Question

"Galvanized iron" is steel sheet that has been coated with zinc; "tin" cans are made of steel sheet coated with tin. Discuss the functions of these coatings and the electrochemistry of the corrosion reactions that occur if an electrolyte contacts the scratched surface of a galvanized iron sheet or a tin can.

Short answer

Zinc on galvanized steel protects by corroding itself, while tin allows steel corrosion if scratched.

Step-by-step solution

Understanding Coating Functions

The coatings on steel, such as zinc on galvanized iron and tin on tin cans, serve primarily as protective barriers. Zinc and tin protect steel from rusting by preventing oxygen and moisture from reaching the steel surface. This protection is crucial as it prolongs the life of the steel structures or containers by preventing oxidation.

Zinc and Galvanized Iron

In the context of galvanized iron, the zinc coating acts as a sacrificial anode. This means that when the surface is scratched and the steel is exposed, zinc will corrode in place of the steel. This occurs because zinc is more reactive (less noble) than iron, so it preferentially oxidizes and provides cathodic protection to the steel.

Electrode Reactions for Galvanized Iron

For galvanized iron with a scratch, the electrochemical reactions are: - **Anodic Reaction (Zinc oxidizes):** \[ Zn ightarrow Zn^{2+} + 2e^- \] - **Cathodic Reaction (Steel reduction is minimized):** \[ O_2 + 2H_2O + 4e^- ightarrow 4OH^- \]These reactions prevent the steel from corroding as zinc corrodes instead.

Tin and Tin Cans

In the case of tin cans, the tin coating acts as a barrier to protect steel from corrosion. However, tin is more noble than steel. This means if the tin surface is scratched, the electrochemical reactions can result in the steel corroding instead of the tin.

Electrode Reactions for Tin Cans

When the tin coating is scratched, the following reactions can occur:- **Anodic Reaction (Iron oxidizes):** \[ Fe ightarrow Fe^{2+} + 2e^- \]- **Cathodic Reaction (Tin prevents its own corrosion):** \[ Sn^{2+} + 2e^- ightarrow Sn \]As a result, the exposed steel might corrode faster than if there were no tin present.

Comparing the Effects

The key difference between the applications is in the electrochemistry: zinc acts sacrificially in galvanization, protecting the steel even when scratched, whereas tin, being more noble, allows steel to corrode when it is exposed. This makes galvanized iron more durable in situations where the surface might be damaged.

Key concepts

Galvanized Iron: A Shield Created by Zinc

Galvanized iron is an essential material in many industries because of its remarkable corrosion resistance. By applying a layer of zinc to the steel surface, the metal is safeguarded against moisture and air that could cause rust. When steel is galvanized, it becomes hardy and durable, even in aggressive environments.
Zinc doesn't just form a physical coating; it acts as a protective layer that sacrifices itself to keep the steel from rusting. This means that even if the surface is scratched, exposing the steel underneath, the zinc will continue to protect it by corroding in its place. This unique feature of zinc makes galvanized iron especially durable and a popular choice for construction and manufacturing.

Sacrificial Anode: Protecting Steel Through Self-Sacrifice

The concept of a sacrificial anode is a cornerstone of corrosion protection. When zinc is used as a coating for galvanized iron, it plays the role of a sacrificial anode. In practical terms, zinc will corrode instead of the iron underneath when subjected to conditions that promote oxidation. This occurs because zinc is more reactive than iron.
In electrochemical terms, zinc readily loses electrons, thus oxidizing and dissolving away. Although this may sound negative, it's actually a helpful process. By corroding before the iron does, zinc spares the iron from oxidation, ensuring the longevity of the steel structure. This self-sacrificial action is the reason why galvanized iron is particularly reliable in outdoor and industrial applications.

Electrochemical Reactions: The Science Behind Corrosion Prevention

Electrochemical reactions are at the heart of how both zinc and tin protect steel from rusting. In an ideal situation where the coating remains intact, these reactions do not occur. However, scratches can expose the underlying steel, necessitating the protective mechanisms of the coatings.
For galvanized iron, the anodic reaction sees zinc oxidizing, while the cathodic reaction involves reducing oxygen to water-producing hydroxides.

• Anodic Reaction:
  1. \( Zn \rightarrow Zn^{2+} + 2e^- \)
• Cathodic Reaction:
  1. \( O_2 + 2H_2O + 4e^- \rightarrow 4OH^- \)

These reactions provide cathodic protection to the exposed steel. On the other hand, when a tin coating is scratched, the iron beneath can oxidize since tin does not behave sacrificially. In both scenarios, the understanding of electrochemical reactions helps in designing better protection strategies.

Zinc Coating: A Strong Defense Against Rust

The zinc coating applied to galvanized iron is pivotal in preventing rust. By effectively creating a barrier, zinc stops oxygen and moisture from reaching the steel, crucial components in the rusting process.
More than just a barrier, zinc serves as a temporary shield that corrodes instead of iron when exposed to the elements. This is particularly valuable for applications involving constant exposure to harsh conditions, such as in construction frameworks, pipelines, and automotive parts.
It is because of these abilities that zinc is preferred as a rust prevention method, not just for its self-sacrificial role but also for its durability and efficiency in safeguarding metal structures over time.

Tin Coating: A Noble Yet Compromised Armor

Tin coatings, often found on food cans, serve as physical barriers to protect steel from corrosion. Tin is less reactive than zinc, and this makes it both a strength and a weakness. Its nobility reduces its inclination to corrode. However, when the tin surface is compromised, the steel underneath may suffer more rapid corrosion than it would without the tin coating.
When scratched, the steel can corrode due to tin being more noble and electrochemically stable, which means it doesn’t corrode itself and does little to protect scratch-exposed steel. This limits the application of tin-coated steel to environments where minimal mechanical damage is expected.
Despite this, tin remains a useful coating due to its resistance to corrosion in non-scratched conditions and its non-toxic nature, making it perfect for food storage.