A 455 mm diam x 8 mm thick wall carbon steel (ASTM A 53) discharge line for a circulating-water system at a cooling tower fractured in service; a manifold section cracked where a Y-shaped connection had been welded. Investigation (visual inspection and photographs) supported the conclusion that the pipe failed by fatigue. Cracks originated at crevices and pits in the weld area that acted as stress raisers, producing high localized stresses because of the sharp-radius corner design. Abnormally high structural stresses and alternating stresses resulting from the pump vibrations contributed to the failure. Recommendations included changing the joint design to incorporate a large-radius corner and improving fitting of the components to permit full weld penetration. Backing strips were suggested to increase weld quality, and the pipe wall thickness was increased from 8 to 9.5 mm.

After 14 months of service, cracks were discovered in castings and bolts used to fasten together braces, posts, and other structural members of a cooling tower, where they were subjected to externally applied stresses. The castings were made of copper alloys C86200 and C86300 (manganese bronze). The bolts and nuts were made of copper alloy C46400 (naval brass, uninhibited). The water that was circulated through the tower had high concentrations of oxygen, carbon dioxide, and chloramines. Analysis (visual inspection, bend tests, fractographs, 50x unetched micrographs, 100x micrographs etched with H4OH, and 500x micrographs) supported the conclusions that the castings and bolts failed by SCC caused by the combined effects of dezincification damage and applied stresses. Recommendations included replacing the castings with copper alloy C87200 (cast silicon bronze) castings. Replacement bolts and nuts should be made from copper alloy C65100 or C65500 (wrought silicon bronze).

One-quarter inch diameter 304 stainless steel cooling tower hanger rods failed by chloride-induced stress-corrosion cracking (SCC). The rods were located in an area of the cooling tower where the air contains drop lets of water below the mist eliminators and above the flow of water The most extensive cracking was observed in the rod nuts and in the portions of the rod which were covered by the nuts. Cracking was transgranular with extensive branching, and some corrosion occurred along the crack paths. The clamping force from the nuts used on both sides of the supported member and residual stresses from thread rolling likely contributed to the stresses for the cracking mechanism, along with the stresses induced by the supported load. The external surfaces of the hanger rods were reportedly exposed to a chloride-containing atmosphere, likely due to the biocide. Type 304 stainless steel is not a suitable material for this application, and materials that resist SCC, such as Inconel, should be considered.

This article focuses on the mechanisms of microbially induced or influenced corrosion (MIC) of metallic materials as an introduction to the recognition, management, and prevention of microbiological corrosion failures in piping, tanks, heat exchangers, and cooling towers. It discusses the degradation of various protective systems, such as corrosion inhibitors and lubricants. The article describes the failure analysis of steel, iron, copper, aluminum, and their alloys. It also discusses the probes available to monitor conditions relevant to MIC in industrial systems and the sampling and analysis of conditions usually achieved by the installation of removable coupons in the target system. The article also explains the prevention and control strategies of MIC in industrial systems.

This article focuses on the mechanisms of microbiologically influenced corrosion as a basis for discussion on the diagnosis, management, and prevention of biological corrosion failures in piping, tanks, heat exchangers, and cooling towers. It begins with an overview of the scope of microbial activity and the corrosion process. Then, various mechanisms that influence corrosion in microorganisms are discussed. The focus is on the incremental activities needed to assess the role played by microorganisms, if any, in the overall scenario. The article presents a case study that illustrates opportunities to improve operating processes and procedures related to the management of system integrity. Industry experience with corrosion-resistant alloys of steel, copper, and aluminum is reviewed. The article ends with a discussion on monitoring and preventing microbiologically influenced corrosion failures.

The first-stage nozzles of a high-pressure turbine section of an industrial gas turbine exhibited leading and trailing-edge deterioration. The nozzles were made of X-40, a cobalt-base alloy, and were aluminide coated. Failure analysis determined that the deterioration was the result of hot corrosion caused by a combination of contaminants, cooling-hole blockage, and coating loss.

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.

A spiral heat exchanger made from 316L stainless steel developed a leak after eight years of service as a condenser on a distillation tower. Examination identified the leak as being located on the cooling water side in the heat affected zone (HAZ) of a weld joining two plates. Cooling water deposits were observed in a V-shaped corner formed by the weld. A metallurgical examination identified the presence of transgranular cracks in the HAZ on the cooling water side. Analysis of the cooling water revealed the presence of chlorides. Based on the metallurgical analysis and other findings, it was determined that the cracks and associated leak were the result of chloride stress-corrosion cracking.

During construction of a revolving sky-tower observatory, a 2.4 m (8 ft) diam cylindrical column developed serious circumferential cracks overnight at the 14 m (46 ft) level where two 12 m (40 ft) sections were joined by a girth weld. The temperatures ranged from 12 deg C (53 deg F) to 7 deg C (45 deg F) that night. The column was shop fabricated in 12 m (40 ft) long sections of 19 mm (3/4 in.) thick steel plate of ASTM A36 steel. Crack initiation was caused by high residual stress during girth welding, and the presence of notches formed by the termination of the incomplete welds. Continuation of the cracks was attributed to the brittle condition of the steel when cooled by the night air. A steel with a much lower ductile-to-brittle transition temperature is essential for this type of structure. Other necessary steps include better control of the girth-welding, choice of a more favorable electrode to avoid porosity, careful termination of all welds to avoid formation of notches, and completion of all welds before other sections of the column are erected.

Six cases of failure attributed to microbiologically influenced corrosion (MIC) were analyzed to determine if any of the failures could have been avoided or at least predicted. The failures represent a diversity of applications involving typical materials, primarily stainless steel and copper alloys, in contact with a variety of liquids, chemistries, and substances. Analytical techniques employed include stereoscopic examination, energy dispersive x-ray spectroscopy (EDS), temperature and pH testing, and metallographic analysis. The findings indicate that MIC is frequently the result of poor operations or improper materials selection, and thus often preventable.

A service water pump in a nuclear reactor failed when its shaft gave way. The fracture originated in the threaded portion of the sleeve nut on the drive-end side of the shaft. Results of the failure analysis showed that the cracking initiated at the thread root as a result of corrosion fatigue. Crack propagation occurred either by corrosion or mechanical fatigue. Evidence was found indicating high rotary bending stresses on the shaft during operation. The nonstandard composition of the En 8 steel used in the shaft and irregular maintenance reduced the life of the shaft. Recommendations included use of a case-hardened En 8 steel with the correct composition and regular maintenance of the pump.

A steel splice plate in a power transmission line tower cracked while in service. Metallographic analysis indicated the presence of a white hard martensite layer near the crack, which occurred in the heel of the plate. Mechanical property tests revealed localized hardening in the area of the crack, supporting the metallurgical findings. A substantial deterioration of the Charpy impact toughness of the material in the heel region was also observed which is believed to have caused the initiation and propagation of the cracks leading to the failure.

In 1975, a manufacturer was awarded a contract to produce modular air-traffic control towers for the U.S. Navy. The specifications called for painted steel siding, but the manufacturer convinced the Navy to substitute aluminum-bonded-to-plywood panels that were provided by a supplier. In less than one year, the panels began to delaminate and the aluminum began to crack. It was found that the failure was the result of chloride-induced intergranular corrosion caused by chemicals in the adhesive and excessive moisture in the wood introduced during manufacturing.

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.

This study examines the role of calcium-precipitating bacteria (CPB) in heat exchanger tube failures. Several types of bacteria, including Serratia sp. (FJ973548), Enterobacter sp. (FJ973549, FJ973550), and Enterococcus sp. (FJ973551), were found in scale collected from heat exchanger tubes taken out of service at a gas turbine power station. The corrosive effect of each type of bacteria on mild steel was investigated using electrochemical (polarization and impedance) techniques, and the biogenic calcium scale formations analyzed by XRD. It was shown that the bacteria contribute directly to the formation of calcium carbonate, a critical factor in the buildup of scale and pitting corrosion on heat exchanger tubes.

This article outlines the basic steps to be followed and the range of techniques available for failure analysis, namely, background data assembling, visual examination, microfractography, chemical analysis, metallographic examination, electron microscopy, electron microprobe analysis, X-ray techniques, and simulations. It also describes the steps for analyzing the data, preparing the report, preservation of evidence, and follow-up on recommendations.