Difference Between
AUS 32 and AUS 40

AUS 32 is a 32.5% aqueous urea solution, the substance sold as AdBlue or Diesel Exhaust Fluid (DEF), standardised under ISO 22241 for on-road diesel vehicles. AUS 40 is a 40% aqueous urea solution standardised under ISO 18611 and India's IS 17661 (Part 1):2022 for marine engines. The "32" and "40" refer directly to the urea concentration by mass. Marine SCR systems are engineered, dosed and calibrated around the 40% figure, so AUS 32 is not an acceptable substitute.

Parameter	AUS 32 (AdBlue / DEF)	AUS 40 (Marine Urea)
Urea concentration	32.5% by mass	40% by mass
Governing standard	ISO 22241	ISO 18611 · IS 17661 (Part 1):2022
Primary application	On-road trucks, buses, cars	Marine vessels & ship engines
SCR system type	Automotive, low reductant demand	Heavy marine, high reductant demand
Regulatory driver	Euro VI / BS-VI road emissions	IMO Tier III · MARPOL Annex VI
Typical packaging	10L cans, 210L drums, IBC	200L drums, 1000L IBC totes, bulk tanker

An SCR (Selective Catalytic Reduction) system injects a precise quantity of urea solution into the hot exhaust stream. The urea decomposes into ammonia, which reacts with nitrogen oxides (NOx) over a catalyst to form harmless nitrogen and water. The amount of ammonia produced is a direct function of the urea concentration in the dosed fluid.

A marine SCR system is designed and flow-calibrated to deliver the ammonia mass that 40% urea provides. Feed it 32.5% fluid and every injection carries roughly 19% less urea than the controller expects. The system has no way to know the fluid is weaker, so it doses the same volume and the engine runs reductant-starved.

What under-dosing causes

• Incomplete NOx conversion. The catalyst receives too little ammonia, NOx slips through, and the vessel can fall out of IMO Tier III / MARPOL Annex VI compliance, risking detention or fines at port state control.
• Ammonia slip and false fault codes. Closed-loop dosing systems may over-correct, triggering NOx-sensor faults and inconsistent after-treatment behaviour.
• Lost certification. Using a non-ISO 18611 / non-IS 17661 fluid can void the SCR system's type approval and the engine's emissions certificate.

Concentration is not the only problem. Marine reductant standards specify tighter limits on contaminants (biuret, aldehydes, phosphate and trace metals) because marine SCR catalysts and dosing hardware operate continuously for far longer service intervals than a road vehicle. Off-spec or automotive-grade fluid introduces two failure modes:

Urea crystallisation and deposit build-up

When dosing parameters and fluid chemistry are mismatched, unreacted urea and biuret form hard crystalline deposits on injector nozzles, mixing pipes and the catalyst face. These deposits restrict flow, distort the spray pattern and progressively choke the after-treatment train, a phenomenon operators describe as "scaling". Clearing it means downtime and manual cleaning of components that were never meant to be opened at sea.

Catalyst poisoning and accelerated wear

Trace contaminants outside the marine specification can poison the SCR catalyst's active sites, permanently reducing conversion efficiency. On a large two-stroke or four-stroke marine engine, an SCR catalyst is a major capital component, and degrading it with the wrong fluid is an expensive error that automotive AdBlue users never face at the same scale.

Road vehicles (trucks, buses, construction or agricultural fleets) need AUS 32 (AdBlue / DEF) to ISO 22241, available from our sister brand PROTICKS DEF (adbluedef.in). Marine vessels running SCR after-treatment for IMO Tier III need AUS 40 to ISO 18611 and IS 17661 (Part 1):2022. The two are never interchangeable. PROTICKS supplies certified AUS 40 in bulk across India.

PROTICKS manufactures certified AUS 40 Marine Urea Solution from Jamnagar, Gujarat, with port-side delivery across India. See where we deliver on our Indian port supply network page.

Related guides: New to marine urea? Start with What Is AUS 40?, then see how marine SCR systems dose urea and the IMO Tier III rules driving it all.