Oxygen Scavengers for Boiler Water: 8 Types, Dosage and Selection by Pressure

Oxygen scavengers for boiler water are 8 reducing chemicals in 3 classes, from sodium sulfite to hydrazine, carbohydrazide and DEHA, that react away the dissolved oxygen a deaerator leaves in feedwater before it pits steel tubes. In theory 7.88 mg/L of sodium sulfite but only 1.0 mg/L of hydrazine removes each mg/L of oxygen, so which scavenger fits which boiler?

The 3 classes are the inorganic sulfite scavenger, sodium sulfite (also spelled sulphite); the volatile scavengers hydrazine, carbohydrazide, DEHA and MEKO; and the non-volatile organic scavengers hydroquinone, sodium erythorbate and tannin. Together, oxygen scavengers are one of the 16 families of water treatment chemicals.

Boiler pressure decides the choice: sulfite below 1,000 psig (69 bar), hydrazine or an organic volatile scavenger above it, and no reducing agent at all in all-ferrous high-pressure cycles under IAPWS guidance. Doses, residuals, oxygen limits, 21 CFR 173.310 status and hazards follow.

Every value on this page is checked against primary sources (eCFR, IAPWS, PubChem, ECHA, NIOSH and the handbooks named in the text); see our methodology and fact-checking process.

What Is an Oxygen Scavenger in Water Treatment?#

An oxygen scavenger in water treatment is a reducing chemical, such as sodium sulfite, hydrazine or DEHA, that reacts with dissolved oxygen in boiler feedwater, condensate or a closed loop and converts it into water, nitrogen or a sulfate salt. The Veolia Water Technologies Handbook (chapter 11) names sodium sulfite, sodium bisulfite, hydrazine, catalyzed sulfites and hydrazine, hydroquinone and ascorbate as the most common scavengers.

Oxygen scavengers remove the oxidant itself, while corrosion inhibitors, such as nitrite and molybdate in closed loops, protect the metal surface. Both chemical groups are compared by mechanism on corrosion inhibitors for water treatment.

Why must dissolved oxygen be removed from boiler water?#

Dissolved oxygen must be removed because it corrodes the feedwater train, economizer and boiler tubes, and water at 15.6 °C (60 °F) and 1 atm holds about 8 mg/L (ppm) of it, while feedwater above 450 psig (31.0 bar) may carry only 0.007 mg/L (7 ppb) before scavenging.

Solubility follows Henry's law: 8 ppm at 0.2 atm oxygen partial pressure, 4 ppm at 0.1 atm. At the economizer, 3 to 10 ppb is the level the same handbook calls not significantly damaging.

Oxygen pitting is one of the failure modes covered under boiler corrosion.

How is oxygen removed from boiler feedwater?#

Oxygen is removed in 2 steps: a deaerator strips 97 to 98 % of the incoming oxygen with steam, and a chemical scavenger reacts away the remaining traces, because nearly complete removal needs both (same handbook, chapter 10).

Deaerator performance sets the scavenger load. A pressure deaerator reaches less than 0.005 cm³/L (7 ppb) of oxygen, a vacuum deaerator 0.25 to 0.50 cm³/L (330 to 650 ppb). In Spirax Sarco figures, an atmospheric feedtank at 85 to 90 °C (185 to 194 °F) still holds about 2 mg/L, while a pressurized deaerator at 0.2 bar g (3 psig) and 105 °C (221 °F) reaches less than 20 ppb.

The feedwater chemicals appear in Table 2 below; the equipment side is explained under deaerators.

How Do Oxygen Scavengers Work?#

Oxygen scavengers work in 3 ways: they reduce dissolved oxygen in a fixed stoichiometric ratio, they react faster with heat, alkalinity and a metal catalyst, and several of them also reduce iron and copper oxides to protective magnetite and cuprous oxide.

Reduction reactions and the theoretical dose#

Each scavenger removes oxygen in a fixed ratio set by its reaction, from 1.0 mg/L of hydrazine to 7.88 mg/L of sodium sulfite per mg/L of oxygen in theory. The 4 reactions below set the theoretical dose of the most used scavengers.

• Sodium sulfite: 2 Na2SO3 + O2 → 2 Na2SO4, 7.88 mg/L per mg/L O2.
• Hydrazine: N2H4 + O2 → N2 + 2 H2O, 1.0 mg/L per mg/L O2.
• Carbohydrazide: (N2H3)2CO + 2 O2 → 2 N2 + 3 H2O + CO2, 1.41 mg/L per mg/L O2.
• DEHA: 4 DEHA + 9 O2 → 8 CH3COOH + 2 N2 + 6 H2O, 1.24 mg/L per mg/L O2.

Sulfite adds sulfate solids, hydrazine adds none, and carbohydrazide (CO2) and DEHA (acetic acid) both raise cation conductivity.

Temperature, pH and catalysts set the reaction speed#

Oxygen scavengers react faster as water gets hotter: the reaction rate doubles with every 10 °C (18 °F) and becomes rapid at 100 °C (212 °F), which is why a catalyst is added to sulfite for cold or poorly deaerated feedwater.

The pH windows are 8.5 to 10.0 for sulfite and 9.0 to 10.0 for hydrazine. Iron, copper, cobalt, nickel or manganese cations catalyze sulfite, cobalt most commonly; in the Veolia handbook (chapter 11), catalyzed sulfite removed all oxygen in 25 s, uncatalyzed less than 50 %. Sipilä et al. (2024, Corrosion Science 240, 112476) rank carbohydrazide first, hydrazine about equal to erythorbic acid, then DEHA, with MEKO far behind at an 8-fold excess.

Metal passivation: magnetite and cuprous oxide#

Passivating scavengers, such as hydrazine, carbohydrazide and hydroquinone, also reduce red iron oxide to magnetite (Fe3O4) and cupric oxide to cuprous oxide (Cu2O), while sodium sulfite removes oxygen without passivating the metal.

For hydrazine the reactions are N2H4 + 6 Fe2O3 → 4 Fe3O4 + N2 + 2 H2O and N2H4 + 4 CuO → 2 Cu2O + N2 + 2 H2O. In the firerod test of US patent 4,269,717, carbohydrazide at 1 ppm ranked first, ahead of sodium sulfite at a 20 to 40 ppm residual and hydrazine at 2 ppm.

8 Types of Oxygen Scavengers in 3 Classes#

The 8 oxygen scavengers used in boiler water fall into 3 classes: 1 inorganic sulfite scavenger, 4 volatile scavengers and metal passivators, and 3 non-volatile organic scavengers. Table 1 compares the 3 classes by volatility, dissolved solids, passivation and service.

1. Inorganic sulfite scavengers (1)#

Inorganic sulfite scavengers are sulfur(IV) salts that oxidize to sulfate, add dissolved solids and serve boilers below 1,000 psig (69 bar); this class has 1 scavenger with its own entry, sodium sulfite, plus catalyzed sulfite and bisulfite blends.

Sodium bisulfite and sodium metabisulfite carry the same sulfite chemistry, at 6.5 and 5.94 mg per mg of oxygen in theory, and their main role in water treatment is removing chlorine. That role, and their dose ratios per mg of chlorine, are covered under dechlorination chemicals.

Sodium sulfite#

Sodium sulfite (Na2SO3) is the standard scavenger for boilers below 1,000 psig (69 bar): 7.88 mg/L removes 1 mg/L of oxygen in theory, about 10 mg/L does in practice with technical grade, and the boiler water holds more than 20 mg/L as SO3 as a reserve. The practical figures come from the Veolia handbook (chapter 11); supply is dry or as a catalyzed liquid.

2. Volatile oxygen scavengers and metal passivators (4)#

Volatile oxygen scavengers are nitrogen-based reducing agents that add no dissolved solids and travel with the steam into the condensate; this class has 4 scavengers: hydrazine, carbohydrazide, DEHA and MEKO.

Hydrazine decomposes to ammonia and DEHA distributes with the steam, so both also raise condensate pH, a job that neutralizing amines, such as morpholine and cyclohexylamine, do on purpose. Amine selection and distribution ratios are compared on neutralizing amines for condensate treatment.

Hydrazine#

Hydrazine (N2H4) is the reference high-pressure scavenger: 1.0 mg/L removes 1 mg/L of oxygen in theory and 1.5 to 2.0 mg/L in practice, and it adds no solids, but its harmonised EU classification as Carc. 1B (H350) limits its use. Boiler grade is a 35 % solution. Hydrazine decomposes to ammonia from about 204 °C (400 °F), rapidly at 316 °C (600 °F).

Carbohydrazide#

Carbohydrazide (CH6N4O) is the fastest of the hydrazine alternatives in the 2024 VTT ranking and needs 1.41 mg/L per mg/L of oxygen in theory. Above about 135 °C (275 °F) it hydrolyzes in part to hydrazine and CO2, and above 200 °C (392 °F) to ammonia, nitrogen, hydrogen and CO2.

DEHA (diethylhydroxylamine)#

DEHA (N,N-diethylhydroxylamine) is a volatile scavenger that distributes with the steam and protects condensate lines, needing 1.24 mg/L per mg/L of oxygen in theory. It is sold mainly as an 85 % solution. Its oxidation product is acetic acid, and in VTT loop tests conductivity rose rapidly above about 180 °C (356 °F).

MEKO (methyl ethyl ketoxime)#

MEKO (methyl ethyl ketoxime) is an oxime scavenger studied as a hydrazine alternative, with the slowest initial oxygen reaction of the alternatives in the VTT 2024 tests. Like hydrazine, it carries a harmonised EU Carc. 1B (H350) classification.

Hydroquinone#

Hydroquinone (C6H6O2) is the scavenger that reacts quickly with oxygen even at ambient temperature, which suits start-up, shutdown and condensate service, and it also catalyzes other scavengers. It needs 6.9 mg/L per mg/L of oxygen in theory, oxidizes to benzoquinone and degrades to acetates and CO2 at high pressure.

Sodium erythorbate#

Sodium erythorbate (C6H7NaO6), the salt of erythorbic acid, is an ascorbate-type scavenger for low- and medium-pressure boilers, needing 12.4 mg/L per mg/L of oxygen in theory (11.0 mg/L as erythorbic acid). In the VTT 2024 work it reacts about as fast as hydrazine and interacts more strongly with the oxide film.

3. Non-volatile organic oxygen scavengers (3)#

Non-volatile organic oxygen scavengers are carbon-based reducing agents that stay in the boiler water; this class has 3 scavengers: hydroquinone, sodium erythorbate and tannin.

Tannin#

Tannin, including quebracho extract, is a polyphenolic plant extract that scavenges oxygen and conditions sludge in low-pressure boilers, and 21 CFR 173.310(c) lists it with no numeric limit. The fluid sludge it forms settles in the mud drum and leaves with bottom blowdown.

Which Oxygen Scavenger for Which Boiler? Selection by Pressure and System#

Oxygen scavenger selection depends on 3 factors: boiler pressure, the metals in the feedwater and condensate system, and whether the water circulates as steam, in a closed loop or stands idle in lay-up.

Below 1,000 psig (69 bar): sulfite and catalyzed sulfite#

Boilers below 1,000 psig (69 bar) use sodium sulfite or catalyzed sodium bisulfite, because the sulfate solids they add are removed by blowdown and the reaction is fast at deaerator temperature (Veolia Water Technologies Handbook, chapter 11).

Catalyzed sulfite should be chosen under 5 conditions: low feedwater temperature, incomplete deaeration, a need for rapid reaction, short residence time and an economizer. Sodium erythorbate and tannin are the organic options in this range.

1,000 psig (69 bar) and above: volatile scavengers or no reducing agent#

Boilers at 1,000 psig (69 bar) and above stop using sulfite, because it decomposes to SO2 and H2S, and either feed hydrazine or an organic volatile scavenger or, in all-ferrous cycles, follow the IAPWS preference for all-volatile treatment without any reducing agent.

The SO2 and H2S corrode the condensate system and are linked to turbine stress corrosion cracking. IAPWS TGD3-10(2015) keeps a reducing agent only for mixed-metallurgy cycles under AVT(R), at economizer inlet pH 9.1 to 9.3 and oxygen below 10 µg/kg. Carbohydrazide, DEHA and erythorbate replace hydrazine where its Carc. 1B classification rules it out.

By system: hot-water boilers, closed loops, condensate and lay-up#

Each system outside the steam drum has its own scavenger practice: sulfite with pH control and polymer in high-temperature hot water, sulfite, erythorbate or DEHA in closed loops, DEHA in condensate lines, and high scavenger residuals in wet lay-up.

High-temperature hot-water systems run up to 34 bar (500 psig), usually 2.4 to 24 bar (35 to 350 psig). For tight closed loops, an AWT closed-system guide gives DEHA at 200 mg/L initial and 125 mg/L maintenance as product, at pH 9.5 to 10.0. Copper-alloy heaters in lay-up take half the scavenger dose at pH 9.5. Table 2 sets out where each scavenger is used.

Oxygen Scavenger Dosage by Chemical#

Oxygen scavenger doses start from a theoretical ratio per mg/L of dissolved oxygen, from 1.0 mg/L for hydrazine to 12.4 mg/L for sodium erythorbate, and the fed dose adds an excess and a residual reserve on top. Table 3 compares the ratios in mg/L per mg/L O2 (equal to ppm per ppm and lb per lb), as the pure chemical.

Theoretical ratios are calculated from balanced reactions and PubChem molecular weights. A 35 % hydrazine or 85 % DEHA solution needs the dose divided by its strength, and the fed amount follows feedwater oxygen plus chemical demand.

Worked example: from feedwater oxygen to a sulfite feed rate#

At 10 m³/h (44.0 gpm) of feedwater carrying 2 mg/L of oxygen, sodium sulfite at the practical 10 : 1 ratio needs 20 mg/L, or 4.8 kg/d (10.6 lb/day), before any residual reserve is added. Worked example inputs: 10 m³/h (44.0 gpm), 2 mg/L oxygen from an atmospheric feedtank, technical-grade sodium sulfite treated as 100 %, 24 h/d.

1. Multiply 2 mg/L of oxygen by 10 to get 20 mg/L of sodium sulfite.
2. Convert the flow: 10 m³/h x 24 h = 240 m³/d (0.0634 MGD).
3. Multiply 20 g/m³ by 240 m³/d to get 4.8 kg/d.
4. Check in US units: 20 x 0.0634 x 8.34 = 10.6 lb/day.

For other scavengers and strengths, the chemical dosage calculator converts any scavenger dose in mg/L into kg/d, lb/day and pump settings.

Where is an oxygen scavenger fed, and with what equipment?#

An oxygen scavenger is fed continuously to the storage section of the deaerator, so that it has the longest possible contact time before the feedwater reaches the economizer and boiler. The 4 feed rules below keep oxygen scavengers effective and safe.

• Feed point: deaerator storage section or drop leg, plus extra sulfite ahead of hot zeolite units.
• Attemperation water: never sulfite; hydrazine and organic scavengers are acceptable.
• Feeding alone: catalyzed sulfite, diluted only with condensate or demineralized water; hydrazine, never with organics, amines or nitrates.
• Materials: 304 stainless tanks for sulfite; polyester, FRP or polyethylene tanks with PVC or 316 stainless lines for acidic liquid sulfite; an injection quill.

How Is Oxygen Scavenger Feed Controlled and Tested?#

Oxygen scavenger feed is controlled in 2 layers: a residual of the scavenger in boiler water or feedwater shows a reserve, and feedwater oxygen measured close to the line proves the result.

Residual targets: sulfite, hydrazine and DEHA#

Residual targets exist for 3 scavengers, and each has its own test method. The 3 targets are listed below.

• Sulfite: above 20 mg/L as SO3 in low- and medium-pressure boiler water, by iodide-iodate titration.
• Hydrazine: 0.05 to 0.1 mg/L excess in feedwater, by the p-DMAB method (Hach Method 8141).
• DEHA: by a modified PDTS ferrous-iron colorimetric method.

Boiler residuals of volatile scavengers are unreliable, so their feed is compared with feedwater oxygen instead (Veolia handbook, chapter 11). Test frequency and the full parameter list are on boiler water testing.

Feedwater oxygen measurement and sampling#

Feedwater oxygen is measured by ASTM D888-18(2026) or, at microgram levels, ASTM D5543-21, on a sample line kept short and flowing at 1.5 to 1.8 m/s (5 to 6 ft/s) so that the scavenger cannot keep reacting inside it.

Sample flow of 800 to 1,000 mL/min runs continuously, ideally to a continuous oxygen meter, at least once per shift by grab test otherwise.

Long lines read low when the scavenger keeps reacting and high when air leaks in. Units and solubility tables are on dissolved oxygen in boiler, cooling and wastewater systems.

A deaerator check stops scavenger feed briefly and reads the deaerator outlet directly.

Under AVT(R), the deaerator inlet also has to read reducing, below 20 mV.

How that reading is measured is explained under oxidation-reduction potential (ORP).

Which Standards and Regulations Apply to Oxygen Scavengers?#

Oxygen scavengers are governed in 3 layers: feedwater oxygen limits from ASME and IAPWS guidance, the 21 CFR 173.310 list for steam that contacts food, and EU CLP and US workplace rules for the hazardous members. No AWWA or EN product standard exists for the 8, NSF/ANSI/CAN 60 does not cover boiler use, and the European shell-boiler water standard is EN 12953-10:2003.

What feedwater oxygen does a scavenger have to reach?#

ASME guidance limits feedwater oxygen to 0.040 mg/L (40 ppb) up to 450 psig (31.0 bar) and 0.007 mg/L (7 ppb) above it, measured before the scavenger, so the scavenger handles what the deaerator leaves and drives the residual toward zero. Table 4 compares the ASME consensus values (CRTD-34), industry guidance and IAPWS TGD3-10(2015).

The ASME values apply to industrial watertube, high-duty, drum boilers with superheater or turbine drive and a deaerator. Iron, copper, silica and conductivity limits sit in boiler water limits by pressure.

21 CFR 173.310 listing status for food-contact steam#

21 CFR 173.310 lists 3 of the 8 oxygen scavengers for steam that contacts food, sodium sulfite, tannin and, at zero in steam, hydrazine, plus sodium metabisulfite and cobalt sulfate as a catalyst (eCFR text as of 2026-10-01). Table 5 gives the status of each chemical.

The section was last amended by 88 FR 17710 (2023-03-24).

The amine caps and polymer limits are listed under 21 CFR 173.310 boiler water additives.

Hazard classification and process-safety thresholds#

3 of the 8 oxygen scavengers carry a harmonised EU carcinogen or mutagen classification: hydrazine and MEKO as Carc. 1B (H350), and hydroquinone as Carc. 2 (H351) and Muta. 2 (H341). Table 6 gives the classification basis and US workplace limits of each scavenger (ECHA and EUR-Lex, checked 2026-10-20).

Formulated oxygen scavenger products are classified on their own SDS, provided by the supplier.

Hydrazine is listed under the EPA RMP at 15,000 lb but not under OSHA PSM; the threshold is explained under EPA RMP and OSHA PSM.

Complete List of Oxygen Scavengers: All 8 Chemicals in One Table#

The complete list of oxygen scavengers below gives all 8 chemicals with their CAS number, class, function, pressure range, theoretical ratio and 21 CFR 173.310 status.

Two related sulfites carry the same chemistry but are counted in the dechlorination family; Table 8 compares them.

Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.

Who Supplies Oxygen Scavengers and Boiler Treatment Programs?#

Oxygen scavengers are bought either as part of a boiler treatment program, where the provider sets chemical, residual and test frequency, or as a commodity chemical at a stated strength, such as 35 % hydrazine or 85 % DEHA solution.

A supply request names the form, such as solid carbohydrazide or dry or catalyzed liquid sulfite. Quotes depend on concentration and basis, volume and container (drum, IBC or bulk), freight distance, contract term and any 21 CFR 173.310 listing required. Makers and distributors by region are listed under water treatment chemical suppliers.

A program provider also reports feedwater oxygen and residuals, while a chemical supplier delivers a specified strength.

How to compare boiler program providers is covered under water treatment companies.

Do All Boilers Need a Chemical Oxygen Scavenger?#

No: most boilers need a chemical scavenger after the deaerator, but IAPWS prefers all-volatile treatment without a reducing agent for all-ferrous high-pressure cycles, and oxygenated treatment even doses oxygen, 30 to 50 µg/kg at the economizer inlet of drum units.

The oxygen gas used for oxygenated treatment is covered under oxygen (LOX) in water treatment.

What chemical is used as an oxygen scavenger?#

Sodium sulfite is the most widely used oxygen scavenger in boilers below 1,000 psig (69 bar), and hydrazine, carbohydrazide and DEHA are the usual scavengers at higher pressure (Veolia Water Technologies Handbook, chapter 11).

Can sodium sulfite be used as an oxygen scavenger at any pressure?#

No: sodium sulfite is used below 1,000 psig (69 bar), because at higher pressure it decomposes to SO2 and H2S that corrode the condensate system, and it never goes into attemperation water.

Is sodium bisulfite listed in 21 CFR 173.310?#

No: sodium bisulfite is not listed in 21 CFR 173.310, while sodium sulfite (neutral or alkaline) and sodium metabisulfite are listed (eCFR text as of 2026-10-01).

Which chemicals must never be mixed with oxygen scavengers?#

Oxygen scavengers must never meet oxidizers, such as chlorine or hydrogen peroxide, and sulfite must never meet acids, which release sulfur dioxide gas. Hydrazine also reacts with metallic oxides and acids and can ignite on porous materials, such as earth, wood or cloth. Every incompatible pair is listed under water treatment chemical compatibility.