America's Definitive Marine Engine Magazine

marine engine digest logo

How to Stop Corrosion On
Outboard Motor and Sterndrive
Lower Units

Volvo Penta QL Corrosion Protection for marine engines

Even when outboard motors and sterndrives live in saltwater there are a number of things you can do to prevent corrosion.

Prevent corrosion on Outboard and Sterndrive Lower Units with Electronic Modules and Sacrificial Anodes.

Let's deal with the ravages of corrosion. Saltwater boats can be rigged with special corrosion resistant power packages. Good examples include MerCruiser's SeaCore gas engines (also available from Cummins MerCruiser Diesel). Similarly, Volvo Penta's Ocean Drives were specifically designed to resist the ravages of corrosion, by using special coatings and stainless steel components. You can get them in new boats or repower.

Volvo Penta QL Corrosion Protection for marine engines

Volvo Penta QL Active Corrosion Protection
for marine engines

Besides building corrosion resistant engine and drive packages, the engine-building companies also offer electronic protection kits that effectively ward off galvanic corrosion by flowing a reverse current into the water. The units are fairly inexpensive (a couple hundred bucks) and are especially recommended for boats spinning stainless steel propellers. Installation of either MerCruiser MerCathode or Volvo Penta's QL Active Corrosion Protection System is quick and does not require special tools. With no moving parts, the systems work their magic by sensing water temperature, the water's chemical composition and then emitting ions into the water surrounding the drive and propeller. The ions are an electronic barrier to galvanic corrosion. Because these units draw current 24/7 (in the milliamp range) it's wise to wire a battery charger to shore power in order to keep the battery from going dead.

mercathode prevent corrosion on lower units

The MerCathode Module Fights Corrosion
With Low Voltage

Some boats swing aluminum propellers instead of stainless props because aluminum alloy's galvanic activity number is significantly closer to that of the aluminum drive housing. In other words, aluminum offers less opportunity for corrosion. Stainless steel is at the extreme of galvanic activity so it renders drives more vulnerable to corrosion. That's why it's smart to install electronic corrosion protection.

You already know how important it is to regularly inspect sacrificial anodes. When one is eaten away to less than half the original size, it's time to replace them. When bolting the new one in place don't make the mistake of coating the threads with grease or anti-seize compound to make them easier to remove down the road. Smearing anything on the threads effectively insulates the anodes from the drive legs, rendering them worthless. Obviously, never paint an anode. And don't use saltwater anodes (zinc) in fresh or brackish water, because after about a month, crud builds up on the surface rendering them inert.



© Copyright by Tim Banse