Question

Pennies manufactured after 1982 are made of zinc metal, \(\mathrm{Zn}\), within a coat of copper metal, Cu. Zinc is more easily oxidized than copper. Why, then, don't these pennies quickly corrode?

Short answer

The copper coating acts as a protective barrier against corrosion for the zinc core.

Step-by-step solution

Understand the Material Composition

Pennies post-1982 are primarily made of zinc with a thin coating of copper. The zinc composition allows for a cost-effective material while maintaining the appearance and initial protective properties of copper.

Consider the Oxidation Tendency

Zinc is indeed more easily oxidized than copper, which generally means zinc would corrode faster when exposed to elements such as air and moisture.

Assess the Role of the Copper Layer

The outer copper coating protects the underlying zinc. Copper is less reactive and forms a protective barrier against direct exposure to environmental factors that could cause oxidation.

Evaluate the Role of Passivation

Copper can form a stable oxide layer when oxidized, which further protects the zinc core. This passivation layer limits the penetration of oxygen and moisture to the zinc.

Analyze Any Environmental Factors

In usual environmental conditions, the copper layer remains intact, preventing significant exposure of zinc to create a rapid corrosion process.

Key concepts

Material Composition

The composition of a material is crucial in determining its properties and behavior. Modern pennies are primarily made from zinc with an external layer of copper. This combination is chosen for several reasons:

• Cost-effectiveness: Zinc is cheaper than copper, making it economically beneficial to use as the primary material.
• Appearance: The copper coating gives pennies the traditional metallic shine and color associated with older, fully copper pennies.

This clever design helps balance both economic considerations and aesthetic expectations.

Corrosion Resistance

Corrosion resistance is a key property that prevents rapid deterioration of metals. In the context of post-1982 pennies:

• Copper's Role: The copper layer prevents the zinc core from direct exposure to harmful environmental elements like oxygen and water, which can lead to corrosion.
• Barrier Function: This thin copper layer acts as a first line of defense, providing a stable exterior surface resistant to environmental degradation.

By employing a material that naturally resists corrosion, the longevity of the pennies is greatly enhanced.

Metal Passivation

Metal passivation is the process by which a metal surface becomes less reactive when exposed to environmental factors. Copper is known to undergo a passivation process:

• Oxide Layer Formation: When copper oxidizes, it forms a stable and protective oxide layer on its surface.
• Protection of Zinc Core: This oxide layer reinforces the barrier against moisture and oxygen, providing an additional shield to the underlying zinc.

Thus, passivation plays a critical role in maintaining the protective qualities of the copper coating over time.

Environmental Chemistry

Environmental conditions greatly influence the corrosion and oxidation processes in metals. For pennies:

• Stable Conditions: In normal environmental conditions, the copper layer remains intact and does not degrade quickly, thus protecting the zinc core from corrosion.
• Pollutants and Variability: Factors such as pollution, humidity, and temperature changes could affect corrosion rates, but typically, the copper barrier suffices to mitigate these effects.

Understanding environmental chemistry is essential for predicting long-term metal behavior and durability.