Engine and Outdrive Anodes

Engine and Outdrive Anodes

Zinc Anodes & Protection

What are you protecting? The type of boat that you have will determine how careful you need to be. A fibreglass hull with an inboard engine will need much less protection than an aluminium hull or a boat with an aluminium Sterndrive for example. The surest way to determine whether your boat is fully protected is to measure its cathodic protection voltage or “hull potential”. This involves using a voltmeter, with one probe connected to the bonding system (red terminal) and the negative terminal connected to a reference electrode. You probably don’t have this equipment but don’t worry there are some simple guidelines.

Inboard boats with mainly bronze and stainless metal parts can be protected using zinc or aluminium anodes. Don’t worry about overprotecting them. You are only overprotected when the weight of the anodes is so great that your boat sinks! The voltage generated by zinc or aluminium anodes will not cause any damage – no matter how much anode material is added, the maximum voltage that can be generated is the voltage of the anode itself. You could also use magnesium in freshwater locations on fibreglass-hulled boats. Be careful using magnesium on aluminium or wooden hulled boats since you can overprotect them. Steel hulls can also be overprotected to the point where excessive protection voltage rapidly lifts the paint off the hull.

Sterndrives and Outboard Motors require a little more care. The sacrificial anodes have a difficult task since they have to protect what is already a very active aluminium assembly. Initially, the anodes for these units were made of zinc, but in response to corrosion problems, Mercury and Johnson/Evinrude/OMC started selling the aluminium anodes in the early 1990’s. Other manufacturers are switching to aluminium too. The small increase in protective voltage helps ensure that the sterndrive is protected. If you use zinc anodes you may even invalidate your warranty! Again, be careful using magnesium anodes since you can overprotect your sterndrive or outboard. Where do you do your boating? 

WHAT’S THE TRUTH ABOUT ALUMINUM SACRIFICIAL ANODES?

A NEW SACRIFICIAL ANODE MATERIAL IS APPEARING ON THE SCENE – AN ALLOY OF ALUMINUM, ZINC AND INDIUM.A
IS IT AN IMPROVEMENT ON THE OLD TRIED AND TESTED ZINC?

By Martin Wigg and Paul Fleury

First, what do sacrificial anodes do?
Sacrificial anodes “sacrifice” themselves by wasting away instead of the propeller, the propeller shaft, rudder, engine or other metal parts in or connected to the water being” eaten” away or wasting away due to the electrochemical process of corrosion.
Sacrificial anodes are relatively cheap pieces of metal. They are designed specifically to corrode instead of the more expensive metal parts of your boat corroding. Sacrificial Anodes, which are known as “active” metals, are chosen for this task.
Traditionally sacrificial anodes were usually made of Zinc, hence the name “zincs” but they can also be made from magnesium or a special alloy of aluminum. Anodes are very important, indeed essential, even when you trailer your boat, and will help to prevent very expensive damage to the metal parts of your boat that are underwater.
Sacrificial anodes come in all shapes and sizes but they all perform the same task. They have to be connected “electrically” - usually metal to metal - to the valuable metal parts on your boat that need protecting. Thus you will see these grey pieces of metal attached to rudders, outboard motors, propeller shafts etc.
The sacrificial anode forms the negative terminal of a battery and is connected electrically to the metal to be protected. Because the metal of the sacrificial anode is more active electrically, i.e. it wastes away; it forms the negative terminal (anode) of the battery in the electrolyte which is, for example, sea water. The anode is the terminal that will always corrode with its metal literally dissolving into the water. The positive terminal of this “battery” is, of course, the metal (cathode) that is being protected, e.g. the propeller shaft, propeller, engine etc. This whole process is known as galvanic corrosion. The saltier or more polluted the water, the more it will conduct electricity and the faster the sacrificial anode will be corroded.
You may hear the term “electrolysis”, which is often used to describe “galvanic corrosion” or “stray current” corrosion. This is an inappropriate use of the term, and has no relevance in the marine industry.
What metals are used in sacrificial anodes?
The three most active materials used in sacrificial anodes are zinc, aluminum and magnesium. They have different properties and uses.
The first property to consider is their electrical potential. All metals generate a negative voltage (as compared to a reference electrode) when immersed in water. 

The lower – the more negative - the voltage, the more active the metal is considered to be, for example:
Magnesium generates -1.6 Volts, i.e. negative 1.6 volts.
Aluminum sacrificial anode alloy generates -1.1Volts
Zinc, -1.05 Volts

Now compare these voltages, by comparison, to metals commonly used in marine use:-
Bronze generates -0.3 Volts
Steel, -0.6 Volts

Aluminum, such as that used in hulls, sterndrives or outboard motors generates -0.75 Volts.

VOLTAGES IN SEAWATER OF MARINE-USE METALS WITH REFERENCE TO A SILVER-SILVER-CHLORIDE (AgAgCl) REFERENCE ELECTRODE

In order to provide protection, the highest practicable voltage difference possible is required between the sacrificial anode and the metal to be protected.
For example, if zinc is used to protect a bronze propeller, a “driving or protecting voltage” of negative –0.75 volts will be available, i.e. zinc at -1.05 volts minus bronze at -0.3 = - 0.75 volts.
If aluminum anodes are used this increases to -0.8 Volts.
Magnesium anodes increase this to -1.3 volts. 

The bigger the difference in voltage, the more protection you get. But, beware, some materials (aluminum) can be “overprotected” – more about that later.
The second property that is important is the current capacity of the anode material. The anode generates a voltage difference and this drives a current between the anode and the protected metal and through the water. 

It’s like having a bigger battery, the more capacity you have the longer it will keep protecting. Incidentally, for a particular anode, the rate of current flow is dependant on the surface area of the anode and the longevity depends on the mass. For the same size anode the relative capacities are:
Zinc: 100 (Taken as a datum e.g. this could be 100 days)
Magnesium: 30
Aluminum anode: 130 – 150 (Different manufacturers quote different ranges)

So if you used a magnesium anode in place of the “100 day” zinc anode it would only last 30 days.
The aluminum anode would last between 130 and 150 days.
The third property is Quality of the Anode Alloy
A word of caution about the metals used !!

Not just any zinc or any aluminum will work. Beware ! There are some imported anodes which are of questionable quality. It is important to ensure that the anodes you buy are made to the appropriate military or marine specification. Installing cheap or sub-standard anodes will undoubtedly cause increased and potentially very expensive corrosion problems. The most common material specifications are:
Zinc: MIL-A-18001K
Aluminum: MIL-A-24779(SH)
Magnesium: MIL-A-21412
The Winner so far?
It’s hard to pick a winner yet. Magnesium gives the highest voltage protection, but the aluminum lasts a lot longer (5 times as long as magnesium). The aluminum anode alloy is also a bit more active than the zinc.

So why is zinc is the most common material? Well, it was the simplest and cheapest material to use for many years, even centuries and it does do a good job in most saltwater applications. Pure aluminum is useless as an anode material because it immediately forms an oxide layer that insulates and protects it. Aluminum alloy was not developed until the middle of the 20th century after it was discovered that adding small quantities of zinc and indium (one method) to aluminum overcame the problem of aluminum forming an insulating oxide coating – which stopped the anode working effectively. It takes time for new ideas to catch on!

What are you protecting?
The type of boat that you have will determine how careful you need to be. A fiberglass hull with an inboard engine will need much less protection than an aluminum hull or a boat with an aluminum Sterndrive for example. The surest way to determine whether your boat is fully protected is to measure its cathodic protection voltage or “hull potential”. This involves using a voltmeter, with one probe connected to the bonding system (red terminal) and the negative terminal connected to a reference electrode. You probably don’t have this equipment but don’t worry there are some simple guidelines.

Inboard boats with mainly bronze and stainless metal parts can be protected using zinc or aluminum anodes. Don’t worry about overprotecting them. You are only overprotected when the weight of the anodes is so great that your boat sinks! The voltage generated by zinc or aluminum anodes will not cause any damage – no matter how much anode material is added, the maximum voltage that can be generated is the voltage of the anode itself. You could also use magnesium in freshwater locations on fiberglass-hulled boats. Be careful using magnesium on aluminum or wooden hulled boats since you can overprotect them. Steel hulls can also be overprotected to the point where excessive protection voltage rapidly lifts the paint off the hull.

Sterndrives and Outboard Motors require a little more care. The sacrificial anodes have a difficult task, since they have to protect what is already a very active aluminum assembly. Initially the anodes for these units were made of zinc, but in response to corrosion problems, Mercury and Johnson/Evinrude/OMC started selling the aluminum anodes in the early 1990’s. Other manufacturers are switching to aluminum too. The small increase in protective voltage helps ensure that the sterndrive is protected. If you use zinc anodes you may even invalidate your warranty! Again, be careful using magnesium anodes since you can overprotect your sterndrive or outboard.

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