Search Results for Transgranular corrosion

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.

A segment of a stainless steel clad bottom cone of an acid sulfite pulping batch digester failed from severe corrosion loss. The digester was fabricated of 19 mm ( 3 4 in.) low-carbon steel with 3.8 mm (0.15 in.) type 317L stainless steel cladding. The manufacturing method for the cladding was unknown. Visual and metallographic analyses indicated that the failure was from transgranular stress-corrosion cracking (TGSCC), which caused extensive cracking and spalling of the cladding and was localized in a segment of the bottom cone. The remainder of the digester cladding was unaffected. The TGSCC was attributed to high, locked-in residual stresses from the cladding process. It was recommended that the bottom cone replacement segment be stress relieved prior to installation.

A large pressure vessel that had been in service as a hydrogen sulfide (H2S) absorber developed cracks and began leaking at a nozzle. The vessel contained a 20% aqueous solution of potassium hydroxide (KOH), potassium carbonate (K2CO3), and arsenic. The vessel wall was manufactured of ASTM A516, grade 70, low-carbon steel plate. A steel angle had been formed into a ring was continuously welded to the inside wall of the vessel. The groove formed by the junction of the lower tray-support weld and the top part of the weld around the nozzle was found to have a crack. Pits and scale near the crack origin were revealed by microscopic examination and cracking was found to be transgranular. Periods of corrosion alternated with sudden instances of cleavage, under a tensile load, along preferred slip planes were interpreted during examination with a microscope. It was concluded that the combination of the residual plus operating stresses and the amount of KOH present would have caused stress corrosion as a result of caustic embrittlement. It was recommended that the tray support should be installed higher on the vessel wall to prevent coincidence of the lower tray-support weld with the nozzle weld.

A 5000-gal (20,000-L) hot-water holding tank fractured at a large automotive manufacturing plant. The tank was made from Type 304 austenitic stainless steel. The inner diameter of the tank displayed a macroscopic, web-like network of cracks that deceptively suggested intergranular stress-corrosion cracking. The problem, however, originated on the outside surface of the tank where a tensile stress (due to low applied stress and fabrication-induced residual stresses) accelerated the growth of numerous stress corrosion cracks that eventually broke through to the inner surface, causing leakage and ultimately failure.

Leaks developed in 22 admiralty brass condenser tubes. The tubes were part of a condenser that was being used to condense steam from a nuclear power plant and had been in operation for less than 2 years. Analysis identified three types of failure modes: stress-corrosion cracking, corrosion under deposit (pitting and crevice), and dezincification. Fractures were transgranular and typical of stress-corrosion cracking. The primary cause of the corrosion deposit was low-flow conditions in those parts of the condenser where failure occurred. Maintenance of proper flow conditions was recommended.

An 8 in. diam stainless steel black liquor feed pipe to a carbon steel digester had failed within one year of service. The material was type 316 molybdenum-containing austenitic stainless steel. The service environment was alkaline black liquor at 175 deg C (350 deg F). The pipe had developed cracks on the inside surface coincident with an external support gusset. The cracks initiated at wide corrosion grooves. The early stages were corrosion-assisted fatigue cracks. The cracks initiated at the corrosion grooves and propagated as transgranular SCC with characteristic branching. Evaluation indicated the cracks were localized in an area of high cyclic stresses as a consequence of geometrical constraints on the piping and unsupported cantilever loads. No cracks were found elsewhere in the pipe. In the absence of highly localized service stresses (exceeding yield strength of the material), the corrosion grooving and subsequent SCC would not have occurred in this service environment. The pipe support system was modified with additional gussets to reduce the magnitude of cyclic stresses at the critical areas. The modification was apparently successful.

A process vessel heating coil, consisting of several 3 ft diam turns, was supplied with steam at 400 psi and a temperature of 343 deg C (650 deg F). At bi-weekly intervals well water was introduced to effect rapid cooling of the contents. After about eight months, leakage developed from a circumferential crack on the underside of the uppermost turn. Shorter cracks were found at a similar location on the bottom turn, and further leakage occurred at pinhole perforations adjacent to the crack in the top turn and near to a butt-weld in the coil. Microscopic examination revealed that the cracks were predominantly of the intergranular variety. In addition, transgranular cracks were present. Material was an austenitic stainless steel of the type specified but the absence of columbium and titanium in significant amounts showed that it was not stabilized against intergranular carbide precipitation. The transgranular cracks indicated that failure was due partly to stress-corrosion. It was concluded that the chlorides provided the main corrodent for both the stress and intercrystalline-corrosion cracking.

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.

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.

A pair of steam generators operating at a pressurized water reactor site were found to be leaking near a closure weld. The generators were the vertical U-tube type, constructed from ASTM A302 grade B steel. The shell material exhibited high hardness values prior to confirmatory heat treatment, indicating high residual stresses in the area of the weld. All cracks were transgranular and were associated with pits on the inside surfaces of the vessels. It was concluded that the cracking was caused by a low-cycle corrosion fatigue phenomenon, with cracks initiating at areas of localized corrosion and propagating by fatigue. The cause of the pitting/cracking was related to the unit's copper species in solution.

Field metallography and replication were performed on a type 316 stainless steel column in diglycol amine vacuum service to determine the cause of visible OD pitting on the column in several areas above the insulation support rings. The examination revealed transgranular stress-corrosion cracking beneath the pitted areas on the OD. The likely cause of the cracking was chloride stress corrosion, with chlorides deriving from the marine atmosphere and concentrating under the insulation around the support rings. A complete insulation evaluation, including repair or replacement, was recommended to prevent chloride buildup. Painting of the steel surface with an epoxy-phenolic or epoxy-coal tar was also suggested.

Weld-decay and stress-corrosion cracking developed in several similar all-welded vessels fabricated from austenitic stainless steel. During a periodic examination cracks were revealed at the external surface of one of the vessels. External patch welds had been applied at these and several other corresponding locations. Cracks visible on the external surface developed from the inside in a region close to the toe of the internal fillet weld to the deflector plate, and another deep crack associated with a weld cavity is visible slightly to the right of the main fissure. Microscopic examination revealed that precipitation of carbides at the grain boundaries had taken place in the vicinity of the cracks, but that the paths of the cracks were not wholly intergranular. Conditions present in the vicinity of the internal fillet weld must have been such as to favor both inter- and transgranular cracking. It is probable that the heating associated with the repair welds made from time to time also contributed to the trouble. The transgranular cracks, however, were indicative of stress-corrosion cracking.

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.

After being in service for ten years, two admiralty brass heat-exchanger tubes from a cooler in a refinery catalytic reforming unit cracked circumferentially in the area of U-bends. A blunt transgranular cracking with minimal branching propagating from the inside surface of the tube was revealed by metallography which was typical of cracking by corrosion fatigue mechanism. Corrosion deposits on both the inside- and outside-diam surfaces were found in the tubes. The presence of copper, zinc, iron, and small amounts of chloride, sulfur, silicon, tin, and manganese was revealed by energy-dispersive analysis of the deposits. It was interpreted by the hardness values (higher than typical for annealed copper tubing) that the tubes may not have been annealed after the U-bends were formed and thus the role of residual stresses in the crack was revealed. It was concluded that the tubes failed by corrosion fatigue initiated by pitting at the inside-diam surface. The tubes were recommended to be annealed after bending to reduce residual stresses from the bending operation to an acceptable level.