Technical definition
Chloride is a measurement of the concentration of dissolved chloride ions (Cl⁻) present in the heating system water, expressed in milligrams per litre (mg/L).
Chloride occurs naturally in mains water and may also be introduced through filling water, contamination, treatment chemicals or external sources. Unlike hardness-forming minerals, chloride does not normally form scale, but it is important because elevated concentrations can increase the corrosiveness of the system water.
Chloride is particularly relevant to metals that rely on a protective oxide layer, such as stainless steel and aluminium, because sufficiently high chloride concentrations can damage this protective layer and increase the risk of localised corrosion.
Simplified definition
Chloride is a dissolved salt naturally found in water.
Small amounts are normal, but high chloride levels can make system water more aggressive towards certain metals and may increase the risk of corrosion, particularly localised pitting corrosion.
Chloride is important because elevated concentrations can significantly influence the corrosion behaviour of metals within a heating system.
An abnormal or elevated chloride result may indicate:
Naturally high chloride levels in the original filling water.
Repeated topping-up with mains water.
Contamination from another water source or substance.
Introduction of chloride-containing chemicals or residues.
Incorrect filling or treatment practices.
Changes in system water chemistry following maintenance or refilling.
A potential increased risk of corrosion to susceptible system materials.
One of the main concerns associated with chloride is pitting corrosion. Rather than causing uniform corrosion across an entire surface, chloride can attack small areas of a metal's protective oxide layer, producing highly localised pits.
This type of corrosion is important because significant penetration of a component can occur while the majority of its surface still appears unaffected.
Elevated chloride levels may contribute to:
Pitting corrosion of susceptible metals.
Damage to stainless-steel components and heat exchangers.
Increased corrosion risk for aluminium and aluminium alloys under unsuitable water conditions.
Localised attack on pipework, fittings and other metallic components.
Increased concentrations of corrosion products within the circulating water.
Leakage or premature component failure in severe cases.
The effect of chloride is influenced by several other factors, including pH, temperature, oxygen concentration, system materials and overall conductivity. Higher temperatures and unsuitable water chemistry can make some materials more vulnerable to chloride-related corrosion.
Chloride testing is therefore useful for establishing whether the water contains an unusually high concentration of potentially aggressive dissolved salts and should be considered alongside pH, conductivity, iron, copper, aluminium and inhibitor concentration when assessing overall corrosion risk.
Chloride should not normally be assessed in isolation. The presence of chloride does not automatically mean that corrosion is occurring, and some chloride is naturally present in many mains-water supplies.
The significance of the result depends on the concentration present, system materials, operating temperature, pH, treatment regime and appliance manufacturer's water-quality requirements.
It is also important to distinguish chloride from chlorine. Chloride (Cl⁻) is a stable dissolved ion commonly found in water, whereas chlorine is an oxidising chemical that may be used during water treatment or disinfection.
A chloride result is therefore most useful when compared with the expected filling-water quality, previous system results and the requirements applicable to the components installed within the heating system.
