Low-Carbon Steel Alloying — Effect on Yield Point and Atmospheric Corrosion In low carbon steels, which element, when added in small amounts, raises the yield point and improves resistance to atmospheric corrosion (weathering)?

Introduction / Context:
Minor alloying additions in low-carbon steels can significantly modify mechanical properties and environmental resistance. Phosphorus, while often controlled due to embrittlement concerns at higher levels, provides beneficial effects in small amounts for structural sheet and weathering steels.

Given Data / Assumptions:

• Carbon content is low (typical mild steel range).
• Small phosphorus additions, typically below levels that cause cold-shortness.
• Atmospheric (not marine or chloride) corrosion considered.

Concept / Approach:
Phosphorus in small quantities strengthens ferrite by solid-solution effects, raising yield point and improving bake hardenability. It also enhances resistance to atmospheric corrosion by promoting the formation of a more adherent rust layer, a principle leveraged in weathering steels (with additional Cu, Cr, Ni). Conversely, sulphur usually harms ductility (hot shortness) unless balanced by Mn; Mn primarily improves hardenability and deoxidation; Si acts as a deoxidizer/strengthener but is not chiefly responsible for the atmospheric corrosion improvement in mild steels.

Step-by-Step Solution:
Target effects: higher yield point + improved atmospheric corrosion resistance.Match element: phosphorus provides both benefits in small additions.Therefore, select phosphorus.

Verification / Alternative check:
Automotive sheet steels (BH/IF grades) and certain weathering steels use controlled P to raise yield and enhance corrosion resistance in air exposure.

Why Other Options Are Wrong:

• Sulphur: causes hot shortness unless neutralized; not used to raise yield/corrosion resistance.
• Manganese: improves hardenability and sulphur control; not the principal atmospheric corrosion improver.
• Silicon: strengthens and deoxidizes but is not the primary contributor to weathering behavior in mild steels.

Common Pitfalls:
Assuming any strengthener will also improve atmospheric corrosion; specific chemical effects and passivation behavior matter.

Final Answer:
phosphorus