Because of the tough engineering environment of the railroad industry, fatigue is a primary mode of failure. The increased competitiveness in the industry has led to increased loads, reducing the safety factor with respect to fatigue life. Therefore, the existence of corrosion pitting and manufacturing defects has become more important. This article presents case histories that are intended as an overview of the unique types of failures encountered in the freight railroad industry. The discussion covers failures of axle journals, bearings, wheels, couplers, rails and rail welds, and track equipment.

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.

High-level radioactive wastes generated during the processing of nuclear materials are kept in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate and sodium hydroxide. Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors without letting radionuclides escape. Several intergranular cracks were discovered in the vent pipe of one such system. The pipe, made of galvanized steel sheet, connects to an exhaust fan downstream of high-efficiency particulate air filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking was the cause of the failure.

This paper describes the investigation of a corrosion failure of bottom plates on an aboveground tank used for the storage of potable water. The tank was internally inspected for the first time after six years of service. Paint blisters and rust spots were observed on the bottom plates and first to third course shell plates. Sand blasting and repainting of the bottom plates and first course shell plates was to be used as a remedial measure. However, during the sand blasting, holes and deep pitting were observed on the bottom plates. On-site visual inspection, magnetic flux leakage (MFL) inspection, ultrasonic testing (UT), and evaluation of the external cathodic protection (CP) system were used in the failure analysis. The corrosion products were analyzed using energy-dispersive X-ray analysis (EDAX). The failure is attributed to the ingress of water and its impoundment under the tank bottom along the periphery inside the ring wall and failure of water side epoxy coating. Various measures to prevent such failures in the future are recommended.

This study examines the role of calcium-precipitating bacteria (CPB) in heat exchanger tube failures. Several types of bacteria, including Serratia sp. (FJ973548), Enterobacter sp. (FJ973549, FJ973550), and Enterococcus sp. (FJ973551), were found in scale collected from heat exchanger tubes taken out of service at a gas turbine power station. The corrosive effect of each type of bacteria on mild steel was investigated using electrochemical (polarization and impedance) techniques, and the biogenic calcium scale formations analyzed by XRD. It was shown that the bacteria contribute directly to the formation of calcium carbonate, a critical factor in the buildup of scale and pitting corrosion on heat exchanger tubes.

Several mercury diffusion pump stages in the Tritium Purification process at the Savannah River Site (SRS) have been removed from service for scheduled preventive maintenance. These stages have been examined to determine if failure has occurred. Evidence of fatigue around the flange portion of the pump has been seen. In addition, erosion and cavitation inside the throat of the venturi tube and corrosion on the other surface of the venturi tube has been observed. Several measures are being examined in an attempt to improve the performance of these pumps. These measures, as well as the noted observations, are described. Six stages [two machined (MP) and four electron beam (EB) welded] from the mercury diffusion pumps operating in the Tritium Purification process at SRS have been analyzed to determine their condition after nine months of usage. Several cracks were found around the necked region of the two MP stages. The EB welded stages, however, seemed to perform better in service; only two of four stages showed cracking. The cracking is caused by fatigue that has been enhanced by high stresses and tritium in the flange area. The EB welded stage appears to be a step in the right direction. Since the EB weld is a shrink fit, the surface is in compression, thereby eliminating crack propagation. In addition, shot peening has been employed to produce a compressive material surface since fatigue usually originates at the surface. Pitting was observed down the throat of the venturi. This pitting was caused by cavitation and erosion along the length of the venturi tube. Corrosion and pitting was seen on the exterior walls of the diffuser tubes. Stress-corrosion cracks were observed emanating from these corrosion pits. The corrosion likely occurred from the chloride ions present in the process cooling water. Shot peening is now being used in an attempt to place the outside of the diffuser tube in compression to eliminate the stress-corrosion cracking.

At a power plant, C-276 nickel alloy welds (N10276) on a C-276 duct floor completely disappeared in less than half a year. A continuous supply of flue gas came in contact with the closed bypass duct. The unscrubbed combustion products condensed on the cold duct, then the closed damper conducted heat from the chimney and reheated the condensate. Investigation (visual inspection and welded coupon testing) supported the conclusion that the corrosion was caused by “Green Death,” a corrosive medium used to test for pitting resistance (11.9% H2SO4 + 1.3% HCl + 1% FeCl3 + 1% CuCl2) at 103 deg C (217 deg F). Such conditions exist at power plants. Recommendations included repairing the C-276 plates with a 686CPT weld alloy, and if that did not correct the situation, replacing the plates with 686 plate (N06686) welded with 686 CPT.

A heat exchanger made of a pipe in which oil was heated from the outside from approximately 90 deg C to 170 deg C, by superheated steam of about 8 to 10 atmospheres had developed a leak at the rolled joint of the pipe and pipe bottom. The pipes were supposed to be made from St 35.29 steel and annealed at the rolled joint to 100 mm length. The outer pipe surface was strongly pitted by corrosion all around the rolled joint. In the vicinity of the steam chamber the pipe wall had oxidized through from the exterior to the interior at one spot. Adjoining this spot, grooves caused by erosion were noticeable. This was a typical case of crevice corrosion. The rolled joint evidently was not entirely tight, so that saturated steam condensate could penetrate into the gap.

The Advanced Photon Source (APS) is a state-of-the-art synchrotron light source. The storage ring vacuum chamber is fabricated from 6061 extruded aluminum. Water connections to the vacuum chambers that were fabricated from 3003 aluminum had developed water leaks, which were subsequently remedied after considerable investigations.

Equipment in which an assembly of in-line cylindrical components rotated in water at 1040 rpm displayed excessive vibration after less than one hour of operation. The malfunction was traced to an aluminum alloy 6061-T6 combustion chamber that was part of the rotating assembly. Analysis (visual inspection, 100x/500x/800x micrographic examination, spectrographic analysis, and hardness testing) supported the conclusions that, as a result of improper heat treatment, the combustion-chamber material was too soft for successful use in this application. Misalignment of the combustion chamber and one or both of the mating parts resulted in eccentric rotation and the excessive vibration that caused malfunction of the assembly. Irregularities in the housing around the combustion chamber and temperature variation relating to the combustion pattern in the chamber were considered to be possible contributing factors to localization of the cavitation erosion. Recommendations included adopting inspection procedures to ensure that the specified properties of aluminum alloy 6061-T6 were obtained and that the combustion chamber and adjacent components were aligned within specified tolerances. In a similar situation, consideration should also be given to raising the pressure in the coolant in order to suppress the formation of cavitation bubbles.

A resistance-welded carbon steel superheater tube made to ASME SA-276 specifications failed by pitting corrosion and subsequent perforation, which caused the tube to leak. The perforation was found to have occurred at a low point in a bend near the superheater outlet header. It was found that the low points of the superheater tubes could not be completely drained during idle periods. Water-level marks were noticed on the inside surface above the area of pitting. It was revealed by microscopic examination that localized pitting had resulted from oxidation. It was concluded that water contained in the tube during shutdowns had accumulated and cumulative damage due to oxygen pitting resulted in perforation of one of the tubes. Filling the system with condensate or with treated boiler water was suggested as a corrective action. Alkalinity was suggested to be maintained at a pH of 9.0 and 200 ppm of sodium sulfite should be added to the water.

Screens made of stainless steel X5 Cr-Ni-Mo 18 10 (Material No. 1.4401), which were exposed to cooling water from the mouth of a river, became unserviceable after a few months because of the breaking out of parts of the bars. The multiple fracturing of the screen bars in the brackish water of the mouth of the river was attributed to stress corrosion and pitting. The steel used, which contained molybdenum, would have withstood the severe corrosive conditions in the heat-treated condition, i.e. quenched after high temperature anneal. However, the stresses caused by deformation and welding, as well as the intensification of corrosive conditions brought about by design, i.e. creation of corrosion currents in the poorly aerated gaps (Evans elements), made this impossible.

A steel galvanizing vat measuring 3 x 1.2 x 1.2 m (10 x 4 x 4 ft) and made of 19 mm thick carbon steel plate (ASTM A285, grade B)) at a shipbuilding and ship-repair facility failed after only three months of service. To verify suspected failure cause, two T joints were made in 12.5 mm thick ASTM A285, grade B, steel plate. One joint was welded using the semiautomatic submerged arc process with one pass on each side. A second joint was welded manually by the shielded metal arc process using E6010 welding rod and four passes on each side. The silicon content of the shielded metal arc weld was 0.54%, whereas that of the submerged arc weld was 0.86%. After being weighed, the specimens were submerged in molten zinc for 850 h. Analysis (visual inspection, chemical analysis, 100x 2% nital-etched micrographs) supported the conclusions that the vat failed due to molten-zinc corrosion along elongated ferrite bands, possibly because silicon was dissolved in the ferrite and thus made it more susceptible to attack by the molten zinc. Recommendations included rewelding the vat using the manual shielded metal arc process with at least four passes on each side.

An austenitic stainless steel (type 316/316L stainless steel, schedule 40, 64 mm (2.5 in.) diam and larger) piping network used in the fire-sprinkler system in a large saltwater passenger and car ferry failed by rapid leaking. Operating conditions involved stagnant seawater at ambient temperatures. The pipe was in service for four weeks when three leaks appeared. Investigation (visual inspection and photographic images) supported the conclusion that the failure was caused by attack and corrosion damage of Cl ions in conditions that were ideal for three modes of highly accelerated pitting of austenitic stainless steel: the bottom surface, weld or HAZ pits, and crevices. Recommendations included proper material selection for piping, flanges, and weld rods with greater corrosion resistance. Proper filtering to prevent entrained abrasives and timely breakdown inspections were also advised.

Plate perforation occurred in the cylindrical section and walls of the inlet foot (2.38 mm thick Incoloy 825 plate welded using INCO welding rod 135) of an inert gas fire prevention system in an oil tanker. Cross-sectional microprobe analysis showed the corrosion product to contain sulfur, mainly from the flue gas, and calcium and chlorine, mainly from the sea water. The gray corrosion product was interspersed with rust and a black carbonaceous deposit. Corrosion pitting and poor weld penetration, with carbide precipitation and heavy etching at grain boundaries, indicated sensitization and susceptibility to aqueous intergranular corrosion. Chemical analysis showed the predominant acid radical to be sulfate (6.20% in the carbonaceous deposit and 0.60% in the corrosion product), suggesting that oxidation of SO2 in the flue gas caused the corrosion. Moisture condensation, the carbon acting as a cathode, and alloy susceptibility to intergranular corrosion contributed to the corrosion.

While in the stationary mode, capillary action at the contact line between roller and race in a steel rolling mill taper bearing caused a concentration of lubricant and moisture to occur. This lead to lines of corrosion pits at roller intervals. During subsequent operation, the individual corrosion pits acted as stress raisers and initiated coarse grain spalling. Due to a bending moment on the rotating element, this in turn initiated bending fatigue normal to the longitudinal axis, which propagated through to the bore of the inner ring. Stain marks were visible in the bore at a spacing corresponding to roller intervals where lubricant had flowed through the cracks from the race.

The metallurgical condition of a cylindrical induction melter (CIM) vessel was evaluated after approximately 375 h of operation over a two-year span at temperatures between 1400 to 1500 deg C. Wall thinning and significant grain growth was observed in the lower portion of the conical section and the drain tube. No through-wall penetrations were found in the cylindrical and conical sections of the CIM vessel and only one leak site was identified in the drain tube. Failure of the drain tube was associated with localized overheating and creep. The observed degradation resulted from cumulative service at elevated temperature. A recommendation was made to implement a support for the conical section of the CIM and to increase the wall thickness of the drain tube. Thus, the possibility of drain tube misalignment in the induction coils and localized over heating will be minimized. In addition, the use of grain stabilized Pt/Rh alloy should be evaluated as a method to prevent grain growth.

The drive-shaft hanger bearings failed after 300 to 400 h in service. The shaft, supported by labyrinth-sealed single row radial ball bearings of ABEC-1 tolerances, was made of aluminum 2024-T3 tubing (2.5 cm diam and 1.2 mm wall thickness). The bearings were lubricated with a paste-type mineral-oil lubricant (containing molybdenum disulfide and polytetrafluoroethylene particles) or grease conforming to MIL-G-81322 (containing thickening agent and synthetic hydrocarbons) and had two-piece spot-welded retainers. On visual examination, the balls were observed to be embedded in the inner-ring raceway which had been softened by the elevated temperatures reached during the failure. Broken retainers and worn and bent out of shape seals were found. Penetration of gritty particles, water and other corrosive agents and leakage of lubricant out of the bearing permitted by the worn seals was observed. It was concluded that overheating was caused by lubricant flow was permitted by wear of the labyrinth seals. Positive rubbing seals and MIL-G-81322 grease lubricant were found to have longer life than those with the labyrinth seals and mineral-oil-paste lubricant on testing under simulated environmental conditions and were installed as a corrective measure. Importance of dirt free supply and drainage of oil was discussed.

Examination of a connecting-rod shell bearing from a six-cylinder gasoline engine was done after it was returned to the factory. Copper-lead alloy SAE 485 bonded to a low-carbon steel backing was used to make the bearing and the oil used in the engine was the recommended one. Measurable material loss was visible over most of the bearing halves particularly in a wide region at the centerline. A brittle waxlike substance identified to be a mixture of copper and lead sulfides covered the visible shallow pits and the darkened region. Change of oil with greater frequency to prevent the buildup of sulfur compounds or bearing halves that have corrosion-resistant overlay materials were recommended as best solutions.

Type 302 stainless steel springs used in a printing operation failed by breaking into several pieces after two months in service. The springs were operating over a very small deflection and were regulating the flow of ink, in which they were constantly immersed. Fatigue fractures on every piece of the spring were revealed by visual examination. Each of the fractures was found to be oriented at 45 deg to the wire axis. Clear evidence of pitting corrosion at the fatigue fracture origin was also observed. Free chloride ions were revealed to be present in the ink in which the spring was operating. An alternative ink that contained no free chloride ions was recommended.