The Important Role of Oxygen Scavengers in Boiler Systems
In the world of boiler systems, there is one invisible enemy that can have a devastating impact: dissolved oxygen. Even in small amounts, the presence of oxygen in boiler water can cause corrosion that eats away at pipes, drums, and expensive equipment, leading to costly downtime and repairs.
This article will explore why dissolved oxygen is so dangerous at high temperatures, and how a chemical called an oxygen scavenger plays a crucial role in protecting your boiler system investment and reliability.
The Invisible Threat: Why is Dissolved Oxygen so Corrosive at High Temperatures?
Dissolved oxygen (DO) is O₂ molecules that are naturally dissolved in water. Under normal conditions, oxygen causes iron to rust. However, in the high-temperature and high-pressure environment of a boiler, this corrosion reaction occurs very quickly and aggressively.
Here are the reasons why dissolved oxygen is so destructive:
Accelerates Electrochemical Reactions
Corrosion in boilers is essentially an electrochemical reaction in which iron (Fe) acts as an anode and oxidises. Oxygen acts as a powerful cathode. In hot water, oxygen readily accepts electrons from iron, accelerating the oxidation of iron into rust (iron oxide).
The simple reaction is: 2Fe + O₂ + 2H₂O → 2Fe(OH)₂
This ferrous hydroxide compound is then further oxidised to form rust (Fe₂O₃ or Fe₃O₄).
Causes Localised Corrosion (Pitting Corrosion)
This is the most dangerous impact. Corrosion caused by oxygen is often uneven. It attacks specific areas, creating small, deep holes (pitting) in the metal. These holes can act as stress concentration points, which ultimately cause pipe failure or boiler drum leaks even though the surrounding metal thickness is still good. Pitting corrosion is like an ‘enemy within’ that weakens the structure from within.
Impact of Damage Caused:
What is an Oxygen Scavenger and How Does it Work?
Leaks in Pipes and Tubes: Pitting corrosion can penetrate the walls of economiser pipes, boiler pipes, or condensate pipes.
Damage to Boiler Drums: Scale deposits and corrosion attacks can weaken the structure of the drum.
System Contamination: Detached rust is carried by water and settles in other areas, causing blockages and reducing heat transfer efficiency.
High Repair Costs and Downtime: Replacing boiler parts damaged by corrosion is costly and halts plant operations.
Since mechanical deaerators (oxygen removal devices) alone are often not 100% effective, additional chemical defences are required. This is where oxygen scavengers come into play.
Oxygen scavengers are chemicals added to boiler water or feed water with the primary purpose of chemically binding residual dissolved oxygen and converting it into non-corrosive compounds.
Working Principle:
Oxygen scavengers react with dissolved oxygen in a rapid and irreversible chemical reaction. The result of the reaction is a stable compound that no longer has corrosive properties, so that the oxygen is completely ‘neutralised’ before it has a chance to attack the boiler metal.
Conclusion: Small Investment for Big Protection
Dissolved oxygen in boiler systems is a real threat that should not be taken lightly. The damage it causes—from destructive pitting corrosion to equipment failure—is far more costly than the cost of prevention.
The use of Oxygen Scavenger is a crucial and highly cost-effective chemical defence measure. The selection of the appropriate type of oxygen scavenger (sulphite, hydrazine, or organic) must be tailored to:
Boiler operating pressure
Health and safety (H&S) considerations
Overall cost and reaction efficiency
With a comprehensive water treatment programme, which includes mechanical deaeration and controlled oxygen scavenger injection, you can ensure a longer boiler system life, optimal energy efficiency and, most importantly, the operational reliability of your plant.
Consult with us to determine the most appropriate programme and chemicals for your boiler system.