Fun with Rusting

Why don’t ships rust? 

It sounds like a simple enough question: ships do rust, just not quickly enough to notice. Yet, if one thinks about it, it becomes more of a curiosity. Even iron that’s just been exposed to the air starts to rust; what do boats go through? They never come out of the water, the paint doesn’t stay on forever, and any speed they gain just serves to quicken the reaction. By all accounts, some of the most magnificent ships on the sea should be having a much harder time riding the surf in two or three years. How do these giant boats stay afloat while facing the constant threat of corrosion?

It all has to do with electrolytes.

Yeah, those things.

The term ‘electrolytes’ refers to any solution that contains stray ions floating around it; in other words, atoms that have too many or too few electrons. Small batteries called Galvanic cells utilize this conductivity to create a constant flow of electrons from one metal to the other, like this: 

This is actually how all batteries are designed; the only difference lies in the type of metals used and the number of cells. 

When one gets down to the nitty-gritty science of the concept, metal corrosion goes through the exact same process. In sea water, the salt is naturally broken up by the water into ions; this imbalance of electrons makes the whole solution electrically conductive. Therefore, the ocean acts as one big electrolytic solution. The metal of the boat reacts with the electrolytes so much, that eventually there isn’t any more metal to react. Covering the hull with paint only does so much. 

To prevent this, ship builders play a clever trick on nature: focus all of the electrolytic attention to one metal in order to spare the other. Down the hull of a 45-foot steel boat, one can usually see four to six large zinc bricks nailed down into the side. These bricks act as super-anodes in the electrically-conductive sea, working to slow down corrosion on the steel by corroding more rapidly themselves (hence why these zinc pieces are often called ‘sacrificial bricks’). This whole process, attaching two different metals together to slow down decay of the better one, is called Galvanic Corrosion. 

In larger boats, blocks of zinc don’t quite do the job anymore; more advanced technology is needed. The US Navy uses a nifty piece of machinery called ICCP, or Impressed Current Cathodic Protection. ICCP feeds electrons from the ship’s internal electrical system into the water around the bottom of the ship, reducing the corrosion the same way blocks would. Yet this system is considered an improvement over sacrificial blocks in many ways, since it doesn’t weigh anything, doesn’t deplete, and its voltage can be actively controlled to best suit the environment. However, this heightened control also comes with a small caveat: let the voltage drop too low, and it’s like the boat has no protection at all. 

Fascinating, no? The same technology that powers a Gameboy helps keep the world's largest ships on the sea. So, next time you’re out roaming on the seven seas in your fifty-foot yacht, remember: it’s science that’s keeping you afloat.

Also, don’t forget sunscreen. Seriously.

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