Types of Marine Corrosion (Part 2)

Stray Current Corrosion

We’ve discussed what galvanic corrosion can do, using just the electrical potential in metals. Imagine what happens if you add more electricity. That’s exactly the basis for stray current corrosion.

Stray current corrosion occurs when metal with an electrical current flowing into it is immersed in water that is grounded (such as in any lake, river, or ocean). The current can leave the metal and flow through the water to ground. This will cause rapid corrosion of the metal at the point where the current leaves. Stray direct current (or battery current) is particularly destructive. Stray current corrosion can cause rapid deterioration of the metal. If the metal in question happens to be an aluminum part like your drive unit, it can be destroyed in a matter of days.

Stray current corrosion is different from galvanic corrosion in that galvanic corrosion is caused by connections between dissimilar metals of your boat’s drive components, and utilizes the electrical potential of those dissimilar metals. Electrons flow from one dissimilar metal (the anode) to another dissimilar metal (the cathode). In stray current corrosion, electricity from an outside source flows into your boat’s metal components and out through the water for a ground.

For example, your boat may be sitting between a boat leaking DC current and the best ground for that current. Rather than the DC current moving through the water to ground, your boat could provide a path of lower resistance. The DC current could enter a throughhull fitting, travel through the bonding system, and leave via your drive to the ground. Remember that corrosion occurs at the locations where DC current leaves metal and enters water.

Stray current can come from an outside source either internal or external to your boat. Internal sources involve a short in your boat’s wiring system, such as a poorly insulated wire in the bilge, an electrical accessory that may be improperly wired, or a wire with a weak or broken insulation that is intermittently wet.

External sources are almost always related to shorepower connections. A boat with internal stray current problems can cause accelerated corrosion to other boats plugged into the same shorepower line if they provide better ground. The stray current would be transmitted to other boats through the common ground wire, but can and should be blocked by installing a galvanic isolator.

A much more subtle, but potentially more damaging cause of stray current corrosion can occur without any electrical problems. Supposed you cruise back to your marina after a weekend on the water, and plug into shorepower to recharge batteries using your automatic trickle charger. Then you go to work for the week. On Monday, a large steel hulled boat (with scratched and scraped paint) ties up next to your boat. This boat is also plugged into shorepower and goes visiting onshore for a few days. A battery has just been formed—the large steel hull and your small aluminum drive connected by the shorepower and ground wire. Depending on the proximity, relative sizes, and how long your neighbor is ashore, when you go out the next weekend you may find your drive highly deteriorated. This unfortunate scenario can also be prevented by the installation of a galvanic isolator.

There is greater danger for boats that connect to AC shorepower: destructive, low-voltage galvanic currents (DC) passing through the shorepower ground wire. Normally, AC is not a corrosion problem, but because the boat, pier, and wire are all connected, or due to a leakage, there can be direct current (DC) also present. This is potentially very damaging and requires additional protection.

Safety regulations require a three-wire cable for carrying shorepower aboard any boat, and that one of these leads grounds all electrical and propulsion equipment to shore. This safety procedure reduces the danger of shock, but also connects the underwater metal components on your boat with metal on neighboring boats using shorepower, steel piers, and metal objects on shore that extend into the water. This interconnecting of dissimilar metals allows destructive galvanic currents to flow between them. If these currents are allowed to continue, your drive unit will experience severe corrosion damage in a very short time—as little as a few days.

There is a common misconception that you can overprotect your drive by using too many zinc or sacrificial aluminum anodes. This is not true. The corrosion potential of any metal is a voltage that can be measured by a reference electrode. Such measurements in water commonly are made with a silver/silver chloride reference electrode. The corrosion potential of a sacrificial anode is a characteristic value for that metal, and it does matter if you have one piece of the metal or 100 pieces. The corrosion potential stays the same. Of course, 100 anodes would be expensive, heavy, and a considerable drag under water. Only by increasing the corrosion potential by using a different anode material (such as magnesium in seawater) can you overprotect your drive.

Crevice Corrosion

There is also a form of corrosion that affects many metals, particularly stainless steel, called crevice corrosion. A crevice may be formed under any of the following: deposits (such as silt or sand), plastic washers, fibrous gaskets, or tightly wrapped fishing line. It can also form where moisture can get in and not back out, forming a stagnant zone. Stainless steel is an iron-based alloy containing chrome and nickel. The quality that causes it to be stainless (nonrusting) is its formation of a thin, tightly adhering surface layer of chrome oxide. If this surface is deprived of oxygen, the oxide layer breaks down and the stainless steel will rust just like plain steel. In other words, stainless steel is only stainless when it has access to oxygen. In a crevice where there is moisture depleted of oxygen, stainless steel rusts. The simplest prevention for this condition is to seal out the moisture or clean off any deposits.

Antifouling Paint On Drives

Fouling is a major concern in many situations. Marine animals (barnacles, mussels, etc.) and vegetation can make life miserable for boaters. Antifouling paints are available, but some can affect corrosion protection or even accelerate corrosion.

In the past, tributyltin-(sometimes referred to as TBT or organotin) based antifouling paints controlled fouling and did not cause corrosion problems for aluminum drives. However, environmental concerns and legislation have restricted or prohibited the use of tributyltin paints. Presently, tributyltin-based paints must be applied by a state-liscensed repair shop. In the United States and Canada, tributyltin is prohibited for vessels less than 25 meters with an exemption for aluminum hulls, fittings, and drives. If TBT paint can be obtained, it is still recommended for drives.

Galvanic Isolators

Galvanic isolators are solid-state devices that are part of a series connected in line to the boat's green safety ground lead ahead of all grounding connections on the boat. This device functions as a filter, blocking the flow of destructive low voltage galvanic (DC) currents but still maintaining the integrity of the safety grounding circuit.

Inactive Sacrificial Anodes

If the underwater portion of the drive unit shows signs of corrosion, but the sacrificial anodes are not being consumed, the problem may be due to the following:

• The sacrificial anodes may not be making good electrical contact with the drive unit. Remove the anode, scrape the mounting surfaces on the part to be protected down to bare metal, and reinstall anodes.
• Zinc sacrificial anodes may have a protective coating of a very dense oxide film on their surface (which usually has a charcoal-gray appearance). This condition usually occurs in freshwater, but it can also happen in saltwater areas.

To confirm this condition, test for continuity between the anode and the drive using a multimeter set to "ohms" on the R x 1 scale. If the anode must be scraped with a knife in order to get a conductive reading, the anode is oxidized and should be replaced. Sanding the surface with coarse sandpaper provides a temporary solution, but the oxide will form again.

Some Cautions

Due to the location of the sacrificial trim tab, the drive unit must be kept in the "in" position when the boat is moored. If the drive unit is raised, the trim tab may be out of the water and, therefore, unable to act as a galvanic corrosion inhibitor. (Note from BoatUS editor: it is possible and preferable to place other additional sacrificial zincs on the bracket or elsewhere so that the motor can be raised.)

Do not paint anodes. Painting them will render them inoperative. The anodes will not provide corrosion protection when the boat is removed from the water, therefore the drive unit should be flushed with freshwater to remove saltwater and pollutants prior to storage. For example, dried salt deposits can react with moisture in the air or create a cell and corrode metal.

Do not attempt to use magnesium anodes in saltwater. They will provide overprotection. Over protection will result in a different electrochemical reaction that will create hydrogen on the metal surface of the drive, under the paint. The paint will blister and peel completely off the surface of the overprotected drive.

Corrosion Protection Testing and Troubleshooting

For diagnostic tests, a simple digital volt/ohm meter (multimeter) is necessary. An analog version may be used, but it must be a high-impedance model. Even the most inexpensive digital volt/ohm meter has high impedance.

One of the most helpful methods for determining if corrosion below the waterline is occurring is through the measurement of the hull potential. This is done by immersing a reference electrode, usually silver/silver chloride (a silver wire with a coating of silver chloride) into the water about six inches behind the drive. This electrode is connected to the positive terminal of a digital volt/ohm meter. The negative lead from the meter is attached to the battery ground. With the meter on a two-volt DC scale, the hull potential is displayed. When performing tests, be sure to make sure your battery is fully charged. Also, new boats will usually produce higher readings than normal. This is because the drive unit is being protected by a new finish and new sacrifical anodes. To obtain an accurate diagnosis, the test should be performed after the boat has been used at least one or two weeks. All boats should be moored for at least eight hours before performing the test. This is necessary in order to allow the cathode system and sacrificial anodes to polarize the water molecules in direct contact with the drive. Be careful not to rock the boat excessively while boarding to perform the test, as this will alter the reading. (Note from BoatUS editor: If you are not thoroughly familiar with safety procedures in dealing with electricity on the water, leave this to a qualified professional.)

The first signs of corrosion below the waterline are paint blistering, usually on sharp edges, and the formation of powdery white corrosion material on exposed aluminum surfaces. If the corrosion is allowed to continue, pitting of the aluminum will occur. The chart below may help you determine the cause of the corrosion and the corrective action needed to prevent its continuance.

Article courtesy of Quicksilver Marine.

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