Questions: Corrosion and Environmental Degradation

Question 1

Aluminum and iron both oxidize readily in air, yet aluminum objects last indefinitely outdoors while uncoated iron rusts through. What determines this difference?

• A. Aluminum is more thermodynamically stable than iron, so Al₂O₃ forms but Fe₂O₃ does not form spontaneously
• B. The Al₂O₃ film is dense and adherent (Pilling-Bedworth ratio ≈ 1.28), forming a continuous barrier, while Fe₂O₃ has a ratio > 2, causing tensile stress, cracking, and spalling
• C. Iron oxidizes at a faster rate because it has more valence electrons available for reaction with oxygen
• D. Aluminum's lower density means the oxide layer covers more surface area per unit mass, providing better protection

Answer: (B)

Both metals oxidize thermodynamically — neither is inherently stable as a pure metal. The difference is entirely in the oxide product. The Pilling-Bedworth ratio (volume of oxide / volume of metal consumed) governs whether the oxide seals or cracks. Al₂O₃ at ~1.28 is mildly compressed, adheres tightly, and blocks further oxygen access after just nanometers of growth. Fe₂O₃ at > 2 is under tensile stress, cracks, and flakes off, continuously exposing fresh iron. Corrosion resistance is determined by kinetics of the oxide layer, not thermodynamics.

Question 2

A marine engineer attaches zinc anodes to the steel hull of a ship. Over time, the zinc anodes corrode away but the steel hull remains intact. Which principle explains this?

• A. Passivation: zinc forms a protective oxide film that also coats and protects the steel
• B. Galvanic protection: zinc is more anodic (active) than steel, so it corrodes preferentially while providing cathodic protection to the steel
• C. Galvanic protection: steel is more anodic than zinc, so the zinc acts as a noble cathode, drawing corrosion current away from the hull
• D. The zinc coating physically blocks seawater from contacting the steel, acting as a barrier rather than an electrochemical protector

Answer: (B)

This is sacrificial anode cathodic protection. Zinc is more active (anodic) than steel on the galvanic series, so when both are electrically connected in seawater (the electrolyte), zinc oxidizes (loses electrons = corrodes) while the steel is protected as the cathode. The zinc 'sacrifices' itself to protect the steel. Option C has the galvanic series reversed — the more active metal (zinc) is always the anode, not the cathode.

Question 3

Stainless steel owes its corrosion resistance to a chromium oxide passive film, not to thermodynamic stability — it would corrode rapidly if this film were removed and prevented from reforming.

• T. True
• F. False

Answer: True

This is a crucial distinction between thermodynamic and kinetic corrosion resistance. Stainless steel thermodynamically 'wants' to corrode — the chromium and iron in it would prefer to be in oxide form. The passive film is a kinetic barrier: it forms rapidly when oxygen is available, and its adherence and density make ion transport through it extremely slow, shutting down further oxidation. In reducing environments or when chloride ions competitively displace oxygen from the film, the passive film breaks down and stainless steel can corrode aggressively.

Question 4

Attaching a zinc sacrificial anode to an iron pipe protects the zinc from corrosion while the iron pipe corrodes preferentially.

• T. True
• F. False

Answer: False

This has the roles reversed. Zinc is more anodic (more active) than iron on the galvanic series. When electrically connected in an electrolyte, the more active metal (zinc) corrodes preferentially as the anode, while the less active metal (iron) is protected as the cathode. The zinc sacrifices itself to protect the iron — hence the term 'sacrificial anode.' Attaching copper to iron, by contrast, would make the iron corrode preferentially because iron is more active than copper.

Question 5

Explain why pitting corrosion is considered more dangerous than uniform corrosion, and under what conditions a passivated metal like stainless steel can experience pitting.

Model answer: Pitting corrosion is more dangerous than uniform corrosion because it is localized, rapid, and hard to detect. Uniform corrosion removes material predictably across a surface, allowing engineers to measure the corrosion rate and schedule replacement. Pitting creates small but deep penetrations into the metal while the surrounding surface appears intact, concentrating mechanical stress and potentially causing sudden structural failure. Stainless steel is vulnerable to pitting when its passive chromium oxide film breaks down locally. Chloride ions (found in seawater, deicing salts, and many industrial chemicals) are particularly aggressive: they competitively adsorb onto the oxide surface, displacing oxygen and preventing the film from maintaining itself. In strongly reducing environments, the film also cannot form or sustain itself. Once a pit initiates, the local chemistry inside the pit becomes acidic and depleted of oxygen, which further prevents repassivation and accelerates growth.

The core insight is that passivation is a kinetic effect that requires sustained oxidizing conditions. Any local disruption to the passive film — by chlorides, reducing agents, or mechanical damage — creates a tiny active anode surrounded by a large passive cathode. This galvanic couple focuses all corrosion current into the pit, accelerating its growth while the rest of the surface corrodes imperceptibly.