Stray current corrosion is similar to galvanic corrosion, except that a current source is added to the circuit. Source: USPS.
Electrolytic or stray current corrosion is related to galvanic corrosion, discussed last month. It functions similarly, but with the addition of an external electrical source to the corrosion equation, which significantly accelerates the reaction. It occurs when metal with an electrical current flowing into it is immersed in water that is grounded (all water in the marine environment). This type of corrosion occurs when a short develops between:
a) An external DC or AC current source (such as the 12-volt DC system on your boat or a nearby one).
b) Something on the boat that is connected to boat’s underwater metals.
The current flow to ground through the water does not necessarily start with the zinc sacrificial anodes. Any metal that is at a lower potential than the incoming current is susceptible to stray current corrosion. This could be the prop shaft, the propeller or a through-hull fitting. The current flow is likely to attack one specific piece of metal at a time. As the current leaves the piece of metal on its way to ground, it is carrying bits of the metal with it This stray current is several orders of magnitude higher than the current produced by a galvanic cell. The corrosive effect is rapid and able to destroy a lower unit in mere days.
External Current Sources
If your boat is near another vessel leaking DC current, your boat could provide a path of lower resistance than simply moving straight through the water to ground. If so, the DC current could enter a through-hull fitting, travel through the bonding system (including the battery ground), and corrode the drive’s grounded or bonded aluminum structure as it passes into the water to ground. Galvanic isolators will stop only DC current in the AC service cable’s green (ground) wires. Next month, we will cover testing the slip environment, which is the only way to detect and prevent stray current corrosion damage in wet slips.
Internal Current Sources
Stray current from a nearby boat overwhelmed the sacrificial anodes and destroyed this outdrive in a very short space of time. Source: BoatUS.
Internal sources of corrosive current are normally shorts in your boat’s wiring system. Most often this is a poorly insulated wire or connection in the bilge, or an electrical accessory that may be improperly wired. Wiring faults cause accelerated corrosion on the host boat, especially with metal hulls. You will notice corrosion at the current leakage points. These may be at fuse clips, service cord connector bodies and loose or poor electrical connections.
Wiring faults with current leakage are damaging to other boats plugged into the same shore power line if they provide better ground. The stray current would be transmitted to other boats through the common ground wire, unless blocked by a galvanic isolator.
New boats normally do not experience stray current corrosion problems, unless they are victimized by nearby boats or ground wiring faults on the dock. Most problems show up in older boats with jury rigged wiring and systems that owners add using incorrect wiring practices.
Practical Implications
Coastal cruises. If you plan to take your boat that is normally dry stored at an inland lake on a coastal cruise, be sure to install a galvanic isolator. Why? You may be docked at a guest slip in a coastal marina, connected to shore power to keep your air conditioner running. During the night, a metal hulled transient vessel docks near you and also connects to shore power. Your two boat’s electrical systems are connected by a common ground. If he has a ground fault in his electrical system, your boat faces serious risk from stray DC current in the other boat’s electrical system. Your anodes could be rapidly consumed, and at the same time your underwater metal components could be damaged. Rapid damage to props, prop shafts and lower units could result. Also, stray current to the bonding system in the other boat could find entry through your wetted parts and cause damage.
This mast footplate was not insulated from the mast or not bonded to the grounding system and was destroyed by stray current corrosion. Source: BoatUS.
Aluminum outboard boats. Never ground any circuit to the metal hull, unlike the automotive practice, which grounds everything conveniently to the frame and then to the negative battery pole. You do not want any stray current flowing through your hull to ground. You will observe pinholes at the least, holes in your transom, or maybe corrosion around rivets. Ground all circuits back to a common ground bus connected to the negative pole of the battery. This pole is also connected to your outboards which are protected by sacrificial anodes.
Note that on a metal boat, the outboard ground is unavoidably connected to the hull through the mounting bracket and bolts—you cannot avoid this. However, you do not want the current path to go first through the hull and then hopefully to the outboard ground. You want the current path to go back to the battery negative pole and then through insulated cables to the outboard.
Bilge pump wiring. Bilge pumps and float switches are common sources of stray currents that cause corrosion. Most of these are straight wired to the battery for unattended operation. Their conductors are necessarily exposed to moisture and bilge water. They are replaced every few years, which brings risk if the new pump is not spliced into the circuit correctly. Also, no wire insulation is 100% effective. Some current leakage is unavoidable. Use wire restraints to keep as much of the conductor from lying in residual bilge water. Any splice should be done with shrink wrap crimp connectors using a marine crimping tool.
Sources:
https://www.boatus.com/seaworthy/magazine/2015/july/marine-corrosion-101.asp
http://www.boatus.com/boattech/articles/marine-corrosion.asp
https://www.goldeagle.com/tips-tools/understanding-corrosion-around-boat/
https://www.jetdock.com/knowledge-center/prevent-aluminium-boat-corrosion.asp
Marine Electrical Systems, AC/DC-Electrical Circuits, Grounding, Lighting, & Safety; United States Power Squadrons; 2008.
Next month: testing for corrosion potential.
Learning More
A nine-session course is well worth the time and effort expended to manage your electrical system for safety and longevity. (USPS Education Department)
Does marine corrosion seem complicated to you? Are you not sure where to start in applying best practices to your boat? United States Power Squadrons has a solution for you. Marine Electrical Systems is an elective course dedicated to onboard electrical power–-both AC and DC. You will learn about electrical circuits, wiring, terminating, and routing wiring for safety and trouble-free operation. You will learn how to use multimeters and tools to check, diagnose and repair electrical problems. You also will learn about galvanic corrosion, stray currents, isolation, grounding, and safety of electrical systems.
The course material is organized in seven chapters and is intended to be presented in the classroom over nine two-hour sessions:
Day 1 – Introduction, Chapter 1: Properties of Electricity
Day 2 – Chapter 2: Boat Electrical Wiring Practices
Day 3 – Chapter 3: Direct Current Power
Day 4 – Chapter 4: Alternating Current Power
Day 5 – Chapter 5: Galvanic and Stray Current Corrosion
Day 6 – Chapter 6: Lightning Protection
Day 7 – Chapter 7: Troubleshooting Part 1; on-the-boat optional
Day 8 – Chapter 7: Troubleshooting Part 2
Day 9 – Review and Examination
This course is the best way to learn about electrical factors that can save you from sustaining considerable damage, especially if you berth your boat at a marina, or if you make any repairs or modifications to your OEM electrical system.
If you are interested in taking this class, indicate your interest using the contact page.
