Dissolved oxygen is one of the biggest causes of boiler corrosion. Even a small amount of oxygen in feed water can pit steel, damage pipes, and shorten the life of your boiler. That is why plants use oxygen scavengers.
Two chemicals lead this market: carbohydrazide and hydrazine hydrate. Both remove oxygen well. But they are not the same. This guide compares them side by side, so you can pick the right one for your boiler system.
What Is an Oxygen Scavenger?
An oxygen scavenger is a chemical added to boiler feedwater. It reacts with dissolved oxygen before the oxygen can attack metal surfaces. This protects pipes, drums, and heat exchangers from rust and pitting.
Good oxygen scavengers should:
- React quickly with oxygen
- Leave no harmful residue
- Protect metal surfaces (this is called passivation)
- Be safe to store, handle, and dose
Carbohydrazide and hydrazine hydrate both do this job. The difference is in how they do it.
What Is Hydrazine Hydrate?
Hydrazine hydrate (N₂H₄·H₂O) has been used in boiler water treatment for decades. It reacts with dissolved oxygen to form water and nitrogen gas. This reaction is fast and complete, even at low temperatures.
Strengths of hydrazine hydrate:
- Reacts almost instantly with oxygen
- Low dosing cost per unit of oxygen removed
- Long track record in high-pressure boilers
- Passivates metal surfaces well
Drawbacks of hydrazine hydrate:
- It is a highly toxic compound and can be hazardous to people working around the boiler.
- Classified by many agencies as a probable carcinogen
- Needs strict handling, storage, and ventilation controls
- Facing tighter regulation in the EU, US, and other regions
What Is Carbohydrazide?
Carbohydrazide (CH₆N₄O), also known as carbonic dihydrazide, offers a similar oxygen scavenging capability to hydrazine but with a much-improved safety profile. It was developed as a safer alternative to hydrazine for many boiler water treatment applications.
Strengths of carbohydrazide:
- Not classified as a carcinogen, and poses lower risk to health and the environment.
- Ships as a solid or liquid concentrate, easy to store
- Breaks down into nitrogen, water, and small amounts of ammonia — no harmful byproducts
- Also passivates metal surfaces during operation and layup
- Easier to justify to safety officers and regulators
Drawbacks of carbohydrazide:
- Reacts more slowly than hydrazine at low temperatures, unless catalyzed
- Slightly higher cost per kilogram than hydrazine hydrate
- Needs a higher dosage in some low-temperature systems to match hydrazine’s speed
Carbohydrazide vs Hydrazine Hydrate: Side-by-Side Comparison
| Factor | Carbohydrazide | Hydrazine Hydrate |
|---|---|---|
| Chemical formula | CH6N4O | N2H4·H2O |
| CAS Number | 497-18-7 | 7803-57-8 |
| Reaction speed | Slower at low temperature, fast when catalyzed | Very fast, even at low temperature |
| Toxicity | Low; not classified as carcinogenic | High; probable human carcinogen |
| Handling requirements | Standard PPE | Strict PPE, ventilation, and storage controls |
| Regulatory status | Fewer restrictions | Increasingly restricted in many countries |
| Physical form | Liquid or solid | Liquid (hydrate) |
| Metal passivation | Yes | Yes |
| Typical boiler pressure range | Low to high pressure | Low to high pressure |
| Best fit | Plants prioritizing worker safety and compliance | Plants with strict oxygen limits and existing hydrazine systems |
Which Oxygen Scavenger Reacts Faster?
Hydrazine generally provides faster oxygen removal, especially in low-temperature conditions.
However, modern carbohydrazide formulations often use catalysts to improve reaction performance.
In many industrial boiler applications, catalyzed carbohydrazide can provide effective oxygen control while reducing safety concerns associated with hydrazine.
The actual performance depends on:
- Operating pressure
- Boiler temperature
- Dissolved oxygen concentration
- Feedwater chemistry
- Dosage control
Is Carbohydrazide a Good Hydrazine Replacement?
For many industrial users, yes.
The transition from hydrazine to carbohydrazide is mainly driven by:
- Workplace safety requirements
- Environmental regulations
- Chemical handling concerns
- Corporate sustainability targets
However, replacement decisions should always consider:
- Boiler operating conditions
- Oxygen removal requirements
- Existing chemical dosing systems
- Water chemistry program
A professional water treatment supplier should evaluate the complete system before recommending a replacement.
Cost Comparison: Carbohydrazide vs Hydrazine Hydrate
Hydrazine hydrate usually has a lower purchase price.
However, the total cost of ownership includes more than chemical price.
Factors affecting real operating cost:
Chemical handling
Hydrazine may require:
- Additional safety equipment
- Special storage facilities
- Operator training
Compliance requirements
Some regions require stricter management procedures for hydrazine-containing chemicals.
Operational risk
A safer oxygen scavenger can reduce potential workplace exposure risks.
Therefore, carbohydrazide can become economically attractive when considering the complete lifecycle cost.
Which Oxygen Scavenger Should You Choose?
Here is a simple way to decide:
Choose hydrazine hydrate if:
- Your boiler already runs on hydrazine and switching is not urgent
- You need the fastest possible oxygen removal at very low temperatures
- Your plant has strong existing safety controls in place
Choose carbohydrazide if:
- Worker safety and reduced chemical exposure are priorities
- Your region has tightened rules on hydrazine use
- You want a solid or easy-to-handle liquid product for storage
- You are upgrading your water treatment program
Many plants around the world have already made the switch from hydrazine to carbohydrazide for exactly these reasons. Hydrazine has been largely replaced by carbohydrazide as an oxygen scavenger in several high-pressure boiler systems due to safety and health concerns.
Dosing Tips for Both Chemicals
- Always base dosage on your feed water’s dissolved oxygen level, not on a fixed rule
- Test residual levels regularly to avoid under- or over-dosing
- For carbohydrazide, consider a catalyzed grade if your system runs at lower temperatures
- For hydrazine, monitor exposure limits closely and log usage for compliance records
- Work with your chemical supplier to set the right dosing curve for your boiler pressure and load
Frequently Asked Questions
Is carbohydrazide as effective as hydrazine?
Yes. Carbohydrazide removes oxygen effectively in most boiler systems. At low temperatures, a catalyzed grade closes most of the speed gap with hydrazine.
Can I switch from hydrazine to carbohydrazide without changing equipment?
In most cases, yes. Carbohydrazide can be dosed through the same feed systems used for hydrazine, with adjustments to dose rate.
Is carbohydrazide safe to store long term?
Carbohydrazide is more stable and easier to store than hydrazine, and it does not carry the same carcinogen classification.
Does carbohydrazide leave any residue in the boiler?
It breaks down mainly into nitrogen, water, and small amounts of ammonia. It does not add dissolved solids to the system.
Which chemical is better for high-pressure boilers?
Both work in high-pressure systems. Many desalination and power plants now use carbohydrazide in high-pressure boilers as a safer substitute for hydrazine.
Choose the Right Boiler Oxygen Scavenger with HOO CHEMTEC
Selecting between carbohydrazide and hydrazine hydrate depends on your boiler system, safety requirements, operating conditions, and regulatory environment.
HOO CHEMTEC supplies industrial-grade carbohydrazide and hydrazine hydrate for boiler water treatment applications, providing:
✓ Reliable export supply
✓ Technical data sheets and SDS
✓ Customized packaging solutions
✓ Support for oxygen scavenger selection
Whether you are replacing hydrazine or optimizing your existing boiler treatment program, our technical team can help recommend the right oxygen scavenger solution.
Contact HOO CHEMTEC today for product specifications, quotation, and technical consultation.
