Admiralty gun metal — identify the standard composition In non-ferrous alloy practice, “Admiralty gun metal” is a well-known copper–tin–zinc alloy used for corrosion-resistant marine fittings and bearings. Which of the following compositions best represents Admiralty gun metal (values in percent by mass)?

Introduction / Context:
Admiralty gun metal is a classic bearing and marine-service bronze. It is prized for good castability, resistance to dezincification in seawater, and reasonable strength with anti-friction properties. Recognizing its composition helps engineers select suitable alloys for pump bodies, valve seats, and condenser components where corrosion resistance and sliding performance matter.

Given Data / Assumptions:

• All options list nominal compositions in percent by mass.
• “Admiralty gun metal” refers to the traditional Cu–Sn–Zn bronze used in marine service.
• The application context is corrosion-prone (e.g., brine, seawater).

Concept / Approach:
Gun metals are essentially tin bronzes with a small addition of zinc to improve fluidity and strength while maintaining corrosion resistance. Admiralty gun metal is typically around copper 88%, tin 10%, and zinc 2%. This tin level distinguishes it from brasses (Cu–Zn) and from silicon bronzes, and the small zinc content separates it from nearly binary tin bronzes.

Step-by-Step Solution:

Verification / Alternative check:
Materials handbooks and marine standards cite Admiralty gun metal near 88Cu–10Sn–2Zn, sometimes with trace Pb for machinability in specific grades.

Why Other Options Are Wrong:

• 60Cu–35Zn–5Mn: a manganese brass, not a tin bronze.
• 76Cu–20Si–4Zn: silicon level is unrealistic; silicon bronzes use ~3% Si, not 20%.
• 82Cu–12Zn–6Mn: again a manganese brass variant.
• 70Cu–20Sn–10Zn: much higher tin; not the standard Admiralty grade.

Common Pitfalls:
Confusing brasses (Cu–Zn) with bronzes (Cu–Sn); assuming “Admiralty” implies high zinc like naval brass (which is different: Cu–Zn with small Sn).

Final Answer: