A forging die in a 250-ton press producing brass valves began to show signs of fatigue after a few thousand hits. By the time it reached 30,000 hits, the die was badly damaged and was submitted for analysis along with one of the last forgings produced. The investigation included visual and macroscopic inspection, metallographic and chemical analysis, SEM imaging, optical profilometry, mechanical property testing, and EDX analysis. The die was made of chromium hot-work tool steel and the forgings were made of CuZn39Pb3 heated to an initial working temperature 700 deg C. The entire surface of the die was covered with fatigue cracks and many fillets had been plastically deformed. Several other types of damage were also observed, including areas of oxidation, corrosion pits, voids, abrasive wear, die adhesion, and thermal fatigue. Fatigue cracking was the primary cause of failure with significant contributions from the other damage mechanisms.

After about 17 years in service, copper alloy C27000 (yellow brass, 65% Cu) innercooler tubes in an air compressor began leaking cooling water, causing failure and requiring replacement. The tubes were 19 mm in diam and had a wall thickness of 1.3 mm (0.050 in.). The cooling water that flowed through the tubes was generally sanitary (chlorinated) well water; however, treated recirculating water was sometimes used. Analysis (visual inspection, 9x and 75x unetched micrographs, and spectrochemical analysis) showed a thick uniform layer of porous, brittle copper on the inner surface of the tube, extending to a depth of about 0.25 mm (0.010 in.) into the metal, plug-type dezincification extending somewhat deeper into the metal. This supported the conclusion that failure of the tubes was the result of the use of an uninhibited brass that has a high zinc content and therefore is readily susceptible to dezincification. Recommendations included replacing the material with copper alloy C68700 (arsenical aluminum brass), which contains 0.02 to 0.06% As and is highly resistant to dezincification. Copper alloy C44300 (inhibited admiralty metal) could be an alternative selection for this application; however, this alloy is not as resistant to impingement attack as copper alloy C68700.

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).

An arsenical admiralty brass (UNS C44300) finned tube in a generator air cooler unit at a hydroelectric power station failed. The unit had been in operation for approximately 49,000 h. The cooling medium for the tubes was water from a river. Air flowed over the finned exterior of the tubes, while water circulated through the tubes. Investigation (visual inspection, leak testing, history review, 100X micrographs etched in potassium dichromate, chemical analysis, and EDS and XRD analysis of internal tube deposits) supported the conclusion that the cause of the tube leaks was ammonia-induced SCC. Because the cracks initiated on the inside surfaces of the tubes and because the river water was not treated before it entered the coolers, the ammonia was likely present in the river water and probably concentrated under the internal deposits. Recommendations included either eliminating the ammonia (prohibitively expensive in cost and time) or using an alternate material (such as a 70Cu-30Ni alloy or a more expensive titanium alloy) that is resistant to ammonia corrosion as well as to chlorides and sulfur species.

An aftercooler was of conventional design and fitted with brass tubes through which cooling-water circulated. Air at 100 psi pressure was passed over the outsides of the tubes, entering the vessel near to the upper tubeplate on one side and leaving it by a branch adjacent to the lower tubeplate on the opposite side. After a mishap, the paint had been burned off the upper half of the shell. Internally, most of the tubes were found to be twisted or bent. The casing of the pump used to circulate the cooling water was also found to be cracked after the mishap. All the evidence pointed to the probability that a fire had occurred within the vessel. Some months before the failure, one of the tubes situated towards the center of the nest developed a leak. Owing to the difficulty of inserting a replacement tube, the defective one was scaled by means of a length of screwed rod fitted with nuts and washers at each end. This assembly became loose, thereby allowing air under pressure to enter the waterside of the cooler and expel the water, leading to overheating and ultimately to the damage described.

The horizontal heat-exchanger tubes made of copper alloy C70600, in one of two hydraulic-oil coolers in an electric power plant, leaked after 18 months of service. River water was used as the coolant in the heat-exchanger tubes. Several nodules on the inner surface and holes through the tube wall, which appeared to have formed by pitting under the nodules, were revealed by visual examination. Steep sidewalls, which indicated a high rate of attack, were revealed by microscopic examination of a section through the pit which had penetrated the tube wall. The major constituent of reddish deposit on the inner surfaces of the tubes was revealed to be iron oxide and slight manganese dioxide. Effluent from steel mills upstream was indicated by the presence of these and other constituents to be the source of most of the solids found in the tubes. It was concluded that the tubing failed by crevice corrosion. The tubing in the cooler was replaced, and cooling-water supply was changed from river to city water, which contained no dirt to deposit on the tube surfaces. An alternate solution of installing replacement tubes in the vertical position to make deposition of solids from river water less likely was suggested.

Failures occurred in admiralty brass condenser tubes in a nuclear plant cooled by freshwater. About 2500 tubes had to be replaced over a span of six years. Investigation (visual inspection, chemical analysis, water chemistry (for both intake and outfall), and corrosion products in the operating system and on test coupons exposed to the operating environment) supported the conclusion that the failure was caused by microbe-initiated SCC. No recommendations were made.

Tubes in heat exchangers, made of copper alloy C44300 and used for cooling air failed after 5 to six years of service. Air passed over the shell-side surface of the tubes and was cooled by water flowing through the tubes. Water vapor in the air was condensed (pH 4.5) on the tube surfaces during the cooling process. Air flow over the tubes reversed direction every 585 mm as a result of baffling placed in the heat exchangers. An uneven ridgelike thinning and perforation of the tube wall on the leeward side of the tube was revealed by visual examination. Undercut pits on the outer surface of the tube were revealed by metallographic examination of a cross section of the failed area. Impingement attack which led to perforation was revealed by both the ridgelike appearance of the damaged area and the undercut pitting. The heat exchanger was retubed with tubes made of aluminum bronze (copper alloy C61400).

Some of the admiralty brass tubes were failing in a heat exchanger. The heat exchanger cooled air by passing river water through the inside of the tubes. The wall thickness of all tubes ranged between 1.19 to 1.27 mm (0.047 to 0.050 in.). General intergranular corrosion occurred at the inside surfaces of the tubes. Transgranular stress-corrosion cracking, probably the result of sulphates under basic conditions, and dezincification occurred also as the result of galvanic corrosion under the deposits in the tubes. Recommendations were to use a closed-loop water system to eliminate sulphates, ammonia, etc., and to run trials on one unit with tubes of other alloys such as 80-20 Cu-Ni or 70-30 Cu-Ni to evaluate their performance prior to any large scale retubing operations.

Dezincification is a particular form of corrosive attack which may occur in a variety of environments and to which some brasses are susceptible. It is favored by waters having a high oxygen, carbon dioxide, or chloride content, and is accelerated by elevated temperatures and low water velocities. In the present study, steam turbine condenser tubes had to be renewed after 25 years of service. The tubes were nominally of 70:30 brass. The appearance of a typically corroded one showed uniform dezincification attack on the bore, extending from one-half to two-thirds through the tube wall thickness.

Admiralty brass (Alloy C44300) cooling tubes which were part of a heat exchanger in a turbogenerator that provided electricity to a manufacturing plant failed. A mixture of non-recirculating city and “spring pit” water flowed through bundles of tubes to cool the oil in which they are immersed. However, a problem developed when several of the brass tubes cracked transversely, allowing cooling water to mix with the oil. The presence of a tensile stress, intergranular cracks, and a corrosion product suggested the tube failures resulted from stress-corrosion cracking. The main corrosion product was cupric hydroxychloride. In addition to switching to a more corrosion-resistant alloy, extreme care should be taken in the manufacturing of the replacement tube bundles to avoid imparting any residual tensile stresses in the tubing. Analyses of city and spring-pit water were recommended also, to determine which contained the least-harmful corrosive chemicals.

After being in service for ten years, two admiralty brass heat-exchanger tubes from a cooler in a refinery catalytic reforming unit cracked circumferentially in the area of U-bends. A blunt transgranular cracking with minimal branching propagating from the inside surface of the tube was revealed by metallography which was typical of cracking by corrosion fatigue mechanism. Corrosion deposits on both the inside- and outside-diam surfaces were found in the tubes. The presence of copper, zinc, iron, and small amounts of chloride, sulfur, silicon, tin, and manganese was revealed by energy-dispersive analysis of the deposits. It was interpreted by the hardness values (higher than typical for annealed copper tubing) that the tubes may not have been annealed after the U-bends were formed and thus the role of residual stresses in the crack was revealed. It was concluded that the tubes failed by corrosion fatigue initiated by pitting at the inside-diam surface. The tubes were recommended to be annealed after bending to reduce residual stresses from the bending operation to an acceptable level.

Several fuses made of nickel silver (57 to 61% Cu, 11 to 13% Ni, bal Zn) exposed to air containing ammonium and nitrate ions failed by SCC. Test solutions of 1 N ammonium nitrate (NH4NO3) and a 1:1 mixture of 1 N sodium nitrate (NaNO3) and 1 N calcium nitrate (Ca(NO3) 2) were prepared. In addition, stressed fuses made of nickel silver and of cupro-nickel (80Cu-20Ni) were exposed to a drop of corrosive solution in the stressed area. All nickel silver specimens failed after two days of exposure to NH4NO3 solution. However, 17% of them failed and 67% showed crack initiation but no failure after 42 days of exposure to NaNO3 + Ca(NO3)2 solution. None of the cupro-nickel specimens failed, but among those exposed to NH4NO3, 17% displayed crack initiation and 83% showed partial dealloying after 42 days. Based on the test results, the fuse material was changed from nickel silver to cupro-nickel, solving the SCC problem.

Electrical contact-finger retainers blanked and formed from annealed copper alloy C65500 (high-silicon bronze A) failed prematurely by cracking while in service in switchgear aboard seagoing vessels. In this service they were sheltered from the weather but subject to indirect exposure to the sea air. About 50% of the contact-finger retainers failed after five to eight months of service aboard ship. Investigation (visual inspection, 250x images etched with equal parts NH4OH and H2O2, emission spectrographic analysis, and stereoscopic views) supported the conclusion that the cracking was produced by stress corrosion as the combined result of: residual forming and service stresses; the concentration of tensile stress at outer square corners of the pierced slots; and preferential corrosive attack along the grain boundaries as a result of high humidity and occasional condensation of moisture containing a fairly high concentration of chlorides (seawater typically contains about 19,000 ppm of dissolved chlorides) and traces of ammonia. Recommendations included redesign of the slots, shot-blasting the formed retainers, and changing the material to a different type of silicon bronze-copper alloy C64700.

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.

The failure of a 90-10 cupronickel heat exchanger tube resulted in flooding of the vessel and subsequently sinking it. The corrosion of the cupronickel alloy was facilitated by the high sulfur content of the seawater in which it operated. The failure modes were anodic dissolution and copper reprecipitation.

Copper shortening has been found to occur in the rotor windings of turbo alternators and takes the form of a progressive reduction in the length of the coils leading to distortion of the end windings. The trouble results from the high loading which develops between successive layers of the strip conductor due to centrifugal force. This leads to a high frictional binding force between turns and prevents axial expansion under normal heating in service. Rotor trouble which proved to be due to copper shortening was found in a set rated at 27.5 MW. It was manufactured in 1934 at which time silver-bearing copper was not available. The use of hard-drawn silver-bearing copper for a rewind, in conjunction with special attention to blocking up the end windings, is confidently expected to effect a complete cure.

A substantial number of copper alloy C27000 (yellow brass, 65Cu-35Zn) ferrules for electrical fuses cracked while in storage and while in service in paper mills and other chemical processing plants. The ferrules, made by three different manufacturers, were of several sizes. One commonly used ferrule was 3.5 cm long by 7.5 cm in diam and was drawn from 0.5 mm (0.020 in.) thick strip. Investigation (visual inspection, metallographic examination, and a mercurous nitrate test, which is an accelerated test used to detect residual stress in copper and copper alloys) of both ferrules from fuses in service and storage in different types of plants, and ferrules from newly manufactured fuses, supported the conclusion that the ferrules failed by SCC resulting from residual stresses induced during forming and the ambient atmospheres in the chemical plants. The atmosphere in the paper mills was the most detrimental, and the higher incidence of cracking of ferrules there was apparently related to a higher concentration of ammonia in conjunction with high humidity. Recommendations included specifying that the fuses meet the requirements of ASTM B 154.

A welded joint between lengths of 4 in. OD x 13 SWG copper pipe which formed part of a cold-water main failed by cracking over one-third of the circumference. Microscopic examination of the filler metal showed that it had a structure corresponding to a brass of the 60:40 type commonly used for bronze welding. Failure resulted from dezincification of the joint material from the internal side of the tube. Also, a selective attack on the beta phase had occurred. It was evident that the loss in mechanical strength arising from the corrosion had resulted in the development of cracking in service. The filler metal used was not resistant to the conditions to which it was exposed. Copper welding rods as per BS 1077 or a Cu-Ag-P brazing alloy as recommended in BS 699, would have been preferable.

An interstage radiator gas coil began leaking after only 45 days of service. The original brass coil with several aluminum fins was replaced three times but each replacement lasted less than a day. After removing the fins, leaks were found at circumferential cracks. A section of a tube was removed and split, revealing a series of cracks, evenly spaced. Crack spacing coincided with fin spacing, indicating that stresses incurred during installation of the fins promoted failure. Metallographic examination showed intergranular, branched cracking, characteristic of stress corrosion failures, with the cracks starting on the inside surfaces of the tubes. There was no known corrosive agent in the system, and no other corrosion damage could be found. Qualitative tests and spectrographic analysis gave a positive indication for mercury. The spacing of the cracks, the branched intergranular cracking, the rapid failure, and presence of mercury led to the conclusion of stress-corrosion cracking. It was impossible to remove mercury from the system so carbon steel coils were substituted for the brass ones. The carbon steel coils gave failure-free service for over nine years.