Failure Analysis of Casting Ni-Cr-Mo Impeller After Long-Term Weak Acid Exposure

The failure of the circulation pump in a sulfur recovery unit occurred after 11 years of operation, during which it was exposed to the weak acid process stream. The A494-CW6M impeller of the pump was found to be completely shattered, leading to the failure. Despite the long operating period, the failure appearance was unusual, and further investigation was needed to understand the cause of the failure. To this end, the detailed metallurgical characterization was performed using the electron microscope and commercially available thermodynamic software. The experimental results showed that the leached regions exhibited more than 50% of the impeller thickness, and those were matched with topological closed phases (M6C, Sigma, P phase), which commonly had low Cr (10 similar to 11 wt.%) and high Mo (56 similar to 58 wt.%) composition. The failure was attributed to the weak corrosion resistance of the low Cr phases when exposed to oxidizing weak acid streams. In addition, thermodynamic phase calculation showed that a large amount of TCPs precipitation was likely due to the malpractice of the heat treatment process. The detailed failure analysis result will be discussed in the present paper.