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Why Metal Rusts

October 20th, 2015 by Mike Meyerhoff

Don’t let the clever headline fool you, this process is quite simple…and probably rather boring for most.  But, if you’re reading this blog post then metal probably plays a big role in your life and ipso facto so does galvanic corrosion, ferric oxide, Fe2O3, Texas suntan…rust.

 

Quick Breakdown of the science stuff:

Atoms attach to other atoms to make up most all the stuff in the universe (H2O…two hydrogen atoms attach to each oxygen atom to make a water molecule.  There are about 8.36 X 10^24 molecules of water in a cup…but who’s counting).

Atoms are made of protons (positively charged particles) and neutrons (non-charged particles) surrounded by a cloud of orbiting electrons (negatively charged particles).  The electrons orbit around the nucleus of the atom (made of protons and neutrons) like the planets go around the sun, until they are ripped away by a stronger force than that which holds them to the atom.

An electric current is really just the flow of electrons from one point to another.

An electrolyte is basically any media (typically in solution form) which carries the electric current.  Marty and Doc needed a time machine to get them to and from 1985…Marty and Doc were electrons and the Delorian was the electrolyte.

An ‘anode’ is the source of an electric current (where the electrons come from) and a ‘cathode’ is where the electrons end up.

 

How the science stuff, plus metal, makes rust:

The steel mill takes coal, coke, and all the specific alloy elements, melts them all together in a big pot to get your molten steel soup.  The molten mixture is then poured into whatever shape the customer needs (slabs, billets, etcetera) and from there made into plate, coils, pipe, beams…whatever.  While that steel is still hot, all of the atoms in that steel are full of energy and bouncing around.  As the steel cools, the atoms loose energy and bounce around less and less.  After a while, the atoms stop and where they stop is where they stay.  At this point heat treatments may be applied to re-excite the atoms in an effort to get them to line up in a more beneficial way, but no matter what there will never be a truly uniform or predictable distribution of all the different atoms.  Just as when you add nutmeg and cinnamon to your pancake batter, no matter how well you mix it all up you will never achieve a truly uniform distribution of everything in the batter.

Fast forward to your finished product…say steel pipe for example.  To the eye the new piece of pipe looks pretty uniform, but we chemists know better.  A LOT of energy went into making that pipe; turning those atoms into a piece of steel.  And now that energy wants out…it wants to go back into being in balance.  There are ga-jillions of micro sections of the pipe which want to give up electrons (anodes), and there are just about as many which WANT electrons (cathodes).  When that pipe gets exposed to an electrolyte (like the wet air around the gulf coast) those electrons now have a way to travel from anode to cathode.  This is called an oxidation-reduction reaction…electrons are moving from one type of atom to another.  Most of us just call this simply ‘oxidation’.  And naturally the stronger the electrolyte, the more (quicker) electrons move from various anodes to cathodes.  Steel left under water will oxidize much quicker than steel just sitting in the desert…or on the moon where there is no electrolyte.  And as the piece of pipe is not chemically uniform, the anodes and cathodes will pop-up almost at random creating rust spots.  When enough electrons have left an anode, that portion of the steel falls apart…after all it’s now missing these significant electrons which were holding it all together in the first place.  Over time, the energy once all pinned up in that finished piece of new pipe is completely released as the electrons flow out, leaving behind a crumbled pile of dust…fully oxidized Iron…Iron oxide…rust.

 

But Mike, I don’t want that rust on my steel and I have no customers on the moon.  What do I do?

The basic answer to rust prevention is simple…remove the anode, cathode, or electrolyte and oxidation cannot occur.  Elections won’t flow out, the pipe will keep all its original energy, and all will be well.  But since we live on a planet full of electrolytes all around us, and because we have so much energy in that pipe, its near impossible to eliminate any of the mechanisms causing oxidation…so we do what we can to slow the reaction down.

Liners and coatings are often applied to pipe to keep the steel from being exposed to electrolytes in the atmosphere, but most all of these coatings fall apart over time (by much the same process as the steel itself).

Galvanizing is a process of adding zinc to the steel (either during the steel making process, or by dipping finished product in a molten zinc bath).  The zinc bonds chemically to the steel, and creates a barrier between the steel and any electrolytes.

Some folks fight rust with cathodic protection whereby the steel is bombarded with electrons from a nearby anode.  Here the steel is showered with electrons so that its quite difficult for electrons to leave the pipe in the first place…kinda like trying to walk the opposite way through a huge crowd.