Magnesium on Ships

Why do ocean-going ships hang large bars of Magnesium? What is this process called?
- Lindsay (age 17)
Tallassee, AL, US

The large bars of magnesium are used to help protect the ship against rusting. Like the zinc on a galvanized trash can, the magnesium gets oxidized instead of the iron structure it protects. The chemical free energy can be lowered more by forming magnesium oxide than by forming iron oxide, as you can look up in a table of half-reaction potentials in a chemistry book. It might still seem surprising that a bar of magnesium could protect the hull. Water with dissolved oxygen in it can still reach the hull without going near the magnesium, so you might think that BOTH the magnesium and the iron would oxidize.

However, rusting is a complicated process. The first step involves metal ions (electrically charged) coming off into solution. That generates an electrical potential. If one of the two types of ions (say Mg++) dissolves more readily, that generates an electrical potential which keeps the other type (Fe++) from coming off, since the Mg and the Fe are electrically connected. At least that’s how we think it works. We weren’t able to find direct information on this process with ships, and so we don’t know a special name for it.

We did find a useful site on .

Mike W.

(published on 10/22/2007)

I don’t know exactly what the "process" is called, as asked in the question, but I know that the Magnesium bars are called "sacrificial anodes" - if that helps.
- Anonymous

Thanks!

(published on 10/22/2007)