Everything seems perfect, yet pitting still appears in the economizer and the boiler. What happened?
Oxygen scavengers were designed to complete the action of deaerators, and they are good at it. However, they are not the solution to every oxygen-related problem — they need to operate within their characteristics and constraints. If operation fails to meet the requirements of the scavenger used, the cost is severe.
Beyond scavenging O₂, most scavengers also act as passivating agents, helping to build a protective film on the metal surface. Several products are available — sodium sulfite, DEHA, carbohydrazide, hydrazine and sodium erythorbate, among others. The main ones will be covered individually in Part II.
The choice of scavenger should be based on boiler pressure, boiler type, oxygen content, environmental requirements and steam use. In higher-criticality installations, selection should also consider decomposition products, condensate protection and the contribution to forming and maintaining the magnetite layer.
Deaerator Failure — Depletion and Kinetic Limitation
The first issue to assess is operational failure of the deaerator. When it occurs, the oxygen load increases and two effects happen simultaneously: accelerated depletion of the scavenger residual and kinetic limitation of the reaction process.
Depletion is direct: scavenger consumption is proportional to the amount of O₂ entering the system, following the stoichiometry of the reaction.
Kinetic limitation is a less-discussed concept. Reaction speed depends on combined factors:
- Water temperature
- System pH
- Scavenger concentration
- O₂ concentration
With sudden O₂ spikes, typical of deaerator failures, reaction kinetics cannot keep up enough to scavenge all the O₂ before it reaches metal surfaces such as carbon steel tubing and economizers.
ASME recommends less than 7 ppb O₂ for industrial boilers. The scavenger should provide the final fine adjustment — not compensate for a faulty deaerator. When the deaerator operates with failures, the O₂ load can exceed the stoichiometric and kinetic capacity of the available scavengers, making chemical treatment alone insufficient.
Even with increased scavenger dosing, unscavenged residual O₂ causes:
- Localized corrosion (pitting) concentrated at specific points
- Preferential attack in low-flow zones and condensate return lines
- In high-pressure boilers: real risk of tube failure
When deaerator malfunction persists for weeks or months, the treatment system enters structural collapse. The scavenger program, sized to act as polishing for an efficient deaerator, starts operating under permanent overload.
Scavenger Preparation
The second critical point is incorrect product preparation. Liquid products must be dosed neat. Dry products, such as sodium sulfite, must be prepared to minimize contact with air and never mixed with other products.
In practice, it is common to find scavenger solution tanks prepared several days earlier, often uncovered and, more rarely, without agitation — conditions that compromise product effectiveness even before it enters the system.
pH, Dosing Point and Frequency
pH matters: most scavengers work best above 8.5, but the ideal range depends on the product used. It is essential to check the water pH at the dosing point.
The dosing point must provide fast, adequate mixing, plus enough reaction time to reach completion. Below the deaerator there is a storage tank whose residence time must be sufficient for all the oxygen to react with the scavenger.
The most suitable point is the line leaving the deaerator and entering the storage tank. If that is not possible, the second-best point is in the deaerator vessel itself, just below the water line — feeding from both ends to avoid a concentration gradient.
Dosing should be continuous and variable according to the system load.
Potential Consequences
The deaerator/scavenger set must operate as specified. When there is imbalance or failure, the aftereffects persist even after normal operation is restored:
- Advanced pitting in economizers and feedwater lines, with reduced wall thickness
- Magnetite (Fe₃O₄) and hematite (Fe₂O₃) deposits inside boiler tubes, acting as thermal insulation and raising wall temperature
- Stress corrosion cracking in copper alloys (heat exchangers and condensers) from the combined presence of O₂ and ammonia
- Steam contamination with corrosion products, compromising processes that use steam as a utility
The oxygen scavenger does not fail due to an inherent product problem. The fundamental limitation is that it was sized to act as polishing for an efficient deaerator, removing residual traces of O₂ — not to absorb the load of a malfunctioning piece of equipment.
In medium-pressure (20 to 60 bar) and high-pressure (above 60 bar) industrial boilers, an undiagnosed failure often results in an unplanned shutdown for tube replacement, at a cost that can easily exceed the investment in preventive diagnostics for several years.
If your plant shows recurring corrosion, abnormal scavenger consumption or a history of pitting in economizers, diagnostics start with the deaerator.
Next Steps
This is the tenth article in the Kakama Wera Industrial Water Treatment series, covering water as a heat transport medium, water sources, water treatment plants, reverse osmosis (I and II), demineralization (I and II) and deaeration (I and II).
Part II will cover the main scavengers available, their characteristics and selection criteria.
References
- ASME. Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers.