ADVANCED MATERIALS & PROCESSES | OCTOBER 2025 23 TABLE 1 — PLUMBING FITTING AND UNS C27451 COMPOSITION Element Fitting, wt% UNS C27451, wt% Tin 0.15 -- Lead 0.09 0.25 max Zinc 38.3 Remainder Phosphorus 0.14 0.05 – 0.20 Iron 0.06 0.35 max Silver 0.01 -- Copper 61.3* 61.0 – 65.0 *Copper value calculated by difference. for NPT fittings consists of applying a sealant such as pipe joint compound and/or thread sealant tape, tightening the connection by hand, and applying additional torque using a wrench. Later, after the crack occurs, some of these sustained tensile stresses are relieved (and the dimensions of the component change). Quantitative measurements of the sustained tensile stresses that caused the failure are thus impossible once a failure has occurred. Assessment of the likely stresses at a connection in the as-installed conditions often becomes a point of contention between the various parties when litigating or subrogating a claim related to the failure of a brass component in domestic water service. Ideally, to provide a reliable basis for the opinions of an expert witness, determinations of accepted practices for tightening and use of thread sealants may be identified in product installation guides from the manufacturer or from accepted practices of trade groups documented in technical standards. In this case, no installation guides for NPT fittings had been published by the fitting manufacturer, nor were any universally accepted, comprehensive industry standards for installation practices of NPT fittings identified in a literature review conducted during this investigation. One attribute of the as-installed threaded fittings that does not change due to the occurrence of crack(s) is the number of engaged threads between the two mating components. This technique (counting the number of exposed threads of the mating component that remained visible at the open end of the fitting after tightening) was considered as potential evidence of the extent of tightening for this fitting. Exemplar testing using similar, unused fittings on a shower mixing valve like the one at the subject property revealed that the number of engaged threads required to create a water-tight seal for the mixing valve-to-adapter connection could vary substantially between individual connections. Exemplar fit testing indicated that, for four similar hand-tightened Susceptible metal – The composition of the fitting metal included 38.3 wt% zinc and 61.3 wt% copper (Table 1). The composition was consistent with UNS C27451 yellow brass. High-zinc brass alloys, i.e., alloys with greater than ~30 wt% zinc, are known to be highly susceptible to SCC. Copper and brass alloys with lower concentrations of zinc offer better resistance to SCC than highzinc brass alloys[4]. Incompatible chemical environ- ment for the metal – SCC failures for high-zinc brass alloys have been documented even in relatively benign service environments, including some potable waters[3]. Water testing revealed that the municipal water delivered to the home was typical for municipal water and was neither unusually corrosive nor unusually scale-forming. The technical literature indicates that myriad common chemical species can produce SCC in copper alloys, such as ammonia, nitrates, amines and oxides of nitrogen, sulfates, sulfur dioxide, and steam[4]. The specific chemical agent responsible for SCC in this case could not be identified. However, the inability to identify a specific aggressive chemical agent is common for failure investigations of high-zinc brass components in domestic water service. Sustained tensile stress – The longitudinal orientation of the cracks in the fitting indicated that the causative stresses for the cracks were in the circumferential direction (i.e., hoop stress). Three potential sources of hoop stress for the adapter include installation stress, residual stress from manufacturing, and service stress. Hoop stresses are applied on the fitting during installation due to the tapered thread geometry of NPT fittings. These stresses are an inherent byproduct of the interference that creates the watertight seal and are therefore necessary for the fitting to perform its intended function. In general, the installation process Fig. 5 — Typical microscopic fracture features as viewed in SEM-SEI (left) and a prepared metallographic section through the primary crack near the fracture origin area (righthand side of right image).
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