The paper discusses the analysis of a coating defect on a high phosphorus electroless nickel (Ni-11 wt. % P) deposit plated on an aluminum alloy substrate. Preliminary investigations had indicated that the elongated defects were possibly caused by the entrapment of long fibers or particles during the plating. The possible sources of fibers were identified. The SEM/EDS analysis of fibers collected from the air duct filters correlated very well with the defect shape and the EDS profile collected from under the defect site. It appears that the fibers from air duct filters directly above the plating line were blown into the plating tank and getting co-deposited. The paper describes the step-by-step analysis of the defect that led to successful identification of the root cause of the defect.

During 5.7 years of service, dye penetrant inspection of Inconel 800H pigtail connections regularly showed cracks at weld toes. Weld repairs were not able to prevent reoccurrence but often aggravated the condition. Samples containing small, but detectable, reducer-to-pigtail cracks showed intergranular cracks originating at weld toes and filled with oxidation product, which precluded determination of the cracking mechanism. All weldments exhibited high degrees of secondary precipitates, with original fabrication welds exhibiting higher apparent levels than repair welds. SEM/EDS analysis showed base metal grain boundary precipitates to be primarily chromium carbides, but some titanium carbides were also observed. Failure was believed to result from the synergism of thermally driven tube distortion, which resulted in over-stress, and from the intergranular oxidation products and intergranular carbides which contributed to cracking. It was recommended that stresses be reduced and /or that materials and components be changed. Refinements in welding procedures and implementation of preweld/postweld heat treatments were recommended also.

A storage tank had been in service at a petrochemical plant for 13 years when inspectors discovered cracks adjacent to weld joints and in the base plate near the foundation. The tank was made from AISI 304 stainless steel and held styrene monomer, a derivative of benzene. The cracks were subsequently welded over with 308 stainless steel filler wire and the base plate was replaced with new material. Soon after, the tank began leaking along the weld bead, triggering a full-scale investigation; spectroscopy, optical and scanning electron microscopy, fractography, SEM-EDS analysis, and microhardness, tensile, and impact testing. The results revealed transgranular cracks in the HAZ and base plate, likely initiated by stresses developed during welding and the presence of chloride from seawater used in the plant. It was also found that the repair weld was improperly done, nor did it include a postweld heat treatment to remove weld sensitization and minimize residual stresses.

The operator of an electric transit system purchased a large number of tin-plated copper connectors, putting some in service and others in reserve. Later, when some of the reserve connectors were inspected, the metal surfaces were covered with spots consisting of an ash-like powder and the plating material had separated from the substrate in many areas. Several connectors, including some that had been in service, were examined to determine what caused the change. The order stated that the connectors were to be coated with a layer of tin-bismuth (2% Bi) to guard against tin pest, a type of degradation that occurs at low temperatures. Based on the results of the investigation, which included SEM/EDS analysis, inductively coupled plasma spectroscopy, and x-ray diffraction, the metal surfaces contained less than 0.1% Bi and thus were not adequately protected against tin pest, which was confirmed as the failure mechanism in the investigation.

A 13/16-in. hex socket failed while in use. Analysis (hardness testing, optical and scanning electron microscopy, and EDS) revealed that the socket was made of low carbon steel formed in a powder metallurgy process. A number of flaws were found including nonuniform wall thickness, poor geometric design with sharp corners as stress raisers, and incomplete sintering evidenced by unsintered particles. These were determined to be the primary cause of failure, although inclusions on the fracture surface containing S and Al may have played a role as well.

A flanged 100 mm (4 in.) diam low-carbon steel spool piece lined with Teflon was removed from a sulfuric acid denitrification system after cracks were observed in the painted coating. Visual and microstructural examination along with SEM fractography revealed scaled iron oxides on all opened crack surfaces. The surfaces had a faceted morphology, indicating intergranular fracture. Cracks originated at the interface between the tube and the Teflon liner Corrosion products were found caked into the intergranular region between the liner and the spool. The portion of the liner that had been exposed to the process stream was discolored. Failure of the spool was attributed to stress-corrosion cracking promoted by the presence of nitrates. Nitric acid contaminant in the sulfuric acid stream had diffused through the liner and accumulated in the annular space. Use of a liner that is more impermeable to the diffusion of ionic species was recommended.

Several 304H stainless steel superheater tubes fractured in stressed areas within hours of a severe caustic upset in the boiler feedwater system. Tests performed on a longitudinal weld joint, which connected two adjacent tubes in the tertiary superheater bank, confirmed caustic-induced stress-corrosion cracking, promoted by the presence of residual welding stresses. Improved maintenance of check valves and routine inspection of critical monitoring systems (conductivity alarms, sodium analyzers, etc.) were recommended to help avoid future occurrences of severe boiler feedwater contamination. Additional recommendations were to eliminate these short longitudinal weld joints by using a bracket assembly joint between the tubes, use a post-weld heat treatment to relieve residual welding stress or select a more stress-corrosion cracking resistant alloy for this particular application.

Cracking was discovered in an in-service, second-stage turbine impeller during a downtime inspection. The fabricated 4300 series low-alloy steel impeller was used in a compressor in an industrial petrochemical plant. It was also reported that a process upset had allowed a 10% NaOH solution to be ingested by the unit. Routine magnetic particle inspection revealed numerous cracks in the hub area and vane tips of the second-stage impeller Additionally, the outside surface of the backing plate showed a cyclic pattern of cracks. An overview of a conventional, systematic metallurgical approach to failure analysis to confirm that the cracking was caused by a caustic stress-corrosion cracking mechanism is presented.

An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

Radiant tubes that failed prematurely in an ethylene cracking furnace were analyzed to determine the cause of their early demise. The tubes were made from austenitic heat-resistant steel and cracked along their longitudinal axis. New and used tubes were compared using scanning electron microscopy, energy dispersive x-ray spectrometry, and mechanical property testing. This provided critical information and revealed that improper coking and decoking had removed the protective oxide layer (Cr 2 O 3 ) that normally prevents coke deposits from forming on exposed surfaces. Without this layer, coke readily accumulates on the surface of the tubes, fueling carbon diffusion into the metal and a corresponding degradation in microstructure and loss of ductility at high temperatures.

Failure of three C22000 commercial bronze rupture discs was caused by mercury embrittlement. The discs were part of flammable gas cylinder safety devices designed to fail in a ductile mode when cylinders experience higher than design pressures. The subject discs failed prematurely below design pressure in a brittle manner. Fractographic examination using SEM indicated that failure occurred intergranularly from the cylinder side. EDS analysis indicated the presence of mercury on the fracture surface and mercury was also detected using scanning auger microprobe (SAM) analysis. The mercury was accidentally introduced into the cylinders during a gas-blending operation through a contaminated blending manifold. Replacement of the contaminated manifold was recommended along with discontinued use of mercury manometers, the original source of mercury contamination.

Two large tension springs fractured during installation. The springs were manufactured from a grade 9254 chromium-silicon steel spring wire. The associated material specification allows wire in the cold-drawn or oil-tempered (quenched-and-tempered) condition. The specified wire tensile strength range was 1689 to 1793 MPa (245 to 260 ksi). The finished springs were to be shot peened for greater fatigue resistance. Investigation (visual inspection, 3x images, 2% nital etched 148x SEM images, chemical analysis, hardness testing, and EDS analysis) supported the conclusion that the springs failed during installation due to the presence of preexisting defects. Crack surfaces were found to be corroded and phosphate coated, indicating that the cracks occurred during manufacture. Installation, which presumably entailed some axial extension, resulted in ductile overload failure at the crack sites. Recommendations included evaluating the manufacturing steps to identify the process(es) wherein the cracking was likely occurring. It was further recommended that a suitable nondestructive method such as magnetic particle inspection be implemented.

Several surgical tool failures were analyzed to understand why they occur and how to prevent them. The study included drills, catheters, and needles subjected to the rigors of biomedical applications such as corrosive environments, high stresses, sterilization, and improper cleaning procedures. Given the extreme conditions to which surgical tools can be exposed, and the potential for misuse, failures are inevitable and systematic methods for analyzing them are necessary to keep them in check.

A type 410 stainless steel circulating water pump shaft used in a fossil power steam generation plant failed after more than 7 years of service. Visual examination showed the fracture surface to be coated with a thick, spalling, rust-colored scale, along with evidence of pitting. Samples for SEM fractography, EDS analysis, and metallography were taken at the crack initiation site. Hardness testing produced a value of approximately 27 HRC. The examinations clearly established that the shaft failed by fatigue. The fatigue crack originated at a localized region on the outside surface where pitting and intergranular cracking had occurred. The localized nature of the initial damage indicated that a corrosive medium had concentrated on the surface, probably due to a leaky seal. Reduction of hardness to 22 HRC or lower and inspection of seals were recommended to prevent future failures.

Failure analysis of a mobile harbor crane wheel hub that included SEM and EDS analyses demonstrated that the mechanism of failure was fatigue. The wheel hub was a ductile cast iron component that had been subjected to cyclic loading during a ten-year service period. The fracture surface of the fatigue failure also contained corrosion deposit, suggesting that cracking occurred over a period of time sufficient to allow corrosion of the cracked surfaces. Replacement and alignment of the failed wheel hub was recommended along with inspection of the nonfailed wheel hubs that remained on the crane.

A cast silicon bronze (UNS C86700) impeller that had been severely corroded was submitted for failure analysis. The failed part was used to pump potable water, but service life and chlorine content of the water were unknown. The impeller displayed a Cu-rich red phase on its surfaces and showed a pattern very similar to dezincification. Further investigation to determine the cause of damage using light microscopy and SEM-EDS techniques revealed that the microstructure consisted of multiple phases and that a Si-rich phase was being preferentially attacked, leading to increased porosity. After a thorough examination, it was concluded that the part had failed due to dealloying via desiliconification.

A type 304 stainless steel tube that failed in a boiler stack economizer was analyzed to determine the cause. The investigation consisted of visual, SEM/EDS, and metallographic analysis. Several degradation mechanisms appeared to be at work, including pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. Investigators concluded that the primary failure mechanism was fatigue fracture, although either of the other mechanisms may have eventually caused the tube to fail in the absence of fatigue.

Gas turbines and other types of combustion turbomachinery are susceptible to hot corrosion at elevated temperatures. Two such cases resulting in the failure of a gas turbine component were investigated to learn more about the hot corrosion process and the underlying failure mechanisms. Each component was analyzed using optical and scanning electron microscopy, energy dispersive spectroscopy, mechanical testing, and nondestructive techniques. The results of the investigation provide insights on the influence of temperature, composition, and microstructure and the contributing effects of high-temperature oxidation on the hot corrosion process. Preventative measures are also discussed.

A high-speed pinion gear shaft, part of a system that compresses natural gas, was analyzed to determine why it failed. An abnormal wear pattern was observed on the shaft surface beneath the inner race of the support bearings. Material from the shaft had transferred to the bearing races, creating an imbalance (enough to cause noise and fumes) that operators noted two days before the failure. Macrofeatures of the fracture surface resembled those of fatigue, but electron microscopy revealed brittle, mostly intergranular fracture. Classic fatigue features such as striations were not found. To resolve the discrepancy, investigators created and tested uniaxial fatigue samples, and the microfeatures were nearly identical to those found on the failed shaft. The root cause of failure was determined to be fatigue, and it was concluded that cracks on the pinion shaft beneath the bearings led to the transfer of material.

X-ray spectroscopy is generally accepted as the most useful ancillary technique that can be added to any scanning electron microscope (SEM), even to the point of being considered a necessity by most operators. While “stand-alone” x-ray detection systems are used less frequently in failure analysis than the more exact instrumentation employed in SEMs, the technology is advancing and is worthy of note due to its capability for nondestructive analysis and application in the field. This article begins with information on the basis of the x-ray signal. This is followed by information on the operating principles and applications of detectors for x-ray spectroscopy, namely energy-dispersive spectrometers, wavelength-dispersive spectrometers, and handheld x-ray fluorescence systems. The processes involved in x-ray analysis in the SEM and handheld x-ray fluorescence analysis are then covered. The article ends with a discussion on the applications of x-ray spectroscopy in failure analysis.