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.

A 12.7 mm (0.5 in.) diam tube was removed from a potable water supply due to leaks. The tube wall thickness was 0.711 mm (0.028 in.) with a thin layer of chromium plate on the OD surface. The tube had been in service for approximately 33 years. Investigation (visual inspection, EDS deposit analysis, metallurgical examination, and unetched magnified images) supported the conclusion that failure occurred due to porous material typical of plug-type dezincification initiating from the inside surface. Where the dezincification had progressed through the tube wall, the chromium plate had exfoliated from the base material and cracked. Recommendations included replacing the piping with a more corrosion-resistant material such as red brass (UNS C23000), inhibited Admiralty brass (UNS C44300), or arsenical aluminum brass (UNS C68700).

Inhibited admiralty brass (UNS C44300) condenser tubes used in a natural-gas-fired cogeneration plant failed during testing. Two samples, one from a leaking tube and the other from an on leaking tube, were examined. Chemical analyses were conducted on the tubes and corrosion deposits. Stress-corrosion cracking was shown to have caused the failure. The most probable corrosive was ammonia or an ammonium compound in the presence of oxygen and water. All of the tubes were replaced.

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. Stereomicroscopic examination revealed two small transverse cracks that were within a few millimeters of the tube end, with one being a through-wall crack. Metallographic examination of sections containing the cracks showed branching secondary cracks and a transgranular cracking mode. The cracks appeared to initiate in pits. EDS analysis of a friable deposit found on the inside diameter of the tube and XRD analysis of crystalline compounds in the deposit indicated the possible presence of ammonia. Failure was attributed to stress-corrosion cracking resulting from ammonia in the cooling water. It was recommended that an alternate tube material, such as a 70Cu-30Ni alloy or a titanium alloy, be used.

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).

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 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.

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.

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 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.

Stress-corrosion cracking (SCC) is a form of corrosion and produces wastage in that the stress-corrosion cracks penetrate the cross-sectional thickness of a component over time and deteriorate its mechanical strength. Although there are factors common among the different forms of environmentally induced cracking, this article deals only with SCC of metallic components. It begins by presenting terminology and background of SCC. Then, the general characteristics of SCC and the development of conditions for SCC as well as the stages of SCC are covered. The article provides a brief overview of proposed SCC propagation mechanisms. It discusses the processes involved in diagnosing SCC and the prevention and mitigation of SCC. Several engineering alloys are discussed with respect to their susceptibility to SCC. This includes a description of some of the environmental and metallurgical conditions commonly associated with the development of SCC, although not all, and numerous case studies.