Passivation of Evaporative Condensers in Ammonia Refrigeration Systems

From Experience Newsletter: August 2026

For evaporative condensers, passivation is the controlled formation of a thin, stable protective layer on galvanized steel surfaces, including the refrigerant coil. Most evaporative condensers used in industrial ammonia refrigeration are constructed from hot-dip galvanized steel, where a zinc coating protects the underlying carbon steel from corrosion. When a condenser is first placed into service, the new zinc surface is highly reactive when first exposed to recirculating water. Passivation intentionally creates a passive zinc carbonate film under controlled conditions that protects the coating and improves long-term corrosion resistance.

The zinc coating is the condenser’s primary defense against corrosion. Without a properly formed passive layer, the zinc coating can develop white rust corrosion that consumes the protective coating. This can:

• Negatively affect heat-transfer performance.
• Expose underlying carbon steel to corrosion.
• Reduce condenser service life.
• Result in a refrigerant leak.
• Increase future maintenance costs.

The timing of passivation is critical but often overlooked. Condenser passivation should be included in the planning of commissioning and startup activities and coordinated with the equipment manufacturer, installation contractor, and water-treatment vendor. Most manufacturers recommend a passivation period of 4 to 12 weeks, depending on water quality, climate, and equipment design. Ideally, it should be completed immediately after installation and before the condenser is subjected to sustained refrigeration load. (Heat load increases water evaporation, which concentrates dissolved solids, raises pH, and makes white rust formation more likely.) Failing to plan sufficient time and resources can lead to insufficient passivation and the development of white rust.

Passivation should be performed in coordination with a qualified water treatment specialist. The goal is to maintain water conditions that encourage the formation of a protective zinc film while preventing aggressive corrosion. Typical guidelines include maintaining a pH between approximately 6.5 and 8.0, controlling alkalinity and hardness, limiting chlorides, and using blowdown to control conductivity and dissolved solids. Continuous water circulation is also required to keep all galvanized surfaces evenly wetted and exposed to the conditioning water. Water chemistry must be monitored frequently during the first several weeks, with adjustments made as necessary to maintain target conditions. Consult with the condenser manufacturer for specific requirements.

Successful passivation is usually confirmed visually. New galvanized steel has a bright, shiny appearance. As passivation progresses, the surface develops a uniform dull gray color, indicating the formation of a stable protective film. Once this passive layer is established, the condenser is much better protected against corrosion and can transition to its normal operating water treatment program.

Passivation requires time, planning, and coordination, but the payoff is significant: By incorporating passivation into the commissioning plan, facility owners can help protect their investment and avoid costly corrosion-related issues down the road.