Under-Deposit Caustic Corrosion on Sodium Carbonate Pipeline

Under deposit corrosion (UDC) is defined as a localized corrosion that develop beneath or around deposit on a metal surface. UDC represents an important threat in the integrity matrix of petroleum and petrochemical production and transportation facilities, and has been cited as responsible for high corrosion rates and loss of containment during operations. For a sodium carbonate production process, crude caustic from the caustic plant enters the sodium carbonate storage tanks. When the batch cycle begins, crude caustic is pumped to the top of the carbonation tower. The caustic stream flows down through two packed sections where it contacts boiler's stack flue gas rising up through the tower. The caustic (sodium hydroxide) reacts with the carbon dioxide in the flue gas to form sodium carbonate. A carbon steel pipeline then transports sodium carbonate. This carbon steel pipeline experiences frequent leaks. An investigation was conducted to identify the types and possible causes of the corrosion on the sodium carbonate pipeline. XRF (X-Ray Fluorescence Spectrometry) and carbon/sulfur analyzer and XRD were used to chemically identify the deposited materials in the pipeline and to study the damage morphology. It was concluded the failure of the pipeline is attributed to caustic corrosion aggravated by deposited of process carry-over materials.