Simple Corrosion

Corrosion requires a few things. An anode, a cathode, a metallic path, and an electrolyte. When those conditions are met, there is going to be the breakdown and the loss of metal. That’s corrosion, defined about as directly as possible. Electrolytic corrosion, which we’ll tackle in a few posts, takes the same formula and adds one important variable: an electric current.

Anodes and cathodes are usually thought of as separate objects. The anode corrodes, protecting the cathode in the process. An aluminum collar anode on a steel propeller shaft, as an easy example. The aluminum, being less noble (which is to say, more easily corroded) than the steel, breaks down one ionized atom at a time. The anode and cathode here are obvious. The path is provided by direct metal-to-metal contact. The electrolyte, facilitating ion flow, is the water.

An anode, a cathode, a metallic path, and an electrolyte.

But the conditions for corrosion can be met with a single piece of metal. This is what’s known as simple corrosion, and it’s actually happening at all times. To all metals. Very, very slowly. Let’s take another example now. Think about a bronze propeller. An unmounted one, fresh from the foundry and waiting for installation. Bronze itself isn’t really one metal, of course. It’s an alloy of zinc and copper. That same process will play out again: zinc serving as anode, copper as cathode. They’re already in direct contact with one another, they just need an electrolyte now. One would obviously become available if the propeller is mounted and submerged in the water. But that wouldn’t even be necessary for the corrosion process to begin. The humidity in the air would do it.

There’s always an anode, there’s always a cathode. Metallic paths are easily established, just by touching. And an electrolyte is (usually) never too far away.

Next week, we’re going to dig deeper into the science at work with galvanic corrosion. That’s going to be a fun one, see you there.