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.).
The mechanism for crevice corrosion is very often described as consisting of four stages [1,2]:
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.
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.
Moreover localized processing is one of the most important advantages of the laser surface treatment.
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.
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 mm/s.
The average composition of the laser alloyed layer was Al-5Cr-1.7Cu-0.7Mg-3.1Zn.
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.
The specimen was immersed in naturally aerated 3% NaCl solution.
After the crevice corrosion tests the corrosion morphology was observed by optical microscopy.
Identical crevice corrosion tests on untreated aluminium were carried out in the same experimental conditions for comparisonpurpose.
Fig. 1 - Scheme of the artificial crevice specimen
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.
All the samples were mounted in epoxy resin and carefully polished with emery paper to 800 grit before the tests.
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).
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% NaCl solution
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.
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, Fig. 3.
Fig. 3 - Pitting corrosion in the partially laser treated specimen outside the crevice, after immersion in 3% NaCl solution for 1 week (larger image)
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.
Fig. 4 - General aspect of untreated Al 7175 inside the crevice after immersion in 3% NaCl solution for 1 week (larger image)
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 not present.
The experimental results have shown that laser alloying the area inside the crevice with chromium was able to change this situation, avoiding crevice corrosion.
The explanation for this fact becomes clear if the processes involved in laser alloying and crevice corrosion initiation are considered.
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 diffuse away.
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 .
Fig. 5 - Cellular structure at the top of the AlCr alloyed and remelted layer  (larger image)
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.
Fig. 6 - Scanning electron micrography showing the pitting morphology on AlCr alloy  (larger image).
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.
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 corrosion.
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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