Closed System Inhibitors

Water treatment for Closed Systems

Closed systems cover chilled water systems, condenser circuits, Low/Medium/High Temperature Heated Water (LTHW/MTHW/HTHW) systems and heat pump systems. The problems encountered in these systems are similar to those found in open recirculating systems, such as cooling waters/towers. The main issues being corrosion, scale, and microbiological fouling, which can lead to microbiologically induced corrosion (MIC). Due to their generally lower makeup and greater retention times, treatment levels can be run significantly higher with a much superior level of corrosion inhibition and biological control. In a closed system, a thorough appreciation of the metallurgy and system water chemistry are crucial in selecting the correct treatment package. Careful choosing of the treatments and subsequent management of the treatments are fundamental in maintaining efficiencies and prolonging the system life.

We recommend referring to the following BSRIA guides – BG29 and BG50. You may also want to check out BS 8552:2012

Molybdate Inhibitors

Molybdate inhibitors are one of the most popular options when it comes to closed system inhibitors due to their ability to protect a broad range of materials, including ferrous metals (steel, iron) and non-ferrous metals (aluminium, copper). They work by promoting the formation of a passive oxide layer on the metal surfaces, which acts as a protective barrier against corrosion. Furthermore, the anodic molybdate ions (MoO₄²⁻) form stable complexes with metal ions on the metal surface, further reducing the system’s susceptibility to corrosion. It is important to note that molybdate inhibitors require the presence of dissolved oxygen to form the passivating layer. It is crucial to monitor the concentration of molybdate inhibitor in the system regularly, ensuring an adequate reserve to protect your system.

Nitrite Inhibitors

Nitrite inhibitors are another great option as a closed system inhibitor. Although they are not as effective at protecting non-ferrous metals, compared to molybdate inhibitors, they are perfect for systems consisting of ferrous metals only. Like with molybdate inhibitors, they also form a passive oxide layer to protect the metal, but this does not require dissolved oxygen. However, their anodic inhibition mechanism adds to the protection of ferrous metals as the nitrite ions (NO₂⁻) preferentially react with the anodic sites on ferrous metals, further stabilising the metal surface by inhibiting the anodic dissolution of the metal. Be sure to check the metallurgy of your system before adding a nitrite inhibitor as a system containing aluminium will be compromised by the nitrite.

Molybdate-Nitrite Inhibitors

As a blended product, molybdate-nitrite inhibitors are arguably the best on the market due to their versatility and synergistic benefits. A dual protection mechanism where the molybdate component protects the wider range of metals, especially the non-ferrous, in the system, and the nitrite component can focus on protecting the ferrous metals, resulting in effective corrosion control. As with any inhibitor, regular monitoring of the inhibitor concentration, system parameters (e.g., pH, conductivity), and system performance is essential for maintaining the effectiveness of molybdate-nitrite blended inhibitors.

Azole Inhibitors

Azoles are primarily used in systems that contain yellow metals such as copper and brass. To provide enhanced inhibition in a system, azoles are typically used alongside other corrosion inhibitors such as nitrite and molybdate, ensuring all metals in the system are protected. Azole inhibitors adsorb onto the metal surface to form a protective film, which acts as a barrier to prevent corrosive agents from reaching the metal. This film inhibits the electrochemical reactions that lead to corrosion, such as oxidation and reduction processes.