On 13 Dec 1994, two massive detonations leveled portions of an ammonium nitrate plant near Sioux City, IA. The primary explosion allegedly occurred in defectively-designed titanium sparger piping inside the neutralizer vessel. Investigation however, revealed the explosion occurred because of unsafe plant operations and poor maintenance procedures. Specifically, the ammonium nitrate within the 18,000 gal capacity neutralizer vessel had become contaminated and made highly acidic. The operators then injected superheated steam directly into the ammonium nitrate in the neutralizer vessel.

After 10 to 20 months of service, the carbon steel hoppers on three trucks used to transport bulk ammonium nitrate prills developed extensive cracking in the upper walls. The prills were discharged from the steel hoppers using air superchargers that generated an unloading pressure of approximately 11 kPa (7 psi). Each hopper truck held from 9,100 to 11,800 kg (10 to 13 tons) of prills when fully loaded and handled approximately 90,700 kg (100 tons) per month. The walls of the hoppers were made of 2.7 mm (0.105 in.) thick flat-rolled carbon steel sheet of structural quality, conforming to ASTM A 245 (obsolete specification replaced by A 570 and A 611). Investigation (visual inspection and 100x micrographs polished and etched with nital) supported the conclusion that failure of the hoppers was the result of intergranular SCC of the sheet-steel walls because of contact with a highly concentrated ammonium nitrate solution. Recommendations included the cost-effective solution of applying a three-coat epoxy-type coating with a total dry thickness of 0.3 mm (0.013 in.) to the interior surfaces of the hoppers.

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.

Several tubes in a tube bundle in an evaporator used to concentrate an acid nitrate solution failed by leakage. The feed to the evaporator contained about 6% nitrate, and the discharge about 60% nitrate. The tube bundle was comprised of type 309S (Nb) stainless steel drawn-and-welded tubes expanded and welded into two type 304L stainless steel tube sheets. The tubes failed by crevice corrosion. The failed tubes were defective as-received, and the establishment of concentration cells within the longitudinal cracks in the seam welds led to ultimate corrosive penetration of the wall. There was no evidence of crevice corrosion or any localized penetration of tubes that had sound welds. The leaking type 309S (Nb) welded tubes should be replaced with seamless tubes of type 304L stainless steel to minimize the areas requiring welding and to provide maximum weldability for the tube-sheet joints.

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.

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 flanged 100 mm (4 in.) diam low-carbon steel spool piece lined with Teflon was removed from a sulfuric acid denitrification system after cracks were observed in the painted coating. Visual and microstructural examination along with SEM fractography revealed scaled iron oxides on all opened crack surfaces. The surfaces had a faceted morphology, indicating intergranular fracture. Cracks originated at the interface between the tube and the Teflon liner Corrosion products were found caked into the intergranular region between the liner and the spool. The portion of the liner that had been exposed to the process stream was discolored. Failure of the spool was attributed to stress-corrosion cracking promoted by the presence of nitrates. Nitric acid contaminant in the sulfuric acid stream had diffused through the liner and accumulated in the annular space. Use of a liner that is more impermeable to the diffusion of ionic species was recommended.

Tube sheets (found to be copper alloy C46400, or naval brass, and 5 cm (2 in.) thick) of an air compressor aftercooler were found to be cracked and leaking approximately 12 to 14 months after they had been retubed. Most of the tube sheets had been retubed several times previously because of unrelated tube failures. Sanitary (chlorinated) well water was generally used in the system, although filtered process make-up water (river water) containing ammonia was occasionally used. Investigation (visual inspection, chemical analysis, mercurous nitrate testing, unetched 5X micrographs, and 250X micrographs etched in 10% ammonium persulfate solution) supported the conclusion that the tube sheets failed by SCC as a result of the combined action of internal stresses and a corrosive environment. The internal stresses had been induced by retubing operations, and the environment had become corrosive when ammonia was introduced into the system by the occasional use of process make-up water. Recommendations included making a standard procedure to stress relieve tube sheets before each retubing operation. The stress relieving should be done by heating at 275 deg C (525 deg F) for 30 min and slowly cooling for 3 h to room temperature.

An air compressor was installed at a chemical plant in which nitric acid was produced by burning ammonia with air. It was a 5000 hp, 5-stage centrifugal machine running at 6000 rpm, compressing air to 5 atm. Failure of the first stage impeller occurred due to a segment from the back plate becoming detached. On the remaining portion, cracks were visible running between the holes for rivets by which the vanes were attached. Metallographic examination of selected sections from the backplate revealed the material to be in the hardened and tempered condition, and the cracking to have initiated on the internal surface of the plate at the crevice between the plate and the vane. It was evident that the impeller failed by stress-corrosion cracking, which initiated in the crevice between the vanes and back plate and propagated through the plate along the line of the rivets where working stresses would be greatest. The compressor intake was situated in the vicinity of nitric acid pumps which had a history of leakage troubles, and which had evidently given rise to the nitrates found on the impeller.

A steam-condensate line (type 316 stainless steel tubing) began leaking after five to six years in service. The line carried steam condensate at 120 deg C (250 deg F) with a two hour heat-up/cool-down cycle. No chemical treatment had been given to either the condensate or the boiler water. To check for chlorides, the inside of the tubing was rinsed with distilled water, and the rinse water was collected in a clean beaker. A few drops of silver nitrate solution were added to the rinse water, which clouded slightly because of the formation of insoluble silver chloride. This and additional investigation (visual inspection, and 250x micrograph etched with aqua regia) supported the conclusion that the tubing failed by chloride SCC. Chlorides in the steam condensate also caused corrosion of the inner surface of the tubing. Stress was produced when the tubing was bent during installation. Recommendations included providing water treatment to remove chlorides from the system. Continuous flow should be maintained throughout the entire tubing system to prevent concentration of chlorides. No chloride-containing water should be permitted to remain in the system during shutdown periods, and bending of tubing during installation should be avoided to reduce residual stress.

Coaxial cable connectors made of brass were failing at a high rate after less than one year of service in an outdoor industrial environonment. The observed failures, which consisted of cracks in the body and end cap, were analyzed and found to be brittle fractures due to stress-corrosion cracking. Two common stress-corrosion cracking tests for copper materials were conducted on new connectors from the same manufacturing lot, confirming the initial determination of the fracture mode. Additional testing as was done in the investigation is often helpful when analyzing corrosion failures.

Eddy-current inspection was performed on a leaking absorber bundle in an absorption air-conditioning unit. The inspection revealed crack-like indications in approximately 50% of the tubes. The tube material was phosphorus-deoxidized copper. Investigation (visual inspection, chemical analysis, 0.75x images, 2x macrographs after light acid cleaning to remove corrosion product, and 75x micrographs) supported the conclusion that the absorber tubes failed by SCC initiated by ammonia contamination in the lithium bromide solution. No recommendations were made.

Two freshwater tanks (0.81 mm (0.032 in) thick, type 321 stainless steel) were removed from aircraft service because of leakage due to pitting and rusting on the bottoms of the tanks. One tank had been in service for 321 h, the other for 10 h. There had been departures from the specified procedure for chemical cleaning of the tanks in preparation for potable water storage. The sodium hypochlorite sterilizing solution used was three times the prescribed strength, and the process exposed the bottom of the tanks to hypochlorite solution that had collected near the outlet. Investigation (visual inspection, 95x unetched images, chemical testing with a 5% salt spray, chemical testing with sodium hypochlorite at three strength levels, samples were also pickled in an aqueous solution containing 15 vol% concentrated nitric acid (HNO3) and 3 vol% concentrated hydrofluoric acid (HF) and were then immersed in the three sodium hypochlorite solutions for several days) supported the conclusion that failure of the stainless steel tanks by chloride-induced pitting resulted from using an overly strong hypochlorite solution for sterilization and neglecting to rinse the tanks promptly afterward. Recommendations included revising directions for sterilization and rinsing.

Banding wires of the rotor of an 1800 hp motor were renewed following replacement of the banding rings. After about six months of service, a breakdown occurred due to bursting of the banding wires in several places. The 0.064 in. diam wire was nonmagnetic and of the 18/8 Cr-Ni type of austenitic stainless steel. The fractures were short and partially crystalline, with no evidence of slowly developing cracks of the fatigue type. Microscopical examination of sections taken through the fractures showed the cracking to be of the multiple branching type. Because the material was in the heavily cold-worked condition, it was not possible to determine with certainty if the cracks were of the inter- or trans-granular type. It was concluded that failure was due to stress-corrosion cracking in a chloride environment. Failure of the wires was likely due to the use of a chloride-containing flux during the soldering operation.

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.

Brass pipe couplings submitted for examination were deep-drawn from disks then annealed and subsequently cold threaded. Chemical analysis confirmed that the specified alloy Ms 63 was used for fabrication. Some of the pipe already showed fine cracks prior to their installation. In most cases however the cracks were detected after a certain period of operation. The intercrystalline course of the cracks indicated stress-cracking as it often appears in brass after heavier cold deformation. The splitting of the couplings could have been avoided by a tempering heat treatment at temperatures between 230 and 300 deg C after rolling the threads. This procedure would have reduced the internal stresses while maintaining strengthening gained by the cold deformation.

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.

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.

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.

Upon arrival at the erection site, an AISI type 316L stainless steel tank intended for storage of fast breeder test reactor coolant (liquid sodium) exhibited cracks on its shell at two of four shell/nozzle fillet-welded joint regions. The tank had been transported from the manufacturer to the erection site by road, a distance of about 800 km (500 mi). During transport, the nozzles were kept at an angle of 45 deg to the vertical because of low clearance heights in road tunnels. The two damaged joints were unsupported at their ends inside the vessel, unlike the two uncracked nozzles. Surface examination showed ratchet marks at the edges of the fracture surface, indicating that loading was of the rotating bending type. Electron fractography using the two-stage replica method revealed striation marks characteristic of fatigue fracture. The striations indicated that the cracks had advanced on many “mini-fronts,” also indicative of nonuniform loading such as rotating bending. It was recommended that a support be added at the inside end of the nozzles to rigidly connect with the shell. In addition to avoiding transport problems, this design modification would reduce fatigue loading that occurs in service due to vibration of the nozzles during filling and draining of the tank.