| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 86 === ABSTRACTIn this study, intergranular corrosion(IGC), stress-corrosion crackin
g (SCC) and electrochemical behaviors of the superplastic and superplastically
-formed 5083, 7475 and 8090 aluminum alloys with various heat treatments were
systematically investigated. The specimens of nonsuperplastic 5083, 7475 and
8090 aluminum alloys were also studied in order to evaluate the effect of grai
n refinement on the corrosion behaviors of these alloys. The NAMLT method was
employed for evaluating the IGC of both superplastic and nonsuperplastic 5083
aluminum alloys. Experimental results showed that the IGC susceptibility of th
ese alloys increased due to the sensitization effect during the thermal proces
s of superplastic forming. It was also demonstrated that the sensitization eff
ect can be eliminated by post-annealing at 345℃ for 1 hour. The IGC of superp
lastic 8090 and 7475 aluminum alloys was tested by immersing the specimens in
a 5.7% NaCl solution containing 0.3% H2O2. Both IGC and pitting corrosion were
found in the superplastic 8090 aluminum alloy with T6 or T7 temper. The T6 te
mpered specimens were attacked more deeply than the T7 tempered specimens. How
ever, pitting was found in the W tempered specimens even though no IGC existed
. The results for nonsuperplastic 8090 aluminum alloys with the same temper co
nditions showed a trend similar to that of the superplastic specimens but wit
h deeper pits.The slow strain rate test (SSRT) was further applied to evaluate
the SCC susceptibility of the superplastic aluminum alloys in a 3.5% NaCl sol
ution. Experimental results showed that thermal processes during superplastic
forming sensitized the 5083 aluminum alloys and led to an increase of their SC
C susceptibility. However, post-annealing treatment of the above specimens at
345℃ for 1 hour eliminated the sensitization effect. The SCC susceptibility o
f the superplastic 8090 aluminum alloy with various heat treatment conditions
decreased in the sequence of T6, T7, T4 and W. The SCC mechanisms of T4 and T7
specimens were planar slip and IGC, respectively, while the SCC of T6 specime
n was caused by both planar slip and IGC. The SCC susceptibility of the non-su
perplastic 8090 aluminum alloy tested under the same conditions showed a trend
similar to that of the superplastic one, but the former was about 10% higher
than the latter. For the superplastically-formed 8090 aluminum alloy, both T5
and T6 post-heat-treated specimens possessed similar strength. However, the SC
C susceptibility of the former was 30% lower than that of the latter. In addit
ion, T5 temper was more favorable for practical application since no distortio
n would occur, and it also possessed the advantages of energy saving and opera
ting convenience. Both potentiodynamic polarization and electrochemical i
mpedance spectroscopy (EIS) were also employed to study the corrosion behavior
s of these alloys in 3.5% NaCl solution. Experimental results showed that the
superplastic and non-superplastic 5083 aluminum alloys possessed similar elect
rochemical properties. Moreover, the corrosion rate of both alloys were reduce
d to one-third of their original when 1 ppm of sulfide was added into the 3.5%
NaCl solution. However, their corrosion rate increased with the increase of s
ulfide concentration from 100 ppm to 1000 ppm. In the 3.5% NaCl solution conta
ining 100 ppm sulfide, electrochemical tests also revealed that the corrosion
reaction was under diffusion control when the DC applied potential was more ne
gative than the breakdown potential (Eb); However, the corrosion reaction chan
ged to activation control as the DC applied potentials were kept at Eb or were
nobler than Eb. For the superplastic 7475 aluminum alloy, the corrosion curre
nt densities of the specimens after W and T73 heat treatments were lower than
that after T6 heat treatment. The result was attributed to the different micro
structures in specimens after various heat treatments. The major precipitates
in W and T73 specimens were GP zone and MgZn2, respectively. The GP zone conta
ined active elements of Mg and Zn, while MgZn2 was a non-passive compound. For
the superplastic 8090 aluminum alloy, both corrosion potential and breakdown
potential shifted to more active values in the sequence from W, T4, T6 to T7.
The corrosion current density and passive current density increased with the i
ncrease of the aging degree, especially for the specimen under T7 heat treatme
nt. Since the major precipitate in W, T4 and T6 heat-treated specimens was Al3
Li, their electrochemical properties were similar. However, the more active Al
Li also existed in the specimen after T7 heat treatment, which led to the incr
ease in corrosion rate. On the other hand, at the applied potential of -650 mV
, the transient current density and the steady-state current density of the sp
ecimen after T7 heat treatment were higher than those of the other specimens.
Similar results were also obtained from the EIS measurements. In addition, in
-situ monitoring of the electrochemical behaviors during a slow strain rate te
st showed that the corrosion potential gradually shifted toward the active dir
ection and the corrosion rate increased with an increase of strain.
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