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Corrosion and surface treatment

Check standard electrode potentials, local cells and differential aeration cells, galvanic corrosion, passivity, pitting and stress corrosion cracking, and protection by sacrificial anodes, plating and painting with multiple-choice questions, and practice calculating corrosion amounts with Faraday's law.

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Corrosion and surface treatment: key points

Corrosion of metals is an electrochemical reaction that proceeds as a pair: an anodic reaction in which the metal gives up electrons and becomes cations, and a cathodic reaction in which dissolved oxygen or hydrogen ions accept electrons. Metals with more active standard electrode potentials tend to dissolve more readily. Even on the surface of a single metal, nonuniformities and oxygen concentration differences create local cells, and when dissimilar metals are in contact, galvanic corrosion occurs in which the more active one dissolves faster.

Stainless steels and titanium are protected by passive films a few nanometers thick, but localized corrosion requires care: pitting and crevice corrosion caused by chloride ions, and stress corrosion cracking when tensile stress and environment coincide. Protection methods include sacrificial anodes, plating and painting, and the key point is how active zinc protects steel. Amounts of corrosion and plating can be calculated from current and time with Faraday's law.

All Corrosion and surface treatment questions (12)
  1. 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)?
  2. When local cells form on an iron surface in neutral, aerated water and corrosion proceeds, which reaction mainly occurs at the cathode?
  3. When the concentration of dissolved oxygen varies from place to place on steel in water, a differential aeration cell forms. Where does corrosion tend to concentrate?
  4. A zinc sacrificial anode is attached to a steel structure in seawater. Treating it as a cell under standard conditions with standard electrode potentials of −0.44 V for Fe²⁺/Fe and −0.76 V for Zn²⁺/Zn, which correctly gives the emf and the metal that dissolves?
  5. Steel and copper are used in electrical contact in seawater. In which case is galvanic damage to the steel likely to be most severe?
  6. What is the main reason stainless steel resists rusting in air and water?
  7. When SUS304 (JIS; AISI 304) is used in chloride-rich seawater, which correctly pairs the localized corrosion likely to occur with a countermeasure?
  8. Which statement about stress corrosion cracking is correct?
  9. Which of the following statements about the corrosion protection of steel sheet is correct?
  10. 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.
  11. 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.
  12. 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.

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