aluminum and saltwater

[boater]

heres alittle artical i found on the subject

Corrosion

Any alluminium

will be susceptable cat man, just that somes more susceptible than others is all.

The marine grading refers to it's ability to form an oxidising layer that protects it from further corrosion, and is an indication of it's properties as far as engineering aspects of vessel design go.

Electron flow in galvanic cells is imutable, ions will flow from least to most noble metal when they are immersed in an electrolyte thats an imutable law, much like Einsteins E=MC2, Beardens E=DeltaTC2, and troutys M=Delta T.

The "trick" with ANY alluminium, in saltwater, is to have sacrificial anodes that get eaten away in preference to the alloy getting eaten away being the least noble metal in the system.

Also make sure you don't have stray current on your boat (Battery Isolator switch). Ensure if you have shorepower it's well maintained, properly earthed.

IF you have all your anodes in place, and keep foreign disimilar metals out of your bilge you don't have to worry at all.

Alloy is used by many commercial vessels of great age and now also mega yachts..so it's NOT a bad material.

It is still flamable just like fibreglass is - except it's flashpoint of some 1200 C is almost double that of fibreglass. (But it still will burn!)

Aloy Glass steel they all have their drawbacks as hull materials.

Woods starting to look good, no?

Cheers!

another corrosion post;

Now, on your glass boat the Outboard, is bolted thru the transon, which is an inert material of wood/glass or some other similar inert matrix.

On an alloy boat - your OB is also bolted thru the transom, only this time, the transoms alloy )conductor), hence the entire hull is now part of the earth ground circuit, since the outboards electrics have an earth strap that uses the engine bloc as the ground, and the engine blocks connected to the tilt/mounting bracket via the leg.

As you can see - the entire alloy hull is a ground...

Then people with alloy boats go along mounting special plastic spacers between all the metal accesories (lets say a 316 stainless cleat), they mount on their hull, trying to "insulate" the metalic component from the alloy hull to avoid dissimilar metal contact (and hence they beleieve eliminate electrolysis or bi metallic corrosion).

What is a lead acid battery?

Its a bi metallic cell of two dissimilar metals, separated with an inert spacer & imersed in an electrolyte, so that electrons can flow (current).

Take the metalic accessory separated from the alloy hull with an inert plastic spacer, plus the alloy hull and throw a cup of saltwater over the whole thing.

What you've now got is a defacto bimetallic cell imersed in electrolyte (saltwater).

What was the point of "insulating" the dissimilar metal accessory from the hull?.... in the act of placing the plastic inert spacer between the alloy hull and dissimilar metal accessory, in a salt water spray environment - your creating a bimetallic cell where electrons must flow from the most noble to the least noble metal.

If the alloy hull is the least noble of the two, then it will be eaten away adjacent to the metal accessory, and "bubbling of the paint" will be observed with a white powder contained within the bubbles.

Now - lets go the other way - lets attach the dissimilar metal fixture, direct to the hull, which is earth, and throw in the salt water...

Now what happens - we still have disimilar metals BUT they are now equipotentially bonded (in direct contact), so now - the most noble metal in the binary system will lose it's electrons to the least noble.

In our system - which is the least noble metal????

Is it the disimilar metal accessory?,
or the alloy hull?
Or
Is it the sacrificial anode, made of least noble metal Zinc, Hydral or Mercatel, on your OB Bracket and anode/fin?

Remember?...

The outboards equipotentially bonded to the hull which is the earth or ground...so it's sacrificial anode gets eaten away in the saltwater in preference to the more noble metals in the system, alloy of the hull, stainless steel of the accessory.

That is the role of the sacrificial anode (leat noble metal) - to lose electrons in place of the other metals in the system.

Not equipotentially bonding accessories on an alloy hull, (i.e. trying to insulate them with inert spacers) means that in the presence of an electrolyte [saltwater] they become part of a dissimilar metal binary cell..where the alloy becomes the least noble of the two...

It's a difficult concept for most to grasp, who've spent a lifetime wiring glass/wooden boats etc...

Any marine electrical contractor who's ever wired a steel hull will know tho...

Would anyone in their right mind get on a metal huled boat with 240V (110V) genset, where the metal hull in contact with the water wasn't grounded?

Electrolysis, disimilar metals, electrolysis, and wiring vessels, do some homework on it...

Alloys NOT the most versatile boat building wonder material some claim, it's OK, but not without it's problems from a maintenance / longevity point of view.

Lets take a Perhaps "common" situation.

One assumes you have some marina's slips where the penns (docks) are made of steel reinforced concrete?

Lets say the concretes aged a little, and taken an occasional "bump" from a novice learning to back his rig into the slip.

Lets say this concrete pillar, has a slight crack in it sufficient to allow water to seep into contact with the steel reinforcing inside.

Your new alloy hulled vessel is moored in the slip, alongside this concrete pillar with the crack and steel reinforcing in contact with the saltwater.

What have we got?

Alloy (metal 1) in saltwater (electrolyte) with a steel reinforcing rod (metal 2).

On your boats a stainless steel prop and prop shaft (metal 3).

Theres a few Galvanised fittings (or nickel plated?)on the docks that get wet for a few hours a day with the tidal movement (Zinc Metal 4, Nickel Metal 5)

And then theres, your sacrificial anodes (zinc/Hydral or Mercatel 4 or 6,& 7)

Now, which of these metals is the least nobel, which is going to give up it's electrons to the other metals in the system?..

Hopefully it's your sacrificial anodes, but, with all that dissimilar metal around it's not going to be long before that annodes eaten right out so whats next?

Your alloy hull? & Your alloy lower unit (or alloy prop if you have one).

See, alloys NOT the low maintenance material some would have you believe IF you don't know it's potential shortcommings.

Lets say theres NOT a cracked concrete pier, BUT some kids stolen steel bike was dumped in your pen the year before you bought your new alloy hulled boat, and parked her in the slip.

Or failing that lests say an old cast iron engine block used as a mooring now laying idle on the marine floor..etc etc.

Alloy takes constant surveilance to ensure it's not dissolving like an asprin.

Leaking shore power and Galvanic corrosion is another subject entirely and just as damaging for alloy hulls.

Whatabout Caustic cleaners on alloy?

Ever seen what happens when you mix caustic soda (one of the main ingredients in Caustic cleaning solutions) and alluminium?

Try it - get a few ring pulls of alloy cans and put em in a bottle/glass jar, add a few tablespoons of water and half a handfuull of neat caustic soda flakes.

You'll have enough hydrogen to fill a kids party balloon in under 3 mins and no aluminium left in the same time (plus a lot of heat!).

Imagine using a strong caustic cleaner on your alloy boat!

People do it all the time...unknowingly of course and if you don't rinse it off totally - lookout...

Those of you with alloy T Tops / hardtops should pay attention to this...what do you use to clean them?

Are they equipotentially bonded with your OB and hence sacrificial anodes?...if not are they in near contact with any other dissimilar metals?..(tinned copper wire in your T/Top electricals, nickel plated switches on your console etc etc) that ALL get wet with salt spray together, and become in essence a wet cell battery?.

article about aluminum and corrosion

Changing Image of Aluminum Boats
"Aluminum boat technology is relatively new," says Steve Daigle, president of Daigle Welding & Marine Ltd., in Campbell River, British Columbia, noting that welded aluminum boats first came on the market en masse as recently as the late 1960s. Prior to that, riveting was the standard technique for joining aluminum sheets on boat hulls—a technique still used for many smaller boats that use thinner-gauge aluminum.

The image of a leaky, riveted runabout with ungainly lines persists for some people when picturing an aluminum boat. In fact, since the dawn of the welding age for aluminum boats, they have grown in stature to the point where they are the top choice in a number of categories of boats, including small recreational craft, small to medium-sized commercial fishing boats in the northwestern U.S., crew boats in the Gulf of Mexico, and fine yachts—both sail and motorized. Daigle himself notes that in just his 16 years in the boat business, the number of welded aluminum boat manufacturers in British Columbia has jumped from 6 to 24.

But whether aluminum can make inroads into the large mid-sized recreational boat market remains to be seen.

Corrosion-Resistant Alloys
Central to aluminum's suitability for use in boat construction is the high resistance to corrosion of its marine alloys. The bread-and-butter marine alloys—5083, 5086, and 5456—resist corrosion in fresh water, saltwater, and polluted water.

Except for cosmetic reasons, 5XXX-series alloys don't even have to be painted above the waterline; the unpainted metal reacts with air (as would any other aluminum alloy) to form aluminum oxide—a hard, protective coating that protects the underlying aluminum. The bottom of an aluminum hull needs only compatible antifouling paint to prevent the growth of performance-robbing barnacles and weeds and zinc anodes to prevent galvanic corrosion

[roadsend]

Good article. It would be nice to have a what to do or not to do instruction list with every metal accessory bought.