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.
Diagnosis first. The symptom — no general pitting or etching, but cracking concentrated at every sharp bend — is textbook stress-corrosion cracking (SCC) of the brass, not general corrosion. SCC needs three things acting together: a susceptible alloy, a specific environment, and tensile stress. Brass (a Cu–Zn alloy; the paper's "330" is almost certainly a 70/30-type cartridge brass) is highly SCC-prone; the sharp bends are exactly where cold-work residual tensile stress is highest; and hot, moist HCl vapour with "a little air" supplies both the corrodent and the oxidiser that copper alloys need to crack. That combination cracks the liner even though uniform metal loss looks minor.
Can we braze the cracks? No. Brazing is the wrong repair on three counts: (i) it does nothing about the root cause — the alloy is simply unsuitable for this environment, so cracks will re-initiate beside every repair; (ii) brazing adds a fresh heat-affected zone and new residual stresses, which accelerate further SCC; and (iii) the liner is already embrittled and cracked throughout at the high-stress locations, so a braze cannot restore integrity. A brazed patch buys days, not a campaign.
Do we have to replace it? Yes — reline or replace with the right material. Hot HCl vapour contaminated with air is one of the most demanding services in the plant. Options, roughly in order of increasing cost and capability:
Recommendation. Stop trying to save the brass. Reline (or replace the cooler) with a fluoropolymer- or graphite-lined unit for the general case, or with Hastelloy C-276 / tantalum for the wetted metal parts if a metallic cooler is required. Protect the carbon-steel outer casing in the interim, since any breach exposes it to HCl.
