Brazed aluminum composite
BACKGROUND OF THE INVENTION 
     Brazed aluminum equipment is subject to the severe problem of intergranular corrosion in corrosive environments on surfaces coated with the brazing alloy. The corrosive environments which can cause this problem include water containing dissolved chloride, bicarbonate or sulfate ions, especially if the pH of the water has a relatively low value. Such waters may condense as films on the fins of heat exchanger equipment used for automotive or aircraft air conditioners, automotive radiators, gas liquefaction equipment or the like. 
     Intergranular corrosion has also been encountered in other applications, as on brazed headers inside automotive radiators and heat exchangers generally. In such cases, the coolant is usually corrosive. For example, if automotive antifreeze solutions are used, poor maintenance can often result in the solution becoming corrosive for a variety of reasons. Chief among these reasons are that the antifreeze may have been allowed to remain in the radiator for a number of years without replacement while replenishing the level with mixtures of fresh antifreeze solution with hard natural water. These practices would deplete the corrosion inhibitors and reserve alkalinity components, permitting the coolant pH to drop and allowing heavy metal ions to accumulate from reaction of the acids with copper alloy and cast iron surfaces in the coolant system. 
     U.S. Pat. Nos. 3,898,053 and 3,853,547 describe certain aluminum-silicon brazing compositions for joining aluminum alloy components; however, these compositions do not solve the problem of intergranular corrosion described hereinabove. 
     The problem of intergranular corrosion may occur whether flux brazing or vacuum brazing techniques are employed. There is evidence in the case of flux brazed aluminum Alloy 3003 (an aluminum base alloy containing from 0.05 to 0.20% copper, from 1 to 1.5% manganese, up to 0.6% silicon and up to 0.7% iron) clad with aluminum Alloy 4343 (an aluminum base alloy containing from 6.8 to 8.2% silicon, up to 0.8% iron, up to 0.25% copper, up to 0.1% manganese, up to 0.2% zinc and the balance essentially aluminum) that the silicon rich eutectic formed when the Alloy 4343 brazing alloy is brazed can migrate into the grain boundaries of the Alloy 3003 component and can cause increased susceptibility to intergranular corrosion. A similar silicon rich eutectic migration into the parent metal can occur in the case of vacuum brazed assemblies made from aluminum Alloy 3003 clad with the silicon rich aluminum vacuum brazing Alloy MD 150 (an aluminum base alloy containing about 9.5% silicon, 1.5% magnesium, up to 0.3% iron, up to 0.05% copper, up to 0.07% manganese, up to 0.01% titanium and the balance essentially aluminum) or aluminum vacuum brazing Alloy MD 177. The MD 177 alloy has substantially the same composition as MD 150 containing in addition from 0.08 to 0.1% added bismuth. In both MD 150 and MD 177 the magnesium addition is used to getter traces of oxygen in the vacuum brazing furnaces. 
     Flux brazed assemblies made from No. 12 brazing sheet (aluminum Alloy 3003 clad on both sides with aluminum Alloy (4343) and monolithic aluminum Alloy 3003 components are sensitized to corrosion by prolonged holding at elevated temperatures below the brazing temperature. This practice is used to assure that the final relatively short time brazing step will liquify the brazing alloy everywhere in very large assemblies. The effect of this holding time, which may be up to 5 hours at 1000° F for large gas liquifaction heat exchangers, is to coarsen cathodic iron rich second phase particles in the metal. This causes increased susceptibility to both intergranular corrosion and pitting corrosion. 
     Accordingly, it is a principal object of the present invention to provide an improved brazed aluminum composite which is characterized by substantial resistance to intergranular corrosion. 
     It is a still further object of the present invention to provide an improved brazed aluminum composite as aforesaid which is inexpensive and convenient to use on a commercial scale. 
     Further objects and advantages of the present invention will appear hereinbelow. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily obtained. The brazed aluminum composite of the present invention is characterized by improved intergranular corrosion. The core or parent metal is an aluminum base alloy consisting essentially of from 0.05 to 0.4% chromium, from 0.2 to 0.9% manganese, up to 0.2% iron, up to 0.1% silicon and the balance essentially aluminum. The cladding or brazing alloy is a silicon containing aluminum brazing alloy containing from 4 to 14% silicon, up to 3% magnesium, up to 0.2% bismuth, balance essentially aluminum. 
     The present invention resides in a brazed aluminum composite with the brazing alloy clad on one or both sides of the core material. It is a particular and surprising feature of the present invention that the composites of the present invention are characterized by greatly improved susceptibility to intergranular corrosion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more readily understood from a consideration of the following illustrative drawings in which: 
     FIGS. 1A, 1B and 1C are photographs of clad and unclad sides of samples following exposure to a corrosive environment; 
     FIGS. 2A, 2B, 2C and 2D are photomicrographs at a magnification of 200X showing cross sections through composites after exposure to a corrosive environment; and 
     FIGS. 3A, 3B, 3C and 3D are photomicrographs at a magnification of 200X showing cross sections of composites after exposure to a corrosive environment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described hereinabove, the composite of the present invention is characterized by improved resistance to intergranular corrosion. It is a finding of the present invention that the susceptiblity of brazed assemblies to intergranular corrosion due to silicon migration and coarsening of iron containing second phase particles may be drastically reduced by using a parent alloy containing significantly restricted concentrations of both iron and silicon to which specific amounts of manganese and chromium have been added as purposeful additions. The effect of restricting the iron and silicon concentrations is to reduce the size and population density of second phase particles rich in iron which are most frequently alpha phase particles containing iron, silicon and manganese. Restricting the silicon concentration in the parent metal makes the parent metal a good solvent for the silicon rich eutectic which tends to migrate into the parent metal from the brazing alloy. The effect of this is to drastically reduce the depth to which such migration may occur into the parent metal and thereby greatly reduce intergranular corrosion. 
     In accordance with the present invention, the aluminum alloy core material contains from 0.05 to 0.4% chromium and preferably from 0.15 to 0.30% chromium. The manganese content is from 0.2 to 0.9% and preferably from 0.3 to 0.6%. The iron content is up to 0.2%, preferably up to 0.1% and optimally from 0.02 to 0.08%. The silicon content is up to 0.1% and preferably from 0.02 to 0.08%. 
     Any silicon containing aluminum brazing alloy may be employed as the cladding material wherein the silicon content ranges from 4 to 14%, such as, for example, the MD 150 and MD 177 alloys listed hereinabove and also aluminum Alloy 4045 (an aluminum base alloy containing from 9 to 11% silicon). It is preferred to include additives in the brazing alloy, such as from 0.05 to 0.2% bismuth and from 0.5 to 3% magnesium. The cladding material may also contain up to 0.8% iron, up to 0.5% copper, up to 0.3% manganese, up to 0.1% titanium, others up to 0.05% each, total up to 0.15%. 
     The deliberate manganese addition in the core alloy perform a beneficial role by preventing formation of the highly cathodic FeAl 3  phase which is present in commercial manganese free alloys. Concentrations of manganese beyond the desired range, such as manganese concentrations in excess of 1% which are present in commercial aluminum Alloy 3003,  cause excessive precipitation of MnAl 6  particles. These particles have an electrode potential almost the same as the aluminum matrix in a substantially iron free system. However, in commercial purity aluminum base alloy matrices, which are adequate for the core alloy of the present invention, there is sufficient iron present to cause the MnAl 6  particles to dissolve enough iron to become cathodic to the matrix aluminum and cause localized corrosion. The chromium addition in the core alloy of the present invention shifts the electrode potential of the parent metal in the noble direction. This may, in some corrosive media, be enough to make the parent metal more noble than the cladding alloy and thereby prevent anodic dissolution of the parent metal by a galvanic couple. A second and perhaps more important role of the chromium is to act as a corrosion inhibitor at sites of localized corrosion, such as pits, grain boundaries or crevices. Where such corrosion occurs, the corrosion product contains soluble chromate ions which may migrate to the anodic sites where they act as anodic type corrosion inhibitors. 
     The composite of the present invention is particularly useful in the manufacture of brazed equipment by mass production methods involving either flux or vacuum brazing. The composites of the present invention also have particular value for equipment which is expected to encounter corrosive conditions which could cause intergranular corrosion of the parent metal layers of conventional brazing sheets. Vacuum brazed aluminum heater cores have been found to have severe intergranular corrosion problems when made using conventional brazing sheets with Alloy 3003 parent metal. These heater cores are used, for example, to provide warm air to warm the passenger compartment of passenger cars by abstracting excess heat from the automotive engine coolant. The engine coolant passes through channels formed by parallel plates of brazing sheet which are brazed to the inlet and outlet header tanks of the heater core units. The intergranular corrosion results from contact between the corrosive aqueous engine coolant and the internal surfaces of the plate channels. The composite of the present invention significantly reduces the intergranular corrosion which occurs in this type of application. Other automotive applications exist for which the composite of the present invention is quite suitable, including automotive radiators and oil coolers in the engine systems, and also evaporators and condensers in automotive air conditioning systems. Alternatively, the core could be used as a monolithic sheet in an assembly with the brazing alloy in the form of another sheet or foil associated with the core. The assembly can then be brazed together to form the final composite article. 
     The present invention will be more readily understandable from a consideration of the following illustrative examples. 
     EXAMPLE 1 
     Two core ingots were cast having the composition set forth below, with Alloy A representing the material of the present invention and Alloy B being a comparative alloy. 
     Alloy A 
     Silicon -- 0.04% 
     Chromium -- 0.3% 
     Manganese -- 0.4% 
     Iron -- 0.035% 
     Titanium -- 0.01% 
     Aluminum -- Balance 
     Alloy B 
     Chromium -- 0.15% 
     Iron -- 0.04% 
     Silicon -- 0.04% 
     Aluminum -- Balance 
     Direct chill castings of ingots of Alloys A and B were homogenized at 1125° F for 8 hours using a maximum heat up rate from 600° F of 50° F per hour. The ingots were cooled from 1125° F to 600° F at 25° F per hour and air cooled to room temperature. The core materials of Alloys A and B were scalped to 1.5 inches thickness and were brushed on one side. 
     EXAMPLE II 
     Durville ingots were cast to the compositions shown in Table I below. 
     
                       TABLE I                                                     
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             Percentage of Element                                        
Alloy Type Alloy   Si    Fe  Cu  Mn   Ti  Mg   Bi                         
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4343       C       7.5   .35 .05 --   .01 --   --                         
MD 150     D       9.7   .3  .05 .07  .01 1.5  --                         
MD 177     E       9.7   .3  .05 .07  .01 1.5  .1                         
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     The foregoing Alloys C, D and E represent silicon rich cladding alloys. The Durville ingots of Alloys C, D and E were scalped to 1.5 inches thickness. They were reheated to 800° F for 1 hour and hot rolled to 0.15 inches gage. The hot rolling scale was removed by caustic etching and rinsing. 
     EXAMPLE III 
     Brazing sheets of Alloy C clad on Alloy A and Alloy C clad on Alloy B were fabricated. In addition, for comparative purposes a brazing sheet of Alloy 4343 (Alloy C) clad on Alloy 3003 was prepared. All brazing sheets were one side clad only. The brazing sheets were fabricated by welding the appropriate brazing alloy to the wire brushed side of the parent alloy and hot rolling the sandwich. An 800° F entry temperature was used. One side was left unwelded to permit air to be expelled from the mating surface. Hot rolling was continued until the sandwiches were 0.15 inch thick. They were then cold rolled to 0.030 inch and annealed by heating to 660° F at a rate of 25° F per hour from 300° F, held at 660° F for 2 hours, cooled to 400° F per hour, and air cooled from 400° F to room temperature. 
     EXAMPLE IV 
     The brazing sheets were subjected to a simulated flux brazing cycle such as might be applied to a bulky assembly. The simulated flux brazing cycle included stacking 4 × 4 inches sheets in a tray and placing the tray in a muffle furnace. A thermocouple was present in a dummy load in the furance in order to determine metal temperature. Samples were heated to 1000° F and held at this temperature for 5 hours to simulate the preheat step. They were then heated to 1115° F for periods of 15 minutes, 30 minutes, 60 minutes and 300 minutes with some samples unheated. The samples were then cut to 1 × 1/2 inch specimens and a hole was punched near one of the 1/2 inch edges to permit them to be supported on a nylon threaded rod support. All of the specimens were immersed for 140 hours at 40° F in a solution having composition set forth in Table II below. 
     
                       TABLE II                                                    
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         0.1  Molar    NaCl                                               
         0.01 Molar    NaNO.sub.3                                         
         2.0  cc glacial acetic acid                                      
         4.0  cc 30% H.sub.2 O.sub.2                                      
all in 1 liter of distilled water                                         
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     The foregoing solution was designed to simulate the particularly corrosive service to which air liquidation heat exchangers are exposed. The photographs of FIG. 1 show the appearance of the clad and unclad sides of the samples following this exposure. FIG. 1A represents Alloy C clad on Alloy A. FIG. 1B represents Alloy C clad on Alloy B. FIG. 1C represents Alloy C clad on Alloy 3003 corresponding to No. 11 brazing sheet. It is apparent that the Alloy C clad on Alloy A sample of the present invention is completely free of visual evidence of corrosion on both its clad and unclad surfaces. The Alloy C on Alloy B comparative material exhibits some pits on the unclad side, but no attack on the clad side. These pits were found to have the depths set forth in Table III below depending upon the time at 1115° F. 
     
                       TABLE III                                                   
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Pitting into the                                                          
unclad side of the                                                        
C on B brazing sheet                                                      
Time at 1115° F                                                    
                    Average pit depth                                     
in minutes          in mils                                               
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0                   0                                                     
15                  7.0                                                   
30                  8.6                                                   
60                  7.5                                                   
300                 12.2                                                  
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     The No. 11 brazing sheet (FIG. 1C) is severely affected by the exposure on the unclad side where the brazing time is either zero or 300 minutes and is moderately affected for the 15, 30 and 60 minute brazing times. The form of attack was blister formation. Examination of the clad side of the No. 11 brazing sheet (FIG. 1C) shows severe attack only in the case of the sample which had not been subjected to simulated brazing at 1115° F. 
     EXAMPLE V 
     FIGS. 2A, 2B, 2C and 2D represent photomicrographs of cross sections through specimens of Alloy C clad on Alloy A and No. 11 brazing sheet following exposure for 140 hours to the corrosive solution of Table II at 25° F and 40° F. The photomicrographs are at a magnification of 200X. FIG. 2A represents Alloy C clad on Alloy A following exposure at 25° F, FIG. 2B represents Alloy C clad on Alloy A following exposure at 40° F, FIG. 2C represents No. 11 brazing sheet following exposure at 25° F and FIG. 2D represents No. 11 brazing sheet following exposure at 40° F. The results shown in FIG. 2 indicate that over a range of flux brazing conditions the Alloy C clad on Alloy A brazing sheet of the present invention was much more resistant to intergranular corrosion on the clad and unclad surfaces than the No. 11 brazing sheet. The results in FIG. 1 show severe pitting on the core element on the comparative Alloy C clad on Alloy B brazing sheet. The Alloy A element of the Alloy C on Alloy A composite is the alloy containing 0.4% manganese and 0.3% chromium of our invention. The B component of the Alloy C on Alloy B brazing sheet is the 0.15% chromium, balance aluminum plus about 0.04% iron and 0.04% silicon. The severe pitting of this material shows that it is not sufficient to incorporate chromium in a base with restricted iron and silicon contents. The deliberate manganese addition incorporated into Alloy A of the present invention is essential for adequate corrosion resistance. 
     EXAMPLE VI 
     Brazing sheets of Alloy D (MD 150) clad on Alloy 3003 and Alloy E (MD 177) clad on Alloy 3003 plus brazing sheets of Alloy D on Alloy A of the present invention and Alloy E on Alloy A of the present invention were all subjected to simulated vacuum brazing treatment. The treatment consisted of holding the material for a total of 12 minutes in a vacuum furnace set at 1100° F at a pressure of 2 × 10 -4  Torr. The specimens were then removed from the furnace and air cooled. Samples were evaluated for susceptibility to intergranular corrosion by immersing same for 24 hours in a boiling solution prepared by dissolving the materials set forth in Table IV below in 10 liters of distilled water. 
     
                       TABLE IV                                                    
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        1.48 grams                                                        
                Na.sub.2 SO.sub.4                                         
        1.65 grams                                                        
                NaCl                                                      
        1.40 grams                                                        
                NaHCO.sub.3                                               
        0.29 grams                                                        
                FeCl.sub.3                                                
        0.39 grams                                                        
                CuSO.sub.4                                                
                         7H.sub.2 O                                       
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     The specimens were allowed to remain in the solution for a further 24 hour period during which time the solution cooled to ambient temperature. The specimens were then removed from the solution and examined for intergranular corrosion. The specimen surfaces were marked in some places by white corrosion product which corresponded to internal intergranular corrosion. Metallographic cross sectioning of the specimens was carried out in the most extensive areas covered by the white corrosion product. Photomicrographs at 200X magnification of the polished cross sections are shown in FIG. 3. FIG. 3A represents Alloy D clad on Alloy A, FIG. 3B represents Alloy D clad on Alloy 3003, FIG. 3C represents Alloy E clad on Alloy A and FIG. 3D represents Alloy E clad on Alloy 3003. The results clearly show that the composite of the present invention is unaffected by the corrosive test medium using two types of vacuum brazing alloy. In contrast, composites using Alloy 3003 suffered varying degrees of intergranular corrosion depending upon whether the vacuum brazing alloy is Alloy D (silicon plus magnesium) or Alloy E (silicon plus magnesium plus bismuth). 
     This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.