Corrosion is something no one wants to think about, until their boat is hauled and they find the prop eaten and the thru-hulls crumbling. Corrosion has been a problem for boaters since the first metal was introduced to hold wooden boats together. It has only gotten worse as we’ve added more electrical conveniences.

Let’s break down the different types of corrosion and look at some case samples.

Dissimilar Metals

We can credit Alessandro Volta for discovering the usefulness of placing two dissimilar metals in an electrolyte back in 1800. Volta realized that if one metal was more anodic and one metal was more cathodic, and if the two were connected and immersed in an electrolyte, it would create a battery.

This reaction is called galvanic corrosion. And while a battery is useful for storing and discharging energy, you don’t really want the metals in your hull to behave like a battery.

To prevent galvanic corrosion, you can coat the metals and isolate them from the electrolyte. Or, you can connect them and add a metal that is more anodic and sacrificial, and have that corrode instead of the metal you want to protect. While coating can be effective, it must be maintained. Once the coating starts to break down and expose the metal to seawater, corrosion will begin.

Utilizing an anode (traditionally zinc) works so well that “zinc” has become the generic word for anodic protection. Zinc is low on the nobility scale, which is a measure of how active a metal is compared to a reference metal. Passive stainless steel and bronze are on the other end of the scale. They are more noble. Zinc is relatively inexpensive as well. Boatbuilders have learned how much to place on a boat for the anode to corrode at a rate that lasts a year or two, providing protection for the other metal.

Manufacturers also produce anodes in magnesium, which is even more anodic than zinc and useful for freshwater protection. More recently, they’ve ramped up production of aluminum alloy anodes, which work well in salt and brackish water. This isn’t a normal aluminum used for other parts on your boat. The anode alloy includes some zinc, indium and silicon, along with the aluminum alloy.

Anodes made of aluminum alloy are slightly less noble than zinc (.05 volt) but are also less harmful to the environment. A recent check at the local West Marine revealed far more aluminum anodes in the bins than zinc. Most people are making the switch. The only caveat is to make sure that if there are multiple anodes installed on your boat, they are all made of the same material.

Another galvanic corrosion process common on boats occurs when stainless steel fasteners are used in aluminum. If the stainless steel screw, bolt or rivet is installed dry, then any moisture (especially if it is salty) will start the corrosion process, damaging the aluminum and locking the fastener in place. It will also bubble any paint or coating on the aluminum around the fastener. An effective sealant must be used to isolate the stainless from the aluminum. Products such as Loctite, Tef-Gel and LanoCote, or high-quality polyurethane sealants, can give years of protection by sealing out the moisture. Attempting to later remove fasteners that were improperly sealed can require impact hammers, heat, penetrating oils, cobalt drill bits and active swearing.

Aluminum also suffers from another type of damage called poultice corrosion. If aluminum is dry, it develops a hard, durable surface. But if it’s kept wet from a damp rag, insulation, carpet or neglect, then it will develop a white pasty corrosion that can pit the aluminum to the point of perforation. This can be especially damaging to aluminum tanks. It is a good reason to inspect your tanks, to make sure they are clean and dry on the outside. They should also be resting on raised stringers that are padded with rubber that’s glued to the aluminum.

Bonding

A fiberglass boat’s bonding system is what keeps all the underwater metals at the same electrical potential. These are typically the green 8-gauge wires or copper strips you see connecting thru-hulls, tanks and other metal equipment. This bonding connects that metal to a common anode, so we don’t need an individual anode for each piece of metal—although there can be some gear, such as large metal trim tabs, where using separate anodes is preferred.

In a properly operating bonding system, all connections will have less than 1 ohm of resistance. This is easiest to check when your boat is hauled out (since you can easily locate all the metal from the outside). If the boat is floating and you know where all your thru-hulls are located, you can check that way too.

Set your meter to ohms (Ω). Connect one end to a jumper wire that’s connected to the propeller, rudder shaft or hull-mounted anode bolt. Then, touch the other probe to each distinct underwater metal. You may need to scratch through the antifoulant to get a good connection.

If the resistance is higher than 1 ohm or if there is no resistance at all, then you’ll need to check inside the boat for the cause. Chances are that the connection will be cruddy or altogether missing.

There are two additional advantages to connecting all the underwater metals with bonding wiring. In a properly wired bonding system, the bonding wiring is connected to the DC negative bus and the AC safety ground bus. If an AC hot wire on your boat were to contact a bonded metal, most of that fault current would be carried ashore on the AC ground wire to the marina’s electrical system grounding rod. This setup protects the crew on board. Conversely, if your boat is fitted with an isolation transformer, then the fault aboard would travel back to the transformer. The other benefit is that the bonding system can help carry a measure of lightning current out of the boat, should you be so unlucky.

One exception is that it is better not to bond a traditionally planked wooden boat. Connecting all the underwater metals (except the fasteners) can encourage electrical activity that can damage the structure of the vessel. The electrical activity can cause salts to form around the wet wood and around underwater metals that can deteriorate the wood.

To mitigate corrosion on a traditionally planked wooden boat, use similar metals in the hull fasteners, thru-hulls and propulsion gear, with bronze being the preference. It’s better to sacrifice the prop after a few decades than to have to replace all the fasteners in the planks or, worse yet, the planks themselves.

How Much Is Too Much?

It is possible to have too much anode for the underwater metal load for any hull material. This encourages more electrical activity than is necessary to protect the metal, and can damage metal-based antifoulants, usually showing up as “rings” around thru-hulls or other metals.

To prevent this, have a professional check your boat with a corrosion meter. This is a calibrated voltage meter that utilizes a silver/silver chloride half-cell reference transducer. The transducer is hung over the side in the water while the other probe is attached to each underwater metal in turn. Metal-hulled boats are often fitted with permanent versions of a corrosion meter so the voltage can be regularly monitored to be sure the anodes are doing their job.

As a side note, Mercury and Volvo Penta both produce active corrosion protection systems. Unlike a passive anode, an active system is a solid-state device powered by the boat’s 12-volt battery. It provides protection by impressing a reverse-blocking current in the water that stops the destructive flow of galvanic currents. This becomes useful if you are trying to protect a large aluminum casting such as an outdrive, but it does require a continuous source of power, or the protection will need to resort to a backup anode.

The corrosion meter will also register higher voltage if there is a DC fault sending some positive current through the water and back to the battery through the bonding system. A prime example is a bilge pump that has run for hours in a dry bilge. If the motor shaft sealing O-ring has been compromised and moisture can reach the motor, then positive DC current can flow through the bilge water toward a bonded piece of gear the next time the pump is energized. Damaged float-switch wiring can do the same thing. This unwanted, impressed current can quickly destroy underwater metals.

Because of the damage that can be caused by DC stray current, it is important to try to limit the faults to just your boat. Consider that if you leave your boat connected to shore power, you are basically connected through your green safety wire to all the other boats on the dock that are plugged into shore power. A fault on a nearby boat, for example, can try to reach ground through your boat wiring and underwater metals. That is why galvanic isolators (or even isolation transformers) are recommended.

A galvanic isolator can be installed in your green AC safety ground between the boat’s inlet and any other connection. It blocks DC current from other boats without blocking any of the possible AC faults the wire is supposed to carry. For even more separation, an isolation transformer completely disconnects the boat from the shore power and re-establishes ground on the secondary side of the transformer.

Leaking AC is extremely dangerous. It can also cause corrosion issues as the current tries to find its way back to shore through the boat’s underwater metals. Prime candidates are malfunctioning water heater elements or thermostats, compromised battery chargers, damaged refrigeration or air conditioning compressors, and faulty wiring. Reversed polarity where the hot and neutral wires are switched, and improper neutral-to-ground connections aboard the boat (possibly in a household appliance), are two additional common problems. A telltale sign is AC leakage that produces rings around underwater metals in metal-based antifouling (similar to what is found when utilizing too much anode). 

Recipes for Corrosion Resistance

The composition of the metal alloy used underwater makes a big difference. Brass should never be used below the waterline, inside or outside the boat. Brass has a high zinc content and, as you might expect, the zinc will protect the rest of the alloy by corroding away, leaving the metal fragile and susceptible to breaking. This is a process called dezincification. Bronze has a much lower zinc content and, while not quite as strong as brass, will usually outlast the boat.

In some instances, a stronger, harder metal is useful. Stainless steel alloys were developed to improve corrosion resistance over plain steels. Adding varying amounts of chromium and nickel produces an alloy that, in most circumstances, is much less likely to rust. This is the 304 alloy. Adding molybdenum resulted in the 316 alloy, which is even more corrosion-resistant.

These alloys are corrosion-resistant because they form a hard outer surface coating when exposed to oxygen. The metal is termed “passive” when it has this coating, and it works very well above the waterline. Underwater, especially if the water can become stagnant and deprived of oxygen, the metal can de-passivate and begin to corrode. This will show up as pits that can travel deep into the metal. It is called crevice corrosion. We see this mostly with stainless steel propeller shafting, strut bolts and swim platform bolts, making 304 and 316 alloys less useful below the waterline.

To combat corrosion, more expensive alloys such as Aquamet 22 or Aqualoy 22 have been developed. These metals are austenitic and non-magnetic. They have lower carbon, more chromium, manganese, nickel, molybdenum and nitrogen than the standard 316 alloy. The 22-series alloys have become the industry standard for high-quality propeller shafting.

The metal industry has a standard called the Pitting Resistance Equivalent Number. As boaters, we can find this useful because it determines which alloys are more resistant to pitting in seawater. Higher PREN numbers denote increased corrosion resistance.

It’s Science

We can sometimes feel like we’re involved in a cruel science experiment. We take beautiful, piano-type sculptures and plop them into acid. We pelt them with UV radiation
and agitate them to speed up the reactions.

If we arm ourselves with an understanding of the processes, however, we can slow the ultimate deterioration and give ourselves plenty of time to enjoy boating in the first place.

This article was originally published in the September 2024 issue.