC growth rate of AA-7075 aluminium alloys in a 0% NaCI 
fog chamber 
crack velocity increased when the concentration of chlorides in 
the environment was higher, as was expected since the 
concentration of chlorides specifically causes SCC in this alloy 
[1,4]. The increase was less than an order of magnitude for all the 
heat treatments, this behaviour is quite similar to the crack 
velocity observed in 7075-T6 in aqueous environments between 
distilled water and a saturated sodium chloride solution [2, 4]. In 
7075 aluminium alloy, the effect of the sodium chloride 
concentration over the increase in the SCC in a fog atmosphere 
has been quite similar to that of an aqueous environment. 
The heat treatment precipitation process was analysed by 
transmission electrón microscopy (TEM). The geometry and 
morphology of precipitation particles around the grain-boundary 
are shown in Figs. 5-7. T6 heat treatment produces the smallest 
grain-boundary precipitates (21 nm length and 9 nm width). RRA 
heat treatment precipitates are bigger (28 nm length and 13 nm 
Figure 5. AA-7075-T6. Grain-boundary precipitation morphology 
Figure 6. AA-7075-T73. Crain-boundary precipitation morphology 
width). T73 heat treatment presents the biggest precipitates (54 
nm length and 26 nm width). The number of grain-boundary 
precipitates is inverse of the size, i.e. T6 heat treatment presents 
more and smaller precipitates than RRA, and RRA presents more 
and smaller precipitates than T73. The precipitate-free zone 
width, as reported by others, was found to be independent of 
ageing [2]. 
The crack morphology was intergranular in all cases as it is 
shown in Fig. 8. The propagation was produced in several parallel 
planes. The crack surface presented an exfoliation morphology. 
The results of the tensile and hardness mechanical tests are 
shown in Table 3. RRA heat treatment maintains the strength and 
hardness cióse to T6 properties and is significantly higher than 
T73. 
4 Discussion 
The method of fog testing is generally applied for specification 
purposes as accelerated corrosión test to materials exposed to 
atmosphere. The fail of 7075 alloy component by SCC process has 
been generally reported in atmospheric environment [2]. The fog 
Figure 7. AA-7075-RRA. Crain-boundary precipitation morphology 
u 20 jim 
Figure 8. AA-7075-T6. Keller attack. Intergranular cracking 
Table 3. Mechanical test results 
Temper 
7075-T6 
7075-RRA 
7075-T73 
Elastic limit 
(MPa) 
476 
450 
408 
Tensile strength 
(MPa) 
555 
550 
502 
Hardness 
(HB) 
180 
175 
138 
chamber testing has permitted a good evaluation of SCC 
susceptibility in 7075 alloy. 
The T73 artificial-ageing heat treatment was the least 
susceptible to SCC and the íCiscc w a s the highest, the cracks 
grew only when a significant load was applied (19 MPa m1 '2). 
The coarsest precipitation particles during over-aged T73 heat 
treatment as shown in Fig. 6 reduces the susceptibility to SCC, 
but at the same time, reduces the mechanical properties of the 
alloy as shown in Table 3. 
In all the environments tested, the T6 artificial-ageing heat 
treatment was the most susceptible to SCC, even in less 
aggressive conditions such as with vapourised distilled water at 
30 °C. The valúes of crack velocity are high if the load and 
environmental conditions that exist are taken into account. 
If the results of SCC in each heat treatment process is 
compared to the individual mechanical characteristics, we can see 
that there is a direct relation between individual mechanical 
characteristics and a greater susceptibility to SCC when the 
ultímate tensile stress increases. 
The RRA heat treatment was developed to obtain a higher 
resistance to SCC [10]. Although SCC velocity is significantly 
reduced compared to the T6 condition, along with a small 
reduction of the mechanical properties of the T6 temper, it does 
not attain the more resistant T73 behaviour and remains within 
an intermedíate zone in all the environmental conditions tested. 
The RRA treatment carried out has modified the size, geometry 
and distribution of the precipitation particles around the grain-
boundary, as shown in Fig. 7, but it was unable to limit the crack 
velocity to T73 temper behaviour. 
The SCC process has an intergranular morphology which is 
favoured by the granular structure since it was produced in the 
short-transverse direction. This process is verified not only on one 
plañe as it takes place on various parallel planes in the direction of 
the cracking. The aggressive environmental conditions triggered 
the formation of corrosive products within the crack with the 
appearance of mud patterns. The fracture-surface morphology 
was very similar in all the heat treatments analysed. In general, 
the samples whose crack velocity was slower, were more intensely 
attacked at the surface level in the región of the fracture since the 
said attack was in a smaller área. 
5 Conclusions 
i. The susceptibility to SCC of 7075 aluminium alloy was 
analysed for different temper heat treatments in a fog 
chamber with two spraying solutions. All tempers were 
susceptible to SCC in all the environments studied. 
ii. The increase in sodium chloride concentration in the fog 
chamber produces an increase in SCC velocity that is less in 
magnitude, although quite similar to the increase reported in 
aqueous environments. 
iii. The 7075-T6 temper alloy was the most susceptible to SCC. 
The crack velocity, obtained as a function of the stress-
intensity factor, was higher for this heat treatment in all the 
aggressive environmental conditions that were analysed. The 
7075-T73 temper alloy, however, was the one which exhibited 
the best SCC behaviour. 
iv. Although the RRA heat treatment lowers SCC susceptibility 
in comparison with the T6 state and slightly lowers the 
performance of its mechanical properties, it does not succeed 
in achieving the results obtained with the T73 treatment. 
v. It was observed that there is a direct relationship between 
increased susceptibility to SCC and improved mechanical 
properties. 
vi. The crack morphology, which was intergranular in all cases, 
developed in planes that were parallel to the main cracking 
direction. 
6 References 
[1] ASM Special Handbook, Aluminum and Aluminum Alloys, 
J. R. Davis, (Ed.), ASM Handbook, Materials Park, Ohio, 
USA 1993. 
[2] N. J. H. Holroyd, in: Environment-Induced Cracking of Metals, 
NACE Houston, Texas, USA 1989, pp. 311-345. 
[3] J. Schneider, H. Haindl, Mater. Corros. 2008, 59, 753. 
[4] M. O. Speidel, Metallur. Trans. 1975, 6A, 631. 
[5] T M. Yue, C. F. Dong, L. J. Yan, H. C. Man, Mater. Lett. 2004, 
58, 630. 
[6] M. Puiggali, A. Zielinski, J. M. Olive, E. Renauld, D. Des-
jardins, M. Cid, Corr. Sci. 1998, 40, 805. 
[7] G. M. Scamans, N. J. H. Holroyd, C. D. S. Tuck, Corr. Sci. 
1985, 27, 329. 
[8] N. C. Danh, K. Rajan, W. Wallace, Metallur. Trans. 1983, 14A, 
1843. 
[9] R. N. Lumley, I. J. Polmear, A. J. Morton, Mater. Sci. Technol. 
2005, 21, 1025. 
[10] M. U. Islam, W. Wallance, Metals Technol. 1985, 11, 320. 
[11] F. Viana, A. M. P. Pinto, H. M. C. Santos, A. B. Lopes, 
J. Mater. Process Technol. 1999, 92-93, AUG 54. 
[12] R. N. Parkins, Corrosión Processes, Applied Science Publish-
ers, London 1982. 
[13] J. Oñoro, A. Moreno, C. Ranninger, J. Mater. Sci. 1989, 24, 
3888. 
[14] D. McNaughtan, M. Worsfold, M. J. Robinson, Corr. Sci. 
2003, 45, 2377. 
[15] ASTM Standards, G 85-02, Philadelphia, USA, ASTM, 2002. 
(Received: January 26, 2009) 
(Accepted: February 13, 2009) 
W5255 


The stress corrosion cracking behaviour of heat-treated Al–Zn–Mg–Cu Alloy in Modified salt Spray Fog Testing

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The stress corrosion cracking behaviour of heat-treated Al–Zn–Mg–Cu Alloy in Modified salt Spray Fog Testing

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