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Rust, what is it?

Discussion in 'Question of the Day' started by ship, Oct 25, 2005.

  1. ship

    ship Senior Team Emeritus Premium Member

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    Just finished sorting a screw bin with lots of rusty stuff in it including brass screws that somehow had surface rust on them when in the same bin and adjacent to a rusty screw. Have had aluminum plate that became pock marked with a coating of rust when it spent time toucing a rusting steel plate. Rust tends to spread to other surfaces in even an air born way once it's introduced to an area where there is none.

    So what is rust anyway? Is it some form of metal eating virus, some form of decomposition of the metal caused by what is in the metal or is it something other than what the metal is made up of as if some form or termites?

    How is it that you can remove rust by way of DC current, yet batteries that get old tend to rust when the acid leaches thru the container? How is it that high temperatures tend to rust un-treated metal in an expediated way at times over that of metals that got and stayed moist? This all at times no matter what type of metal or surface treatment is in use.
     
  2. Radman

    Radman Well-Known Member

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    Well I know that rust is formed by oxidization, a chemical reaction between iron and oxygen. I think it might be considered a burn as well, or am I thinking of frostbite? Well the rust is the product of a reaction between iron and oxygen. It might happen with other metals too, I don't know. (Just started chem this year. Thank goodness for all those experiments I did when I was younger. Curses for my teacher not knowing anything about chemistry!)

    So:

    Fe + O -> RUST

    I wish I could remember more.
     
  3. TheHeadhunter

    TheHeadhunter Member

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    Iron + Oxygen --> Iron Oxide
    4Fe(s) + 3O2(g) --> 2Fe2O3(s)
    NOTE: This is a redox reaction. The half-equation for the oxidisation of Iron is: Fe(s) --> Fe+2(aq) + 2e-

    Iron Oxide is Rust. Nothing else is rust. Other metals do corrode, however. eg Lead Oxide, Tin Oxide, Aluminium Oxide etc.
     
  4. bahaha

    bahaha Member

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    It's been a while since i've done redox. The iron is being oxidized, it it losing electrons. Those electrons are gained by oxygen, which is being reduced. By supplying energy (DC) you can make the reaction run in reverse, removing the rust. This is how metal plating works. Batteries also work on the principle of redox reactions. With a lump of iron left in the rain, the electrons are free to move however they please. But if we force the electrons to move through a wire, we've made a battery.
     
  5. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    If I recall--the correct formula is actually Iron + Oxygen + Water or moisture to create rust.. You need to count that third part to create Iron Oxide. The water or moisture is the medium for allowing the oxidation transfer for electrolytes creating an acid.. I know in a DRY situation (completely free of moisture in the air or surroundings) that rusting occurs MUCH much slower or very little at all...

    Been a while since I had chemistry....

    -w
     
  6. kingfisher1

    kingfisher1 Active Member

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    really? i thought that water just provided lots of O2.
    actually though, now that i think about it WD-40 is hydraphobic and a anti rust agent, connection?
     
  7. ship

    ship Senior Team Emeritus Premium Member

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    So how than is it that rust also acts like a virus?

    Under normal conditions of not too hot or cold, not too moist or dry, put something in a sealed container with something that is rusting and even if not touching it, it most likely will also develop rust?

    Still just curious about this virus that has effected many of the bolts and nuts and washers and things I have stored up for years.

    Much less the cat pee that instantantly rusted what ever at a young age he marked his territory on.

    What is it about cat pee that instantly rusts zinc plated steel?

    Just some thoughts....
     
  8. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    I think I can answer ya about the cat pee probably--pee contains urea (which can form ureaic acid--and an acid needs to form to create the oxidation procees for Rust to occur)--and Urea is also a SALT. Salts are corrosive to begin with and speed up the rusting process significantly, and salt stays around on many surfaces long after the moisture has dried--so when it gets moisture around it again--the salt starts back up again and accellerates the rusting...

    Someone else will have to try and explain the transfer to other metals that are not touching--I believe it has to do with the free electrons flying around during the oxidation process...but as I said--its been a long time since I had chemistry class....

    -w
     
  9. chieftfac

    chieftfac Active Member

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    Rust

    Good one wolf....I think the acid in the cat pee reacts with the zinc (removes its bond with the steel) and aids the oxidation proccess....and rust transmital through the air is a result of free electrons (those dam electrons, can't they behave??) I'd add more but it would just be reduntant.

    I do know this

    Rust never sleeps...
     
  10. ricc0luke

    ricc0luke Active Member

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    As people before have stated, rust is a iron oxide.
    This is mostly correct, but you forgot that iron in this case needs to be written as Fe(III) not just Fe. This is because iron is a metal that could have a positive charge of +2 or +3. In the case of rust, the charge is +3, hence the reason it is written Fe(III).

    Rewritten it is ==> 2Fe(III)(s)+3O2(g) --> 2Fe2O3(s)

    Alright, since I get the feeling only some and not all are adapt at chemistry... he's the jist of what that ment--

    You take 2 atoms of iron with a +3 charge and take 3 molcules of O2, put them together, and you get 2 molcules of iron oxide... to be more specific since there are two types of iron oxide, its Hematite.


    Wolf825
    kingfisher1 is right, technically, you don't have to have water, it just works a hell of alot faster with water... Here's what that equation would look like ==>

    Fe(III)(s)+3H2O(a) --> Fe2O3(s)+3H2(g)


    Ok... and I'm not sure where you guys started talking about acids at... those are another story... Acids have much more complex rules regarding how they bond and what happens when they do. Besides, I doubt ship dumped acid on his screws.

    Ship, as far as your brass screws go... It is very possible that A) the rust is that from one of the other steel screws that simply got onto the brass screw... it won't be easy to clean off the brass, but solid brass cannot rust, which brings me to the next seneiro... B) The screw is not solid brass and contains iron in some form that is exposed. Or, there is alway the last thing I can think of though very unlikely unless those screws have been there for a long long time... C) When 2 metals are touching each other, they act like liquids in really really slow motion in that the property of diffusion applies to them. This means that the brass on the brass screw with start to move over onto the steel screw and the steel will start to move over on the brass... This is normally a very very very slow process, but is speed up by heat. This is why when you change the spark plugs in your car you but anti-seizing stuff on them, because at high tempertures, this process is speed up and after time, the sparkplug with meld with the engine. But my bets goto A and B first.


    Another thing about the moisture... Rust is speed up even more when it does to wet or humid and then dry cycles. Example: A plate of iron that is dipped in water, allowed to dry and then this is repeatly done, will rust faster than a peice of iron that is left submerged in water... This has to deal with the evaporation of the water and the bonding... Because the electrons are all happily bonded in body of water, but when it evaporates those bonds break.


    Anyway, as far as rust being on another peice of metal that is not touching a rusting peice of metal, it is simple. They are both rusting. Rust isn't a virus. It won't jump from one peice to another. Both peices of metal are simply rusting. Rust can start anywhere, anytime where there is iron and oxygen.

    The reaction is speed up by heat because heat is energy. They extra heat excites the electrons in the iron. They get excited, and they are more likely to bond with oxygen. And like bahaha said, the electricity splits the Fe2O3 molcule and leaves with Fe(III)(s) and O2 again.. Why you can't do that with AC I'm not sure... maybe you can... I don't know, that's more a weird physics problem...


    Does that clear things up abit? Please, ask questions...


    Alrighty, I'm outtie-
    -Luke
     
  11. jwl868

    jwl868 Active Member

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    Hey, hey. My, my.

    Wait

    Headhunter's original equation is correct:

    4Fe(s) + 3O2(g) --> 2Fe2O3(s)

    This is a balanced oxidation-reduction reaction. Rust (and corrosion) is oxidation of metal. In the common example, iron reacts with oxygen and turns to iron oxide when it rusts (oxidation at its most obvious). But an atom of a pure element, like iron and oxygen, has all of its electrons, is not charged, and has a valence number of 0. When an element reacts with another element, electrons can be shared with or transferred to the other element. And an electron has a charge equal to -1. When iron reacts with oxygen to produce rust, it gives up some of its electrons to the oxygen. Subtract a negative number and you get a positive; the iron's valence number goes up, to +I, +II, or +III. (The increase in the valence is oxidation.) (With rust, iron goes to Fe(III)). Oxygen's valence number is reduced to –II. (Roman numerals are usually used for valence, to distinguish it from a charge.)

    The opposite of oxidation is reduction, and reduction refers to the valence number. In rust, oxygen is reduced. In fact, any reaction where a substance gets oxidized, another gets reduced. (The balance of these oxidation-reduction reactions can often be elegant, but I digress.)

    Thus, with the valence numbers included, and excluding the (s) for solid and (g) for gas:

    4Fe(0) + 3O2(0) --> 2{Fe(III)2 O(-II)3}



    Joe
     
  12. TheHeadhunter

    TheHeadhunter Member

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    ricc0like:
    THe DC current provides the electrons used in the reaction - the negative part 'pumps out' electrons, the posotive 'sucks up' electrons. In ac, polarity is constantly reversed so this ballances out and dosn't cause a reaction.

    Also, about the other metals... In chem thisavo some-one mentioned the Galvanic table. A metal that is higher up on this table corrodes metals below it. Maybe this has to do with the rust 'spreading'. Anyone who understands this more please explain...

    Also, I think things are usually brass-plated, not only brass as this would be very expensive.

    -jono
     
  13. drumbum

    drumbum Member

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    Really bad if you poke yourself with it . . .
     
  14. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    Hi Luke--great explanation--thanks for writing it. =)

    As for my input about acids--I recall that when moisture and air mixed around the metal it would form a weak carbonic acid--a mild corrosive (from the CO2 in the air mixing with the moisture), that would become electrolytic and help speed up the rusting process...that is why I brought up acids..and also why I brought up the water factor, since most air most places has some moisture in it. Don't know whether I am correct or not in presuming that I am recalling this correctly (and please correct me if I am mistaken) or that it was a major contributing factor--just figured I would toss that info out there and see if it could jog some other discussions. Best things I learned and retained from chem class had to do with fire and explosive reactions..hehehe... :)

    -w
     
  15. kingfisher1

    kingfisher1 Active Member

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    i was just reading one of my fishing cataloggs and i saw a book for flys that prevents the hook from rusting, it said it was "chemically infused..."
    any idea which chemical?
    something that keeps the e- from mis behavin' ;)
     
  16. jwl868

    jwl868 Active Member

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    Corrosion is an electrochemical process and water and oxygen are necessary components. The typical water vapor in the air is enough to allow these reactions to proceed. But rust will proceed faster where its wet compared to dry.

    Corrosion of iron (and steel) actually follows several steps, and the earlier post showed the net reaction:

    1. 2Fe + 2H2O + O2 --> 2Fe(OH)2 [Fe(II)]

    (Note that the oxygen in the water is already reduced.)

    2. 4Fe(OH)2 + 2H2O + 02 --> 4Fe(OH)3 [Fe(III)]

    (Note that the oxygen in the hydroxide [OH] is already reduced.)

    And with time, the Fe(OH)3 crystallizes to:

    3. 2Fe(OH)3 --> Fe2O3*3H2O (although some of the water is not bound)


    So you see with corrosion you need a base metal (okay, that's obvious), you need another element to be reduced (typically oxygen), and you usually need a water solution for the ions to complete the reactions. If you can prevent either of the last two things, you'll stop corrosion, or at least slow it down. (Okay, you could eliminate the corrosion issue by choosing a different material…)

    Keep things dry or coated (with paint, for example, or plated with another metal, or even oiled) and corrosion will be minimized or eliminated.

    ***

    That "chemically infused" sounds like marketing-speak. Could be plated, anodized (a process like plating), conversion-coated (another plating-type process that proceeds chemically, or painted.

     
  17. jwl868

    jwl868 Active Member

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    To address a few items:

    The metals (and alloys) are arranged according to a "galvanic series". (I don't know the chemical or physical reasons why this is so, you'll have to look that one up.) Less active metals (noble) are at one end of the list, and more active metals at the other. A partial list: (from noble to active) is platinum-certain stainless steels-copper-brass-iron-steel-zinc. [Some authors arrange the noble metals at the top of the list; other authors arrange the active metals at the top of the list.]

    This galvanic series affects the corrosion properties of dissimilar metals in contact with each other, either touching or with some other electrical conductor in contact with them, and in a conducting solution, like tap water. In this situation, the more noble metal will be protected from corrosion at the expense of corrosion of the more active metal. Steel is less active on the series than zinc and this is partly why zinc coating protects steel (galvanized steel). On one hand, the coating keeps oxygen and water off the steel. On the other, the zinc will oxidize instead of the steel. Eventually the zinc metal is entirely oxidized, and the iron is then exposed to corrosive conditions, although somewhat protected by the zinc elsewhere on the piece.

    If the proportion of the more noble metal is large compared to the active metal, then the deterioration of the active metal will be fast. An example of galvanic corrosion is using a steel screw (small amount of active metal) to attach the washer to a brass valve stem (large amount of relatively noble metal) in a faucet. While steel is a bad choice in the first place, the galvanic corrosion accelerates the corrosion of the screw. (The closer the dissimilar metals are to each other and the greater the distance apart on the galvanic series increases the rate of the galvanic corrosion.)

    ***

    Not sure about ship's aluminum pitting problem. It could just be moisture, droplets of water, or imperfections in the sheet that lead top the localized corrosion. (Aluminum is available is several grades.) Or, aluminum is slightly more active than steel. If other metals were resting on the aluminum sheet and conditions favorable to corrosion were present (moisture and oxygen), then the aluminum will corrode in lieu of the steel while in contact with steel.

    ***

    Rust and corrosion do not spread from object to object through the air. If the conditions are suitable for corrosion – water and oxygen – then rust will occur. (It can spread along a surface of an object because the reactions themselves often create conditions more favorable for corrosion at the interface between the pure metal and the corrosion products.) But without doing a control, I would expect identical pieces of steel sealed in identical bags under identical conditions to rust to the same extent, regardless of the presence on a rusted piece of steel in the bag. The sealed bags still contain moist air and oxygen, so you could get some rust, but eventually the oxygen will get used up. [I suppose some of the water from the hydrated rust (Fe2O3*xH2O) could increase the water vapor concentration in the bag with the rusty steel; that could increase the rate of the rust reaction, but not necessarily cause the rust.]



    Joe
     
  18. ship

    ship Senior Team Emeritus Premium Member

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    Wow, what a discussion! In theater, those with some semblence of science in understanding it well by way of at least a general understanding of it's principles will go much further than someone out for fun in it. Remember what you learn in science class because it's important for using it in the career.

    Yep, plates of steel and aluminum were stacked together in the trunk of my car for a few months thus the moisture also.
    The surface are of the 6" square steel plates was for the most part coated in rust. The aluminum adjacent to it was more or less pock marked with recessed areas where the steel rusted more. This and in general there was a almost blead off of the rust that no doubt came with moisture that attached itself in general to the aluminum.

    Just heard my current assistant tell me today about the wonders of cola on cleaning rusting tools. Seems he has somehow gotten a hold of a electrician in his apartment complexes tool kit whic while in general is a good thing, some are rusting. He thus talked about a Coke bath for them. One would think the acid in it would further etch the metal and any coatings on it would be more succeptable to rusting.
     
  19. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    Hey SHip--yup this is a great discussion.... As for the cola bath-I think the boys at Mythbusters did something with cola...and if I recall correctly--I think it had to do with cola and tooth decay--and if I am remembering right--the cola bath dissolved a tooth to nothing in a few days. It can be pretty tough corrosive and strong stuff..and mighty tastee over ice. :) Speaking of which--time for a refill...

    -w
     
  20. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Yep – great discussions and I have enjoyed reading the posts in this forum.

    Mythbusters did in fact conduct several experiments involving Coke and these included, cleaning coins, dissolving a tooth, dissolving a steak, cleaning chrome and loosening bolts. The only one that they confirmed was the cleaning of chrome with Aluminium foil and Coke, when compared to other available cleaners. All the others failed. Thus your assistant is simply wasting his drink and making the tools stick when other agents do a much better job.

    I actually give all my tools and metal work surfaces a wipe down with a mixture of 50% air tool oil and 50% white spirits to prevent rusting.
     

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