The surface composition of oxides formed on Alloy 600 under conditions similar to those in the primary side of PWR heat exchangers has been studied as a function of potential using Rutherford backscattering and proton inelastic scattering. Electropolished samples of Alloy 600 were exposed at several potentials to a solution of 0.18M H/sub 3/BO/sub 3/(2000 ppM B) with 0.28M LiOH (1.4 ppM Li) at 300/sup 0/C for 450 hours. The potentials relative to an internal hydrogen electrode ranged from -.09 to 750 mV. RBS analysis showed little or no oxide formation on samples exposed at 0 mV. Above 0 mV oxide layers formed whose thicknesses increased with potential. In addition the RBS showed a significantly enhanced concentration of aluminum and silicon in oxide. Both the oxygen and the sum of the aluminum and silicon content appeared to maintain a fixed surface concentration independent of the oxide thickness. Boron and lithium concentration were analyzed with proton inelastic scattering. No lithium was found in any sample. The boron concentration was found to follow the thickness of the oxide

Hanson, A.L.

Isaacs, H.S.

Kraner, H.W.

Hanson, A. L.

Isaacs, H. S.

Kraner, H. W.

UNT Digital Library

VjiY/fCC;"* -—)3L-IL—3 38 30DE84 C02872Determination of Surface Oxide Compositions on Alloy 600 UsingRutherford BackscatteringA. L. Hanson, H. S. Isaacs, and H. W. KranerBrookhaven National Laboratory, Upton, Hew York 11973, USADISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.Presented atCorrosion 84New Orleans, LouisianaApril 2-6, 1984The submitted manuscript has been authored under contract DE-AC02-76CH00016with the Division of Basic Energy Sciences, US Department of Energy.Accordingly, the US Government retains a nonexclusive, royalty-free license topublish or reproduce the published form of this contribution, or allow othersto do so, for US Government purposes.D I S T r M M OF THIS DOCUMENT I? UHUMilEODetermination of Surface Oxide Compositions on Alloy 600 UsingRutherford BackscatteringA.L. Hanson, H.S. Isaacs, H.W. KranerBrookhaven National Laboratory, Upton, N.Y. 11973ABSTRACTThe surface composition of oxides formed on Alloy 600 underconditions similar to those in the primary side of FWR heat exchangershas been studied as a function of potential using Rutherfordbackscattering and proton inelastic scattering. Electropolishedsamples of Allov 600 were exposed at several potentials to a solutionof 0.18M H3BO3(2000ppm B) with 0.28M LiOH (1.4ppm Li) at 300°C for 450hours. The potentials relative to an internal hydrogen electroderanged from -.09 to 750 mV. RBS analysis showed little or no oxideformation on samples exposed at 0 nV. Above 0 mV oxide layers formedwhose thicknesses increased with potential. In addition the RBSshowed a significantly enhanced concentration of aluminum and siliconin oxide. Both the oxygen and the sum of the aluminum and siliconcontent appeared to maintain a fixed surface concentration independentof the oxide thickenss. Boron and lithium concentration were analyzedwith proton inelastic scattering. No lithium was found in anysample. The boron concentration wan found to follow the thickness ofthe oxide.INTRODUCTION:The nickel based Alloy 600 has been widely used in the nuclearpower industry for heat exchanger tubing because of its highresistance to general corrosion, which is controlled by the formationof a passivating surface oxide layer. Only a few studies have beencarried out on the composition of the oxide layer or how it isinfluenced by the presence of boron or lithium, which are generallyadded to the primary coolant. The presence of hydrogen or possiblyoxygen would be expected to alter the potential at which the oxidegrows which in turn may alter the susceptibility of the alloy tostress corrosion cracking. The corrosion of Alloy 600 is difficult tocharacterize since there are many factors affecting the performance ofthe alloy. These seem to include strong dependences on temperature(the iron dissolution peaking at roughly 200°C), physical state of thewater (liquid or steam), impurities in the water, cover gas, surfaceroughness, and the potential of the particular surface influenced bythe presence of dissolved oxygen, hydrogen, or other oxidizing orreducing species.While corrosion from the seconda^ i side is the major problem ofsteam generators, understanding the exiting and possible future prob-lems from the primary side is important. We previously reported (1)work studying corrosion products formed on electropolished Alloy 600exposed to caustic solution at 315°C for 10 days in a nickel autoclaveat several potentials. The autoclave had been pressurized at 1.38MPawith a %5H2 95%N2 cover gas. That work showed an enhancement ofnickel on the surface with potentials of -90 mV and 0 mV; with thenickel layer being thicker at the more negative potential. Neither ofthese samples showed any significant oxide formation. At 170 and225 mV a significant highly stressed oxide layer had formed withapproximately a 50 at% metal concentration. The total oxygen contentwas not determined because the oxide was found to flake off thesurface of the material. The basic stoichometry of the alloy wasessentially maintained through the oxide without a major buildup ofany specific element. At and above 270 mV, however, a duplex oxidewas found. The oxide in contact with the solution was high in nickel,and the oxide adjacent to the the metal was high in chromium. Adepletion of chromium was also found by Hclntyre et al.(2) on exposingAlloy 600 to oxygen containing neutral and caustic solutions at285°C. In their work there was apparently no major buildup of anyminor alloying elements on their specimens prepared by abrasion andexposed to either deionized water at pH 7 or solutions of higher pHvalues with LiOH additives.The purpose of this work was to characterize the effects ofpotential on the surfaces formed on Alloy 600 exposed to conditionssimilar to those of the primary side of PWR steam generators. Theprimary coolant consists of borated water (less than 4400 ppm boron)with a trace of lithium hydroxide (0.2 to 1.0 ppm Li) (3). Thetemperature of the water ranges from 290~C to 325°C with hydrogenadded to the covergas to minimize the oxygen content. Theconcentration of boron in oxide films formed on type 316 stainlesssteels have been measured after exposure to 340°C water with 1000 ppmboron. After about 600 hours the boron content of the oxide reached astable value of about 0.03 jig/cm , or about O.Oi % of the corrosionfilm, a value similar to that in crud. This level was substantiallylower than previous results which had maxia of 0.75 and 1.7 pg/cmafter about 300 hour exposures with and without 1 pom Li as anadditive (4,5).EXPERIMENTAL:Rectangular samples, 10x20x0.8 mm in size, were cut from an asreceived annealed sheet of Alloy 600 with weight percent composition:75.31 Ni, 14.59 Cr, 9.45 Fe, 0.33 Cu, 0.20 Mn, 0.20 Al, 0.20 Ti,0.11 Si, 0.01 C and 0.001 S, determined by chemical analyses. Thesesamples were electropolished in a mixture of 60% H3PO^, 20% H2S04, and20% H20, by volume, at 35 to 40cC with a cell potential of 15 volts.After polishing the samples were washed repeatedly in distilled waterand dried after a methanol wash.The electropolished samples were exposed in a borate solution of0.18 M boric acid (2000ppm B) and 5 ppm lithium hydroxide (1.4 ppm Li)for 450 hours at 300°C in a nickel autoclave. The pH of the solutionwas 6.6. Prior to heating, the solutions were deoxygenated. Theautoclave was pressurized at 9.7 MPa (1400 psi) with a 5% H 2 and95% H2 covergas. The samples were exposed at a series of controlledpotentials using the technique of Seys and van Haute (6) as developedby Roberge et al. (7). The controlled potential range was -0.03 toto 710 mV relative to a nickel electrode at its mixed or corrosionpotential. A freely corroding Allo/ 600 specimen also exposed in theborate solution gave potentials close to that of the nickel. Theaverage potential of this sample was -0.09 raV. At the termination ofthe test the potential controls were switched off and the autoclaveallowed to cool prior to removing the samples.The samples were analyzed with Rutherford BackscatteringSpectrometry (RBS), to study the relative elemental composition of thesurface layers; SEM, to study the microstructure of the surfaces; andnuclear reaction analysis (NRA), to determine the boron and lithiumcontent of the surfaces. The RBS measurements were made using a2.8 MeV beam from the Brookhaven National Laboratory 3.5 MVelectrostat- accelerator. The alpha beam size was 1 mm square, thescattering angle 165°, and the energy resolution of thedetector-analyzer system 14 keV. The typical He currents were 30 nAand the total dose accumulated was approximately 50 uC. The energy ofthe system was calibrated with an Am a source along with He+scattering edges from aluminum and copper. This type of analysis hasbeen discussed thoroughly in the past and will not be reproduced here(for example see refs. 8,9 ).Nuclear reaction analysis is a nondestructive technique mostlysensitive for the analysis of light elements. In this experiment,energetic protons were used to excite the nuclei of the target atoms.The nuclear reactions used for the analyses were the 10B(p,p'-y) 10B and7 7Li^i.p'Y) Li inelastic scattering reactions. The resulting gammarays used in the analyses were the 2125- and 478-keV gamma rays fromthe boron and lithium, respectively. As has been discussed in detailpreviously (10,11), the gamma ray yield is directly proportional tothe concentration of target atoms. In this particular experiment theresulting gamma rays were counted with a 90-cm3 Ge(Li) detector, 14%afficient for Co-60 gamma rays, placed 4 cm from the target at 90° toa 3.4 MeV proton beam. Beam currents of 3 nA were used with countingtimes on the order of 1 to 2 hours.RESULTS:Microscopy:All samples wera examined with scanning electron microscopy toobserve the surface morphology. The micrographs of the samples didnot show any notable, visible features resulting from exposure.These results are in sharp contrast to the results from exposure tocaustic solution (1) where etching was evident up to +70 mV and oxideformation did not develop until 170 mV. The oxide layers on thesamples exposed to caustic solution were also poorly adhered to thesurface and above 225 mV had a soluble outer layer. With the boratesolution the oxides were thinner over most of the potential regionsand there was no evidence of spalling up to potentials of 750 mV. Inaddition there was no development of the grain structure indicatingthat the oxides were far more protective than in the caustic solution.Nuclear Reaction Analysis:Boron was determined in each sample with nuclear reactionanalysis. The results of the analyses are given in table I. Lithiumcan also be analyzed with this method, but the level of lithium in thesurface layer was below the sensitivity limit of 0.05 ug/cm . Thistype of analysis does not give any depth information nor does itprovide information on the element's chemical state. There was nomeasurable concentration of boron in the two 0 mV samples and only atrace in the sample exposed at 165 mV. The concentration shows aplateau as a function of potential between 250 mV and 500 mV and thenstarts to increase with potential.Rutherford Backscattering Spectrometry (RBS):RBS spectra from the samples is shown in figure 1. Each curve isidentified with the potential at which each sample was exposed to theborate solution. All of the samples were held at their respectivepotentials except the -0.09 mV sample, which was allow to float; the-0.09 mV being its average potential. The conversion from channel toenergy is 1.4 keV/channel. The leading edge of the 0 mV samplescorrespond to the surface concentration of nickel (channel 1520). Thestep at channel 1470 corresponds to the surface concentration ofchromium, and the iron edge occurs at channel 1500. There are twolower energy edges in the spectra of the higher potential samples.The one at channel 1100 is from aluminum and/or silicon and the one atchannel 700 is from oxygen.The sample exposed at 740 mV shows a nickel edge that correspondsto a concentration of only 18 at%. The chromium does not appear atany significant concentration in this layer nor at any potential above250 mV. It also shows an Al+Si surface concentration of 20%. Oxygenaccounts for the remaining surface concentration as deduced from theheight of the oxygen edge. The height of the edge was fairly constanton all the oxidized samples.' Only the width of the oxygen "peak" andhence the thickness of the oxygen layer increased- The Al+Si edgeincreased in height until 415 mV indicating an increase in the surfaceconcentration. After that most of the increase in the amounts wereagain due to increasing the width and hence the depth below thesurface of the Al+Si layer. The amount of the Al+Si in the surfacelayer always followed the amount of the oxygen. There certainly is acorrelation between the oxygen content, the Al+Si content, and theboron content. This is illustrated in figure 2, which is a plot ofoxygen, Al+Si, and boron content as a function of potential.DISCUSSION:Samples of electropolished Alloy 600 exposed to borate solutionshow a considerably different behavior than samples exposed in asimilar manner to a caustic solution. This experiment and the causticexperiment, reported earlier, exposed samples of Alloy 600 atdifferent potentials to simulate the effects of oxidizing and reducingconditions which could arise by the presence of other impurities orgalvanic effects in the primary and secondary sides, respectively, ofPWR steam generators. Sanples exposed to the caustic environmentexperienced active dissolution and etching of the surface between -90and 75 mV. In contrast, the samples exposed to the borate solution at0 mV potential showed no etching or dissolution effects with the metalsurface free of oxide.At higher potential, with oxides present, a significantdifference between the two experiments was the mechanical performanceof the oxide layers. The oxide on samples exposed to the causticenvironment displayed cracking, spalling, and dissolution of oxidelayers after reaching 170 mV, becoming more pronounced at higherpotentials. In contrast the samples exposed to the borate solutionsshowed no signs of cracking, spalling, or dissolution of the oxidelayers even up to 740 mV. The effect of potential was to increase thethickness of the oxide present on the samples. In the causticsolutions the effects of potential were more complex and involvedchanges in the nature of the oxides as well as their internalstresses.An extremely interesting result of this experiment was theenhancement of Al+Si in the surface oxide. As is illustrated infigure 2, the Al+Si concentration increases with potential. Theconcentration of aluminum and silicon in the base alloy is only.65 at% and with the higher potential exposures finish atapproximately 52 (metal) at%. This increase would appear to occurwith the dissolution of the other metallic alloying components leavinga relatively small fraction of the original nickel and most if not all10of the aluminum and silicon. As is also illustrated in figure 2 theAl+Si concentration increases as the oxygen content increases. Thesemeasurements demonstrated that for the 740 mV sample at least 5 mg/cmof the alloy had dissolved. In the samples exposed to a causticsolution there was no increase in A.l+Si since aluminum and silicon aresoluble in caustic solutions. The loss of 5 mg/cm of alloy isconsiderably higher than losses usually reported (12) and provides anindication of the large effects of a relative small increase in thepotential of the surface. The sample exposed to 0 mV, as many otherexperiments measured, experienced no detectable (with RBS) loss ofmetal atoms.The next qutestion to be asked is what are the chemical forms ofthe components of the surface oxide? Even though the techniquesutilized in this experiemnt cannot determine the chemical state of thecomponents we have enough information to make some comments aboutlimits. The metal composition of the surface is only 38 at% and theoxygen makes up the majority of the balance. If the silicon were tobe dissolved, leaving the aluminum-silicon conctentration to be 100%aluminum, the most probable form is AI2O3. This would then leave 1.8oxygen atoms for each remaining nickel atom. The chemical form ofnickel could not be NiO, but would have to be something on the orderof Ni(0H)2 and NiO. This type of surface layer has been reported inthe past (2). If the aluminum-silicon concentration is 100% silicon(as SiO2) then the nickel/oxygen ratio would be unity leaving NiO as apossible candidate. Of course, then any mixture of aluminum and11silicon would likewise lead to a mixture of nickel oxide and nickelhydroxide.CONCLUSIONS:Ue have demonstrated that there are significant differences inthe performance of surfaces and their passivating oxides formed onAlloy 600 depending on the type of environment the alloy was exposedto. The oxides formed on the samples exposed to borate solution wereclearly more mechanically stable and thinner than the oxides formedfrom exposure to caustic environments. It is not clear if thedifferences are simply that the caustic eiivirom^nt is more aggressive,if the boron (or borate) in the oxide film helps in passivation, or ifthe nickel compound (probably NiO in the caustic andNiO+Ni(OH)2+Al2O3+SiO2 in the borate) formed is more stable. We alsohave shown that the amount of corrosion is strongly dependent on thepotential of the sample with respect to its surroundings.12ACKNOWLEDGMENTS:The authors wish to thank Dr. D. Cubicciotti for his many helpfuldiscussions during the course of this work, Dr. R. Roberge forexposing the samples, Mr. D. Becker for his experimental assistance,and Mr. R. L. Sabatini for the scanning electron microscopy.Research supported by Electric Power Research Institute, PaloAlto, California, under Contract PR1167-08 and by the Processes andTechniques Branch, Division of Chemical Sciences, Office of BasicEnergy Sciences, US Department of Energy, Contract No.DE-AC-2-76CH00016.13REFERENCES:1. Isaacs, H. S., Kraner, H. W., and Hanson, A. L. EnvironmentalDegradation of Materials in Nuclear Power Plants-Water Reactors,J. T. A. Roberts, ed., NACE (to be published).2. Mclntyre, N. S., Zetaruk, D. G., and Owen, D. J. Electrochem.Soc. 126, p. 750 (1979).3. Green, S. J., and Paine, J. P. N. Nucl. Tech. 55, p. 10 (1981).4. Cohen, P. Hater Coolant Technology of Power Reactors, Am. Nucl.Soc. p. 232 (1980).5. Fletcher, W. D., Kreig, A., and Cohen, P. The Behavior ofAustinic Stainless Steel Corrosion Products in High TemperatureBoric Acid Solutions, WCAP 1689 (rev), Westinghouse ElectricCorp., Atomic Power Dept., (Hay, 1961).6. Seys, A. A., and van Haute, A. A. Corrosion 29, p. 329 (1973).7. Roberge, R., Bandy, R., and van Rooyen, D. Private Conversation.8. Chu, W. K., Mayer, J, W., and Nicolet, M. A. Backs cat ter ingSpactrometry, Academic Press, NY (1978).9. Foti, G., Mayer, J. W., and Rimini, E. Ion Beam Handbook forMaterial Analysis, J. W. Mayer and E. Rimini, eds., AcademicPress, NY, chapter 2 (1977).10. Hanson, A. L., Jones, K. W., and Corbin, D. R. Anal. Chem.(submitted).11. Hanson, A. L., Kraner, H. W., Shroy, R. E., and Jones, K. W.Nucl. Instrum. Methods (submitted).12. Cohen, P. Water Coolant Technology of Power Reactors, Am. Nucl.Soc., chapter 8 (1980).14Table IBoron Concentration as a Function of PotentialPotential(mV)-.09 ± 0.02-.03165250330415490570740Boron concentrs(pg/cm )NTNT<0.20.2 ±0.040.35±0.040.22+0.030.31+0.030.64±0.201.3 ±0.315FIGURE CAPTIONS:Figure 1. RBS spectra of the samples exnosed to the borate solution atdifferent potentials. The beam was 2.8 MeV a particles with thedetector at 165°. The nickel edge is at channel 1520, thealuminum is at channel 1110 and the oxygen is at channel 700.Figure 2. Amount of each element analyzed as a function potential.0xygen=0, aluminum+s±licon=*, and boron=A (which was multipliedby 10).COHOo>I-UJcc400 1000CHANNEL1600Figure 1EOO)IDCD-200 200 400POTENTIAL (mV)BOO 800Figure 2

Determination of surface oxide compositions on Alloy 600 using Rutherford backscattering

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