On the long list of complex technical topics that boat owners face, corrosion of underwater metals is one of the most complicated, potentially most expensive and least well understood. While it is not possible to ease the complexity or terminology of the topic, I can at least describe several related “stray current” metal corrosion phenomena in this one place.
Some readers may feel this topic is “beyond their pay grade.” Like it or not, we all have a stake in understanding the basics. At some time in boat ownership, most owners will face one or more corrosion issues. Even for those for whom the topic is both uninteresting and obscure, all boaters should know how these phenomena are similar and how they are different. Some familiarity will allow the affected owner to hire the right expert, understand remediation recommendations, and possibly avoid problems in the first place.
AC and DC “stray” electric currents flow in the water. Because these currents flow outside their normal electrical conductors and devices, they are referred to as “stray currents.” Worst case, all types of fault currents can be present at the same time. Boaters should consider all electrical currents that flow in the water as a bad thing.
FUNDAMENTAL CONCEPT OF CORROSION:
The basic concept in all corrosion is always the same: there is a voltage difference between two or more different metals, or alloys of metal, which are a) connected together electrically and b) immersed in an electrically active liquid.
The three major “stray currents” flowing in water (or in the earth’s crust) are:
- AC “ground fault” currents, resulting mostly from wiring errors aboard boats and occasionally from inadequate equipment design, incorrect equipment selection or AC appliance/equipment malfunction,
- DC “Galvanic” currents, resulting from the natural behavior of dissimilar metals in mineral-containing ground water, fresh surface water or sea water, and
- DC “Electrolysis” currents, a DC “ground fault” current, resulting from wiring errors, equipment faults, and improper equipment use.
While the basic electro-chemical processes and terminology of corrosion are always the same, the cause is context-specific. Understanding the context (AC fault current, DC galvanic current, or DC Electrolysis current) is essential to avoiding confusion caused by the shared terminology.
“Electrolysis,” or “Electrolytic corrosion,” is frequently confused with, but very different from, “Galvanic corrosion.” To repeat, the concepts and terminology are shared and common to both phenomena; it is the “cause and origin” of the driving voltage that is different.
No matter the terminology, corrosion currents are a silent attack on every boat, and can cost many hundreds or thousands of dollars for those who don’t mount an appropriate and effective defense.
AC FAULT CURRENTS:
AC fault currents flowing in the water are often dangerous to people, pets and wildlife.
Worst case, AC fault currents can lead to death by “Electric Shock Drowning:” (ref: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/electric-shock-drowning/). Children and pets must never swim in a marina’s basin. Boat owners and professional divers performing in-water maintenance on boats must be alert to the causes and consequences of AC electric currents in the water.
In general, AC fault currents DO NOT deteriorate the underwater metals of boats and do not cause rapid zinc wasting. There is a great deal of technical understanding about environmental AC ground fault currents that comes from the utility and transportation industries (power transmission, buried utilities, pipeline and railroad). The 60 Hz AC power found across North America changes polarity 120 times per second. Whatever molecular metal material that might be removed from a metal in one half-cycle is re-deposited in the second-half cycle. (Cit: “DC Currents in the Bilge – Not AC – Is the Culprit When Metal Fittings Corrode,” Robert Loeser, Seaworthy Magazine, October, 1996). AC fault currents must be very large before metal corrosion results. The AC voltages found around pleasure craft docks (less than 600V) DO NOT cause zinc wasting.
Aluminum can be an exception. Aluminum can be damaged by AC stray currents IF the density of the fault current is greater than 40 Amps per square meter of aluminum surface area (40A/M2). What that means in English is: a relatively high AC fault current in the water will cause erosion to a relatively small chunk of underwater aluminum. This combination would be unusual, but not impossible, in pleasure craft marinas. One m2 is equal to about 10.75 ft2, so it would only take 3.7 amps of AC stray current to cause corrosion damage to a 1 ft2 aluminum part. This is not an extraordinary leakage current, but 1 ft2 is a small piece of aluminum. So trim tabs and outdrives may be “relatively” “safe” at levels that would waste aluminum anodes installed for galvanic protection. On boats without other aluminum parts, aluminum anodes can waste quite rapidly in proportion to the size of a moderate in-water AC fault current.
Readers can find information on testing for AC ground and leakage fault currents in layman’s language on this website, here: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/ac-safety-tests-for-boats/. The reference article helps owners bring their boat into compatibility with National Electric Code (NEC) standards that require ground fault sensors on docks, and it also dovetails well with identifying and eliminating corrosion issues.
Be aware that some in-water AC stray currents can originate from sources on land. In that case, the fault current will flow on the green AC safety ground wire (a component of the boat’s bonding system), originating in the basin water and flowing back into the shore power infrastructure. This situation is not caused by a problem on the boat, and in general, is not something a boat owner can fix. Always report this finding to marina management.
WHAT THE BOAT OWNER CAN DO: AC FAULT CURRENTS:
- Assess the boat for AC ground fault and leakage fault conditions. (ref: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/ac-safety-tests-for-boats/)
- Correct all issues in order to establish a defect-free starting-point baseline.
- Consider installing Equipment Leakage Current Interrupter (ELCI) sensors on boat shore power AC service circuits. (ref: ABYC E-11, 11.11.1 and https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/elci-primer/)
- Where automatic ELCI sensors are not installed, perform frequent manual monitoring of AC shore power cords with a decent-quality clamp-on Ammeter. (ref: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/ac-safety-tests-for-boats/)
- Correct any newly discovered issues as soon as they present themselves.
DC GALVANIC CURRENT TERMS AND CONCEPTS:
- DC galvanic currents are associated with small voltage potentials that are a naturally-occurring characteristic of all metals. The specific voltage is determined by the atomic structure of the individual metal (or metal alloy).
- Different metals have different naturally-occurring electro-potentials.
- A “Galvanic Series” is a list of metals sorted by their naturally-occurring characteristic electro-potentials. Different “references” can be used for ordering a “galvanic series.” The best reference for salt water is a silver/silver chloride cell.
- A “galvanic couple” is any combination of two or more dissimilar metals or metal alloys connected together electrically and immersed in an electrolyte.
- An “electrolyte” is an electrically conductive liquid (generally) medium.
- “Dry Corrosion” is the direct attack on a metal by dry gasses (air, oxygen) through chemical reactions which result in surface oxidation.
- “Wet Corrosion” is the direct attack on a metal by an aqueous solution (strong or dilute, acidic or alkaline) through electro-chemical reaction. Moisture and oxygen can act by themselves.
DC GALVANIC CURRENTS:
The underwater metal alloys on a boat together with the minerals in the surface water in which the boat is floating create the elements of a “galvanic cell” (a battery). Galvanic currents will always be generated when a boat with dis-similar metals occupies water containing dissolved minerals. A zinc/copper galvanic couple (common “dry cell” flashlight battery) is a “galvanic cell.” A “lead/acid” automotive or boat battery (wet cells, AGM or Gel) are examples of “galvanic cells.”
When the metals making up the galvanic cell (battery) are actually the underwater component parts of a boat (bronze, aluminum, stainless steel), the naturally-occurring galvanic currents result in corrosion of some of the underwater metal. The mineral concentration of sodium, calcium and magnesium salts and many others in the surface water affect the speed at which galvanic corrosion proceeds.
The flow of electrons in a DC galvanic current is always from a more active metal (anode) to a less active metal (cathode) on a Galvanic Series. All environmental surface water, whether fresh or salt, acts as an electrolyte. Salt water carries more mineral ions than fresh water, so is more “efficient.”
Galvanic corrosion is a slow process that occurs over many months. Since it’s the anode in a galvanic cell that dissolves, the point of avoidance/remediation is to artificially force the metal(s) to be protected relatively more cathodic compared to a sacrificial metal present in the electrolyte (water). This is done by adding a “sacrificial anode” made of a very active metal (aluminum, magnesium, zinc) to the mix of less active but more valuable underwater metals on a boat.
Perhaps a ”before” and ”after” view:
Figure 1 shows a ”before view” of a galvanic couple lying in seawater. The stainless steel (SS) is the anodic alloy, so it erodes due to the natural galvanic voltage between it and it’s cathodic bronze couple-mate.
Figure 2 shows an ”after view” of the same galvanic couple with the addition of a sacrificial zinc anode. The zinc forces the SS relatively more cathodic (relative to the zinc), so the SS part is now protected from corrosion. The zinc is the most active (anodic) metal in this new couple. By corroding, the zinc acts to protect all of the more “valuable” metals.
Ongoing zinc maintenance is required to provide continuing protection of the more important components of the couple. Additional valuable corrosion control techniques include the installation of galvanic isolation devices in shore power ground conductors, cable TV coax ground sheathes, and the ground conductors of (now pretty much obsolete) wired telephone and wired Ethernet connections.
ENGINE COOLING SYSTEMS AND GALVANIC CORROSION:
The propulsion and genset drive engines on most cruising-sized pleasure craft are fit with two-stage engine cooling systems. In diesel cooling systems, a coolant (“fresh water”) circulates through the engine block, heads, oil-cooler, turbo-charger, and intercooler. A heat exchanger transfers waste heat from the fresh water coolant to environmental raw water, where it is eliminated via the raw water exhaust. Commonly, a second heat exchanger transfers waste heat from transmission fluid into exhausted raw water.
Electro-chemically, the raw water passing through the heat exchanger is an electrolyte. Heat exchangers contain several different alloys of copper and nickel. The alloys used in heat exchangers are designed to have galvanic voltage potentials that are close to one another on the salt-water galvanic series. That greatly slows, but does not stop, the galvanic corrosion which occurs within heat exchangers. The dissimilar metals of the heat exchanger act as the galvanic couple and the raw water is the electrolyte.
If galvanic corrosion in heat exchangers is allowed to continue uninterrupted, pinpoint leaks will develop in the shell or tubes of the exchanger. Similarly, pinpoint leaks can develop in raw-water cooled oil coolers, transmission coolers and intercoolers. The result over time is damage to expensive heat exchangers, as well as the possibility of secondary damage to the engine itself. Boat owners must be aware that there are zincs located within the raw water channels in engines and heat exchangers.
Boats with wood, steel and aluminum hulls require special anti-corrosion techniques. Many sacrificial anodes are required to protect the surface area of metal hulls. Too many anodes can cause paint to peel from a metal hull, and cause damage to the woods of a wooden hull. Alternatively, systems such as Electro-Guard (http://electro-guard.com/index.html) apply a voltage to a metal hull. These “Impressed Current Cathodic Protection” (ICCP) systems protect the hull plates and welded joints from galvanic attack by making the hull cathodic to its surrounding environment. This is one of many areas that are “different” for owners of metal-hulled boats vs hulls of fiberglass reinforced plastic (FRP).
SINGLE-METAL GALVANIC CORROSION:
SS, bronze, brass and galvanized steel are metallic alloys that contain several elemental metals within their compounding mix. Dissimilar metals within the alloy can experience galvanic corrosion. “Single metal” corrosion results in micro-fractures in the material’s structure, and often results in surface pitting. The process can proceed to structural failure.
Anodic and cathodic areas form on the surface of alloys due to surface imperfections in the alloy mix, lack of oxygen and/or other environmental factors. The anodic areas in the matrix give up electron(s). The ions left behind form into the visible hydroxyl oxidation residue that is shed. Corrosion currents flow at the expense of the anodic metal of the circuit, which corrodes continuously.
SS shaft logs and propeller shafts, SS rudders and rudder posts, SS fasteners that attach swim platform brackets to an FRP hull, SS keel bolts, SS exhaust port fasteners, etc, etc, are all candidates for a form of single metal galvanic corrosion called “crevice corrosion.”
In brass that contains more than 15% zinc, like the manganese bronze alloy often used in propellors, unprotected fittings can undergo a single metal galvanic corrosion process called “dezincafication.” Zinc within the brass alloy erodes away, leaving behind a weak matrix of copper and small percentages of other metals (such as nickel, chromium, manganese) of the original casting. What’s left is structurally weak and can fail catastrophically. “Dezincification” leaves a characteristic “pinkish” color to what once had a golden bronze color; particularly so in broken, exposed areas of a part.
In stainless steel, this process is called “CREVICE CORROSION.” In aluminum, the analogous process is called “POULTICE CORROSION.” When stale water lies against stainless steel for long periods or time, the water looses it’s content of dissolved oxygen. Oxygen-depleted water in prolonged contact with stainless steel promotes crevice corrosion, leading to possible structural failure in stainless steel parts and fittings. Similarly, water that lies in contact with aluminum for long periods of time promotes poultice corrosion. Poultice corrosion can result in pinpoint leaks in aluminum fuel tanks.
For thruhulls especially, boaters should use fittings of bronze or Marelon; BRASS FITTINGS SHOULD NEVER BE USED UNDERWATER.
For those interested, I have more details on Galvanic Corrosion and the Galvanic Series for salt water on this website, here: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-dc-topics/zincs-and-galvanic-corrosion/.
BoatUS has a good article on electrochemical corrosion on their website, here: http://www.boatus.com/boattech/articles/marine-corrosion.asp.
David Pascoe has a good article on electro-chemical corrosion on his website, here: http://www.yachtsurvey.com/corrosion.htm.
WHAT THE BOAT OWNER CAN DO: DC GALVANIC CURRENTS:
- Install a complete bonding system if one is not currently present.
- Install zincs to protect bonded underwater metals.
- Perform routine maintenance of zincs on underwater metals: propellor shaft, rudder, and other underwater metal structures.
- Maintain the “master” zinc that protects the boat’s bonding system.
- Maintain zincs protecting engine and transmission cooling system components.
- Use deck fill screw-on covers that are galvanically compatible with under-deck fittings to avoid galvanic corrosion and hidden fuel leaks.
- Install an appropriately rated Galvanic Isolator in the shore power safety ground if one is not already present.
- Install galvanic isolators to telephone, Ethernet and TV Cable feeds that come onto a boat from shore.
DC ELECTROLYSIS CURRENTS:
The source of the voltage that drives the process is what distinguishes a “Galvanic current” from “Electrolysis,” or an “Electrolytic” current. Recall that galvanic voltages are a function of the natural atomic electro-potential of the metals of a galvanic couple. The voltages which cause electrolysis are man-made, not naturally-occurring. The voltages that drive electrolytic corrosion are often significantly larger than galvanic voltages, and the destructive impact of a DC fault causing electrolytic damage is much faster and more aggressive than galvanic currents.
In Figure 3, the metallic actors (SS and bronze alloys) are the same as shown in Figure 1. In this case, the elemental voltage polarity of the couple has been reversed by the application of an outside source of DC voltage. This is a DC fault scenario. The bronze thruhull in this example will disintegrate, freely giving up it’s copper content into the surrounding sea water.
Electrolysis voltages originate with an externally- supplied DC source; i.e., a battery or its equivalent. Causes can be a wiring error, chafed/frayed DC conductor, defect or age-deteriorated insulation on a bilge pump B+ wire lying in bilge water, a defect in a DC power supply or a DC generator, a failed piece of DC equipment or misapplication of use of DC equipment. A common wiring error that can lead to electrolysis currents results from incorrectly wiring the neutral return circuit of a DC device to the boat’s bonding system. NEVER USE THE DC BONDING SYSTEM FOR THE ELECTRICAL RETURN PATH FOR DC CIRCUITS.
DC electrolysis currents are equivalent to the industrial process called “electroplating.” In a marina, failed DC equipment can deliver a DC voltage into the basin water. On a boat, wiring error or a failed piece of equipment can apply a DC fault voltage to the boat’s ground buss. Electrolysis current flows IN ONE DIRECTION through the ground path and into the surrounding basin water. The anode literally dissolves. The fault can be on the same boat as the failed equipment, on a neighboring boat or in nearby land based equipment. Or it can be, simply, in between a source point and a return point. The fault can be located in shore-side infrastructure wiring, or it can be because of misuse of equipment by a contractor, such as a welder or a DC motor on a marine railway or travel lift.
It is a law of physics that electric current always seeks the path of least-resistance back to their source. Scenario: imagine three adjacent slips on a dock. In slip #1 is a boat with a fault and dumping a DC electrolysis current into the basin water. Slip #2 is empty. In slip #3 is a boat providing a path to ground for the fault current via it’s shore power cord. So far, only boat #1, the boat with the fault, has a corrosion problem.
Now a transient boat comes into slip #2. The fault current previously passed through slip #2 on the way to ground, but when the transient arrived, that boat’s protective electrical system (Bonding System) becomes inserted into the path of the fault current. The transient’s bonding system has a lower resistance than the surrounding basin water. The fault current passes into the transient via one or more underwater metals, passes on through the transient’s bonding system, and exits back into the basin “on the other side” of the transient. The fitting(s) where the current exits the transient will corrode. That fault current is now causing damage to boat #1 and boat #2. Boat #2 is a true victim, safe if plugged into shore power, potentially damaged if not.
All fault currents are always opportunistic. They simply follow the rules of physics to find the “path of least resistance” back home.
The rate at which metal loss occurs is proportional to the voltage involved and to the many Ohms Law factors that determine the magnitude of accompanying current flow. At its worst case, this process can sink a boat astonishingly quickly (a matter of hours/days), because with large uni-directional electrolytic currents (electroplating), metal mass can erode away from the anodic terminal(s) very quickly. The part that gives up metal mass will ultimately suffer structural failure if the process is not interrupted. If it happens to a thruhull on a boat in the water, the boat will sink. All underwater metals – propellors, rudders, struts, trim tabs and radio ground planes – can be effectively “dissolved” by these stray electrolysis currents.
The best articles I know of for an understanding of this topic are by Capt. David Rifkin, who has good reference articles on his website, here: http://www.qualitymarineservices.net/corrosion.html.
Nigel Calder, Ed Sherman of ABYC and Steve d’Antonio have also written about these phenomena, mostly in fee-based subscription publications like Professional Boatbuilder and Passagmaker Magazine, or in their own for-fee publications.
RAPID ZINC WASTING:
Like all metal corrosion, zinc wasting is a form of electro-chemical corrosion, always due to DC currents. Electrical measurements of the basin occupied by the boat would be necessary to determine which mix of stray currents are present at any given time, but zinc wasting is a DC phenomena, and by far most commonly, a galvanic phenomena.
Boat owners can do their own basin-water testing, but it is not a process I would recommend for the electrical layman. An understanding of the theory of these types of faults, understanding the probes and tools that are necessary, and the skills to correctly interpret test results are necessary. This can be quite confounding, even to experts.
Boat owners that experience corrosion issues would be better served to hire an ABYC CORROSION-CERTIFIED MARINE ELECTRICIAN. That said, skilled and knowledgeable boat owners can do their own DC testing with a silver/silver chloride half-cell. High quality Digital Voltmeters (DVM) can detect AC ground fault currents, DC galvanic currents and electrolysis currents, but detection and evaluation is highly specific and sensitive to the placement of the measurement electrodes, quality of the test equipment, and conductivity of the surrounding basin water.
By the time a layman has bought the tools, learned to use them, and learned to interpret the results, said layman would be better off financially in hiring a professional who could provide the diagnosis and remediation recommendations as a one-time service.
WHAT THE BOAT OWNER CAN DO: DC ELECTROLYSIS CURRENTS:
- Diligently avoid having DC wiring wetted or submerged in bilge water.
- Never use the boat’s bonding system as a B- “neutral” return circuit for DC attachments.
- Avoid facilities (marinas, municipal or private docks, boatyards, etc) where the infrastructure appears to be poorly maintained.
- Be alert in marinas located in industrial neighborhoods where ground fault currents from shore sources may be more likely; check with the dockmaster for known issues in the basin.
- Avoid facilities with numerous boats that are in a poor state of maintenance and repair.
CHEMISTRY OF GALVANIC COUPLES:
CHEMICAL REACTION AT THE ANODE:
Oxidation occurs with the release of electrons and the simultaneous shedding of positively charged metal atoms which detach from the surface of the metal. These particles enter the electrolyte solution as positively charged ions. Chemically:
Fe → Fe++ + 2e– (example with Iron);
Zn → Zn++ + 2e– (example with Zinc);
Pb → Pb++ + 2e– (example with lead).
CHEMICAL REACTION AT THE CATHODE:
Free electrons reach the cathode and react with hydrogen ions in the electrolyte. Hydrogen bubbles will often form on the cathode; clearly visible in lead/acid batteries. Chemically:
2H+ + 2e– → 2H
CHEMICAL REACTION IN THE ELECTROLYTE:
If acid is not available, water itself will break down (dissociate) to generate hydrogen ions (H+). The specific chemistry here depends on the composition of the electrolyte. Assuming water, water dissociates, forming free hydrogen and hydroxyl ions:
H2O ⇌ H+ + OH–
Then, metal ions combine to form metallic oxide, which is the corrosion product:
Fe++ + 2(OH)– → Fe(OH)2, or
Zn++ + 2(OH)– → Zn(OH)2, or
Pb++ + 2(OH)– → Pb(OH)2