We'd been struggling to remove the lower unit of my friend's outboard for an hour, with distressingly bad results. Three of the four bolts had pulled out of the cast aluminum drive, and we were now looking suspiciously at the fourth. With great trepidation (and a flood of WD-40), the socket wrench went back on and my friend gave it a tug. It moved slightly, and the entire bolt pulled out, dropping the drive on the garage floor.
From inside the hollow casting came an explosion of fine grit. When we picked up the drive and turned it upside down, a sizable pile of the grit formed on the floor.

"What the heck's that stuff?" my friend asked me. "I think," I replied sadly, "that it's your lower unit after electrolysis ate everything else away." We had solved the mystery of why his prop shaft seemed loose, and we'd rediscovered why it's so important to make sure the zinc anode on the lower unit is checked regularly.

What Is Corrosion?

Corrosion is a major problem aboard every boat, and not just on metal under water, but every metallic item on or inside your boat. Dampness, salt spray and humidity make corrosion breed. The most common type on your boat is galvanic corrosion. This takes place when two dissimilar metals are submerged in an electrolyte (a liquid that conducts electricity) with the result being a battery that generates electrical current. Flowing from one metal to the other, it eats away at the weakest (least "noble") of the two metals. A second type of corrosion is electrolytic, caused by currents from outside your boat, such as leakage from your shore-power system or a mild electrical current from a nearby boat. There are more ordinary types of corrosion as well, such as rusting and pitting. Iron and steel (your engine block) get a common reddish-brown rust wherever bare metal is exposed to dampness. Aluminum doesn't "corrode" but rather forms a layer of oxidation on the surface that looks like dull grime and whitish lumps.

Preventing Corrosion

The primary method to defeat both galvanic and electrolytic corrosion is a system of zinc anodes. These are bolted to the outdrive, rudders, prop shafts, trim tabs and any other piece of metal that might be attacked by corrosion. Because the zinc is "sacrificial," it will be eaten away before your more expensive metal parts. So be sure that you have zincs on everything, and check them often. Galvanic corrosion can also be eliminated by keeping dissimilar metals apart. Mounting an aluminum fitting with a bronze bolt is simply asking for trouble, which is why stainless-steel bolts are commonly used for boating gear. If you have to place a metal fitting on a dissimilar metal, insulate the two metals with a layer of electrical tape.

Recognizing The Signs

One sure sign of galvanic or electrolytic corrosion is the pitting of your zincs. A second indicator is when your metal parts, such as a bronze prop shaft, turn a pinkish color. Some boats use a central bonding system that ties together all the underwater metal parts with a heavy cable that is then bonded to a central zinc. To keep this system effective, you have to check the connections regularly so that no parts are left exposed by a broken or loose cable. If your zinc is disappearing at a rapid rate, call in an experienced marine electrician to see if your boat is "leaking" the stray currents or if your marina and nearby boats are at fault. Rust can be prevented by painting the parts to keep them protected from dampness. If it's already started to rust, you can't just paint over it since the rust will continue to blossom under a layer of paint that will soon blister off. Instead, use a wire brush, bronze wool (not steel wool, which leaves pieces that will rust!), or sandpaper to get down to bare metal, apply a coat or two of metal primer, and follow it up with paint. Aluminum is a problem unto itself, since that grayish film is actually oxidation that protects the metal. Sand it off, and it will reform again. Aluminum parts exposed to the elements, such as windshield frames, should be anodized. This process creates a smooth layer of oxidation that becomes the protective layer and can even be done in colors. If your aluminum corrosion wipes off like chalk, use a specialized aluminum restoration product that will clean the surface and allow the new oxide layer to form smoothly. Another alternative is to sand the aluminum smooth, cover it with aluminum primer, and paint it. Either way, aluminum will remain a fairly high maintenance item on your boat. Stainless steel, despite what you might think, isn't. After all, it's "stain-less," not "stain-free," and you'll frequently see weepings of rust on a stainless-steel rail. Often that is from tiny particles of iron used to cut or machine the fitting, but even stainless steel can bleed a little corrosion. Using wax can help protect the stainless steel, and it can be cleaned with a mild abrasive such as Bon Ami. Chrome is bright, shiny and tough, but even so, it's not impervious to pitting, especially in areas where you have acid rain. To prevent pitting, wipe the chrome parts regularly with an alcohol-based cleaner like Windex, and use one of the polishes such as Flitz to remove discoloration caused by heat on chrome exhaust pipes. Finally, you're likely to find that brass on your boat tarnishes quickly. In this case, use the remedy applied by the British Navy for more than a century: polish it to a sparkle with a slice of lemon dipped in salt crystals. You'll be amazed at how quickly even black tarnish wipes off. Besides, you can use the rest of the lemon to make lemonade or a Tom Collins to celebrate how you have done your best to protect your boat against corrosion.

September 2000