Technical definition
Molybdate is a measurement of the concentration of dissolved molybdate ions (MoO₄²⁻) present in the heating system water, expressed in milligrams per litre (mg/L).
In closed heating systems, molybdate is commonly associated with corrosion inhibitor formulations. Fernox Protector F1, for example, contains a sodium molybdate compound as part of its inhibitor package, and Fernox uses molybdate measurement as a means of assessing the concentration of Fernox Protectors and other molybdate-based inhibitors within system water.
Molybdate-based inhibitors help protect metallic system components against corrosion as part of a broader inhibitor formulation. Measuring molybdate can therefore provide an indication of whether an appropriate corrosion-protection treatment is present and maintained at a sufficient concentration.
Simplified definition
Molybdate is a chemical commonly found in heating-system corrosion inhibitors.
Testing the amount of molybdate in the water helps determine whether the system contains enough inhibitor to protect boilers, radiators, pipework and other metal components from corrosion.
Molybdate is important because it can act as a useful marker for the concentration of molybdate-based corrosion inhibitor within the heating system.
Fernox specifically uses molybdate as the measurement parameter in its Express Inhibitor Test and Instant System Test. Its Express Inhibitor Test can detect Fernox Protectors and other molybdate-based inhibitors, with Fernox stating that an inhibitor test result of 100 ppm or above represents a sufficiently protected system, while a result below 100 ppm indicates that re-dosing may be required.
A low or unexpected Molybdate result may indicate:
Insufficient corrosion inhibitor within the system.
An inhibitor that has become diluted through repeated topping-up with fresh water.
Loss of treated system water through leaks, draining or maintenance.
A system that has been refilled without being correctly re-dosed with inhibitor.
Incorrect initial dosing of the treatment chemical.
A system that may be using a non-molybdate-based inhibitor, in which case a low molybdate result does not necessarily mean that no inhibitor is present.
A change in the system's treatment regime since a previous water test.
Maintaining sufficient inhibitor is important because untreated or inadequately treated heating-system water can allow internal corrosion to occur. Fernox Protector F1 is specifically formulated to provide long-term protection against both internal corrosion and limescale formation and is suitable for mixed-metal systems, including aluminium.
Insufficient inhibitor protection can contribute to:
Formation of iron oxide and magnetite sludge.
Corrosion of radiators and steel pipework.
Corrosion of aluminium and other metallic components under unsuitable water conditions.
Increased levels of dissolved and suspended corrosion products.
Radiator cold spots and restricted circulation.
Fouling of pumps, valves and heat exchangers.
Reduced system efficiency.
Leakage and premature component failure in more severe cases.
Testing molybdate is particularly valuable because the presence of inhibitor cannot reliably be determined simply by looking at the system water. A laboratory or suitable on-site test provides objective evidence that an appropriate concentration of a molybdate-based inhibitor is present.
Molybdate should therefore be considered alongside parameters such as Iron, Copper, Aluminium, pH, Conductivity and Suspended Solids when assessing whether the system is adequately protected against corrosion.
Molybdate should not normally be assessed in isolation. Its significance depends heavily on the type of inhibitor used within the heating system.
A low molybdate concentration is particularly significant where a molybdate-based treatment such as Fernox Protector is expected to be present. However, some inhibitor formulations use different corrosion-inhibiting chemistries, so a low molybdate result does not automatically prove that the system is completely untreated. Fernox describes its Express Inhibitor Test specifically as a test for Fernox Protectors and other molybdate-based inhibitors.
It is also important to distinguish between Molybdate concentration and overall inhibitor performance. Molybdate can provide an indication of inhibitor concentration, but the overall protection of the heating system also depends on factors including correct dosing, pH, water chemistry, contamination levels and the condition of the system.
Where a molybdate-based Fernox Protector is being used, the result can therefore provide an important indication of whether the system has been correctly dosed and remains adequately treated.
