Corrosion-resistant clad material made of aluminum alloy

Abstract: A corrosion-resistant clad material is made of aluminum alloys, and in a preferable mode, comprises: a core which is either formed of an Al-Mn alloy containing 0.5 - 1.5 % by weight of Mn; or alternatively formed of an Al-Mg-Si alloy containing 0.05 - 1.0 % by weight of Mg and 0.05 - 0.3 % by weight of Si. The Al-Mn alloy and Al-Mg-Si alloy further contain, as additional components, 0.05 - 0.3 % by weight of Ti and/or 0.05 - 0.4 % by weight of Zr. The clad material further comprises skin layers which are formed of an Al-Zn alloy clad onto at least one surface of the core. In another preferable mode, the clad material comprises: a core formed of an Al-Mn alloy containing 0.5 - 1.5 % by weight of Mn; and skin layers clad onto at least one surface of the core and formed of an aluminum alloy containing 1.3 - 5.0 % by weight of Zn, with or without 0.2 - 1.0 % by weight of Mg, and a balance of aluminum and unavoidable impurities. Cu content as one of the impurities is controlled to 0.05 % by weight or less, with the skin layers having a thickness of 30 µm or less.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a corrosion-resistant clad material which has a multi-layered structure composed of aluminum alloys and is used to manufacture for example the tubes employed in various types of heat exchangers such as radiators, car heaters, intercoolers or the like. The term "sheet" in this specification is meant to include "films" which is of a thickness less than 0.25 mm. 2. Description of Prior Art In manufacture of such heat exchangers which must be highly corrosion-resistant, there has been used an aluminum alloy clad material comprising a core and skin layers which are formed on at least one surface of core by the so-called cladding process. The core is a sheet made of an aluminum alloy selected from Al-Mn alloys (i.e., aluminum-manganese alloys including "A3003" series) or Al-Mg-Si alloys (i.e., aluminum-magnesium-silicon alloys called "6000" series) which are easy to process mechanically. The skin layers serving as "sacrificially corroding layers" are made of for example an Al-Zn alloy (i.e., aluminum-zinc alloy). The core is protected from corrosion due to the sacrificial corrosion of the Al-Zn alloy forming the skin layers. Composite tube material of this kind having a thickness of 0.4 mm is disclosed in JP-A-62 208 938. While compositions of the aluminum alloys used for the core and a skin layer are similar to those according to the invention the Examples disclosed in the JP-A depart grossly in at least one essential compositional feature. It is however noted that the prior art aluminum alloy clad material is disadvantageous due to its drawbacks summarized below. In general, the constituent parts are brazed to become integral with each other to form such heat exchangers as exemplified above. Zinc atoms in the skin layer tend to diffuse into the core at high temperatures when the clad sheets as said heat exchanger parts are brazed. With the zinc atoms which probably diffuse deep in the core, it is likely that the corrosion resistance of the core itself becomes so poor that the corrosion resistance of the heat exchanger as a whole cannot be improved to a desirable degree. Heat treatments other than the hot brazing will likewise cause such a problem. In a case wherein a corrosive medium such as water contacts with the inner surfaces of the tubes incorporated in for example a radiator, another type of clad material has been used which comprises the core of "A3003" alloy having its inner surface coated with a skin layer of another aluminum alloy "A7072". The core is easy to work, and the skin layer serves as the sacrificial corrosion layer more effectively than in the general cases as mentioned above. However, this alternative clad material is also problematic from the following aspect. Lighter weight and lower manufacture cost are recently required for the tubes and other parts of the heat exchangers, and therefore efforts have been made to provide thinner parts including the tubes. A thinner core must be covered with thinner "A7072" skin layers 30 µm thick or less. Such extremely thin skin layers will not produce a sufficient effect of sacrificial corrosion, thus causing the core to rapidly corrode impairing the corrosion resistance of the parts. OBJECTS AND SUMMARY OF THE INVENTION An object of the invention is to provide an aluminum alloy clad material which is of such an improved corrosion resistance that can be maintained even in the case of thinner skin layers having a thickness of 30 µm or less. The object is achieved in the invention with a corrosion-resistant aluminum alloy clad material comprising an Al-Mn alloy core and skin layers covering at least one surface of the core and having a thickness of 30 µm or less, which skin layers are formed of an aluminum alloy composed of: 1.3 - 5.0 % by weight of Zn with or without 0.2 - 1.0 % by weight of Mg; and the balance which consists of aluminum and unavoidable impurities, wherein content of Cu as one of the unavoidable impurities is controlled to be 0.05 % by weight or less. ("Cu" denotes copper.) Accordingly, the invention is defined in claim 1, optional features thereof being mentioned in dependent claims 2 - 5. Other objects, features and advantages of the present invention will become clear from the following description made referring to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing in part an aluminum radiator which is made of a clad sheet provided in the invention. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a corrosion-resistant aluminum alloy clad sheet comprising a core and Al-Zn alloy skin layers which are clad to cover at least one surface of the core. The core is formed of an Al-Mn alloy. The Al-Mn alloy contains 0.5 - 1.5 % by weight of Mn. Mn (i.e., manganese) contained in the Al-Mn alloy as the core material enhances to the core and the clad material a high mechanical strength, easiness of processing and corrosion resistance, and therefore is regarded as an element effective to improve these properties of the composite clad material composed of the core. A content of Mn below 0.5 % by weight renders insufficient the effect, and due to saturation thereof, a superfluous Mn content above 1.5 % by weight gives no additional merit which compensates an increasing cost. Furthermore, an excessive content of Mn will produce coarser crystalline grains which impair the easiness of processing the core. Thus, it is most preferable that Mn content falls within a range of 0.7 - 1.2 % by weight. The aluminum alloy of the core may contain other elements such as Fe (i.e., iron) and Cu (i.e., copper), in addition to those which are already described above. Fe contributes to produce finer crystalline grains, improves mechanical strength, and inhibits intercrystalline corrosion at high temperatures. These effects are poor with a Fe content below 0.03 % by weight, but due to saturation of effects, reach an uppermost limit at a content of 1.0 % by weight or higher. Thus, the content of Fe which may exist in the alloy must fall within a range of about 0.03 - 1.0 % by weight, and more desirably within a narrower range of 0.1 - 0.6 % by weight. Cu prevents cathode corrosion of the skin layer, and suppresses corrosion from a core's outer surface (that is, from the surface not covered with a skin layer). Cu content below 0.05 % by weight does not give such an effect, and a higher Cu content above 0.2 % by weight causes diffusion of Cu atoms into the skin layer so that a decreased potential difference between the core and skin layer does not sufficiently prevent the so-called cathode corrosion. Therefore, the content of Cu which may exist in the alloy must be about 0.05 to 0.2 %, and more desirably 0.07 - 0.15 % by weight. Cu may be contained in the aluminum alloy of the skin layer as an impurity element, but it is necessary to control its content to 0.05 % or less. With a higher content above 0.05 % by weight, the potential of skin layers becomes high relative to that of the core and as a result the sacrificial corrosion effect of Zn is lowered. Therefore, Cu content is controlled more preferably to 0.01 % by weight or less. To manufacture the aluminum alloy clad material in the first mode of the invention, any special condition need not be employed but such ordinary manners are available as in the case of prior art clad materials. Aluminum alloy ingots are subjected to the homogenizing treatment and are cut into raw thick sheets, i.e., a raw skin sheet and raw core sheet, are successively hot-rolled and cold-rolled together to clad the former onto the latter. The finished clad material provided in the first mode may be used with its skin layer exposed to a corrosive environment. For example, an aluminum radiator comprising tubes 1, fin members 2 and header pipes 3 as shown in part in Fig. 1 may comprise each tube made of this clad sheet, wherein an inner skin layer 1b formed of an Al-Zn alloy will be disposed inside as an inner lining to contact with water. It will be understood that it may suffice to clad the skin layer 1b onto at least one surface of a core 1a, with another surface thereof being clad with an outer skin layer, i.e., a brazing agent layer 1c which facilitates the brazing of each fin member 2 to the tubes. The core 1a is formed of Al-Mn alloy containing 0.5 - 1.5 wt% of Mn; skin layers 1a, 1b cover at least one surface of the core. The skin layers are formed of an aluminum alloy which contains 1.3 - 5.0 wt% of Zn, with or without 0.2 -1.0 wt% of Mg incorporated in addition to Zn; and the balance which is aluminum and unavoidable impurites therein. Cu content as one impurity is controlled to 0.05 wt% or less, and the thickness of the skin layers is controlled to 30 µm or less. The reason for delimiting the material of the core to the Al-Mn alloy resides in a fact that the incorporation of Mn can give the core a high mechanical strength as well as easiness of processing and a high corrosion resistance to thereby improve these properties of the clad material as a whole. However, a lower Mn content below 0.5 wt% makes insufficient such effects, and on the contrary, a higher content above 1.5 wt% will be useless because of saturation of the effects. Such a superfluous content of Mn will raise manufacture cost to an intolerable degree which cannot be compensated with a slight marginal effect. The range of Mn content is therefore 0.5 -1.5 wt%, and more desirably 0.7 -1.2 wt%. The alloy forming the core may further contain for example Si (i.e., silicon), Fe (i.e., iron), Cu (i.e., copper) or other elements. If a content of Si which contributes to a higher mechanical strength of the brazed core is below 0.05 wt%, then this effect is rendered poor, and a superfluous Si content above 0.3 wt% will lower the corrosion resistance. Thus, Si content preferably falls within a range of 0.1 - 0.2 wt%. Fe contributes to produce fine crystal grains, improves strength, and inhibits intercrystalline corrosion at high temperatures. These effects are poor with a Fe content below 0.03 wt%, but due to saturation of effects, reach an upper limit at a content of 1.0 wt% or higher. Thus, the content of Fe which may exist in the alloy must fall within a range of about 0.03 - 1.0 wt%, and more desirably within a narrower range of 0.1 - 0.6 wt%. Cu prevents cathode corrosion of the skin layer, and suppresses corrosion from a core's outer surface (that is, the surface not covered with a skin layer). Cu content below 0.05 wt% does not give such an effect, and a higher Cu content above 0.2 wt% causes diffusion of Cu atoms into the skin layer so that a decreased potential difference between the core and skin layer does not sufficiently prevent the so-called cathode corrosion. Therefore, the content of Cu which may exist in the alloy must be about 0.05 to 0.2 wt%, and more desirably 0.07 - 0.15 wt%. The thickness of skin layers is restricted to 30 µm or less because a thickness greater than 30 µm renders difficult to maintain the lightness and reduce manufacture cost of the clad materials and heat exchangers made therefrom. Corresponding to the thickness of skin layers limited to or below 30 µm, the aluminum alloy contains 1.3 - 5.0 wt% of Zn. Zn (i.e., zinc) contained in the Al-Zn alloy of the skin layers contributes to their sacrificial corrosion so that the core and clad material resist corrosion. A poor Zn content below 1.3 wt% is however insufficient, while a higher content above 5.0 wt% will undesirably accelerate sacrificial corrosion of the skin layers to rather result in a lower corrosion resistance of the core. Such an excessive Zn content will also impair the rolling property of the alloy or clad material. A preferred range of Zn content is 2 to 4 wt%. Mg (i.e., magnesium) is an optional element which may be added to the alloy forming the skin layers for the purpose of improving the corrosion resistance due to sacrificial corrosion effect. Its content below 0.2 wt% is insufficient for this purpose, but its content above 1.0 wt% is superfluous and useless due to saturation of said effect. The most preferable Mg content is thus from 0.3 to 0.8 wt%. About 0.3 wt% or less of Si and about 0.3 wt% or less of Fe may also be included in the unavoidable impurities. Cu content as one of the impurities must be controlled to 0.05 wt% or less With a higher content above 0.05 % by weight, the potential of skin layers becomes high relative to that of the core and as a result the sacrificial corrosion effect of Zn is lowered. Therefore, Cu content is controlled more preferably to 0.01 % by weight or less. The aluminum alloy clad material of the invention can be manufactured under no special condition but in such an ordinary manner available to the prior art clad materials. Aluminum alloy ingots are subjected to the homogenizing treatment and are cut into raw thick sheets, i.e., a raw skin sheet and raw core sheet, which raw sheets are successively hot-rolled and cold-rolled together to clad the former onto the latter. The finished clad material may be used with its skin layer exposed to a corrosive environment. For example, an aluminum radiator comprising tubes, fin members and header pipes as shown in part in Fig. 1 may comprise each tube made of this clad sheet, wherein an inner skin layer 1b formed of an Al-Zn alloy will be disposed inside in contact with water. It will be understood that it may suffice to clad the skin layer 1b onto at least one surface of a core 1a, with another surface thereof being clad with an outer skin layer, i.e., brazing agent layer 1c which facilitates the brazing of the tubes to fin members 2. The members constituting the radiator in Fig. 1 have the following compositions. The core 1a of tube 1 having a wall 0.18 mm thick is formed of an aluminum alloy containing: 1 wt% of Mn; 0.6 wt% of Fe; 0.2 wt% of Si; and 0.13 wt% of Cu (the balance being Al). The inner skin layer 1b is formed of another aluminum alloy containing 2.5 wt% of Zn ( the balance being Al ), and the outer skin layer 1c is formed of a "BA4343" alloy. Each fin member 2 having a wall 0.07 mm thick is formed of an Al-Mn alloy containing 0.01 - 0.1 wt% of In (i.e., indium), and more particularly an alloy containing 1 wt% of Mn and 0.05 wt% of In is suitable for the fin member. Each header pipe 3 having a wall 1.6 mm thick is composed of: a core 3a formed of an "A3003" alloy; an inner skin layer 3b formed of an "A7072" alloy; and an outer skin layer 3c formed of a "BA4045" alloy. It will now be apparent that the clad material is excellent in corrosion resistance due to sacrificial corrosion effect of the the skin layers, in spite of their small thickness of 30 µm or less. Such thin skin layers are advantageous in that lighter clad sheet can be manufactured at a lower manufacture cost. This will in turn contribute to production of lighter, cheaper and more corrosion resistant heat exchangers or other apparatuses. THE PREFERRED EMBODIMENTS # Examples Table 1 gives the composition of the aluminum alloy cores and the skin layers respectively clad onto one surfaces thereof, wherein the finished thickness of the cores and skin layers are 1 mm and 20 µm, respectively. Samples denoted by Nos. 38 and 39 are naked cores comprising no skin layer. Manufacture conditions were as follows. Raw core sheets 40 mm thick and slabs for the skin layers were homogenized at 560 °C for 10 hours before cut or shaved into thinner sheets having a thickness of 32 mm. Subsequent to heat treatment at 500 °C for 30 minutes, the thinned raw sheets for the cores were hot rolled to become 10 mm thick, and those for the skin layers were also hot rolled to become 0.2 mm thick. Then each core and each skin layer were adjoined one to another to form a raw clad sheet. After the raw clad sheets were heated again at 500 °C for 30 minutes, they were repeatedly hot rolled until the thickness of each core became 2 mm, and successively cold rolled to give the core 1 mm thick. The thus finished clad sheets were thereafter subjected to a heating test which, as a simulation of the brazing process, was carried out in N 2 gas at 605 °C for 10 minutes. Corrosion test to determine the depth of corrosion was also performed for 250 hours according to ASTM D1384-87 method using glass wares, wherein "OY" water (containing: 200 ppm of Cl -, 60 ppm of SO 4 2- , 30 ppm of Fe 3+ and 1 ppm of Cu 2+ ) was employed as a corrosive solution maintained at a temperature of 95 ± 1 °C. Results of those tests are given on Table 1. As shown on this Table, the clad sheets manufactured in accordance with the invention clearly showed a smaller value of maximum pin-hole corrosion depth than the prior art clad sheets. In other words, the clad sheets of the invention have proved excellent in their corrosion resistance. Samples Composition of Core (wt%) Composition of Skin (wt%) Maximum Pin-hole Depth(µm) Mn Si Fe Cu Al Zn Mg Cu Al Inv. 21 1.0 0.3 0.6 0.1 * 2.5 - - * 50 or less 22 1.0 0.3 0.6 0.1 * 5.0 - - * 50 or less 23 1.0 0.1 0.1 0.1 * 2.5 - - * 50 or less 24 1.0 0.1 0.05 0.1 * 5.0 - - * 50 or less 25 0.8 - - - * 1.7 - - * 50 or less 26 1.2 - - - * 3.0 - 0.03 * 50 or less 27 1.3 0.2 - - * 3.5 - 0.03 * 50 or less 28 1.0 - 0.3 - * 2.0 - 0.02 * 50 or less 29 0.7 - - 0.15 * 4.0 - 0.04 * 50 or less 30 1.0 - - - * 2.5 0.5 0.02 * 50 or less 31 1.0 0.3 - - * 4.0 0.6 0.04 * 50 or less 32 1.0 - 0.1 - * 3.0 0.8 0.03 * 50 or less 33 1.2 - - 0.1 * 2.5 0.5 0.02 * 50 or less 34 1.0 0.3 0.1 0.1 * 2.5 0.4 0.02 * 50 or less Ref. 35 1.0 0.3 0.6 0.1 * 1.0 - 0.05 * 200 or more 36 1.0 0.1 0.1 0.1 * 0.5 - - * 300 or more 37 1.0 0.3 0.6 0.1 * 2.5 - 0.30 * 300 or more 38 1.0 0.3 0.6 0.1 * ( no skin layer ) * 700 or more 39 1.0 0.1 0.1 0.1 * ( no skin layer ) * 700 or more Notes: (1) Cu in the skin layers is contained as an impurity component. (2) The symbol * denotes the balance.

Claims: A corrosion-resistant clad material made of aluminum alloys, the clad material comprising a core formed of an aluminum alloy containing 0.5 - 1.5 % by weight of Mn, and optionally at least one of 0.05 - 0.3 % by weight of Si, 0.03 - 1.0 % by weight of Fe, and 0.05 - 0.2 % by weight of Cu, a balance of aluminum and unavoidable impurities and skin layers clad onto at least one surface of the core and formed of an aluminum alloy containing 1.3 - 5.0 % by weight of Zn, and optionally 0.2 - 1.0 % by weight of Mg, a balance of aluminum and unavoidable impurities, characterized in that Cu content as one of the impurities in the skin layers is controlled to 0.05 % by weight or less, with the skin layers having a thickness of 30 µm or less. A corrosion-resistant clad material according to claim 1, characterized in that the Mn content in the core falls within a range of 0.7 - 1.2 % by weight. A corrosion-resistant clad material according to claim 1, characterized in that the alloy forming the skin layers contains 2 - 4 % by weight of Zn, with the Cu content being controlled to 0.01 % by weight or less. A corrosion-resistant clad material according to claim 3, characterized in that the Mn content in the core falls within a range of 0.7 - 1.2 % by weight. A corrosion-resistant clad material according to claim 1, characterized in that the alloy forming the skin layers contains 2 - 4 % by weight of Zn, and 0.3 - 0.8 % by weight of Mg, with the Cu content being controlled to 0.01 % by weight or less.

Inventors: HAYASHI TADAYOSHI, INOUE SUSUMU, IWAI ICHIRO, KAWABE TSUYOSHI, KOBORI KAZUHIRO, OTSUKA RYOTYATSU, TAKEUCHI TOSHIHIRO, TSUKUDA ICHIZO, YUASA HIDEKO, HAYASHI, TADAYOSHI, INOUE, SUSUMU, IWAI, ICHIRO, KAWABE, TSUYOSHI, KOBORI, KAZUHIRO, OTSUKA, RYOTYATSU, TAKEUCHI, TOSHIHIRO, TSUKUDA, ICHIZO, YUASA, HIDEKO
Applicants: HONDA MOTOR CO LTD, SHOWA ALUMINIUM CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA, SHOWA ALUMINUM KABUSHIKI KAISHA
Application year: 1991
Main classification: B32B15
IPC codes: F28F19, B32B15