The question asks about the chemical composition of the self-healing protective layer on stainless steel that prevents rusting. This layer is known as a passive film, and its formation is crucial to stainless steel's corrosion resistance.
D) Chromium oxide — The passive film on stainless steel that provides corrosion resistance and is self-healing is predominantly composed of chromium oxide (\( \text{Cr}_2\text{O}_3 \)).
The question asks to identify a specific bronze variant known for its enhanced strength, wear resistance, and suitability for springs and electrical contacts, due to the addition of phosphorus. This requires knowledge of different bronze alloys and their properties.
Correct Option: B) Phosphor bronze is a type of bronze alloyed with 0.5% to 11% tin and 0.01% to 0.35% phosphorus. The phosphorus acts as a deoxidizing agent during casting and also improves the alloy's strength, stiffness, wear resistance, and elasticity. These properties make it highly suitable for applications requiring good spring qualities, such as springs, bellows, and electrical contacts, as well as for bearings and gears.
The question asks to identify a specific type of bronze that uses aluminium as the main alloying element instead of tin, known for high strength and use in marine hardware. This requires knowledge of different bronze alloys and their compositions and applications.
B) Aluminium bronze — This alloy is characterized by copper and aluminium (typically 5-11%) as the primary alloying elements. It exhibits high strength, excellent corrosion resistance (especially in seawater), and good wear resistance, making it ideal for marine hardware, propellers, and other demanding applications where tin bronzes might not perform as well.
The question asks about the effect of increasing the proportion of zinc in brass on its mechanical properties. Brass is an alloy primarily composed of copper and zinc. The properties of alloys are significantly influenced by the proportions of their constituent elements.
B) Increase its hardness and strength, though excessive zinc reduces ductility. This statement accurately describes the general effect of increasing zinc content in brass. Zinc acts as a hardening agent, but too much zinc can make the alloy brittle, reducing its ductility.
The question asks why bronze was preferred over pure copper for casting bells and statues. This involves understanding the material properties of bronze (an alloy) compared to pure copper and how these properties relate to the functional requirements of bells and statues.
Correct Option: D) Bronze is harder, more corrosion-resistant, and gives better sound resonance than pure copper. This option accurately describes the key advantages of bronze over pure copper for these specific applications. The addition of tin to copper significantly increases its hardness and strength, making it more durable for bells and statues. Its improved corrosion resistance ensures longevity, especially for outdoor statues. Crucially, the metallurgical structure of bronze contributes to its excellent acoustic properties, producing a clear, sustained, and resonant sound ideal for bells.
The question asks about the specific benefit of adding molybdenum to stainless steel, particularly in conjunction with chromium and nickel. This points to understanding the role of alloying elements in enhancing the corrosion resistance of stainless steel, especially in challenging environments.
D) Pitting and corrosion in chloride-rich (e.g., marine) environments — Molybdenum is specifically added to stainless steel to significantly improve its resistance to pitting and crevice corrosion, especially in environments containing chlorides. This makes grades like 316L stainless steel highly valued for marine applications, chemical processing, and other aggressive environments where chloride attack is a concern.