In the atmosphere of Earth, oxygen makes up around 21% of the air. It is presents in the gaseous phase; however, it can dissolve in water like in rivers, lakes, and seawater.
Dissolved oxygen concentration in water is influenced by many factors:
- The flow rate of water or stagnancy.
- The turbulence of the flow.
- The presence of biological activities.
- Water temperature.
- Depth of water.
For example, the dissolved oxygen level decreases when the temperature of water increases. In addition, the oxygen concentration is lower at deeper water levels.
Dissolved oxygen in water is one of the factors contributing to the corrosion cell of a metal.
In electrochemical reactions, the metal surface is exposed to the active oxygen and thus it will be oxidized and dissolved in water.
The reaction is a redox reaction, where the metal is oxidized at the anode and released into the water. The electrons from oxidation are then released and then absorbed by oxygen at the cathode.
Anode: Fe° → Fe+2 + 2e–
Cathode: ½ O2 + H2O + 2e– → 2OH–
Electrochemical Reaction: Fe° + ½ O2 + H2O → Fe(OH)2
As this reaction continues, due to the presence of free oxygen, a corrosion cell is formed which
eventually forms the classic oxygen pit, while oxygen provides a major pitting challenge on its own, additional forms of corrosion are accelerated by its presence.
To stop this reaction, dissolved oxygen can be reduced mechanically, chemically, or through a combination of the two methods. Other popular methods of protecting the metal are passivation and filming inhibitors.
An oxygen scavenger is a chemical compound used to decrease or completely remove oxygen in fluids to eliminate corrosion related to the presence of dissolved oxygen. Many terms are also used to refer to oxygen scavengers’ function, such as oxygen absorber, antioxidant, and interceptor.
The purpose of an oxygen scavenger is not limited to being used in water treatment applications, it can also be used in oil and gas production, pharmaceuticals, metals, and food industries:
- To prevent oxygen-induced corrosion, it can be used as a corrosion inhibitor in applications like oil and gas production installations and seawater systems, thus extending their service life.
- For pharmaceutical products and many foods, deteriorative reactions due to oxygen include lipid oxidation, nutritional loss, changes in flavor and aroma, alteration of texture, and microbial spoilage.
Oxygen scavengers can be classified based on chemical structure into organic and inorganic scavengers. The main criterions used while a section of suitable oxygen scavengers is listed below:
- The level of oxygen needs to be removed.
- The cost of the treatment.
- Characteristics and field conditions (pressure and temperature).
- Ability to passivate the metallic surface.
- The carcinogenic risk or special handling.
- Compatibility with other chemicals used.
- Not affecting the pH or lowering it to corrosive levels (pH<8)
Types of Oxygen Scavengers:
In the water treatment field, the most common use for oxygen scavengers is in boiler systems and dichlorination of reverse osmosis field, the oxygen scavengers are considered as a secondary treatment of defense against oxygen corrosion in boilers, while primary protection requires adequate facilities for mechanical deaeration of the feed water.
Sulfite and hydrazine were the main materials used for years to protect boiler systems from oxygen attacks. However, sulfite increase the boiler water solids making it undesirable in high-pressure boilers, on the other hand, due to the carcinogenic properties of hydrazine it became restricted to be used in many countries. Other scavengers were introduced to the market to replace hydrazine in high-pressure boilers.
Nowadays it is more common to use an oxygen scavenger that can do more than react and remove dissolved oxygen, especially when treating high-pressure boilers, scavengers that can also form protective oxide films on the metal surfaces of the steam/water system are preferred.
Generally, oxygen scavengers are classified into inorganic and organic types:
Inorganic scavengers are mainly based on sulfite products such as sodium sulfite, and sodium bisulfite, this type of scavenger is suitable for low-pressure boilers and should be fed continuously based on boiler feed water, these products can be mixed with a catalyst to accelerate the reaction between the dissolved oxygen and the scavenger when needed.
Organic scavengers such as carbohydrazide and diethylhydroxylamineare mainly organic compounds that have the ability to react with oxygen and do not contribute to the boiler solids, which makes it suitable for high-pressure boilers, it is dosed at much lower levels compared to sulfite type. Some of these scavengers like DEHA have the ability to passivate the metal surfaces in the boiler, then pass out of the boiler with the steam, and
act as a metal passivating agent in the return line system.
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