A Boatman's Primer on the Essentials
by David Pascoe, Marine Surveyor
With 5 photos
The electrical systems on boats have improved sufficiently over the last 20 years that stray current corrosion is much less of a problem. It begins to show up in older boats because of all the jury rigged wiring and systems that get added on over the years. In newer boats, it usually occurs due to ground wiring faults on the dock.
The basic mechanism of stray current corrosion is the same as all other types, only this time the introduction of an outside source of electricity screws up the normal electrical balance of the boats metals, plus adding one other pernicious problem. Energizing all the underwater metals in the boat, that raised electrical potential will seek a path to ground. And rest assured that it will find one. That path will be the piece or pieces of metal below the water line that are the weakest like. It will start with the zincs and once those are destroyed, it will move onto the next lest noble metal. That's usually a poor quality alloy of prop shaft, propeller or through hull fitting. In other words, it's likely to attack one specific piece of metal. As the current leaves the piece of metal on its way to ground, it is carrying bits of the metal with it, as well as the erosion that may occur from part to part.
The owner of this stern drive has a big problem, though he doesn't know it. A dry stored boat, the white stuff all over this drive is zinc oxide from the zinc collars around the hydraulic rams. This is the result of stray current and it won't be long before he's looking at a $3500 bill. This one is less than a year old.
Classic crevice corrosion. In this case caused by a carbon rubber exhaust flapper clamped onto a stainless exhaust flange plate. Notice that the worst corrosion is where the clamp pressed the rubber the tightest.
Stray current is the most difficult corrosion problem of all to identify and correct because the source could anywhere amongst that mass of wiring in your boat. The first thing that must be done is to meter out the dock circuit to make sure that it isn't feed-back from the dock grounding system (i.e. the grounding system is positively charged, which can feed back into your boat), or that the grounding wires are not reversed.
The conditions created by AC and DC current is not the same, with DC current being the most damaging. The reason for this is that AC current is pulse current that moves in two directions, greatly reducing the corrosion potential for reasons I won't get into here. Technically, AC current requires some kind of diode that converts it into DC current before it causes metallic corrosion. There are lots of naturally occurring diodes in crystalline forms of metallic oxides. Aluminum oxide is VERY good at converting AC to DC current. The aluminum oxides that form inside aluminum boats!
Yet AC current may be just as damaging because the voltage is so much higher. If you ever wondered why there is so much misinformation concerning AC current corrosion, this is why. Unless the right conditions exist to covert the AC to DC current, the corrosion does not occur. Thus, only some boats on a dock with faulty AC wiring suffer damage, while others don't. These are almost always older boats where corrosion is already present, as it is the crystalline nature of metallic oxides that exacerbates the rate of corrosion.
AC current corrosion occurs much less frequently, mainly due to the fact that the high voltage is dangerous, and it is treated with more respect. Moreover, if there is a 125 volt leak, the chances are that it's going to be found rather quickly as the boat owner doesn't appreciate the shocks he gets occasionally. Amongst the thousands of boats I have surveyed, the number that I have found with AC ground faults is amazingly few.
DC Current leaks are the most common form of stray current problem. The fact is that any boat that utilizes high quality underwater metals and has a good bonding system can tolerate a fairly high amount of stray current leakage. This is because the low voltage current rather quickly dissipates throughout the system. Like trying to wash your car with a cup of water, there isn't enough to go around to do the job. The small amount that finds its way to underwater metals is usually taken care of by the zincs, or dissipated by a large amount of metal.
Substandard bilge pump wiring is the most common source of stray current. So are batteries mounted in a wet environment, such as sitting on a wet deck. Take a fully charged battery and sit it on the floor of your garage. Come back a month later and measure the voltage. Surprise! It's dead! Where did the power go? Well, right out through the casing and into the damp concrete, that's where. Put that battery up on dry wood blocks and this won't happen. The same thing happens when you have wiring laying in a wet bilge. Plastic, you see, is not an absolute isolator. Just as your fiberglass hull will absorb water, battery casings and wiring insulation will absorb some water too, just enough to leak out a small amount of current. And cause you a lot of problems. Having said that, I needn't say any more about batteries and wiring. You now know what to look for.
Forty years ago, the paint companies tried using pure copper in bottom paint. It stopped marine growth cold, but it turned the entire bottom of the boat into a bonding system. Well, the EPA has us back to copper again, only this time cupric oxides are the toxic agent. Fortunately, it works pretty good. It works even better as a telltale for stray current, as the photo above shows. These oxides are still highly conductive, and still contain not completely reacted copper, so that the paints will corrode. Don't worry about that because there is no material in this world that does not corrode, including you and me.
If you are getting white and/or green halos around your zincs or underwater metals, you have a stray current leak. Even a very small amount of current will cause a paint reaction, so we have a built-in litmus tester here in our bottom paint. You can judge the severity by these photos, as this degree of paint reaction is associated with very rapid wastage of the zinc. Also make note of the fact that differing paint formulations react differently. I've seen some that are so reactive that you'd think the whole boat was being dissolved, and yet only a small current (1/2 volt) created this result with no damage to underwater metals. Therefore, judge the severity by the condition of the zincs, as well as the paint.
The evidence of corrosion appears in one of three forms. The one that we are all familiar with is the appearance of oxides, the byproduct of a metal that has chemically changed. Copper-based metals like brass and bronze leave green oxides, white for aluminum, and reddish-brown for stainless steels. These are the telltales of ordinary oxidation corrosion.
However, we have these other forms of corrosion to deal with, and these have different affects. Galvanism and stray current are abnormal, the result of something that shouldn't be happening. When the current generated by galvanic action is weak, it will generate the usual corrosion byproduct, the oxide of the metal. Stronger galvanic and stray current will more often result in rapid erosion of the metal, usually to the point where there are no oxides present, but will leave an appearance of bright metal. If you see any part of any underwater metal that is showing bright, regard this as a red flag. It's the indicator of very rapid erosion. Examples of this would be eroded, but shiny surfaces on zincs, or propeller blade tips that are bright yellow and showing a crystalline texture or pattern. It will look like frost on the inside of a window, only it is bright yellow.
The effects of galvanism most often occur very slowly with bronze or brass. Here, a condition known as dezincification occurs. Since copper is alloyed with zinc to make bronze, zinc is the weak link and will leach out of the alloy to leave raw copper. The result is metal that is pinkish in color, is granular in texture and tends to crumble when probed. It is soft and very weak. Any pinkish looking copper-based metal is waving a red (pink) flag.
Problems with stainless is typified by the appearance of pits or very coarse tunneling, creating cavities that are very rough, and which have very sharp edges. Stainless can be very quickly destroyed by stray current, so if you see any sign of pitting on things like shafts, rudders or trim tabs, you need to seek out the cause and eliminate it quickly. This is one of the reasons why we don't recommend the use of stainless for any sea water plumbing systems. Screws and bolts underwater will usually end up with rusty looking oxides around them when crevice corrosion is involved, but no telltale oxides when stray current is the culprit.
You can have serious stray current problems and yet your zincs seem unaffected. Once again, understanding this is not easy. First, there are many alloys of zinc -- some are more durable than others. You may have gotten the wrong kind of zinc. Secondly, like aluminum, zincs tend toward self protecting. That means that as the oxide layers build up, the metal becomes insulated from the water, so that the corrosion rate diminishes or stops altogether. This layer can get as much as 1/4" thick. At this point, the zincs have lost their effectiveness. If there is still a large amount of zinc left, you can just scrape off the oxide and the effectiveness will be restored. This condition means that you don't have a stray current problem.
Very rapid zinc loss that results in bright, shiny metal being exposed is a clear indication of electrical activity, be it galvanic or stray current, usually the later, since galvanism rarely creates enough current to destroy zincs quickly . Bright zinc in association with heavily corroded bottom paint means you have a problem that needs to be addressed immediately (see photo above). The brightness of the zinc is telling you that there is too much current for the zincs to handle. Adding more zinc is NOT the solution.
This is how you can tell if your going to have an aluminum fuel tank problem. Sitting on a flat deck that gets wet, here you can see the corrosion stains coming out from under the tank. This one's days are numbered.
Crevice corrosion of through hull bolts. In this case, the water got at the bolts from the inside. The use of nuts with nylon inserts (shakeproof type) accelerated the process. Notice that the acid etching leaves the metal very bright. This is a total lack of oxygen to the cavity, whereas the exposed threads look rusty and had more oxygen available. That's because the nut completely wasted away.
The vast majority of inboard powered and sail boats don't even need zincs. We put a few on as a precautionary measure, and to serve as a telltale should a problem develop. Why? Because on most boats everything is in balance without a lot of dissimilar metals creating electrical current. Do you need to put a zinc on every piece of underwater metal? No, you don't if your bonding system is in good condition. That means that the wire connections are still making a good electrical connection. If your boat is 10 years old and you have never serviced the wire connections, then rest assured that your bonding system is now useless. The wire ends to components like sea cocks, rudders, struts, etceteras, should be serviced every few years. If you have wire splicing tools, it will take you about an hour to redo your entire system.
Stainless trim tabs are the exception to this. Most often tabs are NOT tied onto the bonding system unless they are bolted, rather than screwed to the hull. That's because there's nothing to attach a bonding wire to. The screws are all on the outside.
Over zincing, putting too much zinc, on causes the opposite problem. It will reverse the flow of current in the other direction and actually cause corrosion. Over zincing is discernable when you find your props all covered with a coarse layer of zinc oxide. It will feel like sandpaper. This condition will reduce your boat's speed significantly and increase fuel consumption. I'll bet that statement got your attention!!!
Which is better: using one big zinc on a central bonding system, or putting zincs on rudders, shafts, etceteras? A single large zinc tied to the internal bonding system is unreliable because of the internal corrosion problems to wiring. And you have no way of knowing, short of doing a conductivity test, to know if the system is functional. Surface area of zinc is the most important criteria, and you'll get more surface are with more small ones. Multiple zincs are far more reliable.
The purpose of a bonding system is to equalize the electric potential of dissimilar underwater metals by tying them all together with wire or copper straps. The benefits of a bonding system are wide ranging but little perceived. One is that it serves to dissipate stray current leaks. 12 volts of current focused on a small piece of metal will result in rapid destruction. But that same 12 volts spread over a much larger surfaces, causes less damage in proportion to the size of the water exposed surfaces of the metal. Bonding systems can reduce the corrosion potential of metals inside and on the bottom of the boat. Boat which have all the hardware bonded, such as the railings, will suffer much less corrosion.
As mentioned above, bonding systems are not maintenance free. The wire connections corrode too, and need to be reestablished periodically. This is done by cutting off the old terminal or connection, and then establishing a new one. It's as easy as standing on your head in the bilge ;-)
Corrosion in Systems
Okay, so now well know all about corrosion and the underside of the boat, but we haven't even touched on all those internal systems through which water passes. Yep, I mean the engines, airconditioning, and other expensive stuff like that. Anything that water touches has a potential corrosion problem. Some of these things we can perform preventative maintenance on; for others you wait until the part fails and replace it.
Those metallic components through which water flows also need some corrosion protection. That includes the engines, pumps, A/C units and so on. The metal chassis or housings of things like pumps and air conditioners also need to be tied into the bonding system. On an A/C system, the sea cock, strainer, and compressor chassis all need to be wired together. The same applies to all other metal housing pumps, but not to plastic housing pumps. The primary reason pumps fail so often is because (a) they're not bonded, and (b) they are located in places where they get wet. Pumps tend to have a lot of dissimilar metals in them, which is why bonding is so important. I'd be willing to bet that the pump motors on your heads aren't bonded, which is one reason why you've replaced so many pumps.
The general rule is that anytime a piece of metal plumbing or hardware is isolated in a system, as with a sea strainer that is joined by two hoses is electrically isolated, needs to be wired into the system. This can be done by daisy chaining items together, but it's a good idea not to include too many items in a chain. Obviously, at any point where a connection is broken, all those items upstream will be unprotected.
More On Stainless
Stainless steel comes in a very wide range of alloys. Naturally the best are the most expensive because they contain higher percentages of nickel, which is very expensive. Ergo, builders don't like to pay for this stuff, and that includes all you screws, nuts and bolts. Stainless is quite vulnerable to crevice corrosion, and it's most often to be found on low grade fasteners. The better grades -- this doesn't mean the best and most expensive -- when used for deck hardware do not rust. If you have rusting stainless hardware, it's because it's a low grade. Most corrosion occurs via crevices, such as around screw holes, stanchion bases and sockets, rub rails, etceteras -- any place that can trap water and create a closed cell. There isn't anything you can do about this.
This stern drive cavitation plate was lightly scraped by a fork lift truck. With the aluminum exposed, the metal now begins to erode strictly as a result of stray current which has left the metal bright.
Previously banned, ABYC has recently approved stainless for use as fuel tanks. This is unfortunate for stainless has a very poor track record for use as tanks. Stress, corrosion cracking and welding problems are among the reasons why. Place a stainless tank flat on a deck and the same thing will happen to it as an aluminum tank. Water tanks are even worse; metallurgically altered metallic structure around the welds results in unacceptably high corrosion failure rates. Weld failures in stainless water tanks are legion and can't be stopped.
Stainless piping and other fittings for sea water use is not recommended. Not so much because it isn't any good, but because no one can tell what grade of material it is. If it's a low grade, there always exists the danger of failure. Bronze is easy to make and is not subject to the alloying problems like stainless, so when buying plumbing hardware, it's best to opt for bronze. The stainless that looks so pretty today, probably won't after a few years anyway.
Hose connections to metal pipes and nipples, and threaded pipe fittings are all subject to crevice corrosion. That's because of the tendency for water to work its way between hose and the part it's clamped onto. One of the reasons marine hose is so expensive is because it has to be made of butyl rubber, and not carbon rubber which is cheap while the former is expensive. The problem of leaking connections is particularly acute on engines because of the effects of heating and cooling, which changes the dimensions of the parts. When water gets under the connection, crevice corrosion begins. This is the reason why teflon tape is used on pipe connections, and why it's a good idea to use silicon gasket cement on hose-to-metal connections. These materials will greatly reduce the propensity for leakage, bearing in mind that small leaks ALWAYS become large leaks. Always.
Stiffer reinforced plastic hose is very good for plastic to plastic connections, but it should NOT be used for plastic to metal connections. Plastic hose is too stiff to form up a good seal with a metal pipe.
No doubt you have wondered why they can't make a hose clamp that doesn't corrode like crazy. Well, they can't and the reason is that old bugaboo stress corrosion. No mater how good the stainless, stress corrosion will take its toll. For this reason, it is extremely important that all hose connections be properly fitted. If you have to use a ton of clamp pressure to make the seal, you will only cause the clamp to fail that much quicker because of the increased stress on the clamp. This is another reason why you shouldn't use plastic hose unless the plastic is quite soft and will deform easily like a rubber hose will. A good fit is one where little clamp pressure is needed.
Double clamping. For decades surveyors have parroted the need to double clamp hoses. The truth is that you don't, although it's a very good idea to have two clamps in place, the other not tightened up. Two very tight clamps will fail just as fast as one. Moreover, a properly fitted hose will fuse itself to the pipe connection so hard that you'll have a hard time getting it off. Tighten the second clamp just enough to hold it in place. When it's time to replace the first clamp, just tighten up the new one and remove the old one. If you're trying to clamp a hose with a bad fit, THEN you need more than one clamp. All engine cooling system and exhaust systems should be double clamped for obvious reasons. You NEED more clamping surface area here.
Ugh. This inexpensive material has its uses on a boat, but it's used in too many unsuitable applications just because it is cheap. It makes lousy hardware but good sail boat masts and great fuel tanks, so long as they are installed properly. It makes for lousy trim and moldings; it makes for lousy machinery casings like winches and spotlights. Aluminum cannot be cast in good marine grade alloys, thus all aluminum castings corrode like a banshee. Aluminum is extremely vulnerable to crevice corrosion, which is why so many aluminum tanks fail. Don't blame the material, blame the installer for not doing it right. Done right, aluminum tanks will last forever. Fuel tanks, that is, not water tanks. AL should not be used for water tanks, period.
Aluminum is similar to Corten steel in that it develops a layer of self-protecting oxide. This layer is so thin you can't even see it, but it's there. Quality marine alloys don't corrode and fail, except where crevices may exist, and when joined to dissimilar metals. That's why it's nearly impossible to keep paint on aluminum window frames when secured with stainless screws.
Keeping paint on aluminum is extremely difficult, and requires very careful and proper preparation. All these boats with painted but corroding window and door frames are the result of just shooting the raw aluminum without proper preparation procedures, many without even the proper primers that are indispensable to making the paint stay on. Worse, many boats these days simply use residential or recreational grade windows and doors that are not even a marine grade aluminum. There's no hope for this stuff.
Needless to say, this is what makes maintaining stern drives so difficult. Not only are they vulnerable to galvanism because of all the different metals in the drive, but it only takes a very small amount of stray current to cause serious damage, as shown in the photos above.
Related article: Corrosion in Marinas
First posted January 4, 1999
Page design changed for this site.
Posted July 12, 1999