Abstract:
This invention relates to a sacrificial anode, comprising: a first layer of a first material; and, a second layer of a second material which is electrically connected to the first layer, wherein the first material is more anodic with respect to a galvanic series than the second material. The invention also relates to a body including the sacrificial anode.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    This invention relates to a sacrificial anode. In particular, this invention relates to a sacrificial anode made from two materials, one material being higher galvanic series relative to the other. 
       BACKGROUND OF INVENTION 
       [0002]    It is well known to use sacrificial anodes to prevent corrosion of metallic bodies in corrosive environments, such as sea water. Such sacrificial anodes are typically metallic members which are mounted local to or on the body they are to protect and are more susceptible to galvanic corrosion in the given environment in which they are located and thus more anodic. As the sacrificial anode is more anodic (less noble) than the metal of the parent structure a small localised electrochemical cell is set up between the anode and the body which is to be protected when placed in an electrolyte such as sea water. In this way, corrosion of the metallic body is reduced, if not entirely prevented. The anodes are sacrificial in that they corrode during the process and require periodic replacement. 
         [0003]    It is common practice to use surface mounted sacrificial anodes which are readily replaced when necessary. However, surface mounted sacrificial anodes represent a hydrodynamic penalty in the form of increased drag in conditions where the body is subjected to a constrained flow of water, such as a pipe or duct or in unconstrained flow such as on the rudder of a ship. The additional drag is generally undesirable. 
         [0004]    One option for overcoming the hydrodynamic penalty is to use an impressed current cathodic protection system which utilises a permanent (non consumable) anode through which a current is passed during operation. This has the advantage that the anode can have a much reduced profile and represents a lower hydrodynamic penalty. However, the complexity and cost of such a system is too high for many applications. 
         [0005]    The present invention seeks to provide a sacrificial anode which seeks to overcome some of the problems of the known systems. 
       STATEMENTS OF INVENTION 
       [0006]    In a first aspect the present invention provides a sacrificial anode, comprising: a first layer of a first material; and, a second layer of a second material which is closely connected to the first layer, wherein the first material is more anodic with respect to a galvanic series than the second material. 
         [0007]    Providing a first and second material in this way provides a sacrificial anode in which can be recessed into a body whilst the underside of the anode corrodes and the upper side remains intact, thereby preserving the hydrodynamic shape of the body in which the anode is recessed. 
         [0008]    The first material and second material may be directly bonded together. The first material may be zinc. The second material may be magnesium. It will be appreciated, with reference to the electrochemical series, that other combinations of material may be used. The combinations of materials must ensure the galvanic relationship between the two is preserved such that the first material is more anodic than the second material. And, where the anode is recessed within a body, the second material is more anodic than the body. 
         [0009]    The ratio of the first material to the second material may be between approximately 1:5 and 1:12. 
         [0010]    In a second aspect, the present invention provides a metallic body comprising: a recess; and, the sacrificial anode as claimed in any preceding claim located within the recess and separated from the body by a channel, wherein the body is more cathodic with respect to a galvanic series than the first and second materials. The channel may substantially surrounds the anode. 
         [0011]    The recess may have an opening to a fluid flow in normal use. The opening may have a first dimension. The sacrificial anode may extend across up to 90% of the first dimension. 
         [0012]    The recess may be located in a fluid washed surface and a surface of the first material is located in the same plane as the fluid washed surface. 
         [0013]    At least one edge between the fluid washed surface and a surface of the recess may be shaped to encourage a flow of fluid into the recess. 
         [0014]    The at least one edge may have a curved profile which subtends between the fluid washed surface and surface of the recess. 
         [0015]    In a third aspect, the present invention provides a water jet propulsion unit comprising the body according to the second aspect. 
         [0016]    The body may form at least part of a duct through which water may be propelled when the propulsion unit is in normal use. 
         [0017]    In a fourth aspect, the present invention may provide a method of inspecting a sacrificial anode as claimed in claim any preceding claim, comprising: visually inspecting the first material; determining whether the corrosion of the first material is greater or lesser than a predetermined acceptable amount; and, replacing the anode if the corrosion of the first material is greater than the predetermined amount. 
         [0018]    Initiation of corrosion on the first material indicates consumption of the second material, indicating the need to replace the entire anode. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0019]    Embodiments of the invention will now be described with the aid of the following drawings in which: 
           [0020]      FIGS. 1   a, b  and  c  shows a sacrificial anode according to the present invention prior to, during and after a period of corrosion 
           [0021]      FIG. 2  shows a water jet propulsion unit with a sacrificial anode. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]      FIG. 1   a  shows a body  10  having a recess  12  located in a fluid washed surface  14 . A sacrificial anode  16  is located within the recess such that it is surrounded by a channel  18 . The channel  18  is formed by the anode  16  being located within the recess  12  and separated from its sides such that a fluid can flow around and contact the sides of the anode  16 . 
         [0023]    The sacrificial anode  16  is constructed from a first material  20  and a second material  22 . The first material  20  is more anodic than the second material  22  meaning that it has a higher anodic potential in a particular aqueous environment. In the present embodiment, the first material  20  is made from Magnesium and the second material  22  from Zinc and the body  10  is a steel structure and thus more cathodic than the first  20  and second materials  22  of the sacrificial anode  16 . The electrolytic environment is provided by sea water. It will be appreciated that other anode-cathode material combinations are possible as exampled in table 1 below and that in some cases pure metals may be substituted with alloys which are commonly used for sacrificial anodes as known in the art. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 A list of suitable anode combinations. 
               
             
          
           
               
                 Bimetallic  
                 Top surface  
                 Bottom (bulk)  
               
               
                 anode pair 
                 of anode 
                 of anode 
               
               
                   
               
               
                 1 
                 Zinc 
                 Magnesium 
               
               
                 2 
                 Aluminium 
                 Zinc 
               
               
                 3 
                 Aluminium 
                 Magnesium 
               
               
                 4 
                 Mild steel 
                 Magnesium 
               
               
                 5 
                 Mild steel 
                 Zinc 
               
               
                 6 
                 Mild steel 
                 Aluminium 
               
               
                   
               
             
          
         
       
     
         [0024]    The first  20  and second materials  22  are directly bonded together so as to prevent the ingress of water and allow a good electrical connection between the two. Providing a good electrical connection allows an electrical circuit to be formed out of the steel, the anode and the sea. This allows the corrosion of the preferential corrosion of the first material and thus protects the second material from corrosion until the second material has been consumed. There are numerous techniques which can be used to bond dissimilar metals together such as ultrasonic welding, diffusion bonding, brazing, rotary friction welding and fiction stir welding, to mention a few. 
         [0025]    The proportion of second material  22  to first material  20  will depend on the application but will be a balance between the expected amount of corrosion and the desired maintenance interval for example. The thickness of the second material  22  should be sufficient enough to be able to withstand mechanical damage which results from debris in the fluid flow and any hydrodynamic loads once the first material  20  has been consumed. Typically, the thickness ratio of the first material  20  to the second material will be approximately 1:9. However, the skilled person will appreciate that it may be preferential to have a range between 1:5 and 1:12. 
         [0026]    The recess  12  is in the form of a well having straight sides and a flat bottom which is parallel to the fluid washed surface  14 . However, other shapes and configurations of recesses will be possible within the scope of the invention. 
         [0027]    The sacrificial anode  16  is mounted to the body  10  within the recess  12  on spacers in the form of pillars  26 . The pillars  26  separate the anode  16  from the sides and bottom of the recess  12  within the body  10  so as to preserve the channel  18  which surrounds the anode  16 . The size of the channel  18  will depend on the amount of fluid displacement required to provide satisfactory ionic exchange between the anode  16  and body  10 . 
         [0028]    The sacrificial anode  16  is fixed to the body  10  using bolts  28  which pass through the apertures in the anode  16  which extend from an upper surface of the anode to the underside, through the pillars  26  and which engage with threaded bores within the body  10 , The bolts  28  are metallic and provide an electrical connection between the anode  16  and the body  10 . It will be apparent to the skilled person that the pillars  26  and bolts  28  are made from a non-corrosive material such that mechanical support can be maintained throughout the life of the anode  16 . 
         [0029]    Providing an electrical connection between the anode and the body in this way allows an electron flow between the body  10  and anode  16  in use. Thus, there is an ionic flow between the anode and the body through the sea water and an electron flow through the bolts  28 . It will be appreciated that the electrical connection can be made in other ways as known in the art. 
         [0030]    The anode  16  is mounted within the body  10  such that the upper surface of the anode  16  lies in approximately the same plane as the fluid washed surface. In this way, the hydrodynamic profile of the fluid washed surface can be maintained. 
         [0031]    An edge  30  of the body which is defined by the fluid washed surface and recess is rounded so as to have a curved profile which subtends at an angle of approximately 90° in the described embodiment. This feature encourages the flow of fluid through the channel  18  between the body  10  and anode  16 , thus improving the flow of water around the anode, maintaining efficient operation. It will be appreciated that other features may be included to improve the flow of water in the channel  18 . 
         [0032]    In use, the body  10  is placed in a fluid flow (indicated by arrows  32 ) with the sacrificial anode  16  mounted a within the recess  12 . The curved portion of the body  10  is placed upstream of the sacrificial anode  16  such that a flow of fluid is encouraged into the recess  12  and around the sacrificial anode  16 . The presence of the seawater around the anode  16  and the galvanic relationship between the sacrificial anode  16  and the body  10  results in an electrochemical cell being set-up between the anode  16  and the body which prevent corrosion of the body  10  as described above. 
         [0033]    The ionic and electron flow results in the corrosion and consumption of the first of material  20  because it is more anodic than the body  10  and the second material  22 . This is shown in  FIG. 1   b  where the first material  20  is partially corroded, but the second material  22  is preserved. Once the first material  20  has been completely consumed, the second material  22  then becomes the sacrificial anode as shown in  FIG. 1   c  and starts to corrode, while still providing protection to the body  10 . 
         [0034]    Because the corrosion of the second material  22  only occurs after the first material  20  has been entirely consumed, this provides a clear indication that the anode  16  needs to be changed. Thus, a person carrying out maintenance to the body  10  can readily identify whether the anode  16  needs to be replaced by assessing the condition and amount of corrosion of the second material. This may include determining whether the corrosion is greater or less than a predetermined amount. The predetermined amount may be related to the physical dimensions of the second material or to the surface appearance. Further, in one embodiment, there may be markers embedded in the second layer which become exposed after a particular amount of corrosion. This system of maintenance would not be possible if the second material  22  corroded at the same time as the first material  20  which is not readily observable as it is located within the recess  12 . 
         [0035]    Having a second material  22  which is less anodic than the first material  20  also means that it provides a protective layer for the fluid washed surface of the sacrificial anode  16 . This means that the first material  20  corrodes from within the recess  12  and helps preserve the hydrodynamic profile of the body  10  and sacrificial anode  16 . 
         [0036]    The skilled person will appreciate that the clearance between the sacrificial anode  16  and the recess will be determined by the number of factors. For example, the salinity, temperature, and velocity of the fluid flow to name a few. Another important factor is the metal oxide which is formed as a part of the anode corrosion and dissolution process which will likely have a bigger volume than the parent metal and will partially fill the clearance round the anode. As will be appreciated, the volume of the oxide depends on the type of oxide formed and whether it is soluble or friable which may result in the oxide naturally eroding over time. 
         [0037]    In one embodiment, the clearance is the same around all sides of the anode  16  and approximately between 10 and 20% of the minor dimension of the anode to account for possible variations in the oxide formation and maintain some water flow even under worst case conditions. For example, for an anode which is 10 cm thick and 40 cm long, the corresponding recess  12  in the body  10  should be approximately 11 to 12 cm deep and 42 to 44 cm long. A typical radius for the curved edge  30  of the recess in this case may be in the region of approximately 7 to 20 mm, depending on the operating environment. 
         [0038]      FIG. 2  shows a water jet propulsion unit  210  for a marine vessel which represents a typical example of an environment in which the sacrificial anode  212  of the invention may be used. The water jet includes a duct  214  having an inlet  216  for ingesting water, an outlet  218  for exhausting water so as to provide propulsion and a shaft driven impeller  220  arrangement for accelerating the water towards the outlet  218 . The anode  212  can be seen as being recessed in a wall  222  of the duct  214 . 
         [0039]    The above described embodiments are examples of the invention defined by the claims and should not be taken as limiting. For example, although the first and second layers are described as being electrically connected together, this is an optional feature which prevents the protective second layer from corroding until all of the first material has corroded. The second layer may be provided simply to protect the sacrificial anodic layer and maintain the hydrodynamic profile.