The rubber-lined ISO tank uses soft natural rubber, bromobutyl, or chlorobutyl liner systems 4 to 8 mm thick to handle concentrated hydrochloric acid (30 to 37%), dilute sulphuric acid up to 70%, and a range of dilute mineral acids. Marflex’s PG70 natural rubber, HB50HT bromobutyl, and VE621BC chlorobutyl are the standard grades on the European fleet; Blair Rubber supplies FDA-compliant rubber grades for food-adjacent applications. The build sits between PE-lined (cheaper, narrower chemistry) and PTFE-lined (expensive, broader chemistry) on the cost-versus-resistance curve.
What rubber-lined is built for
Concentrated hydrochloric acid 30 to 37% (UN 1789, Class 8 PG II) is the workhorse cargo. The natural-rubber liner has been the historical solution for HCl across decades of industrial trade, with bromobutyl as the upgraded variant for slightly tougher service. Dilute sulphuric acid up to 70% (UN 1830) where the cargo doesn’t passivate stainless and where PE would fail at the higher concentrations. Various dilute mineral acids that fall in the rubber-compatibility zone.
Construction and materials
Carbon-steel Q345R shell, typically 6 mm thick, with a 4 to 8 mm soft natural or synthetic rubber liner vulcanised to the steel substrate. Liner thickness varies by chemistry and operating temperature: 4 mm for mild service, 8 mm for heavier-duty fleet rotation. Marflex PG70 is the standard natural-rubber grade; HB50HT bromobutyl and VE621BC chlorobutyl provide better resistance to specific chemistry combinations.
A particular caveat: natural rubber is permeable to water at the molecular level. Dilute hydrochloric acid (below about 25% concentration) can permeate through the liner over months of service, attacking the steel substrate from behind. Operators handle this through liner thickness specification and through service rotation that limits residence time. For dilute HCl below 25% the trade-off may favour PE-lined (better water-barrier properties).
When rubber-lined is the right choice
Rubber-lined is the right tank for concentrated HCl 30 to 37% on routes where the chemistry profile is well-understood and the operator fleet is established. The cost / lead-time / chemistry combination is competitive with PE-lined for some cargoes, and superior for HCl specifically because the rubber tolerates the cargo’s mechanical and thermal cycling better than rotomoulded PE on long ocean transits.
When rubber-lined is the wrong choice
Rubber-lined is the wrong tank for HCl below 25% concentration (water permeation issue), for HF at any concentration (rubber dissolves in HF), for organic solvents that swell rubber (most chlorinated solvents, many aromatic hydrocarbons), and for hot caustic above 60 deg C (rubber softens and degrades).
How a rubber-lined booking is verified
Pre-loading inspection covers the standard lined-tank plate stack plus a rubber-lining condition check (visual through the manlid, looking for cracks, splits, or vulcanisation-bond delamination). Marflex or Blair Rubber liner certificate provides installation date, rubber grade, thickness, and the vulcanisation log. Liner life on concentrated HCl runs 6 to 10 years on standard fleet rotation; longer for less-aggressive cargoes.