A group of control valves that regulate production in a field of sour gas wellheads performed satisfactorily for three years before pits and cracks were detected during an inspection. One of the valves was examined using chemical and microstructural analysis to determine the cause of failure and provide preventive measures. The valve body was made of A216-WCC cast carbon steel. Its inner surface was covered with cracks stemming from surface pits. Investigators concluded that the failure was caused by a combination of hydrogen-induced corrosion cracking and sulfide stress-corrosion cracking. Based on test data and cost, A217-WC9 cast Cr–Mo steel would be a better alloy for the application.

Nineteen out of 26 bolts in a multistage water pump corroded and cracked after a short time in a severe working environment containing saline water, CO 2 , and H 2 S. The failed bolts and intact nuts were to be made from a special type of stainless steel as per ASTM A 193 B8S and A 194. However, the investigation (which included visual, macroscopic, metallographic, SEM, and chemical analysis) showed that austenitic stainless steel and a nickel-base alloy were used instead. The unspecified materials are more prone to corrosion, particularly galvanic corrosion, which proved to be the primary failure mechanism in the areas of the bolts directly exposed to the working environment. Corrosion damage on surfaces facing away from the work environment was caused primarily by chloride stress-corrosion cracking, aided by loose fitting threads. Thread gaps constitute a crevice where an aggressive chemistry is allowed to develop and attack local surfaces.

Three separate corrosion mechanisms were involved in the failure of an AISI type 304 stainless steel pipe elbow. The major cracks, including the one that penetrated the wall, tend to be wide-mouthed, tapering to a blunt tip, with corrosion products filling much of the crack space. This was characteristic of corrosion fatigue. The second type of cracking originated at some of the major cracks. These cracks were branched and transgranular, which is characteristic of stress-corrosion caused by chlorides. The third crack mode, an intergranular network, was most probably the result of hydrogen sulphide attack. The 13-year service life of the elbow made it difficult, if not impossible, to determine the order of the corrosion mechanisms or the length of time it took to reach the present state of degradation after the initiation of corrosion. Based on the long service life the present material has given, it was recommended that it be used again.

Some of the admiralty brass tubes were failing in a heat exchanger. The heat exchanger cooled air by passing river water through the inside of the tubes. The wall thickness of all tubes ranged between 1.19 to 1.27 mm (0.047 to 0.050 in.). General intergranular corrosion occurred at the inside surfaces of the tubes. Transgranular stress-corrosion cracking, probably the result of sulphates under basic conditions, and dezincification occurred also as the result of galvanic corrosion under the deposits in the tubes. Recommendations were to use a closed-loop water system to eliminate sulphates, ammonia, etc., and to run trials on one unit with tubes of other alloys such as 80-20 Cu-Ni or 70-30 Cu-Ni to evaluate their performance prior to any large scale retubing operations.

Copper electrical feedthrough pins used in a bolting application in a refrigeration compressor had functioned without failure for years of production and thousands of units. When some of the pins began to fail, an investigation was conducted to determine the cause. Visual examination revealed that the observed fractures were mixed brittle intergranular with ductile microvoid dimples. An extensive analysis of failed samples combined with a process of elimination indicated that the fractures were due to stress-corrosion cracking caused by an unidentified chemical species within the sealed compressor chamber. A unique combination of applied stress, residual stress, stress riser, and grain size helped isolate the failure mechanism to a single production lot of material.

Small paddles used to mix pulp had experienced a high incidence of breakage through the shafts. In some of the shanks, shrinkage was found relatively close to the surface where threads had been cut all the length of the shaft. Chemistries were within normal CF-8M ranges. Metallography showed the parts to be correctly heat treated. Cross sections of several of the parts showed pitting corrosion, and beneath the pits, stress-corrosion cracks in areas where the shafts had been bent during use. All the samples showed deep SCC in the areas where bending had occurred. In several cases, centerline shrinkage from inadequate risering had decreased life by reducing the cross-sectional area. Type CF-8M is not resistant to chloride SCC where the chloride concentration is considerable. The biggest problem was the bending of these parts. Deformed material with high residual stresses would always be susceptible to SCC. Redesign to lower stresses was essential. In addition, change to a high-strength duplex stainless steel with its higher strength and greater resistance to chlorides was recommended. Finally, the part must be adequately risered to produce solid shanks free from shrinkage.

A CF-8M (cast type 316) neck liner or manway was removed from the top of a digester vessel. Repeated attempts to repair the part in the field during its life cycle of many years had failed to keep the unit from leaking. The casting was a CF-8M modified with the molybdenum level at the top end of the range. The plate was standard 317L material. The filler metal was type 316, although marginal in molybdenum content. Investigation (visual inspection, chemical analysis, micrographs, and metallographic examination) supported the conclusion that the damage to the neck liner was due to Cl-SCC in an area of debris buildup. It appeared the original casting suffered SCC in a low-oxygen area high in chlorides from repeated wet/dry cycles where there was a buildup of debris. Recommendations included redesigning the neck liner to eliminate the abrupt change where there was debris buildup. If redesign was impossible, an alloy more resistant to Cl-SCC, such as a duplex stainless steel or a high-molybdenum (4 to 6%) austenitic stainless steel, should be used.

Several air heat exchangers failed in service in a pulp and paper operation. The tubes were made from AISI 316 stainless steel with an extruded aluminum fin mechanically bonded to the outside. Originally, the failures were blamed on poor tube to header welds. The units were sent back to the manufacturer for repair. Some of the units failed the hydrostatic test after they were repaired. Microscopic examination revealed the presence of branched transgranular cracks characteristic of stress-corrosion cracking. Only some of the tubes failed and these did so by stress-corrosion cracking. The most probable primary cause of the stress-corrosion cracking was local high residual stresses indicated by the areas of high hardness in the tubes. Low halogens in the water and airborne corrodents found normally in a pulp and paper mill were all that were required in the presence of high residual stresses in the tubes to initiate stress-corrosion cracking. Use of a low-carbon grade of stainless steel such as 316L was recommended to facilitate formation of the tube without producing excessive residual stresses. It was recommended also that failed units be segregated until it can be determined if the failure was related to operating pressure or some other unique cause.

The Pandia digester is a long cylindrical vessel which uses alkaline sulfite liquor to cook sawdust for pulping. The inlet cone was fabricated from AISI 304L stainless steel with E308 welds. Typical liquor concentration was approximately 80% NaOH, 20% Na2SO3 with chloride concentrations at 2 grams per liter. The operating pressures in the inlet cone were up to 1.2 MN/sq m (170 psig). The inlet cone had developed leaks within a year of service. Liquid penetrant inspection showed significant through-wall cracking near the fillet welds joining the bottom flange and side wall and the butt welds. Metallographic specimens were prepared from the welds to examine the microstructure and nature of the cracks. The cooking liquor at the inlet cone contained over ppm chlorides and was aggressive to 304 stainless steel. The cracking was identified as chloride-induced SCC. The inlet cone was replaced with an Inconel clad carbon steel inlet cone to combat the SCC.

A falling film black liquor evaporator consisted of flat twin plate heat exchangers and was used to increase black liquor solids content prior to its burning in the recovery boiler. Several plate heat exchangers were fabricated of AISI type 316L stainless steel by electric resistance welding. Cracks initiated at the inside surface of the welded areas and penetrated through the wall thickness. In several locations, the weld fractured and the plates separated with significant spring back, indicative of high residual stresses attributed to fabrication and weld procedures. The cracks had extended radially from the electric resistant weld into the base metal. Metallographic examination revealed the cracks were transgranular and branching, characteristic of SCC in austenitic stainless steels. The fracture surfaces had a brittle cleavage-like appearance, typical of SCC in austenitic stainless steels. Chlorides in the service environment were a contributory factor. The primary factor causing SCC localized at the electric resistant welds was substantial residual stresses as a result of fabrication procedures. It was recommended that the heat exchanger plates be subjected to stress-relief heat treatment following fabrication and welding.

An arsenical admiralty brass (UNS C44300) finned tube in a generator air cooler unit at a hydroelectric power station failed. The unit had been in operation for approximately 49,000 h. Stereomicroscopic examination revealed two small transverse cracks that were within a few millimeters of the tube end, with one being a through-wall crack. Metallographic examination of sections containing the cracks showed branching secondary cracks and a transgranular cracking mode. The cracks appeared to initiate in pits. EDS analysis of a friable deposit found on the inside diameter of the tube and XRD analysis of crystalline compounds in the deposit indicated the possible presence of ammonia. Failure was attributed to stress-corrosion cracking resulting from ammonia in the cooling water. It was recommended that an alternate tube material, such as a 70Cu-30Ni alloy or a titanium alloy, be used.

Two AISI type 316 stainless steel dished ends failed through the formation of intergranular stress-corrosion cracks (IGSCC) within a few months of service. The dished ends failed in the straight portions near the circumferential welds that joined the ends to the cylindrical portions of the vessel. Both dished ends were manufactured from the same batch and were supplied by the same manufacturer One of the dished ends had been exposed to sodium at 550 deg C (1020 deg F) for 500 h before failure due to sodium leakage was detected. The other dished end was used to fabricate a second vessel that was kept in storage for 1 year Clear evidence of sensitization was found in areas where IGSCC occurred. Sensitization was extensive in the dished end that had been exposed to sodium at high temperature, and it occurred in a narrow band similar to that typical of weld decay in the dished end that had been kept in storage. Solution annealing was recommended to relieve residual stress, thereby reducing the probability of failure. It was also recommended that the carbon content of the steel be lowered, i.e., that a 316L grade be used.

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.

Original carbon steel and subsequent replacement austenitic stainless steel superheater tube U-bend failures occurred in a waste heat boiler. The carbon steel tubes had experienced metal wastage in the form of caustic corrosion gouging, while the stainless steel tubes failed by caustic-induced stress-corrosion cracking. Sodium was detected by EDS in the internal deposits and the base of a gouge in a carbon steel tube and in the internal deposits of the stainless steel tube. The sodium probably formed sodium hydroxide with carryover moisture and caused the gouging, which was further aggravated by the presence of silicon and sulfur (silicates and sulfates). It was recommended that the tubes be replaced with Inconel 600 or 601, as a practical option until the carryover problem could be solved.

A type 430Ti stainless steel flue gas expansion joint cracked because of caustic-induced stress-corrosion cracking. Energy-dispersive X-ray spectroscope analysis of the fracture surface deposits revealed the presence of sodium and potassium—caustics in hydroxide form. Primary fracture surfaces were all similar in appearance, and a primary crack origin could not be identified. A secondary crack brought to fracture in the laboratory showed brittle, cleavage features rather than classic, tensile overload features. This suggested that the material was embrittled.

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.

A section of type 304 stainless steel pipe from a stand by system used for emergency injection of cooling water to a nuclear reactor failed during precommissioning. Leaking occurred in only one spot. Liquid penetrant testing revealed a narrow circumferential crack. Metallographic examination of the cracked area indicated stress-corrosion cracking, which had originated at rusted areas that had formed on longitudinal scratch marks on the outer surface of the pipe. The material was free from sensitization, and there was no significant amount of cold work. It was recommended that the stainless steel be kept rust free.

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.

A cold-formed Grade TP 304 stainless steel swaged region of a reheater tube in service for about 8000 hours cracked because of sulfur-induced stress-corrosion cracking (SCC). Cracking initiated from the external surface and a high sulfur content was detected in the outer diameter and crack deposits. Comparison of the microstructure and hardness of the swaged region and unswaged Grade TP 304 stainless steel tube metal indicated that the swaged section was not annealed to reduce the effects of cold working. The high hardness created during swaging increased the stainless steel's susceptibility to sulfur-induced SCC. Because SCC requires water to be present, cracking most likely occurred during downtime or startups. To prevent future failures, the boiler should be kept dry during downtime to avoid formation of sulfur acids, and the swaged sections of the tubes should be heat treated after swaging to reduce or eliminate strain hardening of the metal.

An aluminum bronze propeller tap bolt from a twin-screw vessel fractured just below the bolt head. Liquid penetrant testing revealed a large network of cracks that extended radially from sites in and just below the bolthead. Metallographic analysis indicated that the tap bolt failed by stress-corrosion cracking. It was surmised that seawater or some other corrosive substance was present in sufficient quantity to induce intergranular cracking at regions of high stress concentration. It was recommended that all tap bolts be replaced with new bolts made from an alloy with a higher copper content and at least the same yield strength. Steps to exclude seawater and any possible source of ammonia from the bolt shank were also suggested.