HEDP (Etidronic Acid) Storage & Tank Compatibility

Storing HEDP (Etidronic Acid)? Start Here

HEDP, or etidronic acid (C₂H₈O₇P₂), is one of the most widely used organophosphonate scale and corrosion inhibitors in water treatment. As a diphosphonic acid it strongly chelates calcium, iron, copper, and zinc, suppressing scale deposition and stabilizing metal ions across a broad pH range. It is supplied as a white crystalline powder or, more commonly, as a clear 60 percent aqueous solution used in cooling-water programs, boiler treatment, reverse-osmosis antiscalant blends, detergents, and oilfield service. In its concentrated acidic form it is corrosive to skin, eyes, and metals and is harmful if swallowed, so it is handled as a corrosive chemical. Because the working material is an aqueous acid rather than a solvent, polyethylene storage is straightforward and reliable when metal contact is avoided.

Is HEDP (Etidronic Acid) Compatible with Polyethylene Tanks?

Yes. Aqueous etidronic acid and its salt solutions are compatible with both high-density polyethylene (HDPE) and cross-linked polyethylene (XLPE), which is why phosphonate scale inhibitors are routinely stored and dosed from polyethylene tanks. HEDP is a water-soluble acid that does not swell, soften, or chemically degrade polyethylene the way hydrocarbons, aromatics, chlorinated solvents, ketones, or strong oxidizers would. The real compatibility risk is on the metal side: as an acid and a powerful chelant, HEDP attacks carbon steel and aluminum and can sequester metal ions even from stainless steel under the wrong conditions. For storage and feed systems, pair an HDPE or XLPE tank with polypropylene, PVC/CPVC, or fluoropolymer fittings, EPDM or Viton seals (verified for concentrate), and keep wetted metal out of the system. Vent fittings should account for the acidic vapor space, and concentrate tanks benefit from chemical-resistant secondary containment.

Material compatibility at a glance

HEDP (etidronic acid) is an aqueous corrosive organophosphonic acid and an aggressive metal chelant. Polyethylene (HDPE / XLPE) is the workhorse material and is fully compatible, along with PP, PVC/CPVC, PVDF/PTFE, and EPDM. Metals are the problem: carbon steel and aluminum are unsuitable, and even stainless steel is only conditionally acceptable in dilute, ambient service. Specify a polyethylene tank with polymer or fluoropolymer wetted hardware.

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

• Corrosive: causes severe skin burns and serious eye damage. Wear chemical-splash goggles, a face shield, and acid-resistant gloves and apron when handling the concentrate.
• Harmful if swallowed and may cause organ damage on exposure; never eat, drink, or store food in the handling area, and wash thoroughly after contact.
• Corrosive to metals: store and transfer only in polyethylene or other approved polymer equipment; isolate from carbon steel and aluminum to prevent attack and contamination.
• Provide adequate ventilation, eyewash stations, and a safety shower; in case of eye or skin contact, flush with water for at least 15 minutes and seek medical attention.
• May cause long-lasting harmful effects to aquatic life; prevent release to drains and waterways and contain spills with inert absorbent and proper neutralization.
• Keep containers tightly closed in a cool, well-ventilated area away from incompatible bases and strong oxidizers.

Common questions

Can I store HEDP (etidronic acid) in an HDPE or XLPE tank?

Yes. Aqueous etidronic acid and its salt solutions are compatible with both HDPE and XLPE polyethylene, which is the standard storage choice for phosphonate scale inhibitors. Use polymer or fluoropolymer fittings and keep wetted metal out of the system.

Why can't I store HEDP in a steel tank?

HEDP is an acid and a strong metal chelant. It corrodes carbon steel and aluminum and can sequester metal ions even from stainless steel under unfavorable conditions, leading to tank attack and product contamination. Polyethylene avoids the problem entirely.

Is HEDP flammable?

No. Etidronic acid is non-combustible, with an NFPA flammability rating of 0 and a predicted flash point above 300 C. The hazard is its corrosivity, reflected in the NFPA health rating of 3.

What fittings and seals work with HEDP?

Polypropylene, PVC/CPVC, and PVDF/PTFE all perform well, and EPDM is a good seal choice. Viton/FKM is generally acceptable but should be confirmed with the seal maker for concentrate or heated service. Avoid carbon steel and aluminum hardware.