Overview

Ferric chloride (FeCl₃) at 40% to 50% commercial concentration is the dominant coagulant in municipal drinking water treatment, wastewater clarification, and etching of printed circuit boards. It is a strongly acidic solution (pH 1.0-1.5), aggressively corrosive, dark red-brown in color, and stains essentially anything it touches. Storage in polyethylene is well-established but the hardware stack is more restrictive than most acids.

Why HDLPE or XLPE Both Work

Snyder approves BOTH HDLPE and XLPE for 50% ferric chloride at 1.9 ASTM specific gravity. Either resin will give 15+ years service in ambient ferric chloride storage. XLPE is slightly preferred for outdoor installations where trace condensate and UV exposure combine on the tank exterior.

Gasket: Viton

EPDM fails in ferric chloride — the chloride attacks the EPDM matrix. Viton (FKM) is required. Service life in ferric chloride is 3–5 years between replacements, shorter than in phosphoric but comparable to hypochlorite service.

Bolts: Hastelloy or Titanium — NOT 316SS

This is the critical departure from simpler acid specs. 316 stainless steel pits aggressively in ferric chloride service — chloride-driven crevice corrosion can start within months. Snyder's chart calls out Hastelloy OR Titanium for ferric chloride bolting. Both are comparable; Hastelloy is typically less expensive per bolt, Titanium gives slightly better long-term performance. Do NOT substitute 316SS to save cost — it will fail on a timeline that is predictable enough to be a known failure path.

Fittings: PVC

PVC bulkhead fittings work well for ferric chloride at ambient. PVC unions and valves are standard. CPVC is acceptable for warmer service but not required for ambient water-treatment use. Do not use polypropylene threaded fittings — ferric can penetrate PP thread bodies and create slow leaks.

Venting Plan

Ferric chloride has low vapor pressure and does not require special vent design. A standard atmospheric vent sized for fill/draw is adequate. The tank headspace may collect a faint orange-brown mist over time — this is normal aerosolization during fills and does not indicate tank failure.

System-of-Construction Table (Snyder Industries)

This is the exact specification Snyder Industries publishes for this chemistry. Every column is required — changing any of them voids the service rating.

Concentration-Band Compatibility (Enduraplas / Equistar Data)

Polyethylene chemical resistance by concentration and service temperature. Satisfactory (S) = long-term service. Limited (O) = occasional only. Unsatisfactory (U) = do not use.

Frequently Asked Questions

Can I use 316SS bolts for short-term ferric service?

No. Even short-term (weeks) exposure initiates chloride pitting that propagates even after service stops. Order Hastelloy or Titanium from day one; don't retrofit later.

Is ferric chloride dangerous to skin?

Yes. The pH is 1.0-1.5 — strong acid. Skin contact causes immediate burning and staining. Eye exposure is medical emergency. PPE: acid-resistant gloves, face shield, apron. Treat handling the same as strong sulfuric or hydrochloric.

Does the tank get stained inside?

The tank wall gets a permanent orange-brown tint from iron deposits. This is cosmetic and does not affect service. On cleaning/replacement, expect a persistent iron film that does not fully rinse off — it is bonded at the polymer surface.

Can I dilute ferric chloride with plant water?

If water is clean and deionized, yes. If water contains calcium or carbonates, expect precipitation of iron oxides. Most commercial ferric chloride customers dilute in the dosing loop, not in the storage tank, for this reason.

Source Citations

• Snyder Industries — Chemical Resistance Recommendations (current edition)
• Enduraplas / Equistar Technical Tip — Chemical Resistance of Polyethylene (12-page reference)

Chemical-Service Tanks

These HDPE vertical chemical-storage tanks from Snyder Industries ship pre-engineered for industrial chemistry service at 1.9 ASTM design specific gravity. When you order for ferric chloride service, our team verifies the full materials-of-construction stack (resin grade, fittings, gaskets, bolts) against the OEM recommendations above before shipment — no surprises at commissioning.

120 Gallon

120 Gallon HDPE Vertical Chemical Storage Tank

SII-5700100N45

$738.78

550 Gallon

550 Gallon HDPE Vertical Chemical Storage Tank

SII-1820000N45

$1,158.00

1,100 Gallon

1100 Gallon HDPE Vertical Chemical Storage Tank

SII-1830000N45

$2,468.99

1,500 Gallon

1500 Gallon HDPE Vertical Chemical Storage Tank

SII-8120100N45

$2,047.00

2,500 Gallon

2500 Gallon HDPE Vertical Chemical Storage Tank

SII-5090000N39

$3,352.00

5,500 Gallon

5500 Gallon HDPE Vertical Chemical Storage Tank

SII-7000000N43

$10,975.00

Ferric Chloride Compatibility Matrix — 30% to 45% FeCl3

Ferric chloride is supplied as a 38–42% solution (the commercial "40%" grade) and is strongly acidic (pH 1–2) and chloride-rich. Polyethylene is the tank-material default. Stainless steels and titanium are all unacceptable — chloride pitting and stress-corrosion cracking initiate quickly even at room temperature. Tank selection in ferric chloride service is narrower than most acid services because the chloride eliminates the usual stainless fallback.

Vinyl-ester FRP is the industry-preferred material for bulk ferric chloride storage above 50,000 gallons. Polyester-based FRP is B-rated; spec vinyl ester for any new build. PVDF is the universal choice for piping and valves. Do not specify stainless steel anywhere in the wetted path — even passivated 316L, 304L, 2205 duplex, and 254 SMO all pit under ferric chloride. Tantalum, Hastelloy C-276, and Alloy 20 are the acceptable alloy choices for critical wetted metal components (instruments, pump impellers).

Real-World Industrial Use Cases

Ferric chloride is the workhorse coagulant in wastewater treatment and a major steel-industry byproduct. US consumption is approximately 300,000 tons per year. Five dominant verticals:

• Municipal wastewater (phosphorus removal): 40% FeCl₃ in 5,000–50,000 gallon HDPE, XLPE, or FRP bulk tanks at POTWs; dosed to the primary clarifier or activated sludge basin to precipitate phosphate as iron phosphate. Federal and state nutrient limits (NPDES) drive most of the demand.
• Drinking water coagulation: 40% FeCl₃ in 2,000–25,000 gallon HDPE or XLPE tanks at surface-water plants; alternative to alum and PAC where high-turbidity, high-color raw water favors iron coagulation.
• Steel pickling & etching: Concentrated FeCl₃ (40%+) in HDPE or FRP etch-bath day tanks at PCB manufacturers and metal-finishing shops; pickling line waste streams generate ferric chloride that is often reclaimed and resold into the coagulant market.
• Sludge conditioning: 40% FeCl₃ in 500–5,000 gallon day tanks dosed before belt-filter or plate-and-frame dewatering to improve sludge cake solids.
• H₂S odor control (lift stations & force mains): 40% FeCl₃ in 500–3,000 gallon HDPE tanks for continuous dosing into wastewater collection; precipitates sulfide as iron sulfide and eliminates the corrosion and odor at the downstream treatment plant.

Bulk ferric chloride storage tank sizes run larger than most coagulants because POTW dose rates scale with sludge solids and phosphorus load. A 20-MGD plant dosing 40 mg/L runs through 40,000+ gallons of 40% FeCl₃ per week.

Hazard Communication — GHS, NFPA 704, DOT, TSCA

CAS: 7705-08-0. UN: 2582 (solution) / 1773 (anhydrous). TSCA: listed, active.

• GHS pictograms: Corrosion, Exclamation Mark. Signal word: Danger.
• GHS hazard statements: H290 (may be corrosive to metals), H302 (harmful if swallowed), H314 (causes severe skin burns and eye damage), H318 (causes serious eye damage).
• NFPA 704: Health 3, Flammability 0, Instability 1, Special — (none).
• DOT hazard class: Class 8 (corrosive), PG III for 40% solution / PG II for more concentrated grades.
• EPA CERCLA RQ: 1,000 lb reportable quantity.
• OSHA PEL: 1 mg/m³ TWA (as iron).

The dominant hazard beyond corrosivity is the stain and cleanup burden. A ferric chloride release oxidizes quickly to iron oxide (rust), permanently staining concrete, asphalt, steel, and clothing. Secondary containment surfaces in FeCl₃ service develop a rust patina that is cosmetic only — but neighboring equipment, building walls, and grading are routinely damaged by minor leaks and drips. Drip pans and trays under every fitting are standard good practice.

Storage Protocol — Freeze Protection, Venting, Stainless Exclusion

Secondary containment: 110% of largest tank with an acid-resistant liner. HDPE geomembrane (60 mil minimum) is standard. Concrete should be sealed with an acid-resistant coating — untreated concrete is slowly attacked by FeCl₃ and develops surface spalling over years. The containment area will stain rust-red over the tank life; treat that as cosmetic, not a failure indicator.

Freeze protection: 40% FeCl₃ freezes at approximately -4°F (-20°C) — lower than PAC but above the design minimum for northern climates. A partially frozen tank does not fully re-dissolve; iron hydroxide precipitates out and settles as sludge. Heat-trace and insulate outdoor tanks to hold above 32°F. Frozen FeCl₃ expansion can crack the tank wall or rupture feed piping.

Venting: Atmospheric breather adequate. FeCl₃ off-gasses trace HCl vapor, more noticeable at warm temperatures and during fill. Indoor tank rooms benefit from active ventilation; scrubbed venting is rarely required for 40% solutions. Goose-neck the vent outdoors and stage the exit above any occupied walkway.

Temperature control upper limit: 100°F. Above that the HCl off-gas rate climbs and the polyethylene long-term creep margin tightens. Outdoor tanks in hot climates should be shaded.

Gaskets & fittings: Viton (FKM) preferred. PTFE universal. EPDM acceptable below 20% FeCl₃. Avoid Buna-N — it swells and cracks within weeks. No stainless steel anywhere in the wetted path — spec CPVC, PVDF, PVC, or lined carbon steel for piping; spec Alloy 20, Hastelloy C-276, or tantalum for wetted metal instruments. Bolts on wetted manways should be FRP, Hastelloy, or coated for corrosion service.

Transfer & mixing: Dilution is mildly exothermic but not at the sulfuric-acid scale; standard dilution in a stirred HDPE vessel is safe. The handling hazard is splash — pH 1–2 contact with skin causes immediate burns and permanent staining. PPE (acid suit, face shield, gloves) is mandatory for any hose connection or sample pull.

Ferric Chloride FAQs — Field-Tested Answers

Why can't I use a 316L stainless tank for ferric chloride?

Chloride pitting. 40% FeCl₃ contains approximately 26% chloride by weight at pH 1–2 — an aggressive pitting environment even for the duplex and super-austenitic stainless grades. Pit initiation happens in days and the first visible perforations follow within months. Industry consensus is no stainless steel in FeCl₃ wetted service. Spec HDPE, XLPE, vinyl-ester FRP, PVDF, or — for wetted metal in critical positions — Alloy 20, Hastelloy C-276, or tantalum.

What pH does my 40% ferric chloride show on a test strip?

pH 1 to 2. Commercial 40% FeCl₃ is strongly acidic from the HCl equilibrium in solution. Dilute 40% to 1% and the pH climbs to roughly 2.5–3. Dose the coagulant into raw water at 10–50 mg/L and the pH depression at the point of dose is typically 0.2–0.5 units. Recarbonation or lime dose downstream restores pH for regulatory compliance.

Why does my ferric chloride tank leave a rust patina on the containment floor?

Expected and cosmetic. FeCl₃ drips oxidize on air exposure to iron oxide (rust), staining concrete and HDPE geomembrane a deep red-orange. The stain is cosmetic; the underlying liner integrity is unaffected if the material specifications match. The cleanup burden is real — expect to repaint or re-coat adjacent equipment over the tank life. Drip trays under every fitting limit the spread.

Can ferric chloride freeze in an outdoor tank?

40% FeCl₃ freezes at -4°F. In the northern tier of the US and all of Canada, outdoor tanks must be heat-traced and insulated to hold above 32°F. A frozen tank does not cleanly thaw — iron hydroxide crystallizes and settles as sludge that does not re-dissolve. Annual tank cleanout is recommended after any winter with sustained sub-zero exposure.

What's the difference between ferric chloride and ferric sulfate for phosphorus removal?

Same iron chemistry, different anion. Ferric sulfate (Fe₂(SO₄)₃) avoids the chloride pitting problem and is kinder to stainless equipment. It also depresses pH less per unit of iron dosed. The downside is higher cost and lower commercial availability at large scale. Most large POTWs settled on FeCl₃ a generation ago and designed the containment, piping, and tank materials around the chloride — the switch back to ferric sulfate is usually a tank replacement decision, not a feed change.

Related Chemistries in the Water-Treatment Coagulant Cluster

Related chemistries in the water-treatment coagulant cluster (municipal + industrial + paper-mill coagulation + flocculation):

• Ferric Sulfate (Fe2(SO4)3) — Sulfate-based iron coagulant
• Ferrous Chloride (FeCl2) — Reduced-iron byproduct for odor control
• Aluminum Sulfate (alum) — Aluminum-based coagulant alternative
• Polyaluminum Chloride (PAC) — High-charge Al coagulant