Greg K
04-14-2004, 04:06 PM
What do sacrificial anodes do?
All metals immersed in an electrolyte (sea water for example) produce an electrical voltage. When two dissimilar metals are in contact (electrically connected) they produce a galvanic cell (like a battery), with the less noble metal (a bronze propeller for example) forming the anode and the more noble metal (stainless steel shaft) forming the cathode.
If you want to protect both metals you need to connect a third metal that is more active than the first two. The most active metal (zinc for example) becomes the anode to the others and sacrifices itself by corroding (giving up metal) to protect the cathode - hence the term sacrificial anode.
What factors increase corrosion?
The voltage difference between the two metals will affect the rate of corrosion. For example a stainless steel prop, which is a relatively noble metal, will cause more corrosion of a set of zincs than a bronze prop.
Corrosion will increase the saltier the water is. Increasing temperature will also increase the conductivity of water and the resulting corrosion. The corrosion rate doubles with every 10 degrees Celsius (18 degrees Fahrenheit) increase in temperature. Pollution can also increase corrosion. For example, many freshwater lakes have been contaminated by acid rain, which increases the conductivity of the water and therefore corrosion rates.
When should sacrificial anodes be replaced?
Anodes should be changed, at least, on an annual basis (including anodes in fresh water) or when they have corroded to half their original size.
What precautions should I take when installing new anodes?
Make sure they make good electrical contact with the metal that is being protected. Remove any paint and clean the metal surface that will be in contact with the anode.
DON’T paint anodes! They can’t work if they are covered up.
What type of anodes should I use for my sterndrive?
Sterndrives and outboards are made from aluminum alloys, which are particularly prone to corrosion (less noble metal). The use of aluminum anodes is strongly recommended. They give much better protection in both salt and fresh water.
What else should I do to help protect my sterndrive?
Keep paint (on engines, sterndrive units etc.) in good condition. A small scratch will corrode rapidly.
Leave the sterndrive unit immersed in the water. If you don’t the anodes can’t work.
Don’t use anti-fouling paint containing copper or mercury on a sterndrive unit. The metal in the paint will increase galvanic corrosion.
Don’t mix zinc anodes on the hull with aluminum anodes on the drive. The aluminum anodes will protect the zinc anodes in addition to the unit.
What anodes should I use on Trim Tabs?
Trim tabs are usually made of stainless steel and they should be protected with small zinc trim tab/rudder anodes. Do not bond the trim tabs to the main internal bonding system! A drive unit may corrode trying to protect the trim tab and its zincs. An unprotected trim tab will corrode rapidly.
What anodes should I use in freshwater?
Where possible aluminum anodes are recommended over zinc. Zinc anodes can become inactive after only a few months due to the build up of an insulating film of zinc hydroxide. Aluminum anodes will remain active.
Zinc anodes should be changed on an annual basis, even if they look OK – remember the coating!
Why aluminum?
As discussed above, aluminum is a more active alloy creating a greater protective voltage difference (-1.10V compared with –1.05V). It also has a much higher current capacity for the same weight (1150Ahr/lb compared with 368 Ahr/lb for zinc). The result - an anode made in aluminum gives better protection and lasts longer than the same anode made in zinc. Also, aluminum does not suffer from the problem of becoming inactive in freshwater.
MORE...
What do zincs do?
The correct name for them is sacrificial anode and they are used to protect the metal parts of a boat that are in the water. They do this by corroding instead of the metal.
When two different metals are immersed in an ELECTROLYTE (A liquid that can carry electricity) such as salt water, they set up an electrical voltage and a current flows between them, just as in a battery. The most "active" metal (the ANODE or positive) deteriorates by losing material into the electrolyte and the less active metal (or CATHODE) is protected. This is known as GALVANIC corrosion. If you want to protect both metals you need to connect a third metal that is more active than the first two. This is where the zinc comes in. It is one of the most active metals and is commonly used as a sacrificial anode.
Facts and Advice
General Corrosion Facts
Corrosion will increase the saltier the water is.
Increasing temperature will also increase the conductivity of water and the resulting corrosion. The corrosion rate doubles with every 10 degrees Celsius (18 degrees Fahrenheit) increase in temperature.
Pollution can also increase corrosion. For example, many freshwater lakes have been contaminated by acid rain, which increases the conductivity of the water and therefore corrosion rates.
Installing Sacrificial Anodes
Anodes should be changed, at least, on an annual basis (including anodes in fresh water).
You should change your anodes when they have corroded to half their original size if this occurs sooner than 1 year.
When you fit new zincs, make sure they make good electrical contact with the metal that is being protected.
Always use noble metal, such as stainless steel (preferably 316 grade), for bolts or fasteners. The fasteners are very small compared with the surrounding metal and if they corrode first you will lose something!
DON’T paint anodes! They can’t work if they are covered up.
It is near impossible to overprotect with zincs (except wooden boats).
Sterndrives and Outboards
Sterndrives and outboards are made from aluminum alloys, which are particularly prone to corrosion (active metal). The use of aluminum anodes is strongly recommended. They give much better protection in both salt and fresh water.
Magnesium anodes can be used in fresh water areas with severe corrosion problems, but caution is advised. If magnesium anodes are used in salt or brackish water the paint will be “blown off” the drive in a very short time!
Keep paint (on engines, sterndrive units etc.) in good condition. A small scratch will corrode rapidly.
Leave the sterndrive unit immersed in the water. If you don’t the anodes can’t work.
If you have an outboard tilt it out of the water (as long as it is completely clear).
If you remove your boat from the water, flush the metal components and/or drive unit with fresh water to remove any pollution or saltwater.
Don’t use anti-fouling paint containing copper or mercury on a sterndrive unit. The metal in the paint will increase galvanic corrosion.
Don’t mix zinc anodes on the hull with aluminum anodes on the drive. The aluminum anodes will protect the zinc anodes in addition to the unit.
Fresh Water
Where possible aluminum anodes are recommended over zinc.
Magnesium anodes, if used, must not be used in salt or brackish water!
Zinc anodes should be changed on an annual basis, even if they look OK. Anodes can become inactive after only a few months due to the build up of an insulating film of zinc carbonate.
Shore Power
If you leave your boat connected to shore power, the grounding conductor from the shore connects your boat’s ground to the grounds of surrounding boats on the dock. You may then be protecting neighboring boats with your anodes (or worse still with your aluminum outdrive).
The solution is to install a galvanic isolator in the grounding conductor of the shore power cable. This ensures that you are isolated from the galvanic dc currents without creating a safety hazard. DO NOT DISCONNECT THE GROUND (GREEN) CONDUCTOR. You could create a potentially lethal electrical hazard.
All metals immersed in an electrolyte (sea water for example) produce an electrical voltage. When two dissimilar metals are in contact (electrically connected) they produce a galvanic cell (like a battery), with the less noble metal (a bronze propeller for example) forming the anode and the more noble metal (stainless steel shaft) forming the cathode.
If you want to protect both metals you need to connect a third metal that is more active than the first two. The most active metal (zinc for example) becomes the anode to the others and sacrifices itself by corroding (giving up metal) to protect the cathode - hence the term sacrificial anode.
What factors increase corrosion?
The voltage difference between the two metals will affect the rate of corrosion. For example a stainless steel prop, which is a relatively noble metal, will cause more corrosion of a set of zincs than a bronze prop.
Corrosion will increase the saltier the water is. Increasing temperature will also increase the conductivity of water and the resulting corrosion. The corrosion rate doubles with every 10 degrees Celsius (18 degrees Fahrenheit) increase in temperature. Pollution can also increase corrosion. For example, many freshwater lakes have been contaminated by acid rain, which increases the conductivity of the water and therefore corrosion rates.
When should sacrificial anodes be replaced?
Anodes should be changed, at least, on an annual basis (including anodes in fresh water) or when they have corroded to half their original size.
What precautions should I take when installing new anodes?
Make sure they make good electrical contact with the metal that is being protected. Remove any paint and clean the metal surface that will be in contact with the anode.
DON’T paint anodes! They can’t work if they are covered up.
What type of anodes should I use for my sterndrive?
Sterndrives and outboards are made from aluminum alloys, which are particularly prone to corrosion (less noble metal). The use of aluminum anodes is strongly recommended. They give much better protection in both salt and fresh water.
What else should I do to help protect my sterndrive?
Keep paint (on engines, sterndrive units etc.) in good condition. A small scratch will corrode rapidly.
Leave the sterndrive unit immersed in the water. If you don’t the anodes can’t work.
Don’t use anti-fouling paint containing copper or mercury on a sterndrive unit. The metal in the paint will increase galvanic corrosion.
Don’t mix zinc anodes on the hull with aluminum anodes on the drive. The aluminum anodes will protect the zinc anodes in addition to the unit.
What anodes should I use on Trim Tabs?
Trim tabs are usually made of stainless steel and they should be protected with small zinc trim tab/rudder anodes. Do not bond the trim tabs to the main internal bonding system! A drive unit may corrode trying to protect the trim tab and its zincs. An unprotected trim tab will corrode rapidly.
What anodes should I use in freshwater?
Where possible aluminum anodes are recommended over zinc. Zinc anodes can become inactive after only a few months due to the build up of an insulating film of zinc hydroxide. Aluminum anodes will remain active.
Zinc anodes should be changed on an annual basis, even if they look OK – remember the coating!
Why aluminum?
As discussed above, aluminum is a more active alloy creating a greater protective voltage difference (-1.10V compared with –1.05V). It also has a much higher current capacity for the same weight (1150Ahr/lb compared with 368 Ahr/lb for zinc). The result - an anode made in aluminum gives better protection and lasts longer than the same anode made in zinc. Also, aluminum does not suffer from the problem of becoming inactive in freshwater.
MORE...
What do zincs do?
The correct name for them is sacrificial anode and they are used to protect the metal parts of a boat that are in the water. They do this by corroding instead of the metal.
When two different metals are immersed in an ELECTROLYTE (A liquid that can carry electricity) such as salt water, they set up an electrical voltage and a current flows between them, just as in a battery. The most "active" metal (the ANODE or positive) deteriorates by losing material into the electrolyte and the less active metal (or CATHODE) is protected. This is known as GALVANIC corrosion. If you want to protect both metals you need to connect a third metal that is more active than the first two. This is where the zinc comes in. It is one of the most active metals and is commonly used as a sacrificial anode.
Facts and Advice
General Corrosion Facts
Corrosion will increase the saltier the water is.
Increasing temperature will also increase the conductivity of water and the resulting corrosion. The corrosion rate doubles with every 10 degrees Celsius (18 degrees Fahrenheit) increase in temperature.
Pollution can also increase corrosion. For example, many freshwater lakes have been contaminated by acid rain, which increases the conductivity of the water and therefore corrosion rates.
Installing Sacrificial Anodes
Anodes should be changed, at least, on an annual basis (including anodes in fresh water).
You should change your anodes when they have corroded to half their original size if this occurs sooner than 1 year.
When you fit new zincs, make sure they make good electrical contact with the metal that is being protected.
Always use noble metal, such as stainless steel (preferably 316 grade), for bolts or fasteners. The fasteners are very small compared with the surrounding metal and if they corrode first you will lose something!
DON’T paint anodes! They can’t work if they are covered up.
It is near impossible to overprotect with zincs (except wooden boats).
Sterndrives and Outboards
Sterndrives and outboards are made from aluminum alloys, which are particularly prone to corrosion (active metal). The use of aluminum anodes is strongly recommended. They give much better protection in both salt and fresh water.
Magnesium anodes can be used in fresh water areas with severe corrosion problems, but caution is advised. If magnesium anodes are used in salt or brackish water the paint will be “blown off” the drive in a very short time!
Keep paint (on engines, sterndrive units etc.) in good condition. A small scratch will corrode rapidly.
Leave the sterndrive unit immersed in the water. If you don’t the anodes can’t work.
If you have an outboard tilt it out of the water (as long as it is completely clear).
If you remove your boat from the water, flush the metal components and/or drive unit with fresh water to remove any pollution or saltwater.
Don’t use anti-fouling paint containing copper or mercury on a sterndrive unit. The metal in the paint will increase galvanic corrosion.
Don’t mix zinc anodes on the hull with aluminum anodes on the drive. The aluminum anodes will protect the zinc anodes in addition to the unit.
Fresh Water
Where possible aluminum anodes are recommended over zinc.
Magnesium anodes, if used, must not be used in salt or brackish water!
Zinc anodes should be changed on an annual basis, even if they look OK. Anodes can become inactive after only a few months due to the build up of an insulating film of zinc carbonate.
Shore Power
If you leave your boat connected to shore power, the grounding conductor from the shore connects your boat’s ground to the grounds of surrounding boats on the dock. You may then be protecting neighboring boats with your anodes (or worse still with your aluminum outdrive).
The solution is to install a galvanic isolator in the grounding conductor of the shore power cable. This ensures that you are isolated from the galvanic dc currents without creating a safety hazard. DO NOT DISCONNECT THE GROUND (GREEN) CONDUCTOR. You could create a potentially lethal electrical hazard.