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.

Several mercury diffusion pump stages in the Tritium Purification process at the Savannah River Site (SRS) have been removed from service for scheduled preventive maintenance. These stages have been examined to determine if failure has occurred. Evidence of fatigue around the flange portion of the pump has been seen. In addition, erosion and cavitation inside the throat of the venturi tube and corrosion on the other surface of the venturi tube has been observed. Several measures are being examined in an attempt to improve the performance of these pumps. These measures, as well as the noted observations, are described. Six stages [two machined (MP) and four electron beam (EB) welded] from the mercury diffusion pumps operating in the Tritium Purification process at SRS have been analyzed to determine their condition after nine months of usage. Several cracks were found around the necked region of the two MP stages. The EB welded stages, however, seemed to perform better in service; only two of four stages showed cracking. The cracking is caused by fatigue that has been enhanced by high stresses and tritium in the flange area. The EB welded stage appears to be a step in the right direction. Since the EB weld is a shrink fit, the surface is in compression, thereby eliminating crack propagation. In addition, shot peening has been employed to produce a compressive material surface since fatigue usually originates at the surface. Pitting was observed down the throat of the venturi. This pitting was caused by cavitation and erosion along the length of the venturi tube. Corrosion and pitting was seen on the exterior walls of the diffuser tubes. Stress-corrosion cracks were observed emanating from these corrosion pits. The corrosion likely occurred from the chloride ions present in the process cooling water. Shot peening is now being used in an attempt to place the outside of the diffuser tube in compression to eliminate the stress-corrosion cracking.

Cracking was found in the heads on large Yankee dryers, large, cylindrical, rotating, pressurized, high-temperature, cast iron pressure vessels (ASME Boiler and Pressure Vessel Code Section VIII, Rules for Construction of Pressure Vessels), used to remove moisture from sheets of tissue paper during manufacturing. The typical components consist of a cast iron shell, two cast iron concave heads, and a large cast iron internal center stay attached to journals. The heads are attached to the shell and center stay with high-strength bolts. FEA and metallurgical investigation supported the conclusion that the cracking was caused by an unexpected type of load placed on the machine, namely corrosion product buildup at the head/shell interface causing the joint to displace open. It was also found that compressive bolting loads could slightly open the head/shell interface at the periphery. Recommendations included design changes in the head/shell joint, and detailed preventive maintenance inspection procedures were also suggested.

The US Army Research Laboratory performed a failure investigation on a broken main landing gear mount from an AH-64 Apache attack helicopter. A component had failed in flight, and initially prevented the helicopter from safely landing. In order to avoid a catastrophe, the pilot had to perform a low hover maneuver to the maintenance facility, where ground crews assembled concrete blocks at the appropriate height to allow the aircraft to safely touch down. The failed part was fabricated from maraging 300 grade steel (2,068 MPa [300 ksi] ultimate tensile strength), and was subjected to visual inspection/light optical microscopy, metallography, electron microscopy, energy dispersive spectroscopy, chemical analysis, and mechanical testing. It was observed that the vacuum cadmium coating adjacent to the fracture plane had worn off and corroded in service, thus allowing pitting corrosion to occur. The failure was hydrogen-assisted and was attributed to stress corrosion cracking (SCC) and/or corrosion fatigue (CF). Contributing to the failure was the fact that the material grain size was approximately double the required size, most likely caused from higher than nominal temperatures during thermal treatment. These large grains offered less resistance to fatigue and SCC. In addition, evidence of titanium-carbo-nitrides was detected at the grain boundaries of this material that was prohibited according to the governing specification. This phase is formed at higher thermal treatment temperatures (consistent with the large grains) and tends to embrittle the alloy. It is possible that this phase may have contributed to the intergranular attack. Recommendations were offered with respect to the use of a dry film lubricant over the cadmium coated region, and the possibility of choosing an alternative material with a lower notch sensitivity. In addition, the temperature at which this alloy is treated must be monitored to prevent coarse grain growth. As a result of this investigation and in an effort to eliminate future failures, ARL assisted in developing a cadmium brush plating procedure, and qualified two Army maintenance facilities for field repair of these components.

Reliability-centered maintenance (RCM) is a systematic methodology for preventing failures. This article discusses the history of RCM and describes the key characteristics of an RCM process, which involves asking seven questions. The first four questions comprise a form of failure modes and effects analysis (FMEA), and therefore, the article explains the approach of RCM to FMEA and the failure management policies available under RCM. It reviews the ways that RCM classifies failure effects in terms of consequences and details how RCM uses failure consequences to identify the best failure management policy for each failure mode. The article concludes with a discussion on some practical issues pertaining to RCM that lie outside the scope of SAE JA1011.

Reliability-centered maintenance (RCM) is a systematic methodology for preventing failures. This article begins by discussing the history of RCM and uses Society of Automotive Engineers (SAE) all-industry standard JA1011 as its model to describe the key characteristics of an RCM process. It then expands on questions involved in RCM process, offering definitions when necessary. Next, the article describes the approach of RCM to failure modes and effects analysis (FMEA), the failure management policies available under RCM, and the criteria of RCM for deciding when a specific failure management policy is technically feasible. Then, after discussing the ways that RCM classifies failure effects in terms of consequences, it describes how RCM uses failure consequences to identify the best failure management policy for each failure mode. Next, the building blocks of RCM are put together to create a failure management program. The article ends with a discussion on some practical issues pertaining to RCM that lie outside the scope of SAE JA1011.

A hollow, splined alloy steel aircraft shaft (machined from an AMS 6415 steel forging – approximately the same composition as 4340 steel – then quenched and tempered to a hardness of 44.5 to 49 HRC) cracked in service after more than 10,000 h of flight time. The inner surface of the hollow shaft was exposed to hydraulic oil at temperatures of 0 to 80 deg C (30 to 180 deg F). Analysis (visual inspection, 15-30x low magnification examination, 4x light fractograph, chemical analysis, hardness testing) supported the conclusions that the shaft cracked in a region subjected to severe static radial, cyclic torsional, and cyclic bending loads. Cracking originated at corrosion pits on the smoothly finished surface and propagated as multiple small corrosion-fatigue cracks from separate nuclei. The originally noncorrosive environment (hydraulic oil) became corrosive in service because of the introduction of water into the oil. Recommendations included taking additional precautions in operation and maintenance to prevent the use of oil containing any water through filling spouts or air vents. Also, polishing to remove pitting corrosion (but staying within specified dimensional tolerances) was recommended as a standard maintenance procedure for shafts with long service lives.

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.

High-chromium steel pipes 42.25 x 3.25 mm from a blast furnace gas fired recuperator for the preheating of air were heavily oxidized and perforated in places. It was found that the blast furnace gas had a high sulfur content. Both the carburization and the formation of sulfide proved that in addition, from time to time at least, combustion was incomplete and the operation was carried out in a reducing atmosphere, with the result that oxygen deficiency prevented the formation or maintenance of a protective surface layer on the external surface of the pipes. The sulfur would probably not have damaged the nickel-free steel used here at the given temperatures if it had been present as sulfur dioxide in an oxidizing atmosphere. The damage was therefore caused primarily by an incorrectly conducted combustion process.

A pinion gear made of AMS 6470 steel, nitrided all over, lost internal splined teeth due to wear. Spline failure of the power turbine gear caused an engine overspeed and disintegration. Excessive spline wear resulted from a new coupling being mated during overhaul with a worn gear spline. Wear on the spline teeth flanks of the coupling was attributed to severe wear on the mating gear (internal) spline teeth. The assigned cause was an inadequate maintenance procedure which resulted in a wear-damaged component being retained in the power train during engine overhaul. To prevent reoccurrence, specific inspection criteria were issued defining maximum limits for spline wear. A procedure and requirements were specified for installing the coupling and pinion gear at the next overhaul.

One of three valves in a dry automatic sprinkler system tripped accidentally, thus activating the sprinklers. Maintenance records showed that the three valves had been in service less than two years. The valve consisted of a cast copper alloy clapper plate that was held closed by a pivoted malleable iron latch. The latch and top surface of the clapper plate were usually in a sanitary-water environment (stabilized, chlorinated well water with a pH of 7.3) under stagnant conditions. Process make-up water that had been clarified, filtered, softened, and chlorinated and had a pH of 9.8 was occasionally used in the system. Analysis (visual inspection and 250x micrograph) supported the conclusions that failure of the latch was caused by plastic deformation from extensive loss of metal by galvanic corrosion and the sudden loading related to the tripping of the valve. Failure in some regions of the contact area was by ductile (transgranular) fracture. Recommendations included changing the latch material from malleable iron to silicon bronze (C87300). The use of silicon bronze prevents corrosion or galvanic attack and proper adjustment of the latch maintains an adequate contact area.

This article briefly introduces the concepts of failure analysis, including root-cause analysis (RCA), and the role of failure analysis as a general engineering tool for enhancing product quality and failure prevention. It initially provides definitions of failure on several different levels, followed by a discussion on the role of failure analysis and the appreciation of quality assurance and user expectations. Systematic analysis of equipment failures reveals physical root causes that fall into one of four fundamental categories: design, manufacturing/installation, service, and material, which are discussed in the following sections along with examples. The tools available for failure analysis are then covered. Further, the article describes the categories of mode of failure: distortion or undesired deformation, fracture, corrosion, and wear. It provides information on the processes involved in RCA and the charting methods that may be useful in RCA and ends with a description of various factors associated with failure prevention.

This article briefly introduces the concepts of failure analysis and root cause analysis (RCA), and the role of failure analysis as a general engineering tool for enhancing product quality and failure prevention. It reviews four fundamental categories of physical root causes, namely, design deficiencies, material defects, manufacturing/installation defects, and service life anomalies, with examples. The article describes several common charting methods that may be useful in performing an RCA. It also discusses other failure analysis tools, including review of all sources of input and information, people interviews, laboratory investigations, stress analysis, and fracture mechanics analysis. The article concludes with information on the categories of failure and failure prevention.

Metallography is an important component of failure analysis. In the case of a liquid metal embrittlement (LME) failure it is usually conclusive if a third phase constituent can be formed inside of the cracks after failure. In the case where it is necessary to characterize the third phase material, one can use various x-ray spectrographic techniques in conjunction with a scanning electron microscope (SEM). This study describes those metallographic and SEM analysis techniques for determining the mode of failure for a locomotive traction motor by LME.

During a routine shear-pin check, the end lug on the barrel of the forward canopy actuator on a naval aircraft was found to have fractured. The lug was forged from aluminum alloy 2014-T6. Investigation (visual inspection, 2x views, and 140X micrographs etched with Keller's reagent) supported the conclusion that the cause of failure was SCC resulting from exposure to a marine environment. The fracture occurred in normal operation at a point where damage from pitting and intergranular corrosion acted as a stress raiser, not because of overload. The pitting and intergranular attack on the lug were evidence that the surface protection of the part had been inadequate as manufactured or had been damaged in service and not properly repaired in routine maintenance. Recommendations included anodizing the lug and barrel in sulfuric acid and giving them a dichromate sealing treatment, followed by application of a coat of paint primer. During routine maintenance checks, a careful examination was suggested to look for damage to the protective coating, and any necessary repairs should be made by cleaning, priming, and painting. Severely corroded parts should be removed from service.

Ground maintenance personnel discovered hydraulic fluid leaking from two small cracks in a main landing gear cylinder made from AISI 4340 Cr-Mo-Ni alloy steel. Failure of the part had initiated on the ID of the cylinder. Numerous cracks were found under the chromium plate. A 6500x electron fractograph showed cracking was predominantly intergranular with hairline indications. Leaking had occurred only 43 h after overhaul of the part. Total service time on the part was 9488 h. It was concluded that cracking on the ID was caused by hydrogen embrittlement which occurred during or after overhaul. The specific source of hydrogen which produced failure was not ascertainable.

In a helicopter engine connecting rod, high-cycle, low-stress fatigue fractures in bolts and arms progressed about 75% across the section before the final rupture. Factors involved were insufficient specified preload, inadequate tightening during assembly, and engine overspeed. The assigned main causes were design deficiency, improper maintenance during overhaul, and abnormal service operation. The problem can be solved by proper overhauling that ensures bolted assemblies are tightened evenly and accurately, in accordance with recommended torque values. Also, the manufacturer made various modifications, such as a thicker rod, fatigue resistant bolts, and more accurate preload measurements. The configuration of these rods were changed to a tongue-and-groove design to increase service life.

Fretting and/or fretting corrosion fatigue have been observed on such parts as main rotor counterweight tie rods, fixed-pitch propeller blades, propeller blade clamps, pressure regulator lines, and landing gear support brackets. Microcracks started from severe corrosion pits in a failed control rotor spar tube assembly made of cadmium-plated AISI 4130 Cr-Mo alloy steel. Inadequate design was responsible for the failure. A lower tine of the main rotor blade cuff failed in fatigue. The rotor blade cuff was forged of 2014-T6 aluminum alloy. Initial stages of crack growth displayed features typical of low stress intensity fatigue of aluminum alloys. The fatigue resulted from abnormal fretting owing to inadequate torquing of the main retention bolts. Aircraft maintenance engineers and owners were advised to adhere to specifications when torquing this joint.

Despite extensive aircraft landing gear design analyses and tests performed by designers and manufacturers, and the large number of trouble-free landings, aircraft users have experienced problems with and failures of landing gear components. Different data banks and over 200 failure analysis reports were surveyed to provide an overview of structural landing gear component failures as experienced by the Canadian Forces over the last 20 years on more than 20 aircraft types, and to assess trends in failure mechanisms and causes. Case histories were selected to illustrate typical problems, troublesome failure mechanisms, the role of high strength aluminum alloys and steels, and situations where fracture mechanics analyses provided insight into the failures. The two main failure mechanisms were: fatigue occurring mainly in steel components, and corrosion related problems with aluminum alloys. Very few overload failures were noted. A number of causes were identified: design deficiencies and manufacturing defects leading mainly to fatigue failures, and poor materials selection and improper maintenance as the principal causes of corrosion-related failures. The survey showed that a proper understanding of the failure mechanisms and causes, by thorough failure analysis, provides valuable feedback information to designers, operators and maintenance personnel for appropriate corrective actions to be taken.