Effect of sulphur dioxide (SO₂) in furnace flue gases on ferrous alloys: In an industrial heating furnace atmosphere containing SO₂, how does sulphur attack typically manifest on steels and other ferrous metals at elevated temperature?

Introduction / Context:
Sulphur-bearing combustion products such as sulphur dioxide (SO₂) and, under some conditions, sulphur trioxide (SO₃) are common in furnaces firing high-sulphur liquid or solid fuels. These species can degrade ferrous materials by accelerating oxidation, promoting hot corrosion, and weakening the metal microstructure. Understanding the combined effects is essential for selecting fuels, refractories, and protective atmospheres in reheating and heat-treatment furnaces.

Given Data / Assumptions:

• Furnace processing steels at high temperature.
• Flue gas contains SO₂ from fuel-bound sulphur.
• Normal excess air operation, with typical oxygen present.

Concept / Approach:
At elevated temperature, SO₂ participates in reactions that increase oxide scale growth on iron and steel. Sulphur also diffuses to grain boundaries where it forms low-melting sulphides or segregates, reducing cohesion and increasing embrittlement risk. These effects are more pronounced in low-carbon and some nickel-bearing alloys where grain boundary chemistry is sensitive.

Step-by-Step Solution:

Verification / Alternative check:
Plant evidence includes thicker, friable scale; increased metal loss; and intergranular fracture features on metallography following exposure to sulphur-laden gases.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming sulphur effects are negligible at moderate sulphur levels; even ppm-level sulphur can matter at high temperature and long residence times.

Final Answer:
all (a), (b) and (c).