Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Examinations, December 2019 — 16-Mec-B8 Engineering Materials. Three hours, open book; any non-communicating calculator is permitted. Seven problems, all of equal value; any five of them constitute a complete paper, so each problem carries 20 marks. Candidates are urged to submit a clear statement of any assumptions made. All seven problems are solved below, because the complete set is the study resource. Problems 2, 5 and 6 are to be answered against the figures reproduced on page 4 of the examination paper — the Callister cold-work curves, the eutectoid isothermal-transformation diagram and the aluminium-rich Al–Cu phase diagram.
Askeland & Wright, The Science and Engineering of Materials, 7th ed. — the primary syllabus text.
Callister & Rethwisch, Materials Science and Engineering: An Introduction, 10th ed. — the source of the three figures attached to this paper.
Shackelford, Introduction to Materials Science for Engineers, 8th ed.
Dieter, Mechanical Metallurgy, 3rd ed. — true stress–strain and the flow curve.
Kalpakjian & Schmid, Manufacturing Engineering and Technology — drawing schedules, cold work and process annealing.
Fontana, Corrosion Engineering, 3rd ed. — the galvanic series in sea water.
Ashby, Materials Selection in Mechanical Design, 5th ed. — selection by property profile and material index.
Figure values (the copper cold-work curves for Problem 2, the eutectoid isothermal-transformation curves for Problem 5 and the Al–Cu phase boundaries for Problem 6) are read from the printed figures. Graph readings carry the usual chart-reading tolerance of roughly one part in twenty, which is stated wherever it changes an answer.
Question 4: Galvanic couples in sea water (20 marks)
Given. Five two-metal couples, each immersed in sea water and in electrical contact. Sea water is a strong, well-aerated electrolyte of roughly 3.5% NaCl, so the relevant ranking is the galvanic series in sea water (Fontana; Callister Table 17.2), not the standard emf series, because the passive alloys in this list owe their nobility to an oxide film rather than to their standard electrode potentials.
Find. For each pair, whether galvanic corrosion is possible and, if so, which member of the pair is the anode and corrodes.
The five couples plotted on the sea-water galvanic series. In every pair the lower (more active) member is the anode and corrodes; the wider the vertical separation, the larger the driving potential.
Approach. Locate both members of each couple on the sea-water galvanic series. Because the two are in electrical contact in a common electrolyte they must sit at a single mixed potential, so the more active (lower) member is polarised anodic and dissolves while the more noble (upper) member becomes the cathode and is protected. Corrosion is "possible" whenever there is a real separation between them; where the two lie close together the driving potential is small and attack is slow and of little practical consequence.
Part (a) — aluminium and magnesium. Magnesium and its alloys sit at the very bottom of the sea-water series, below every aluminium alloy. Corrosion is therefore possible and the magnesium corrodes, the aluminium acting as cathode. This is the largest separation of the five couples, of the order of 0.7 V, and it is the basis of magnesium sacrificial anodes. The practical warning that goes with it is the area effect: magnesium fastenings in a large aluminium structure present a small anode to a large cathode, so the anodic current density and the penetration rate are very high.
Part (b) — zinc and low-carbon steel. Zinc lies below iron and steel in the series, so corrosion is possible and the zinc corrodes while the steel is cathodically protected. This is not a design fault but the design intent of galvanising and of zinc anodes on ships and buried pipe: the coating is chosen to be sacrificial, which is why a scratch through a galvanised layer does not undercut the way a scratch through tin plate does.
Part (c) — brass (60Cu–40Zn) and Monel (70Ni–30Cu). Monel 400 is a nickel-rich alloy that sits well above the copper alloys in sea water, so corrosion is possible and the brass corrodes. There is a second mechanism to note for this particular brass: 60Cu–40Zn is a duplex α–β alloy above the 35% zinc limit, and in a chloride water it is susceptible to dezincification, in which zinc is leached selectively and leaves a porous copper sponge of little strength. The galvanic couple with Monel accelerates that attack. An inhibited (arsenical) alpha brass or a cupro-nickel is the usual fix.
Part (d) — titanium and 304 stainless steel. Both are passive film-formers and both sit high in the series, but titanium is the more noble of the two, so corrosion is possible in principle and the 304 stainless steel is the anode. The driving potential is small, perhaps 0.1 V, and while the 304 remains passive the current is negligible, so in clean, moving, aerated sea water the couple behaves well. The real risk is not the galvanic potential but the passivity of the 304: chloride attacks its film, and in crevices, under deposits or in stagnant water the 304 can depassivate locally. It then drops to its active position near cast iron in the series, the driving potential jumps by several hundred millivolts, and the whole titanium surface acts as cathode to a pit of very small area. Type 304 is therefore not a sound choice for permanent sea-water immersion, with or without the titanium.
Part (e) — cast iron and 316 stainless steel. Passive 316 sits near the top of the series and cast iron well below it, so corrosion is possible and the cast iron corrodes. The separation is large, of the order of 0.5 V, and the geometry is usually unfavourable as well: a stainless trim, seat or fastener in a cast iron body is a large cathode driving a small anode, and cast iron in sea water also suffers graphitic corrosion, in which the iron matrix is consumed and a weak graphite skeleton is left that still looks like sound metal. Insulating the joint, or coating the cathode rather than the anode, is the standard mitigation.
Question 4 — which member of each couple corrodes
Quantity
Symbol / basis
Value
(a) Aluminium / magnesium
Mg is far more active
corrosion possible — magnesium corrodes
(b) Zinc / low-carbon steel
Zn below iron
corrosion possible — zinc corrodes
(c) Brass 60Cu–40Zn / Monel 400
Monel is the more noble
corrosion possible — brass corrodes
(d) Titanium / 304 stainless (passive)
Ti is the more noble; small separation
possible but slow — 304 corrodes
(e) Cast iron / 316 stainless (passive)
316 is far more noble
corrosion possible — cast iron corrodes
Check: the answers above rank the alloys on the galvanic series in sea water, with the stainless steels in their passive positions, which is what the question's "coupled in sea water" wording requires. Two of the five verdicts would change if the stainless steels were taken in their active positions: an active 304 lies below titanium by several hundred millivolts rather than about a hundred, and an active 316 lies close to cast iron, so couple (e) would become nearly indifferent. State the passive assumption explicitly on exam day. The rate of attack in every case also depends on the cathode-to-anode area ratio, not on the potential alone.
