When metals are exposed in the presence of water, whether fresh, brackish or salt, marine corrosion is inevitable. From the moment they are manufactured, nearly all metals are trying to return to the original form in which they existed when we dug them out of the ground.
The three types of corrosion boaters typically work to manage are:
- Simple electrochemical corrosion
- Galvanic corrosion
- Electrolytic/stray current corrosion
All three types of marine corrosion are the result of an electrochemical process, the difference is how fast the corrosion occurs. The process speeds up when multiple metals are involved (galvanic corrosion) and faster still when unwanted electrical currents are applied (stray current corrosion). While the result of any of these types of corrosion is pretty much the same – exposed metal damage – there can be many reasons why the corrosion occurred in the first place. The challenge we face is identifying the type of corrosion and finding its true cause to minimize future corrosion.
Electrolytic - Stray Current Marine Corrosion
Direct current (DC) stray current corrosion is the corrosion that occurs when current flows through metal and water as it seeks a path back to battery ground. The corrosion rate caused by stray DC current can be 100 times the rate of galvanic corrosion, and it can cause significant damage in day, or even hours. In extreme cases of stray current corrosion a lower unit and outdrive housing can be eaten almost completely away, exposing the shafts and gears. While this certainly does happen, there are other more common cases of stray current corrosion that occur, some that are easily – but mistakenly – attributed to galvanic corrosion. There is a fairly common stray current situation which occurs on multi-engine boats. Often, the stainless-steel propeller on one motor has a dull surface and the other is still shiny and clean.
The first clue that this is a stray current issue can be deduced from these photos. If this was a galvanic corrosion situation, the stainless-steel propellers would not be affected. The stainless steel would be protected by any active anodes or, if no anodes are active, by the aluminum housing of the lower units and midsections. In a stray current situation, electrical current is trying to find a path back to ground. So as it leaves one metal, regardless of type, the current will erode that metal. When the current reaches the next metal on its path back to ground, deposits will form on the surface of that metal.
The second clue, if the boat operators are observant, is that the issue typically occurs while the motors are running, not while they are moored. They will clean their props, but the next time they return from a day of use one prop will be covered with buildup. So what could cause this type of stray current corrosion? Often, it’s due to poor battery cable connections. This is a problem because all motors, of multi-engine applications, must be at the same electrical potential but, due to charging output variations between running motors, as well as battery condition, they may not be. If they’re not, they will attempt to equalize.
Good, clean cable connections and battery cables of the correct size are necessary to maintain all motors and batteries at the same potential. The battery cables which connect between the ground terminals of all batteries are particularly important to allow equalization. If the connections at the batteries and the motors are poor, stray current can flow from one motor to the other through the water in an attempt to equalize the potential. One common factor in this type of stray current corrosion is missing or undersized negative battery cables between the starting and house batteries. This condition doesn’t usually cause significant damage in a short period of time, but customers will often comment on how one prop looks different than the other.
There is another similar situation with battery cables which could occur, though, and one that can cause significant stray current damage in a very short period of time: incorrectly connected battery cables. All negative battery terminals should be connected together. But what happens when one motor’s positive battery terminal is connected to another motor’s negative battery terminal?
You might think this would cause an immediate short circuit. Not really. What does happen is one motor will have a 12V positive potential on all grounded metals and stray current will flow from that motor to the other. An incorrect connection can arise when boat owners, an inexperienced apprentice technician, or yard guy is installing new batteries or reinstalling batteries when preparing a boat for use after offseason storage. Watch for these situations, they are easy to spot and correct once you know where the evidence is leading you.
Electrochemical Marine Corrosion
Electrochemical corrosion, also referred to as self-corrosion, only requires a piece of metal to be in contact with an electrolyte. In this case, the metal is both the anode and the cathode, as well as the conductive path. How can a single piece of metal be both an anode and cathode? Most metals are actually alloys, meaning they contain multiple base metals and impurities. In an alloy, one base metal functions as the anode, while another base metal functions as the cathode. The electrolyte can be plain old air, a liquid, or a combination of both. Examples we commonly see would be the formation of rust on steel or a layer of oxide on bare aluminum. This process, although extremely slow, begins as soon as most metals are manufactured and may be the easiest form of corrosion to slow down. A protective coating, such as the correct kind of paint, can slow the process way down.
Anodes are placed on outboard motors as a sacrifice for the natural process that occurs to metals trying to return to their original form. Other than a visual check for corrosion, how can we tell if there are sufficient anodes to protect the motor and other metal components?
The best way is to use a silver/silver chloride reference electrode to measure the hull potential. The instructions that come with the silver/silver chloride reference electrode will guide you through a procedure. The general overview of the procedure is to connect the voltmeter’s negative lead to the reference electrode.
Then place the reference electrode in the water near the submersed metals. With the voltmeter’s positive lead connected to the boat’s DC ground or bonding system, you will observe a very low voltage reading between the reference electrode and the boat. The voltage, typically in the -750 to -1100 mV range, is the hull potential. There are established ranges for different hull materials and drive systems, which you should be within when the correct amount of anode protection is present. Anytime you are working with a rapid corrosion issue, the hull potential must be checked to get to the root of the problem. Replacing corroded parts without resolving the real problem will only result in continued corrosion issues.
Galvanic Marine Corrosion
Galvanic corrosion typically involves two dissimilar metals, aluminum and stainless steel. These metals are bonded together either by direct contact or by the electrical system and are submerged in an electrolyte – the water the boat is in. The assortment of these elements essentially becomes a big wet cell battery.
What happens in a battery when a connection is made between the positive and negative posts? Electrons flow between the battery’s plates, which are anodes and cathodes. On a boat without any corrosion protection, aluminum, being the most active metal, will become the anode and the stainless steel, a less active metal, will be the cathode. Electrons will flow from the anode to the cathode, resulting in a loss of anode material, visible as corrosion on the aluminum components.
Galvanic corrosion typically appears as paint blisters with a white powdery residue on the exposed metal. Corners and edges of components, such as the leading edge of the lower unit and the sides of the anti-ventilation plate, will usually be the first areas affected. Galvanic corrosion is much more destructive than electrochemical corrosion but can be controlled and minimized when you understand the corrosion process.
Be sure to test for deficiencies and apply sound preventative measures. The best first line of defense is to apply a layer of good paint to insulate the metals from the electrolyte. Another method is to introduce an alternate metal, one more active than aluminum, into the system. A metal that is more active will become a sacrificial anode and will provide protection to both the aluminum and stainless steel components. The trick is installing the right amount of sacrificial anode material to protect all of the aluminum and stainless steel, connected either by direct contact or connection to the boat’s electrical system.
Sacrificial Anodes Facts
- The factory-installed anodes must be in the water to provide protection.
- Trim tab anodes may not be in the water when the lower unit is tilted up.
- Transom bracket anodes may not be in the water on motors installed in applications requiring extremely high mounting.
- Additional anodes are required when other metal components are in the system: aluminum hulls, jack plates, trim tabs*, trolling motors, etc. The motor’s anodes not only will be unable to protect the other components, but will also be unable to protect the motor. They will also erode extremely quickly. Additional anodes may be required on a motor when used in applications other than what it was originally designed for. Example: SHO® models, primarily a freshwater bass boat motor, may need more anodes when used in saltwater, brackish water, and even when regularly moored in freshwater.
- Anodes must be clean and free of paint to work properly.
- Anodes must be electrically conductive and connected to the boat and motor’s grounding system. Conductivity through the anode material and between the anode and the boat’s electrical system deteriorates with time and exposure.
- By design, anodes deteriorate as they provide protection and must be replaced periodically. Replace anodes when they have eroded to 2/3 their original size. Beware, looks can be deceiving. Anodes can lose mass (weight) without visibly looking smaller. Did you ever see one that looked like it was full of worm holes?
- Water flow around a moored boat can disrupt the flow of electrons from the anode to the cathode. If the electrons from the anode do not make it to the cathode, the anode will shed even more electrons at a faster rate in its valiant effort to protect the cathode.
- Boats connected to shore power could be electrically connected to other boats and dock structures through the common grounding wire (green) in the AC power system, forming one huge system. Anodes on one boat will try to protect other boats or submerged dock structures that do not have sufficient anode protection of their own. A galvanic isolator installed near the boat’s shore power receptacle in the AC grounding (green) wire blocks the flow of galvanic current between boats and dock structures connected to a shore power system.
- Not only is too little anode protection a problem, too much protection is also not good. Too many anodes or the incorrect type of anode material can create a different electrochemical reaction which creates hydrogen on the metal surface. This will cause paint to blister and peel completely off overprotected surfaces. Metal components electrically isolated from the rest of the boat are not bonded. These components may require their own standalone anode protection to minimize electrochemical corrosion. Zinc, magnesium, and aluminum anodes are available for many sources. How can aluminum protect aluminum? The aluminum alloy used for anodes is more active than the aluminum alloys typically used for outboards, drives, and hulls. Zinc and the aluminum anodes are used most frequently. Magnesium is a very good anode material for freshwater, but should never be used in saltwater or brackish water as magnesium will overprotect and erode very quickly when used in saltwater.
