Technical definition
Silicates are a measurement of the concentration of dissolved silicon-containing compounds, typically present in water as soluble silicate species, expressed in milligrams per litre (mg/L).
Silicates can occur naturally in mains water as a result of contact with minerals and rocks, but they may also be introduced into a heating system through water-treatment chemicals, corrosion inhibitors or certain cleaning products.
Within closed heating systems, silicates can form part of some corrosion-inhibitor formulations, where they can help create a protective layer on metal surfaces. However, if present at excessive concentrations or under unsuitable water-chemistry conditions, silicates can also contribute to deposits or precipitates within the system.
Simplified definition
Silicates are naturally occurring mineral compounds that may also be present in some heating-system treatment chemicals.
At the correct level they can help protect metal surfaces from corrosion, but unusually high levels can indicate over-treatment, contamination or conditions that may lead to deposits forming inside the heating system.
Silicates are important because their presence can provide useful information about both the original filling water and the chemical treatment used within the heating system.
An abnormal or elevated Silicates result may indicate:
Naturally high silicate levels in the original mains or filling water.
The presence of a silicate-containing corrosion inhibitor.
Excessive dosing of a treatment product containing silicates.
Residual cleaning or treatment chemicals remaining within the system.
Contamination from another water or chemical source.
Changes to the system water following refilling, treatment or maintenance.
Conditions that could encourage silicate-containing deposits to form.
Where silicates are deliberately included in an inhibitor formulation, they may help protect metallic surfaces by contributing to the formation of a thin protective film, reducing direct contact between the metal and the circulating water.
However, silicate chemistry is strongly influenced by factors such as pH, temperature, concentration and the presence of other dissolved minerals. Under unsuitable conditions, silicates can become less soluble and form deposits.
Excessive or unstable silicate concentrations may therefore contribute to:
Deposits on heat-transfer surfaces.
Fouling of heat exchangers.
Restricted flow through narrow waterways.
Deposits within valves and other small components.
Reduced heat-transfer efficiency.
Unwanted interaction with other water-treatment chemicals.
Cloudiness or particulate material within the system water where precipitation occurs.
Silicate testing can also be diagnostically useful when the treatment history is uncertain. A significantly higher silicate concentration in the system water than in the original filling water may indicate that a silicate-containing chemical product has been added.
Silicates should therefore be considered alongside parameters such as pH, Conductivity, Suspended Solids, Sodium, Molybdate and other treatment-related indicators when evaluating the condition and treatment history of the heating system water.
Silicates should not normally be assessed in isolation. Their significance depends heavily on the source of the silicate and the treatment chemistry used within the system.
A measurable silicate concentration is not automatically a problem. Some silicate may naturally be present in the incoming water, while certain corrosion-inhibitor products may deliberately introduce silicate as part of their protective chemistry.
Likewise, a high silicate concentration does not necessarily mean that deposits are currently forming. The likelihood of precipitation or deposition depends on several interacting factors, including pH, temperature, overall concentration, water hardness and other treatment chemicals.
It is therefore particularly useful to compare the system-water result against the original filling-water silicate level, previous laboratory results and the known inhibitor or treatment product used.
