# Difference Between AUS 32 and AUS 40

Both are clear, colourless aqueous urea solutions, but they are **not interchangeable**. Using automotive AdBlue (AUS 32) in a marine SCR system is a costly mistake.

## Short answer

- **AUS 32**: 32.5% aqueous urea, sold as **AdBlue / Diesel Exhaust Fluid (DEF)**, standardised under **ISO 22241** for on-road diesel vehicles.
- **AUS 40**: 40% aqueous urea, standardised under **ISO 18611** and India's **IS 17661 (Part 1):2022** for marine engines.

The "32" and "40" are the urea concentration by mass. Marine SCR systems are calibrated around 40%, so AUS 32 is not an acceptable substitute.

## Comparison

| 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 | IMO Tier III · MARPOL Annex VI |
| Typical packaging | 10L cans, 210L drums, IBC | 200L drums, 1000L IBC totes, bulk tanker |

## Why concentration matters

An SCR system injects urea, which decomposes to ammonia and reacts with NOx over a catalyst. Ammonia output is a direct function of urea concentration. A marine SCR system is flow-calibrated for 40% urea; feeding it 32.5% delivers roughly **19% less urea** per injection, starving the catalyst of reductant.

Under-dosing causes:

- **Incomplete NOx conversion**: NOx slips through; risk of IMO Tier III / MARPOL Annex VI non-compliance, detention or fines.
- **Ammonia slip and false fault codes**: closed-loop systems over-correct.
- **Lost certification**: non-ISO 18611 / non-IS 17661 fluid can void SCR type approval.

## Engine scaling & deposit risk

Marine standards set tighter limits on biuret, aldehydes, phosphate and trace metals because marine catalysts run far longer than road vehicles. Off-spec fluid causes:

- **Urea crystallisation / deposit build-up ("scaling")** on injectors, mixing pipes and the catalyst face, restricting flow and requiring downtime.
- **Catalyst poisoning**: trace contaminants permanently reduce conversion efficiency of an expensive marine SCR catalyst.

**Bottom line:** Never top up a marine SCR system with automotive AdBlue (AUS 32). Always use ISO 18611 / IS 17661 (Part 1):2022 certified AUS 40.

## Which one does your fleet need?

- **Road vehicles** → AUS 32 (AdBlue / DEF), ISO 22241.
- **Marine vessels with SCR** → AUS 40, ISO 18611 / IS 17661 (Part 1):2022.

PROTICKS manufactures certified AUS 40 from Jamnagar, Gujarat, with port-side delivery across India.

## Related

- [Home](https://aus40.in/)
- [Marine Urea Supply at Indian Ports](https://aus40.in/marine-urea-supply-ports-india.html)

Request bulk pricing: https://aus40.in/#contact · +91 74909 17799 · info@proticks.com