AME 7 Q2

Correct Answer:

A. The formation of an electrolytic cell where two dissimilar metals in contact in the presence of an electrolyte leads to one metal corroding preferentially while the other is protected.

Explanation:

• A. Correct: Galvanic corrosion occurs when two dissimilar metals are in electrical contact within an electrolyte (such as seawater). The metal with a lower electrochemical potential (anodic) corrodes preferentially, while the metal with a higher electrochemical potential (cathodic) is protected. This creates a galvanic cell.
• B. Incorrect: This describes metal degradation due to high temperatures without an electrolyte, which is not galvanic corrosion. High-temperature corrosion is a different process.
• C. Incorrect: This describes general corrosion or rusting, which occurs due to chemical reactions with air and moisture, not specifically galvanic corrosion. It does not involve an electrolytic cell.
• D. Incorrect: This describes corrosion caused by the accumulation of salt crystals leading to mechanical stress, not galvanic corrosion. Galvanic corrosion involves electrochemical reactions rather than mechanical stress.

Correct Answers:

A. The electrochemical potential difference between the dissimilar metals.
D. The presence and conductivity of the electrolyte.

Explanation:

• A. The Electrochemical Potential Difference: This is correct because a larger potential difference between the dissimilar metals increases the severity of galvanic corrosion. Metals with a greater difference in electrochemical potential will result in more pronounced corrosion of the anodic metal.
• D. The Presence and Conductivity of the Electrolyte: This is correct because the presence of an electrolyte (such as seawater) facilitates the flow of electric current between the metals. The higher the conductivity of the electrolyte, the more severe the galvanic corrosion.

B. The Color of the Metals: This is incorrect because the color of the metals does not influence galvanic corrosion. The key factors are the electrochemical properties and the environment.

C. The Temperature of the Surrounding Environment: While temperature can affect the rate of corrosion in general, it is not a direct factor in the severity of galvanic corrosion specifically. The key factors are the potential difference and the electrolyte conductivity.

E. The Thickness of the Metal Layers: This is incorrect because the thickness of the metal layers does not directly impact the severity of galvanic corrosion. The primary factors are the electrochemical potential difference and the presence of an electrolyte.

Explanation:

• A. Temperature of the Seawater: Higher temperatures accelerate corrosion by increasing reaction rates and reducing oxygen solubility.
• B. Presence of Dissolved Oxygen: More dissolved oxygen can enhance corrosion rates due to its role in electrochemical reactions.
• C. Type of Metal Used: Different metals have different corrosion rates based on their electrochemical properties.
• D. Salinity of the Seawater: Higher salinity increases electrical conductivity, which accelerates corrosion.
• F. Flow Rate of Seawater: Increased flow rates enhance the delivery of corrosive agents to the metal surface, increasing corrosion.
• G. Presence of Biological Organisms: Biofouling can create corrosive microenvironments and increase corrosion rates.
• H. Duration of Exposure: Longer exposure allows more time for corrosion to occur, increasing overall corrosion.

E. Presence of Protective Coatings or Paints: Incorrect because protective coatings or paints actually reduce corrosion rates.

I. Metal’s Color or Paint Scheme: Incorrect because the color or paint scheme of the metal does not influence the corrosion rate of the unprotected metal surface.

J. Amount of Sunlight Exposure: Incorrect because sunlight exposure does not directly affect the rate of corrosion in a marine environment.