Corrosion Failure Analysis of the Aluminum Heat Exchanger in a Mercury-Containing Natural Gas Processing Plant

In order to explore the corrosion failure mechanism of an aluminum heat exchanger in the Yakela Gas Processing Plant, operating conditions, metallographic structure, mechanical properties, and macroscopic and microscopic corrosion characteristics of the heat exchanger were analyzed. The phase analysis of corrosion products was performed with a scanning electron microscope, an energy-dispersive spectrometer, and an X-ray photoelectron spectroscopy (XPS). The physicochemical properties of the aluminum alloy heat exchanger still met the performance requirements in related standards after 18 years of service. However, localized corrosion occurred at the bottom of the exchanger and corrosion pit depth was up to 18.75 mu m. Corrosion products were identified as Al(OH)3 and contained 1.46% mercury, which mainly existed in the form of elemental mercury. The corrosion mechanism of the heat exchanger was primarily mercury corrosion, in which liquefied mercury accumulated at the bottom, blind sections, and welding seams of pipes, thus causing aluminum to dissolve in mercury to form amalgam. In addition, dissolved aluminum reacted with water to produce the corrosion product Al(OH)3. In this way, the exfoliation and dissolution effect caused by mercury resulted in pitting on the aluminum alloy surface and finally the corrosion failure of the heat exchanger.