Aluminium Hull protection

Hi Guys , i have seen several system to protect the Aluminium Hull in the last years like the use of magnesium , zinc anodes or only coated but what is the best method ? There are a lot of aluminium boat without coating what are the advantages ?

 

Aluminum hulls will form there own oxide if they are not painted. Many crewboats see years of service without any paint. The problem is that the oxide will rub off on clothes or most anything that it touches leaving a black mark. Ie you do not have to paint aluminum boats

If you paint the hull, obviously, this is not an issue.

But painted or not, you still need anodes to protect against corrosion as it is almost impossible not to have some water to metal contact somewhere around the boat.
Use Zinc, not magnesium, at least according to ABYC to keep the negative potential below 1200 mv.

All shafts etc should also be protected by anodes and do not use any fittings containing copper or brass or serious corrosion will occur

The aluminum hull should be connected above a wet area on the hull to the negative terminal of the engine. Note this is not to provide a ground for any electrical circuit. Ie all circuits must have a positive feed and a negative ground feed wire.
Do not ground circuits to the hull

There are various systems that provide a battery powered isolation transformer to help reduce corrosion. ABYC calls them "impressed current cathodic protection system

And of course do not paint the anodes

Bottom Paint
Do not use any of the copper based antifouling paints. Illegal in North America.
If you use an antifouling paint, I would recommend an ablative paint. After a proper prime, you apply maybe 3 - 4 coats of ablative paint. Ablative paint rubs off with the water moving over it taking
pesky marine organisms with it. After a few years, when you begin to see some primer coming through, simply high pressure wash the surface and give it a quick scotch pad scrub, wipe it with whatever the manufacturer recommends, often the same thinner as the paint and re apply the coating.

 

I see no reason why the hull should not be used as a return path for electrical circuits. Aluminum is an excellent conductor, more that 6 times better than steel, so the voltage drop for all usual DC loads (except starter motors) is virtually zero.
There is however another issue that needs to be addressed: any fasteners used must also be aluminum. Especially stainless bolts and nut must be avoided unless they are packed in grease and protected against moisture.

 

The comments about not using the hull as a return ground were based on the ABYC standards. I did not type them out as they can tend to be lengthy

1)E11.16.1 DC Grounding-**************-the DC grounding conductor shall be used to connect metallic non-current carrying parts of those direct current devices identified in ***** to the engine negative terminal or bus for the purpose of minimizing STRAY CURRENT CORROSION and ensuring a FAULT CURRENT PATH in the event of a short

E)2.10.6 The hull shall not be used as a current carrying conductor as per ABYC*****


These two standards specifically require a ground wire from devices back to a ground terminal for two reasons a)minimizing corrosion and b) providing a precise fault path in case of a short.

I would hazard a guess that in case of a short, a dedicated fault path would protect sensitive equipment if they shared a common ground.




E2.10.4 Fasteners used for connections to aluminum hulls shall be 300 series stainless steel

H33.18.3.5 Fasteners used to couple fittings such as fuel senders to aluminum tanks, shall be of 300 series stainless steel.

As most of the veteran contributors are aware, and I know that you are CDK from earlier contributions, but perhaps some of the newer subscribers are not, when you take two metals that are in contact with each other and immerse
them in salt water the least noble or anodic metal will begin to corrode. Aluminum is extremely anodic and the selection of fasteners, wire, fittings etc is very important.

The purpose of a zinc anode is to allow the anode to corrode before the aluminum hull.

While I could not find specific fastener information in the ABYC book, I know that almost all aluminum boat manufacturers at least in the north west, use stainless fasteners and ABYC's standards allow them on aluminium fuel tanks.

I owned a 40 foot aluminum boat for many years. Stainless fasteners were used and I did not see any corrosion around the bolts during normal maintenance work and the boat was about 12 years in service before we sold it. Stainless components in contact with the aluminum, hand rails, radar arches, cleats etc also did not appear to be a problem for corrosion

 

Thank you Barry for you clarification and precious support.
Regarding an aluminium planing hull like the following pic
http://www.powerandmotoryacht.com/s...tal_shark_boats/MetalShark40Fearless_600w.jpg

Where do you suggest to apply the anodes ? I have always seen this type of boat without anodes on the bottom.

 

Normally, they are fastened to the transom below the waterline so they are immersed/wet when the boat is floating.

If the anode is 6 inches wide by 12 inches long, then a channel is formed 6 inch x 12 inches with say one inch legs off the 6 inch side and welded legs toward the transom. This permits the anode to be bolted to the channel, without mounting holes going through the transom. This channel or anode must not be painted and be in contact with the anode,

 

The aluminum hull should never be used as a current carrying path (that is the negative return path on DC systems) In addition to the risks of metal corrosion (very small risk on aluminum) galvanic corrosion, (that's what the anodes are for, as well as galvanic isolation) there is the risk of stray current corrosion. If you use the hull as a current carrying path you are directly introducing stray currents into the water. Stray current corrosion can destroy aluminum outdrives very rapidly (i've seen cases where in a few weeks the Outdrive was wrecked) or an aluminum hull. That is why you never use the hull as a current path. All DC circuits should have a positive wire and a negative wire. The only reason for connecting the hull to the engine block is because the engine and hull need to be at ground potential. that is zero voltage. This prevents galvanic corrosion cause by dissimilar metals submerged in an electrolyte (water) forming a battery cell and corroding the hull or other fittings. That's also what the anodes are for. All underwater metal fittings should be connected (bonding) to keep them all at the same zero potential. see http://newboatbuilders.com/pages/electricity14.html The Connection Between Bonding, Grounding And Lightning

 

The concept of stray current is grossly misunderstood. In aluminum, stray currents only exist in the presence of a changing magnetic field. The best example is the mechanical Kw meter where a thin aluminum disc is placed between the poles of an electromagnet. Because of stray currents generated within the aluminum it behaves like a (weak) magnet.

It is also used for the voltage drop between the wheels of electric trains where massive currents cause a voltage drop in the rails, resulting in increased corrosion.

Although the calculation electrical resistance in a metal plate is quite complicated (Thévenin's theorem), for practical purposes where currents are low and the aluminum hull has a substantial thickness, the resistance can be assumed to be (almost) zero, so there will also be no voltage drop, certainly not in the wet area.

Zinc anodes.

I prefer to call them sacrificial electrodes. They make no sense on a bare aluminum hull; the substantial galvanic current will rapidly eat them without any benefits for the hull material. In an ideal situation where the hull is protected by a paint layer, they are useful to maintain a bias voltage in case of scratches or other damage.

 

An interesting comment regarding anodes not making sense on a bare aluminum hulls as thousands or tens of thousands of bare aluminum hulls in service use zinc anodes with corresponding corrosion of the anode.

I would say that the comment is at odds with itself

You say that the current will eat them RAPIDLY but that is the purpose of the sacrificial anode. If the anode corrodes while in contact with the aluminum hull, then the aluminum in the hull is protected by the anode. Therefore when a large aluminum boat is coupled with a small sacrificial anode, then the anode will corrode quickly.

There was an article that I read maybe 5 years ago, I suspect that it might have been in the Boat Building magazine about sizing anodes for service.
I hate relying on memory but could not find the article so I cannot defend my comments with calculations or reference this article.
But my understanding is that if you have a large surface to protect, then you need a large (by weight) anode(s) to protect it.

The articles calculations surrounded perhaps a barge or a large steel boat. They took the wetted area and worked out maybe a current density. If the area is large and the anode is small, then the anode will erode quickly.

Attached are some links that I found after the initial comments as I was looking for the Boat Builders article

http://www.boatzincs.com/anode_weight_calculation.html
http://www.katradis.com/anodes/calculations


and some others that add in other elements in the anode sizing process

http://www.boatingmag.com/how-to/choosing-right-sacrificial-anode This is a short video that shows us the process used to check a fittings exposure to galvanic corrosion

 

Barry, I recall the article you speak of, although I don't think it was in ProBoat. If memory serves me (sometimes not well, I am 72 and not getting any younger) I believe it was written by Steve D'Antonio, and may have been in PassageMaker Magazine, but I could be wrong. Anyway, any search on the internet will turn up hundreds of articles on sacrificial anodes, Even a search on Steve's name with anodes, brings up pages of hits.

And you are right, there are thousands of aluminum boats in use all protected by sacrificial anodes. I have been to a number of seminars on this subject and it becomes painfully obvious that it is a complicated subject and even many professionals disagree on how it should be done, but not many ever say they aren't needed. As I said you can do a search on the subject and get thousands of hits.

 

Of course there are 1000's of bare aluminum hulls with sacrificial electrodes. And there are also millions of people wasting money on food supplements they do not need.
Zinc is big business. It is one of the cheapest metals on this planet, but small pieces with two mounting holes become real moneymakers.

Zinc is necessary to protect steel, on aluminum, grp and wood it is a waste of money.

 

So zinc anodes on outboards, which are designed in to them, are not necessary in your view ?

 

We were discussing zinc for hull protection.

On an outboard there is a shaft and sometimes a prop and ss bolts or nuts, each of them creating a galvanic element with the light alloy. Without zinc, the outboard body would be the sacrificial electrode as soon as the paint is scratched.
Zinc does not give 100% protection; in the 80's there have been Mercruiser drives that were eaten within a few years although they all had a zinc trim tab and a zinc plate. The alloy itself also contained a lot of zinc, so the electrical potential between the drive and the trim tab was too low to be effective.

 

Certainly aluminum can form a non-conductive surface if left bare. But at a thickness of 3 nanometers, or 10 millionths of an inch one would wonder how effective this thickness would be as an insulator.

I could not find a dielectric constant for aluminum oxide which is .0000001 of an inch thick. I also could not find any modulus of elasticity for the thin coating to see if it could follow minor flexing of the boat hull underway and maintain a zero porosity condition to eliminate wetting of the aluminum.

Can you answer this question?

If I were to hook up a copper block to the bare aluminum hull and the copper is immersed with salt water, will corrosion of the aluminum take place?

 

Such data is not available because the compounds formed on aluminum are not uniform.
In air Al2O3 is formed, which is a very good isolator with a high dielectric strength, strong adherence to the base material and with approximately the same thermal expansion. Anodized aluminum has a non-conductive surface, so no further oxidation takes place.
In water Al(OH)3 is formed, a soft compound with totally different properties. It reacts with other elements like and forms colloids. Because the volume is much larger than the base material it offers some protection against corrosion.

Aluminum is also used for electrodes to protect steel. Your copper block connected to aluminum will increase the corrosion rate of the latter.