In recent years, as air and water pollution has increased, the quality of the circulated water in evaporative condensers and cooling towers has declined. As regulations limit the use of many corrosion inhibitors for the hot dip galvanizing process, such as chromates, the passivation process for protection has become even more critical.
Passivation is a treatment process that forms a very thin protective layer that reduces chemical activity with air and water or other material that comes into contact with a surface. The passivation process varies depending on the type of material to be protected and the substances with which they contact. The most common circumstance is passivation for coils and casings for evaporative condensers and cooling towers. Passivation provides maximum protection from corrosion on newly installed evaporative condensers and cooling towers that have hot dipped galvanized steel tube coil surfaces. Hot dip galvanizing produces a coating of zinc-iron intermetallic alloy layers on steel with the outer layer being purely zinc.
The zinc provides cathodic (sacrificial) protection and a physical barrier protection. Passivation helps prevent the development of corrosion that could result in rapid penetration through the zinc coating to the steel. If the corrosion makes it entirely through the protective coating to the carbon steel condenser tubes, they will rapidly corrode. Note that fouling material, such as scale or white rust on the tubes, can hide corrosion on the surface of the tubes. Corrosion control of galvanized steel depends on forming and maintaining a stable and passive oxide layer on the surface of the galvanized steel during the installation of the equipment.
Passivation is a chemical process that forms a metal oxide bond that enhances the original corrosion resistant, hot dipped galvanized surface by forming a thin transparent oxide coating. It dissolves any embedded or slightly rubbed-in iron pickup on the surface, and reduces the transport of corrosive elements to the underlying metal surfaces.
It is important that proper precautions are planned and taken for passivation when a new condenser or tower with hot dip galvanized tubes is installed. The passivation must take place prior to startup with a full heat load. The heat load should be avoided since water evaporation can concentrate corrosive ions and increase the pH and fouling potential.
Water quality varies by location, so passivation is best performed on site or at a nearby controlled environment by the designated treatment company—not at the factory. Many contractors will not accept responsibility for passivation because water treatment is an ongoing process that is the responsibility of the owner. It is important to develop a schedule early in the startup process that includes time to involve a qualified water treatment provider to minimize problems and optimize the investment. The passivation process can take weeks or months (typically 6-8 weeks or even more in some locations).
For a quality assurance check on your water treatment service provider, have the company provide references for which equipment they have successfully passivated and have them provide and explain the passivation process used to service your equipment. Hold the company accountable as the expert to provide the passivation process, even if they have to get the process details from the manufacturer. Make sure they have enough insurance in case an unfortunate situation occurs while your equipment is in their hands. Passivation must extend the life of the galvanized protective coating on the tubes and not damage it in any way. If an onsite water treatment provider is currently servicing existing similar equipment at your facility, it is best they provide a proven passivation process for your new equipment.
It is also important to have a good preventive maintenance (PM) program for existing equipment that checks the conditions of the tubes early enough to consider enhanced water treatment, re-passivation or the scheduling of equipment replacement before it becomes a risk for a refrigerant release. Typically, condenser or cooling tower replacement projects need to start at least six months in advance to ensure the involvement of a qualified water treatment company. If the project schedule gets too crunched, the passivation process is at risk of elimination because the new unit will need to be online as quickly as possible. A sign that the passivation process is successful and completed is when the new shiny zinc coating on the tube surfaces turns to a dull gray color.
The target life expectancy of a newly installed condenser should be a minimum of 15 years with a stretch target of up to 25 years. It is noteworthy that some condensers are still in operation after more than 40 years. When originally purchased, these condensers likely had chromate corrosion inhibitors applied as part of the hot dipped galvanizing process and the galvanizing itself was likely thicker. If a new condenser is installed without passivation, the tubes may only last 5-15 years. This quickly identifies passivation as a great investment to optimize your equipment’s life expectancy.