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Cast Stainless Steel Impeller Failure

Background: Severe corrosion damage was experienced in the lines carrying water in and out of the scrubbers. Impeller was operating at 125-165° F temperature in environment containing ammonium sulfate, sulfuric acid, and ammonia with chlorides and at pH between 1.8-3.0.

Figure 1: Visual Inspection of As-Received Cast Impeller


Figure 2: SEM Micrographs of the Cavitation Damage on Cast Impeller

Summary: This impeller failure was associated with severe erosion-corrosion assisted cavitation damage. In general, cast stainless steels materials possess excellent corrosion resistance. However, in this case the change in cross-section of the impeller likely caused turbulent conditions, whicih coupled with thpresence of calcium hypochlorite and sulfuric acid in the system, caused erosion-corrosion and cavitation damage. Uniform thinning with cavitation was observed on the impeller blades.
Cavitiation is a typical damage mechanism in high-velocity fluid environments which experiences pressure change creating boiling conditions at lower temperatures. Collapsing bubbles produced during boiling on the material surface generates shock waves that causes material removal.

Take Away: The presence of Cl- ions in a stainless steel system, in conjunction with the high-velocity fluid flow, can cause severe erosion-corrosion damage. Pressure fluctuations in the process that create boiling conditions, accelerate material removal through a cavitation mechanism.