Marine Corrosion 101

Understanding a bit of science will help you protect your boat.

By Ed Sherman

Photo of outdrive corrosionWhich of the three types of corrosion caused the damage to this outdrive? The answer depends upon how long it took to get into this condition. (Photo: Peter Spang)

If ever there were a corner of the nautical universe shrouded in mystery, mystique, and just plain lies and misinformation, it has to be the galaxy labeled marine corrosion. Contrary to popular belief, this really isn't witchcraft, and it can all be explained by employing some fairly basic science.

Whether corrosion is a problem or not is largely a function of time. If an acceptable service life has not been reached before the material fails, then there's a problem. Time is also the key factor in determining which of the three major types of corrosion is occurring and what you have to do to fix the problem.

You'll notice that I will never use the term "electrolysis" in this article. That's because electrolysis has become a catchall phrase that is not specific enough to be helpful. In this article, we'll put to rest some common misunderstandings by looking at the different types of corrosion that can occur on boats, how to protect your boat's metals from corrosion's effects, and how to recognize if you have a serious corrosion problem.

Types Of Corrosion And Causes

The differences between the types of corrosion we experience on our boats has to do with how the corrosion occurs and how quickly the metal is compromised.

Simple Corrosion

The degradation of metal as molecules on the surface combine with oxygen to create a more stable metal oxide. Over time, almost all metals will corrode to some degree with exposure to oxygen in the presence of moisture — think of the red discoloration of rusted iron or the gray dust that forms on bare aluminum. In some cases, as with aluminum and stainless steel, the metal oxide coating actually protects the underlying metal by stopping oxygen from reaching the base metal beneath. Simple corrosion takes years or decades to weaken the metal, and it is generally not considered problematic. A bronze propeller that slowly corrodes over 30 years of service is not a cause for alarm.

Galvanic Corrosion

Occurs when two metals with different electrical potentials are connected together and submerged in a common electrolyte pool. Examples include anything from a stainless steel bolt threaded directly into an aluminum mast regularly doused in saltwater, to a bronze propeller on a stainless steel shaft immersed in seawater. In both situations — and dozens more on boats — dissimilar metals in a common electrolyte create a battery with an anode and a cathode. Which metal is the anode and which is the cathode depends upon their relative electrical potential, or voltage, when submerged in seawater (Table 1). The metal with the more negative voltage has an excess of electrons relative to the other metal, and it will act as the anode, sending negatively charged ions to the cathode, the metal with the higher (less negative charge) voltage. Over time, that ion flow will result in the loss of material in the anode. How quickly this occurs depends upon how large the difference in electrical potential is between the two metals, or how far apart they are on what is called the galvanic series of metals. The farther apart, the faster corrosion occurs. So platinum will do a number on zinc in no time at all. To be comparable, electrical potential has to be measured using the same reference cell (a silver-chloride electrode) with a standard digital multimeter in water within a specified range of temperature, salinity, and water flow rate. If left unchecked, galvanic corrosion can cause serious damage in a matter of months, so your boat must be protected against it by using sacrificial anodes as described in the next section.

Table 1. Galvanic Series Of Metals In Sea Water With Reference To Silver/Silver Chloride Reference Cell [Sea water flowing at 8 to 13 ft./sec. temperature range 50 degree F (10 degree C) to 80 degree F (26.7 degree C)]


Anodic or Least Noble
Corrosion Potential Range in Millivolts
*The range shown does not include sacrificial aluminum anodes. Aluminum alloy sacrificial anodes are available that have a maximum corrosion potential of -1100 mV.
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)-600 to -710
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% Mo0.0 to -100
Titanium-50 to +60
Hastelloy C-30 to +80
Stainless Steel Shafting (Bar) (UNS 20910)-250 to +60
Platinum+190 to +250
Graphite+200 to +300

Electrolytic Corrosion

More commonly called "stray current" corrosion, adds an external electrical source to the corrosion equation, rapidly accelerating the reaction. It occurs when metal with an electrical current flowing into it is immersed in water that is grounded (which would include any lake, river, or ocean). This can happen if a short develops between an external current source (almost always the 12-volt electrical system on your boat or someone else's) and some part of the electrical system that is tied into the boat's underwater metals. The stray current will exit the boat from an underwater metal fitting, which can be several orders of magnitude higher than the "natural" voltage levels shown in Table 1. The result is a very rapid reaction that can cause profound metal damage in a matter of days or even hours.

Crevice Corrosion

An all too common type of corrosion that affects stainless steel. Simply put, stainless steel is not particularly stainless unless it is exposed to oxygen. Only in the presence of oxygen does an oxide coating form on the surface of the metal; it's this coating that mitigates rust or corrosion. In situations where stainless steel gets deprived of oxygen but moisture is still present, crevice corrosion will occur. This can happen, for example, when a chainplate starts to leak, introducing water into the space where the chainplate passes through the deck — an area where there is very little oxygen.

Photo of stray current corrosionStray current corrosion can destroy underwater metals in a matter of days.

Protecting Your Boat From Galvanic Corrosion

Protecting the metals you don't want to corrode means turning them into the cathode of the corrosion "battery." We can do that by providing a sacrificial anode that is less noble (lower on the galvanic table) than the rest of the metals on the boat. This would all be relatively straightforward if we had only one metal to protect. But most boats have a variety of metals, many of them under the waterline, each with its own voltage. Aluminum alloys fall at the least noble end of the spectrum while stainless steel falls at the most noble end. To make it easier to turn all of these disparate metals into a single cathode, American Boat & Yacht Council (ABYC) Standard E-2 recommends tying underwater metals together using an 8 AWG green insulated wire in a process known as bonding.

Bonding allows all of the dissimilar metals to achieve an equalized potential (voltage). By connecting all of the bonded metals to an adequately sized anode or anodes that have a voltage potential of at least -200mV relative to the corrosion potential of the least noble metal being protected, we create a cathodic protection system. This is what the anodes on your boat do — give up their lives for the good of their more noble metal mates.

Bonding can be controversial. Why? One reason is that a DC fault into the bonding system could lead to electrolytic corrosion. Theory says that if there were a DC fault into the bonding system, it would migrate equally throughout the bonding system due to its designed low electrical resistance. This would cause voltage potentials at all points within the system to equalize, which means no current could flow. If there is no current flow, then there can be no corrosion. Sound theory. But many bonding systems in the real world are a corroded mess. My contention is that things may not be quite as "equal" as theory would wish, so voltage potentials could vary, and electrolytic corrosion could occur.

Note that it certainly is possible to protect a boat without bonding the metals together. But this method runs the risk of some other metal than the one you intended acting as the sacrificial anode — a thru-hull made of a lower-quality bronze alloy, for example. It's all great fodder for an interesting debate about bonding versus no bonding. I've owned boats that were bonded and boats that were not. I've never had a problem in either case because I kept track of my sacrificial anodes and conducted periodic inspections of my boat's underbody.

Sometimes sacrificial anodes can't get the job done. This could be because the differences between the voltages of the anode and cathode (the metals to be protected) are too close or because the area to be protected is large relative to the boat. In these cases, impressed current systems measure potentials in real time via a sensor, and a non-sacrificial anode emits adequate current from the boat's battery to protect the metals connected to the system. Aluminum sterndrives are a good example. In the early years after they were introduced, many had serious corrosion issues. Today, to supplement the protection from sacrificial anodes, the Mercury Marine sterndrives are protected by the Mercathode systems and Volvo uses the Active Corrosion Protection (ACP) system.

Levels of Protection

It used to be the case that most boats were made out of fiberglass, and most underwater metals were bronze except for the prop shaft, which was marine-grade stainless. In that world, zinc was the perfect anode material. How do I know that? If you look at Table 2 below, you'll see that to protect a fiberglass boat, the anode needs a voltage of between -550 and -1,100 mV. If you compare that to Table 1, you'll see that zinc has a reference voltage of -980 to -1030 mV, right within the specified range. At -240 to -310 mV, all bronze alloys are significantly above zinc on the cathodic scale, and shaft-grade stainless is even more noble, at -30 to 130 mV. So zinc will offer plenty of protection. In fact, until just a few years ago, sacrificial anodes were called "zincs," and many boaters still use that term. But our world has gotten a lot more complicated.

Zinc anodes have actually begun to fall out of favor in recent years. Zinc works just fine in a true saltwater situation. But as water becomes more brackish to fresh, zinc becomes less effective. In fresh water, it actually forms an oxide on its surface that stops it from working as a sacrificial anode. The two other anode materials that have come to the fore in recent years are magnesium and aluminum.

Magnesium is the most expensive anode material but also the least noble metal on the list, so it runs out of electrons quickly; in fresh water it lasts only about a third as long as zinc. It can also overprotect other metals that are chemically active, like aluminum, creating too much current, especially in such chemically active water as polluted fresh water or saltwater. The reaction between the aluminum and the magnesium can even result in an alkaline solution that will start eating away at the aluminum. Magnesium should be used only in clean fresh water, never in brackish, polluted, or saltwater.

Aluminum anodes, on the other hand, will work nicely in both salt and brackish water. That's because the alloy used in anodes includes iridium and other metals that interfere with the oxidation of the aluminum. This is important because many people who keep their boats in the water in coastal communities are often migrating from pure saltwater into brackish and even fresh water on a daily basis. The aluminum alloy in high quality anodes will protect aluminum hulls and sterndrives, so follow your manufacturer's recommendation when replacing anodes. So once you decide what anode material will work best with your hull material and boating environment, how do you know how many anodes you need? And how do you determine if your boat's cathodic protection system is in order?

If you own a good multimeter or Digital Volt Ohm Meter (DVOM), then you already have one of the key pieces of equipment you'll need. You'll also need the silver-chloride reference electrode mentioned earlier. You really can't measure your boat's hull potential accurately without one, at least not using the potential values used throughout ABYC's E-2 standard. BoatZincs.com is a good source for a reference cell. The cell costs about $125 and includes a publication that explains how to hook it up to perform tests and what to expect in your readings. I think this is money well spent.

Table 2. Recommended range of cathodic
protection for boats of different hull
materials in saltwater

Hull MaterialMillivolt Range
Fiberglass-550 to -1100
Wood-550 to -600
Aluminum-950 to -1100
Steel-850 to -1100
Non-metallic w/Aluminum drives-950 to -1100

Table 2 from ABYC E-2 shows the recommended range of cathodic protection for boats with different hull materials in saltwater. Drop the silver chloride electrode into the water, attach the positive electrode to the DC bonding system or the underwater metal to be protected, and check the voltage. If the reading is higher (less negative) than shown in Table 2, then you need more anodes. Once the maximum negative voltage potential for the anode material in use is reached as shown in Table 1, adding more anodes will increase anode life but will not have any impact on voltage.

If Protection Fails

In spite of your best efforts, sooner or later you may be the victim of underwater metal corrosion that is caused by something other than your boat or the level of cathodic protection you have provided. This is where the experts on every dock weigh in with not-so-expert advice that is sure to drive you crazy!

Of course, the obvious question is how do you know if you even have a problem? Without the proper measurement equipment, the only way to judge is visual. Keep in mind that if things were done correctly before a spring launch, you should expect to get a full season's use from your sacrificial anodes. If you suddenly start running through anodes every four weeks, don't jump to conclusions. The appendix of ABYC E-2 includes a list of things that can alter anode consumption rate.

If environmental conditions haven't changed, start looking for signs of galvanic corrosion. The first sign is paint blistering (starting on sharp edges) below the waterline, and a white powdery substance forming on the exposed metal areas. As the corrosion continues, the exposed metal will become deeply pitted.

Before it gets to that point, you need a genuine expert with the proper training and some specialized tools to make sure you get a solid diagnosis of the problem(s) that may be causing either accelerated anode consumption or serious corrosion. The ABYC has a list of certified corrosion specialists that is searchable by state at its website. This should be your first stop in my opinion. Go to www.abycinc.org and use the "Certified Tech Search" button on the home page. 

— Published: July 2015

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