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Carbonic Acid Corrosion

Carbonic Acid Corrosion

Carbonic Acid Corrosion

We have a project that will take ethane from a Deethanizer overhead and blends it into a propane product rundown piping circuit. The ethane could contain up to 40,000 ppm of CO2 at its highest concentration, with a more balanced composition of 1,000 ppm of CO2 during normal operations. The propane rundown system was designed to be dry, however, we do have few intermittent spikes of caustic carryover, which we are not sure of the caustic concentration during these events (estimated concentration = <25% NaOH). The propane rundown piping (carbon steel) is not stress relieved, nor insulated, and operates around 100 degrees F, with a design temperature of 150 degrees F. The ethane flow rate will be ~ 7ft/sec via a 6" diameter circuit.

I know the CO2 mixed with water will form carbonic acid, which promotes pitting on exposed surfaces of the carbon steel piping systems. I also understand that an iron carbonate surface layer can protect the piping to minimize corrosion over time if the operating conditions favor film formation. With the occasional caustic carryover and the known flow velocities we are not sure how it will affect the carbonate layer.

Any shared knowledge or assistance regarding this scenario would be greatly appreciated.

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