Volume 1 Paper 5
Modification of the Crevice Corrosion Behaviour of Al7175 Alloy by Surface Alloying
M.G.S. Ferreira and R.Li
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JCSE Volume 1 Paper 5
Submitted 15 November 1996, revised version submitted 5 June 1997
Modification of the Crevice Corrosion Behaviour of Al7175 Alloy by Surface
M.G.S. Ferreira and R.Li,
Instituto Superior Técnico, 1096 Lisboa Codex, Portugal
Crevice corrosion is a dangerous form of localized corrosion that leads to
the attack of creviced (covered) areas of metallic structures, where the
access of oxygen is difficult. In this work it is shown that laser surface
alloying of Al7175-T7351 with chromium can modify the situation since surfaces
more corrosion resistant than the substrate alloy are obtained. Therefore, if
this treatment is applied to the creviced area, crevice corrosion is avoided.
Crevice corrosion is a dangerous form of localized corrosion that usually
takes place in a small volume of stagnant solution trapped in crevices formed
in components of engineering structures in service (gaskets, lap joints,
deposits, narrow spaces between bolts and metallic surfaces, etc.).
§3 The mechanism for crevice corrosion is very often described as consisting
of four stages [1,2]:
comment(4)Stage 1, where the oxygen inside the crevice is depleted due to its
consumption in the cathodic reaction. The depletion of oxygen inside the
crevice makes the metal there an anode relatively to the metal outside
the crevice by forming a differential aeration cell. This cell
accelerates the anodic process of metal dissolution inside the crevice
and exists through all the stages of crevice corrosion.
comment(5)Stage 2, where there is an increase of the acidity and chloride
content inside the crevice due to the hydrolysis of the cations
originated in the anodic process. The geometry of the crevice makes the
exchange of solution between the inside and outside of the crevice
difficult, creating a local solution chemistry inside the crevice.
comment(6)Stage 3, when the local solution inside the crevice attains a critical
value of pH that corresponds to the breakdown of the passive film.
Stage 4, that corresponds to the propagation of the attack.
§7 Taking into consideration the galvanic character of crevice corrosion, and
the localized effect of the phenomenon, involving usually small anodic areas
(inside the crevice), it seems to be plausible for overcoming the problem, to
modify the surface of the metal inside the crevice in a way that it becomes
more noble than the metal outside the crevice.
§8 Laser surface treatments, such as laser surface alloying [3,4] and laser
surface cladding [5,6] could be adequate to this aim. Alloying or cladding the
inside part of the crevice, forming surface alloys more noble than the
substrate alloy, can reduce or even eliminate crevice corrosion.
§9 Moreover localized processing is one of the most important advantages of
the laser surface treatment.
§10 In this work the effect of adding chromium to an aluminium alloy (Al
7175-T7351) in the crevice area was studied. Chromium was chosen because it is
one of the alloying elements that improves the corrosion resistance of
aluminium and its alloys. However as its solubility in equilibrium conditions
is very low, to have a certain amount of chromium dissolved in the alloy it is
necessary to use techniques that allow for the formation of metastable alloys.
Among these techniques are the laser treatments, since they are rapid melting
and solidifying processes.
The Al7175-T7351 samples (60x20x10 mm) after being sand blasted were laser
alloyed with chromium in an area of 15x20 mm. A 2 kW CO2 laser was
used . The spot size of the laser beam was 5 mm and the alloy moved under
the laser beam at a transverse speed of 5 mm/s.
§12 The alloying powder with a composition of 25% Cr + 75% Al was fed to the
substrate at a rate of 0.03 g/s. To obtain a uniform composition and eliminate
porosity and cracks in the surface layer the specimens after alloying were
remelted. The transverse speed of the specimens during this operation was 10
§13 The average composition of the laser alloyed layer was
§14 The creviced specimen consisted of the Al7175-T7351 sample, mounted in
epoxy resin, and a Perspex plate, covering the laser treated part of the
surface, and separated from it by a hard plastic frame with a thickness of 0.1
mm, Fig. 1. The limits of the crack were sealed with epoxy resin, and the
dimension of the crevice was 15x20x0.1 mm.
§15 The specimen was immersed in naturally aerated 3% NaCl solution.
§16 After the crevice corrosion tests the corrosion morphology was observed by
§17 Identical crevice corrosion tests on untreated aluminium were
carried out in the same experimental conditions for comparisonpurpose.
§18 Fig. 1 - Scheme of the artificial crevice specimen
§19 In order to simulate what happens in the initial stage of the crevice
process, the potential of the Al7175-T7351 aluminium alloy was monitored in
naturally aerated 3% NaCl solution and the potential of the chromium alloyed
alloy (AlCr) was monitored in deaerated 3% NaCl solution. These measurements
were performed on 15 x 20 x 10 mm uncreviced specimens relatively to a
saturated calomel electrode (SCE) using a HP 3478A multimeter connected to a
HP 9121 computer.
§20 All the samples were mounted in epoxy resin and carefully polished with
emery paper to 800 grit before the tests.
§21 RESULTS AND DISCUSSION
Fig. 2 shows the variation with time of the corrosion
potential of Al 7175-T7351 alloyed with chromium (AlCr) in deaerated 3% NaCl
solution (curve A). In the same figure the variation of the potential with
time of the same alloy in aerated 3% NaCl solution is presented (curve B).
§22 Fig. 2 - Variation of the corrosion potential with time of: A.
Al Cr in deaerated 3% NaCl solution, B. Al 7175 in naturally aerated 3%
§23 The corrosion potential of the 7175 aluminium alloy increases at the
beginning and then reaches a relative stable value of about -0.82 V. The
corrosion potential of AlCr also increases with time reaching a stable value
of -0.68 V, which is 0.14 V higher than that of the Al 7175. Then the AlCr
alloy is cathodic relatively to the Al 7175 alloy.
§24 Immersion tests
Using the crevice specimen described in the experimental
section an immersion test was carried out on the specimen partially laser
treated for one week in quiescent solution. The observation of the specimens
shows that inside the crevice where the substrate is alloyed with Cr (AlCr),
the surface is still bright, as before the test, i.e. uncorroded. Outside the
crevice (untreated part) the Al 7175 alloy suffered severe pitting corrosion,
§25 Fig. 3 - Pitting corrosion in the partially laser treated
specimen outside the crevice, after immersion in 3% NaCl solution for 1 week (larger
§26 The same crevice test was performed on a crevice specimen of
plain Al 7175, without laser alloying in any part of the specimen. After the
test the alloy inside the crevice is severely corroded, as shown in Fig. 4.
The alloy outside the crevice is not corroded. This case is the normal crevice
corrosion situation where the smaller area inside the crevice works as anode,
compared to the larger area outside the crevice that works as cathode.
§27 Fig. 4 - General aspect of untreated Al 7175 inside the
crevice after immersion in 3% NaCl solution for 1 week (larger
§28 A small ratio (<<1) of anodic area/cathodic area is very
unfavourable from the galvanic corrosion point of view, since as the system is
electrically isolated, there should be an equality between the current leaving
the anodic area and the current entering in the cathodic area, which has as
consequence a large anodic current density, i.e., a large corrosion rate. The
area inside the crevice (anode) corrodes much faster than if the crevice was
§29 The experimental results have shown that laser alloying the area inside the
crevice with chromium was able to change this situation, avoiding crevice
§30 The explanation for this fact becomes clear if the processes involved in
laser alloying and crevice corrosion initiation are considered.
§31 During laser surface alloying the substrate and the alloying elements are
melted together and then they solidify rapidly, in comparison with the
equilibrium solidifying process. The solid-liquid interphase moves more
rapidly than in the equilibrium solidifying process and entraps more solute
elements than it would do in the latter case, because they have less time to
§32 As a consequence laser surface alloying of Al 7175 with
chromium leads to the formation of a very fine microstructure, constituted by
an a-Al matrix, which is much richer in chromium
than in the alloy formed in equilibrium conditions , and intermetallic
compounds dispersed in the matrix. The amount of chromium in the matrix
depends on the amount of chromium used for alloying but it has been reported
to be in some circumstances 30 times higher than in the alloy obtained in
equilibrium conditions . The intermetallic compounds formed during the
alloying process were identified as q-Al7Cr, h-Al11Cr2
and e-Al4Cr [7,8], and in the outermost layers of
the treated specimens they are radially distributed around an aluminium rich
central particle, forming an equiaxed cell structure, Fig. 5 .
§33 Fig. 5 - Cellular structure at the top of the AlCr alloyed
and remelted layer  (larger image)
§34 In contact with air aluminium alloys are covered with a film
that contains aluminium oxide and for the most of the cases oxides of the
alloying elements. The film has defects (flaws, pores, etc) that act as pit
and then crevice corrosion nuclei when the alloy is exposed to an environment
susceptible to cause localized corrosion. In the pre-existing film defects the
dissolution of the alloy is easier with consequent formation of Al3+,
that suffering hydrolysis lowers the pH of the local solution. When the pH of
this solution reaches a critical value, breakdown of the film occurs due to
its dissolution, making the progress of corrosion possible . At a
microstructural level the pits in Al7175 alloy nucleate in the a-Al
matrix as can be seen in Fig. 6. Therefore the presence there of solute
elements that makes film breakdown more difficult will lower the tendency of
the alloy to localized corrosion.
§35 Fig. 6 - Scanning electron micrography showing the pitting
morphology on AlCr alloy  (larger image).
§36 In Al-Cr alloys the passive film contains chromium oxide  which makes
it less soluble in acid solution. Thus the breakdown of the film only occurs
at a lower pH, making the onset of corrosion more difficult.
§37 The chromium alloyed part of the specimen (crevice region) becomes, thus,
more corrosion resistant to localized attack than the untreated alloy,
avoiding crevice corrosion inside the covered area. Outside the crevice the
extent of the attack (pitting) would be influenced by the crevice geometry
that affects the partial potential drop down the crevice and hence the
available cathodic area and the bulk solution conductivity. However this
aspect is beyond the scope of this paper that deals only with crevice
Laser surface alloying of Al 7175-T 7351 alloy with chromium produces
surfaces more resistant to localized corrosion than the substrate alloy. If
the part of the component of the structure that is prone to crevice corrosion
is alloyed, crevice corrosion can be avoided. As laser surface processing is
specially adequate for treating small areas there is an enormous potential in
this technology to modify the crevice corrosion behaviour of materials
susceptible to this type of corrosion, particularly when crevices of small
size are present.
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