Failure analysis of one peculiar ‘Yin-Yang’ corrosion morphology on heat exchanger tubes in purified terephthalic acid (PTA) dryer

It is a common knowledge that localized corrosions occur on austenitic stainless steels such as 316L, exposed to halide ions. This paper will discuss such a localized failure that took place not long after beginning of service on the heat exchanger tubes inside a purified terephthalic acid (PTA) dryer in a petrochemical works. Particularly, one peculiar corrosion morphology termed ‘Yin-Yang’ corrosion was observed, i.e. the upside surface of the failed tubes was severely corroded while the downside was intact. Consequently, in order to ascertain the actual causes of this premature failure, samples including the failed tubes and the process media were investigated by a variety of characterization methods. Metallographic structures and chemical compositions of the tube matrix materials were inspected by optical microscope (OM) and photoelectric direct reading spectrometer; both the surfaces and the cross-sections of the corroded areas were microscopically analyzed through scanning electron microscope (SEM) and energy disperse spectroscopy (EDS); and the chemical constituents of the process media were detected via the gas chromatography–mass spectrometric (GC–MS). Finally, the localized corrosion mechanisms were discussed in detail and the pertinent countermeasures were proposed.

► A peculiar ‘Yin-Yang’ corrosion morphology was observed on the heat exchanger tubes.
► Upside surfaces of the failed tubes were corroded while their downsides were intact.
► Interaction between operation parameters and materials processing was the root cause.