Initially, two vertical double-acting two-stage compressors delivering chlorine gas at a pressure of 100 psi appeared to be running satisfactorily. About six months later the LP piston-rod of the No. 2 compressor failed due to burning, the compressor being worked double-acting at the time. About five months later, the HP piston rod of the No. 1 compressor failed in a similar manner. Specimens for microscopic examination were cut from the rod in the region of the failure and from the extreme end that had been situated above the piston and hence not subjected to an appreciable rise in temperature. The material was a steel in the normalized condition with a 0.35% C content. It appears probable that deficient lubrication of the gland resulted in overheating of the rod due to friction. The presence of a sprayed-metal coating was probably an additional factor in promoting failure, as it would present to the gas a surface area considerably greater than that of a homogeneous material.

Leakage was identified around a coupling welded into a stainless steel holding tank that stored condensate water with low impurity content. The tank and fitting were manufactured from type 304 stainless steel. The coupling joint consisted of an internal groove weld and an external fillet weld. Cracking was found to be apparent on the tank surface, adjacent to the coupling weld. Chlorine, carbon, and oxygen in addition to the base metal elements were revealed by energy-dispersive x-ray spectrometric analysis. A great number of secondary, branching cracks were evident in the weld, heat-affected zone, and base metal. The branching and transgranular cracking was found to emanate primarily from the exterior of the tank. It was concluded that the tank failed as a result of stress-corrosion cracking that initiated at the exterior surface as aqueous chlorides, especially within an acidic environment, have been shown to cause SCC in austenitic stainless steels under tensile stress.

A tie rod, nut, and bellows from a failed 610 mm (24 in.) diam tied universal expansion joint that carried tail gases consisting of N 2 + O 2 with slight traces of nitrogen oxides and water were examined. The materials were SA 193-B7 (AISI 4140), SA 194–214, and Incoloy 800H, respectively. Visual examination of the bellows revealed cracks in heavily cold-worked areas (both inside and outside) and considerable corrosion. SEM analysis showed a classical intergranular failure pattern with microcracking. The threaded tie rod microstructure contained spheroidized carbide that was more pronounced at the tie rod end of the failure. Energy-dispersive X-ray analysis of fracture surfaces from the bellows showed the presence of chlorine and sulfur. Failure of the bellows was attributed to stress-corrosion cracking, with chlorine and sulfur being the corroding agents. The rod damage was the result of failure of the bellows, which allowed escaping hot gases to impinge on the tie rods and heat them to approximately 595 deg C (1100 deg F). It was recommended that the insulation be analyzed to determine the origin of the chlorine and sulfur and that it be replaced if necessary.

Catastrophic pitting corrosion occurred in type 304L stainless steel pipe flange assemblies in an industrial food processor. During regular service the pumped medium was pureed vegetables. In situ maintenance procedures included cleaning of the assemblies with a sodium hypochlorite solution. It was determined that the assemblies failed due to an austenite-martensite galvanic couple activated by a chlorine bearing electrolyte. The martensitic areas resulted from a transformation during cold-forming operations. Solution annealing after forming, revision of the design of the pipe flange assemblies to eliminate the forming operation, and removal of the source of chlorine were recommended.

The thin plates within a type 309 stainless steel chlorinated solvent combustion preheater/heat exchanger designed to process fumes from a solvent coating process showed severe corrosion within 6 months of service. Within a year corrosion had produced holes in the plates, allowing gases to shunt across the preheater/exchanger. Metallographic examination of the plates showed that accelerated internal oxidation had been the cause of failure. Corrosion racks of candidate alloys (types 304, 309, and 316 stainless steels, Inconel 600, Inconel 625, Incoloy 800, Incoloy 825, and Inco alloy C-276) were placed directly in the hot gas stream, containing HCl and Cl2, for in situ testing. Results of this investigation showed that nickel-chromium corrosion-resistant alloys, such as Inconel 600, Inconel 625, and Inco alloy C-276, performed well in this environment. Laboratory testing of the same alloys, along with Inconel alloys 601, 617, and 690 and stainless steel type 347 was also conducted in a simulated waste incinerator nitrogen atmosphere containing 10% Co2, 9% O2, 4% HCl, 130 ppm HBr and 100 ppm SO2 at 595, 705, 815, and 925 deg C (1100, 1300,1500, and 1700 deg F). The tests confirmed the suitability of the nickel-chromium alloys for such an environment. Inconel 625 was selected for fabrication of a new preheater/exchanger.

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 bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, energy dispersive x-ray analysis (EDXA) indicated significant segregation of sulfur and chlorine along the grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O 2 that entered the tube during shutdowns and startups. Residual stresses, originating from in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries made the material more susceptible to cracking.

Six ASTM A-574 steel cap screws from a hydraulic coupling failed after 3 months in service. The screws were replacements for smaller-diameter cap screws that had been installed during an outage. Six new cap screws were examined along with the failed screws. Eight fracture locations were identified—three at the head-to-shank fillet, four at the eighth thread root from the cap, and one at the sixth thread root from the cap. Fracture surfaces were examined using a stereomicroscope and SEM, and the fracture mode was shown to be transgranular. EDS on the fracture surfaces showed sulfur and chlorine in the surface deposits. The observations indicated that the screws had failed by fatigue. Insufficient preloading was considered to be the most likely cause of the fatigue cracking. It was recommended that the proper preload on the screws be verified and maintained.

Radial cracking occurred adjacent to 11 vanes in a 19-vane impeller operating in a chemical plant environment. The impeller vanes were fillet welded to both the disk and the cover Cracks were next to the fillet welds and near the cover outer diameter They generally did not extend to the outer diameter. The entire impeller surface was tested by the dry magnetic particle method. Visual and microstructural examinations revealed intergranular cracking. Energy-dispersive spectroscopy of corrosion products contained in the cracks disclosed the presence of chlorine and sulfur The failure was attributed to stress-corrosion cracking caused by a corrosive atmosphere.

A type 321 stainless steel downcomer expansion joint that handled process gases was found to be leaking approximately 2 to 3 weeks after installation. The expansion joint was the second such coupling placed in the plant after failure of the original bellows. The failed joint was disassembled and examined to determine the cause of failure. Energy-dispersive x-ray analysis revealed significant peaks for chlorine and phosphorus, indicating failure by chloride stress-corrosion cracking (SCC). Cracks in the liner and bellows exhibited a branched pattern also typical of SCC. Cracks through the inner liner initiated on the outer surface of the liner and propagated inward, whereas cracks in the bellows originated on the inner surface and propagated outward. Stress-corrosion cracking of the assembly was caused by chloride contaminants trapped inside the bellows following hydrostatic testing. Checking the test fluid for chloride and removing all fluids after hydrostatic testing were recommended to prevent further failure.

Several type 304L stainless steel dished ends used in the fabrication of cylindrical vessels developed extensive cracking during storage. All of the dished ends had been procured from a single manufacturer and belonged to the same batch. When examined visually, several rust marks were observed, indicating contamination by rusted carbon steel particles. Liquid penetrant testing was used to determine the extent of the cracks, and in situ metallographic analysis was performed over the cracked region. The morphology of the cracks was indicative of transgranular stress-corrosion cracking (TGSCC). Conditions promoting the occurrence of the TGSCC included significant tensile stresses on the inside of the dished ends, the presence of surface contamination by iron due to poor handling practice using carbon steel implements, and storage in a coastal environment with an average temperature of 25 to 32 deg C (77 to 90 deg F), an average humidity ranging from 70 to 80%, and an atmospheric NaCl content ranging from 8 to 45 mg/m2 /day. Recommendations preventing further occurrence of the situation were strict avoidance of the use of carbon steel handling implements, strict avoidance of cleaning practices that cause long-term exposure to chlorine-containing cleaning fluid, and solution annealing of the dished ends at 1050 deg C (1920 deg F) for 1 h followed by water quenching to relieve residual stresses.

Severe pitting was found on the internal surfaces of SA-210 Grade C waterwall tubing of a coal-fired boiler at a cogeneration facility. Metallographic examination showed the pits to be elliptical, having an undercut morphology with supersurface extensions,. a type of pitting characteristic of acidic attack. Energy-dispersive X-ray spectroscope revealed the presence of chlorine in the pit deposits, indicating that the pitting was promoted by underdeposit chloride attack. The presence of copper in deposits on the internal surface of the tubing may have acted as a secondary factor. Acidic conditions may have formed during a low-pH excursion that reportedly occurred several years prior. To prevent future failures, severely damaged tubing must be replaced. Internal deposit buildup must be removed by chemical cleaning to prevent further pitting. Water quality needs continued monitoring and maintenance to ensure that another low-pH excursion does not occur.

An investigation of the impeller and deposit samples from a centrifugal compressor revealed that an aluminum IR-12 refrigerant reaction had occurred, causing extensive damage to the second-stage impeller and contaminating the internal compressor components. The spherical surface morphology of the impeller fragments suggested that the aluminum had melted and resolidified. The deposits were similar in composition and were identified by XRD as consisting primarily of aluminum trifluoride. In addition, EDS analysis detected major amounts of chlorine and iron. Results of a combustion test indicated that the compressor deposit was comprised of a 9. 8 wt% carbon and that the condenser deposit contained 8.7 wt% carbon. It was concluded that the primary cause of failure was the rubbing of the impeller against the casting and that a self-sustaining Freon fire had occurred in the failed compressor

After about 17 years in service, copper alloy C27000 (yellow brass, 65% Cu) innercooler tubes in an air compressor began leaking cooling water, causing failure and requiring replacement. The tubes were 19 mm in diam and had a wall thickness of 1.3 mm (0.050 in.). The cooling water that flowed through the tubes was generally sanitary (chlorinated) well water; however, treated recirculating water was sometimes used. Analysis (visual inspection, 9x and 75x unetched micrographs, and spectrochemical analysis) showed a thick uniform layer of porous, brittle copper on the inner surface of the tube, extending to a depth of about 0.25 mm (0.010 in.) into the metal, plug-type dezincification extending somewhat deeper into the metal. This supported the conclusion that failure of the tubes was the result of the use of an uninhibited brass that has a high zinc content and therefore is readily susceptible to dezincification. Recommendations included replacing the material with copper alloy C68700 (arsenical aluminum brass), which contains 0.02 to 0.06% As and is highly resistant to dezincification. Copper alloy C44300 (inhibited admiralty metal) could be an alternative selection for this application; however, this alloy is not as resistant to impingement attack as copper alloy C68700.

Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected to visual inspection, SEM, x-ray diffraction, microhardness measurement, and chemical and microstructural analysis. It was determined that extended non-operation of the plant resulted in the settlement of corrosive species on the tubes in Stage 3. The complete failure of the tube occurred due to diffusion of these elements into the base metal and precipitation of potassium and chlorine compounds along the grain boundaries, with subsequent dislodging of grains. The nonmetallic inclusions acted as nucleating sites for local pitting bursting. Nonuniform heat transfer in Stage 3 operation accelerated the selective corrosion of front-end tubes. The relatively high heat transfer in this stage resulted in condensation of some corrosive gases and consequent corrosion. Continuous operation of the plant with some precautions during assembly of the tubes reduced the corrosion problem.

Tube sheets (found to be copper alloy C46400, or naval brass, and 5 cm (2 in.) thick) of an air compressor aftercooler were found to be cracked and leaking approximately 12 to 14 months after they had been retubed. Most of the tube sheets had been retubed several times previously because of unrelated tube failures. Sanitary (chlorinated) well water was generally used in the system, although filtered process make-up water (river water) containing ammonia was occasionally used. Investigation (visual inspection, chemical analysis, mercurous nitrate testing, unetched 5X micrographs, and 250X micrographs etched in 10% ammonium persulfate solution) supported the conclusion that the tube sheets failed by SCC as a result of the combined action of internal stresses and a corrosive environment. The internal stresses had been induced by retubing operations, and the environment had become corrosive when ammonia was introduced into the system by the occasional use of process make-up water. Recommendations included making a standard procedure to stress relieve tube sheets before each retubing operation. The stress relieving should be done by heating at 275 deg C (525 deg F) for 30 min and slowly cooling for 3 h to room temperature.

A corrosion resistant chromium nickel steel (X 2 Cr-Ni-Mo 18 10) worm drive used in a chemical plant at 80 deg C and 100 to 200 atm pressure to transport media containing chloride failed during normal operation. Visual inspections showed that the entire surface of the gear was covered with fine branching cracks and was flaking off. Microscopic examination showed that the unetched polished material had disintegrated to an average depth of 1 mm below the surface. A micrograph of the etched surface revealed numerous deformation lines and transgranular cracking. The failure was thus due to stress-corrosion cracking and additional corrosion due to ventilation elements. Because austenitic chromium nickel steels are prone to stress-corrosion cracking, particularly in the presence of chlorine compounds at high temperatures, and because austenitic rust- and acid-resistant steels are prone to smearing and work hardening during machining, it was recommended that these types of steels be machined only with sharp, short tools mounted in rigid structures. In addition, residual stresses should be eliminated by post-process annealing in a protective atmosphere.

AISI type 321 stainless steel heat exchanger tubes failed after only three months of service. Macroscopic examination revealed that the leaks were the result of localized pitting attack originating at the water side surfaces of the tubes. Metallographic sections were prepared from both sets of tubes. Microscopic examination revealed that the pits had a small mouth with a large subsurface cavity which is typical of chloride pitting of austenitic stainless steel. However, no pitting was found in other areas of the system, where the chloride content of the process water was higher. This was attributed to the fact that they were downstream from a deaeration unit. It was concluded that the pitting was caused by a synergistic effect of chlorine and oxygen in the make-up water. Because it was not possible to install a deaeration unit upstream of the heat exchangers, it was recommended that a molybdenum-bearing stainless steel such as 316L or 317L be used instead of 321.

An aluminum brass seawater surface condenser failed due to pitting after less than one year of service. Large pits filled with a green deposit were evidenced under the nonuniform black scale present over the entire inside surface of the tube. The black deposit was identified as primarily copper sulfide, with zinc and aluminum sulfides while the green deposit was revealed to be copper chloride. The combination of sulfide and chloride attack on the tubes was concluded to have resulted in the failure. Injection of ferrous sulfate upstream of the condenser which could aid the formation of protective oxide films was recommended.

A cracked, martensitic stainless steel, low-pressure turbine blade from a 623 MW turbine generator was found to exhibit fatigue cracks during a routine turbine inspection. The blade was cracked at the first notch of the fir tree and the cracks initiated at pits induced by chloride attack. Examination of the blade microstructure at the fracture origins revealed oxide-filled pits and transgranular cracks. The oxide filled cracks appeared to have originated at small surface pits and probably propagated in a fatigue or corrosion-fatigue fracture mode. It was recommended that the sources of the chlorides be eliminated and that the remaining blades be inspected at regular maintenance intervals for evidence of cracking.