The Research Laboratory of the Development  and Research Department of The Mond Nickel Company Limited,  has been concerned is developing new heat and  corrosion-resistant

alloys to meet the demands of  designers  of  gas-turbine chemical plant and electrical equipment.Premature service failures in these materials are not common, and when they do occur they can usually be  traced  to  misuse or  mal-treatment of the material during  fabrication  or  appli-cation, or to  some  form of  accidental contamination or damage. Important lessons have nevertheless been learned from post-mortem  examinations of  service  failures, and 

the following  five  examples  have  been  selected as of interest not  only  to  works'  metallurgists but also to designers, to  whom  these  special  materials  offer new possibilities, and to fabricators, to  whom  they present certain problems

Cotton, R F

Franklin, A W

English

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SOME SERVICE FAILURES ARISING FROM VARIOUS TYPES OF CORROSIONB1R.F. Cotton , B.Sc., and A.W. Franklin , M.Sc., A.I.M.SYNOPSIS.Five examples of service failures, in heat-and corrosion-resisting alloys, involving variousforms of corrosion. are briefly described, and recommendations made to avoid their recur-rence are indicated.Introduction.The Research Laboratory of the Development and Research Department of The MondNickel Company Limited, has been concerned is developing new heat and corrosion-resistantalloys to meet the demands of designers of gas-turbine chemical plant and electrical equipment.Premature service failures in these materials are not common, and when they do occur they canusually be traced to misuse or maltreatment of the material during fabrication or application, orto some fiirm of accidental contamination or damage. Important lessons have neverthelessbeen learned from post-mortem examinations of service failures, and the following five exampleshave been selected as of interest not only to works' metallurgists but also to designers, to whomthese special materials offer new possibilities, and to fabricators, to whom they present certainproblems.1. Welded Stainless -Steel Tank.The first case concerned 100 gal, welded stainless-steel tanks, used fins storing flavouringessences. These tanks developed serious Icaks after a relatively short time in service, owing tothe special lbrm of corrosive attack known as "weld decay." This effect occurs in unstabilisedstainless steels, and is avoided by the use of steels containing titanium or niobium. The leaksoccurred near welds around the bottom outlet sumps of the tanks. Examination of one tankrevealed a large crack in the bottom plate, running alongside the weld joining this plate to theconical sump. Microscopical examination showed the crack to be intercrystalline and associatedwith grain-boundary' corrosion. The crack and corrosion were in a zone where the grainswere outlined by precipitated carbides. Sections cut through the weld between thebottom plate and one of the side plates also revealed a similar band of grain-boundary carbidesin the bottom plate, about 1 mm. away from the weld head. Here, however, only slight inter-granular corrosion had occurred. The zones of carbides did not appear on the other side of theweld in the side plate. Analysis of the component sheets showed that the side sheets and the partsof the sump were made from 18 °,, chromium, 8 °,, nickel austenitic stainless steel containing3% molybdenum. The botttom plate, however, was a straight 18 °" chromium 8%'o nickel steeland did not contain molybdenum or any other carbide-stabilising elements.The localised corrosion in the zone of grain-boundary carbide is typical "Weld decay" andand had obviously occurred due to the inadvertent use of unstabilised stainless steel. This effectwas not, however, sufficient to account for the early failure of the tank, as shown by the relative-Iv slight corrosion in the affected zone along the weld in the side of the tank. It seems likely thatthe corrosion near the sump had been greatly accelerated by stresses which could have beenresidual, resulting from the welding, or vibratory, transmitted to the sump along connectingpiping.2. Stainless-Steel Bubbie Caps.Stress-corrosion can occur in stainless steel in the absence of such conditions as the carbideprecipitation in the first example. As interesting case recently investigated was the failure, bysevere cracking, of stainless-steel bubble caps after only a few months' service in distillation149II '--L em1 ,11 ____ 1_ _1 . ON, 1^11Ii - u"" -1ISOcolumns at an oil refinery. The caps were deep-drawn from stainless-steel sheet: slots had thenbeen punched out of the rim, which was afterwards flared out. The failures occurred in caps onthe top tray only of the columns, where they were exposed to reflex containing small proportionsof ammonium hydroxide and ammonium chloride, at a temperature of about 110 C.Stress-corrosion was suspected, and microscopical examinations of one of the failed capsshowed branched cracks, both inter-and transcrystalline, which are typical of this firm of failurein stainless steel (Figures I and 2). As shown in Figure 1, severe cracking had occurredl aroundthe slots and this also extended to the radius between the wall and the flat top of the cap.In order to find out what changes in material or processing procedure were necessary toovercome this trouble. the bubble cap manufacturers asked The Mond Nickel Company to carryout laboratory stress corrosion tests on unused caps made from three different grades of austeniticstainless steel, the analyses of which are given in Table 1. The caps from each grade of steelwere produced by three different fabrication procedures, namelySeries I Pressed, serrated, annealed, and flared.Series 2 -- Pressed, serrated, flared, finally annealed.Series 3 - Pressed, serrated, flared.In each case the annealing treatment was 15 minutes at 1085-C:., followed by air colling. Series3 corresponded to the normal procedure used f o r the caps which had failed in service. I t shouldbe stated that the usual annealings were, of course, carried of during the deep drawing operationfiir all three series.The stress-corrosion tests were carried out by totally immersing the caps, supported onrubber insulators, in a tank of boiling -I2"„ magnesium chloride solution boiling point154- C., as described by Scheil.*In this solution branched cracks typical of' stress-corrosion failure gradually developed inall but the specimens which had receeed the final annealing treatment. The extent of the crack-ing, the time until the first appearanc of cracks, and the resonance on testing for "ring" at theend of the exposure, are summarised in Table 2. The cracks appeared mainly on the curvedregion between the wall and the flat top, as seen in Figure 3. Since they were not obviated bythe annealing applied before flaring, it would appear that this intermediate annealing treatmentwas inadequate.Steel B showed less susceptibility to cracking than the other two grades. Since it containedmolybdenum, it was expected to be better than Steel A., which did not contain the element.Its superiority to Steel C, however, was difficult to explain, although it was possibly clue todifferent proceessing techniques applied to the original sheet which might have produceddifferences in grain size and differing amounts of ferrite in the structure.The manufacturer of the bubble caps was recommended to use steel of type, B. and to applythe final annealing treatment to the finished caps. Since final annealing became standard practice,by design or coincidence no further failures have occurred.An interesting feature of this investigation was the close similarity between the cracks pro-duced in the laboratory test and those which occurred during service. Not only did they occurin the same positions on the caps. but their appearance under the microscope was very similar.3. Nickel -chromium alloy Heat-exchanger tubes.'Pubes made of a nickel-chromium base heat-resisting alloy failed after only 85 hours'service in a heat-exchanger at a temperature at which the alloy should have behaved quitesatisfactorily. The fracture was intercrystalline and had occurred with no detectable elongation.This feature ruled out failure by creep or simple overstressing and suggested that the prints causeof' the failure was fatigue.Patches of brown-coloured deposit were found irregularly distributed inside the tube andspectrographic analysis of portions of the brown material gave very strong indications of the* M.A. Scheil and R .:\. Husehv. "Stress Corrosion Cracking of .\ustenitic Stainless Steels," \\ Icling Journal,1911. Vol . 2:3. pp . :361-363.151Figure ISTAINLESS-S'1'I:EL BUBBLE' CAPPiece of failed cap. shaming stress-corro;ium crackFigure 2STAINLESS-STEEL BUBBLE CAPStress-corrosion crack ( x 100)152presence of lead, tin and bismuth. It was known that a fusible-alloy filler had been used inbending the tubes to their final form, and it was concluded that the brown deposits were theoxidised remains of residual particles of the alloy left in the tubes. "Traces of the same elementswere detected on the surfaces of the fracture. Microscopical examination showed that, beneathseveral of the deposits, alloying with the tube material had occurred.It was concluded that the failure of this tube was due to fatigue under the corrosiveinfluence of traces of lead, tin and bismuth.A failed air-heater tube in the same alloy examined subsequently revealed a very similartype of fracture. In this case fatigue failure had been accelerated by the notch effect of defectsin the bore, arising during manufacture of the tube, but traces of bismuth and lead were againfound, and probably contributed to the failure. In this instance no fusible alloy had been usedand contamination probably arose from residual traces of drawing compounds left in the tube.4. Electric furnace heating element.A nickel-chromium alloy heating element failed after only 100 hours' service at 1000°C.in a furnace used for gasification of coal.The surface of the failed element was found to be covered with patches of corrosion in theform of "Wart-like" scale excrescences. The microstructure beneath these corroded areas con-tained light-and dark-grey constituents. The dark-grey constituent had penetrated along thegrain boundaries below the main corroded layer.Similar corrosion of this type had been seen before on nickel-chromium alloys which hadbeen exposed to sulphur-hearing gases at high temperatures. A sodium azide iodine spot teston the polished microspecimen confirmed the presence of sulphides in this instance also.Investigation showed that although the heating element was enclosed in a silica muffle,it was possible for gases from the coal to come into contact with it, and it was concluded that thesulphur in these gases was the cause of the failure. The remedy in this case was obvious.5. Nickel -Chromium alloy tubes.A number of nickel-chromium alloy tubes were separated by asbestos from other tubesduring their final heat-treatment. When they were taken from the furnance it was noticed thatvery heavy scaling had taken place where the tubes had been in contact with the asbestos. Theheavily attacked sides of two tubes are shown in Figure 4. This material normally has but athin uniform oxide skin after heat-treatment. Micro-examination of transverse sections throughthe tubes showed that, beneath the layer of scale, a layer of the metal contained corrosion pro-ducts resembling those formed by sulphur attack. At the most heavily scaled part of the tubesthe wall thickness had been reduced by about 8°c„ and penetration of the corrosion, mainly alongthe grain boundaries, extended to a depth of equal to a further 8°„ of the original wall thickness.The presence of sulphides was confirmed by chemical spot tests on the micro-section andalso by chemical analysis of a sample of scale removed from one of the tubes, which had a sulphurcontent of over 1°,. It was concluded that sulphur present in the asbestos had caused thecorrosion. The amount of sulphur present must have been considerable, since parts of the tubewall which had not been in direct contact with the asbestos showed some signs of sulphur attack.It is hoped that these few examples have served to indicate the wide variety of factors whichcan cause failure of nickel-chromium and nickel-chromium-iron alloys. Contamination bytraces of foreign matter, surface damage through mishandling, undetected residual stresses,and unforeseen corrosive influences can all lead to serious and expensive failure, and it is only bycontrolling such factors in manipulation and use of these newer alloys that full advantage of theirspecial properties is to be obtained.ACKNOWLEDGEMENT.The authors wish to thank the Directors of The Mond Nickel Company Limited for permis-sion to publish this paper. Thanks are also due to the authors' colleagues, Mr. M.A. Comleyand Mr. H. Lewis, who carried out some of the investigations described.153Figure 3STAINLESS-STEEL BUBBLE CAP7b) view, showing, cracks produced during laljorator% ,tn s,-corrosion tcstFigure 4CORRODED NICKEL-CHROMIUM ALLOY TUBES154TABLE I.ANALYSIS OF BUBBLE CAP MATERIALS SUBJECTED TO STRESS CORROSIONTESTS.Chemical composition per centCarbon Silicon Manganese Nickel (.hro-miumMolvh- 'denuniTitaniumSteel A. 0. 10 0.78 0.34i I8.25 18.12 0.24 0.61Steel B. I 0.07 0.22 0.40 8.40 18.85 2.82 NilSteel C. 0.06 0.63 0.68 9. 18 17.87 2.83 NilTABLE 2.RESULTS OF STRESS-CORROSION CRACKING TEST OF STAINLESS STEEL BUBBLECAPS.The times stated are the hours of exposure to boiling 42','c magnesium chloride solution(B.P. 154 ° C) before cracking was first observed. All specimens were previously tested in magne-sium chloride solution boiling at 143 °C for a period of 44O hours without fracture.Series 1. Pressed, serrated, Series 2. Pressed, serrated, series 3. Pressed,MATERIAL annealed and flared. flared and finally annealed. Serrated and flared.Steel A Branched cracks, in places Apparentlycompletely penetrating the unaffected.metal. Affected area(about 2' x " ") located onflat and severely curvedregion, not at serrations.(236 hours",Steel B Nearly unaffected except Apparentlyfor some fine superficial unaffected.hair cracks at a late stagein the test causing slightloss of resonance.(265 hours)Severe branched cracks pene-trating the metal over alength of more than 2".No cracks at serrations.Poor resonancc. 27 hours)-One large branched crack2." long over flat andcurved regions. Poorresonance. (96 hours).Steel C Very numerous branched Apparently Numerous severe cracks incracks around central hole unaffected. flat and curved regions andand over flat and curved in sides. Very poor reson-regions. Very poor reson - ance. ( 27 hours').ance. (48 hours.)

Some Service Failures arising from Various Types of Corrosion

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Some Service Failures arising from Various Types of Corrosion

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