“Galvanic Corrosion” is a normal, predictable and definitely unwanted electro-chemical phenomenon. However, it does not have to be inevitable! The science of Galvanic Corrosion spans chemistry, physics and electricity. There is an entire body of specialty engineering knowledge on the subject arising from the electrical utility, railroad, shipping and pipeline industries. For boater’s, there are at least four ABYC Standards that relate to it:
- A28, “Galvanic Isolators,” July, 2008,
- E2, “Cathodic Protection,” July, 2013,
- E11, “AC and DC Electrical Systems on Boats,” July, 2012, and
- TE4, “Lightening Protection.” July, 2006.
This post is written as an introduction to the key terms and concepts of Galvanic Corrosion. It is written for boaters and others who have little or no prior electrical background. In struggling with the technical concepts and unfamiliar terminology used to describe them, one may find that skilled people slip almost casually from one contextual use of terms to another. For beginners and bystanders, that can be confusing, confounding and frustrating. My hope is this post will wrap some perspective and understanding around the topic.
There is enormous confusion about corrosion among boaters. There are two very different causes of corrosion that boaters face: 1) galvanic corrosion and 2) “electrolysis,” or “stray current” corrosion. Galvanic corrosion is virtually universal – by far the most common – and is the subject of this article. However, many of the technical concepts, terminology and materials are the same between the two. The big difference between Galvanic Corrosion and Electrolytic Corrosion are their very different cause. Symptoms of the two can be the same, but can be very different. Galvanic corrosion always happens slowly, over the course of months. “Electrolytic” (DC stray current” corrosion) can happen astoundingly fast, with catastrophic failures in a matter of hours. The techniques that slow and stop galvanic corrosion may or may not work with stray current corrosion.
For boats, of course, galvanic corrosion is a real and ever-present concern simply because boats float in environmental surface water which contains varying degrees of impurities (dissolved minerals, salts, organic materials and chemical pollutants). Residential electricians have little to no familiarity with galvanic corrosion issues. Corrosion is a complex and challenging electrical sub-specialty.
If experiencing corrosion on boat metals, or if corrosion symptoms have noticeably changed in recent days/weeks, boat owner/operators are advised to engage a “certified” marine corrosion expert. Do not, under any circumstances, rely on a residential electrician to diagnose this phenomenon. For non-professionals, galvanic corrosion is a somewhat obscure topic. It is generally not familiar to people who live in – or provide professional services to – residential single family homes or multi-family buildings. Electrochemical corrosion is generally beyond the experience of the general public.
In any discussion of “Galvanic Corrosion,” there are several inescapable and frequently-encountered terms one must know and understand.
- “Galvanic Table,” or “Galvanic Series.” Because of the atomic structure of metals, every metal has a natural electrochemical ‘static charge,’ or “electrical potential.” The magnitude of this electrical potential is unique to the particular metal. Each metal’s electrical potential is different from every other metal. The electrochemical “galvanic potential” of metals is determined by measurement against a “reference cell.” Reference cells of different chemical composition can be used for different purposes. The reference cell most commonly used for marine purposes in surface water containing varying amounts of dissolved minerals is a silver/silver chloride (Ag/AgCl) half-cell. The “galvanic series” is a table sorted based on the electro-physical charge potentials of metals against the reference half-cell. Alloys of metals also carry unique electro-potentials. The proportions of metals that make up an alloy will affect the absolute magnitude of the electrical potential of the resulting alloy.
- “Galvanic Couple.” Any two of the different metals and alloys in the galvanic series are referred to as a “galvanic couple.” The difference in electro-chemical potentials between any two metals in the galvanic series represents a voltage that can be measured with a high quality Digital Multimeter (DMM). When several metals are all joined together in an electrical network – as is the case of props, prop shafts, trim tabs, thru hulls all bonded together – they are referred to as a “galvanic collection.”
- “Anode,” “cathode,” “anodic” and “cathodic.” These terms are always used in a particular context. The context will either be a description of:
- the position of a specific metal in the galvanic series; that is, reference to the “Galvanic Potential” of a specific metal or alloy, or
- the relative relationship of one metal to another on a “Galvanic Series of Metals,” or a “Nobility Scale.” For example, “zinc is anodic referenced to bronze,” and “bronze is cathodic referenced to zinc.”
- In all electrically connected galvanic couples and galvanic collections, one metal will be “anodic” to the other(s). Electrically, the anode gives up electrons to the galvanic system. More importantly, the anode also sheds positive ions of its metallic structure into the surrounding electrolyte solution (surface water containing dissolved minerals). That shedding is the galvanic corrosion we observe as boaters.
The concepts these terms represent are fundamental to understanding galvanic corrosion. Relative to any “Galvanic Series,” or “Nobility Scale:”
- “cathodic” metals are highly “noble,” or considered “passive” metals. Compared to an Ag/AgCl half-cell, their natural electrochemical potential is more positive with respect to other metals in the galvanic series. They are naturally more resistant to galvanic corrosion. Examples include titanium, gold and graphite.
- “anodic” metals are “less noble,” or considered “active” metals. Compared to an Ag/AgCl half-cell, their natural electrochemical potential is more negative with respect to other metals in the galvanic series. They are moderately to highly subject to galvanic corrosion. Examples with which all boaters are familiar include magnesium, aluminum and zinc, all used as “sacrificial anodes” on boats.
So, context is critical to understanding.
Metals incur galvanic corrosion only when they are in electrical contact with other metals. Therefore, galvanic corrosion should always be thought of as involving two or more dissimilar metals; that is, a “couple” or a “collection.”
For “Galvanic Corrosion” to occur, three conditions are necessary:
- Metals must be “well separated” – moderately or greatly – on the “Galvanic Series:”
The larger the galvanic potential difference between the metals involved, the greater the probability of galvanic corrosion, and the faster that corrosion will progress.
- The metals must be electrically connected together:
The metals can be wired together, or pressed, riveted, bolted, welded, clamped, or even piled-upon each other. Normally on boats, galvanic corrosion occurs when metals that are bonded together, so the path connecting them is low resistance. However, galvanic currents can and do flow when only high resistance paths connect two dissimilar metals.
- Both metals must be simultaneously immersed in an “electrolyte:”
An electrolyte is an electrically conductive medium. The electrolytic medium acts to complete the electrical circuit. If the conductivity of the medium is high, the metal-to-metal corrosion of the less noble metal will be dispersed over a larger area. If the conductivity of the electrolytic medium is low, the corrosion will be localized to the part of the less noble metal nearest to the mechanical connection between the metals. Sea water is an excellent electrolyte, brackish water, less, fresh water, not so much.
When all of the above conditions are met, a difference in “galvanic potential” (a voltage) exists between the different metals in the electrolyte solution. That voltage is the driving force for electrons to flow from one metal to the other metal through the electrolyte. This current results in positive charged ions of the anodic metal of the couple being shed into the electrolyte. Inside a carbon/zinc dry cell battery, when the anode (zinc) is fully depleted, the battery is thought of as “dead.” Between metals in any mechanical system, this process is thought of as “destructive galvanic corrosion.” Be aware, “in any mechanical system” in a boat IS NOT limited to prop shafts and propellors. This deterioration can and does occur inside engines and transmissions, and in metallic structures like potable water and fuel tanks, thru hulls, seachests and strainers, etc.
SILVER/SLIVER CHLORIDE GALVANIC SERIES:
The following table shows galvanic potentials of many common marine metals in free-flowing sea water as measured with a silver/silver chloride reference cell:
|TABLE I – GALVANIC SERIES OF METALS IN SEA WATER WITH REFERENCE TO SILVER/SILVER CHLORIDE REFERENCE CELL [Sea water flowing at 8 to 13 ft./sec. (except as noted), temperature range 50°F (10°C) to 80°F (26.7°C)]|
|(ANODIC OR LEAST NOBLE) CORROSION-POTENTIAL||RANGE IN MILLIVOLTS|
|Magnesium and Magnesium Alloys||-1600 to –1630|
|Zinc||-980 to –1030|
|Aluminum Alloys||-760 to –1000|
|Cadmium||-700 to –730|
|Mild Steel||-600 to –710|
|Wrought Iron||-600 to –710|
|Cast Iron||-600 to –710|
|13% Chromium Stainless Steel, Type 410 (active in still water)||-460 to –580|
|18-8 Stainless Steel, Type 304 (active in still water)||-460 to –580|
|Ni-Resist||-460 to –580|
|18-8, 3% Mo Stainless Steel, Type 316 (active in still water)||-430 to –540|
|Inconel (78%Ni, 13.5%Cr, 6%Fe) (active in still water)||-350 to -460|
|Aluminum Bronze (92% Cu, 8% Al)||-310 to -420|
|Nibral (81.2% Cu, 4% Fe, 4.5% Ni, 9% Al, 1.3% Mg)||-310 to –420|
|Naval Brass (60% Cu, 39% Zn)||-300 to –400|
|Yellow Brass (65% Cu, 35% Zn)||-300 to –400|
|Red Brass (85% Cu, 15% Zn)||-300 to –400|
|Muntz Metal (60% Cu, 40% Zn)||-300 to –400|
|Tin||-310 to –330|
|Copper||-300 to –570|
|50-50 Lead- Tin Solder||-280 to –370|
|Admiralty Brass (71% Cu, 28% Zn, 1% Sn)||-280 to –360|
|Aluminum Brass (76% Cu, 22% Zn, 2% Al)||-280 to –360|
|Manganese Bronze (58.8% Cu,39%Zn,1%Sn, 1%Fe, 0.3%Mn)||-270 to –340|
|Silicone Bronze (96% Cu Max, 0.80% Fe, 1.50%Zn, 2.00% Si, 0.75% Mn, 1.60% Sn)||-260 to –290|
|Bronze-Composition G (88% Cu, 2% Zn, 10% Sn||-240 to –310|
|Bronze ASTM B62 (thru-hull)(85%Cu, 5%Pb, 5%Sn, 5%Zn)||-240 to –310|
|Bronze Composition M (88% Cu, 3% Zn, 6.5% Sn, 1.5% Pb)||-240 to –310|
|13% Chromium Stainless Steel, Type 410 (passive)||-260 to –350|
|Copper Nickel (90% Cu, 10% Ni)||-210 to –280|
|Copper Nickel (75% Cu, 20% Ni, 5% Zn)||-190 to –250|
|Lead||-190 to –250|
|Copper Nickel (70% Cu, 30% Ni)||-180 to –230|
|Inconell (78% Ni, 13.5% Cr, 6% Fe) (passive)||-140 to –170|
|Nickel 200||-100 to –200|
|18-8 Stainless Steel, Type 304 (passive)||-50 to –100|
|Monel 400, K-500 (70% Ni, 30% Cu)||-40 to –140|
|Stainless Steel Propeller Shaft (ASTM 630:#17 & ASTM 564: # 19)||-30 to +130|
|18-8 Stainless Steel, Type 316 (passive) 3% Mo||0.0 to –100|
|Titanium||-50 to +60|
|Hastelloy C||-30 to +80|
|Stainless Steel Shafting (Bar) (UNS 20910)||-250 to +60|
|>Platimium||+190 to +250|
|Graphite||+200 to +300|
|(CATHODIC OR MOST NOBLE)|
|†The range shown does not include sacrificial aluminum anodes. Aluminum alloy sacrificial anodes are available that have a maximum corrosion potential of -1100 mV.|
|1. Metals and metal alloys are listed in the order of their potential in flowing sea water as determined in tests conducted by a nationally-recognized corrosion research laboratory.|
|2. The galvanic series may be used to predict whether galvanic actions are likely between two metals. Other factors (e.g., area of the material, flow rate, composition of the electrolyte, crevices, the coupling of copper alloys with aluminum, etc.) affect the relative corrosion rates in seawater.|
Source: American Boat and Yacht Council, Standard E-2, July, 2013, Page 10.
A boat with underwater metal fittings in the water is a natural battery (a “galvanic cell”). That natural battery produces a very small DC voltage between the under-water “cathodic” and “anodic” metals of the couple/collection. The water in which the boat is floating is the necessary “electrolyte” in this natural battery, and the dissimilar metals of the propeller, drive shaft, reduction gear/transmission, rudder, thruster components, outdrives, trim tabs, thru-hulls, radio ground plane, speed and sounder sensor bodies, etc., etc., etc., are the relatively anodic (+) and relatively cathodic (-) terminals of the battery.
Electrons carry a negative electrical charge. Galvanic electric currents consist of a “flow of electrons” out of, and back into, the galvanic cell created by your boat. In a common galvanic corrosion scenario, the flow of electrons that make up a galvanic current originate in the under-water anodic (+) metals of the boat, flow through the electrolyte (water), and return to the under-water cathodic (-) metals of the boat. Part of the mechanism of these small DC currents is the shedding of positive ions of the metal, with the ultimate destruction of the anode, the least “noble” metal of the galvanic collection. Hopefully, that anode will be a sacrificial anodes (zinc) and not the more noble metals of bronze props, aluminum outdrives, steel transmissions, SS rudders, steel thrusters, etc. This flow of galvanic current can be interrupted, and anode wasting (corrosion) stopped, by installing a “Galvanic Isolator” or an “Isolation Transformer.”
The galvanic potential (voltage) that causes the above flow of electrons is determined by the position of the specific cathodic and anodic metals involved in the galvanic series, and a variety of other factors related to metal mass and the physical characteristics of the electrolyte. The flow of electrons from an anodic metal leaves behind a positive ionic charge. To balance that charge, anodic metal ions are shed into the electrolyte. That shedding is the corrosive deterioration we see with sacrificial anodes (zincs).
There are very subtle factors that affect the rate at which anode wasting occurs. These factors vary greatly from place-to-place. Boaters will fit into all of the affected subcategories, so there is simply no “one size fits all” formula. Examples:
- Galvanic currents increase with water circulation over the hull. The protection requirement can be several times that required in still water. Zincs do not have the capability to automatically respond to changing needs as water velocity increases, as active protection devices (“impressed current devices”) do. So a boat in the Beaufort River in Beaufort, SC, or the Ashley or Cooper Rivers in Charleston, SC, may need more protection than the same boat would need in Marblehead, MA, or Miami, FL, or Marsh Harbour, BS.
- The ratio of cathode/anode surface area. The larger the relative surface area of the anode, the lower the galvanic current density on the anode, so the lesser the attack. The amount of galvanic corrosion may be considered as proportional to the ratio of Cathode/Anode surface area.
- Boat use. More frequently-operated boats (cruisers) require more cathodic protection than vessels infrequently used (floating condos, dock mavens). Relates to item 7, following.
- The conductivity of the water. As conductivity increases, the rate of galvanic activity increases. Related to item 5, following.
- Water salinity. Proportionally more protection is required on a given boat in salt water than in fresh water.
- pH of the water. As pH decreases (acid rain fresh water lakes), the cathodic corrosion rate increases.
- Condition of bottom paint. Deteriorating bottom paint increases exposed cathodic surface area, which increases anodic protection requirements.
Furthermore, when connected to shore power, your boat is part of a electrical network of boats – a collection of metals – that are electrically interconnected by the shore power safety ground system. The underwater metals on the collection can dramatically alter the cathodic potential (the amount of protection) of your boat. This is particularly true if your neighbor has aluminum (outdrives, trim tabs) and you do not.
A very common rapid zinc wasting condition occurs when a nearby neighboring boat is poorly maintained. If you have good, well maintained zincs on your boat, but your dock neighbor does not, you will be glad to know that the noble metals of the neighboring boat are protected. Your neighbor’s boat is being protected by your zincs, through your generosity, via the shore power network of shared safety ground connections. Since your zincs are the sacrificial metals in this system, they are likely to deteriorate at a faster-than-normal rate. You may or may not consider this generosity to be a good thing. This is a particularly common situation at marinas where a high number of absentee owners reside.
Occasionally we hear that we need to be careful not to “over-zinc” a boat. It is possible to “over-zinc,” but the term is frequently misused in context. In the context of that statement, “zinc” does not refer to the metallic substance; it uses the term “zinc” as a synonym for “anode;” i.e., a mechanical object. So more properly, one should say, “it’s possible to ‘over-anode’ a boat.”
Anodes used on boats are available in Magnesium, Aluminum and Zinc metals. Magnesium is best for boats kept long term in fresh water. Zinc is best for boats kept long term in brackish and salt water. Aluminum is often regarded as acceptable for use in all types of water. Using the wrong anode material in the wrong environment can reverse the galvanic potential between some dissimilar metals or non-metallic, electrically conductive materials under some conditions. Magnesium anodes should not be used long term in salt water. Aluminum anodes can cause harmful over-protection which may result in cathodic corrosion of aluminum parts (outdrive, trim tabs) and possible hydrogen blistering of paint, also known as cathodic disbondment. Some oxides of a few metals, including aluminum, tin, lead, and zinc, are “amphoteric,” meaning they are capable of reacting chemically in both acid (low pH) and basic (high pH) environments. These metals are more susceptible to corrosion in alkaline high pH electrolytes (fresh water) than other metals.
Conclusions: my PERSONAL OPINIONS:
- In general, most boats are better off having Bonding systems than not having bonding systems. If an owner chooses not to have a bonding system, that should be a deliberate, intentional decision made with great forethought. It should be backed up by thorough, professional cathodic measurement testing and with due consideration to dissipate of static electricity and lightening as a related technical matter.
- In general, absent an Isolation Transformer, all owners of boats that are fit with shore power service should install a Galvanic Isolator. Galvanic Isolators block the flow of galvanic currents. They greatly extend the life of the boat’s anodes, but more importantly, extend the protection of the most noble (and expensive) underwater metals on the boat.
- In general, the best protection an owner can afford their boat investment is to diligently maintain their anodes (“zincs”).
If you aren’t familiar with this language and these advanced electrical and materials concepts, you are in the clear majority of the general public and boat owners! Frankly, only true, insanely devoted electrical geeks venture into these “techno-weeds!” Most electrical service professionals – including the great majority of “marine-certified” electrical service professionals, do not really “get into this stuff.” When faced with an issue, most “professionals” call in “experts” to handle remediation. This is why there can be, and is, a lot of misunderstanding and confusion around this subject.