A brackish water pump impeller was replaced after four years of service, while its predecessor lasted over 40 years. The subsequent failure investigation determined that the nickel-aluminum bronze impeller was not properly heat treated, which made the impeller susceptible to aluminum dealloying. The dealloying corrosion was exacerbated by erosion because the pump was slightly oversized. The investigation recommended better heat treating procedures and closer evaluation to ensure that new pumps are properly sized.

A fan support casting failed unexpectedly while running at 1800 rpm in pulp at 65 deg C (150 deg F). The leading edge of the blade exhibited deep spongy holes leading to reduced section and finally to fracture of the part when the remaining section size was insufficient to support the load. Analysis showed the support casting to be a standard 8620 type composition with a hardness of 311 HRB. The design of the casting was not streamlined. There were several square corners present where great pressure differences could be generated. This was a case of erosion-corrosion with the classic spongy appearance of cavitation. Two changes were proposed: streamlining the part to avoid abrupt changes in fluid flow; and a change in alloy to a more corrosion-resistant material (304 or preferably 316) to increase the tenacity of protective films.

Copper alloy (C83600) impellers from two different feed pumps that supplied water to a 2-year-old boiler failed repeatedly. Examination by various methods indicated that the failures were caused by sulfide attack that concentrated in shrinkage voids in the castings. Two alternatives to prevent future failures were recommended: changing the impeller composition to a cast stainless steel, or implementing stricter nondestructive evaluation requirements for copper alloy castings.

A microstructural analysis has been made of a burner nozzle removed from service in a coal gasification plant. The nozzle was a casting of a Co-29wt%Cr-19wt%Fe alloy. Extensive hot corrosion had occurred on the surface. There was penetration along grain boundaries, and corrosion products in these regions were particularly rich in S, and also contained Al, Si, O, and Cl. The grain boundaries contained Cr-rich particles which were probably Cr23-C6 type carbides. In the matrix, corrosion occurred between the Widmanstatten plates. Particles were found between these plates, most of which were rich in Cr and O, and probably were Cr2-O3 oxides. Other matrix particles were found which were rich in Al, O, and S. The corrosion was related to these grain boundary and matrix particles, which either produced a Cr-depleted zone around them or were themselves attacked.

Two suction rolls at the first press section of a 25 ft. wide paper machine developed cracks within two years of service. The rolls were austenitic stainless steel castings made of ASTM A 351 Grade CF8M alloy containing molybdenum. The rolls were exposed to slightly acidic white water (pH approximately 4.7) containing chlorides (45 ppm). Visual and liquid penetrant inspections of the rolls revealed extensive cracking at the roll inside surface. The cracks penetrated more than 30 percent of the wall thickness and a few cracks were several inches long. The cracks were preferentially oriented along the roll length and primarily at the roll inside surface. Field metallographic examination showed significant grain boundary chromium-carbide precipitation and intergranular corrosion. The roll failures were attributed to chromium depletion along the grain boundaries (sensitization) resulting from slow cooling of the casting to avoid large residual stresses. The roll manufacturer recommended a proprietary ferritic/austenitic stainless steel as the replacement material for the rolls.

This article addresses the forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. It describes the mechanisms of corrosive attack for specific forms of corrosion such as galvanic corrosion, uniform corrosion, pitting and crevice corrosion, intergranular corrosion, and velocity-affected corrosion. The article contains a table that lists combinations of alloys and environments subjected to selective leaching and the elements removed by leaching.

A 1-in. diam pump spindle fractured within the length covered by the boss of the impeller which was attached to the spindle by means of an axial screw. The pump had been in use in a chemical plant handling mixtures of organic liquids and dilute sulfuric acid having a pH value of 2 to 4 at temperatures of 80 to 90 deg C (176 to 194 deg F). The fracture was unusual in that it was of a fibrous nature, the fibers-which were orientated radially-were readily detachable. The surface of the spindle adjacent to the fracture had an etched appearance and the mode of cracking in this region suggested that failure resulted from an intergranular attack. Subsequent microscope examination confirmed the generally intergranular mode of failure. A macro-etched section near the fracture revealed a radial arrangement of columnar crystals, indicating that the spindle was a cast and not a wrought product as had been presumed. Spectroscope examination showed this particular composition (Fe-23Cr-18Ni-1.8Mo-1.2Si) did not conform to a standard specification and is apparently a proprietary alloy. It was evident that the particular mode of failure was related to the inherent structure of the material.

Corrosion is the electrochemical reaction of a material and its environment. This article addresses those forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. Various forms of corrosion covered are galvanic corrosion, uniform corrosion, pitting, crevice corrosion, intergranular corrosion, selective leaching, and velocity-affected corrosion. In particular, mechanisms of corrosive attack for specific forms of corrosion, as well as evaluation and factors contributing to these forms, are described. These reviews of corrosion forms and mechanisms are intended to assist the reader in developing an understanding of the underlying principles of corrosion; acquiring such an understanding is the first step in recognizing and analyzing corrosion-related failures and in formulating preventive measures.

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.

A recuperator for blast heating of a cupola furnace became unserviceable because of the brittle fracture of several finned tubes made of heat resistant cast steel containing 1.4C, 2.3Si and 28Cr. The service temperature was reported as 850 deg C. This led to the suspicion that the fracturing had something to do with the precipitation of sigma phase. Metallographic examination showed that the multiaxial stresses caused by sigma phase formation and the related embrittlement was the cause for the fracture of the recuperator. A steel of lower chromium content with no or little tendency for sigma phase formation would have had adequate corrosion resistance at the relatively low service temperature.

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.

Post-service destructive evaluation was performed on two commercially pure zirconium pump impellers. One impeller failed after short service in an aqueous hydrochloric acid environment. Its exposed surfaces are bright and shiny, covered with pockmarks, and peppered with pitting. Uniform corrosion is evident and two deep linear defects are present on impeller blade tips. In contrast, the undamaged impeller surfaces are covered with a dark oxide film. This and many other impellers in seemingly identical service conditions survive long lives with little or no apparent damage. No material or manufacturing defects were found to explain the different service performance of the two impellers. Microstructure, microhardness and material chemistry are consistent with the specified material. Examination reveals the damage mechanism to be corrosion-enhanced cavitation erosion, the most severe form of erosion corrosion. Cavitation damage to the protective oxide film caused the zirconium to lose its normally outstanding corrosion resistance. The root cause of the impeller failure is most likely the introduction of excessive air into the pump due to low liquid level, a bad seal or inadequate head. Corrosion pitting, crevice corrosion, and solidification cracks (casting defect) also contributed to the failure.

After operating for six months, a pump impeller (of nickel-containing cast iron) showed considerable corrosion. Cross sections showed substantial penetration of the wall thickness without loss of material. The observed supercooled structure implied low strength but would not affect corrosion resistance. Etching of the core structure showed a selective form of cast iron corrosion (spongiosis or graphitic corrosion) which lowered the strength of the cast iron enough that a knife could scrape off a black powder (10.85% C, 1.8% S, 1.45% P). Analysis showed that some of the “sulfate” found in the scrubbing water was actually sulfide (including hydrogen sulfide) and was the main cause of corrosion.

Practical examples of stress-corrosion cracking (SCC) and methods for its prevention were presented. Cracks in chloride-sensitive austenitic steels were very branched and transcrystalline. Etched cross sections of molybdenum-free samples showed chloride-induced cracks running out of the pitted areas. Alternatively polishing and etching micro-sections for viewing at high magnification made crack detail more visible. Optical and scanning electron micrographs showed cracking in austenitic cast steel and cast iron due to both internal tensile and critical residual stresses; the latter causes flake-like spalling. Measures to prevent SCC include stress reduction, use of austenitic steels or nickel alloys not susceptible to grain boundary attack, use of ferritic chromium steels, surface slag removal, control of temperature and chloride concentration, and cathodic protection.

Grate bars in the traveling grate indurators in several taconite processing units suffered excessive corrosion following a conversion from acid to fluxed pellet production procedures. The campaign life of the HH grade cast stainless steel bars was reduced from more than 7 years to approximately 9 months. Several corroded grate bars were examined metallographically and by electron microscopy to determine the causes of the accelerated corrosion. Chemical and X-ray diffraction analyses were also conducted, along with simulation tests to assess the role of alkali chlorides in the corrosion process. The basic cause of degradation was found to be hot corrosion caused by the deposition of alkali sulfates and chlorides. However this degradation may have been aggravated by thermal cycling and abrasion. The source of the salt was impurities in the flux. Two potential solutions were proposed: modification of the processing parameters to reduce the salt deposition and / or change of bar materials to a more resistant alloy.

The information provided in this article is intended for those individuals who want to determine why a casting component failed to perform its intended purpose. It is also intended to provide insights for potential casting applications so that the likelihood of failure to perform the intended function is decreased. The article addresses factors that may cause failures in castings for each metal type, starting with gray iron and progressing to ductile iron, steel, aluminum, and copper-base alloys. It describes the general root causes of failure attributed to the casting material, production method, and/or design. The article also addresses conditions related to the casting process but not specific to any metal group, including misruns, pour shorts, broken cores, and foundry expertise. The discussion in each casting metal group includes factors concerning defects that can occur specific to the metal group and progress from melting to solidification, casting processing, and finally how the removal of the mold material can affect performance.

A desuperheater diffuser nozzle in the steam supply line failed within nine months of service in an 8.25 MN/sq m (1200 psig) steam line. The nozzle was an austenitic stainless steel casting in conformance to material. The nozzle had numerous cracks on the inside and outside surfaces, and the cracks had penetrated through the wall thickness in several areas. The fracture surfaces had distinct beach markings delineating the crack front, representative of crack propagation stages. The cracks were transgranular and, unlike classical corrosion-fatigue cracks, exhibited branching, characteristic of chloride-induced SCC in austenitic stainless steels. The failure resulted from chloride-induced SCC, possibly assisted by cyclic stress. The recommendation for alternate material for the desuperheater nozzle included nickel base alloys per ASTM B 564, Grades 600 or 800 titanium alloy per ASTM B 367, Grades C3/C4, or ferritic stainless steel alloy per ASTM 182, Grade FXM27.

Material samples collected from failed booster pumps were analyzed to determine the cause of failure and assess the adequacy of the materials used in the design. The pumps had been in service at a power plant, transporting feedwater from a deaerator to a main turbine boiler. Samples from critical areas of the pump were examined using optical and scanning electron microscopy, electrochemical analysis, and tensile testing. Based on microstructure and morphology, estimated corrosion rates, and particle concentrations in the feedwater, it was concluded that cavitation and erosion were the dominant failure mechanisms and that the materials and processes used to make the pumps were largely unsuited for the application.

This article presents a failure analysis of 37.5 mW gas turbine third stage buckets made of Udimet 500 superalloy. The buckets experienced repetitive integral tip shroud fractures assisted by a low temperature (type II) hot corrosion. A detailed analysis was carried out on elements thought to have influenced the failure process: a) the stress increase from the loss of a load bearing cross-sectional area of the bucket tip shroud by the conversion of metal to the corrosion product (scale), b) influence of the tip shroud microstructure (e.g., a presence of equiaxed and columnar grains, their distribution and orientation), c) evidence of the transgranular initiation, and d) intergranular creep mechanism propagation. The most probable cause of the bucket damage was the combination of increased stresses due to corrosion-induced thinning of the tip shroud and unfavorable microstructures in the tip shroud region.

A failure analysis was conducted in late 1996 on two rolls that had been used in the production of iron and steel powder. The rolls had elongated over their length such that the roll trunnions had impacted with the furnace wall refractory. The result was distortion and bowing of the roll bodies which necessitated their removal from service. The initial analysis found large quantities of nitrogen had been absorbed by the roll shell. Further research indicated nitrogen pickup accounted for 3% volumetric growth for every 1% by weight nitrogen absorption. This expansion was sufficient to account for the dimensional change observed in the failed rolls. This paper details the failure analysis and resulting research it inspired. It also provides recommendations for cast material choice in highly nitriding atmospheres.