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Corrosion and pH

  1. Feb 3, 2013 #1
    1. The problem statement, all variables and given/known data
    Hello guys. We are given a hypothetical data table and this is pretty much what it contains. It tells that when an iron nail is submerged in various aqueous solutions, the following observations were made. We are to give explanations to each why these things were observed. There is a supply of oxygen in all of those. So, corrosion would occur whenever it can.

    Solution pH Observations

    NaCl Neutral corrosion was observed, red-brown solids are seen

    NaSCN Acidic corrosion observed, red brown solids observed

    HCl Acidic corrosion observed, black solids, gas evolved(H2)

    H2SO4 Acidic corrosion was observed, black solids, gas evolved(H2)

    KNO3 Neutral corrosion observed, red-brown solids

    Na3PO4 Basic no corrosion observed

    NaOH Basic no corrosion observed
    2. Relevant equations



    3. The attempt at a solution

    The first thing I noticed were the bases. No corrosion was observed. I don't really know why. I saw on some source(http://www.corrosion-doctors.org/Electrochemistry-of-Corrosion/Cathodic-processes.htm[/URL]) [Broken] that reduction of oxygen would still occur at basic environment. Fe would still oxidize as usual, and that further oxidation of the Fe2+ ions that were to form would still occur to form rust. Or does Fe metal do not oxidize at pH>7 solutions? So my question here is that why were there no corrosion observed on basic environment?

    On the acids, observations are should be what it is. H2 gas were observed since the strong acids would react with the pure metal to that would reduce the hydrogen ions on HCl and produce H2 gas. The black solids are particularly interesting. Black means that it would be a "magnetite" that was formed right? How can this be? Is this because of the presence of a very strong acids (thus more H+) and thats why it was a magnetite and not a hematite?

    On the neutral solutions, corrosions were still observed. But now, solids formed are reddish brown, like what they usually seen are. This means that they're hematite. Corrosion happens on neutral solutions because they contain (in the given table's case) strong electrolytes which dissociates completely and acts as a pathway for e- to walk through so that it can react to Oxygen (O2) without actually being too close to the metal. Is that right? And why were the solids hematite and not magnetite?

    On the other note, one acidic solution produced the result of a neutral solution. How can this happen? Is this because the salt solution of NaSCN has SCN- ions that hydrolyzes, giving a small amount of H+ and HSCN?? And that it is just weak acid, giving a result as if it was a neutral solution? Is my 'hypothesis' correct?

    Please help me. I'm really confused on how pH level affect corrosion.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Feb 3, 2013 #2

    Astronuc

    User Avatar

    Staff: Mentor

    What is the significance of red-brown solids and black solids, as opposed to the absence of these?
     
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