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Zorluk: OrtaCorrosion of Metals and Prevention Methods

An underground steel pipeline carrying natural gas is buried in moist soil. To protect the iron in the steel from corrosion, blocks of another metal are electrically connected to the pipeline at regular intervals. Given the standard reduction potentials below:

Fe(aq)2++2e−→Fe(s),E∘=−0.44 V\text{Fe}^{2+}_{(\text{aq})} + 2e^- \rightarrow \text{Fe}_{(\text{s})}, \quad E^\circ = -0.44\text{ V}
Cu(aq)2++2e−→Cu(s),E∘=+0.34 V\text{Cu}^{2+}_{(\text{aq})} + 2e^- \rightarrow \text{Cu}_{(\text{s})}, \quad E^\circ = +0.34\text{ V}
Mg(aq)2++2e−→Mg(s),E∘=−2.37 V\text{Mg}^{2+}_{(\text{aq})} + 2e^- \rightarrow \text{Mg}_{(\text{s})}, \quad E^\circ = -2.37\text{ V}
Ag(aq)++e−→Ag(s),E∘=+0.80 V\text{Ag}^{+}_{(\text{aq})} + e^- \rightarrow \text{Ag}_{(\text{s})}, \quad E^\circ = +0.80\text{ V}

Which metal can be attached to the pipeline to serve as an effective sacrificial anode, and what chemical change occurs to it during protection?

  1. Magnesium, because it is oxidized preferentially due to its more negative reduction potential.Cevap
  2. B
    Copper, because its positive reduction potential allows it to supply electrons to the iron structure.
  3. C
    Silver, because metals with higher reduction potentials readily undergo sacrificial oxidation.
  4. D
    Magnesium, because it undergoes reduction at the cathode while keeping the iron oxidized.

Cevap

Magnesium serves as an effective sacrificial anode because it has a more negative standard reduction potential than iron, meaning it oxidizes more readily and supplies electrons to protect the iron pipeline.
Magnesium has a more negative standard reduction potential (-2.37 V) than iron (-0.44 V). In an electrochemical cell formed in moist soil, magnesium oxidizes preferentially (Mg→Mg2++2e−\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-), supplying electrons to the steel pipe. This makes the iron pipeline cathodic and prevents iron from oxidizing into rust.

Adım Adım Çözüm

1
Compare the standard reduction potentials (E∘E^\circ) of iron and the candidate metals.
E∘(Mg2+/Mg)=−2.37 VE^\circ(\text{Mg}^{2+}/\text{Mg}) = -2.37\text{ V}, E∘(Fe2+/Fe)=−0.44 VE^\circ(\text{Fe}^{2+}/\text{Fe}) = -0.44\text{ V}, E∘(Cu2+/Cu)=+0.34 VE^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}, and E∘(Ag+/Ag)=+0.80 VE^\circ(\text{Ag}^{+}/\text{Ag}) = +0.80\text{ V}.
A metal acts as a sacrificial anode only if it is more easily oxidized (more electropositive) than iron, which corresponds to having a more negative standard reduction potential.
2
Identify which metal will oxidize preferentially over iron.
Magnesium has the most negative reduction potential (-2.37 V < -0.44 V), so it oxidizes preferentially: Mg(s)→Mg(aq)2++2e−\text{Mg}_{(\text{s})} \rightarrow \text{Mg}^{2+}_{(\text{aq})} + 2e^-.
The metal with the lower (more negative) reduction potential readily loses electrons to the iron structure, making the iron the cathode in the electrochemical cell.
3
Determine the chemical process occurring at the sacrificial block.
Oxidation takes place at the sacrificial magnesium block (the anode), sacrificing the block while preserving the steel pipeline.
By definition, oxidation occurs at the anode (anode=oxidation\text{anode} = \text{oxidation}).

Anahtar Kavram

Sacrificial Anodic Protection
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Konular

Corrosion of Metals and Prevention Methods

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