Hydrochem marketing lead, Neil Cox, explains why the passivation process is essential to prolong the operational lifespan of galvanised steel equipment.
Galvanised steel is renowned as a durable metal.
It’s cost-efficiency also makes it one of the materials of choice for manufacturers of evaporative cooling systems.
It is imperative that these galvanised steel components undergo a passivation process during commissioning and that this action applies appropriate chemistry.
Properly executed, passivation creates a robust, protective barrier on the steel’s surface, and this effectively shields it against corrosion and prevents white rust.
Galvanised steel equipment is highly susceptible to corrosion and physical damage.
The passivation process converts a new component’s existing zinc oxide layer to zinc carbonate. Converting zinc oxide to zinc carbonate effectively ‘hardens off’ the material to a more tolerant metal surface.
The chemistry of the water used is critical to the passivation process. Water chemical parameters must remain within defined ranges for the entire duration of the specified passivation period.
Changes in both temperature and total dissolved solids (TDS) in the water will negatively impact the process.
It is important throughout the course of the treatment that the newly galvanised equipment is operated without load as this will impact the temperature and water chemistry.
Equally important are the chemicals introduced during the process as they must be compatible with other equipment sharing the system loop.
A passivation plan is an important part of the passivating process. The plan will call for regular inspections of both the newly galvanised equipment and the system’s ancillary equipment.
A successful passivation process will provide a visual indicator when the original bright galvanised steel changes colour, becoming duller and more grey to the eye.
White Rust
The white, chalky substance that forms on galvanised steel if it is not appropriately passivated is called white rust (zinc hydroxide). This indicates that the zinc oxide surface coating on the metal has failed. The cooling system water comes into contact with bare steel once the galvanised coating has failed.
White rust will form when water that is not within the defined parameters comes into contact with a newly galvanised surface that has not been passivated.
The chemical interaction between the water and zinc oxide surface forms zinc hydroxide or white rust.
Completing a passivation process during commissioning will ensure white rust does not form on the galvanised equipment’s surface.
Components that are in constant contact with system water, such as cooling towers, need to develop a stable protective coating on the galvanised steel.