Reducing furnace-scale formation: Which gas condition in the flue or furnace atmosphere helps suppress oxide scale growth on hot steel surfaces?

Introduction / Context:
Scale forms on steel when an oxidizing atmosphere reacts with hot metal surfaces to form iron oxides. Controlling the furnace atmosphere to be slightly reducing minimizes oxidation and scale pickup, improving product quality and reducing material loss.

Given Data / Assumptions:

• Hot steel stock in a reheating or heat-treatment furnace.
• Atmosphere composition affects oxidation-reduction reactions.
• Target is to lower the oxidation potential.

Concept / Approach:
Carbon monoxide (CO) and hydrogen (H2) are reducing agents at high temperature. A high CO/CO2 ratio indicates a more reducing carbon potential, which favors reduction of iron oxides and suppresses further oxidation. Balancing safety and emissions, industries tune excess air and, when applicable, inject controlled amounts of reducing species.

Step-by-Step Solution:

Verification / Alternative check:
Ellingham-type considerations and practical furnace trials show scale thickness decreases as the atmosphere becomes more reducing (within metallurgical limits).

Why Other Options Are Wrong:

Common Pitfalls:
Driving the atmosphere too reducing can cause soot or carbon pick-up; control must be precise to avoid surface defects.

Final Answer:
all (a), (b) & (c)