Question

High temperature corrosion mainly involves

• A. diffusion of gases only
• B. mechanical wear
• C. plastic deformation
• D. oxidation reactions

Hint:

The Pilling-Bedworth ratio (PBR) is used to predict whether a metal oxide scale will be protective or non-protective based on the volume ratio of oxide to metal.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks about the primary chemical or physical process that drives high-temperature corrosion of metals.
High-temperature corrosion is a form of corrosion that occurs in the absence of a liquid aqueous electrolyte, often termed "dry corrosion" or "scaling".

Step 2: Key Formula or Approach:
The primary reaction during high-temperature oxidation of a metal \( M \) in an oxygen environment is:
\[ x M + \frac{y}{2} O_2 \rightarrow M_xO_y \]
This reaction is thermodynamically driven by the negative Gibbs free energy of formation of the metal oxide.

Step 3: Detailed Explanation:

• Oxidation Mechanism: At elevated temperatures, metals directly react with gaseous species in the environment (such as oxygen, sulfur, or carbon).
The most common and widespread high-temperature corrosion process is oxidation, where a solid metal oxide scale forms on the metal surface.

• Role of Diffusion: Although the growth of the oxide scale is often limited by the solid-state diffusion of metal cations or oxygen anions through the scale, the core chemical process is fundamentally an oxidation reaction.
Therefore, calling it "diffusion of gases only" (Option A) is incorrect as it neglects metal ion transport and the chemical reaction itself.

• Distinction from Mechanical Processes: Mechanical wear (Option B) and plastic deformation (Option C) are physical degradation modes, not chemical corrosion phenomena, though they can interact with corrosion at high temperatures.


Step 4: Final Answer:
Hence, high-temperature corrosion mainly involves oxidation reactions, which corresponds to Option (D).