Alloy 600 Aqueous Corrosion at Elevated Temperatures and Pressures: An In Situ Raman Spectroscopic Investigation

Abstract

Two alloy 600 coupons with different surface treatments were exposed in an optically accessible flow cell to air-saturated, high-purity water: one coupon at temperatures up to 543°C at a pressure of 25.4 MPa and one coupon at temperatures up to 392°C at 15.65 MPa. In situ Raman spectra were collected at various temperatures as each coupon was heated and then cooled. Each coupon was also characterized ex situ with Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry. A NiFe 2 O 4 phase was identified on both coupon surfaces while Cr III -containing phases, including α-CrOOH and α-Cr 2 Ο 3 , were identified only on the coupon exposed at higher temperatures, at a higher pressure, and with a rougher surface finish. In addition, the results from the alloy 600 coupon exposed at higher temperatures and a 304L stainless steel coupon exposed under nominally identical conditions were compared. Both NiFe 2 Ο 4 and α-CrOOH phases were observed on the 304L stainless steel coupon. In general, in situ Raman spectroscopy was found to be sensitive to differences in alloy 600 corrosion processes attributed to different surface treatments and differences in alloy 600 and 304L stainless steel corrosion processes attributed to the different oxidation resistance of each alloy.