Technical definition
Molybdenum is a measurement of the concentration of the element molybdenum (Mo) present in the heating system water, expressed in milligrams per litre (mg/L).
Within closed heating systems, molybdenum is particularly relevant because molybdate compounds are used in certain corrosion-inhibitor formulations. When a molybdate-based inhibitor is added to system water, laboratory elemental analysis can detect the molybdenum associated with that treatment. ICP-AES, for example, is an established analytical technique for measuring molybdenum as an element in water.
Fernox laboratory data demonstrates this treatment relationship. In a published system-cleaning trial, molybdenum increased from essentially zero in the fill water to 183.4 mg/L in the system water following cleaning and treatment with Protector F1. Fernox described the post-treatment water as containing optimum levels of water-treatment species, specifically giving molybdenum as an example.
Molybdenum is therefore primarily useful as a treatment-related indicator, helping establish whether molybdate-containing inhibitor chemistry is present and whether its concentration is consistent with the treatment being used.
Simplified definition
Molybdenum is an element commonly associated with certain heating-system corrosion inhibitors.
Testing the amount present can help determine whether the system contains the expected treatment chemical and whether that treatment may have been diluted, lost or incorrectly dosed.
Molybdenum is important because it can provide a useful indication of the presence and concentration of molybdate-based corrosion protection within the heating system.
Fernox's published laboratory results classify the acceptable Molybdenum concentration as “Depends on treatment”, rather than assigning one universal pass/fail limit. This is because the expected concentration depends on the inhibitor product and dosage present in the system.
A low or unexpected Molybdenum result may indicate:
Insufficient molybdate-based inhibitor within the system.
Dilution of the treatment through repeated topping-up with fresh water.
Loss of treated water through leakage, draining or maintenance.
A system that has been refilled without being correctly re-dosed.
Incorrect initial treatment dosage.
A change in treatment chemistry since the previous water test.
Use of an inhibitor that does not contain molybdate, in which case a low molybdenum result may be entirely expected.
An unusually high Molybdenum result may indicate:
A high concentration of a molybdate-containing treatment product.
Over-dosing relative to the intended treatment level.
Concentration changes resulting from the system's filling or treatment history.
The use of a different molybdate-containing chemical than expected.
Maintaining the correct concentration of corrosion inhibitor is important because inadequate protection can allow corrosion of metallic components to progress, potentially contributing to:
Iron and magnetite sludge formation.
Corrosion of radiators, pipework and heat exchangers.
Increased dissolved and suspended corrosion products.
Radiator cold spots and restricted circulation.
Fouling of pumps, valves and heat exchangers.
Reduced heating-system efficiency.
Leakage and premature component failure.
Molybdenum testing can therefore be particularly valuable when assessing whether the known inhibitor treatment is still present at an appropriate concentration. Fernox's published trial showed a substantial increase in molybdenum after Protector F1 treatment and associated this with optimum treatment-species levels and compliance with Fernox recommendations.
Molybdenum should therefore be considered alongside parameters such as Molybdate, pH, Iron, Copper, Aluminium, Conductivity, Suspended Solids and other inhibitor-related measurements when assessing system protection.
Molybdenum should not normally be assessed in isolation. Its significance depends heavily on the treatment product used within the heating system.
A low Molybdenum concentration does not automatically mean that the system is untreated. Some corrosion inhibitors use alternative chemistries and may contain little or no molybdenum. Conversely, a high Molybdenum result may be entirely expected where a correctly dosed molybdate-based treatment is present. Fernox therefore categorises the recommended Molybdenum level as dependent on treatment.
It is also important to distinguish between Molybdenum and Molybdate:
Molybdenum (Mo) reports the amount of the elemental molybdenum component detected by the laboratory.
Molybdate (MoO₄²⁻) refers to the complete chemical ion containing molybdenum and oxygen.
The two values are therefore related but not numerically interchangeable. Where both fields are reported by the laboratory, their units and analytical calculation should be considered before comparing the results.
The most useful interpretation is obtained by comparing Molybdenum with the known inhibitor product and dosage, previous test results, filling-water result and other corrosion indicators.
