A group of control valves that regulate production in a field of sour gas wellheads performed satisfactorily for three years before pits and cracks were detected during an inspection. One of the valves was examined using chemical and microstructural analysis to determine the cause of failure and provide preventive measures. The valve body was made of A216-WCC cast carbon steel. Its inner surface was covered with cracks stemming from surface pits. Investigators concluded that the failure was caused by a combination of hydrogen-induced corrosion cracking and sulfide stress-corrosion cracking. Based on test data and cost, A217-WC9 cast Cr–Mo steel would be a better alloy for the application.

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.

Three separate corrosion mechanisms were involved in the failure of an AISI type 304 stainless steel pipe elbow. The major cracks, including the one that penetrated the wall, tend to be wide-mouthed, tapering to a blunt tip, with corrosion products filling much of the crack space. This was characteristic of corrosion fatigue. The second type of cracking originated at some of the major cracks. These cracks were branched and transgranular, which is characteristic of stress-corrosion caused by chlorides. The third crack mode, an intergranular network, was most probably the result of hydrogen sulphide attack. The 13-year service life of the elbow made it difficult, if not impossible, to determine the order of the corrosion mechanisms or the length of time it took to reach the present state of degradation after the initiation of corrosion. Based on the long service life the present material has given, it was recommended that it be used again.

Nineteen out of 26 bolts in a multistage water pump corroded and cracked after a short time in a severe working environment containing saline water, CO 2 , and H 2 S. The failed bolts and intact nuts were to be made from a special type of stainless steel as per ASTM A 193 B8S and A 194. However, the investigation (which included visual, macroscopic, metallographic, SEM, and chemical analysis) showed that austenitic stainless steel and a nickel-base alloy were used instead. The unspecified materials are more prone to corrosion, particularly galvanic corrosion, which proved to be the primary failure mechanism in the areas of the bolts directly exposed to the working environment. Corrosion damage on surfaces facing away from the work environment was caused primarily by chloride stress-corrosion cracking, aided by loose fitting threads. Thread gaps constitute a crevice where an aggressive chemistry is allowed to develop and attack local surfaces.

A routine examination on a seat ejection system found that the catapult attachment swivel fabricated from 7075-T651 aluminum alloy plate contained cracks on opposite sides of the part. This swivel, or bath tub, does not experience extreme loads prior to activation of the catapult system. Some loads could be absorbed however, when the aircraft is subjected to G loads. Visual examination of the part revealed that cracks through the wall thickness initiated on the inner walls of the fixture. Scanning electron microscopy (SEM) and electron optical examination revealed that the cracking pattern initiated and progressed by an intergranular failure mechanism. It was concluded that failure of the catapult attachment swivel fixture occurred by SCC. It was recommended that the 7075 aluminum ejection seat fixture be supplied in the T-73 temper to minimize susceptibility to SCC.

Military aircraft use a cartridge ignition system for emergency engine starts. Analysis of premature failures of steel (AISI 4340) breech chambers in which the solid propellant cartridges were burned identified corrosion as one problem with an indication that stress-corrosion cracking may have occurred. A study was made for stress-corrosion cracking susceptibility of 4340 steel in a paste made of the residues collected from used breech chambers. The constant extension rate test (CERT) technique was employed and SCC susceptibility was demonstrated. The residues, which contained both combustion products from the cartridges and corrosion products from the chamber, were analyzed using elemental analysis and x-ray diffraction techniques. Electrochemical polarization techniques were also utilized to estimate corrosion rates.

Forged aluminum alloy 2014-T6 hinge brackets in naval aircraft rudder and aileron linkages were found cracked in service. The cracks were in the hinge lugs, adjacent to a bushing made of cadmium-plated 4130 steel. Investigation (visual inspection and 250X micrographs) supported the conclusion that the failure of the hinge brackets occurred by SCC. The corrosion was caused by exposure to a marine environment in the absence of paint in stressed areas due to chipping. The stress resulted from the interference fit of the bushing in the lug hole. Recommendations included inspecting all hinge brackets in service for cracks and for proper maintenance of paint. Also suggested was replacing the aluminum alloy 2015-T6 with alloy 7075-T6, and surface treatment for the 7075-T6 brackets was recommended using sulfuric acid anodizing and dichromate sealing. Finally, it was also recommended that the interference fit of the bushing in the lug hole be discontinued.

A T-bolt was part of the coupling for a bleed air duct of a jet engine on a transport plane. Specifications required that the 4.8 mm diam component be made of AISI type 431 stainless steel and heat treated to 44 HRC. The operating temperature of the duct is 425 to 540 deg C (800 to 1000 deg F), but that of the bolt is lower. The T-bolt broke after three years of service. The expected service life was equal to that of the aircraft. It was found that the bolt broke as a result of SCC. Thermal stresses were induced into the bolt by intermittent operation of the jet engine. Mechanical stresses were induced by tightening of the clamp around the duct, which in effect acted to straighten the bolt. The action of these stresses on the carbides that precipitated in the grain boundaries resulted in fracture of the bolt. Due to the operating temperatures of the duct near the bolt, the material was changed to A-286, which is less susceptible to carbide precipitation. The bolt is strengthened by shot peening and rolling the threads after heat treatment. Avoiding temperatures in the sensitizing range is desirable, but difficult to ensure because of the application.

During a routine shear-pin check, the end lug on the barrel of the forward canopy actuator on a naval aircraft was found to have fractured. The lug was forged from aluminum alloy 2014-T6. Investigation (visual inspection, 2x views, and 140X micrographs etched with Keller's reagent) supported the conclusion that the cause of failure was SCC resulting from exposure to a marine environment. The fracture occurred in normal operation at a point where damage from pitting and intergranular corrosion acted as a stress raiser, not because of overload. The pitting and intergranular attack on the lug were evidence that the surface protection of the part had been inadequate as manufactured or had been damaged in service and not properly repaired in routine maintenance. Recommendations included anodizing the lug and barrel in sulfuric acid and giving them a dichromate sealing treatment, followed by application of a coat of paint primer. During routine maintenance checks, a careful examination was suggested to look for damage to the protective coating, and any necessary repairs should be made by cleaning, priming, and painting. Severely corroded parts should be removed from service.

A nose landing gear cylinder made from AISI 4340 Ni-Cr-Mo alloy steel was found cracked and leaking, causing partial depressurization. Investigation revealed the crack to be a stress-corrosion type, judging by the 6500x electron fractograph. It had started in a region of concentrated, large non-metallic inclusions near the chromium-plated ID of the cylinder. Also, there were breaks in the chromium plate and pits in the underlying base metal. The cylinder had been in service for 18,017 h, and 5948 h had passed since the first and only overhaul. Substandard plating of the ID at this time ultimately resulted in pitting of the metal. The combination of surface pitting and stress concentration at the nearby inclusions resulted in stress-corrosion cracking.

Pitostatic system connectors were being found cracked on several aircraft. Two of the cracked connectors made of 2024-T351 aluminum alloy were submitted for failure analysis. The connectors had cut pipelike threads that were sealed with Teflon-type tape when installed. Longitudinal cracks were located near the opening of the female ends of each connector. A cross section showed intergranular cracking with multiple branching in one connector. Scanning electron microscopy (SEM) showed intergranular cracking and separation of elongated grains. A cross section of connector threads showed an incomplete thread form resulting from improper tapping. It was concluded that the pitostatic system connectors failed by SCC. The stress was caused by forcing the improperly threaded female nut over its fully threaded male counterpart to effect a seal. The one connector tested for chemical composition was not made of 2024 aluminum alloy as reported but of 2017 aluminum. It was recommended that the pitostatic system connector manufacturing process be revised to produce full-depth threads rather than pseudo pipe threads. Wall thickness should be increased to increase the hoop stress bearing area if pipe threads were to be used. A determination of proper torque values for tightening the connectors was suggested also.

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.

Five cylinders out of a group of nine in a drying machine developed leaks after a few months service in a textile mill. Leakage was reported from locations between the hoop and body and from the circumferential welds. The materials in the affected area were 18/8 Ti and 18/10/3/Mo austenitic stainless steels. Examination of the cracks at high magnification revealed them to be of the stress-corrosion type. The welds were of satisfactory quality. Cracking was also visible at these locations, this again being of the stress corrosion type. The method of cylinder construction introduced a crevice between the outer hoop and the cylinder at the inboard edge so that during washing of the rolls, water could penetrate the crevice and subsequent heating would lead to the concentration of chlorides within the crevice. Redesign of the cylinder to eliminate the crevice was recommended.

During a routine inspection, cracks were discovered in several aluminum alloy (similar to either 2014 or 2017) coupling nuts on the fuel lines of a missile. The fuel lines had been exposed to a marine atmosphere for six months while the missile stood on an outdoor test stand near the seacoast. A complete check was then made, both visually and with the aid of a low-power magnifying glass, of all coupling nuts of this type on the missile. Investigation (visual inspection, spectrographic and chemical analysis, and metallographic examination) supported the conclusion that the cracking of the aluminum alloy coupling nuts was caused by stress corrosion. Contributing factors included use of a material that is susceptible to this type of failure, sustained tensile stressing in the presence of a marine (chloride-bearing) atmosphere, and an elongated grain structure transverse to the direction of stress. The elongated grain structure transverse to the direction of stress was a consequence of following the generally used procedure of machining this type of nut from bar stock. Recommendations included changing the materials specification for new coupling nuts for this application to permit use of only aluminum alloys 6061-T6 and T651 and 2024-T6, T62, and T851.

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.

Small paddles used to mix pulp had experienced a high incidence of breakage through the shafts. In some of the shanks, shrinkage was found relatively close to the surface where threads had been cut all the length of the shaft. Chemistries were within normal CF-8M ranges. Metallography showed the parts to be correctly heat treated. Cross sections of several of the parts showed pitting corrosion, and beneath the pits, stress-corrosion cracks in areas where the shafts had been bent during use. All the samples showed deep SCC in the areas where bending had occurred. In several cases, centerline shrinkage from inadequate risering had decreased life by reducing the cross-sectional area. Type CF-8M is not resistant to chloride SCC where the chloride concentration is considerable. The biggest problem was the bending of these parts. Deformed material with high residual stresses would always be susceptible to SCC. Redesign to lower stresses was essential. In addition, change to a high-strength duplex stainless steel with its higher strength and greater resistance to chlorides was recommended. Finally, the part must be adequately risered to produce solid shanks free from shrinkage.

A CF-8M (cast type 316) neck liner or manway was removed from the top of a digester vessel. Repeated attempts to repair the part in the field during its life cycle of many years had failed to keep the unit from leaking. The casting was a CF-8M modified with the molybdenum level at the top end of the range. The plate was standard 317L material. The filler metal was type 316, although marginal in molybdenum content. Investigation (visual inspection, chemical analysis, micrographs, and metallographic examination) supported the conclusion that the damage to the neck liner was due to Cl-SCC in an area of debris buildup. It appeared the original casting suffered SCC in a low-oxygen area high in chlorides from repeated wet/dry cycles where there was a buildup of debris. Recommendations included redesigning the neck liner to eliminate the abrupt change where there was debris buildup. If redesign was impossible, an alloy more resistant to Cl-SCC, such as a duplex stainless steel or a high-molybdenum (4 to 6%) austenitic stainless steel, should be used.

Several air heat exchangers failed in service in a pulp and paper operation. The tubes were made from AISI 316 stainless steel with an extruded aluminum fin mechanically bonded to the outside. Originally, the failures were blamed on poor tube to header welds. The units were sent back to the manufacturer for repair. Some of the units failed the hydrostatic test after they were repaired. Microscopic examination revealed the presence of branched transgranular cracks characteristic of stress-corrosion cracking. Only some of the tubes failed and these did so by stress-corrosion cracking. The most probable primary cause of the stress-corrosion cracking was local high residual stresses indicated by the areas of high hardness in the tubes. Low halogens in the water and airborne corrodents found normally in a pulp and paper mill were all that were required in the presence of high residual stresses in the tubes to initiate stress-corrosion cracking. Use of a low-carbon grade of stainless steel such as 316L was recommended to facilitate formation of the tube without producing excessive residual stresses. It was recommended also that failed units be segregated until it can be determined if the failure was related to operating pressure or some other unique cause.

The Pandia digester is a long cylindrical vessel which uses alkaline sulfite liquor to cook sawdust for pulping. The inlet cone was fabricated from AISI 304L stainless steel with E308 welds. Typical liquor concentration was approximately 80% NaOH, 20% Na2SO3 with chloride concentrations at 2 grams per liter. The operating pressures in the inlet cone were up to 1.2 MN/sq m (170 psig). The inlet cone had developed leaks within a year of service. Liquid penetrant inspection showed significant through-wall cracking near the fillet welds joining the bottom flange and side wall and the butt welds. Metallographic specimens were prepared from the welds to examine the microstructure and nature of the cracks. The cooking liquor at the inlet cone contained over ppm chlorides and was aggressive to 304 stainless steel. The cracking was identified as chloride-induced SCC. The inlet cone was replaced with an Inconel clad carbon steel inlet cone to combat the SCC.