This is completely in left field but I had a question and I couldn't really dig up enough good information to answer it on the net(admittedly I didn't dig really far, but I think it has more to do with search terms than the information being unavailable I just couldn't seem to find just the answer I was looking for, I could find reference to both conditions just not why they are different). So I figured I'd come here and pose the question because with our diverse user base I'd guess someone has first hand information why this is;)

So corrosion in steel or iron creates rust which basically destroys the metal. Where corrosion in copper zinc etc, creates a protective oxide finish which actually from what I understand keeps the metal from being destroyed. Same goes for electroplating different metals etc.

Now I know very little about metallurgy and metals in general beyond very basic knowledge so there is probably a relatively simple explanation. This isn't anything beyond simple curiosity but I started thinking about this after watching a modern marvels about copper and it just struck me as interesting. I guess I've never thought about it before, I knew that some metals corrosion is actually protective but it just seems odd to me and I'd love to know why iron/steel is so different in this case.

Either way just curiosity;)

Luke

I actually did know bluing was rust but a different variety of sorts but I forgot to mention it. Just really curious to me why iron is so prone to it. I've always been interested because this area uses so much salt on the roads in winter, anything steel just gets eaten alive and I'm always interested in why;)

Luke

Iron can and often does form a passive layer of surface oxide. Bluing is one type where Fe₃O₄ (magnetite) deliberately generated.

Rust is a little more complicated because it's not just an iron oxide, it's an iron oxide-hydroxide which requires the presence of water to form. A lot of other metals can undergo a similar reaction with water (copper verdigris for example), but I'm not sure exactly why iron is so much more prone to it.

  If I remember correctly, it has to do with the covalent bonds that form the metal atoms and also the crystalline structure or lack thereof. 

Non crystalline metals ( copper and aluminum for instance ) have a homogenous structure that tends to limit corrosion to a layer a few atoms or molecules deep near the exposed surface ( anodizing on aluminum is an example of this ).   Iron and particularly steel have a crystalline structure which is refined by the forging and tempering process.  As I recall, in carbon steel the rust begins to form at the boundary of the carbides which are precipitated out during the Martensitic transformation as steel cools from forging and quenching.  This can continue into the crystalline structure and eventually forms what we see as pits on a smooth surface.

  If I remember correctly, it has to do with the covalent bonds that form the metal atoms and also the crystalline structure or lack thereof. 

This is most of it.  Iron is particularly susceptible to oxidation because it has a relatively small nuclear size (for a metal) and it also exists in the middle of the periodic table so it isnt particularly electronegative, meaning it doesnt take a whole lot of energy to oxidize it.  Its atomic structure also allows for multiple "angles" of covalent bonds.  This means that when it shares electons with an oxidizer it doesn't have to share a set number, it can share two or three.  This is how iron(II,III)oxide or Fe₂O₄ can exist in multiple forms naturally.  Whether it shares two or three and the ratio between those two affect its crystalline structure and that structure can affect its overall strength.  Other heavy metals (like copper) can do this but I think it's just more noticeable with iron due to how prevalent it occurs in nature. 

Splodge is correct, most solids are crystalline in the sense that the atoms have to arrange themselves in a particular order.  Anyone else here read "Cats Cradle" by Kurt Vonnegut?  The crystalline structure of solids plays a huge role in the book, and is easily explained with the analogy of cannonballs stacked together.

louie the lumberjack