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.
Do not order the Hastelloy B pump. Hastelloy B (a Ni–Mo alloy) is outstanding in reducing acids — hydrochloric and air-free sulfuric — but its corrosion resistance collapses in the presence of oxidisers. The service here is described as "well aerated" 35% H2SO4: dissolved oxygen (and any ferric ion) makes it an oxidising environment, exactly the condition in which Hastelloy B corrodes rapidly. Spending heavily on a Hastelloy B pump would buy an expensive component that still fails.
Re-examine why the Alloy 20 pump failed. Alloy 20 (Carpenter 20Cb-3) was developed specifically for sulfuric-acid service and is normally very good in aerated 35% acid at room temperature — general corrosion should be low. Its "very quick" failure is a red flag that the mechanism was probably not simple general corrosion from a wrong alloy choice, but something localized:
If the real cause is velocity or a defect, a more expensive alloy will fail the same way; the problem must be identified before re-purchasing.
Recommendation. For well-aerated 35% H2SO4 at room temperature, appropriate pump materials are Alloy 20 itself (it should perform well — fix the true failure cause first), or, if a higher margin is wanted, an oxidiser-tolerant alloy such as Hastelloy C-276, or a high-silicon cast iron (Duriron/Durichlor) pump, which is a traditional and economical choice for aerated sulfuric acid. Hastelloy B is the one option to avoid. Investigate the failed pump (metallurgical section, velocity check, acid chloride analysis) before placing any order.
