16-Chem-B12 · May 2017
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.
Assessment of item 4 — it is wrong, and dangerously so. The proposal has the corrosion behaviour of steel in sulfuric acid exactly backwards. Carbon steel is not generally resistant to sulfuric acid; it happens to be usable in concentrated acid (roughly 70–100% at ambient temperature) for one specific reason: concentrated H2SO4 passivates iron, forming a thin, adherent ferrous-sulfate film that stifles further attack. This is why the plant has always been able to store and handle 95% acid in ordinary steel tanks and piping.
Diluting the acid to 50% destroys that protection. In the intermediate-to-dilute range the sulfate film is soluble and no longer protective, so steel reverts to active corrosion, with hydrogen evolution as the cathodic reaction. The classic iso-corrosion chart for iron/steel in H2SO4 shows a deep minimum in the concentrated region and a steep rise as concentration falls below about 70%; at 50% the corrosion rate of plain steel is orders of magnitude higher than at 95%. Switching to 50% acid would therefore increase, not reduce, corrosion of the reaction tank and piping — potentially turning a benign service into rapid general wastage and hydrogen-driven attack.
Two aggravating factors make the risk worse than the chart alone suggests. First, temperature: the corrosion rate of steel in dilute H2SO4 climbs sharply with temperature, so any process heat magnifies the problem. Second, velocity: the protective film in concentrated service is easily swept away at elbows, pump suctions and control valves, giving localized erosion-corrosion; dilute acid removes even the possibility of re-passivation.
Recommendation. Reasons 1–3 (yield, spare capacity, acid cost) may all be valid, but item 4 is false: do not justify the change on reduced corrosion. If the switch to 50% acid is still wanted for the other reasons, the wetted equipment must be reviewed and very likely upgraded — e.g. to a lead lining, a suitable stainless (Alloy 20/904L) or a fluoropolymer/rubber-lined tank and appropriate alloy or lined piping — and the true cost of that upgrade weighed against the $10,000/yr acid saving before proceeding.
