Method for bonding different kinds of metals
The present invention relates to a method for bonding different kinds of metals in a ship or other structures to be used in the sea or in water, and more particularly to a method for bonding such different kinds of metals while maintaining an electrically insulated condition therebetween in order to protect them from electrolytic corrosion. 
     What is to be paid of attention in the bonding of different kinds of metals with each other, is to maintain an electrically insulated condition between the different kinds of metals. More particularly, if a given metallic material bonded with another metallic material is immersed in the sea, then in the event that material electrode potentials (hereinafter abbreviated as potentials) are different, a metal having a lower potential is corroded by an electrolytic corrosion effect. Accordingly, in the case where a bonded portion between different kinds of metallic materials is in the sea, it is important to electrically insulate the different kinds of metallic materials from each other for the purpose of preventing elution of either metallic material from being quickly accelerated. 
     Heretofore, as a method for maintaining electrical insulation a number of methods have been known, but they have shortcomings respectively, and they are not reliable with respect to bonding of a large area, durability over a long period, etc. Now, description will be made on these methods in the prior art and their shortcomings. 
     1. A method in which a binder to be used upon bonding is made to have an insulative nature: 
     This method is effective in the case where an area to be bonded is small and bonding force is also not required to be so large. The reasons exist in that while binders are generally classified into a liquid state binder and a film state binder, the both types of binders necessitate to be press-bonded upon bonding (so that no clearance may be formed between one metal and the other metal to be bonded), and that a binding force of a binder is generally enhanced as it is made thinner (aggregation breakdown of a binder is made to occur scarcely by making it thinner). More particularly, since the binder is painted thin and press-bonded, the larger the bonding area is, the higher becomes the probability of one metal to be bonded being partly brought into contact with the other metal to be bonded, and hence this method is not suitable for bonding a large area. In addition, in the event that foreign matters adhere to or welding beads remain on the bonding surface of either metal to be bonded or the bonding surface is not finished flat, then the respective metals to be bonded may be possible brought into indirect or direct contact with each other, and so, there is fear that the insulation therebetween may become poor. 
     2. A method in which an insulative paint is preliminarily applied onto the bonding surface of either one metal to be bonded: 
     This method is effective in the case where the surfaces of the one or both metals to be bonded are smooth and have no unevenness. However, in the case where the metal to be bonded is, for instance, a welded structure, adhesion of sparking particles produced upon welding onto the surface of the structure as by sputtering as well as unevenness of the surface caused by contact with another structure during transportation of the materials, will result in loss of the effect of the insulative paint. In order to overcome this shortcoming, it is necessary to thicken the film thickness of the paint or to finish the surface of the metal to be bonded flat. However, normally the film thickness of the paint is in many cases several 10 μm, and in order to obtain a film thickness thicker than this, it is necessary that after a paint applied has once hardened, again or again and again repeatedly painting is effected several times, so that this method has a shortcoming that a working period is prolonged and more labor is necessitated. In addition, to finish the surface of the metal to be bonded flat would result in increase of the amount of work and hence enhancement of a cost, in the case where the metal to be bonded has a large area as is the case with an outer plate of a hull. 
     3. A method in which a solid-state insulator such as, for instance, a rubber sheet is sandwiched between the different kinds of metals: 
     This method is a reliable method as the method for maintaining electrical insulation between different kinds of metals. However, it has a shortcoming that since bonding works at two portions, that is, between one metal to be bonded and the insulator and between the insulator and the other metal to be bonded are involved, a cost is raised. 
     It is therefore one object of the present invention to provide a method for bonding different kinds of metals, which has a low bonding cost and which can maintain a bonding force and electrical insulation therebetween over a long period of time. 
     According to one feature of the present invention, there is provided a method for bonding different kinds of metals while maintaining an electrically insulated condition therebetween, in which a paint containing glass flakes is used as an insulating material. 
    
    
     The above-mentioned and other features and objects of the present invention will become more apparent by reference to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic view showing one preferred embodiment of the present invention, 
     FIG. 2 is an enlarged view of a part of FIG. 1, and 
     FIGS. 3 and 4, respectively, are schematic views showing constructions of test pieces employed in two different examples of test experiments of the present invention. 
    
    
     Referring now to FIG. 1, an insulative paint 2 is applied to a metal 1 to be bonded as by spraying. As shown in FIG. 2, this paint 2 consists of a principal agent 2-1 and glass flakes 2-2 mixed in the former. The glass flakes 2-2 have the following two effects. One is the effect that thixotropy is generated in the paint and hence a thick film can be obtained through once of painting. In other words, the glass flakes 2-2 can suppress sagging of the paint when it is applied. The other is the effect that in the case where water 5 is going to enter from the surface of the painted film as shown in FIG. 2, the water would not advance straightly before it reaches the surface of the metal 1 to be bonded but instead it could reach the surface through the path shown by a dotted line in FIG. 2, and this means that the paint containing glass flakes can protect the surface of the metal 1 to be bonded from water over a long period of time. 
     On the above-mentioned insulative paint 2 is applied a bonding metal 4 by means of a binder 3. 
     It is to be noted that modification could be made such that the above-mentioned insulative paint has been preliminarily applied to the other metal 4 to be bonded and then this is applied to the metal 1 to be bonded by the intermediary of a binder. 
     Now, the bonding force obtained in the case of practicing the method according to the present invention will be confirmed by reference to examples of test experiments of the invention. 
     Example of test experiments 1: 
     A test piece having the construction shown in FIG. 3 was prepared. 
     In FIG. 3, reference numeral 11 designates a steel plate of 1.6 mm in thickness, numeral 12 designates an insulative paint layer formed by employing a paint containing glass flakes and principally consisting of polyester resin of vinyl ester series or a paint containing glass flakes and principally consisting of polyester resin of isophthalic acid series (the above paints are both commercially available in the market as a heavy anti-corrosion material for a steel plate in the sea water), numeral 13 designates a binder layer formed by employing a binder of acrylic series, and numeral 14 designates a copper plate of 0.5 mm in thickness (two kinds of copper plates were used, one having its surface Ni-plated, the other having its surface painted with an organic primer). 
     Tensile shearing force tests were conducted by making use of the aforementioned test piece. The directions of loading are shown by arrows in FIG. 3. The results of the test experiments were as indicated in Table-1. 
     
                                           TABLE 1                                 
__________________________________________________________________________
         Tensile Shearing Force Tests                                     
Surface  Insulative Paint                                                 
Treatment                                                                 
         A              B                                                 
of       Tensile        Tensile                                           
Copper                                                                    
      Test                                                                
         Shearing                                                         
                Sheared Shearing                                          
                               Sheared                                    
Plates                                                                    
      No.                                                                 
         Force  State   Force  State                                      
__________________________________________________________________________
a     1  101.6 kg/cm.sup.2                                                
                Cutting of                                                
                        78.2 kg/cm.sup.2                                  
                               Aggregation                                
                Copper Plate   Breakdown of                               
                               Binder                                     
      2  98.8 kg/cm.sup.2                                                 
                Cutting of                                                
                        52.3 kg/cm.sup.2                                  
                               Aggregation                                
                Copper Plate   Breakdown of                               
                               Binder                                     
      3  102.0 kg/cm.sup.2                                                
                Aggregation                                               
                        63.0 kg/cm.sup.2                                  
                               Aggregation                                
                Breakdown      Breakdown of                               
                of Binder      Binder                                     
b     1  64.8 kg/cm.sup.2                                                 
                Breakdown on                                              
                        58.7 kg/cm.sup.2                                  
                               Breakdown on                               
                the Surface    the Surface                                
                of Copper      of Copper                                  
                Plate          Plate                                      
      2  64.0 kg/cm.sup.2                                                 
                Breakdown on                                              
                        59.2 kg/cm.sup.2                                  
                               Breakdown on                               
                the Surface    the Surface                                
                of Copper      of Copper                                  
                Plate          Plate                                      
      3  62.9 kg/cm.sup.2                                                 
                Breakdown on                                              
                        53.3 kg/cm.sup.2                                  
                               Aggregation                                
                the Surface    Breakdown of                               
                of Copper      Binder                                     
                Plate                                                     
__________________________________________________________________________
 a: Ni--plated copper plate                                               
 b: Copper plate painted with an organic primer                           
 A: Paint principally consisting of polyester resin of vinyl ester series 
 B: Paint principally consisting of polyester resin of isophthalic acid   
 series                                                                   
 
    
     Example of test experiments 2: 
     A test piece having the construction shown in FIG. 4 was prepared. 
     Reference numerals in FIG. 4 designate equivalent component parts to those designated by like reference numerals in FIG. 3. 
     Tearing bonding force tests were conducted by making use of the aforementioned test piece. The directions of loading are as shown by arrows in FIG. 4. The results of the test experiments were as indicated in Table-2. 
     
                                           TABLE 2                                 
__________________________________________________________________________
Surface  Tearing Bonding Force Tests                                      
Treatment                                                                 
         Insulative Paint                                                 
of       A             B                                                  
Copper                                                                    
      Test                                                                
         Tear-off                                                         
                Tear-off                                                  
                       Tear-off                                           
                              Tear-off                                    
Plates                                                                    
      No.                                                                 
         Strength                                                         
                State  Strength                                           
                              State                                       
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a     1  17.5 kg/cm.sup.2                                                 
                Aggregation                                               
                       10.1 kg/cm.sup.2                                   
                              Aggregation                                 
                Breakdown     Breakdown                                   
                of Binder     of Insula-                                  
                              tive Paint                                  
      2  19.1 kg/cm.sup.2                                                 
                Aggregation                                               
                       12.2 kg/cm.sup.2                                   
                              Aggregation                                 
                Breakdown     Breakdown                                   
                of Binder     of Insula-                                  
                              tive Paint                                  
      3  17.5 kg/cm.sup.2                                                 
                Aggregation                                               
                       16.8 kg/cm.sup.2                                   
                              Aggregation                                 
                Breakdown     Breakdown                                   
                of Binder     of Binder                                   
                and Break-                                                
                down on the                                               
                Surface of                                                
                Copper Plate                                              
b     1  14.1 kg/cm.sup.2                                                 
                Breakdown                                                 
                       15.7 kg/cm.sup.2                                   
                              Aggregation                                 
                on the        Breakdown                                   
                Surface of    of Binder                                   
                Copper                                                    
                Plate                                                     
      2  11.9 kg/cm.sup.2                                                 
                Breakdown                                                 
                       10.3 kg/cm.sup.2                                   
                               Aggregation                                
                on the        Breakdown                                   
                Surface of    of Insula-                                  
                Copper        tive Paint                                  
                Plate         and Aggre-                                  
                              gation                                      
                              Breakdown                                   
                              of Binder                                   
      3  11.1 kg/cm.sup.2                                                 
                Breakdown                                                 
                       10.4 kg/cm.sup.2                                   
                              Aggregation                                 
                on the        Breakdown                                   
                Surface of    of Insula-                                  
                Copper        tive Paint                                  
                Plate         and Aggre-                                  
                              gation                                      
                              Breakdown                                   
                              of Binder                                   
__________________________________________________________________________
 a, b, A, B} denote the same items as Table 1.                            
 
    
     From the Examples of Test Experiments 1 and 2 above, the following facts would become obvious. 
     According to the tensile shearing force tests in the Example of Test Experiments 1, every test piece has a tensile shearing strength of 52.3 kg/cm 2  or higher, and as it is seen that the obtained bonding force is sufficient as a bonding force for a structure necessitating electrical insulation such as a ship and the like. This is a considerably high bonding strength as compared to the bonding strength of an oily paint, a bituminous paint or the like generally used for rust proofing purpose which is equal to 5˜20 kg/cm 2 . 
     In addition, according to the tearing bonding force tests in the Example of Test Experiments 2, every test piece has a tear-off strength which is equal to 10.1 kg/cm 2  or higher even in the worst case, and so it is seen that the test pieces have a remarkably high strength as compared with the strength of an oily paint, a bituminous paint or the like which is equal to 0.5˜2 kg/cm 2 . 
     Moreover, besides the insulative paints A and B used in the Examples of Test Experiments 1 and 2 above, other paints containing glass flakes which principally consist of polyester resin of bisphenol series, epoxy series resin or modified epoxy series resin provided results of test experiments presenting values approximately close to those given in Tables-1 and 2, and it has been proved that they are also excellent as an insulative paint to be used upon bonding different kinds of metals. These paints are excellent in an electrical insulation property in that their volume specific resistance is 1˜3×10 14  Ω-cm (at an applied voltage of 1000 V), moreover a film thickness of 500˜700 μm can be obtained through once of painting work, furthermore spray painting is also possible, reliable insulation as well as a cheap work for a large area can be realized, these paints are excellent in protection of the surface of a base member in the sea over a long period of time, and it has been proved that from a general point of view these paints (and the bonding method) are remarkably more advantageous than other insulating materials (insulating method) as an insulating material (insulating method) for bonding different kinds of metals in the case of using the bonded metals in the sea. 
     The effects of the present invention as described above are summarized as follows: 
     1. As compared to normal painting, thicker painting is possible even if the body to be bonded is a vertical wall-like body because a paint containing glass flakes is used, and so, reliable insulation can be secured (upon painting work, sagging of a paint would not occur). 
     2. Since painting is employed, the process of working is simple. 
     3. Since a paint containing glass flakes is employed, invasion of the sea water seldom occurs, and so, for the purpose of protecting the surface of the bonded body in the sea water, the method according to the present invention is more excellent than other methods. 
     4. A paint containing glass flakes has a strong bonding force and a sufficient durability against a binder and other chemicals, and therefore, it is most suitable as an insulating material to be used in the sea over a long period of time.