16-Chem-B12 · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Open-book exam, 3 hours; any non-communicating calculator permitted. Eight questions of equal value (10 marks each) constitute a complete paper; full solutions to all eight are given here. Questions 1–3 are quantitative (a galvanic-cell Nernst calculation, a Faraday's-law corrosion-rate conversion, and an impressed-current cathodic-protection circuit); Questions 4–8 are short "corrosion-consultant" case studies answered as reasoned engineering judgements.
Reference texts: M. G. Fontana, Corrosion Engineering (3rd ed., McGraw-Hill) — the classic text behind this syllabus (electrode potentials and the EMF series Ch. 9; corrosion-rate expressions and Faraday's law Ch. 9–10; the eight forms of corrosion Ch. 3; materials selection and the sulfuric-acid/HCl case problems Ch. 12; cathodic protection and inhibitors Ch. 6–11); D. A. Jones, Principles and Prevention of Corrosion (2nd ed., Prentice Hall) — mixed-potential theory, Tafel extrapolation and CP design; A. W. Peabody, Control of Pipeline Corrosion (2nd ed., NACE) — anode-bed resistance and current density; ASM Handbook Vol. 13, Corrosion for materials-selection charts. Canadian practice: potable-water corrosion control follows the CCME/Health Canada guidelines and the AWWA carbonate-saturation approach (Question 8).
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.
The stock “forget it” answer is right about classic industrial inhibitors, but incomplete about corrosion control of potable mains. The three objections are sound as stated: cast iron does corrode slowly in near-neutral water; the public (and health regulators) will not accept chromate, nitrite, benzoate or other industrial inhibitors dosed into drinking water; and continuously inhibiting an entire municipal flow at industrial concentrations would indeed be prohibitively expensive and wasteful. So the naive approach — pick an industrial inhibitor from a corrosion handbook and dose the mains — is correctly rejected.
But municipalities do control internal corrosion of iron mains — with health-approved methods, not conventional inhibitors. If asked to “select an inhibitor,” the professional answer is to reframe it as potable-water corrosion control and choose from the measures that are permitted in drinking water:
Selection procedure. (1) Characterize the water (pH, alkalinity, hardness, chloride/sulfate, DO, and the Langelier or CCPP index). (2) If the water is aggressive (negative LSI), first correct the carbonate balance — the cheapest fix. (3) Where lead/copper or persistent iron release remain, add a health-approved orthophosphate or silicate inhibitor at the minimum effective dose, verified by coupon/pipe-loop testing and confirmed against Health Canada/CCME drinking-water guidelines. (4) For new or relined mains, specify cement-mortar lining rather than relying on dosing. So the honest answer to the client is: don't dose an industrial inhibitor — but “forget it” is too strong; the accepted practice is carbonate-saturation control plus, where justified, an approved orthophosphate/silicate inhibitor.
