NZ603335B - A Sacrificial Anode - Google Patents
A Sacrificial Anode Download PDFInfo
- Publication number
- NZ603335B NZ603335B NZ603335A NZ60333512A NZ603335B NZ 603335 B NZ603335 B NZ 603335B NZ 603335 A NZ603335 A NZ 603335A NZ 60333512 A NZ60333512 A NZ 60333512A NZ 603335 B NZ603335 B NZ 603335B
- Authority
- NZ
- New Zealand
- Prior art keywords
- recess
- anode
- sacrificial anode
- layer
- metallic body
- Prior art date
Links
- 239000000463 material Substances 0.000 claims abstract description 85
- 239000012530 fluid Substances 0.000 claims abstract description 32
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 19
- FYYHWMGAXLPEAU-UHFFFAOYSA-N magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 claims abstract description 6
- 229910052749 magnesium Inorganic materials 0.000 claims abstract description 6
- 239000011777 magnesium Substances 0.000 claims abstract description 6
- HCHKCACWOHOZIP-UHFFFAOYSA-N zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims abstract description 6
- 229910052725 zinc Inorganic materials 0.000 claims abstract description 6
- 239000011701 zinc Substances 0.000 claims abstract description 6
- 238000005260 corrosion Methods 0.000 abstract description 19
- 239000010410 layer Substances 0.000 description 8
- 229910052751 metal Inorganic materials 0.000 description 4
- 239000002184 metal Substances 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 230000037250 Clearance Effects 0.000 description 3
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 3
- -1 Zinc Magnesium Chemical compound 0.000 description 3
- 230000035512 clearance Effects 0.000 description 3
- 238000000034 method Methods 0.000 description 3
- 238000003466 welding Methods 0.000 description 3
- 229910000831 Steel Inorganic materials 0.000 description 2
- 231100000078 corrosive Toxicity 0.000 description 2
- 231100001010 corrosive Toxicity 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 239000010959 steel Substances 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 239000004411 aluminium Substances 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminum Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000005219 brazing Methods 0.000 description 1
- 239000010406 cathode material Substances 0.000 description 1
- 238000004210 cathodic protection Methods 0.000 description 1
- 230000000875 corresponding Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 239000003792 electrolyte Substances 0.000 description 1
- 230000003628 erosive Effects 0.000 description 1
- 238000005755 formation reaction Methods 0.000 description 1
- 230000000977 initiatory Effects 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- 150000004706 metal oxides Chemical class 0.000 description 1
- 230000000737 periodic Effects 0.000 description 1
- 230000001681 protective Effects 0.000 description 1
- 239000011241 protective layer Substances 0.000 description 1
- 238000003756 stirring Methods 0.000 description 1
- 238000011144 upstream manufacturing Methods 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
- C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
- C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
- C23F13/06—Constructional parts, or assemblies of cathodic-protection apparatus
- C23F13/08—Electrodes specially adapted for inhibiting corrosion by cathodic protection; Manufacture thereof; Conducting electric current thereto
- C23F13/16—Electrodes characterised by the combination of the structure and the material
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
- C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
- C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
- C23F13/06—Constructional parts, or assemblies of cathodic-protection apparatus
- C23F13/08—Electrodes specially adapted for inhibiting corrosion by cathodic protection; Manufacture thereof; Conducting electric current thereto
- C23F13/22—Monitoring arrangements therefor
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
- C23F2213/00—Aspects of inhibiting corrosion of metals by anodic or cathodic protection
- C23F2213/30—Anodic or cathodic protection specially adapted for a specific object
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
- C23F2213/00—Aspects of inhibiting corrosion of metals by anodic or cathodic protection
- C23F2213/30—Anodic or cathodic protection specially adapted for a specific object
- C23F2213/31—Immersed structures, e.g. submarine structures
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
- C23F2213/00—Aspects of inhibiting corrosion of metals by anodic or cathodic protection
- C23F2213/30—Anodic or cathodic protection specially adapted for a specific object
- C23F2213/32—Pipes
Abstract
603335 Disclosed is a sacrificial anode to provide cathodic corrosion protection to a metallic body while maintaining the flow characteristics of a constrained fluid flow, particularly .within a water jet propulsion unit. The metallic body comprises a sacrificial anode located within a recess or cavity of the metallic body, where the recess has an opening with a curved edge profile to encourage fluid flow through the recess in normal use, the opening having a first dimension, wherein the sacrificial anode extends across up to 90% of the first dimension. . The sacrificial anode comprises a first layer of a first material; and a second layer of a second material which is located proximate to the first layer so that both layers are electrically connected of directly bonded together, wherein the first material is more anodic with respect to a galvanic series than the second material. The sacrificial anode is separated from the body by a channel that substantially surrounds the anode and electrically connected thereto, wherein the body is more cathodic with respect to a galvanic series than the first and second materials. The recess is located in a fluid washed surface of the body and a surface of the first material is located in the same plane as the fluid washed surface. The first material is magnesium and the second material is zinc. vity of the metallic body, where the recess has an opening with a curved edge profile to encourage fluid flow through the recess in normal use, the opening having a first dimension, wherein the sacrificial anode extends across up to 90% of the first dimension. . The sacrificial anode comprises a first layer of a first material; and a second layer of a second material which is located proximate to the first layer so that both layers are electrically connected of directly bonded together, wherein the first material is more anodic with respect to a galvanic series than the second material. The sacrificial anode is separated from the body by a channel that substantially surrounds the anode and electrically connected thereto, wherein the body is more cathodic with respect to a galvanic series than the first and second materials. The recess is located in a fluid washed surface of the body and a surface of the first material is located in the same plane as the fluid washed surface. The first material is magnesium and the second material is zinc.
Description
A Sacrificial Anode
Technical Field of Invention
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
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.
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.
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.
The present invention seeks to provide a sacrificial anode which seeks to overcome some of the
problems of the known systems.
Alternatively it is an object of the invention to at least provide the public with a useful choice.
Statements of Invention
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.
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.
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
(Followed by page 2)
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.
The ratio of the first material to the second material may be between approximately 1:5 and 1:12.
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.
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.
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.
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.
The at least one edge may have a curved profile which subtends between the fluid washed surface
and surface of the recess.
In a third aspect, the present invention provides a water jet propulsion unit comprising the body
according to the second aspect.
The body may form at least part of a duct through which water may be propelled when the
propulsion unit is in normal use.
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.
Initiation of corrosion on the first material indicates consumption of the second material, indicating
the need to replace the entire anode.
Unless the context clearly requires otherwise, throughout the description and claims the terms
“comprise”, “comprising” and the like are to be construed in an inclusive sense, as opposed to an
exclusive or exhaustive sense. That is, in the sense of “including, but not limited to”.
Description of Drawings
Embodiments of the invention will now be described with the aid of the following drawings in which:
Figures 1a, b and c shows a sacrificial anode according to the present invention prior to, during and
after a period of corrosion
Figure 2 shows a water jet propulsion unit with a sacrificial anode.
Detailed Description of Invention
Figure 1a 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.
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.
Bimetallic anode pair Top surface of anode Bottom (bulk) of anode
Zinc Magnesium
2 Aluminium Zinc
3 Aluminium Magnesium
4 Mild steel Magnesium
Mild steel Zinc
6 Mild steel Aluminium
Table 1 - A list of suitable anode combinations.
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.
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.
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.
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
and body 10.
between the anode 16
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.
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.
The anode 16 is mounted within the body 10 such that the upper surface ofthe 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.
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.
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.
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 Figure lb
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 Figure lc and starts to corrode, while still providing protection to the body 10.
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.
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.
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.
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 10cm 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.
Figure 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.
The above described embodiments are examples ofthe 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.
Claims (11)
1. A metallic body comprising: a sacrificial anode located within a recess of the metallic body, the sacrificial anode comprising: 5 a first layer of a first material; and, a second layer of a second material which is located proximate to the first layer, wherein the first material is more anodic with respect to a galvanic series than the second material, wherein the sacrificial anode is separated from the body by a channel and 10 electrically connected thereto, wherein the body is more cathodic with respect to a galvanic series than the first and second materials; and, wherein the recess is located in a fluid washed surface of the body and a surface of the first material is located in the same plane as the fluid washed surface.
2. A metallic body as claimed in claim 1 wherein the channel substantially surrounds the 15 anode.
3. A metallic body as claimed in claims 1 or 2 wherein the recess has an opening to a fluid flow in normal use, the opening having a first dimension, wherein the sacrificial anode extends across up to 90% of the first dimension.
4. A metallic body as claimed in any one of claims 1 to 3 wherein at least one edge between 20 the fluid washed surface and a surface of the recess is shaped to encourage a flow of fluid into the recess.
5. A metallic body as claimed in claim 4 wherein the at least one edge has a curved profile which subtends between the fluid washed surface and surface of the recess.
6. A metallic body as claimed in claim 1 wherein the first layer and second layer are 25 electrically connected together.
7. A metallic body as claimed in any one of the preceding claims wherein the first layer and second layer are directly bonded together.
8. A metallic body as claimed in any one of the preceding claims wherein the first material is magnesium. 30
9. A metallic body as claimed in claim 4 wherein the second material is zinc.
10. A metallic body as claimed in any one of the preceding claims wherein the ratio of the first material to the second material is between 1:5 and 1:12.
11. A water jet propulsion unit comprising the body as claimed in any one of claims 1 to 10.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GBGB1119446.1A GB201119446D0 (en) | 2011-11-11 | 2011-11-11 | A sacrificial anode |
GB1119446.1 | 2011-11-11 |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ603335A NZ603335A (en) | 2014-04-30 |
NZ603335B true NZ603335B (en) | 2014-08-01 |
Family
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