What Is AUS 40?
The Complete Marine Urea Guide

AUS 40 (Aqueous Urea Solution 40%) is a clear, colourless solution of 40% high-purity technical urea by mass in demineralised water. It is the reductant, the working fluid consumed by Selective Catalytic Reduction (SCR) after-treatment systems fitted to marine diesel engines. Its sole job is to convert harmful nitrogen oxides (NOx) in engine exhaust into harmless nitrogen and water vapour.

The "40" refers directly to the urea concentration. This is the single most important number on the specification sheet: marine SCR systems are flow-calibrated around 40% concentration, which is why automotive AdBlue (32.5%) can never be substituted, a mistake we cover in detail in our guide to the difference between AUS 32 and AUS 40.

You will also see AUS 40 referred to as marine urea, marine urea solution, marine DEF, SCR reagent or marine NOx reduction agent all describe the same ISO 18611 fluid.

Inside the exhaust duct, AUS 40 goes through three reactions in a fraction of a second:

• 1. Thermolysis. Injected into hot exhaust gas (typically 300–450 °C), the water evaporates and urea decomposes: CO(NH₂)₂ → NH₃ + HNCO.
• 2. Hydrolysis. The isocyanic acid reacts with water vapour to release a second ammonia molecule: HNCO + H₂O → NH₃ + CO₂.
• 3. Catalytic reduction. Over the SCR catalyst, ammonia reduces nitrogen oxides: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O.

The outcome: up to 95% of NOx leaving the engine is converted into the same nitrogen and water that make up ordinary air. For the full system view, covering injectors, mixing duct, catalyst layers and dosing control, read our explainer on how marine SCR systems work.

AUS 40 quality is not negotiable. Off-spec fluid can poison a marine SCR catalyst, a major capital component. Two standards define acceptable product:

Standard	Scope	Key Requirements
ISO 18611 (Parts 1–3)	International: ships and marine technology, NOx reduction agent AUS 40	Quality requirements, test methods, handling, transport and storage
IS 17661 (Part 1):2022	Indian Standard: Marine NOx Reduction Agent AUS 40, Quality Requirements	Aligns Indian production with ISO 18611 quality parameters

Both standards set tight limits on urea content (40 ± 0.5%), biuret, aldehydes, insoluble matter, phosphate and trace metals (calcium, iron, copper, zinc, chromium, nickel, aluminium, magnesium, sodium, potassium). Demand a per-batch Certificate of Analysis from any supplier. Reputable manufacturers issue one with every delivery. Full parameter tables are on our specifications page.

Demand for AUS 40 is driven by international law. MARPOL Annex VI and the IMO NOx Technical Code 2008 impose Tier III NOx limits, roughly a 76% reduction versus Tier II, on marine diesel engines installed on ships constructed on or after 1 January 2016 operating inside designated NOx Emission Control Areas (ECAs). SCR is the dominant compliance technology, and SCR runs on AUS 40.

A vessel that runs out of compliant urea inside an ECA, or doses off-spec fluid, risks port state control detention, fines and charter disputes. We break down the rules, the ECA map and the compliance mechanics in our guide to IMO Tier III and MARPOL Annex VI.

AUS 40 is supplied in three standard formats, sized to vessel consumption:

Format	Volume	Best For
HDPE Drums	200 L	Top-ups, smaller vessels, trial orders
IBC Totes	1000 L	Coastal vessels, tugs, offshore support vessels
Bulk Tanker	10,000–30,000 L	Deep-sea vessels, fleet contracts, terminal storage

PROTICKS manufactures certified AUS 40 in Jamnagar, Gujarat. That puts production minutes from India's busiest western anchorages. Delivery is Pan-India, direct to the berth:

• Gujarat: Mundra, Kandla (Deendayal), Pipavav, Dahej, Hazira
• West coast: JNPT (Nhava Sheva), Mumbai
• South coast: Cochin, Tuticorin (V.O. Chidambaranar)
• East coast: Chennai, Visakhapatnam (Vizag), Haldia

Gujarat ports need 48–72 hours notice. South and east coast ports need 5–7 days. Barge transfer follows the same 5–7 day window.

We supply the transfer service, not only the fluid. That covers road tanker unloading, pumped discharge at the berth, and bunker barge supply. See the full delivery network on our Indian port supply page. For onboard practice, read our guidance on AUS 40 storage, handling and shelf life.