Electrons are insidious, and when they start flowing around your boat’s underwater metal, bad things can happen. Here’s how one outboard owner dealt with galvanic damage.
When he hauled his boat in December, 2022, Gary DeSanctis, president of Active Interest Media Marine Group, publisher of Power and Motoryacht, got an unpleasant surprise: The submerged section of the mounting bracket holding his 200-hp outboard was badly pitted by corrosion, and so was the nose of the lower unit. (Like many outboards, the four-stroke doesn’t tip up far enough to lift the lower unit fully clear of the water.) DeSanctis is careful to keep everything up to snuff, yet he was blindsided by rampant electrons flowing through his underwater metal. This could happen to you, too.
Immersing dissimilar metals that are in electrical contact with each other—through the boat’s bonding system, for instance—in salt water, generates a flow of electrons between them. Salt water is an electrolyte, and this creates essentially a low-voltage battery. The lesser-noble, or anodic, metal, e.g., aluminum, magnesium, zinc, gives up electrons to the more noble, cathodic, metal, e.g., silicon bronze, Monel, passive stainless steel, and deteriorates in the process. This is galvanic corrosion, and it’s why every boat with underwater metal has a sacrificial anode—commonly called a zinc, but often made of aluminum, magnesium or some combination—connected to all the underwater metals. The anode dies so the other metals will live. In most cases, replacing the anode as part of scheduled maintenance is enough to prevent damage from galvanic corrosion.
Galvanic corrosion isn’t the same as electrolysis, although many people use the latter term for all underwater corrosion. Electrolysis introduces an electrical current to cause a change in whatever material the current is flowing through—for example, passing electricity through water can produce hydrogen and oxygen by electrolysis. Galvanic corrosion happens slowly; electrolysis usually happens quickly.
The Real Problem: Stray Current
Add an external source of electrons, and galvanic corrosion becomes electrolytic corrosion, and things can go south very quickly. Most folks call this stray current corrosion, and it’s caused by electricity escaping into the water, frequently because of a poor connection, broken wire, etc., in the bilge. When the stray current passes into the water via a metal through hull, the prop shaft, the stern drive or the lower unit of the outboard, accelerated corrosion takes place. Sacrificial anodes dissolve like Alka-Seltzer, and when they’re gone the next-lowest-nobility metal starts corroding. In the case of an outboard or I/O, that’s the aluminum-alloy mounting bracket and/or lower unit, if it’s immersed, as many I/Os are.
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Stray current can come from the boat’s 12-volt or AC wiring. A serious AC failure will usually trip a circuit breaker, but in some cases the AC ground wire becomes energized with low-level current. Since the AC ground is tied to the DC ground, a ground fault can not only destroy underwater metal, but can also energize the water around the boat—so think twice before swimming in the marina. The shore-power ground is shared by all the boats plugged into the same circuit, so a ground fault on your boat can affect other boats, and vice-versa. Or maybe the problem is on the dock: Marina wiring is famous for being poorly maintained, and not all marinas have ground-fault protection.
Electricity will find the easiest path to ground; maybe that’s through your outboard’s mounting bracket, even though the fault is from the boat a couple of slips down. If you’re having accelerated corrosion, don’t try to DIY the problem: Call an experienced marine electrician appropriately certified by ABYC and/or with a long list of positive references—rogue electricity is an insidious beast, and it often takes a crafty technician to hunt it down.
Fixing the Holes
Short of replacing parts, is there any other way to repair corrosion damage like DeSanctis discovered? According to the folks who sell Belzona products, there is, as long as the metal isn’t too far gone; nothing will repair an engine mount dissolved into a paper-thin shadow of its former self. But surface, superficial corrosion can be filled, and a layer of insulation applied that can prevent future problems.
What’s Belzona Founded in the UK in 1952, and still based in Harrogate, North Yorkshire, Belzona manufactures a range of sophisticated metal repair products used in a wide range of industries, including oil and gas refining; distilling; hospitals; power plants; transit systems; commercial shipping, towing and marine construction—you get the picture. There’s a Belzona product for repairing engine blocks in situ, for recoating the insides of fuel, waste and water tanks, for fixing worn or corroded pipework. In short, if it’s metal, Belzona probably has something that can fix it. For repairing the corrosion on DeSanctis’s outboard, Jack Prince, president of Belzona New York, used a three-pronged attack—well, four-pronged, if you count Edwin Martinez, the technician actually doing the job while the rest of us stood around and watched.
Step one is to prep the metal, to remove all traces of corrosion and create a proper anchor profile on the surface, with enough roughness for full adhesion of the coating. Martinez used a Bristle Blaster rotary brush for this, and a more lethal-looking handheld tool I have never seen. The Bristle Blaster spins a spring-steel wire wheel at 3200 rpm; the ends of the bristles are cut to a chisel point for maximum abrasion. (Use protective eyewear!) According to the manufacturer, the Bristle Blaster is a handheld equivalent to traditional grit blasting; it’s also a machine that can make short work of aluminum alloy if mishandled, so it’s best to leave it in the hands of an expert like Martinez. He cleaned the surface to remove any contaminants—Belzona has a product for that, 9111 Cleaner/Degreaser, but you can also use MEK or any cleaner that doesn’t leave a residue. Then Martinez went to work with the Bristle Blaster, and in a few minutes, he had all the corrosion removed from the bracket and the lower unit, leaving fresh aluminum ready for repair. He washed the surface again with Belzona 9111.
As step two, Prince planned to fill the corroded surface with Belzona 1111 Super Metal, a two-part epoxy resin reinforced with silicon steel alloy. It’s like liquid metal—once cured, it can be drilled and tapped and, according to Prince, has the strength of soft metal. With no solvents in the mix, Super Metal won’t shrink or crack as it cures. The ratio is three parts base to one part hardener (Belzona calls it “solidifier”) by volume, followed by thorough mixing until the color is a consistent grey.
Working time depends on temperature; it was cold, so Martinez had about half an hour—more than he needed. He applied the Super Metal with a throwaway paint brush. The product flows on smoothly, but Martinez used a lot of brush pressure to work it into the aluminum surface. “Don’t just float it on,” said Prince. Once the Super Metal hardens, it can be machined to fair the surface, but it’s easier to do that while it’s still malleable. DeSanctis wasn’t concerned with creating a glass-like surface because the damaged part of the motor bracket is under water, so Martinez used a plastic spatula and his gloved fingers. He also coated the nose of the lower unit, then hit all the Super Metal with a heat gun to speed up its reaction in the clammy weather.
Step three was to protect against future corrosion. Once the Super Metal had stiffened, Martinez mixed enough Belzona 1321 Ceramic S-Metal to cover the repair. Also a two-part product, 1321 provides a hard barrier coat to increase abrasion and corrosion protection. It comes in two colors, blue and grey; the blue almost matched the original finish on the outboard. Prince said 1321 can be applied in one coat about 20 mils thick, but it’s best to apply two thin coats, in different colors, with an hour or two between them; if the topcoat wears thin, the undercoat will show through. Once the 1321 has fully cured, it’s very, very hard; any machining or fairing will require diamond-tipped tools.
All the Belzona products used to repair the outboard are approved by ABS, Bureau Veritas, DNV and other classification societies. Prince said they also meet the requirements of the U.S. Navy, the Coast Guard, major engine manufacturers, etc. Bottom line is, they’ve been used all over the world for decades, to repair damaged metal in all kinds of service, so I think it’s a pretty good bet Belzona has rescued DeSanctis’s outboard motor. But we’ll know more at the end of the boating season.
An Ounce of Prevention
How can any boat owner protect his underwater metals? Maintaining the anodes is crucial. Outboards have anodes on the underside of the mounting bracket and on the lower unit. The anodes on the lower unit are easy to see, and to change; the one on the underside of the bracket is difficult to inspect if the boat’s in the water. You have to take a swim. Outdrives often have many anodes, maybe on the propeller hub, the trim cylinders, and so forth. On most I/O boats, the outdrives mostly live underwater; those with stainless steel props are most susceptible to galvanic corrosion.
Replace anodes whenever they’re half gone. Check them regularly, not just at the season’s beginning or end, but don’t just run your eye over them—anodes can deteriorate internally, and still look OK from the outside. Best way is to weigh them vs. a new anode; if more than half the weight’s gone, change them. Anodes are inexpensive, so if there’s any doubt about their condition, replace them. The damage from trying to squeak a bit more life from 50 bucks worth of anodes can run into the thousands if the lower unit fizzles away.
If you don’t need shore power, don’t plug in. If you do, your boat should have a galvanic isolator. (DeSanctis kept his boat plugged in, and he didn’t have an isolator.) Connected to the green grounding wire on the shore power circuit, a galvanic isolator blocks low current flow through the ground wire, while maintaining protection against serious AC shorts. All boats with shore power should have one. See ABYC Standard A-28 for detailed information.
Turn off the battery switches when you’re not onboard to minimize the chance of stray currents from the 12-volt system. Keep your batteries topped-up with a solar panel on the cabin- or hardtop, not with the yellow cord—or just use your boat more and keep the batteries charged with the alternator. Get out on the water: That’s the best solution of all.
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