Boiler Oxygen Scavengers: DEHA vs Sulfite vs Hydrazine
Oxygen scavengers remove dissolved oxygen from boiler feedwater to stop pitting corrosion. Sodium sulfite is cheap and fast but adds solids and breaks down above ~1000 psi; hydrazine is volatile and adds no solids but is toxic/carcinogenic and being phased out; DEHA and carbohydrazide are low-toxicity, volatile, hydrazine-free scavengers that also passivate metal and protect the condensate. Choose by boiler pressure, whether you need condensate protection, and safety.
Why dissolved oxygen must be removed
Dissolved oxygen in feedwater causes pitting corrosion — deep, localized pits that perforate boiler tubes and economizers even when general metal loss is low. Mechanical deaeration removes most oxygen, but a chemical oxygen scavenger mops up the last traces and maintains a protective reducing environment. Scavengers are dosed to the deaerator outlet or feedwater and are a core part of any boiler water treatment program.
Sodium sulfite — low-pressure workhorse
Sodium sulfite (often catalyzed with cobalt) reacts fast with oxygen and is cheap and reliable for low-pressure boilers. It needs about 7.9 ppm of sulfite per 1 ppm of O₂ (plus a boiler-water residual, typically ~30–60 mg/L). Drawbacks: it adds dissolved solids (raising blowdown), is non-volatile so it gives no condensate protection, and it decomposes above roughly 1000 psi into corrosive SO₂/H₂S. Best for boilers below ~600–900 psi.
Hydrazine — effective but phased out
Hydrazine reacts with oxygen roughly stoichiometrically (~1 ppm per 1 ppm O₂), adds no dissolved solids, is volatile (protects condensate) and passivates metal to magnetite — technically excellent for high-pressure boilers. But hydrazine is toxic and a suspected carcinogen, so it has been widely phased out in favor of safer volatile scavengers like DEHA and carbohydrazide.
DEHA and carbohydrazide — the modern choices
DEHA (N,N-diethylhydroxylamine) is a volatile, low-toxicity scavenger that passivates metal and, because it carries into steam, protects the whole condensate system. Its reaction rate is pH-dependent (near-sulfite speed at pH ~11). Carbohydrazide is a hydrazine-free scavenger that reacts quickly, adds no solids and suits medium- and high-pressure boilers and cold feedwater — it decomposes to give a hydrazine-like passivating action without shipping hydrazine. Both are the standard modern replacements for sulfite/hydrazine.
How to choose
- Low-pressure boiler, cost priority → catalyzed sodium sulfite.
- Need condensate protection / no added solids → DEHA (volatile).
- Medium/high-pressure, cold feedwater, hydrazine-free → carbohydrazide.
- Avoid hydrazine for safety unless a legacy program requires it.
Oxygen scavengers compared
| Property | Sodium sulfite | Hydrazine | DEHA | Carbohydrazide |
|---|---|---|---|---|
| Reaction ratio (per ppm O₂) | ~7.9 ppm | ~1.0 ppm | ~1.2–1.5 ppm | ~1.4 ppm |
| Adds dissolved solids | Yes | No | No | No |
| Volatile / protects condensate | No | Yes | Yes | Yes |
| Pressure suitability | Low (<~1000 psi) | High | Low–medium | Medium–high |
| Toxicity | Low | High (carcinogen) | Low | Low |
| Metal passivation | Weak | Strong | Yes | Strong |
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Frequently asked questions
What is the best oxygen scavenger for boilers?
It depends on pressure and needs. Catalyzed sodium sulfite is best for cost-sensitive low-pressure boilers; DEHA is preferred when you need volatile condensate protection and no added solids; carbohydrazide suits medium- and high-pressure boilers and cold feedwater. Hydrazine is effective but toxic and largely phased out.
How much sodium sulfite is needed per ppm of oxygen?
About 7.9 ppm of sodium sulfite reacts with 1 ppm of dissolved oxygen, and a boiler-water sulfite residual of roughly 30–60 mg/L is usually maintained. Because sulfite adds dissolved solids and is non-volatile, it raises blowdown and does not protect the condensate system.
Why is hydrazine no longer used in boilers?
Hydrazine is an excellent volatile oxygen scavenger that adds no solids and passivates metal, but it is toxic and a suspected carcinogen. For safety it has been widely replaced by low-toxicity volatile scavengers such as DEHA and carbohydrazide, which give similar performance without the hazard.
What is the difference between DEHA and carbohydrazide?
Both are volatile, low-toxicity, hydrazine-free scavengers that add no dissolved solids and protect the condensate. DEHA is a cost-effective choice for low- and medium-pressure boilers; carbohydrazide reacts faster and passivates strongly, making it preferred for higher-pressure boilers and cold feedwater.
Does VCYCLETECH supply boiler oxygen scavengers?
Yes. VCYCLETECH supplies DEHA and carbohydrazide oxygen scavengers factory-direct from China, ISO 9001/14001/45001 certified, with a COA on every batch, free samples and OEM/ODM service. Email sales@vcycletech.com for a dosing recommendation and quotation.
About the manufacturer
VCYCLETECH is a China-based manufacturer of water treatment chemicals — boiler and cooling-water inhibitors, oxygen scavengers, amines, antiscalants, biocides, coagulants and flocculants — ISO 9001 / 14001 / 45001 certified, with a COA on every batch and OEM/ODM service. See our quality & certifications.
References
Related: DEHA · Carbohydrazide · Corrosion inhibitors & oxygen scavengers · Boiler water treatment · Condensate amines