Almgmn alloy product with improved corrosion resistance

Abstract

The invention relates to a method for manufacturing an aluminium alloy sheet, wherein an aluminium alloy is prepared with the following composition, in wt%: Mg: 4.0 - 5.2, Mn: 0.40 - 1.0, Zn: 0.15 - 0.40, at least one element selected from Ti, Cr, Cu and Zr, the content of the element, if selected, being 0.01 - 0.15 for Ti, 0.05 - 0.25 for Cr, 0.02 - 0.25 for Cu, 0.05 - 0.25 for Zr, Fe: < 0.40, Si: < 0.40, other elements or impurities < 0.05 each and < 0.15 in total, the remainder being aluminium. A rolling plate is cast by vertical semi-continuous casting, and said optionally homogenised plate is hot-rolled in two successive stages to obtain a sheet. The sheets obtained by the method according to the invention are advantageous, in particular for shipbuilding, since they demonstrate, after being exposed for 7 days to a temperature of 100°C, a weight loss of less than 15 mg / cm2 during a corrosion test according to the ASTM G67 standard.

Description

[0001] Product made of AlMgMn alloy with improved corrosion resistance

[0002] technical field

[0003] The invention relates to the field of rolled products such as aluminum alloy sheets or strips of the AlMgMn type having a Mg content of at least 4% by weight having high mechanical strength, advantageous welding properties and good corrosion resistance for structural applications, such as for example, boats, offshore constructions or industrial vehicles.

[0004] State of the art

[0005] It is well known that the use of AlMg alloys from the 5000 series, according to the Aluminium Association nomenclature, in the work-hardened condition (H condition according to NF EN 515), whether fully work-hardened (H1 condition), partially softened (H2 condition), or stabilized (H3 condition), provides good mechanical properties and corrosion resistance. For example, alloys 5083, 5059, 5383, and 5086 are widely used in mechanical engineering, both welded and unwelded, for applications requiring adequate corrosion resistance, such as shipbuilding.

[0006] However, corrosion requirements are becoming increasingly stringent, and products must be resistant to exfoliation and intergranular corrosion even after long-term exposure, i.e., service use, including applications in hot climates. Therefore, corrosion tests are performed after 7 days of exposure at 100 °C to simulate long-term exposure at ambient temperature. Under these conditions, intergranular corrosion due to the dissolution of the passivation layer of the b-phase (Af Mg2) that segregates at grain boundaries tends to occur.

[0007] ASTM B928 requires the NAMLT test (ASTM G67 - Nitric Acid Exposure Mass Loss) to characterize resistance to intergranular corrosion. Paragraph 10 of this standard specifies the resistance to intergranular corrosion after a 7-day post-production heat treatment at 100°C for the H128 condition. The strength and weld strength must not be reduced.

[0008] French patent application FR2731019 relates to a specific alloy composition, subsequently registered with the Aluminum Association under the designation 5383, containing, among other things, 3 to 5% magnesium and 0.5 to 1% manganese, in which the sum of the contents (by weight %) Mn + 2Zn is > 0.75. This composition makes it possible to obtain rolled or extruded products exhibiting significantly better fatigue resistance and a significantly lower crack propagation rate than known products intended for the same application. However, the cited patent application provides no information on the corrosion resistance of the product.

[0009] French patent application FR2740144 claims a very narrow composition, within the composition ranges of 5083 and 5086 alloys, containing, among other things, 4.3 to 4.8% magnesium and less than 0.5% manganese, allowing for good properties under large deformations. This application also makes no mention of corrosion resistance.

[0010] US patent application 2011017055 relates to 5xxx aluminum alloys and products made from them. The described alloys consist essentially of: 2.5% to 7% by weight Mg; 0.05% to 2% by weight Cu; 0.3% to 1.5% by weight Mn, possibly up to 2.0% by weight Zn; possibly up to 1.0% by weight total additives, in which the additives are selected from the group consisting of Zr, Cr, V, Sc, Hf, Ti, B, C, Ca, Sr, Be, Bi, Cd, Ge, In, Mo, Nb, Ni, Sn, Y; and the remainder being aluminum and unavoidable impurities. The new 5xxx aluminum alloy products may allow for an improved combination of properties due, for example, to the presence of copper. In one embodiment, the new 5xxx aluminum alloy products are capable of achieving an improved combination of properties through solution heat treatment.

[0011] Patent application CN 104404411 relates to a process for producing aluminum alloy sheets which includes the following steps: first, performing a homogenizing heat treatment on an aluminum alloy ingot, in which the mass fraction of magnesium in the aluminum alloy ingot is 4.0%-4.9%, performing a primary deformation by hot rolling to obtain an intermediate aluminum alloy plate; finally, performing a secondary hot rolling on the intermediate aluminum alloy plate to obtain an aluminum alloy plate. The aluminum alloy sheet production process is particularly suitable for the production of marine aluminum alloy plates, and is aimed in particular at the H116 state of an aluminum alloy 5083. The aluminum alloy plate produced by the aluminum alloy sheet production process has high corrosion resistance and mechanical properties.

[0012] Patent application CN104152759 relates to high-strength, corrosion-resistant Al-Mg alloys and a technology for their preparation. The alloy components are as follows, in mass percentages: 5.0 to 6.5% Mg, 1.2 to 2.5% Zn, 0 to 0.4% Cu, 0.4 to 1.2% Mn, 0 to 0.1% Cr, 0 to 0.15% Ti, 0.05 to 0.25% Zr, 0 to 0.4% Fe, 0 to 0.4% Si, and the remainder Al and unavoidable impurities. The manufacturing process includes the steps of casting, homogenization, hot rolling, recrystallization annealing, cold rolling, stabilization treatment, and pre-drawing.

[0013] Patent application CN 106244872 relates to a preparation process for a medium-thickness Al-Mg aluminum alloy panel with high corrosion resistance for marine applications. The preparation process includes the steps of homogenization, heat treatment, annealing, hot rolling, cold rolling, and heat treatment for stabilization.

[0014] Patent application WO2018104004 relates to a process for manufacturing a wear-resistant rolled aluminum alloy product comprising the steps of: supplying an aluminum alloy plate having Mg from 4.20% to 5.5%, Mn from 0.50% to 1.1% up to 0.40%, Si up to 0.30%, Cu up to 0.20%, Cr up to 0.25%, Zr up to 0.25%, Zn up to 0.30%, Ti up to 0.25%, hot rolling to an intermediate thickness of 15 mm to 40 mm and then hot rolling to a final thickness of 3 mm to 15 mm and wherein the exit temperature of the hot rolling mill is between 130 and 285 °C and then cooling to ambient temperature. This request also does not mention corrosion resistance.

[0015] The problem that the present invention attempts to solve is therefore to provide rolled AlMgMn alloy products that, after long-term exposure, exhibit improved corrosion resistance while maintaining good mechanical properties before and after welding, good fatigue resistance, and can be manufactured at the lowest cost. Object of the invention

[0016] The applicant found that certain AlMgMn alloys can be made more resistant to the sensitizing effect of long-term exposure when obtained by a specific manufacturing process.

[0017] The applicant found in particular that in a specific range of Mg, Mn and Zn content these alloys exhibit a particular and well-defined microstructure, which results from a set of manufacturing process parameters.

[0018] A first object of the invention is a method for manufacturing an aluminum alloy sheet in which

[0019] a) An aluminum alloy is prepared with the following composition, in % by weight

[0020] Mg: 4.0 - 5.2,

[0021] Mn: 0.40 - 1.0,

[0022] Zn: 0.15 - 0.40,

[0023] at least one element chosen from Ti, Cr, Cu and Zr, with a content, if chosen, of 0.01 - 0.15 for Ti, 0.05 - 0.25 for Cr, 0.02 - 0.25 for Cu, 0.05 - 0.25 for Zr,

[0024] Fe: < 0.40,

[0025] If: < 0.40,

[0026] other elements or impurities < 0.05 each and < 0.15 total, remaining aluminum, b) a rolling plate is cast by vertical semi-continuous casting,

[0027] c) Optionally, the said rolling plate is homogenized,

[0028] d) The said optionally homogenized plate is hot-rolled in two successive stages to obtain a sheet

[0029] dl) a hot rolling stage on a reversible rolling mill to a thickness between 12 and 35 mm,

[0030] d2) a hot rolling step on a tandem rolling mill to a thickness of between 3 and 12 mm, in which the final temperature is at least 240 °C and is less than 300 °C,

[0031] e) Optionally, the said sheet metal is cold-rolled to a thickness of between 1 and 4 mm,

[0032] f) Optionally, a final heat treatment is carried out on said sheet, optionally cold-rolled at a temperature below 300 °C. A second object of the invention is an aluminum alloy sheet with a thickness between 1 and 12 mm, composition in % by weight,

[0033] Mg: 4.0 - 5.2,

[0034] Mn: 0.40 - 1.0,

[0035] Zn: 0.15 - 0.40,

[0036] at least one element chosen from Ti, Cr, Cu and Zr, with a content, if chosen, of 0.01 - 0.15 for Ti, 0.05 - 0.25 for Cr, 0.02 - 0.25 for Cu, 0.05 - 0.25 for Zr,

[0037] Fe: < 0.40,

[0038] If: < 0.40,

[0039] other elements or impurities < 0.05 each and < 0.15 total, remainder aluminium,

[0040] capable of being obtained by the process according to the invention exhibiting after exposure of 7 days at 100 °C a weight loss of less than 15 mg / cm2 in a corrosion test according to ASTM G67.

[0041] Yet another object of the invention is the use of a sheet metal of the invention in shipbuilding or for the construction of industrial vehicles.

[0042] Description of the Figures

[0043] Figures 1A and 1B describe the analysis of disorientations in granular zones for KAM measurement.

[0044] The Ligure 2 illustrates the relationship between KAM measurement and corrosion result in the test

[0045] NAMLT after 7 days of exposure at 100 °C.

[0046] Description of the invention

[0047] Unless otherwise stated, all indications concerning the chemical composition of alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed as a percentage by weight is multiplied by 1.4. The designation of alloys is in accordance with the regulations of The Aluminium Association, which are familiar to those skilled in the art.

[0048] The definitions of metallurgical states are given in the European standard EN 515.

[0049] The static tensile mechanical properties, in other words the tensile strength Rm, the conventional yield strength at 0.2% elongation Rpo,2, and the elongation at break A%, are determined by a tensile test according to EN ISO 6892-1 / ASTM E8 - E8M-13, the sampling and direction of the test being defined by EN 485-1. Unless otherwise specified, the definitions of EN 12258 (2012) apply.

[0050] Corrosion tests are performed according to ASTM B928 / B928M and ASTM G66 / G67 standards.

[0051] The applicant was surprised to find that a specific composition of Al-Mg alloys combined with a specific rolling process enables the desired corrosion properties to be obtained.

[0052] The process according to the invention comprises the steps of preparing an alloy according to the invention by casting, optionally homogenization, two-stage hot rolling, optionally cold rolling, and optionally final heat treatment.

[0053] The chosen compositional limits are explained as follows:

[0054] The addition of magnesium ensures good mechanical strength. Below 4.0% by weight, the mechanical strength is insufficient. Moreover, below 4.0% by weight, the alloy generally does not experience corrosion problems, and the present invention is of little interest. Above 5.2% by weight, the problem of thermal sensitization to corrosion becomes so significant that even the implementation of the present invention no longer allows for the production of usable products in corrosive environments. In a first embodiment, the magnesium content is between 4.0 and 4.6% by weight, and preferably between 4.1 and 4.5%, the resulting products being particularly corrosion-resistant. In a second embodiment, the magnesium content is between 4.7 and 5.2% by weight, and preferably between 4.7 and 5.0% by weight, the resulting products having, in particular, high mechanical strength after welding.

[0055] Manganese improves tensile strength and reduces the metal's tendency to recrystallize. Below 0.40% manganese by weight, the present invention is of no industrial interest because the tensile strength is too low. Above 1%, the elongation at break, toughness, and fatigue resistance become too low for the intended applications. In the first embodiment, having a magnesium content between 4.0 and 4.6% by weight, the manganese content is advantageously between 0.45 and 0.60% by weight. In the second embodiment, having a magnesium content between 4.7 and 5.2% by weight, the manganese content is advantageously between 0.70 and 0.90% by weight.

[0056] Zinc, in the presence of manganese, improves fracture toughness, but above 0.40% by weight, the applicant observed difficulties with product shaping and / or post-weld corrosion resistance. A presence of at least 0.15% by weight simultaneously improves corrosion resistance and mechanical strength. Advantageously, the zinc content is between 0.15% and 0.35% by weight, and preferably between 0.18% and 0.30% by weight.

[0057] Titanium, chromium, copper, and zirconium also have a favorable effect on the yield strength, and at least one element is selected from Ti, Cr, Cu, and Zr, with a content, if selected, of 0.01–0.15% for Ti, 0.05–0.25% for Cr, 0.02–0.25% for Cu, and 0.05–0.25% for Zr, expressed as a percentage by weight. In an advantageous embodiment, the added elements are titanium, chromium, and copper, with the zirconium content being less than 0.05% by weight and preferably less than 0.03% by weight. Advantageously, the copper content is at least 0.05% by weight and preferably at least 0.06% by weight. In one embodiment of the invention, the copper content is at most 0.15% by weight and preferably 0.10% by weight. Advantageously, the chromium content is less than 0.1% by weight and preferably less than 0.09% by weight.

[0058] The iron content does not have much influence in the context of the present invention; it should be less than 0.40% by weight and preferably less than 0.35% by weight to avoid the formation of primary phases during casting.

[0059] The silicon content is less than 0.40% by weight. In one embodiment of the invention, a minimum content of 0.05% by weight to ensure the formation of silicon phases such as Mg₂Si is preferred. Advantageously, the maximum silicon content is 0.15% by weight.

[0060] The applicant was unable to observe any significant influence from the other elements or impurities, limited to 0.05% by weight per element, their sum not exceeding 0.15% by weight. After the alloy is prepared, a rolling slab is cast by vertical semi-continuous casting. The slab is then optionally homogenized.

[0061] When homogenization is performed, the chosen temperature is between 535°C and 550°C for a duration of at least 12 hours. However, the inventors have found that, surprisingly, excellent results are obtained without homogenization. In one embodiment, the homogenization step is omitted, but a simple reheating is carried out before hot rolling at a temperature between 490 and 535°C, preferably between 495 and 525°C, and most preferably between 500 and 520°C.

[0062] After homogenization and / or reheating, the slab is hot-rolled in two successive stages to obtain a sheet. The first stage involves hot rolling on a reversible rolling mill to a thickness between 12 and 35 mm, and the second stage involves hot rolling on a tandem rolling mill to a thickness between 3 and 12 mm. In the second hot rolling stage, the final temperature must be at least 240 °C and less than 300 °C. A tandem rolling mill is a mill in which several stands supporting rolling cylinders, typically 3, 4, or 5, operate successively ("in tandem"). Typically, the sheets obtained are coiled at the exit of the tandem rolling mill. The inlet temperature during the first rolling stage is advantageously between 470 °C and 525 °C, preferably between 480 °C and 515 °C and preferably between 490 °C and 505 °C.The first step on a reversible rolling mill can be carried out on one or even two reversible rolling mills placed successively. The inlet temperature for the second hot rolling step is preferably between 350 °C and 450 °C. The final temperature of the second hot rolling step must be below 300 °C. The inventors have observed that if this temperature is too high, the corrosion resistance properties are insufficient. Advantageously, the final temperature during the second hot rolling step is between 240 °C and 280 °C. Furthermore, during the second hot rolling step, it is advantageous for the thickness reduction (Re) achieved at a temperature between 240 °C and 380 °C to be sufficient.This criterion can be determined directly if the thicknesses el and e2 are available, for which the sheet was at temperatures of 380 °C and 240 °C, respectively, during rolling. In this case, Re = (el - e2) / el. If the inlet temperature was below 380 °C and / or the outlet temperature was above 240 °C, Re is the thickness reduction between the effective inlet and / or outlet temperatures, respectively. Preferably, in the second hot rolling stage, the final temperature is at least 250 °C or, more preferably, 260 °C. When the thicknesses el and e2 are not available, they can be estimated from the inlet and outlet temperatures and the inlet and outlet thicknesses, respectively Tinlet, Toutlet, Epinlet, and Epoutlet, during the second hot rolling stage, by linear extrapolation.

[0063] The present inventors have observed that the properties are advantageous when a thickness reduction Re of at least 30% is carried out at a temperature between 240 °C and 380 °C.

[0064] In the first embodiment having a magnesium content between 4.0 and 4.6%, the present inventors have found that a thickness reduction Re of at least 65% carried out at a temperature between 240 °C and 380 °C during the second hot rolling stage is particularly advantageous, this advantageous thickness reduction allowing in particular to improve the granular structure and the resistance to corrosion after long-term exposure.

[0065] After hot rolling, the resulting sheet can optionally be cold rolled to a thickness of between 1 and 7 mm and preferably between 2 and 4 mm (MPa).

[0066] A final heat treatment of the hot-rolled and optionally cold-rolled sheet at a temperature below 300 °C can optionally be carried out. Typically, the final heat treatment is performed at a temperature between 180 and 280 °C, preferably between 190 and 220 °C, for a duration typically between 1h and 100. This type of treatment can, in some cases, improve corrosion resistance properties. However, thanks to the process according to the invention, the final heat treatment is not essential, and in an advantageous embodiment, the final heat treatment step is omitted, and the sheet is used in its as-made condition, that is, as hot-rolled or cold-rolled, depending on the case.

[0067] The aluminum alloy sheets that can be obtained by the process according to the invention are advantageous because after exposure for 7 days at 100 °C they exhibit a weight loss of less than 15 mg / cm2 during a corrosion test according to the ASTM G67 standard.

[0068] The granular structure of the samples was characterized by scanning electron microscopy (EBSD) using disorientation analysis in granular zones by the Kernel Average Misorientation (KAM) method, described for example in the article "A review of strain analysis using electron backscatter diffraction". Stuart I. Wright et al. Microsc. Microanal. 17, 316-329, 2011.

[0069] The local misorientation map of each sample is obtained using the kernel method. For a given pixel, the average misorientation between that pixel and all its nearest neighbors (hexagonal pixels) belonging to the same grain is calculated, as illustrated in Figure 1. For this measurement, a grain is defined by a 5° misorientation and a minimum size of 20 mhi, with a measurement step of 0.15 mhi. The average local misorientation value is assigned to the central point. Figure 1A illustrates the case in which all the nearest neighbors, pixels 1 to 6, of the central pixel A belong to the same grain. The average misorientation AgK for pixel A is then obtained by averaging the Ag misorientations Airelative to pixels 1 to 6. Figure 1B illustrates the case in which some of the first neighbors, pixels 5 and 6, of the central pixel A belong to a different grain than the central pixel. The average of the misorientations for pixel A is then obtained by averaging the misorientations relative to pixels 1 to 4.

[0070] Within the scope of the present invention, the mean degree of misorientation (KAM) or KAM measurement is defined as the average value of the misorientations for each pixel of the map. The conditions for acquiring and processing EBSD data are given below. The electron microscope was a Zeiss ULTRA5 microscope used with the following parameters: HT=20kV, tilt=70°, WD=12mm, map area: 250 µm (L) x 200 µm (ST), step size = 0.15 mhi. For data processing, the software EDAX OIM v7.3.0 was used, parameterized with the following conditions: Cleaning and partitioning: Grain tolerance angle = 5°, minimum grain size = 20, minimum CL 0.1; multi-line KAM analysis conditions: nearest neighbor; maximum misorientation = 5° (“Cleaning and partitioning: Grain tolerance angle=5°, mini grain size=20, mini, CL 0,1; multiple rows KAM analysis conditions: lst nearest neighbor; maximum misorientation=5°”).

[0071] In the first embodiment having a magnesium content between 4.0 and 4.6%, the present inventors have advantageously found that the average degree of disorientation KAM is at least 0.75.

[0072] The use of sheet metal according to the invention in shipbuilding or for the construction of industrial vehicles is advantageous.

[0073] Example

[0074] In this example, several rolling plates whose composition is given in Table 1 were cast.

[0075] Table 1: Composition of the plates in % by weight

[0076] The plates were heated to 520°C and then hot-rolled in two successive stages to obtain a sheet.

[0077] In the first stage, hot rolling was carried out on a reversible rolling mill to a thickness of 26 to 29 mm for alloys A to C and 15 to 17 mm for alloys D and E, with an inlet temperature on the reversible mill between 490 °C and 510 °C. In the second stage, hot rolling was carried out on a tandem rolling mill to a thickness of 4 to 7 mm, at which point the sheets were wound into coils. The rolling conditions in the tandem rolling mill are given in Table 2. For examples C#3 and C#4, a heat treatment at 340 °C was carried out after hot rolling to simulate the effect of a tandem mill outlet temperature of 340 °C. For example C#4, a 2-hour treatment at 210 °C was carried out after cold rolling.

[0078] Table 2 Rolling conditions.

[0079]

[0080] * calculated by linear extrapolation

[0081] The mechanical properties obtained are given in Table 3. Table 3, Mechanical properties obtained

[0082]

[0083] The samples were subjected to a corrosion test according to ASTM G67, "Standard Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series Aluminum Alloys by Mass Loss After Exposure to Nitric Acid (NAMLT Test)." Results are given for the as-made condition or after 7 days of exposure at 100 °C.

[0084] The granular structure of the samples was also characterized by scanning electron microscopy (EBSD) as described previously to obtain the KAM value. The results are given in Table 4.

[0085] Table 4: Results of corrosion tests and characterization of grain misorientations by the KAM method.

[0086]

[0087]

[0088] The products obtained with the process according to the invention (A#1, C#1, C#2 and E#1) exhibit a result after such NAMLT H128 of less than 15 mg / cm2. The products according to the invention having a magnesium content less than or equal to 4.6% by weight have a mean KAM degree of disorientation of at least 0.75, as illustrated in Figure 2.

Claims

Demands 1. A process for manufacturing an aluminum alloy sheet in which a) an aluminum alloy is prepared with the composition, in % by weight Mg: 4.0 - 5.2, Mn: 0.40 - 1.0, Zn: 0.15 - 0.40, at least one element chosen from Ti, Cr, Cu and Zr, with a content, if chosen, of 0.01 - 0.15 for Ti, 0.05 - 0.25 for Cr, 0.02 - 0.25 for Cu, 0.05 - 0.25 for Zr, Fe: < 0.40, If: < 0.40, other elements or impurities < 0.05 each and < 0.15 total, remaining aluminum, b) a rolling plate is cast by vertical semi-continuous casting, c) optionally the said rolling plate is homogenized, d) The said optionally homogenized plate is hot-rolled in two successive stages to obtain a sheet dl) a hot rolling stage on a reversible rolling mill to a thickness between 12 and 35 mm, d2) a hot rolling step on a tandem rolling mill to a thickness of between 3 and 12 mm, in which the final temperature is at least 240 °C and is less than 300 °C, e) Optionally, the said sheet metal is cold-rolled to a thickness of between 1 and 7 mm, f) Optionally, a final heat treatment is carried out on the said sheet, optionally cold rolled at a temperature below 300 °C.

2. A method according to claim 1 wherein the Zn content is between 0.15 and 0.35% by weight and preferably between 0.18 and 0.30% by weight.

3. A process according to claim 1 or claim 2 wherein step c of homogenization is not carried out but a simple reheating is performed before hot rolling at a temperature between 490 and 535 °C.

4. A method according to any one of claims 1 to 3 wherein the final heat treatment step f is not carried out and wherein the sheet metal is used in its as-made condition.

5. A method according to any one of claims 1 to 4 wherein the final temperature at step d2 is between 240 °C and 280 °C.

6. A method according to any one of claims 1 to 5 wherein a thickness reduction of at least 30% is carried out at a temperature between 240 °C and 380 °C during step d2.

7. A method according to any one of claims 1 to 6 wherein the Mg content is between 4.0 and 4.6% by weight and wherein a thickness reduction of at least 65% is carried out at a temperature between 240 °C and 380 °C during step d2.

8. Aluminum alloy sheet with a thickness between 1 and 12 mm, composition as a percentage by weight, Mg: 4.0 - 5.2, Mn: 0.40 - 1.0, Zn: 0.15 - 0.40, at least one element chosen from Ti, Cr, Cu and Zr, with a content, if chosen, of 0.01 - 0.15 for Ti, 0.05 - 0.25 for Cr, 0.02 - 0.25 for Cu, 0.05 - 0.25 for Zr, Fe: < 0.40, If: < 0.40, other elements or impurities < 0.05 each and < 0.15 total, remaining aluminum, obtainable by the process according to any one of claims 1 to 7 exhibiting, after 7 days of exposure at 100 °C, a weight loss of less than 15 mg / cm³ 2 during a corrosion test according to ASTM G67.

9. Sheet according to claim 8 in which the Mg content is between 4.0 and 4.6% by weight, capable of being obtained by the process according to claim 7.

10. Aluminium alloy sheet according to claim 9 characterized in that the average degree of misorientation KAM is at least 0.

75.

11. Use of a sheet metal according to any one of claims 8 to 10 in shipbuilding or for the construction of industrial vehicles.

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