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Corrosion and surface treatment

Which of the following statements about the corrosion protection of steel sheet is correct?

  1. AWith galvanized (zinc-coated) steel sheet, even if a scratch exposes the steel, the surrounding zinc dissolves first and protects the steel
  2. BWith tinplate (tin-coated steel sheet), too, if a scratch exposes the steel in air, the tin dissolves first and protects the steel
  3. CPaint prevents corrosion by passing a current through the coating to make the steel the cathode
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Answer: A. With galvanized (zinc-coated) steel sheet, even if a scratch exposes the steel, the surrounding zinc dissolves first and protects the steel

Zinc is more active than steel, so even if the coating is scratched and the steel exposed, the zinc becomes the anode and dissolves, protecting the steel as the cathode (sacrificial protection). In air or water, on the other hand, tin is more noble than steel, so once a scratch exposes the steel, the steel becomes the anode and the large tin surface the cathode, which actually accelerates corrosion of the small exposed area. Inside a sealed can with little oxygen, however, tin forms complexes with the food's organic acids and becomes more active than steel, protecting it, which is why tinplate has been used for can interiors. Paint works as a barrier that keeps water, oxygen and ions from reaching the steel surface, not by passing a current. Zinc-rich paint, which contains a large amount of zinc powder, does also provide sacrificial protection from the zinc in the coating.

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More Corrosion and surface treatment questions

  1. In a steel pipe, a corrosion current in which iron dissolves as Fe → Fe²⁺ + 2e⁻ flowed at an average of 0.10 A for one year (365 days). Which value is closest to the mass of iron dissolved? Take the molar mass of Fe as 55.85 g/mol and the Faraday constant as 96485 C/mol.
  2. When the corrosion current density of iron is uniformly 1.0 µA/cm² over the whole surface, which value is closest to the rate at which the surface recedes? Assume iron dissolves as Fe²⁺, with a molar mass of 55.85 g/mol, a density of 7.87 g/cm³, a Faraday constant of 96485 C/mol, and one year as 3.15×10⁷ s.
  3. Nickel electroplating is carried out at a current density of 2.0 A/dm² for 30 minutes. Assuming 100% current efficiency, which value is closest to the thickness of the deposit? Take Ni²⁺ + 2e⁻ → Ni, a molar mass of Ni of 58.69 g/mol, a density of 8.90 g/cm³ and a Faraday constant of 96485 C/mol.
  4. At 25 °C, the standard electrode potentials are +0.34 V for Cu²⁺/Cu, −0.44 V for Fe²⁺/Fe and −0.76 V for Zn²⁺/Zn. Which of these three metals is thermodynamically the most easily oxidized (most readily becomes a cation)?
  5. When local cells form on an iron surface in neutral, aerated water and corrosion proceeds, which reaction mainly occurs at the cathode?