Question 4 of 6: Selection of Materials of Construction

Question 4: Selection of Materials of Construction (20 marks)

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.

Material selection balances corrosion resistance against cost: use the cheapest material that survives the service. The governing principle in each case is the specific corrosion mechanism — general attack, chloride pitting/stress-corrosion cracking, or acid attack — set against size (which favours cheap carbon steel or reinforced plastic for large vessels) and product-purity needs.

(a) 10,000 m³ toluene storage tank — carbon steel

Toluene is a dry, non-polar, non-corrosive hydrocarbon, so no corrosion-resistant alloy is justified for a tank this large. Use plain carbon steel (e.g. CSA G40.21 / ASTM A36 plate) to API 650 for a field-erected atmospheric tank, with an internal fixed or floating roof to limit vapour loss of the volatile, flammable solvent. Bonding, grounding and a nitrogen or floating-roof vapour seal address the fire hazard rather than corrosion. Cost dominates at 10,000 m³, and carbon steel is entirely adequate.

(b) 5 m³ tank for 30 % w/w aqueous NaCl — lined steel or reinforced plastic

Concentrated chloride brine is aggressively corrosive to steel and, crucially, causes pitting and chloride stress-corrosion cracking of ordinary austenitic stainless steels (304/316) — so stainless is a poor choice here despite being "corrosion resistant." For a small 5 m³ tank the economical answer is a non-metallic barrier: rubber-lined carbon steel, or a moulded GRP/FRP (glass-reinforced polyester/vinyl-ester) or polypropylene tank. All are inert to neutral brine and far cheaper than a chloride-resistant alloy (which would need a high-nickel or duplex grade).

(c) 2 m × 20 m acrylonitrile distillation column — type-304 stainless steel

Acrylonitrile itself is only mildly corrosive, and carbon steel would nominally survive; however, dissolved iron catalyses the polymerisation of acrylonitrile (forming solids/fouling and consuming inhibitor), and product colour/purity matters. The standard industry choice is therefore type-304 (or 304L) austenitic stainless steel for the column body and internals — it avoids iron pick-up, keeps the monomer clean, and withstands the modest distillation temperatures. (Chlorides are absent here, so austenitic stainless is safe, unlike case b.)

(d) 100 m³ strong (concentrated) nitric acid tank — type-304L stainless steel

Concentrated nitric acid is strongly oxidising and passivates austenitic stainless steel, which is the classic material for nitric-acid service. Use type-304L stainless steel (low carbon to resist sensitisation/intergranular attack at welds); for very concentrated (>~95 %) acid, aluminium is an alternative. Ordinary carbon steel is unsuitable (rapid attack at moderate concentrations), and chloride-resistant grades are unnecessary since no chlorides are present.

(e) 500 m³ waste hold-up tank, pH 1–12 with trace organics — rubber-lined steel or vinyl-ester FRP

The swing from strongly acidic (pH 1) to strongly alkaline (pH 12), plus trace organics, rules out any single bare metal and many plastics. A material with broad chemical resistance is needed: rubber-lined carbon steel (soft natural or synthetic rubber lining over an API-650 steel wall) or a vinyl-ester GRP/FRP tank, whose resin resists both acids and alkalis and tolerates the organics better than a polyester resin. The lining/resin provides the corrosion barrier; the steel or glass reinforcement carries the structural load of a large 500 m³ vessel.

(f) 0.5 m × 3 m HCl-absorption packed column — non-metallic (FRP / rubber-lined / plastic)

Wet hydrochloric acid is one of the most corrosive common process fluids and attacks nearly all metals (including stainless steels, which pit and crack in chlorides). At atmospheric pressure and small size a non-metallic construction is both feasible and standard: a rubber-lined or FRP (vinyl-ester) vessel, or moulded PVC/CPVC, with ceramic or plastic (polypropylene) random packing and PTFE/impervious-graphite for any wetted internals. These give complete resistance to hydrochloric acid at the low operating temperature and pressure of an absorber.