How to Prevent Corrosion on a Boat

Corrosion is the arch-enemy of any boat owner. It works to prematurely age your boat, and fighting back to prevent corrosion is an ongoing effort for as long as you're a boat owner. 

There are three common types of corrosion and several things you can do to protect your outboard engine from it.

Simple Corrosion

This type of corrosion starts once a piece of metal starts to rust after being pulled out of water. 

Yamalube external engine cleaner

The best way to fight back against this type of corrosion on your boat's engine is to keep it clean. Start with a good external boat engine cleaner, followed by a silicone spray to protect its metal surfaces. 

Yamalube silicone spray protectant

Flush your boat's engine after every outing, then clean it and spray it with silicone. If you're preparing your boat for long-term storage, go a step further and protect the internal surfaces of the engine with fogging oils for carbureted boats or EFI fogging oil.

Yamalube engine fogging oil

Yamalube EFI Engine Fogging Oil

Galvanic Corrosion

This type of corrosion is the tendency of one metal to corrode or rust when it's in electrical contact with another metal. That electrical contact can happen when the metals are touching, but can also happen when the two metals are in close proximity to each other in an electrolyte such as water. An example of galvanic corrosion is when a stainless-steel housing acting as a cathode eats up a bronze propeller acting as an anode.

Boat propeller corrosion

The trick is to find something softer than the bronze to act as an anode. That's where sacrificial anodes come in. These small blocks of soft metal (usually zinc) are installed in various areas around the boat. They can be placed on the out-drive, the hull or even on the engine itself. 

Sacrificial anodes boat hull

It's important to know where each of these anodes are, and to make sure they're functioning while in the electrolyte.

In the motor pictured below, the anode was removed from the electrolyte when it was tilted up at the dock, so the housing itself began to corrode below the water line.  

Corroded outboard engine anode

It's also important that your entire boat is bonded together, meaning its parts need a common thread to keep them all at the same potential electrical difference. This can be accomplished with a Mercathode system, which is integrated into the engine and out-drive, and runs off of the battery.

Mercury Marine Mercathode kit

The Mercathode produces a reverse current to ward off galvanic corrosion. However, you can’t see this system working, so it needs to be tested every year to make sure it’s doing its job. Otherwise, your outboard can end up looking like this:

Outboard engine corrosion

Stray Current Boat Corrosion

This last type of corrosion occurs when there's an electrical issue, either on the boat or at the dock. DC electrical current looks for a ground, and what it can do is travel through your bonding system or a through-hull fitting searching for water. 

Mercury galvanic isolator

A great way to prevent this type of corrosion is maintaining strict watertight electrical connections, but a galvanic isolator can also help. The galvanic isolator stops low-voltage DC from getting on your boat, and is connected in-line to the shore power connection.

Preventing Corrosion on a Boat

The best ways to prevent corrosion in general are to keep your boat clean, keep your electrical connections secure, and employ the countermeasures detailed above. And keep your eyes open for signs of corrosion. The sooner you catch it, the better your chances of fixing it!

Boat corrosion prevention cleaning

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