21-Mat-B7 Structure and Properties of Polymers · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2016 — 10-Met-B7, Physical Metallurgy of Non-Ferrous Metals and Alloys. Three hours, closed book, approved Casio/Sharp calculator only. Eight questions of 20 marks each; the rubric states that any five questions constitute a complete paper (100 marks total) and that only the first five appearing in the answer book are marked. All eight are answered here, because this set is a study resource rather than an exam script. The rubric explicitly notes that most questions require an essay-format answer and that clarity and organization are marked, so the answers below are written as structured prose rather than as note form.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A brass is fundamentally a copper–zinc alloy, zinc being the principal (and defining) addition, over a range from a few percent up to about 40 percent. Beyond the base Cu–Zn system, "alloy" brasses add a third element for a specific property: lead (up to a few percent, for free-machining brasses), tin (about 1 percent, in admiralty and naval brass, to suppress dezincification), aluminum (aluminum brass, for improved erosion-corrosion resistance in condenser tubing), and small arsenic, antimony or phosphorus additions (dezincification inhibitors in inhibited brasses).
Dezincification is a selective form of corrosion, most commonly seen in high-zinc ($\alpha+\beta$) brasses such as 60/40 brass exposed to stagnant, chloride-bearing or low-pH waters, in which zinc is preferentially removed from the alloy, leaving behind a weak, porous, reddish copper-rich residue that retains the original component's shape and dimensions but has lost most of its strength — a dangerous failure mode because the part can look intact until it fractures or leaks under load. The accepted mechanism is not simple selective leaching of zinc atom-by-atom but an electrochemical dissolution/redeposition process: both copper and zinc go into solution together at the corroding surface, but zinc (the more anodic of the two) stays in solution while the dissolved copper redeposits back onto the surface as a porous, spongy, mechanically weak copper layer or plug, effectively "de-alloying" the material in place. It occurs as either layer-type (a uniform corroded skin, typical of low-Zn/single-phase brasses) or plug-type (localized deep plugs, typical of high-Zn two-phase brasses, where the more reactive $\beta$ phase corrodes preferentially).
Phosphorus is added to tin bronzes primarily as a deoxidizer during melting and casting: tin bronze melts readily pick up dissolved oxygen, which would otherwise react with tin to form SnO⊂2⊂ inclusions and cause gas porosity in the casting; a small phosphorus addition (residual content on the order of a few hundredths to a few tenths of one percent) scavenges this oxygen as a fugitive P⊂2⊂O⊂5⊂ that is removed in the slag. A secondary benefit is that any phosphorus left in solid solution provides modest solid-solution strengthening, and at higher residual levels forms hard Cu⊂3⊂P particles that improve wear and bearing (anti-friction) performance, which is why phosphor bronze is a preferred bearing and spring alloy. The chief disadvantage of tin bronzes relative to brasses is cost and processability: tin is a far more expensive alloying addition than zinc, and the wide freezing range of the Cu–Sn system promotes coring and interdendritic segregation, making tin bronzes more prone to casting porosity/hot-shortness and generally more difficult and costly to hot- and cold-work than the simpler, cheaper, more formable Cu–Zn brasses.
Aluminum's principal contribution is corrosion resistance: at the alloy contents used (roughly 5–11 percent Al), aluminum forms a thin, dense, tightly adherent, self-healing Al⊂2⊂O⊂3⊂ passive film on the alloy surface, giving aluminum bronzes markedly better resistance to seawater, marine atmospheres and many industrial acids than plain brasses or tin bronzes — the reason they are the material of choice for ship propellers, pump and valve bodies, and other marine hardware. Aluminum also strengthens the alloy (solid solution, and in the higher-Al, heat-treatable grades a eutectoid decomposition of the high-temperature $\beta$ phase analogous to steel's eutectoid reaction, allowing quench-and-temper-type heat treatments), so the combination that makes aluminum bronzes useful engineering alloys is corrosion resistance paired with a strength level well above the plain brasses.
