Galvanic corrosion protection

[Ufling]

Hello there
Is there any basic rules on the galvanic corrosion protection of an aluminium hull. I'm thinking sizes/quantities of zinz bars vs. hull size. The material is EN AW-5083
Thanks in advance!
Uffe "Ufling"

[CDK]

In the galvanic series, zinc and aluminium are too close to provide protection. Only magnesium electrodes would be marginally effective.

An aluminium hull alone will show very little galvanic corrosion unless there are stainless steel parts present.

[Frosty]

Outboard motors use zincs!!

Whats in the boat --if its just a rowing boat then youlle have no trouble.

[Ufling]

Hi
It's 6,6meter, to be fitted with a outboard, see pictures here
http://picasaweb.google.com/TeamUfling/AluAngler6605#
I'll use A4 stainless bolts in the boat where rivets can't be used. The outboard is mounted with Stainless bolts.
I have some grease specially made for Aluminium/Stainless connections.

@ CDK, if I use magnesium won't that protect the zinc on the outboard?

Thanks
Uffe "Ufling"

[gonzo]

Outboard motors use magnesium anodes in fresh water, so do outdrives. The term "zinc" is a generic name only. Some manufacturers use aluminum anodes too. Also, it depends on the aluminum alloy your hull is made of.

[CDK]

Outboard motors use zincs!!

Whats in the boat --if its just a rowing boat then youlle have no trouble.

Very good Frosty! If they wouldn't have a steel shaft they wouldn't need the electrodes we call zinc.
But in that case they wouldn't be outboard motors....

[Frosty]

Very good Frosty! If they wouldn't have a steel shaft they wouldn't need the electrodes we call zinc.
But in that case they wouldn't be outboard motors....

Ahh,-- have you seen one with an alluminium shaft?


They made wooden outboards in the war, didnt know that did you?

[gonzo]

Trojan war?
Anodes need to be appropiate for the metals they are protecting. The location is also important.

[Frosty]

Nope 2nd world war wooden engines, for limited use to escape the vicinity of a sinking aircraft.

[Ufling]

Interesting discussion (CDK and Frosty) but it dosen't answer my question..:?:

@ gonzo, if you have seen the pictures of the boat, would it make reason to place only one magnesiun bar below the waterling on the transom?

Thanks
Uffe "Ufling"

[jonr]

Normally zinc in salt water, magnesium in fresh.

[Zed]

Hi
It's 6,6meter, to be fitted with a outboard, see pictures here
http://picasaweb.google.com/TeamUfling/AluAngler6605#
I'll use A4 stainless bolts in the boat where rivets can't be used. The outboard is mounted with Stainless bolts.
I have some grease specially made for Aluminium/Stainless connections.

@ CDK, if I use magnesium won't that protect the zinc on the outboard?

Thanks
Uffe "Ufling"

Can you get Monel Bolts?

Is that an option people?

[CDK]

Ufling, if you want longevity using stainless parts in wet areas must be avoided at all cost. There are aluminium bolts and nuts.
Above waterline, isolate stainless parts by using nylon washers.

If you feel the need for some extra precaution, attach two 4" magnesium electrodes about midships, one on each side and check how they survived at the end of the season. If they have dissolved more than 50%, use larger ones the next time.

As Gonzo wrote, what you call zinc on the outboard may be something else, so the magnesium electrodes may not provide protection there.

There is also an electrical solution to prevent corrosion. Google for "Mercathode".

[jonr]

Impressed current cathodic protection can provide better performance, longer anode life and anti-fouling protection.

[Ufling]

Thanks! I appreciate you guys taking the time to help. The boat will only be in the water when i fish, otherwice on the trailer. Mostly I fish in salt water, but somtimes also in fresh water. I think I'll look in to the impressed current cathodic protection.

[MikeJohns]

Yes magnesium in fresh water but Zinc andodes are fine on a clean hull in salt water.

You may be able to get an alloy base plate with 316 studs that you weld onto the hull, some are cast and some are tapped.
Use a zinc with a steel strap and drill the strap to suit the studs. Use stainless spring washers and nuts.
We usually recommend that the straps and the tightened nuts and studs be painted with epoxy since it reduces the anode corrosion rate to hide the bare metals from the sea water. Painting the backs of the anodes with epoxy is also a good idea.

Stainless would only ever be a problem if the anode were completely depleted, but by then the steel strap will also be problematic.

There are both zinc and magnesium anodes for outboards, the Mercathode system has had some abysmal failures and the alloy used for casting legs is a very poor alloy for marine use.

Passive protection is always significantly more reliable than active systems and since a boat still needs slipping regulalry it's not much extra cost to keep the anodes up to scratch.

Corrosion rates are not just about the level of nobility but also about ratios of areas.