The Effect of Precipitates on Corrosion Resistance of Alloy 28 Tubing in Sour Environments

Alloy 28, an important corrosion-resistant material candidate for use in down-hole tubing in severe corrosive environments, was investigated for the characteristics of its precipitates and their effect on corrosion resistance. The microstructure of the precipitates was observed by optical microscope and scanning election microscopy (SEM), and the type and chemical composition were analyzed by x-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). It was found that the precipitates in the Alloy 28 were rich in C, Cr, and Mo, mostly σ phase and Cr23C6. The corrosion resistance of Alloy 28 with precipitates was markedly reduced in intergranular corrosion tests and stress corrosion cracking tests. So avoiding precipitates in Alloy 28 by the appropriate heat treatment during manufacture of the tubing is key.