An E-Brite /Ferralium explosively bonded tube sheet in a nitric acid condenser was removed from service because of corrosion. Visual and metallographic examination of tube sheet samples revealed severe cracking in the heat-affected zone between the outer tubes and the weld joining the tube sheet to the floating skirt. Cracks penetrated deep into the tube sheet, and occasionally into the tube walls. The microstructures of both alloys and of the weld appeared normal. Intergranular corrosion characteristic of end-grain attack was apparent. A low dead spot at the skirt / tube sheet joint allowed the Nox to condense and subsequently reboil. This, coupled with repeated repair welding in the area, reduced resistance to acid attack. Intergranular corrosion continued until failure. Recommendations included changing operating parameter inlet to prevent HNO3 condensation outside the inlet and replacement of the floating skirt with virgin material (i.e., material unaffected by weld repairs).

A flanged 100 mm (4 in.) diam low-carbon steel spool piece lined with Teflon was removed from a sulfuric acid denitrification system after cracks were observed in the painted coating. Visual and microstructural examination along with SEM fractography revealed scaled iron oxides on all opened crack surfaces. The surfaces had a faceted morphology, indicating intergranular fracture. Cracks originated at the interface between the tube and the Teflon liner Corrosion products were found caked into the intergranular region between the liner and the spool. The portion of the liner that had been exposed to the process stream was discolored. Failure of the spool was attributed to stress-corrosion cracking promoted by the presence of nitrates. Nitric acid contaminant in the sulfuric acid stream had diffused through the liner and accumulated in the annular space. Use of a liner that is more impermeable to the diffusion of ionic species was recommended.

An air compressor was installed at a chemical plant in which nitric acid was produced by burning ammonia with air. It was a 5000 hp, 5-stage centrifugal machine running at 6000 rpm, compressing air to 5 atm. Failure of the first stage impeller occurred due to a segment from the back plate becoming detached. On the remaining portion, cracks were visible running between the holes for rivets by which the vanes were attached. Metallographic examination of selected sections from the backplate revealed the material to be in the hardened and tempered condition, and the cracking to have initiated on the internal surface of the plate at the crevice between the plate and the vane. It was evident that the impeller failed by stress-corrosion cracking, which initiated in the crevice between the vanes and back plate and propagated through the plate along the line of the rivets where working stresses would be greatest. The compressor intake was situated in the vicinity of nitric acid pumps which had a history of leakage troubles, and which had evidently given rise to the nitrates found on the impeller.

This investigation involved two AISI 304L acid storage tanks and one AISI 304L spent solvent tank from a sewage treatment facility. After installation, these tanks were hydrostatically tested using sewage effluent. No leaks were found and after a year or two, the tanks were drained and filled with nitric acid in preparation for service. Three weeks later the two acid tanks were found to be leaking from the bottom. Samples from the spent solvent tank revealed that pitting was located in a depressed area near a suction hole, beneath a black residue. It was concluded that the acid tanks failed by chloride-induced pitting initiated by microbial activity. Further, the spent solvent tank failed by a similar, but anaerobic mechanism. The use of the effluent for the hydrostatic test and the failure to remove it and clean and dry the tanks was the primary cause of failure. Localized carbide segregation in the original plate served as preferential corrosion sites. Had the tanks been hydrostatically tested in a proper manner, the pitting may not have occurred.

Cracks, with no other damage, were observed in a niobium alloy (Nb-106) part when it was pulled from several months of protective storage for assembly into a rocket nozzle. SEM views showed the cracks to be intergranular, with contaminant particles on a large number of the grain facets. EDX analysis showed they consisted of niobium and fluorine. Plastic replicas, prepared by standard TEM techniques, were analyzed with selected-area electron diffraction, showing a pattern match for niobium tetrafluoride. Auger analyses showed electron spectra containing peaks representing carbon, oxygen, nitrogen, fluorine, and chlorine. Investigation into the processing history of the part showed the tenacious oxide film formed by the affinity of niobium for oxygen - even when heat treated in a vacuum – was removed with a combination of strong acids: nitric, hydrochloric, hydrofluoric, and lactic, resulting in the contaminants found on the surface. Thus, residues of the cleaning acid on the part had caused SCC during storage, with the tensile stresses necessary to generate SCC assumed to have been residual stresses from the heat treatment. Recommendation was made that more stringent cleaning procedures to remove any trace of the cleaning acids be used.

Several nickel-base superalloy (UNS N06600) welded heat-exchanger tubes used in processing black liquor in a kraft paper mill failed prematurely. Leaking occurred through the tube walls at levels near the bottom tube sheet. The tubes had been installed as replacements for type 304 stainless steel tubes. Visual and stereoscopic examination revealed three types of corrosion on the inside surfaces of the tubes: uniform attack, deeper localized corrosive attack, and accelerated uniform attack. Metallographic analysis indicated that pronounced dissimilar-metal corrosion had occurred in the base metal immediately adjacent to the weld seam. The corrosion was attributed to exposure to nitric acid cleaning solution and was accelerated by galvanic differences between the tubes and a stainless steel tube sheet and between the base metal of the tubes and their dendritic weld seams. A change to type 304 stainless steel tubing made without dendritic weld seams was recommended.

Numerous cracks observed on the surface of a forged A470 Class 4 alloy steel steam turbine rotor disc from an air compressor in a nitric acid plant were found to be the result of caustic induced stress-corrosion cracking (SCC). No material defects or anomalies were observed in the disc sample that could have contributed to crack initiation or propagation or secondary crack propagation. Chlorides detected in the fracture surface deposits were likely the primary cause for the pitting observed on the disc surfaces and within the turbine blade attachment area. It was recommended that the potential for water carryover or feedwater induction into the turbine be addressed via an engineering evaluation of the plant's water treatment procedures, steam separation equipment, and start-up procedures.

This article commences with a discussion on the characteristics of stress-corrosion cracking (SCC) and describes crack initiation and propagation during SCC. It reviews the various mechanisms of SCC and addresses electrochemical and stress-sorption theories. The article explains the SCC, which occurs due to welding, metalworking process, and stress concentration, including options for investigation and corrective measures. It describes the sources of stresses in service and the effect of composition and metal structure on the susceptibility of SCC. The article provides information on specific ions and substances, service environments, and preservice environments responsible for SCC. It details the analysis of SCC failures, which include on-site examination, sampling, observation of fracture surface characteristics, macroscopic examination, microscopic examination, chemical analysis, metallographic analysis, and simulated-service tests. It provides case studies for the analysis of SCC service failures and their occurrence in steels, stainless steels, and commercial alloys of aluminum, copper, magnesium, and titanium.

On 13 Dec 1994, two massive detonations leveled portions of an ammonium nitrate plant near Sioux City, IA. The primary explosion allegedly occurred in defectively-designed titanium sparger piping inside the neutralizer vessel. Investigation however, revealed the explosion occurred because of unsafe plant operations and poor maintenance procedures. Specifically, the ammonium nitrate within the 18,000 gal capacity neutralizer vessel had become contaminated and made highly acidic. The operators then injected superheated steam directly into the ammonium nitrate in the neutralizer vessel.

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.

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.

This article provides an overview of the electrochemical nature of corrosion and analyzes corrosion-related failures. It describes corrosion failure analysis and discusses corrective and preventive approaches to mitigate corrosion-related failures of metals. These include: change in the environment; change in the alloy or heat treatment; change in design; use of galvanic protection; use of inhibitors; use of nonmetallic coatings and liners; application of metallic coatings; use of surface treatments, thermal spray, or other surface modifications; corrosion monitoring; and preventive maintenance.

Corrosion is the deterioration of a material by a reaction of that material with its environment. The realization that corrosion control can be profitable has been acknowledged repeatedly by industry, typically following costly business interruptions. This article describes the electrochemical nature of corrosion and provides the typical analysis of environmental- and corrosion-related failures. It presents common methods of testing of laboratory corrosion and discusses the processes involved in the prevention of environmental- and corrosion-related failures of metals and nonmetals.

This article describes the characteristics of tubing of heat exchangers with respect to general corrosion, stress-corrosion cracking, selective leaching, and oxygen-cell attack, with examples. It illustrates the examination of failed parts of heat exchangers by using sample selection, visual examination, microscopic examination, chemical analysis, and mechanical tests. The article explains corrosion fatigue of tubing of heat exchangers caused by aggressive environment and cyclic stress. It also discusses the effects of design, welding practices, and elevated temperatures on the failures of heat exchangers.

The principal types of elevated-temperature mechanical failure are creep and stress rupture, stress relaxation, low- and high-cycle fatigue, thermal fatigue, tension overload, and combinations of these, as modified by environment. This article briefly reviews the applied aspects of creep-related failures, where the mechanical strength of a material becomes limited by creep rather than by its elastic limit. The majority of information provided is applicable to metallic materials, and only general information regarding creep-related failures of polymeric materials is given. The article also reviews various factors related to creep behavior and associated failures of materials used in high-temperature applications. The complex effects of creep-fatigue interaction, microstructural changes during classical creep, and nondestructive creep damage assessment of metallic materials are also discussed. The article describes the fracture characteristics of stress rupture. Information on various metallurgical instabilities is also provided. The article presents a description of thermal-fatigue cracks, as distinguished from creep-rupture cracks.

Failures of various types of hydraulic couplings used to connect pipes in a naval vessel are described and used to illustrate some of the general procedures for failure analysis. Cracking of couplings, which were manufactured from nickel-aluminum- bronze extruded bar, occurred in both seawater and air environments. Cracks initiated at an unusually wide variety of sites and propagated in either longitudinal or circumferential directions with respect to the axis of the couplings. Fracture surfaces were intergranular and exhibited little or no sign of corrosion (for couplings cracked in air), and there was very limited plasticity. Macroscopic progression markings were observed on fracture surfaces of several couplings but were not generally evident. At very high magnifications, numerous slip lines, progression markings, and striations were observed. In a few cases, where complete separation had occurred in service, small areas of dimpled overload fracture were observed. It was concluded from these observations, and from comparisons of cracks produced in service with cracks produced by laboratory testing under various conditions, that cracking had occurred by fatigue. The primary cause of failure was probably the unanticipated presence of high-frequency stress cycles with very low amplitudes, possibly due to vibration, resonance, or acoustic waves transmitted through the hydraulic fluid. Secondary causes of failure included the presence of high tensile residual stresses in one type of coupling, undue stress concentrations at some of the crack-initiation sites, and overtorquing of some couplings during installation. Recommendations on ways to prevent further failures based on these causes are discussed.

This article reviews the general characteristics of fretting wear in mechanical components with an emphasis on steel. It focuses on the effects of physical variables and the environment on fretting wear. The variables include the amplitude of slip, normal load, frequency of vibration, type of contact and vibration, impact fretting, surface finish, and residual stresses. The form, composition, and role of the debris are briefly discussed. The article also describes the measurement, mechanism, and prevention of fretting wear. It concludes with several examples of failures related to fretting wear.

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.

Routine inspections of a carbon steel wood pulp digester revealed a sharply increasing number of cracks in the overlay metal on the internal surface of the digester after 1 and 2 years of service. The weld overlay was composed of type 309 stainless steel on the top fourth of the digester and of a proprietary high-nickel material on the bottom three-fourths. Examination revealed three distinct modes of deterioration. General corrosion was linked to the use of unspecified overlay metal. Cracking resulted during installation from the use of a material susceptible to hot cracking. Deep corrosion fissures then developed at hot crack sites as a result of crevice corrosion. Use of the appropriate overlay material was recommended.

Cracks occurred in a new ship hull after only three months in service. It was noted that the 5xxx series of aluminum alloys are often selected for weldability and are generally very resistant to corrosion. However, if the material has prolonged exposure at slightly elevated temperatures of 66 to 180 deg C (150 to 350 deg F), an alloy such as 5083 can become susceptible to intergranular corrosion. Investigation (visual inspection, corrosion testing, SEM images) supported the conclusion that the cracks occurred because during exposures to chloride solutions like seawater, galvanic couples formed between precipitates and the alloy matrix, leading to severe intergranular attack. No recommendations were made.