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Why inorganic zinc silicate is preferred for coating of newly constructed assets whereas zinc rich primer is recommended for in-service ones?

The difference is driven less by corrosion performance and more by surface preparation, application conditions, and long-term practicality. Both inorganic zinc silicate (IOZ) and organic zinc-rich primers provide cathodic protection through sacrificial zinc, but they behave very differently during application and maintenance.
An inorganic zinc silicate coating requires an almost perfect blast-cleaned surface. Such conditions are achievable in fabrication shops where steel is new, blasting is automated, environmental conditions are controlled, coating inspectors monitor every step.
An operating plant rarely offers these conditions.

IOZ coatings are unforgiving.
Problems can occur with low humidity, high even. Then with steel temperature changes, applied coating is thick or think and then curing.
Poor application may lead to mud cracking, pinholes, poor cohesion, insufficient zinc conductivity, topcoat adhesion problems.

Organic zinc-rich primers are much more tolerant of real-world conditions.