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Homework Help: Rusting - Exponential?

  1. Jun 6, 2010 #1
    Sorry if this might be in the wrong section, but I wanted someone's opinion on the formation of rust.

    Do you think that the rate of formation of rust on iron/steel is an exponential growth?...


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    Also, some extra. I'm doing an EEI on rusting - basically, I'm trying to find the best methods of protection.

    In one of my tests, which included a Zinc strip as a sacrificial anode - the iron ended up rusting, even when there was plenty of Zinc remaining. The iron and zinc were touching, so a galvanic cell effect would have been created....I just don't understand why it would have rusted. Also, it was in a sodium chloride solution (Salt Water), so I'm thinking at the moment that the sodium chloride would have had something to do with it.
     
    Last edited: Jun 6, 2010
  2. jcsd
  3. Jun 6, 2010 #2

    Borek

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    Staff: Mentor

    Are you sure there was an electrical contact between iron and zinc? Even if they looked like connected, they could be in reality separated by thin layer of oxides.

    I have a feeling you started experimenting without completing the literature research - why sodium chloride speeds up corrosion should be obvious to you after reading about corrosion mechanisms. It lowers water resistance, closing the circuit.

    No idea what EEI is. After googling I guess you probably don't mean Edison Electric Institute, East of England International, Energy Economics, Inc. nor Egyptian Educational Initiative. There were more examples even on the first page.
     
  4. Jun 6, 2010 #3
    Yes, I understand the effect of the sodium chloride; basically it acts as the salt bridge. What I meant was I thought the sodium chloride might have been the cause of the rust in the iron even when the zinc was present. Sorry if it seemed a bit confusing.

    Back to the Zinc and the Iron, I didn't think about the thin layer of oxides...I presumed that the zinc oxide would still connect the iron and the zinc strips, creating a electric circuit.

    So overall, what you're saying is that the oxides don't close the circuit?

    Oh, and EEI stands for Extended Experimental Investigation (In Australian schools)
     
    Last edited: Jun 6, 2010
  5. Jun 6, 2010 #4

    Borek

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    Staff: Mentor

    Oxides are usually good insulators.

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  6. Jun 6, 2010 #5
    Thanks Borek, you've been of great help :)

    Now does anyone have any opinions on the spreading of rust?...is it exponential?
     
  7. Jun 7, 2010 #6

    Borek

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    Staff: Mentor

    Unlikely - it may occur only at the surface, and surface is not growing, quite the opposite.

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  8. Jun 7, 2010 #7
    Sorry - used to maths.

    Thanks for all the help though :).
     
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