Supercritical carbon dioxide (sCO₂) is moving from power generation (the sCO₂ Brayton cycle), refrigeration and heat pumps into a growing number of industrial piping systems. Its demands on materials are completely different from ordinary CO₂: when dry it barely corrodes carbon steel, but add water and the corrosion rate can multiply more than fivefold. For hoses it is about the liner and its grade; for tubes it is about moisture content and temperature. This article lays out the material boundaries of sCO₂ systems in one pass, with a comparison table and a site checklist you can copy directly.
1. Three pain points from the field
Pain point 1: A carbon steel line ran for two years in dry sCO₂ with no trouble — then a new moisture monitoring point revealed the wall had thinned by 30%.
That is not a made-up story. Dry (unmodified) CO₂ really does corrode carbon steel slowly, and the research literature agrees: carbon steel shows low corrosion rates and good suitability in unmodified CO₂. But the moment "water-containing" appears, the conclusion flips: 1018 carbon steel shows clear corrosion attack in water-saturated sCO₂. Worse, temperature is an amplifier — in some water-containing conditions, raising the temperature from 40 °C to 80 °C pushes the corrosion rate from 0.45 mm/a to 2.43 mm/a. Less than a fivefold temperature rise, yet the corrosion rate multiplied more than five times.
Pain point 2: PTFE-lined hose is the mainstream choice — but mainstream does not mean universal.
PTFE (polytetrafluoroethylene) is extremely chemically inert; reinforced with a stainless steel braid it handles pressures of several hundred bar, making it one of the most mainstream solutions for sCO₂ transfer hose today. But watch for one trap: studies have reported that PTFE coatings may degrade in certain sCO₂ environments. So when specifying, do not stop at the three letters "PTFE" — ask the supplier for compatibility data on the specific grade under your actual conditions.
Pain point 3: 316 should be retired at 600 °C, yet many only think about changing material at 650 °C.
316 stainless steel (including 316H and 316L) is a common candidate for high-temperature sCO₂ systems; it resists corrosion in water-containing environments too and outperforms 304 overall. But its recommended temperature ceiling is roughly 600 °C: above that, the protective oxide layer on the surface may fail and corrosion resistance drops. Beyond that point you have to consider nickel-based alloys such as Inconel or Hastelloy.
2. The common root cause: watching pressure and temperature while ignoring medium purity
The three pain points above look like three separate problems, but they share one root cause — treating sCO₂ as "a gas" when it is really "a medium of variable purity". Its corrosion behavior is governed by two variables:
- Moisture content (medium purity) — decides whether carbon steel is even on the table for metal piping
- Working temperature — decides the grade ceiling for stainless steel
Until these two are pinned down, any "recommended material" is just talk.
3. One table to see the material boundaries
| Operating condition | Recommended material | Key limitation |
|---|
| Dry (unmodified) sCO₂, low to moderate temperature | Carbon steel | Economical; but severe corrosion as soon as moisture appears |
| Water-containing sCO₂ / better corrosion resistance needed | 304 stainless steel | Far more corrosion-resistant than carbon steel; forms oxide layers in high-temperature sCO₂, coatings often required |
| High-temperature sCO₂ (≤ 600 °C) | 316 / 316H / 316L | 600 °C is the recommended ceiling; beyond it the protective oxide layer may fail |
| High-temperature sCO₂ (> 600 °C) | Nickel alloys (Inconel / Hastelloy) | Cost rises significantly; evaluate case by case |
| High-pressure transfer hose | PTFE liner + stainless steel braid | Verify sCO₂ compatibility of the specific PTFE grade |
| CO₂ refrigerant hose | EPDM / IIR rubber tube + metal foil barrier | Rubber itself needs a barrier layer to suppress CO₂ permeation |
| Lightweight / alternative options | Polyamide (nylon) core hose | Long-term stability must be assessed separately |
One more detail on 304: at 550 °C and 20 MPa, 304 stainless steel forms an oxide layer, so real-world applications often rely on coatings to boost corrosion resistance. Its improved variant (e.g. Super 304H) at 600–700 °C is still an active research topic — do not deploy it in unverified engineering projects.
4. Three hard metrics to nail down before selecting
- Moisture content of the medium (dew point / purity) — the first watershed. Dry → carbon steel is viable; wet → jump straight to 304/316.
- Maximum working temperature — up to 600 °C use the 316 family; above 600 °C evaluate nickel alloys.
- Permeation and barrier — a hose solution must address both liner corrosion resistance and CO₂ permeation rate; rubber tubes require a metal foil or similar barrier layer.
5. How KUNGFLEX engineers it
Shanghai Beirun Hydraulic Equipment Co., Ltd. (KUNGFLEX) has specialized in hydraulic hoses and fluid connections since 2009, offering one-stop solutions for emerging media such as sCO₂, liquid cooling and hydrogen — from liner selection to assembly verification:
- PTFE-lined stainless steel braided hose assemblies: multi-layer braid construction for high-pressure fluid transfer
- 316L stainless steel piping and fittings: covering industrial fittings, steel tube assemblies and liquid-cooling connector lines
- Ultra-high-pressure burst verification: 4000 kg / approx. 400 MPa pressure testing, about 10 times standard working pressure
- ISO 9001:2015 quality management system, products exported to 80+ countries
Note that sCO₂ is a "special medium, severe service" scenario. Shanghai Beirun does not just hand over a generic part number — we first obtain your moisture content, temperature, pressure and medium purity, then lock in the liner material and grade, braid structure and fitting seal type, so compatibility risks are eliminated at the selection stage.
6. Site checklist (copy it straight to procurement and engineering)
- Is the moisture content / dew point of the medium confirmed? Could condensation form during operation?
- What is the maximum working temperature? Does it approach 600 °C?
- Metal piping route: dry → carbon steel; wet → 304/316; > 600 °C → nickel alloys
- Hose liner: PTFE / EPDM / IIR / nylon — do you have grade-specific sCO₂ compatibility data?
- Is a metal foil or similar barrier layer needed to suppress CO₂ permeation?
- Do fitting seal materials match the piping material (avoid galvanic corrosion between dissimilar metals)?
- Are overpressure / burst verification tests required?
7. FAQ
Q: Can carbon steel tubing be used in a dry sCO₂ system?
A: Yes — and it is economical. The prerequisite is keeping the medium continuously dry (unmodified). Once moisture enters, the carbon steel corrosion rate climbs sharply: going from 40 °C to 80 °C can push it from 0.45 mm/a to 2.43 mm/a.
Q: Must an sCO₂ transfer hose use a PTFE liner?
A: PTFE liner + stainless steel braid is the most mainstream solution (pressure capability up to several hundred bar), but some studies show PTFE coatings may degrade in certain sCO₂ environments, so grade-specific compatibility must be verified. For CO₂ refrigeration, EPDM / IIR tubes with a metal foil barrier are common.
Q: How to choose between 304 and 316?
A: In water-containing sCO₂ both are far better than carbon steel; for high temperatures prefer 316 (including 316H / 316L), whose recommended ceiling is about 600 °C. 304 tends to form oxide layers in high-temperature sCO₂ and often needs coating reinforcement.
Q: Any solution above 600 °C?
A: The 316 family is basically capped at 600 °C. Beyond that, evaluate nickel-based alloys (Inconel, Hastelloy, etc.) and rework the cost and structural design.
Q: Anything special about hose fittings?
A: Matching the seal face material to the tube material is critical — especially in wet or high-temperature service, avoid galvanic corrosion between dissimilar metals, and confirm the fitting pressure rating matches the hose.
About Shanghai Beirun
Shanghai Beirun Hydraulic Equipment Co., Ltd. (KUNGFLEX) is a source factory for hydraulic hoses and fluid connections, ISO 9001:2015 certified, exporting to 80+ countries. For emerging media such as supercritical CO₂, liquid cooling and hydrogen, we provide one-stop support covering liner selection, piping material matching and ultra-high-pressure verification: we take your moisture content, temperature, pressure and medium purity first, then lock in the liner grade, braid structure and fitting type — keeping compatibility risks out of the selection stage.
- Email: shbeirun@126.com
- Hotline: 400-820-9728 / 13764595878
- Business: hydraulic hose assemblies, industrial fittings, quick couplings, ball valves, steel tube assemblies and ultra-high-pressure hose verification
