Dissolution Behaviour of Laves Phase in P92 High Alloy Steel in Alkaline Solutions

Abstract

The Laves phase precipitates in ASTM A335 P92 (P92) steel were found to dissolve in solutions with NaOH concentration higher than 7 mol l−1. Such behaviour was utilized to detect and quantify the precipitates in P92 specimens aged at 650 °C for times from 0 to 5000 h. The amount of precipitates was acquired by potentiodynamically polarizing the sample and integrating the dissolution current over time. The result was compared to that of a traditional SEM observation method, and a good linear correlation was found between the results. The dissolution mechanism was discussed based on the electrochemical behaviour of an artificially synthesized Fe2Mo laves phase specimen, pure Fe, and pure Mo in the strong alkaline solution.

Electrochemical Characterization Method of Laves Phase in 9Cr Martensitic Heat-Resistant Steel and Creep Life Prediction

“…0.2 V Ag/AgCl , were observed on all curves, which were related to the dissolution of the Laves phase and carbides, respectively [11]. The curves are almost passivated in the range of −0.7 to −0.1 V Ag/AgCl until the concentration of NaOH is higher than 7 mol/L, which is consistent with reported results [13]. The morphology changes before and after anodic polarization in an 8 mol/L NaOH solution observed with SEM are shown in Figure 9a,b.…”

“…It is clear that almost all Laves phase particles were dissolved in the 8 mol/L test. The results confirm that the current peak does correspond to the dissolution of the Laves phase particles [11,13].…”

“…However, these methods need advanced facilities, such as a scanning electron microscope (SEM) or transmission electron microscope (TEM), and the spatial area of investigation is limited. In recent years, it has been found that the electrochemical method can selectively dissolve the Laves phase in 9-12% Cr heat-resistant steel after long-term aging in a strong alkaline solution [11][12][13]. A good linear relationship was established between the area fraction data of the Laves phase collected by SEM imaging and the dissolved peak charge obtained by the electrochemical method.…”