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Corrosione e protezione del composito Al 6061 T6/AL2O3p in ambiente marino - Corrosion and protection of Al 6061 T6/AL2O3p in marine environment (Articolo in rivista)
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- Label
- Corrosione e protezione del composito Al 6061 T6/AL2O3p in ambiente marino - Corrosion and protection of Al 6061 T6/AL2O3p in marine environment (Articolo in rivista) (literal)
- Anno
- 2006-01-01T00:00:00+01:00 (literal)
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- Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
- P. Traverso; V. Romairone; M. Fassin (literal)
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- http://www.aimnet.it/allpdf/pdf_pubbli/10_06/traverso.pdf (literal)
- Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#numeroVolume
- Rivista
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- CNR-ISMAR, U.O.S. di Genova, Via de Marini 6, 16149 Genova (literal)
- Titolo
- Corrosione e protezione del composito Al 6061 T6/AL2O3p in ambiente marino - Corrosion and protection of Al 6061 T6/AL2O3p in marine environment (literal)
- Abstract
- Aluminium composites are technologically advanced materials
with elevated physical mechanical properties. They
can be considered an alternative to conventional aluminium
alloys for offshore applications.
In this work the behaviour of four different types of composites,
having aluminium alloy 6061 T6 as matrix and aluminium
particles as reinforcing, was studied. The composites
were different in aluminium particle contents and in productive
processes (i.e. casting or extrusion). Protective and antifouling
properties of two new generation organic coatings
(see Tab. 1) were also studied.
Experimental tests consisted in electrochemical and free
corrosion measurements after different immersion times in
natural sea water.
Electrochemical tests consisted in anodic and cathodic polarizations.
Corrosion rate (after 2 hours of immersion) (see
Tab. 2) and localized corrosion susceptibility (after 2 hours,
5 and 15 days of immersion) (see Tab. 3) were obtained.
In particular corrosion current density (icorr) and polarization
resistance (Rp) showed a lower corrosive attack on
samples containing more reinforcing phase.
In table 3 it is possible to note that all samples tested were
susceptible to localized corrosion in presence of chloride
ions. In particular: A composite (10% v/v Al2O3, obtained by
casting process) had a significant passivity domain, but a
very limited perfect passivity range; B (10% v/v Al2O3, obtained
by extrusion process), C and D composites (20% v/v
Al2O3, respectively obtained by casting and extrusion process),
revealed a wide and increasing passivity domain, together
with a vast perfect passivity range.
Free corrosion tests had a variable time range (between 5
and 60 days) which allowed a sample weight measurements.
Figure 3 shows aluminium weight loss values versus immersion
times for all tested composites. It can be noted that C
and D composites were subject to a slight corrosive attack.
A and B composites showed a higher weight loss.
Corrosion products were characterized by X-ray photoelectron
spectroscopy (XPS). Morphology information, corrosive
attack type and entity were obtained by metallographic
microscopy (OM) and by atomic force microscopy (AFM).
Optical microscopy analysis revealed that composites with
reinforcing phase at 20% showed a lower number of localized
corrosion areas.
XPS analyses showed that superficial layer is mainly composed
of oxides, oxy-chlorides, and aluminium chlorides. In
particular, a larger quantity of Cu was detected on corrosion
samples with respect to unexposed ones. In figure 4 this
Cu quantity difference, on A composite, before and after 15
days of exposition in seawater, was visible.
Casting composites (in particular A type), observed by optical
microscopy, showed a great porosity, macro and micro
pits, and a small homogenous distribution of the reinforcing
phase. Extruded samples showed an homogenous distribution
of the reinforcing phase, without agglomerates, pits or
defects. Figure 5 is an image made by an atomic force microscope
and showed a micro-defect on uncorroded A composite,
a possible starting site of localized corrosion.
Tests for evaluating protective properties of low surface
energy coatings, also called \"non stick\" or \"fouling release\"
coatings (silicone and perfluorurate based), on composite
materials essentially consisted in electrochemical measurements
(EIS) (figure 1) and in analyses by IR spectrophotometry
and optical microscopy. Tests were carried out before
and after sample immersion in natural sea water. Results
(shown in tab. 4) pointed out that both coatings guarantee a
very high protection from corrosion. Indeed Z module value
is one order of magnitude larger after one day of immersion
and two order of magnitude larger after 6 days of immersion
for the silicone coating than for the perfluorurate.
The protective power of the silicone coating resisted for
long immersion time, as documented by microscope observations
and infrared spectrophotometry analyses. In detail,
no significant variation in IR peaks, nor variation in chemical
surface composition appeared (see figure 6). This behaviour
indicates a coating characterized by an elevated chemical
inertia.
Potential antifouling properties of silicone and perfluorurate
coatings were also examined by contact angle measurements
(figure 2). Good results were obtained for both coatings,
but a larger contact angle, that could be associated to
a lower surface energy, resulted for perfluorurate coating
(see tab. 5).
Figure 7 macroscopically revealed the antifouling protection
of these coatings. In these images (taken after 1 month
of immersion of coated samples in sea water) it could be noted
that the silicone coating acted better than perfluorurate.
This behaviour may be explained by the fact that other parameters
(the thickness of the film, the elastic module and roughness)
are equally important to determine coating antifouling
power. (literal)
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