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Outdrive Corrosion Mysteries


…and How to Unravel Them

Corrosion seems simple — something metallic flakes off a few layers of rust or a cheap fitting crumbles in your hand — but its causes on a boat can be puzzling. Consider this corroded aluminum outdrive. It’s an expensive piece of equipment that’s ruined. After seeing the damage, the first question that would probably pop into a boat owner’s head (after “Will my kids be happy going to community college?”) is, what happened? Followed by: Did I overlook something? Was it affected by my neighbor’s boat? What should I have done to prevent it? In this issue, we’ll explain the obvious — and not so obvious — mistakes that can cause such severe damage and how to avoid them. It should be noted that corrosion is not covered by insurance.


The damage to the outdrive in the picture is severe, but it likely started with a few bubbles in the paint. Corrosion, especially on aluminum, changes the surface of the metal so that coatings no longer adhere. As with most marine-related corrosion, regular inspections can head off major problems. Examine your outdrive frequently, and if you find bubbling, peeling paint, or pitting, don’t ignore it. Take steps to correct the problem before your outdrive crumbles.


Without a galvanic isolator, your aluminum outdrive can become the anode of someone else’s underwater metal since you’re both connected via the shore power ground. Check your anodes (zincs) frequently and if they seem to be suddenly wasting away, you may be a victim of galvanic corrosion (claim # 0101309).

Aside from bubbling paint, the first thing to check is the anodes (“zincs”), which are critical to preventing corrosion in underwater metals.

You don’t need to know the chemistry that causes corrosion, but a simple explanation can help understand and avoid it. When two different kinds of metal are in contact with each other (say, an aluminum outdrive and a stainless steel propeller) and immersed in saltwater — and, to a lesser extent, fresh water — a weak battery is formed between the two. The more noble metal, in this case stainless steel, tends to remove small amounts of the aluminum as the current flows in the “battery.” An anode, which is a metal that is less noble than either the drive or the prop, can be attached to the aluminum outdrive, which will then sacrifice itself as the metal that loses out on the battery circuit, thereby protecting the aluminum. But in order to work, the anode has to be the right kind, good quality, and installed correctly.

Anodes are made of three kinds of metals, and each has a specific use. Zinc is used in saltwater only, aluminum is used in salt or fresh water, and magnesium is only used in fresh water. Zinc won’t be effective in fresh water, and magnesium should not be used in saltwater. If that sounds confusing, don’t worry; in the vast majority of cases, aluminum anodes are effective in fresh, salt, and even brackish water. (Aluminum anodes are a different alloy, which is why they can protect an aluminum outdrive.)

The best anodes are made to military specifications and some will have “milspec” stamped on them. If they don’t, make sure you buy only the best-quality anodes from a reputable marine chandlery like West Marine. Cheap anodes are more likely to contain impurities that will render them far less effective.

Even the best anodes must have good contact with the metal they’re protecting or they will be ineffective. That means they can’t be installed over painted or dirty surfaces. A quick once-over with some fine sandpaper on the metal helps maintain contact. Once installed, anodes should never be painted since this will deactivate them. Finally, anodes have to be replaced when they are about half gone, since they have less surface area and their effectiveness is reduced as they get smaller.

Most anodes are passive, meaning they simply sacrifice themselves as needed, but others, called impressed current anodes, are active, relying on 12-volt battery power and a controller to force small amounts of DC current through a permanent anode. Impressed current anodes are commonly used on aluminum outdrives in addition to sacrificial anodes, when the sacrificial anodes by themselves will not provide sufficient protection. Impressed current systems, like passive anodes, need to be inspected regularly. They commonly fail due to poor or corroded electrical connections. Since many don’t have indicators to show they’re working properly, most manufacturers recommend testing yearly by a qualified shop.

The outdrive shown in the first photo required no less than three sacrificial anodes for protection. When the damage was investigated, it was found that all the anodes were wasted away — gone. And while this by itself could have destroyed the drive, there are other things that could have caused the anodes to disappear, including a neighboring boat.

Shore Power

You might not think that a boat’s power connection could have anything to do with a corroding outdrive, but it can; aside from missing anodes, it is probably the most common cause of severe corrosion. Boats plugged into shore power share the green ground wire with every other boat in the marina that is plugged in. The ground wire is critical to a safe boat because it carries off dangerous current from a fault in the shore power system. But since the shore power ground wire is also connected to the DC ground system (which is critical to preventing shocks to swimmers in the area), it extends to a boat’s bonded underwater metals, including the outdrive.

The outdrive is then connected to the neighbor’s grounded underwater metals (even one several boats away), which may be stainless steel or bronze. Since the two different metals are in the water and connected through the ground wire, it’s the same as if they were in direct contact. If one boat’s underwater metal is not protected by an anode, it will use a neighboring boat’s anode for protection. When that’s gone, it will use other less noble metals, which is likely to be an outdrive, in order to protect itself.

The solution is to install a galvanic isolator, which will prevent low-level DC current from flowing through the green wire — effectively breaking the connection to neighboring boats — but still allow shore power current to flow to ground in an emergency.

Stray Current

Stray current corrosion can ruin an outdrive in weeks or even days. This prop was destroyed in a few days after the owner inadvertently wired his battery backwards, sending current flowing to the outdrive (Claim #0305156).

Another way an outdrive can be destroyed by corrosion is through a voltage leak in a DC system. This problem is less common and is typically caused by a fault in a boat’s 12-volt DC system. For example, if a bilge pump sitting in saltwater were to have a break in the DC wire’s insulation, current could destroy underwater metal fittings. Avoiding stray current corrosion means making sure that there are no wires in the bilge (all wires should be supported above the level of expected bilge water), except the wires to the bilge pump. The latter should be inspected to make sure it’s in good shape. Aside from keeping wires out of the bilge, the most effective way to prevent stray current corrosion is to have all of your underwater metal bonded to the boat’s ground. That way, any leaks will be brought back to your battery before they have a chance to destroy fittings. Most new boats are bonded from the factory, but older boats can be retrofitted. It should be noted that a galvanic isolator will not normally prevent stray current corrosion damage since the damaging voltage exceeds what the isolator can stop.

Preventing Damage to Your Underwater Fittings

To prevent your outdrive, shafts, trim tabs, or props from looking like Swiss cheese, follow these steps:

  • Check your outdrive for signs of corrosion several times each season. Look for bubbling or missing paint and severe sudden wastage of anodes.
  • Make sure you have the recommended number and type of anodes (aluminum works best for the vast majority of cases) and make sure they are in contact with what they are protecting. Never paint an anode. Some outdrives or outboards have three or more anodes — make sure you haven’t overlooked one. Anodes must be replaced when they are about half wasted.
  • If you have an impressed current system on your outdrive, verify it’s working by having it tested yearly. A non-functioning system can cause rapid corrosion.
  • If your boat is in the water and plugged into shore power, be sure you have a galvanic isolator installed. It will prevent other boats from using your underwater metals as sacrificial anodes.
  • Keep wires out of the bilge or any place that might get standing saltwater because they can cause severe stray current corrosion.

AC Corrosion

It used to be thought that AC current didn’t cause corrosion on boats. Recently, however, that thinking has been questioned. Some informal research spearheaded by the BoatUS Technical Services department has shown that AC current may cause corrosion, at least in aluminum, though normally at slower rates than DC current. AC leakages need to be significantly higher than DC to cause corrosion because AC switches from positive to negative 60 times per second. On the positive cycle, corrosion occurs, but on the negative cycle, some re-plating occurs so corrosion can take significantly longer. However, it’s believed that in swift-moving water, like in a marina with current, the re-plating may not always be able to take place, and AC corrosion will be accelerated.

Studies have shown that in most metals, like iron and copper, AC current has about one percent of the effect that DC current has as it relates to corrosion, but it has about 40 percent of the effect that DC has on aluminum. And while this may be important in determining how a corrosion incident occurred, it pales in comparison to what leaking AC current can do to swimmers in the water. Even very small amounts of AC current can disable a swimmer’s muscles and cause drowning. (See “A Preventable Dockside Tragedy,” Seaworthy, October, 2009). New boats are equipped with a device called an equipment leakage circuit interrupter (ELCI) that not only prevents shocks to swimmers in the water in the event of a fault, but also offers some protection against AC current corrosion.

BoatUS Editors

Contributor, BoatUS Magazine