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. A 316 austenitic stainless steel that has cracked in a hot acidic solution is showing stress-corrosion cracking — the failure mode austenitic stainless steels are most notorious for. Above roughly 60 °C, 300-series stainless is susceptible to chloride- and acid-induced SCC, and this service is squarely in the danger zone: 100–110 °C, with sulfuric and sulfurous acids (and likely trace chlorides) providing the corrodent while fabrication and thermal stresses provide the tension. Simply substituting another austenitic grade will not help.
Selection principle. Choose a liner that is immune to austenitic SCC and resists the hot mixed-acid/organic environment. Practical options:
Recommendation. Line the digester with glass (glass-fused-to-steel) as the primary choice, or with a fluoropolymer/impervious-graphite lining; where a bare-metal wetted surface is required, upgrade to Hastelloy C-276. Whatever is chosen, stress-relieve or otherwise minimize residual stress and keep chlorides out of the feed, since those were contributors to the 316 failure.
