Galvanic Corrosion

Today’s post pertains to the most relevant form of corrosion for our company, our customers, and really for the whole of the marine industry.

Galvanic Corrosion occurs when dissimilar metals are in contact with one another in an electrolyte. That’s sure to sound familiar if you tuned into our last entry on simple corrosion, where we identified the conditions necessary for corrosion.  Reminder, corrosion requires an anode, a cathode, a metallic path, and an electrolyte. Simple corrosion plays out within the space of a single piece of metal. Galvanic corrosion needs the same components, but now our dissimilar metals are distinct, separate from one another.

This is an easier idea for most folks to wrap their heads around, because it’s so obvious. Rather than having to visualize the imperceptibly slow interaction between, say, the zinc and copper that make up an alloy of bronze, everything is a good deal easier to see with galvanic corrosion.

Example: galvanic corrosion will reliably play out when a bronze propeller is mounted onto a stainless-steel prop shaft. The metallic path is clear – the pieces are physically connected together. The electrolyte is also readily apparent – being that they’re submerged in the same body of water. The bronze is significantly less noble than the shaft, so it is going to become the anode here. The shaft will act as the cathode, gaining ions that the propeller offers.

With all the conditions being met, the process of galvanic corrosion is underway. And this time, the loss of metal is going to be considerably faster. Several variables will contribute to how long we’ll have before damage occurs (water chemistry, water flow, and water temperature to name a few common ones). But, without intervention, damage is absolutely on the horizon.

The whole process of protecting your boat from corrosion begins with this idea: you cannot prevent corrosion until you tilt the chemistry in your favor. You’ll want to introduce another metal, one that’s even less noble than the bronze propeller. Something like one of the 1,500 products we offer in our warehouse (with same-day shipping, we’d like to add).

Next week, we’ll talk about the difference between zinc, aluminum, and magnesium anodes. We’ll discuss their applications and compare their efficacy in different types of water.