All metals corrode—some faster than others. Marine-grade aluminum is corrosion-resistant, but is still subject to three forms of corrosion: simple corrosion, galvanic corrosion and electrolytic corrosion. Simple corrosion is the slowest form of corrosion, involving the aluminum metal experiencing electro-chemical reduction with oxygen. Marine grade aluminum self-coats with these oxides, which actually inhibit future corrosion. However, stresses to the aluminum metal (welds, scratches, flexing, etc.) may remove and inhibit the growth of these protective oxide coatings, causing the aluminum metal to irreversibly pit. Anodes attached to aluminum will inject electrons into your valuable metal and disrupt this electro-chemical reduction process.
Galvanic corrosion occurs from the use of non-aluminum metals electrically connected to aluminum. The use of stainless steel propellers, shafts and fasteners is the biggest concern, as these metallic items are galvanically incompatible with aluminum. But every outdrive casing is made of aluminum, yet has stainless steel components (and/or propellers). The only solution to prevent this galvanic corrosion to the outdrives aluminum casing is to 'shift' the galvanic corrosion to different metal—your sacrificial anodes. Magnesium anodes are the best choice for protecting aluminum outdrive’s (and hulls) that operate in freshwater.
Electrolytic corrosion is the most rapid form of corrosion. This involves stray DC electrical currents traveling through the aluminum hull or outdrive. Electrons flow through the metal, and ions complete the circuit by flowing through the water that the aluminum is in. The most common source of electrolytic corrosion is either your own boat, or that of your neighbors. Most commonly it's a bad bilge pump or leaking wire insulation. Improperly installed marine electronics is another common source, along with DC inverters and engine alternators. And these problems are readily shared to neighboring boats via the safety ground wire in the AC shore power cable that keeps all boats in a marina at the same electrical potential. It turns out that that green wire is great for electrical safety, but bad for defending against electrolytic and galvanic corrosion. And since electrical safety takes precedence, you've got to have it. The solution: sacrificial anodes to temporarily 'absorb' any stray electrical currents (actually, ion flows) around your boat.
Anodes are installed on your engine stern drives to help divert simple, galvanic and electrolytic corrosion. In short, the replaceable anodes corrode instead of your sterndrives. These are very critical to the longevity of your lower units. Failure to replace them can result in costly repairs, in severe cases, even total replacement.
A good maintenance schedule for these is replacing them on an annual basis. They are very inexpensive and can be purchased in kits. Kits include all replacement anodes, plus the fasteners required for each stern drive. I recommend magnesium replacements in freshwater.
Note: if you run a stainless steel prop or props, I recommend replacing the shaft nut with a magnesium replacement. (I'd recommend magnesium anodes in all instances to aluminum houseboats in freshwater only.)
For example on a Bravo II there will be as follows,
1 flat trim tab anode
2 lift ram anodes
1 plate anode
7 stainless steel fasteners
It's a pretty straightforward repair requiring minimal tools and experience. Allen wrench, pliers, wrench, small hammer and a wire brush.
First you must determine what sterndrives you have. Either refer to the manuals that came with your boat or trim each outdrive up for visual inspection. In most cases it will be displayed on either side of them. Wear a life vest and go under the swim platform to do this. (Be careful; there are sharp edges under there.)
Remove the bolts and old anodes. On the flat trim tab, reinstall the bolt approximately a quarter-inch and take a hammer and hit the bolt to drive the trim tab out of its housing. Then finish removing the bolt and replace them. It's as simple as that.
If yours are so bad that you can't even get to the screws—most likely the anodes on the trim cylinders—it’s going to take more effort. Depending on your mechanical ability to proceed, you may elect to hire a professional to do the job for you. Make note of the overall condition of the outdrive. If there appears to be paint flaking or blistering corrosion on the painted surfaces, you already have a corrosion issue. These areas must be sanded, cleaned and re-painted. DO NOT PAINT THE NEW ANODES!!!