Corrosion

unwanted oxidation of metals

Main points

  • Understand BV mixed corrosion potential and the corresponding Tafel plot.
  • The slope is 1/b(Tafel slope)
  • Fe corrosion: Effects of Fe(ii) and Cl\(^-\), pits and crevices
  • Cathodic and anodic protection

Effects of salt, chloride ions

  • Complexing agent:
    • \(\mathrm{Fe}(s)+\frac{3}{4} \mathrm{O}_{2}(\text { air })+3 \mathrm{H}_{3} \mathrm{O}^{+}(a q)+2 \mathrm{Cl}^{-}(a q) \rightarrow \mathrm{FeCl}_{2}^{+}(a q)+\frac{9}{2} \mathrm{H}_{2} \mathrm{O}(\ell)\)
  • Dissolving protective layers:
    • \(\mathrm{Al}_{2} \mathrm{O}_{3}(s)+8 \mathrm{Cl}^{-}(a q)+6 \mathrm{H}_{3} \mathrm{O}^{+}(a q) \rightarrow 2 \mathrm{AlCl}_{4}^{-}(a q)+9 \mathrm{H}_{2} \mathrm{O}(\ell)\)

Noble metls and other corrosion-resistive metals

  • Noble metals
    • Only gold is thermodynamically stable
    • Pt, other Pt group metals, Ag, Hg, and to some extent Cu are often stable, because of small driving forces for oxidation
  • Corrosion resistive metals
    • Aluminium is very unstable, but therefore forms a very stable protective oxide layer Al\(_2\)O\(_3\).
    • Same is true for many non-noble metals; Mg, Ti, Cr, Ni, Zn, Sn

Corrosion cells: Two electrodes on the same surface

  • Often reduction and oxidation happens at different locations:
    • Electrochemistry: Transport of electrons and ions between the two locations
    • Often av ever-changing mosaic of sites due to fluctuating driving forces and kinetics (p215-216 in textbook)

Corrosion of Iron and its alloys

  • Is often autocatalytic means: it accelerates itself.
  • \(\mathrm{Fe}(s) \rightarrow 2 \mathrm{e}^{-}+\mathrm{Fe}^{2+}(a q)\)
  • \(4 \mathrm{Fe}^{2+}(a q)+\mathrm{O}_{2}(a q)+4 \mathrm{H}_{3} \mathrm{O}^{+}(a q) \rightarrow 4 \mathrm{Fe}^{3+}(a q)+6 \mathrm{H}_{2} \mathrm{O}(\ell)\)
  • \(2 \mathrm{Fe}^{3+}(a q)+\mathrm{Fe}(s) \rightarrow 3 \mathrm{Fe}^{2+}(a q)\)
  • All above is a negative spiral of oxidation
  • Total: \(4 \mathrm{Fe}(s)+3 \mathrm{O}_{2}(a q)+12 \mathrm{H}_{3} \mathrm{O}^{+}(a q) \rightarrow 4 \mathrm{Fe}^{3+}(a q)+18 \mathrm{H}_{2} \mathrm{O}(\ell)\)
  • Agitation(røring) slows corrosion by removing catalyst

Corrosion potential and corrosion current

  • Corrosion potential: Mixed potential caused by two electrode processes
    • Anodic reaction: \(\mathrm{M}(s) \rightleftarrows n \mathrm{e}^{-}+\mathrm{M}^{n+}(a q)\)
    • Cathodic: \(2 \mathrm{H}_{3} \mathrm{O}^{+}(a q)+2 \mathrm{e}^{-} \rightleftarrows 2 \mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{H}_{2}(g)\)
  • Limited by kinetics (no transport or ohmic polarisation)
    • See figure under ##Corrosion cells above.
  • Tafel regions of each of the two reactions

Polarisation curve

  • Linear polarisation
    • \(I=I_{\text {cor }}\left[\exp \left\{\frac{E-E_{\text {cor }}}{0.434 b_{\mathrm{ox}}^{\mathrm{M}}}\right\}-\exp \left\{\frac{-\left(E-E_{\mathrm{cor}}\right)}{0.434 b_{\mathrm{rd}}^{\mathrm{H}}}\right\}\right]\)
  • Polarisation resistance (explains how easily a material corrodes)
    • \(\frac{1}{\text { slope at } E_{\text {cor }}}=\frac{1}{(\mathrm{~d} I / \mathrm{d} E)_{E_{\text {cor }}}}=\frac{0.434 b_{\mathrm{ox}}^{\mathrm{M}} b_{\mathrm{rd}}^{\mathrm{H}}}{\left(b_{\mathrm{ox}}^{\mathrm{M}}+b_{\mathrm{rd}}^{\mathrm{H}}\right) I_{\text {cor }}}=R_{\mathrm{pol}}\)
  • Can also be found form AC Spectroscopy

Pitting corrosion

  • (A lot in the textbook is a bit speculative..)
  • Oxidation of Fe accompanied by reduction of O\(_2\)(g)
  • Catalysed by Fe(iii) and Cl\(^-\) as complexing agent

Crevice corrosion

  • Same mechanism as in pitting corrosion
  • Avoid closed/covered areas

Corrosion protection

  • Coatings
  • Electropaointing with carboxylate COO\(^-\) groups
    • Negatively charged paint polymer particles migrate, are neutralised, and adhere
    • Fe dissolves electrolytically and forms insoluble Fe carbocylates
    • Uniform
  • Electroplating - Electrochemical reduction of plating metal
  • Electroless plating - chemical reduction of plating metal
  • Corrosion inhibitors
    • Oxidising agents - forms protective film
      • Nitric acid
      • Electrochemical anodisation (e.g. for Al)
    • “Phosphating” - hot phosphoric acid or acidic phosphate solution

Cathodic protection

  • Drive the metal (e.g. Fe) towards (cathodic) reduction
  • Use Zn as sacrificial anode (“offeranode”)

Anodic protection: Passivation

  • Drive the metal towards (anodic) oxidation to form a protective oxide layer
  • Some metals are naturally protected by oxidic layers(Al, Cri, Ni)
  • For others, we may aply chemically or electrochemically forced formation of a oxidic scale: passivation

Stress corrosion, H\(_2\), fatigue

  • Crack corrosion
    • High energy in crack tips
  • Stress corrosion cracking
    • Alloys, not pure metlas
  • Hydrogen embrittlement
    • Hydrogen solved in metal, makes it brittle.
  • Corrosion fatigue
    • Dislocations entangle
    • Stress - enhanced corrosion
    • axle with fatigue corrosion:
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