Background
Three segments of stainless-steel waste disposal piping, damaged by pitting and subsequent leakage, were submitted to the Rimkus Materials Testing and Investigation laboratory team to investigate the cause of failure. The pipes had been cleaned and sanitized using high heat and water prior to delivery. A metallurgical failure analysis and corrosion investigation analysis were performed by the materials team to evaluate the cause of the failure.
Services Provided
The leaking waste disposal pipe was subjected to a series of testing and analytical techniques, including visual and stereoscopic examinations, scanning electron microscope (SEM) examinations, elemental composition analysis by energy dispersive spectroscopy (EDS), metallography/microstructure evaluation, and chemical composition analysis (OES).
Results
The results of the tests and analyses indicated that the pitting initiated from the interior surfaces of the piping due to microbiological influenced corrosion (MIC). MIC pitting initiated preferentially at the heat-affected zones (HAZ) of the pipe weld joints. The presence of pitting morphology of undercutting and tunneling through the pipe wall are both positive characteristics of MIC attack. Additionally, the presence of significant levels of sulfur in residue surrounding the pits suggested attack by sulfate reducing bacteria, a form of MIC attack.
Chemical analysis of one of the pipes confirmed the alloy as ASTM A312 Grade 316L low carbon austenitic stainless steel.
Figures
Figure 1: Photo of the three pipe segments in the as-received condition. A segment of pipe “A” was selected for further analysis.
Figure 2: Photo of the interior surface of the selected section from pipe “A”. The pipe was split longitudinally to expose the interior surface. A close-up view of the pitting identified along the weld is shown in Figure 3.
Figure 3: Close-up photo of some of the pitting colonies (indicated by red arrows) that were identified along the butt weld HAZ. A closer view of the pitting (boxed area) is shown in Figure 4.
Figure 4: Photo of the colony of fine pits surrounded by residual brown deposits (indicated by red arrows). MIC will form colonies of pitting that are typically covered by crust-like nodules. These nodules were removed during the sterilization and cleaning processes by the client, leaving behind residual deposits.
Figure 5: Photo of the previously identified surface pitting. The small, surface openings of the pits are indicative of subsurface undercut pitting of the pipe wall and subsequent subsurface linking of several pits withing the MIC colony.
Figure 6: High magnification SEM image of a large pit on the interior surface of the pipe. The pit exhibited characteristics of undercutting of the pipe interior surface with residual deposits left over after the sterilization process.
Figure 7: EDS spectrum of the dark residue identified around the surface pitting. The analysis identified a high concentration of carbon (C), oxygen (O), and smaller concentrations of silicon (Si), phosphorous (P), sulfur (S), calcium (Ca), copper (Cu), magnesium (Mg), aluminum (Al), and chlorides (Cl). The iron (Fe), chromium (Cr), manganese (Mn), and nickel (Ni) were attributed to the pipe base metal.
Figure 8: Low magnification photomicrograph of a large, pitted area along the weld joint on the pipe’s interior surface.
Figure 9: An increased magnification photomicrograph of a second pit location on the pipe’s interior surface adjacent to the weld joint. The undercutting of the pit just below the pipe’s interior surface and the tunneling through the wall thickness is indicative of MIC.