MULTLAYER BRAZING SHEET

Abstract

The present invention deals with a brazing sheet comprising: a core layer made of a AA3xxx alloy comprising, in weight percentages: up to 0.70% Si, up to 0.70% Fe, 0.20 to 1.10% Cu, 0.70 to 1.80% Mn, up to 0.40% Mg, up to 0.30% Zn, up to 0.30% Ti, Zr and/or Cr and/or V each up to 0.30%, other elements less than 0.05% each and less than 0.15% in total, balance being aluminium; a brazing layer, made of a AA4xxx alloy which is present on at least one side of the core layer; and an interlayer, inserted between the core layer and the brazing layer, on at least one side of the core layer, which composition comprises, in weight percentages: from 1.5 to 2.3% Zn, from 0.2 to 0.75% Mn, up to 0.5% Fe, up to 0.5% Si, other elements less than 0.05% each and less than 0.15% in total, balance being aluminium.

Claims

1. A brazing sheet comprising: a core layer made of a AA3xxx alloy comprising, in weight percentages: up to 0.70% Si, up to 0.70% Fe, 0.20 to 1.10% Cu, 0.70 to 1.80% Mn, up to 0.40% Mg, up to 0.30% Zn, up to 0.30% Ti, Zr and/or Cr and/or V each up to 0.30%, other elements less than 0.05% each and less than 0.15% in total, balance being aluminum; a brazing layer, made of a AA4xxx alloy, which is present on at least one side of the core layer; and an interlayer, inserted between the core layer and the brazing layer, on at least one side of the core layer, which composition comprises, in weight percentages: from 1.5 to 2.3% Zn, from 0.2 to 0.75% Mn, up to 0.5% Fe, up to 0.5% Si, other elements less than 0.05% each and less than 0.15% in total, balance being aluminum.

2. The brazing sheet according to claim 1, wherein the core layer comprises 0.45 to 0.51 wt. % Cu.

3. The brazing sheet according to claim 1, wherein the core layer comprises, in weight %: 0.05 to 0.35% Si; up to 0.40% Fe; 0.25 to 0.70% Cu; 1.10 to 1.60% Mn; up to 0.15% Mg; 0.01 to 0.30% Cr; up to 0.30% Zn; 0.01 to 0.20% Ti; other elements less than 0.05% each and less than 0.15% in total, balance being aluminum.

4. The brazing sheet according to claim 1, wherein the brazing layer is present on both sides of the core layer, both brazing layers having the same or a different composition.

5. The razing sheet according to claim 1, wherein the Mn content of the interlayer is 0.30 to 0.40 wt. %.

6. The brazing sheet according to claim 1, wherein the brazing layer and the interlayer have each a thickness of 3 to 30%, optionally of 5 to 15% of the total thickness of the brazing sheet.

7. The brazing sheet according to claim 1, wherein the Zn/Mn ratio in the interlayer is from 2 to 11, optionally from 3 to 7.

8. The brazing sheet according to claim 1, wherein the thickness of the interlayer is up to 65 μm, optionally up to 55 μm.

9. A product comprising the brazing sheet according to claim 1 for production of a heat exchanger of a motor vehicle.

10. The product according to claim 9, in which the heat exchanger is a water charge air cooler comprising a tube or a channel formed by a pair of plates, having an external side where the gas to be cooled flows, said tube or plates being made from the brazing sheet with the interlayer located on said external side, and comprising fins made of an aluminum alloy having a Zn content from 1.25 to 3.00 wt. % fixed on said external side, and in which the Zn content of the interlayer is less than 120% of the Zn content of the fins.

11. A heat exchanger of a motor vehicle, produced at least partly from a brazing sheet according to claim 1.

12. The heat exchanger according to claim 11, in which the heat exchanger is a water charge air cooler comprising a tube or a channel formed by a pair of plates, having an external side where the gas to be cooled flows, said tube or plates being made from the brazing sheet with the interlayer located on said external side, and comprising fins made of an aluminum alloy having a Zn content from 1.25 to 3.00 wt. % fixed on said external side, and in which the Zn content of the interlayer is less than 120% of the Zn content of the fins.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows a schematic longitudinal section of a tube (roll formed and brazed or welded tube) of an air cooled charge air cooler (air CAC).

[0038] FIG. 2 shows a schematic longitudinal section of a tube (channel formed by a pair of plates) of a water cooled charge air cooler (water CAC).

[0039] FIG. 3 shows a schematic three layer brazing sheet architecture.

[0040] FIG. 4 shows a schematic four layer brazing sheet architecture.

[0041] FIG. 5 shows: (a) state-of-the-art long life material before “CAC test”: specimen's back & edges protected with adhesive & silicone to avoid parasite corrosion. (b) state-of-the-art long life material after 2 weeks “CAC test” & hot water rinsing.

[0042] FIG. 6 shows a schematic representation of the cross section cutting process on a SWAAT sample (after removal of the silicone joint protecting the sample edges).

DETAILED DESCRIPTION OF THE INVENTION

Core Layer

[0043] The core layer is made of a 3xxx alloy.

[0044] Preferably, the core layer comprises, more preferably consists essentially of, in weight percentages: [0045] up to 0.70%, preferably 0.05 to 0.35%, more preferably 0.10 to 0.30% Si, [0046] up to 0.70%, preferably up to 0.40%, more preferably up to 0.25% Fe, [0047] 0.20 to 1.10%, preferably 0.30 to 1.00%, more preferably 0.35 to 0.60% Cu, [0048] 0.70 to 1.80%, preferably 1.10 to 1.60%, more preferably 1.20 to 1.50% Mn, [0049] up to 0.40%, preferably up to 0.30%, more preferably up to 0.15%, even more preferably up to 0.10% Mg, [0050] up to 0.30%, preferably up to 0.20% Zn, [0051] up to 0.30%, preferably up to 0.20%, more preferably 0.01 to 0.20%, even more preferably 0.02 to 0.15% Ti, [0052] Zr and/or Cr and/or V each up to 0.30%, preferably 0.01 to 0.30%, more preferably 0.02 to 0.25%, [0053] other elements less than 0.05% each and less than 0.15% in total, [0054] balance being aluminium.

[0055] Preferably, the core layer of the brazing sheet according to the present invention comprises 0.40 to 0.54 wt. %, more preferably 0.45 to 0.51 wt. % Cu.

[0056] Three core layer alloys suitable according to the present invention are described in Table 1 hereinafter, in wt. %:

TABLE-US-00001 TABLE 1 Core layer alloys suitable according to the present invention Core-a Core-b Core-c Si 0.10-0.30 0.10-0.30 0.10-0.30 Fe ≤0.30 ≤0.30 ≤0.30 Cu 0.60-0.90 0.50-0.80 0.35-0.60 Mn 1.20-1.50 1.20-1.50 1.20-1.50 Mg 0.05-0.30 ≤0.10 ≤0.10 Cr ≤0.15 ≤0.15 0.02-0.25 Zn ≤0.20 ≤0.20 ≤0.20 Ti 0.02-0.20 0.02-0.20 0.02-0.15

[0057] According to an embodiment, a core layer alloy suitable according to the present invention consists of, in weight %: 0.05 to 0.35% Si; up to 0.40% Fe; 0.25 to 0.70% Cu; 1.10 to 1.60% Mn; up to 0.15% Mg; 0.01 to 0.30% Cr; up to 0.30% Zn; 0.01 to 0.20% Ti; other elements less than 0.05% each and less than 0.15% in total, balance being aluminium.

[0058] The temper of the core layer may be a recovered structure such as H24 which is partially annealed or O-temper which is fully annealed. As is commonly known by the person skilled in the art, the tempers are defined for example in standard BS EN 515.

Brazing Layer

[0059] Preferably, the Si content of the brazing layer is 5 to 13 wt. %.

[0060] Preferably, the composition of the brazing layer is AA4045 or AA4343.

[0061] For example, the composition AA4045 is, in wt. %: 9 to 11% Si, up to 0.8% Fe, up to 0.30% Cu, up to 0.05% Mn, up to 0.05% Mg, up to 0.10% Zn, up to 0.20% Ti, other elements less than 0.05% each and less than 0.15% in total, balance being aluminium.

[0062] For example, the composition AA4343 is, in wt. %: 6.8 to 8.2% Si, up to 0.8% Fe, up to 0.25% Cu, up to 0.10% Mn, up to 0.05% Mg, other elements less than 0.05% each and less than 0.15% in total, balance being aluminium.

[0063] According to an embodiment, the temper of the core layer is H24 and the brazing layer is present on only one side of the core layer, preferably on the interlayer side.

[0064] Preferably, the brazing layer is present on both sides of the core layer, on the interlayer when present, otherwise directly on the core layer, both brazing layers having the same or a different composition.

Interlayer

[0065] The interlayer according to the present invention comprises, preferably consists essentially of, more preferably consists of, in weight percentages: from 1.5 to 2.3% Zn, from 0.2% (preferably 0.3%) to 0.75% (preferably 0.45%) Mn, up to 0.5% (preferably 0.4%) Fe, up to 0.5% (preferably 0.4%) Si, other elements less than 0.05% each and less than 0.15% in total, balance being aluminium.

[0066] Preferably, the Mn content of the interlayer of the brazing sheet according to the present invention is 0.3 to 0.4 wt. %. The effect of this specific range of Mn in the interlayer is illustrated by the examples hereinafter.

[0067] Preferably, the Zn content of the interlayer of the brazing sheet according to the present invention is 1.5 to 2.3 wt. %. The effect of this specific range of Zn in the interlayer is illustrated by the examples hereinafter.

[0068] Preferably, the Zn/Mn ratio in the interlayer is from 2 to 11, more preferably from 3 to 7.

[0069] Ti may increase the corrosion potential and thus render the interlayer less sacrificial compared to the core layer. Consequently, the content of Ti in the interlayer is preferably less than 0.05 wt. %.

[0070] Preferably, the thickness of the interlayer is up to 65 μm, more preferably up to 55 μm.

[0071] According to an embodiment, the brazing sheet according to the present invention is used in a water charged air cooler, as is for example illustrated by FIG. 2. In this specific embodiment, the interlayer is on the external side of the tube or pair of plates and fins are fixed on said external side. In this case, the external side of the tube or pair of plates is the side in contact with the gas to be cooled. In this embodiment, the Zn content of the interlayer is preferably less than the Zn content of the fins. This specific embodiment will be further described hereinafter.

Sheet

[0072] Preferably, the brazing sheet according to the present invention is characterized in that the brazing layers and the interlayers have each a thickness of 3 to 30%, preferably of 5 to 15%, more preferably 8 to 12% of the total thickness of the brazing sheet.

[0073] The invention consists in a judicious choice of the respective alloys of the core layer, the interlayer and the brazing layer for carrying out a brazing sheet of the multilayer type, adapted to the severe corrosion conditions to which these materials are subjected in use, in particular in charge air coolers or air conditioning evaporators.

[0074] In particular, the concentration ranges imposed on the constituent elements of the alloy of the interlayer are explained by the following reasons: [0075] Si has an unfavorable effect on the resistance to pitting and/or intergranular corrosion. Therefore, its content must be less than 0.5 wt. % and preferably less than 0.4 wt. %; [0076] Fe is generally considered as an impurity for aluminium and constitutes privileged sites for the initiation of corrosion pitting. Therefore, its content must be less than 0.5 wt. % and more preferably less than 0.4 wt. %; [0077] Cu also increases the corrosion potential thereby reducing the sacrificial anode effect of the interlayer. By its non-homogeneous distribution within the alloy, it may also increase the risks of galvanic corrosion and may favor intergranular corrosion by the presence of Al.sub.2Cu-type phases, in particular at grain boundaries. Consequently, its content must be limited to that of an impurity, ie less than 0.05 wt. %; [0078] Mn is a hardening element that has a positive effect on the strength after brazing by hardening in solid solution and in the form of fine dispersoids. Most importantly, it improves the hot flow stress of the alloy, greatly facilitating the co-rolling. But when there is too much Mn, the corrosion resistance is decreased in that the corrosion attack is not lateralized and not maintained in the interlayer level, and the core layer may be attacked by corrosion. Moreover, with too much Mn, the interlayer becomes less sacrificial compared to the core layer; [0079] Mg has a positive effect on mechanical strength, but it is detrimental to brazability, since it migrates to the surface of the brazing layer and, especially in the case of controlled atmosphere brazing (CAB) of the “Nocolok®” type, forming an oxide layer which modifies in an unfavorable way the properties of the brazing. For this reason, and for such difficult applications, its content may be limited to 0.02% or even 0.01%; [0080] Zn has an influence on corrosion resistance. Its content has to be balanced with the content of Mn. If there is too much Zn, the corrosion potential of the interlayer may be too low. In this case, the interlayer may deteriorate too fast, and in particular when the interlayer is located at fins side it could corrode faster than the fins (which are supposed to be protective). The content of Zn in the interlayer is thus preferably from 1.5 to 2.3 wt. %.

[0081] The presence of an interlayer allows to create a decreasing copper profile from the core layer to the brazing layer. This effect reinforces the effect of zinc on corrosion resistance.

[0082] The core layer side opposite to the interlayer side may be cladded directly with a brazing layer made of an alloy of the AA4xxx series. The brazing layers may have the same or a different composition.

[0083] However, an advantageous variant of this configuration is a symmetrical multi-layered composite material, that is to say provided with an interlayer on both sides of the core layer, one ensuring resistance to internal corrosion and the other to external corrosion, as is particularly favorable in the case of CAC type heat exchangers. In this embodiment also the brazing layers may have the same or a different composition. This is also the case for both interlayers.

Process

[0084] The brazing sheet according to the present invention may be produced using any known process. The process may generally comprise the following successive steps: [0085] casting the different alloys to obtain blocks; [0086] scalping the blocks on both sides; [0087] optionally homogenizing the interlayer; [0088] preheating the brazing alloy and the interlayer alloy blocks at 400 to 550° C.; [0089] hot rolling the brazing alloy and the interlayer alloy blocks until the desired clad thickness to get the desired cladding ratio; [0090] optionally homogenizing the core layer alloy block at 550 to 630° C. during at least 1 hour, preferably 1 to 20 hours; [0091] assembling the blocks to obtain a sandwich; [0092] preheating the sandwich at 400 to 550° C.; [0093] hot rolling the sandwich until an intermediate thickness, for example 2 to 4.5 mm; [0094] cold rolling the hot-rolled sandwich until the desired final thickness, for example 0.15 to 1.20 mm, to obtain the brazing sheet; [0095] annealing at 250 to 450° C. for at least 30 minutes.

[0096] The goal of the annealing step is to achieve the desired temper, for example H24 or O-temper.

[0097] Then the brazing sheet may be brazed to other sheets, that could have the same or another configuration. Preferably, the brazing process uses a flux, for example the known process called Nocolok®.

[0098] Such brazing sheets are particularly suitable for the manufacture of heat exchangers, preferably charge air coolers (CAC), exhaust gas recirculation (EGR) coolers, evaporators, condensers or radiators, more preferably charge air coolers (CAC), due in particular to a good behavior in stamping, and also a corrosion behavior significantly improved, as described in the examples below.

[0099] The invention consists of the best compromise between rolling ability and corrosion resistance. It differs from the known prior art at least by a specific selection of the amounts of Mn and Zn in the interlayer.

Use

[0100] The brazing sheet according to the present invention may be used in the production of a heat exchanger of a motor vehicle, preferably a charge air cooler (CAC), an exhaust gas recirculation (EGR) cooler, an evaporator, a condenser or a radiator, more preferably a charge air cooler (CAC). As is known, there are two main kinds of CAC: air CAC and water CAC.

[0101] Air CAC may be illustrated by FIG. 1. FIG. 1 shows a schematic longitudinal section of a tube of an air cooled charge air cooler (air CAC).

[0102] The tube of air CAC as illustrated in FIG. 1 is made of a four layer brazing sheet. The brazing sheet comprises two brazing layers 1, which may have the same or a different composition, a core layer 2 and an interlayer 3. The reference number 4 represents the fins. It is understood that, according to another embodiment, the brazing sheet may comprise a second interlayer, on the opposite side with the same or a different composition compared to the first interlayer.

[0103] The gas to be cooled 5 flows through the internal side 8 of a tube (=the interlayer side). The air 6 flows at the external side 9 of the tube (=the interlayer opposite side). Fins 4 are positioned on the external side 9 of the tube. The interlayer 3 is positioned on the internal side 8 of the tube, where the gas to be cooled 5 flows.

[0104] Water CAC may be illustrated by FIG. 2. FIG. 2 shows a schematic longitudinal section of a tube (or channel formed by a pair of plates) of a water cooled charge air cooler (water CAC).

[0105] The tube (or channel formed by a pair of plates) of water CAC as illustrated in FIG. 2 is made of a four-layer brazing sheet. The brazing sheet comprises two brazing layers 1, which may have the same or a different composition, a core layer 2 and an interlayer 3. The reference number 4 represents the fins. It is understood that, according to another embodiment, the brazing sheet may comprise a second interlayer on the opposite side, with the same or a different composition compared to the first interlayer.

[0106] The coolant 7 flows through the internal side 8 of a tube or a channel formed by a pair of plates (=the interlayer opposite side). The gas to be cooled 5 flows at the external side 9 of the tube or channel formed by a pair of plates (=the interlayer side). Fins 4 are on the external side 9 of the tube or channel formed by a pair of plates. The interlayer 3 is positioned on the external side 9 of the tube or channel formed by a pair of plates, where the gas to be cooled 5 flows.

[0107] According to an embodiment, the brazing sheet according to the present invention may be used for the production of a heat exchanger of a motor vehicle, preferably a charge air cooler (CAC), an exhaust gas recirculation (EGR) cooler, an evaporator, a condenser or a radiator, preferably a charge air cooler (CAC).

[0108] According to another embodiment, the brazing sheet according to the present invention may be used for the production of a heat exchanger, in which the heat exchanger is a water charge air cooler comprising a tube or a channel formed by a pair of plates, having an external side where the gas to be cooled flows, said tube or plates being made from the brazing sheet according to the present invention with the interlayer located on said external side, and comprising fins made of an aluminium alloy having a Zn content from 1.25 to 3.00 wt. % fixed on said external side, and in which the Zn content of the interlayer is less than 120%, preferably less than 100% of the Zn content of the fins.

[0109] Preferably, the fin alloy comprises, more preferably consists of, a 3003 alloy to which Zn is added so that the total content of Zn is from 1.25 to 3.00 wt. %. Generally, a 3003 alloy comprises, in weight percentages: up to 0.60% Si; up to 0.70% Fe; from 0.05 to 0.20% Cu; from 1.00 to 1.50% Mn; up to 0.10% Zn; other elements less than 0.05% each and less than 0.15% in total; balance being aluminium.

[0110] In its details, the invention will be better understood thanks to the examples described hereinafter, which are however not limiting.

[0111] All documents referred to herein are specifically incorporated herein by reference in their entireties.

[0112] As used herein and in the following claims, articles such as “the”, “a” and “an” can connote the singular or plural.

[0113] In the present description and in the following claims, to the extent a numerical value is enumerated, such value is intended to refer to the exact value and values close to that value that would amount to an unsubstantial change from the listed value.

Examples

[0114] FIG. 4 and Table 2 summarize the configurations and compositions of the investigated materials (in weight percentages). Before brazing all solutions were in O-temper and at a thickness of 400 microns. The interlayer thickness was 40 μm.

[0115] According to FIG. 4, the brazing layer 1 was made of AA4343 and represented 7.5% of the total thickness, on both sides of the brazing sheet. The interlayer 3 represented 10% of the total thickness, and the core layer 2 represented 75% of the total thickness.

TABLE-US-00002 TABLE 2 Investigated materials Core Interlayer composition alloy Si Fe Cu Mn Cr Ti Zr Zn Example-1 Core-1 0.204 0.153 <0.005 0.385 <0.0016 <0.005 — 1.95 Example-2 Core-2 0.204 0.153 <0.005 0.385 <0.0016 <0.005 — 1.95 Example-3 Core-1 0.1 0.19 0.69 1.9 Ref.1-Mn Core-1 0.105 0.208 0.007 0.358 0.001 0.019 0.001 — Ref.2-Mn Core-1 0.104 0.158 0.002 0.709 0.001 0.020 0.002 — Ref.3-Mn Core-1 <0.05 <0.1 — 1.75 — — — Ref-Zn Core-1 0.097 0.184 0.7 0.98

[0116] Several 4 layer sheets with different core layer alloys, and interlayer alloys were prototyped. AA4343 alloy was used as brazing layer on both sides for all the prototyped sheets. The sheets marked Example-1, Example-2 and Example-3 are according to the invention. The sheets marked Ref.1-Mn, Ref.2-Mn, Ref.3-Mn and

[0117] Ref-Zn are comparative examples.

[0118] The Core-1 alloy had the following composition, in wt. %:

[0119] Si: 0.18 Fe: 0.15 Cu: 0.65 Mn: 1.35 Ti: 0.08 other elements <0.05 each and <0.15 in total, balance being aluminium.

[0120] The Core-2 alloy had the following composition, in wt. %:

[0121] Si: 0.19 Fe: 0.13 Cu: 0.51 Mn: 1.33 Cr: 0.09 Zn: 0.02 Ti: 0.01 other elements <0.05 each and <0.15 in total, balance being aluminium.

[0122] The alloy AA4343 had the following composition, in wt. %:

[0123] Si: 7.2 Fe: 0.15 Cu: <0.1 Mn: <0.1 Ti: <0.05 other elements <0.05 each and <0.15 in total, balance being aluminium.

[0124] The process for the production of brazing sheets was as follows: [0125] casting the different alloys to obtain blocks; [0126] scalping the obtained blocks on both sides; [0127] preheating the brazing alloy and the interlayer alloy blocks at 500° C.; [0128] hot rolling the brazing alloy and the interlayer alloy blocks until the desired clad thickness to get the desired cladding ratio; [0129] homogenizing the core layer alloy blocks at 620° C. during 8h; [0130] assembling the blocks to obtain sandwiches; [0131] preheating the sandwiches at 500° C.; [0132] hot rolling the sandwiches until a 3.5 mm thickness; [0133] cold rolling until a 0.4 mm thickness; and [0134] annealing at 350° C. during 1h to obtain a O-temper.

[0135] The sheets were then submitted to a brazing cycle simulation comprising a rise in temperature at 40° C./min up to 550° C., and then at 20° C./min up to 600° C. This temperature was kept during 2 minutes. Cooling was then done in the oven at around −25° C./min.

[0136] The obtained materials were then submitted to corrosion test.

Corrosion Test

[0137] As a first and rough evaluation of the durability of investigated materials in a corrosive environment, ASTM G85A3-SWAAT test is generally carried out in a climatic chamber. The procedure is based on the following cycle: 30 min spray+90 min soak. A 5% synthetic sea salt solution at pH3 is used as the condensate. Although SWAAT test is extensively used for testing heat exchangers, this procedure is related to atmospheric corrosion and concerns the durability of the external side of heat exchanger such as air conditioning evaporators.

[0138] Concerning the specific case of Charge Air Cooler (CAC), SWAAT test has very limited value. Therefore, a dedicated corrosion test was developed to simulate corrosion in CAC heat exchangers. Knowing that exhaust gases circulating inside CAC consist mainly of CO.sub.2, H.sub.2O, NOx and SO.sub.2 (depending on the diesel-sulfur level), if condensate formation occurs, this generates strong acid (HNO.sub.3, H.sub.2SO.sub.4) and less corrosive organic acids. Exhaust-gas condensate composition and dew point depend on fuel composition, combustion process, air ratio, load of the engine, exhaust-gas after treatment, engine start-up stage, etc. . . . . Furthermore, EGR (Exhaust Gas Recirculation) system will frequently be exposed to successive wet and dry environments depending on the engine speed and temperature. States in which fluid acidic concentrate can dwell and dry on the components are critical. From these assessment, a corrosion test, hereinafter called “CAC test”, based on a 3-step 4h cycle was set up (see schema hereunder). This corrosion test, aiming at assessing the corrosion resistance seen in-service includes a dry and wet cycle as well as a spraying phase using a synthetic condensate made of sulfuric acid and nitric acid (H.sub.2SO.sub.4+HNO.sub.3) equi-molar solution. Tests were conducted at pH 2 and 1000 ppm Cl.sup.− during 6 weeks.

##STR00001##

[0139] 45 mm (L)×65 mm (TL)×0.48 mm (TC) samples were cut from each reference (see FIG. 5). The samples were degreased with acetone and then masked in order to expose only the front side to be tested. The edges and the back side respectively were protected with silicone and adhesive tape. Therefore, the tested surface was about 40 mm (L)×60 mm (TL) leading to an exposed surface of about 2400±100 mm.sup.2.

[0140] When SWAAT test was completed, cross section micrographs in the L-ST plane were carried out to investigate corrosion morphology on the exposed side. Four 40 mm (L)×10 mm (TL) sections were cut as described in FIG. 6 and observed with optical microscopy using two different magnifications (i.e. ×50 and ×100) in order to obtain representative and reliable pictures of the attack mode.

[0141] The results of the optical microscopy observations are shown in Table 3 hereinafter.

TABLE-US-00003 TABLE 3 Results of the CAC corrosion test, after 6 weeks of exposure Lateralization of corrosion Absence of corrosion in the interlayer of the core Example-1 + ++ Example-2 ++ ++ Example-3 + ++ Ref. 1-Mn − + Ref. 2-Mn − + Ref. 3-Mn − − Ref-Zn − +

[0142] In Table 3 hereinbefore, “−” means the absence of lateralization of corrosion in the interlayer or the presence of corrosion of the core layer in an important manner, “+” means the presence of lateralization or corrosion of the core layer in a moderate manner, and “++” means the presence of lateralization of corrosion in the interlayer in a fully effective manner or the absence of corrosion of the core layer. The results are based on micrograph observations.

[0143] The results presented in Table 3 hereinabove confirm the lateralization of the corrosion in the interlayer which plays its sacrificial role, at the same time as the absence of perforation of the underlying core layer for the compositions according to the present invention.