Crowley’s Tip #9: Know the enemy: Corrosion

Corrosion rates right up there with chafe, moisture, and overzealous birds in terms of universally-despised-by-boaters. But corrosion is not only despised, it is down-right feared. A lot of this fear probably comes from lack of information. If you know what to look for and what you’re dealing with, then you can prevent it from ever being a problem.

Corrosion happens in many types of metals and many types of environments, but there are a lot of misconceptions about what corrosion is and what causes it. Many people mistakenly think that putting metal in water causes corrosion. It is actually a reaction that occurs when certain metals are connected or grounded through water.

For corrosion to occur, five things must be present:

• Two dissimilar types of metals

• A connection between the two metals

• An ion solution to conduct the electricity (electrolyte)

• A potential difference between the metals.

All metals range from very chemically active like zinc and aluminum to less active, like stainless steel or gold. When two or more dissimilar metals are immersed in water, a flow of electrons occurs between them – despite the fact that the metals are not connected to any external source of electric power. The flow of electrons causes the metal that is most chemically active to change. This is called galvanic corrosion. This process speeds up in salt water, polluted water or water with a high mineral content.

Aluminum is especially prone to corrosion. If a drive unit is not protected, galvanic corrosion will destroy components exposed to water. The first signs of galvanic corrosion are paint blistering below the waterline and white corrosion forming on exposed sharp metal edges. As corrosion gets more advanced, exposed metal parts start dissolving away resulting in pitting of the metal. Because gold is the least active metal, it would be the most resistant to corrosion. Unfortunately, most boaters can’t afford a solid gold drive unit. To build a drive unit that delivers the best performance and durability, it takes a variety of metals like lightweight aluminum alloys, hardened steel and stainless steel.

The way to counteract galvanic corrosion is to add a third metal into the circuit, one that is more active than the other two. This piece of metal is called a sacrificial anode, and most often it is zinc. In fact, most boaters refer to sacrificial anodes simply as zincs.

Sacrificial anodes are specifically designed to provide additional protection against galvanic corrosion. Because they’re more active than aluminum, these inexpensive anodes dissolve instead of the expensive drive components. These anodes are easy to replace and must be replaced when erosion reaches 50%. Make sure the anode is tight to ensure electrical contact. Never paint anodes because this will block the anodic protection. For the trim tab anode to work, it cannot inhibit corrosion if raised out of the water. When a boat is removed from the water, anodes do not provide protection. This is why it’s important to flush the engine after each saltwater excursion. Ear muffs, the rubber gaskets that fit on each side of the outdrive’s intake, make this easy. Connect the muffs to a hose, flush the outdrive for two minutes and you're done.