Background
The Rimkus Materials Testing and Investigation laboratory team was tasked with investigating a fractured nickel-plated brass adapter fitting connecting a water hammer arrester to a valve assembly. A metallurgical failure analysis and corrosion investigation was performed by the team to evaluate the cause of the failure.
Services Provided
The failed brass adapter fitting 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 testing and analyses indicated that the adapter failed due to stress corrosion cracking (SCC). The fracture initiated along the adapter’s inner diameter (ID) surface at a sharp, inside radius corner. The sharp condition of the corner, surface deformation from the machining of the ID, and the presence of deep machining marks on the ID combined to produce the stress needed for SCC failure. The lack of uniform nickel plating on the ID surface exposed the brass adapter to media conducive to SCC in brass, likely containing sulfur and/or chloride compounds, which were found in the fracture surface deposits.
The adapter was manufactured from brass similar to the chemical requirements of C27450. This alloy is highly susceptible to SCC in many water environments.
Figures
Figure 1: Photo of the as-received valve and arrester assembly. The fractured adapter connected the water hammer arrester to the valve, as indicated by the red arrows.
Figure 2: Photo of the fractured adapter after removal from the valve and arrester assembly. The adapter had been nickel-plated during manufacturing. The fracture occurred on the valve side of the hex nut section.
Figure 3: Close-up photo of the water arrester exposed fracture surface prior to cleaning. EDS analysis was conducted to identify the elemental composition of the residual residue on the fracture surface.
Figure 4: The above spectrograph shows the EDS elemental analysis of the residue on the fracture surface. The copper (Cu) and zinc (Zn) were attributed to the base metal. The oxygen (O) was attributed to copper oxide. The carbon (C), silicon (Si), calcium (Ca), and smaller concentrations of sulfur (S), chlorides (Cl), potassium (K), iron (Fe), and aluminum (Al) were attributed to carbonaceous deposits and water born contaminants. The trace amount of nickel (Ni) was attributed to the nickel plating on the adapter’s interior and exterior surfaces. Sulfur and chloride compounds are known corrosives to brass alloys.
Figure 5: Low magnification SEM image of the cleaned fracture surface. Deep machining marks were noted along the ID surface circumference of the adapter.
Figure 6: High magnification SEM image of the fracture surface along the OD surface. The fracture features exhibited significant corrosion/oxidation attack.
Figure 7: High magnification SEM image of the surface along the ID. The fracture features exhibited a combination of quasi-cleavage and corroded/oxidized surface. Multiple secondary cracks (indicated by the yellow arrows) were noted amongst the fractured grains.
Figure 8: Low magnification photomicrograph of the longitudinally cross-sectioned adapter through the fracture surface (white brackets). The white arrows indicate the initiation site at the change in ID.
Figure 9: Increased magnification photomicrograph of the branching cracks identified on the ID surface near the fracture initiation site. The cracks propagated predominately in a multi-branching transgranular manner. This crack morphology is consistent with transgranular stress corrosion cracking (SCC). The direction of the cracks indicated ID surface initiation and propagation toward the exterior surface.
Figure 10: High magnification photomicrograph of the branching cracks’ tips within the wall of the adapter fitting. The transgranular propagation is more readily revealed.
Figure 11: High magnification photomicrograph of the surface deformation created by the heavy machining marks along the ID surface. The surface deformation results in high residual stresses in the adapter ID surface.