The 4340 steel main rotor yoke of a helicopter failed during a hovering exercise. Visual examination of the yoke revealed no evidence of gross external damage. Visual fracture surface examination, macrofractography, scanning electron micrography, and metallography of a section cut from the yoke in the region of the cracking indicated that the failure was caused by fatigue-crack initiation and growth from severe corrosion damage to a pillow-block bolt hole. Corrosion occurred because of failure of the protection scheme. An upgraded corrosion protection scheme for the bolt holes was recommended, along with nondestructive inspection of the region at intervals determined by fractographic analysis of the fatigue crack growth.

Welds in two CMo steel catalytic gas-oil desulfurizer reactors cracked under hydrogen pressure-temperature conditions that would not have been predicted by the June 1977 revision of the Nelson Curve for that material. Evidence of severe cracking was found in five weld-joint areas during examination of a naphtha desulfurizer by ultrasonic shear wave techniques. Defect indications were found in longitudinal and circumferential seam welds of the ASTM A204, grade A, steel sheet. The vessel was found to have a type 405 stainless steel liner for corrosion protection that was spot welded to the base metal and all vessel welds were found to be overlaid with type 309 stainless steel. Long longitudinal cracks in the weld metal, as well as transverse cracks were exposed after the weld overlay was ground off. A decarburized region on either side of the crack was revealed by metallurgical examination of a cross section of a longitudinal crack. It was concluded that the damage was caused by a form of hydrogen attack. Installation of a used Cr-Mo steel vessel with a type 347 stainless steel weld overlay was suggested as a corrective action.

An aluminum brass seawater surface condenser failed due to pitting after less than one year of service. Large pits filled with a green deposit were evidenced under the nonuniform black scale present over the entire inside surface of the tube. The black deposit was identified as primarily copper sulfide, with zinc and aluminum sulfides while the green deposit was revealed to be copper chloride. The combination of sulfide and chloride attack on the tubes was concluded to have resulted in the failure. Injection of ferrous sulfate upstream of the condenser which could aid the formation of protective oxide films was recommended.

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.

The truck beam of the left main landing gear (MGL) of a Boeing 707 airplane collapsed on the ground just after the aircraft was unloaded and refueled. The investigation revealed that failure was caused by the propagation of an intergranular crack originating from the bottom of the pit. The crack reached the critical size and caused failure by stress-corrosion cracking (SCC) under static loading conditions in service. The failed beam was protected by a well adhering paint system. However, the presence of adequate amounts of corrosion preventive compound films (CPC) on the surfaces of the failed beam could not be conclusively established because of the long term service exposure and presence of lubricants.

Piping and structural components used in space launch facilities such as NASA's Kennedy Space Center and the Air Force's Cape Canaveral Air Station face extreme operating conditions. Launch effluent and residue from solid rocket boosters react with moisture to form hydrochloric acid that settles on exposed surfaces as they are being subjected to severe mechanical loads imparted during lift-off. Failure analyses were performed on 304 stainless steel tubing that ruptured under such conditions, while carrying various gases, including nitrogen, oxygen, and breathing air. Hydrostatic testing indicated a burst strength of 13,500 psi for the intact sections of tubing. Scanning electron microscopy and metallographic examination revealed that the tubing failed due to corrosion pitting exacerbated by stress-corrosion cracking (SCC). The pitting originated on the outer surface of the tube and ranged from superficial to severe, with some pits extending through 75% of the tube's wall thickness. The SCC emanated from the pits and further reduced the service strength of the component until it could no longer sustain the operating pressure and final catastrophic fracture occurred. Corrosion-resistant coatings added after the investigation have proven effective in preventing subsequent such failures.

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.

Corrosion in potable and nonpotable water systems has been well documented in the past, and new research discusses innovations in water treatment and materials that are designed to enhance the quality of a water system, whether commercial or residential. This paper is a collection of five case histories on the failure of copper and steels as used in potable and non-potable water systems. The case histories cover a range of applications in which copper and steel products have been used. Copper and steel pipes are the two most commonly used materials in residential, commercial and industrial applications. The projects that are discussed cover these three important applications. The purpose of presenting this information is to allow the reader to gain an understanding of real life corrosion issues that affect plumbing materials, how they should have been addressed during the design of the water system, and how a water system should be maintained during service. We share this information in the hope that the reader will gain some limited knowledge of the problems that exist, and apply that knowledge in designing or using water systems in day-to day life.

Sucker-rod pumps are operating in very aggressive environments in oil well production. The combined effect of a corrosive environment and significant mechanical loads contribute to frequent cases of failure of the rod string during operation. Standards and recommendations have been developed to control and avoid those failures. This study presents various failure cases of sucker rods in different applications. The heat treatment of the steel material and the resulting microstructure are an important factor in the behavior of the sucker rod. A spheroidized microstructure presents a weaker resistance to corrosion affecting the rod life. Non-metallic inclusions are a pitting preferential site leading to fatigue crack initiation. Heterogeneous microstructure as banded martensite and ferrite/pearlite decreases the ductility of the material affecting the fatigue propagation resistance.

Gas turbines and other types of combustion turbomachinery are susceptible to hot corrosion at elevated temperatures. Two such cases resulting in the failure of a gas turbine component were investigated to learn more about the hot corrosion process and the underlying failure mechanisms. Each component was analyzed using optical and scanning electron microscopy, energy dispersive spectroscopy, mechanical testing, and nondestructive techniques. The results of the investigation provide insights on the influence of temperature, composition, and microstructure and the contributing effects of high-temperature oxidation on the hot corrosion process. Preventative measures are also discussed.

A series of poppet-valve stems fabricated from 17-4 PH (AISI type 630) stainless steel failed prematurely in service during the development of a large combustion assembly. The poppet valves were part of a scavenging system that evacuated the assembly after each combustion cycle. The function of the valve is to open and close a port; thus, the valve is subjected to both impact and tensile loading. Analysis (visual inspection, hardness testing, and stress analysis) supported the conclusions that the valve stems were impact loaded to stresses in excess of their yield strength. That they failed in the threaded portion also suggests a stress-concentration effect. Recommendations included changing the material spec to a higher-strength material with greater impact strength. In this case, it was recommended that the stems, despite any possible design changes, be manufactured from an alloy such as PH 13-8Mo, which can be processed to a yield strength of 1379 MPa (200 ksi), with impact energies of the order of 81 J (60 ft·lbf) at room temperature.

A straight-tube cooler type heat exchanger had been in service for about ten years serving a coal pulverizer in Georgia. Non-potable cooling water from a local lake passed through the inner surfaces of the copper tubing and was cooling the hot oil that surrounded the outer diametral surfaces. Several of the heat exchangers used in the same application at the plant had experienced a severe reduction in efficiency in the past few years. One heat exchanger reportedly experienced some form of leakage following discovery of oil contaminating the cooling water. This heat exchanger was the subject of a failure investigation to determine the cause and location of the leaks. Corrosion products primarily contained copper oxide, as would be expected from a copper tubing. The product also exhibited the presence of a significant amount of iron oxides. Metallographic cross sectioning of the tubes and microscopic analysis revealed several large and small well rounded corrosion pits present at the inner diametral surfaces. The cause of corrosion was attributed to corrosive waters that were not only corroding the copper, but were corroding steel pipes upstream from the tubing.

A residential subdivision near Tampa, FL was constructed in 1984 through 1985. Several sections of copper pipe were removed from one residence that had reported severe leaking. Visual examination revealed extensive pitting corrosion throughout the ID surfaces of the sample. Microscopic evaluation of a cross section of a copper pipe revealed extensive pitting corrosion throughout the inner diametral surfaces of the pipe. Some pits had penetrated through the wall thickness, causing the pin hole leaks. Analysis of a sample of water obtained from the subdivision revealed relatively high hardness levels (210 mg/l), high levels of sulfate ions (55 mg/l), a pH of 7.6 and a sulfate-to-chloride ratio of 3:1. Analysis of corrosion product removed from the ID surfaces of the pipe section revealed that an environment rich in carbonates existed inside the pipe, a result of the hard water supply. It was concluded that pitting corrosion was a result of the corrosive waters supplied by the local water utility. Waters could be rendered non-pitting by increasing their pH to 8 or higher and neutralizing the free carbon dioxide.

A failed H-11 tool steel pylon attachment stud was found during a routine walk-around inspection. The stud exhibited gross localized corrosion pitting at several different areas on its surface. Light general rust was also evident. Severe pitting occurred near the fracture location. The fracture face contained evidence of intergranular SCC as well as ductile dimples. The protective coating was found to be an inorganic water-base aluminide coating having a coating thickness of 7.5 to 13 micron (0.3 to 0.5 mil). The coating was of a nonuniform mottled nature. It was concluded that the failure of the pylon attachment stud was caused by general corrosion followed by SCC. The stud was not adequately protected against corrosion by the coating. It was recommended that the coating be applied to a thickness of 38 to 75 micron (1.5 to 3 mil) to provide long-time corrosion resistance. The coating must be either burnished or cured at 540 deg C (1000 deg F) to provide cathodic protection to the steel. Other coatings, such as cadmium or aluminum, were also recommended if a thinner coating is needed.

A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures (approximately 177 deg C, or 350 deg F) and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection. Investigation (visual inspection, on-site examination and testing, EDS analysis of scale deposits, and MIC testing of the soil) supported the conclusion that corrosion may have been caused by an interruption in cathodic protection. The effectiveness of cathodic protection on established microbial deposits is questionable. Recommendations included ultrasonically testing the tank floor and replacing portions based on the remaining wall thickness. Doubling the wall thickness of the floor plates was also recommended.

A shopping mall in South Carolina was originally constructed in 1988 and a second phase completed in 1989. The HVAC system inside the mall included an open, recirculating condenser water loop that served various fan coil units located within tenant spaces. The system had a recirculating capacity of about 44,000 gal (166,000 L) of water. It consisted primarily of steel pipes fitted with threaded connectors on the 2 in. (46 cm) pipes and bolted flanged couplings on the larger pipes. Seven years following the completion of the mall, corrosion problems were noted at the outer and inner surfaces of the pipe. Visual observations on the inner diametral surfaces revealed that the pipes were, in almost all cases, filled with corrosion products. A significant amount of base metal loss was documented in all of the samples. The cause of the observed corrosion was determined to be a lack of corrosion monitoring and poor water quality. Pipe replacement and a regular water testing program were recommended.

This paper describes the investigation of a corrosion failure of bottom plates on an aboveground tank used for the storage of potable water. The tank was internally inspected for the first time after six years of service. Paint blisters and rust spots were observed on the bottom plates and first to third course shell plates. Sand blasting and repainting of the bottom plates and first course shell plates was to be used as a remedial measure. However, during the sand blasting, holes and deep pitting were observed on the bottom plates. On-site visual inspection, magnetic flux leakage (MFL) inspection, ultrasonic testing (UT), and evaluation of the external cathodic protection (CP) system were used in the failure analysis. The corrosion products were analyzed using energy-dispersive X-ray analysis (EDAX). The failure is attributed to the ingress of water and its impoundment under the tank bottom along the periphery inside the ring wall and failure of water side epoxy coating. Various measures to prevent such failures in the future are recommended.

A natural gas pipeline explosion and subsequent fire significantly altered the pipeline steel microstructure, obscuring in part the primary cause of failure, namely, coating breakdown at a local hard spot in the steel. Chemical analysis was made on pieces cut from the portion of the pipe that did not fracture during the explosion and from piece 5-1 which contained the fracture origin site. Both pieces were found to have 0.30% carbon and 1.2% Mn with sulfur and phosphorus impurities acceptably low. Fracture mechanics analysis used in conjunction with fractographic results confirmed the existence of a very hard spot in the steel prior to the explosion, which was softened significantly in the ensuing fire. This finding allowed the micromechanism leading to fracture to be identified as hydrogen embrittlement resulting from cathodic charging.

The origins of the casting are unknown. It is included here as a classic case of intergranular corrosion. The part (apparently a pump outlet) was named the “rubber casting” because of the severity of the intergranular attack. Every grain boundary has been attacked to the extent that the casting could be twisted and stretched as through made of rubber. The chemistry of the casting was acceptable for CN-7M. The reason the part failed is a continuous film of carbide with a continuous crack running parallel to the carbides. This sensitized structure produces an area depleted in protective chromium, making it susceptible to corrosion. Two solutions to this problem are available. The simplest is to ensure correct heat treatment to dissolve grain-boundary carbide film and return the protective chromium to the depleted zone. Alternatively, a low-carbon (0.03% maximum C, for example, CF-3) grade can be specified. Procedures are given in a reference for screening castings that may be susceptible to intergranular corrosion due to processing errors.

High-level radioactive wastes generated during the processing of nuclear materials are kept in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate and sodium hydroxide. Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors without letting radionuclides escape. Several intergranular cracks were discovered in the vent pipe of one such system. The pipe, made of galvanized steel sheet, connects to an exhaust fan downstream of high-efficiency particulate air filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking was the cause of the failure.