Protecting Aluminum Boats From Salt Water Corrosion

They’re light, economical, nearly maintenance-free, easy to repair and almost impervious to damage. The Coast Guard and Navy prefer them for small craft, and many commercial boats are aluminum, as are many recreational craft. Their high strength-to-weight ratio means they can be built lighter and therefore can run faster for a given amount of power, and are easier to trailer. If you’re looking for the ideal boat building material, aluminum could be it. The only problem is that it’s the wimp of the electrochemical schoolyard, being beaten up and corroded away by almost every other metal except for zinc and magnesium.

Want to be a saltwater metal head? Aluminum can seemingly dissolve away in salt water when in the presence of other metals. Builders do everything they can to prevent this, but once the boat is in your hands it’s up to you to keep it alive. Here’s how.

How It Happens

We’re talking about galvanic corrosion. Back in science class you’d say that this is where one metal in an electrically conductive solution (such as salt water) gives up atoms when connected to a dissimilar metal in that same solution. Losing atoms means that the metal is falling apart, or corroding. In the slip aboard your aluminum boat, you’d say that this is where your hull becomes pitted because of a bronze through-hull on a neighboring boat.

The rate of corrosion of a metal on its own is determined by how chemically active it becomes when put in salt water. The more active, the more susceptible it is to corrosion. The less active, the more resistant it is to corrosion. When not in contact with anything else, most marine metals such as aluminum, bronze and stainless steel will corrode away at a reasonably slow rate. No danger there. But connect different metals, one active (aluminum) and the other a lot less active (i.e., a copper penny), in water and atoms will start to flow. And the aluminum will start to fall apart.

The Good News

When not in contact with other metals, aluminum can do quite well in both fresh and salt water, needing only ­bottom paint to prevent fouling. However, to play it safe, the American Boat and Yacht Council (ABYC) recommends that “aluminum vessels shall have a protective paint coating that provides a high [electrical] resistance barrier between the aluminum and the water.”

Above the waterline aluminum does even better. When continuously exposed to oxygen, it develops a film of aluminum oxide so dense and well bonded to the metal that it prevents further corrosion. That’s why many commercial and military craft leave aluminum bare from the waterline up: There’s no need for protective paints, cosmetics aside. As you can see, building an aluminum boat for salt water takes thought. The right alloys must be used, welding must be done just right, and parts must be carefully assembled.

Fiesta, a Florida pontoon builder that bills its boats as “built for use in salt water,” isolates hardware and stainless-steel bolts from the aluminum with nylon washers. Hull compartments have drains with silicone-sealed nylon plugs that can be opened to drain accumulated moisture. They also employ dedicated mounting brackets for sacrificial anodes, and their hulls and extrusions are made from thicker metal.

Better aluminum boat builders don’t allow crevices or joints that collect water, and avoid upturned brackets and channels that trap water. Moisture, including condensation, must drain away, with no sealed or dead air spaces. An emphatic spokesman for Premier Pontoons said, “We are constantly doing pre-emptive detective work to stop corrosion before it starts.”

Zinc or Swim

Engine manufacturers, as well as boatbuilders, attempt to save their aluminum products by attaching sacrificial anodes. As long as the anode is electrically connected to the part, either by direct contact or by wire, it stuffs the aluminum with excess electrons so it loses those rather than the aluminum giving up its own electrons. The zinc? It erodes — sacrificing itself for the benefit of the boat or motor.

Unfortunately, there is no formula to tell you how many anodes you will need and what size. In the beginning, it will be a matter of trial and error. A good place to start is the ABYC’s procedure. Inspect the anodes every month. Go with many small ones rather than a few large ones. If you have the right amount, your “zincs” should be about halfway gone by the end of the season, and replaced each spring. If they are not wearing away, they are either too large (rare) or not making good contact with the aluminum (common). Well-made anodes should have cast-in plates and fasteners to maintain good electrical connection throughout their lives, and they should meet military spec Mil-A-18001J (or higher last letter).

Don’t go nuts with zincs everywhere because aluminum won’t tolerate being overprotected. In mild cases, the zinc develops a crust and stops working. But if things get too far out of balance, you can generate an alkaline solution that will start eating away at the aluminum. An early sign of this is the softening or blistering of the bottom paint.

How Much Zinc?

Use too few anodes, and the fittings are not protected. Use too many, and the anodes erode quickly and may blister paint. A multimeter and a reference electrode help to nail down the right amount.

1. Place a silver/silver chloride electrode in the water near the item that is to be protected.

2. Touch the positive probe to the fitting. Note the meter’s reading in DC millivolts.

3. Connect a sacrificial zinc of the proposed size to the metal part to be protected. Put the zinc in the water and note the new meter reading.

4. Protection is adequate when the new voltage is 200 milli­volts (0.20 volts) more negative than the reading noted without the zinc.

The Trouble with Tanks

Even if you don’t have an aluminum boat, you probably have aluminum fuel tanks. While these can be perfectly safe, the U.S. Coast Guard noticed a recurring problem with leaking tanks. So it asked Underwriters Laboratory to see why. Not surprisingly, UL found that corrosion caused 92 percent of the failures. Most of it was caused by how the tanks were installed.

In general, aluminum tanks should be left bare. Paint can help. But if it’s not properly applied, it wears away, scratches or peels off, and moisture gets under the paint, concentrating and accelerating corrosion.

The most common fittings used in fuel systems are usually made from brass. Screw one of these directly into an aluminum tank and add some moisture, and you’ve got serious galvanic corrosion. Isolate these fittings from the tank by using 300-series stainless-steel washers or adapters.

The tank’s supports must not be moisture-absorbent, such as carpeting is. Suitable materials are stiff neoprene, Teflon or any high-density plastic. Water should drain from all tank surfaces when the boat is at rest; the bottom of the tank must be at least a quarter-inch above the hull to let air circulate and above the level normally reached by bilge water. The European standard says “no less than 25 mm [1 inch] above the top of the bilge pump inlet or the bilge pump automatic float switch.”

A fuel tank should be accessible for relatively easy inspection via a screwed-down, caulked hatch, but that is often not the case. Many builders install tanks so that a saw is needed for inspection or replacement. Too bad.

13 Ways to Prevent Galvanic Corrosion

1. Don’t mix metals, or at least use metals as close to each other as possible in the galvanic series.

2. Bolts should be less active than fittings; they’re small, so loss of metal is more serious.

3. Take all measures to electrically isolate fittings from each other, even on small craft.

4. Securely fasten anodes, and ensure there is firm contact with the metal to be protected.

5. Never paint an anode. Be sure the metal to which it is fastened is free of paint, scale and dirt.

6. Impressed current voltage should never exceed 1,300 millivolts when protecting aluminum.

7. Always repair paint chips and scratches that expose bare metal as soon as possible.

8. Avoid using any lubricant made with graphite aboard a boat made from aluminum.

9. Employ an isolation transformer whenever the boat’s connected to AC shore power.

10. Don’t use an automotive battery charger aboard a boat, especially an aluminum boat.

11. Paint only with primers and coatings specifically designed for aluminum.

12. Wash the aluminum boat down with fresh water after every use to remove built-up salts.

13. Keep hooks, sinkers, bottle caps and other metal debris out of the bilge lest they wreak havoc.

DIY the Right Way
Protecting your aluminum boat and equipment from the rigors of the marine environment.

Adding Hardware
Ideally, you’d use aluminum fittings and fasteners, minimizing the chance of galvanic corrosion, but these are hard to find. Instead use 300-series stainless steel. This works fine if you isolate the fitting with plastic washers or pads and keep the water out with a polysulfide or polyether bedding compound like Star brite polyether Boat Caulk.

Bottom Paint
Interlux and Pettit sell copper-free, aluminum-compatible paint systems backed by excellent application instructions and tech support. It takes lots of prep, plus attention to detail, to do this right. 

Ditch Your Carpet
To prevent hull pitting caused by trapped moisture, replace carpeted trailer bunks with vinyl with DeckRite.