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Author Topic: Corrosion and metals  (Read 2155 times)

luke213(adamsholsters)

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Corrosion and metals
« on: August 21, 2013, 11:49:25 am »
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
MichiganI am the owner/proprietor of www.adamsholsters.com Custom holsters made for you. To contact me please use E-mail rather than Private Messages, [email protected]

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    JesseL

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    Re: Corrosion and metals
    « Reply #1 on: August 21, 2013, 12:06:12 pm »
    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.
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    luke213(adamsholsters)

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    Re: Corrosion and metals
    « Reply #2 on: August 21, 2013, 12:10:28 pm »
    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
    MichiganI am the owner/proprietor of www.adamsholsters.com Custom holsters made for you. To contact me please use E-mail rather than Private Messages, [email protected]

    Mississippi556

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    Re: Corrosion and metals
    « Reply #3 on: August 21, 2013, 12:40:42 pm »
    I know little on this subject and what I do "know" is probably wrong.  What I can say is that most forms of corrosion, regardless of the cause are harmful to firearms and their accessories.  An exception would be protective bluing of steel and anodizing of aluminum which are perhaps a form of controlled "corrosion", at least in a general or non-technical sense, compared to rust on steel or iron or aluminum oxidation. 

    Corrosion of electrical connections is a huge, often overlooked problem until too late.  An example is corrosion of electrical connections in the battery compartment of high grade optics and tactical lights due to use of alkaline batteries  or "regular" carbon-zinc batteries and not checking or replacing them regularly.   Switching to lithium batteries, while more costly in the short run,  not only largely eliminates the issue, but also provides longer and more consistent performance up until they give up the ghost.

    I've come to the realization that for anything that needs to work with 100% reliability and anything that is expensive to replace, lithium batteries are no longer an option, they are a necessity.
    Mississippi"When a strong man, fully armed, guards his own palace, his goods are safe"  Words of Jesus, Luke 11:21 (ESV).

    coelacanth

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    Re: Corrosion and metals
    « Reply #4 on: August 21, 2013, 04:30:23 pm »
    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.
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    Splodge Of Doom

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    Re: Corrosion and metals
    « Reply #5 on: August 21, 2013, 07:58:21 pm »
    Actually, I'm fairly certain that both Aluminium and Copper are usually crystalline. They both have a visible grain structure under a microscope, anyway. If they were amorphous, you'd be able to see through them... though the existence of ALON suggests that at the very least you can make Aluminium amorphous - it just usually isn't...

    Corrosion is an electrochemical process between dissimilar materials, forming an anode and a cathode. Whatever material in the mix is most anodic, that will corrode first.

    IIRC, aluminium and copper both oxidise easily, and the oxide then prevents oxygen getting to the material beneath. In that case, the oxygen forms the cathode and the surface of the material the anode.

    There's a whole load of different corrosion types - nine of them, off the top of my head. Each of them is a different case for an anode forming.

    louie the lumberjack

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    Re: Corrosion and metals
    « Reply #6 on: August 21, 2013, 11:01:34 pm »
      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
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    Plebian

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    Re: Corrosion and metals
    « Reply #7 on: August 22, 2013, 09:58:28 am »
    If I recall, it is because iron forms multiple types of oxides in the natural environment. So the crystal structure of the oxides is not consistent. Where in copper it forms a consistent oxide that "lays" flat like plating. So iron oxides being different leave "gaps" for more intrusion of water/oxygen.

    Which is why bluing works on iron. It forces a single oxide to form making a protective "plate".

    I know from my work. That it is pretty important that there are multiple forms of iron oxides. Since there is pretty much iron and oxygen everywhere on Earth. Would be boring as hell if it only went together one way.
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