Which statement best describes magnesium alloys in corrosion concerns?

• A. Magnesium forms a passive oxide that prevents corrosion in all environments.
• B. Magnesium is cathodic to aluminum and thus protects aluminum surfaces.
• C. Magnesium is not used in aircraft due to corrosion risks.
• D. Magnesium is highly anodic and corrodes readily in moisture-containing electrolytes, often requiring protective coatings.

Answer: (D)

Magnesium alloys are highly anodic in the galvanic series, so they corrode readily in moisture-containing electrolytes. In aircraft environments, humidity, salt spray, and de-icing fluids create electrolytes that accelerate corrosion, making protective coatings and barriers essential to keep the material durable. This is why the statement describing magnesium as being highly anodic and corroding easily in moisture-containing electrolytes, often needing protective coatings, is the best description. The other ideas miss key realities: the oxide film on magnesium isn’t a universal, all-environment shield; magnesium is not cathodic to aluminum (it’s more anodic, and would not reliably protect aluminum surfaces by acting as a cathode); and magnesium is indeed used in aviation with appropriate protective measures, not avoided purely due to corrosion risk.