Microstructural Analysis of Corroded Carbon Steel Used in Process Equipment Handling Caustic Soda

Heat exchanger tubes and pipes made of carbon steel to handle aqueous solutions of spent caustic and crude caustic soda, respectively, at a petrochemical plant have prematurely sustained corrosion damage. Microstructural analysis of representative samples has been conducted using scanning electron microscopy combined with energy- and wavelength-dispersive spectroscopy and X-ray diffraction. It is shown that the tubes were corroded due to the presence of sulfate ions in the spent caustic solution. The corrosion damage sustained by the pipes has been related to the presence of carry over chloride ions in the caustic solution. It is concluded that for better performance, both media require the use of Ni or some Ni alloys rather than carbon steels.