DEF (Diesel Exhaust Fluid, AUS 32 / ISO 22241) Storage & Tank Compatibility

Storing DEF (Diesel Exhaust Fluid, AUS 32 / ISO 22241)? Start Here

DEF (Diesel Exhaust Fluid), designated AUS 32 under ISO 22241, is a high-purity aqueous solution of approximately 32.5% urea in deionized water. It is consumed by selective catalytic reduction (SCR) systems on modern diesel engines, where it is metered into the hot exhaust stream and decomposes to ammonia that converts oxides of nitrogen (NOx) into nitrogen and water vapor. Because SCR catalysts are sensitive to contamination, ISO 22241 sets tight limits on metals, aldehydes, biuret and other impurities. That purity requirement, not chemical aggressiveness, is why material of construction (MOC) matters: the solution itself is mild and poly-friendly, but contact with copper, brass, zinc, aluminum or carbon steel corrodes the metal and degrades the urea, ruining the fluid and the catalyst. Correct tank, pump, hose and fitting materials preserve both fluid quality and SCR performance.

Polyethylene (HDPE / XLPE) Compatibility

Polyethylene is the industry-standard storage material for DEF. Both HDPE and cross-linked polyethylene (XLPE) are rated Suitable (S): the mildly alkaline urea solution does not attack the resin, and poly is inert enough that it does not contaminate the fluid — a key concern for ISO 22241 compliance. Rotationally molded poly tanks, IBC totes and bulk vessels are widely used across the DEF supply chain. XLPE is generally preferred for larger bulk tanks for its added impact and stress-crack resistance. Specific gravity of DEF is only ~1.09, so standard 1.5–1.9 SG poly tank ratings are more than adequate. Keep wetted fittings, valves and pumps to compatible materials (poly, stainless, PVDF/PTFE, EPDM) and avoid any copper, brass, zinc or aluminum contact even in trace hardware.

Material compatibility at a glance

DEF is a mild, slightly alkaline aqueous urea solution and is fully compatible with polyethylene (HDPE/XLPE), making cross-linked or high-density poly the standard storage material. The compatibility driver is not aggressiveness toward poly but the fluid's intolerance of contamination: copper, brass, zinc, aluminum and carbon steel either corrode or catalyze urea breakdown and must be avoided. Use poly, stainless steel, FRP, or lined steel with EPDM/fluoropolymer wetted parts.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• Not classified as a hazardous substance under GHS by typical supplier SDS sheets; representative NFPA 704 is Health 1 / Fire 0 / Reactivity 0.
• Practically non-flammable — it is largely water; however thermal decomposition above ~60°C can release ammonia.
• May cause mild skin or eye irritation on prolonged or repeated contact; rinse with water.
• Slight ammonia odor possible, especially if warm or decomposing; provide ventilation in enclosed dispensing areas.
• Spills are slippery and can crystallize white urea deposits as water evaporates; clean with water before residue dries.
• Store between roughly 12°C and 30°C out of direct sunlight; it freezes near -11°C (expands on freezing) and quality degrades with heat and contamination — always confirm hazards on the specific product SDS.

Common questions

Can DEF be stored in a polyethylene tank?

Yes. HDPE and XLPE are the standard, recommended storage materials for DEF. The solution is mild and poly-friendly, and polyethylene does not contaminate the fluid, which is essential for meeting ISO 22241 purity limits.

Why can't DEF touch copper, brass, aluminum or carbon steel?

Those metals either corrode in the urea solution or catalyze urea degradation, introducing contamination that can ruin the fluid and damage the SCR catalyst. Use poly, stainless steel, FRP, or lined steel with EPDM or fluoropolymer wetted parts instead.

Is DEF hazardous or flammable?

Per typical supplier SDS sheets, DEF is not classified as hazardous and is essentially non-flammable since it is mostly water. It may cause mild irritation, and it can release ammonia if overheated. Always confirm with the specific product SDS.

Does the slightly alkaline pH affect tank choice?

No. DEF's mild alkalinity (representative pH around 9–10) is well within the resistance range of polyethylene, polypropylene, fluoropolymers and stainless steel, so it does not narrow standard poly tank selection.