A Plant Manager's Guide to Sodium Bisulfite for Boiler Corrosion Control
Table of Contents
What you will learn
Dissolved oxygen is the invisible saboteur of boiler room operations, causing catastrophic pitting corrosion that leads to expensive, unplanned downtime. This definitive guide for plant managers and facility engineers breaks down the science and strategy behind using sodium bisulfite for corrosion control. Framed by a real-world case study from a prestigious university power plant, we do a deep dive into the chemistry of oxygen scavengers, provide a practical playbook with dosage calculations and troubleshooting tips, and cover the critical logistics that ensure a successful chemical program. This is the ultimate resource for protecting your critical assets.
📋 What You'll Learn
This guide walks you through a plant manager's guide to sodium bisulfite for boiler corrosion control with detailed instructions.
A technical deep dive into the chemistry, application, and logistics of using sodium bisulfite as an oxygen scavenger to protect critical power plant and facility assets.
Key Takeaways for Plant Managers & Facility Engineers
Sodium Bisulfite is an industry-standard oxygen scavenger used to prevent pitting corrosion in steam and heating boilers. It works by chemically reacting with and removing dissolved oxygen from feedwater. Proper application requires calculating dosage based on oxygen levels and maintaining a sulfite residual of 30-60 ppm. This guide covers the chemistry, a practical dosage playbook, and the logistical considerations for safe and effective implementation.
The Ghost in the Boiler Room
A boiler chemical program depends on the whole operation, from the molecule to the delivery. The right sodium bisulfite does no good if a 275-gallon tote cannot come off the truck, so the receiving questions (dock or no dock, forklift, liftgate) belong in the plan next to the dose calculation.
Industrial facilities, hospitals, and commercial buildings rely on sodium bisulfite for one critical reason: to fight the ghost in their boiler room—dissolved oxygen. This invisible saboteur is the primary cause of pitting corrosion that can destroy expensive boiler tubes and cripple a facility. This technical guide will break down the chemistry of how sodium bisulfite works, why it's the scavenger of choice for many facilities, and provide a practical playbook for its application.
The Invisible Threat: Understanding Dissolved Oxygen Corrosion
General corrosion slowly and evenly thins metal over time, but oxygen corrosion is far more insidious. When heated in a boiler, dissolved oxygen (O₂) becomes highly aggressive and initiates an electrochemical process that attacks the steel in a very localized way. This creates small, deep pits in the metal surface, often hidden under a layer of iron oxide.
This pitting corrosion is the leading cause of premature boiler tube failure. Because it concentrates its damage in small areas, it can perforate a tube wall long before any significant metal loss is detected, leading to forced outages, expensive emergency repairs, and catastrophic downtime. The only way to prevent it is to remove the dissolved oxygen *before* it enters the boiler.
Modern power facilities, like those operated by NAES, rely on precise chemical programs to ensure reliability and protect multi-million dollar assets.
The Chemistry of Protection: How Sodium Bisulfite Works
The Deoxygenation Reaction
Sodium Bisulfite (NaHSO₃) is a powerful reducing agent. When injected into the feedwater, it seeks out and rapidly reacts with dissolved oxygen (O₂) to form Sodium Bisulfate (NaHSO₄), a harmless and stable salt that remains dissolved in the boiler water and is easily removed via blowdown.
2 NaHSO₃ + O₂ → 2 NaHSO₄
Catalyzed vs. Uncatalyzed Sulfite: A Critical Distinction
While the above reaction is effective, its speed is temperature-dependent. In colder parts of a system, like makeup water tanks or long condensate return lines, the reaction can be too slow to provide complete protection. To solve this, the industry uses catalyzed sodium bisulfite. A trace amount of a catalyst, typically a cobalt salt, is added to the solution. This catalyst dramatically accelerates the reaction rate, ensuring instantaneous oxygen removal even at ambient temperatures. For systems with a deaerator, uncatalyzed sulfite is often sufficient, but for systems without one, a catalyzed product is essential for complete protection.
Why Sodium Bisulfite? A Comparison of Oxygen Scavengers
While several chemicals can scavenge oxygen, sodium bisulfite remains an industry workhorse because it offers the best balance of performance, safety, and cost for the vast majority of boiler systems.
- vs. Hydrazine: Decades ago, Hydrazine was common. It is an extremely effective scavenger, but it is now heavily regulated as a highly toxic and suspected carcinogen, posing significant health and safety risks to operators. Most facilities have wisely engineered this chemical out of their plants.
- vs. DEHA: Diethylhydroxylamine (DEHA) is an excellent scavenger that offers the additional benefit of being volatile, meaning it can travel with the steam to protect condensate lines from oxygen attack. However, DEHA is significantly more expensive than sulfite-based programs and can break down into ammonia, which is highly corrosive to any copper or copper alloys (like brass) in the system.
For most low and medium-pressure boiler systems where the primary concern is protecting the boiler and feedwater circuit, a sodium bisulfite program provides the most reliable and cost-effective solution.
On hydrazine, NIOSH lists it as a potential occupational carcinogen with a ceiling REL of 0.03 ppm over 2 hours (NIOSH Pocket Guide).
Can sodium sulfite be used as an oxygen scavenger? Sulfite vs. bisulfite vs. metabisulfite
Yes. Sodium sulfite, sodium bisulfite and sodium metabisulfite all deliver sulfite, react with dissolved oxygen, and end as sulfate. They differ in feed rate and pH: per ppm of oxygen you need 6.50 ppm of sodium bisulfite, 5.94 ppm of sodium metabisulfite or 7.88 ppm of sodium sulfite, and sodium sulfite solutions are alkaline while bisulfite and metabisulfite solutions are acidic.
| Scavenger | CAS | Reaction with oxygen | ppm per 1 ppm O₂ (molar masses from PubChem) | pH, 10% solution |
|---|---|---|---|---|
| Sodium bisulfite, NaHSO₃ | 7631-90-5 | 2 NaHSO₃ + O₂ → 2 NaHSO₄ | 2 × 104.06 ÷ 31.999 = 6.50 | 2.5 to 5.5 |
| Sodium metabisulfite, Na₂S₂O₅ | 7681-57-4 | Na₂S₂O₅ + O₂ + H₂O → 2 NaHSO₄ | 190.11 ÷ 31.999 = 5.94 | 4.0 to 5.5 |
| Sodium sulfite, Na₂SO₃ | 7757-83-7 | 2 Na₂SO₃ + O₂ → 2 Na₂SO₄ | 2 × 126.05 ÷ 31.999 = 7.88 | 8.5 to 11.5 |
Ratios are for the pure compound. Divide by the product strength for a solution: 6.50 ÷ 0.40 = 16.3 ppm of a 40% sodium bisulfite solution per ppm of oxygen.
Boiler systems are the heart of many industrial and institutional facilities, requiring a robust chemical treatment program to prevent corrosion and ensure uptime.
The Operational Playbook: Dosage, Testing, and Control
Implementing a successful sodium bisulfite program is a matter of precision and consistency. Here is a practical, step-by-step guide for your facility.
Step 1: Calculate the Theoretical Dosage
The chemistry provides our starting point. Theoretically, it takes 6.5 ppm of sodium bisulfite (100% basis) to remove 1 ppm of dissolved oxygen: two moles of NaHSO₃ (2 × 104.06 g/mol) per mole of O₂ (31.999 g/mol) is 6.50. The figure of about 8 that circulates for sulfite scavengers is the sodium sulfite ratio, 7.88. Your first step is to test the dissolved oxygen level (in ppm) of your boiler feedwater after the deaerator (if present). Your feedwater dose is this oxygen demand plus whatever sulfite leaves the boiler in blowdown. That is the steady-state rate; at startup, or after the residual has dropped, feed extra until the boiler test reaches the target, then trim back.
Why the boiler residual is not added straight to the feed dose
The sulfite residual is measured in the boiler water, where non-volatile chemicals concentrate as steam leaves. At steady state, the only sulfite that leaves the boiler unreacted is what goes out with the blowdown, so the feedwater has to supply the residual times the blowdown fraction, not the full residual. DOE puts typical blowdown at 4% to 8% of feedwater flow, up to 10% with high-solids makeup (DOE Steam Tip Sheet #9). Residuals are reported as sulfite (SO₃), so convert with 104.06 ÷ 80.07 = 1.30 ppm of NaHSO₃ per ppm of SO₃.
Example Calculation:
Feedwater dissolved oxygen 0.5 ppm, blowdown 8% of feedwater flow, target boiler-water residual 40 ppm as SO₃:
Oxygen demand: 0.5 ppm O₂ × 6.50 = 3.25 ppm NaHSO₃
Residual carried out in blowdown: 40 ppm SO₃ × 0.08 = 3.2 ppm SO₃ × 1.30 = 4.16 ppm NaHSO₃
Total feedwater dose = 3.25 + 4.16 = 7.4 ppm NaHSO₃, or 7.4 ÷ 0.40 = 18.5 ppm of Sodium Bisulfite 40% solution
Then trim the pump to the measured residual; feedwater oxygen after a working deaerator is usually far below 0.5 ppm, so the blowdown term often dominates.
Step 2: Maintain a Target Residual
A common target is a sulfite (SO₃²⁻) residual of 30-60 parts per million (ppm) in the boiler water; confirm the range for your boiler pressure with your boiler maker or water-treatment program. This residual is your safety buffer; it confirms that all oxygen has been removed and there is an excess of scavenger ready to handle any system upsets. This should be tested daily with a simple sulfite drop test kit.
Andre's Pro Tip: The Dangers of Over- and Under-feeding
"Chasing a zero sulfite reading is a recipe for disaster—it means you have no protection. But over-feeding is also a problem. Excess sulfite breaks down into acidic gases in the steam, which can cause severe condensate line corrosion. It also adds to the boiler's total dissolved solids (TDS), forcing you to blow down more water, which wastes energy and chemicals. The goal is to stay consistently within that 30-60 ppm sweet spot. It's a balance of protection and efficiency."
Step 3: Troubleshooting Common Issues
- Unstable Sulfite Residuals: If your readings are swinging wildly, it's a sign of a problem. Check for leaks in your system (especially on the suction side of pumps), ensure your chemical metering pump is functioning correctly, and verify that your deaerator is operating at the proper temperature and pressure.
- High TDS / Conductivity: If your total dissolved solids are climbing, you may be over-feeding the sulfite. The first step is to verify your dosage calculation and recalibrate your chemical pump. Then, adjust your boiler blowdown rate to bring TDS back into the recommended range.
What else is sodium bisulfite used for?
Outside boilers, sodium bisulfite is mostly used as a reducing agent. Its main water job is dechlorination, removing residual chlorine ahead of RO membranes or before discharge (see our engineer's guide to sodium bisulfite for dechlorination). PubChem's use entries (HSDB and Haz-Map) list it as an antichlor and reducing agent in textiles, for bleaching groundwood pulp and wool, in vat dye preparation, as an additive in copper and brass plating, in photographic fixing baths and in the manufacture of other chemicals (PubChem CID 23665763). It is also a regulated food additive (INS 222); that use calls for a food-specification product, not the technical-grade solutions sold for water treatment.
Other names for the same compound: sodium hydrogen sulfite, sodium bisulphite, sodium acid sulfite, monosodium sulfite and SBS.
Is sodium bisulfite the same as baking soda or sodium bisulfate?
No to both. Baking soda is sodium bicarbonate, a mild base; sodium bisulfate is an acid salt sold to lower pH. Baking soda contains no sulfur, and sodium bisulfate's sulfur is already at +6, so neither can scavenge oxygen or remove chlorine.
| Compound | Formula | CAS | Solution pH | Scavenges oxygen? |
|---|---|---|---|---|
| Sodium bisulfite | NaHSO₃ | 7631-90-5 | 2.5 to 5.5 (10%) | Yes |
| Sodium bisulfate | NaHSO₄ | 7681-38-1 | Strongly acidic | No |
| Sodium bicarbonate (baking soda) | NaHCO₃ | 144-55-8 | 8.3 (fresh 0.1 M, 25 °C) | No |
What are the hazards of sodium bisulfite?
Sodium bisulfite is harmful if swallowed (GHS H302) and irritates the eyes, skin and mucous membranes. NIOSH sets a recommended exposure limit of 5 mg/m³ as a time-weighted average, and OSHA has no PEL (NIOSH Pocket Guide). Keep it away from acids: on contact with mineral acids the solution releases toxic sulfur dioxide. Bisulfite solutions ship as UN2693, Class 8, Packing Group III (49 CFR 172.101). Read the SDS before the first delivery; it governs PPE and spill cleanup.
Beyond the Drum: Sourcing a Reliable Chemical Partner
The Importance of a Certificate of Analysis (CoA)
Consistent product purity is critical. Contaminants in a low-quality batch of sodium bisulfite can introduce other problems into your boiler. Need the Certificate of Analysis? Just ask, we send it at no charge.
Logistics That Solve the "Last 50 Feet"
We understand that a successful delivery is about more than just getting a truck to your address. Tell us whether the site has a dock or forklift; if not, we arrange liftgate service before the truck leaves. This proactive approach prevents delivery failures and ensures your team can receive material safely and efficiently.
Packaging for Your Scale
We supply Sodium Bisulfite 40% in a range of sizes, from 5-gallon pails for smaller systems to 55-gallon drums and 275-gallon totes for large industrial and institutional power plants.
The Proven Solution for Boiler Corrosion Control
Protecting your boiler system is about more than just chemicals; it's about a reliable strategy supported by a knowledgeable partner. From guaranteed product purity to logistics that meet your site's specific needs, Alliance Chemical is your partner in protecting your critical assets.
Contact our team today for a quote, to request a sample, or to discuss your boiler water treatment program with one of our technical experts.
Products in this guide: Sodium Bisulfite 25% Solution – Technical Grade (NaHSO3) · Sodium Bisulfite 40% ACS Grade
References & Authoritative Sources
Primary sources consulted for this article. Regulatory and physical-property figures above are drawn from these references.
- Sodium bisulfite (CAS 7631-90-5) — compound summary — PubChem, National Center for Biotechnology Information. Accessed 2026-09-02.
- Sodium sulfite (CAS 7757-83-7) — compound summary — PubChem, National Center for Biotechnology Information. Accessed 2026-09-02.
- Sulfur dioxide (CAS 7446-09-5) — compound summary — PubChem, National Center for Biotechnology Information. Accessed 2026-09-02.
- Energy Tips: Steam Tip Sheet #9, Minimize Boiler Blowdown, U.S. Department of Energy, Advanced Manufacturing Office. Accessed 2026-09-28.
- NIOSH Pocket Guide to Chemical Hazards: Sodium bisulfite, National Institute for Occupational Safety and Health. Accessed 2026-09-28.
- NIOSH Pocket Guide to Chemical Hazards: Hydrazine, National Institute for Occupational Safety and Health. Accessed 2026-09-28.
- Sodium metabisulfite (CAS 7681-57-4), compound summary, PubChem, National Center for Biotechnology Information. Accessed 2026-09-28.
- Oxygen (O2), compound summary, PubChem, National Center for Biotechnology Information. Accessed 2026-09-28.
- Sulfite ion (SO3 2-), compound summary, PubChem, National Center for Biotechnology Information. Accessed 2026-09-28.
- Sodium bisulfate (CAS 7681-38-1), compound summary, PubChem, National Center for Biotechnology Information. Accessed 2026-09-28.
- Sodium bicarbonate (CAS 144-55-8), compound summary, PubChem, National Center for Biotechnology Information. Accessed 2026-09-28.
- 49 CFR 172.101, Hazardous Materials Table (entry UN2693), U.S. Department of Transportation, via eCFR. Accessed 2026-09-28.
Key numbers and sources
| Fact | Value | Source |
|---|---|---|
| Formula and molar mass of sodium bisulfite | NaHSO₃, 104.06 g/mol | pubchem.ncbi.nlm.nih.gov |
| CAS number of sodium bisulfite | 7631-90-5 | pubchem.ncbi.nlm.nih.gov |
| Sodium bisulfite per 1 ppm dissolved oxygen | 6.50 ppm (2 × 104.06 ÷ 31.999) | pubchem.ncbi.nlm.nih.gov |
| Sodium sulfite per 1 ppm dissolved oxygen | 7.88 ppm (2 × 126.05 ÷ 31.999) | pubchem.ncbi.nlm.nih.gov |
| NaHSO₃ per 1 ppm sulfite residual (as SO₃) | 1.30 ppm (104.06 ÷ 80.07) | pubchem.ncbi.nlm.nih.gov |
| pH of a 10% sodium bisulfite solution | 2.5 to 5.5 | pubchem.ncbi.nlm.nih.gov |
| Typical boiler blowdown rate | 4% to 8% of feedwater flow (up to 10%) | energy.gov |
| NIOSH recommended exposure limit, sodium bisulfite | TWA 5 mg/m³ (OSHA PEL: none) | cdc.gov |
Frequently Asked Questions
How does sodium bisulfite protect boilers from oxygen corrosion?
Sodium bisulfite (NaHSO3) is an oxygen scavenger that reacts with dissolved oxygen in boiler feedwater: 2 NaHSO3 + O2 -> 2 NaHSO4. Removing dissolved oxygen prevents pitting corrosion on boiler tubes, drums and economizers. Stoichiometrically it takes 6.5 ppm of sodium bisulfite to remove 1 ppm of oxygen, and plants hold an excess sulfite residual in the boiler water to absorb upsets.
What is the correct dosing rate for sodium bisulfite in boiler systems?
Feed 6.5 ppm of sodium bisulfite per ppm of feedwater oxygen, plus enough to replace the sulfite lost in blowdown: boiler residual (as SO3) x blowdown fraction x 1.30. Example: 0.5 ppm O2, 8% blowdown, 40 ppm residual gives 3.25 + 4.16 = 7.4 ppm NaHSO3, or 18.5 ppm of a 40% solution. A common target is 30-60 ppm SO3; confirm it with your water-treatment program, test daily, and feed extra at startup until the residual is built.
Can sodium sulfite be used as an oxygen scavenger?
Yes. Sodium sulfite (Na2SO3, CAS 7757-83-7) scavenges oxygen by the same sulfite chemistry: 2 Na2SO3 + O2 -> 2 Na2SO4. It takes 7.88 ppm per ppm of oxygen, against 6.50 for sodium bisulfite and 5.94 for sodium metabisulfite. Its solutions are alkaline (pH 8.5 to 11.5 at 10%), while bisulfite solutions are acidic (pH 2.5 to 5.5).
Can sodium bisulfite be used in high-pressure boilers?
Sodium bisulfite is suitable for boilers up to 600-900 psi. Above 900 psi, sulfite decomposes to sulfur dioxide (SO2) and hydrogen sulfide (H2S), which cause acid corrosion. High-pressure boilers (>900 psi) use hydrazine, carbohydrazide, or DEHA (diethylhydroxylamine) as oxygen scavengers instead. Check boiler manufacturer specifications.
What are the alternatives to sodium bisulfite for boiler oxygen scavenging?
Within the sulfite family, sodium sulfite (7.88 ppm per ppm O2, alkaline) and sodium metabisulfite (5.94 ppm per ppm O2, acidic). Outside it, DEHA is volatile and travels with steam to protect condensate lines but costs more and can break down to ammonia, and hydrazine is effective but NIOSH lists it as a potential occupational carcinogen. Sodium bisulfite remains a cost-effective choice for low and medium-pressure boilers.
Why is my sulfite feed rate so much lower than the boiler residual?
Because sulfite concentrates in the boiler. It is non-volatile, so it leaves only with blowdown, and boiler water runs at 1 / blowdown fraction cycles (12.5 cycles at 8% blowdown). At steady state the feedwater only has to supply the residual times the blowdown fraction, plus 6.5 ppm of sodium bisulfite per ppm of oxygen. Residuals are reported as SO3; 1 ppm SO3 equals 1.30 ppm NaHSO3.
What is sodium bisulfite used for?
Mostly as a reducing agent. In water treatment it removes residual chlorine and scavenges dissolved oxygen in boiler feedwater. Industrially it is used as an antichlor and reducing agent in textiles, for bleaching groundwood pulp and wool, in vat dye preparation, as an additive in copper and brass plating, in photographic fixing baths and to make other chemicals. It is also a regulated food additive (INS 222).
Is sodium bisulfite the same as baking soda?
No. Baking soda is sodium bicarbonate, NaHCO3 (CAS 144-55-8), a mild base whose fresh 0.1 M solution has a pH of about 8.3. Sodium bisulfite is NaHSO3 (CAS 7631-90-5), an acidic reducing agent (pH 2.5 to 5.5 at 10%). Sodium bisulfate, NaHSO4, is a third, different compound: an acid salt that lowers pH but cannot scavenge oxygen.
What is another name for sodium bisulfite?
Sodium hydrogen sulfite is the systematic name. It is also listed as sodium bisulphite, sodium acid sulfite, monosodium sulfite and SBS, CAS 7631-90-5, formula NaHSO3. It is not sodium metabisulfite (Na2S2O5), although metabisulfite dissolves in water to form bisulfite, and it is not sodium bisulfate (NaHSO4).
What are the hazards of sodium bisulfite?
Sodium bisulfite is harmful if swallowed (GHS H302) and irritates the eyes, skin and mucous membranes. NIOSH sets a recommended exposure limit of 5 mg/m3 as a time-weighted average; OSHA has no PEL. On contact with mineral acids the solution releases toxic sulfur dioxide. Bisulfite solutions ship as UN2693, Class 8, Packing Group III. Read the SDS before handling.
Related Chemical Collections
About the Author
