Technical definition
Dissolved Iron is a measurement of the concentration of iron present in the system water in dissolved ionic or soluble chemical forms, expressed in milligrams per litre (mg/L).
Within a closed heating system, dissolved iron is commonly associated with the corrosion of ferrous components, such as steel radiators, mild-steel pipework, boiler components and other iron-containing materials. During corrosion, iron can enter the circulating water initially in dissolved forms before being converted into insoluble corrosion products such as iron oxides and magnetite.
Dissolved Iron therefore provides an important indication of active or recent corrosion processes within the heating system, particularly when considered alongside pH, inhibitor concentration, dissolved oxygen, suspended solids and other corrosion-related parameters.
Simplified definition
Dissolved Iron measures the amount of iron that has dissolved into the heating system water.
Because many heating-system components contain steel or iron, elevated levels can indicate that these components are corroding and releasing iron into the water. This can be an early warning of corrosion before large quantities of sludge or deposits have formed.
Dissolved Iron is important because it can provide direct evidence that ferrous metals within the heating system are being attacked by corrosion.
An abnormal or elevated Dissolved Iron result may indicate:
Active corrosion of steel or iron-containing components.
Insufficient or ineffective corrosion-inhibitor protection.
Unsuitable system-water pH.
Oxygen entering the system through leaks, frequent topping-up or other routes.
Corrosive contaminants present within the system water.
Residual cleaning chemicals or installation contaminants affecting water chemistry.
Changes in system-water conditions following draining, flushing or maintenance.
Corrosion products being chemically dissolved back into the circulating water.
Dissolved iron is particularly useful because corrosion can begin before obvious signs such as black sludge, radiator cold spots or component failure become apparent.
As corrosion progresses, dissolved iron can react further within the system and form insoluble iron oxides, including magnetite. These corrosion products can then become suspended in the water or accumulate as sludge and deposits.
Continued iron corrosion can therefore contribute to:
Formation of magnetite and corrosion sludge.
Radiator cold spots.
Fouling of boiler and heat-pump heat exchangers.
Blockages within narrow waterways.
Increased wear or interference with pumps and valves.
Reduced water circulation.
Reduced heat-transfer efficiency.
Increased system noise.
Leakage from corroded components.
Premature failure of radiators, pipework and other system components.
Dissolved Iron can also help differentiate between active corrosion and historical contamination. A system may contain a significant amount of existing black sludge while showing relatively low dissolved iron if corrosion is no longer occurring rapidly. Conversely, elevated dissolved iron can suggest that iron is actively entering the water, even before large amounts of solid corrosion debris have accumulated.
Dissolved Iron should therefore be considered alongside parameters such as Suspended Solids, pH, Conductivity, Chloride, Molybdate or other inhibitor indicators, Copper and Aluminium when assessing the overall corrosion condition of the heating system.
Dissolved Iron should not normally be assessed in isolation. An elevated result indicates that soluble iron is present, but further information is needed to determine the precise cause and severity of the corrosion.
It is particularly important to distinguish between Dissolved Iron and suspended or total iron:
Dissolved Iron represents iron that is chemically dissolved in the water.
Particulate iron consists of solid corrosion products, such as rust or magnetite, suspended in the water.
Total Iron, where measured, can include both dissolved and particulate forms.
This distinction can provide valuable diagnostic information. High dissolved iron may indicate ongoing corrosion, whereas high particulate iron may indicate an accumulation or mobilisation of existing corrosion products.
The result should therefore be assessed alongside the system materials, treatment history, pH, inhibitor concentration, oxygen exposure and other corrosion indicators to determine whether corrective action is required.
