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.

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.

A wall section of a carbon steel choke body in gas service at 4400 psig blew out three months after the use of a corrosion inhibitor was stopped. Corrosion damage occurred in ripples, leaving both smoothly polished and unattacked areas. The corrodent in condensate wells was principally carbon dioxide dissolved in water condensed from the gas stream, with organic acids possibly an aggravating factor. A gas analysis showed no other corrosive agents. No metallurgical or fabrication defects were found in the carbon steel part. The mode of attack was corrosion-erosion, caused by the corrosive, high velocity gas flow. The corrosion rate of either the inhibited or uninhibited gas stream was too high for equipment in high pressure gas service. Type 410 (12% Cr) stainless steel was recommended for the choke bodies because other equipment such as valves made of type 410 showed no evidence of corrosion damage after three years' exposure. This change was made five years ago and there have been no failures since.

During an acceptance test of the Apollo spacecraft 101 service module prior to delivery, an SPS fuel pressure vessel (SN054) (titanium Ti-6Al-4V, approximately 1.2 m (4 ft) in diam and 3 m (10 ft) long) containing methanol developed cracks adjacent to the welds. The test was stopped. This acceptance test had been run 38 times on similar pressure vessels without problems. The methanol was a safe-fluid replacement for the storable hypergolic fuels (blend of 50% hydrazine and 50% unsymmetrical dimethyl hydrazine). Investigation (visual inspection and 65X images) showed similarities to stress-corrosion resulting from contamination during misprocessing of the vessels. However, another vessel underwent a more severe testing procedure and failed catastrophically. Further investigation supported the conclusion that the failure cause was SCC of titanium in methanol. Attack is promoted by crazing of the protective oxide film. It was learned that minor changes in the testing procedures could inhibit or accelerate the reaction. Recommendations included replacing the methanol with a suitable alternate fluid. Isopropyl alcohol was chosen after considerable testing. This incident further resulted in the imposition of a control specification (MF0004-018) for all fluids that contact titanium for existing and future space designs.

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.

New aircraft wing panels extruded from 7075-T6 aluminum exhibited an unusual pattern of circular black interrupted lines, which could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrated inspected, shot peened, H2SO4 anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. Scanning electron microscopy and microprobe analysis (both conventional energy-dispersive and Auger analyzers) showed that the anodic coating was applied over an improperly cleaned and contaminated surface. The expanding corrosion product had cracked and, in some places, had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack to a depth of 0.035 mm (0.0014 in.). It was recommended that a vapor degreaser be used during cleaning prior to anodizing. A hot inhibited alkaline cleaner was also recommended during cleaning prior to anodizing. The panels should be dichromate sealed after anodizing. The use of deionized water was also recommended during the dichromate sealing operation. In addition, the use of an epoxy primer prior to shipment of the panels was endorsed. Most importantly, surveillance of the anodizing process itself was emphasized.

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).

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.

Copper tubes from the cooler assemblies of a large air-conditioning unit exhibited leakage upon installation of the unit. Sections from two leaking tubes and one nonleaking tube were subjected to pressure testing and microscopic examination. The cause of leaking was determined to be pitting corrosion. Extensive pitting was found on the insides of all sections examined, with deep and numerous pits in leaking areas. Circumstantial evidence indicated that antifreeze solution left in the tubes from the manufacturing operation was the most likely cause of the pitting.

Several stainless steel bolts used on a Titan Space Launch Vehicle broke at the shank and failure was attributed to stress-corrosion cracking. But results could not be duplicated in the laboratory with salt-solution immersion tests until the real culprit was established: the secondary effect of galvanic coupling, hydrogen embrittlement.

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.

Although field corrosion tests had indicated that type 316L stainless steel would be a suitable material for neutralization tanks, the vessels suffered severe corrosion when placed in service. Welded coupons of type 316L had been tested along with similar Alloy 20Cb® (UNS NO8020) specimens in a lead-lined tank equipped with copper coils that had served in this function prior to construction of the new tanks. Both materials exhibited virtually no corrosion and no preferential weld attack. Type 316L was selected for the project. The subsequent corrosion was the result of the borderline passivity of type 316L in hot dilute sulfuric acid (about 0.1%). Inaccuracy of the testing was attributed to the presence of cupric ions in the lead-lined vessel fluids, which had been released by corrosion of the copper coils. Careful control of both temperature and pH was recommended to reduce the corrosion to an acceptable limit.

In January 1965, a Reaction Control System (RCS) pressure vessel (titanium alloy Ti-6Al-4V) on an Apollo spacecraft cracked in six adjacent locations. It used N2O4 for vehicle attitude control through roll, pitch, and yaw engines, and was protected from the N2O4 by a Teflon positive expulsion bladder. Investigation (visual inspection, pressure testing of 10 similar vessels, and chemical testing of the N2O4) supported the conclusion that the failure was due to stress corrosion from the N2O4, and specifically from a specification change in the military specification MIL-P-26539. Recommendations included revising the specification to require a minimum NO content of 0.6%.

Visual examination, optical and scanning electron microscopy were used to determine the cause of failure in the connector groove of a marine riser coupling. The specified steel was AISI 4142 (0.40 to 0.45% C; 0.75 to 1.00% Mn; 0.20 to 0.35% Si; 0.80 to 1.10% Cr; 0.15 to 0.25% Mo) normalized from 9000C. Microscopic examination revealed the crack's initiation point and subsequent propagation. SEM examination of chemically stripped corrosion showed that corrosion fatigue and stress corrosion might have contributed to the initial slow crack growth. Impact tests revealed a fracture transition temperature in excess of 1000C. The sequence of events leading to failure was detailed. The main recommendation was to quench and temper existing couplings and to use a lower carbon quenched and tempered steel for new couplings.

Two type 420 martensitic stainless steel load cell bodies, which had been installed under two of the four legs of a milk storage tank failed in service. The failure occurred near a change in section and involved fracture of the entire cross section. Examination showed a brittle fracture that was preceded by a small fatigue region. Pitting corrosion was evident at the fracture origin. The areas around the load cells had been subjected to regular washdowns using high-pressure hot water, and the pitting was attributed to crevice corrosion between the load cell and the holddown bolts. Prevention of such corrosion by the use of a flexible sealant to eliminate the crevice was recommended.

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.

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.

A type 316 stainless steel pipe reducer section failed in service of bleached pulp stock transfer within 2 years in a pulp and paper mill. The reducer section fractured in the heat-affected zone of the flange-to-pipe weld on the flange side. The pipe reducer section consisted of 250 and 200 mm (10 and 8 in.) diam flanges welded to a tapered pipe section. The tapered pipe section was 3.3 mm (0.13 in.) thick type 316 stainless steel sheet, and the flanges were 5 mm (0.2 in.) thick CF8M (type 316) stainless steel castings. Visual and metallographic analysis indicated that the fracture was caused by intergranular corrosion/stress-corrosion cracks that initiated from the external surface of the pipe reducer section. Contributory factors were the sensitized condition of the flange and the concentration of corrosive elements from the bleach stock plant environment on the external surface. In the absence of the sensitized condition of the flange, the service of the pipe reducer section was acceptable. A type 316L stainless steel reducer section was recommended to replace the 316 component because of its superior resistance to sensitization.

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.

Three instances involving the failure of aluminum wiring at the service entrance to single-family homes are discussed. Arcing led to a fire which severely damaged a home in one case. In a second, the failure sequence was initiated by water intrusion into the service entrance electrical box during construction of the home. In the third, failure was caused by a marginal installation. Strict adherence to all applicable electrical codes and standards is critical in the case of aluminum wiring. Electrical components not specifically designed for aluminum must never be used with this type of wiring. All doors, panels and similar portions of electrical boxes should be secured to prevent damage to surroundings in the event of an electrical fault. If symptoms of arcing are observed, professional service should be sought. The latest designs of connectors for use with aluminum wiring are less susceptible to deviations in installation practice.