Water treatment inhibitors are crucial for maintaining the efficiency of heating and chilled water systems. Facility managers and engineers should understand these inhibitors to keep systems running smoothly and sustainably.
What are Inhibitors in Water Treatment?
Inhibitors in water treatment are chemical compounds added to water systems to prevent system corrosion and limescale.
Corrosion Inhibitors
Corrosion inhibitors are special chemicals that help prevent or slow down corrosion in heating and chilled water systems. Corrosion occurs when metal surfaces react with water or other substances, leading to metal deterioration. These inhibitors form a protective layer on the metal surfaces, shielding them from corrosive elements. This extends the system’s lifespan and ensures it continues to function efficiently.
Scale Inhibitors
Scale inhibitors are special chemicals that stop or slow down scale formation when added in small amounts to water, which usually causes scale buildup. Similar to preventing scale in washing machines, these chemicals are used in heating and cooling systems. They work by reacting with substances that cause scale, making them stable or stopping crystal growth. These chemicals are helpful because very low doses can prevent scale for a long time, saving on maintenance and repair costs.
Pros and Cons of Water Treatment Inhibitors
Pros
- They can help to improve the efficiency of water treatment systems.
- They can help reduce the maintenance required for water treatment systems.
- They can help protect the environment by reducing the amount of metal released into the water.
Cons
- They can be expensive.
- They can be harmful to aquatic life.
Types of Water Treatment Inhibitors
Anodic Inhibitors
Anodic inhibitors create a film over anodic surfaces by stopping the reaction that makes metal dissolve. They help form an oxide layer on the metal, which protects against corrosion. This film prevents metal from breaking down in water, keeping heating or chilled water systems running well. Common anodic inhibitors include chromates, nitrates, and molybdates. These inhibitors help extend the life of metal parts and keep the water system efficient. It’s important to check their levels regularly, as wrong amounts can lead to corrosion issues.
Cathodic Inhibitors
The cathodic reduction of oxygen creates hydroxide ions (OH-), which can cause metal surfaces to corrode in water systems. Cathodic inhibitors dissolve in water and become effective at higher pH levels, forming an insoluble compound that doesn’t conduct electricity. This layer protects the metal by stopping further reactions that lead to corrosion. When used correctly, common inhibitors like zinc salts and polyphosphates can greatly slow down corrosion and keep heating and chilled water systems running smoothly. It’s important to monitor pH levels to ensure these inhibitors work effectively.
Combined Anodic-Cathodic Inhibitors
Commercial water treatment formulations usually include both anodic and cathodic inhibitors for two main reasons. First, using both types reduces the total amount needed compared to just one type. Second, relying only on anodic inhibitors can lead to pitting corrosion if the treatment is interrupted, under-dosed, or incorrect. This practice began in the 1950s, especially with zinc-chromate formulations. Zinc acts as a cathodic inhibitor, while chromates work as anodic inhibitors. Previously, using only chromates required large amounts, around 400-600 mg/L, to effectively prevent steel corrosion. However, mixing zinc (about five mg/L of Zn2+) with chromates reduced the chromate concentration to just 20-30 mg/L. This approach improved the inhibitors and reduced the risk of localised corrosion.
Oxygen Absorbers
Anodic and cathodic inhibitors work well when there’s dissolved oxygen from air or water contact. However, their effectiveness drops in high-temperature or sealed systems like boilers or central heating due to reduced oxygen levels. Here, chemicals called “oxygen absorbers” come into play. These agents react with dissolved oxygen to neutralise its corrosive effects. Common ones include sodium sulfite, hydrazine, and ascorbic acid, which quickly form non-corrosive by-products, protecting metal surfaces from damage. Keeping oxygen levels low, these absorbers help maintain high-temperature and sealed water systems, improving their lifespan and efficiency. However, monitoring their use is essential to ensure they remain effective and don’t interfere with water treatment.
Organic Inhibitors
These inhibitors work by forming a thin film between the metal and water. They are often surfactants with both water-attracting and water-repelling parts. The water-attracting end sticks to the metal surface, while the water-repelling end creates a barrier. Film-forming amines are used to prevent corrosion in vapour condensate systems. With 4 to 18 carbon atoms, these amines line up side by side, forming a protective film. The typical amount used ranges from 2 to 20 mg·L–1. For protecting condensate return circuits, these amines are helpful when there’s a lot of CO2 release, making neutralising amines expensive.
Corrosion Inhibitors Uses
- Industrial water treatment to prevent corrosion of pipes, valves, and other equipment in industrial processes.
- Cooling water treatment to prevent heat exchangers and other equipment corrosion in cooling systems.
Popular Water Treatment Inhibitors
Water treatment inhibitors are crucial to keeping water systems running smoothly. These chemicals work in-conjunction with biocides to help reduce problems like microbiological growth, as well as scale and corrosion buildup, helping industrial, cooling, and heating systems work with less maintenance. With different inhibitors, we can adjust our approach for each system’s needs, saving time, cutting costs, and protecting valuable equipment.
Molybdate Inhibitors
Molybdate inhibitors are popular for closed systems because they protect different metals, such as steel, iron, aluminum, and copper. They form a thin protective layer on metal surfaces to stop corrosion. Molybdate ions (MoO₄²⁻) also make stable bonds with metal ions, reducing the risk of corrosion. This protective layer needs dissolved oxygen to form. It’s important to regularly check molybdate levels to keep your system safe.
Nitrite Inhibitors
Nitrite inhibitors are an excellent option for protecting closed systems made of ferrous metals. They may not preserve non-ferrous metals or molybdate inhibitors, but they’re excellent for systems with only ferrous metals. Like molybdate inhibitors, nitrite inhibitors form a protective oxide layer on the metal surface, but they don’t need dissolved oxygen to do so. Nitrite ions (NO₂⁻) target anodic spots on ferrous metals, slowing down the metal’s breakdown and keeping it stable. Before using a nitrite inhibitor, check the metal makeup of your system, as aluminium can react badly and cause damage.
Molybdate-Nitrite Inhibitors
Molybdate-nitrite blended inhibitors stand out for their flexibility and teamwork in fighting corrosion. Molybdate protects many metals, including non-ferrous ones, by forming a protective oxide layer. Meanwhile, nitrite targets ferrous metals, stopping them from breaking down and helping keep systems steady. This combined protection makes molybdate-nitrite inhibitors a reliable pick for systems with both ferrous and non-ferrous metals. To keep them working well, regularly check their levels and watch key system factors like pH, conductivity, and general system performance.
Azole Inhibitors
Azole inhibitors work well in systems with yellow metals like copper and brass. For complete corrosion inhibition, azoles are often combined with other inhibitors like nitrite and molybdate. These azole compounds stick to the metal surface, forming a protective layer that blocks corrosive substances. This layer stops harmful reactions that lead to corrosion. Together, these inhibitors offer strong protection for different metals in the system.
FAQs
What chemicals are used as inhibitors?
Water treatment chemicals used as inhibitors in water treatment include different compounds chosen based on a system’s specific needs and the metals involved. Common inhibitors are:
- Molybdate Inhibitors: Protect ferrous and non-ferrous metals by forming a passive oxide layer.
- Nitrite Inhibitors: Work well for ferrous metals, creating a protective film without dissolved oxygen.
- Molybdate-Nitrite Blended Inhibitors: Offer double protection for both ferrous and non-ferrous metals through their combined actions.
- Azole Inhibitors: Target yellow metals like copper and brass, forming a film to prevent corrosion.
What are scale inhibitors in water treatment?
Scale inhibitors are chemicals that stop scale—hard deposits made of minerals like calcium carbonate and silica—from forming on pipes and equipment in water systems. These deposits block flow and reduce heat transfer, making systems less efficient. Common types of scale inhibitors include:
- Phosphates and Phosphonates: They stop scale by disrupting the growth of scale-forming crystals.
- Polyacrylate and Polymaleate Polymers: Keep minerals suspended in water, preventing scale from forming.
- Chelating Agents: Like EDTA, they bind to metal ions to keep them from turning into scale.
What is the most common corrosion inhibitor?
The primary corrosion inhibitor in water treatment varies based on needs, but nitrite inhibitors are common because they protect metals like iron in many industrial and municipal water systems and what’s more they passivate quickly. Nitrite inhibitors are valued for creating a protective layer on these metals, even without dissolved oxygen, making them a reliable choice for many professionals in the field.
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