Technical definition
Total Zinc is a measurement of the concentration of all zinc present in the heating system water, expressed in milligrams per litre (mg/L). Depending on the laboratory method, this can include both dissolved zinc ions and zinc contained within fine suspended particles or corrosion products.
Zinc can be present in heating systems through brass fittings and valves, galvanised components, zinc-containing alloys and certain treatment chemicals. Brass is particularly relevant because it is primarily an alloy of copper and zinc.
An elevated Total Zinc concentration can therefore provide useful information about the corrosion or deterioration of zinc-containing system components, particularly when considered alongside Copper, Dissolved Copper, pH, Chloride and other corrosion-related parameters.
Simplified definition
Total Zinc measures the amount of zinc present in the heating system water, including both dissolved zinc and zinc contained in small particles.
Higher levels can indicate that brass fittings, valves or other zinc-containing components are corroding or deteriorating. Testing zinc helps identify changes in these materials and the overall condition of the system water.
Total Zinc is important because elevated levels can indicate corrosion of zinc-containing components or the presence of zinc-rich corrosion products within the heating system.
An abnormal or elevated Total Zinc result may indicate:
Corrosion of brass fittings, valves or components.
Deterioration of galvanised or zinc-coated materials, where present.
Unsuitable system-water pH.
Elevated chloride or other aggressive dissolved salts.
Insufficient or ineffective corrosion-inhibitor protection.
Residual installation or cleaning chemicals affecting system-water chemistry.
Existing zinc-containing corrosion products circulating within the system.
Changes to the system following draining, flushing, refilling or maintenance.
Contamination from a zinc-containing chemical or external source.
Zinc testing can be particularly useful when assessing brass components. Brass contains both copper and zinc, and under certain corrosive conditions it can undergo a process known as dezincification, where zinc is preferentially removed from the alloy.
This can leave behind a weakened, porous copper-rich structure and may ultimately result in:
Reduced mechanical strength of brass components.
Deterioration of valves and fittings.
Leakage.
Premature component failure.
Release of both zinc and copper into the circulating water.
An elevated zinc concentration alongside elevated Copper or Dissolved Copper may therefore provide useful evidence that brass or another copper-zinc alloy is being affected.
Total Zinc can also help identify:
Corrosion products already circulating in the system.
Material compatibility problems.
Changes in water chemistry that are affecting system components.
Whether corrective treatment or further investigation may be necessary.
Total Zinc should therefore be considered alongside parameters such as Copper, Dissolved Copper, pH, Chloride, Conductivity, Iron, Aluminium, Suspended Solids and inhibitor concentration when assessing the overall corrosion condition of the system.
Total Zinc should not normally be assessed in isolation. A measurable zinc concentration does not automatically mean that significant corrosion is occurring.
It is particularly important to understand that Total Zinc may include both dissolved and particulate zinc. Where a separate Dissolved Zinc measurement is available, comparing the two can provide additional diagnostic information:
Dissolved Zinc can provide a stronger indication of zinc currently being released into solution.
Total Zinc can include both dissolved zinc and existing zinc-containing corrosion particles or debris.
A high Total Zinc result with relatively low dissolved zinc may therefore indicate the presence of historic or particulate corrosion products, while elevated dissolved zinc may suggest more active metal dissolution.
The significance of the result also depends on whether brass, galvanised or other zinc-containing components are present within the system. It should therefore be interpreted alongside system materials, previous test results, filling-water chemistry and the known treatment regime.
