v V..

AECL-5548

ATOMIC ENERGYOF CANADA LIMITED

L'ENERGIE ATOMIQUEDU CANADA LIMITEE

TOWARDS AN UNDERSTANDING OFZIRCONIUM ALLOY CORROSION

by B. COX

Presented on the occasion of the award ofthe William J, Kroll Medal, Quebec City, P.Q.

August 1976

Chalk River Nuclear Laboratories

Chalk River, Ontario

August 1976

Cover Photograph: Large columnar oxidegrains on zirconium oxidised in air at650°C, magnification x20,000.

TOWARDS AW UNDERSTANDING OF ZIRCONIUM ALLOY CORROSION

by

B. Cox (M.A., Ph.D., Cantab.)Head of Materials Science BranchAtomic Energy of Canada LimitedChalk River Nuclear LaboratoriesChalk River, Ontario KOJ 1J0

on the occaiZon o & the

William J. Kroll Medal PresentationQuebec City, P.Q.August 11, 19 76

AECL-5548

^^mjrrejid_re_lji corrosion des alliages de zi rconium

par

B. Cox

à l ' occas ion de la présenta t ion de la

Médai l le Wi l l i am J . Krol 1

Québec, P.Q.

11 août 1976

Résumé

On donne un b re f h i s t o r i q u e du développement d'un programme v isan t

à mieux comprendre les mécanismes de cor ros ion qui jouent dans les a l l i a g e s

de z i rcon ium. Un sommaire général ind ique les progrès r éa l i sés jusqu 'à

présent dans la mise en oeuvre de ce programme.

L'Energie Atomique du Canada, L imi téeLaborato i res Nucléaires de Chalk River

Chalk R ive r , Ontar io

AoQt 1976

AECL-5548

TOWARDS AN UNDERSTANDING OF ZIRCONIUM ALLOY CORROSION

by

B . Cox

on the occasion of th(.

William J. Kroll Medal PresentationQuebec City, P.Q.August 11, 1976

ABSTRACT

A brief historical summary is given of the development

of a programme for understanding the corrosion mechanisms

operating for zirconium alloys. A general summary is given

of the progress made, so far, in carrying through this programme.

Chalk River Nuclear LaboratoriesChalk River, Ontario

August 19 76

AECL-5548

1.

I feel myself greatly honoured to be the second recipientof the William J. Kroll Medal for Zirconium, and very humble atbeing considered worthy to follow so major a personality in thenuclear field as Admiral H.G. Rickover USN. I was a relative late-comer to the zirconium alloy field, having joined the U.K. AtomicEnergy Authority's research establishment at Harwell only inSeptember 19 55. When I arrived the first Geneva Conference onthe Peaceful Uses of Atomic Energy had already taken place theprevious month, and one of my first duties on arrival at Harwellwas to search through a stack of preprints of the conferencepapers for the ones relevant to the area I had been assigned towork in - zirconium alloy corrosion.

For a new Ph.D. from Cambridge, whose work had been mainlyin the area of structural inorganic chemistry, this was quite achange in field, and I eagerly sought a copy of the new bib]e ofthe zirconium aficionado (The Metallurgy of Zirconium, editedby Lustman and Kerze), which had been published simultaneouslywith the Geneva Conference. At that time, copies were rare andprized possessions, and it is a token of the amount and qualityof information that had already been generated on zirconium alloysthat it remains even now the basic reference book of the zirconiumfield.

Considering the picture as it appeared at that time, Imiaht have wondered how I could carve out a niche for myself inthis field. The alloy Zircaloy-2 (still the "reference alloy"for the industry against which others are compared) was alreadydeveloped and in use. The first U.S. nuclear submarine, theNautilus, was already at sea; what more was there still to do inthis field? I conclude that it has been the approach I haveadopted to the study of corrosion mechanisms, and more recently tothe mechanistic aspects of other areas of the physical metallurgyof zirconium, which has lead to this award. I hope my listenerswill find it informative, therefore, if I delve firstly into thosefactors which have conditioned me to this approach, and then summa-rize briefly where this approach has led me.

Undoubtedly one of the primary influences was that of mysupervisor at Cambridge, Dr. A.G. Sharpe, who, himself somethingof an iconoclast, insisted on cultivating a healthy scepticismtowards currently assumed theories in all who worked for him. Ithink such a questioning attitude to received information is aprerequisite for any scientist hoping to solve complicated mecha-nistic problems, and is epitomised for me by the English northcountry expression "believe nowt ye read, and only 'alf ye see".

The second major influence on the subsequent direction ofmy studies was to have been assigned to the U.K. Homogeneous AqueousReactor (HAR) Project, rather than to the parallel light waterreactor project (LEO). In the parallel U.S. Homogeneous ReactorProject enormous increases in the corrosion rate of zirconium alloys

2.

(thousands of times the laboratory rate) were being observed underirradiation conditions. This large irradiation-induced increasein corrosion showed that all was not ideal with the behaviour ofzirconium alloys in all situations.

Thus, there were some major factors affecting the corrosionprocess which could not be ignored and needed to be understood.It was many years before workers in the light water reactor areabecame convinced that irradiation universally affected the corrosionrate of zirconium alloys, and that it was merely- the chemistry ofthe aqueous environment which determined whether these effectswere so large as to kill a reactor project or so small that theyrequired many years of observation to reveal their presence.

The third major factor determining my course of action hasbeen my personal predilection for developing techniques to lookat specific aspects of an overall problem, rather than the morecommonly adopted approach of specialising in one, or a few related,techniques, and then seeing what problems can be tackled usingthese techniques. This line of attack has enabled me to pursue apredetermined attack on the remaining mechanistic problems in thecorrosion field, at least over the last twelve years or so, byfirst identifying a sequence of questions needing to be answeredif the process was to be understood and then trying to find, ordevelop, techniques capable of answering these questions; an exampleis given in Table 1.

The detailed development of this attack has been summarisedin the reviews which I have written recently (1,2). I do notpropose to try to do a further such review here, but I will restrictmyself to a more anecdotal approach, covering first the early years,where I was in the process of deciding what the questions were thatreally needed asking; and will then give a fairly general summaryof the later development of the programme. In learning what werethe important questions there were inevitably many false starts.However, perhaps we learn most from our mistakes, so I hope youwill bear with me if; in the next few minutes, I seem to be relatinga series of failures rather than successes.

Again serendipity played its part in the early days, bothas a result of the specific task I was set on arrival at Harwell -to develop a technique for in situ measurement of the corrosionrate of zirconium alloys in high temperature and high pressure(irradiated) solutions - and of the commercial zirconium alloymaterials with which we had to work at that time in the U.K. Inattacking the main task, the availability of large laboratoryautoclaves led us to study techniques for monitoring the corrosionrate of individual specimens within a large autoclave, rather thanadopt the ORNL approach of monitoring the corrosion rate of a smallautoclave itself (by following the decrease with time ir the oxygenover-pressure). Thus we were led into the field of electrical andelectrochemical corrosion monitoring techniques, and the vagariesof high-pressure electrical seals.

The techniques we used were of two types, firstly measure-ments of the polarisation curves of zirconium alloy electrodes,using early models of potentiostat, which were already available(constructed by the Harwell electronics division). Ultimately,this type of d.c. electrochemical measurement was abandoned afterwe succeeded in repeatedly and unexplainedly electrodepositinggold (from the counter-electrode) on the specimens. Gold, ratherthan platinum, was used at this time because of its supposedgreater stability in these solutions. Thus, it was obvious thatthe polarisation currents we were measuring contained sizeablecomponents representing electrochemical dissolution and depositionof supposedly inert parts of the system.

Our attention was next directed towards a.c. methods/especially measurements of oxide film capacitance, for continuousmonitoring of the corrosion process. We started off using a com-mercial capacitance bridge, and using this we were soon able todemonstrate the major effect which the rectification phenomenon,discovered by John Wanklyn, had on the specimen corrosion rate.Using this technique, all zirconium alloy specimens were sooncovered with thick white oxide films which indicated that rectifi-cation of the a.c. current had caused a large increase in the amountof corrosion occurring. We were not the last investigators to fallinto this trap. We changed to a homemade bridge using a transformerto reduce the a.c. applied voltage, and also studied the use ofthe square wave polarograph (an instrument then having only recentlybeen developed by George Barker). It was possible to derivecapacitance figures using this instrument and we thought that thesmall square wave pulses it used might cause less of a problemthan the continuous a.c. applied by the capacitance bridge. Infact, both instruments seemed to give reasonable plots of 1/Cversus thickness when applied to anodised zirconium at roomtemperature, and we got similar apparently convincing plots versustime during autoclave tests at ?50°C.

I was on the verge of publishing this work, when I beganto have doubts after studying the capacitance behaviour of thesystem during autoclave cooldown - the capacitance of the samplesapparently increased (i.e. film thickness apparently decreased).This led me to study in more detail than previously, the doublelayer capacitance on the gold counter-electrode. We had establishedits variation with temperature, but had assumed that at any giventemperature, it would remain roughly constant with time - it didn't.So we had again been measuring some peculiarities of the electro-chemistry of the counter-electrode rather than the specimen.

These experiments had led to no usable results, but theydid convince me of the importance of the electrical properties ofthe oxide in the oxidation mechanism, and disillusioned me suffi-ciently about the interpretation of high temperature aqueouselectrochemistry that I subsequently decided, when I returned tostudies in this area, to use a fused salt as the electrolyte , anapproach which was to prove far more fruitful.

4.

The materials we had to work with at that time also affectedmy approach to the subject. Although we had some arc-meltedZircaloy-2 manufactured by I.C.I. Metals Division (as it was then),most of it was still melted in graphite crucibles The zirconiumcarbide particles in such alloys were obviously playing a majorpart in the corrosion process, as could be seen from an opticalmetallographic examination of specimens before and after corrosion.However, I was unconvinced that this was simply a case of prefeiren-tial attack on the carbide particle: itself, as was commonly assumed.In order to examine the process in more detail, I was led into thefield of electron microscopy, and particularly the interpretationof electron microscope replicas, which had not up to that time beenused extensively in the study of corrosion processes.

This foray into the field of electron microscopy opened upa new world to me. I was able to see that the corrosion attackdid not start at the carbide particle, but in the matrix adjacentto the matrix-particle interface. Many other interesting thingsalso appeared, such as localised oxidation along what was originallya metal grain or twin boundary. The formation of large pores inthe oxide at prior metal grain boundaries in pre-oxidised specimensbombarded with fission fragments and subsequently re-oxidised wasa phenomenon observed, but still not fully understood - why werethese pores not randomly distributed? This early work on theapplication of the electron microscope led to a number of papers,for one of which I was proud to receive in 1961 the A.B. CampbellAward from NACE. It also developed my continuing interest in theapplication of all types of electron optical techniques to corro-sion studies , an interest which is still evident in the programmeof the Materials Science Branch at Chalk River.

A third area of techniques, which I entered in the earlydays to see whether or not they could contribute to our under-standing of zirconium corrosion, was that which can be broadlyclassified under the heading autoradiographic. These comprisednot only the true autoradiographic techniques using radioactiveisotopes of the species concerned (of the ones we tried at thattime, iron, chromium, nickel and tritium,only the last led to anysuccess), but also techniques such as "Bitter Figures" which dependon imaging magnetic domain boundaries, and by which we triedunsuccessfully to locate iron-containing second-phase particles.However, a continuing interest in this area ensued, and followingmy move to Atomic Energy of Canada Limited at Chalk River NuclearLaboratories in 1963, I was able to pursue the tritium autoradio-graphy further and also to extend my interest into the area ofnuclear reactions for identifying the location of particular species(there being already a strong group at CRNL working in this area).

Thus, by the time I left AERE I had already developed myinterests in the three areas of experimentation (electricalproperties of oxides, electron microscopy, autoradiographic tech-niques) which were to form the base from which I could attempt to

•. :-.>-_ai!i the oxiuatioa mechanism. Other experiments at AERE on thecorrosion properties of numbers of different alloys, and in thereactors there provided the background from which to decide thosequestions which played an important part in the corrosion mechanism.Soon after arriving at CRNL these ideas were formalized into alist of questions to be answered. While this list has seen somemodification (and some answers inserted in it) it still remainsmuch the same (Table 1). Similar sets of questions could be listedfor other phenomena, such as hydrogen absorption during oxidation,or stress corrosion cracking. But for illustrative purposes theone related to oxidation mechanism will be used.

With Table 1 as the basis for discussion, I will now tryto summarise the progress that has been made in elucidating theoxidation mechanism of zirconium alloys. Strictly speaking, aseparate set of answers should be prepared for each alloy, butfor a general discussion it will probably be sufficient to orientthe answers towards the Zircaloy type of alloy (i.e. one containingiron, chromium, and nickel second phase particles). For a moredetailed exposition of the problem,readers should consult references1 and 2.

In order to decide whether the oxidation process is controlledby diffusion through a protective oxide film or not, it is seldomsufficient to rely alone on the kinetic form of the oxidationcurve. There will always be the possibility of misinterpretationsince, for instance, parabolic kinetics imply only that the oxida-tion rate is inversely proportional to the oxide thickness. Suchkinetics can be achieved equally by diffusion through a barrieroxide, or by gaseous flow through an increasing thickness of porousoxide. A technique which can distinguish between these possibili-ties is to instantaneously change the environmental gas pressureduring a continuous microbalance experiment, and look for instanta-neous changes in oxidation rate. This technique proved very usefulin identifying the onset of porosity during the oxidation ofZircaloy--2 with the onset of the kinetic transition, and showedthat this porosity extended essentially up to the oxide-metalinterface. Prior to the rate transition in the oxidation kineticstherefore, the transport process consists of diffusion through abarrier film, except for the influence of local cracking at anysharp edges on the specimen.

In the pre-transition period of the oxidation kinetics wehave to be concerned with both ionic and electronic transportthrough the oxide, since the net charge transfer must be zero, andwe cannot assume that any species migrates with such ease that itcan be ignored as a factor in the overall transport process. Theevidence available on the ionic transport process suggests thatoxygen is by far the most mobile species, while, as we will seelater, the defect structure of the oxide lattice is of relativelylittle concern for oxidation at normal reactor temperatures, becauseof the microcrystalline nature of the oxide film.

6.

By using a nuclear reaction of oxygen (0!7 + i!e] -*• 0: ' + Hesjwe were able to show that the oxygen diffusion process during oxi-dation was essentially a line diffusion process, and that the bulkdiffusion of oxygen in the oxide was several orders of magnitudetoo slow to account for the amount of oxidation which occurred.Incidentally, I have been criticised by other experimental ists ir.the field of applications of nuclear reactions, for employing arare and expensive isotope of oxygen in a reaction that requireda large and even more expensive accelerator. However, as I havealready said I regard techniques as means to an end, and in chisinstance we were approached by physicists at Chalk River to makesome stable 0 1 7 targets for their studies cf transition states inthe neon nucleus. This coincided with our interest in using anuclear reaction to study oxygen migration. So by a happy conso-nance of aims we agreed to provide targets for their study, if thc-yin turn would do some measurements en our diffusion specimens. The-narrow bulk diffusion profile at the specimen surface proved toothin for accurate study by this technique, but usinc the sametargets Paul Pemsler and I were able to measure it successfully,using an ion bombardment mass spectrometer.

Thus, the deduction from this work is thjt the line diffu-sion process represents diffusion along crystalline boundaries inthe oxide, and that this is the primary route for oxygen migrationduring oxidation, diffusion through the bulk being too slow byseveral orders of magnitude. Transmission electron microscopyhas been used extensively to study the development and epitaxy ofcrystallites as the oxide thickens, but the limited transparencyof the oxide to electrons at up to 200 keV, has meant that adifferent technique had to be used to study the crystallite morpho-logy near to the kinetic transition. It was found that a techniquefor rapid fracturing of thick oxide films, coupled with replicationof the fracture surface, gave very good impressions of the largecrystallites which developed in thick oxide films. By the techniquewe were able to observe in unalloyed zirconium (which does notshow a kinetic transition in 600°C oxygen at film thickness up to10 microns) the large columnar crystallites formed, whereas inZircaloy--2 the crystal growth process was interrupted, so that (atthe transition?) renucleation of small crystallites occurred. Thisprocess of growth and renucleation appeared to continue repetitively.

Thus the ionic transport process is apparently one in whichoxvgen ions diffuse along crystallite boundaries in the oxide,the precise kinetics being determined by the manner in which thecrystallites change size in any particular oxide film.

To study the electron transport process we needed a method offorming an electrical contact with the outer surface of the oxidewhich would satisfy the following criteria:

1. It should not affect the oxidation rate L'f the surfacebeneath the contact.

2. It shoulei contact the whole specimen area, or a knownfraction or that area so that the current density couldbo measured.

3. It should be as near as possible to an ohmic reversiblecontact" Fn. both ionic a^d electronic processes.

-1 . y.c add i t i on 31 contributions to the measured current (i.e.fror proccssos other than specimen oxidation) should bepresent:.

Various J orms of metallic or porous conducting oxide wereel i r.'imtod, nn.i our previous experience with hiqh temperature3qi;eoi:Fi e] ectrnciiemi stry suggested that it did not meet some ofthe above conditions. We finally settled on a conducting, oxirfisinrfused salt as the best solution to the problem.

"y using this technique to measure the current-voltage (I-V)characteristics of growing oxide films, and by separating thesecurves '. ;;to their ionic and electronic components we have been ableto show that the electronic transport process is quite variable,although it can often be approximated to a Schottky emission process.It appears that the electron transport process may differ at differentlocations ovi the specimen surface with the measured charactertisticbeing a weighted mean of the processes contributing over the wholea r e a .

rsoth electron and hole conduction have been identified underdifferent conditions, again showing the variability of the electronconduction processes which occur. Using an imaging technique forthe electron curie.*t it was possible to show that, at least at roomtemperature, conduction was mainly localised at intermetallic parti-cles in Zircaloy type alloys. It is inferred that this is also themain route for electron conduction during oxidation. From observa-tions in a number of oxidising media of differing conductivity,and from direct measurement we have also been able to deduce thatsurface resistivity is a major part of the overall electronicresistance, especially for unalloyed zirconium.

Putting this evidence together we find that in zirconium andthe Zircaloys the electronic component of the oxidation currentflows primarily at a few sites, commonly identified with the inter-metallic particles. Since the reduction of oxygen and its diffusionthrough the oxide are occurring more uniformly over the surface(because of the small crystallite size) the electrical circuit mustbe closed by surface conduction. The oxidation field across theoxide is set by that voltage needed to equalize the two componentsof the oxidation current. In practice, because of the steeplyrising nature of the electronic current as a function of increasingvoltage, this oxidation field is determined largely by the electronicconduction. The ionic flux is controlled by this field, and hencethe oxidation rate is fixed, within limits set by the oxygen diffusion

process, ,jy tne electronic properties of che oxide. In such ^situation neither process can be said to be uniquely controllingthe oxidation. Thus a change in conditions which causes a largechange in only one of the two components of the I-V curve willnot result in a big change in oxidation rate. Both processesmust be changed significantly before any large changes in oxidationrate can be observed. This argument formed the basis of anhypothesis I proposed to explain effects of irradiation on theoxidation rate. So far this hypothesis seems to have withstoodthe test of time and further experimental observations.

We have still to consider the processes occurring during thepost-transition oxidation period when the whole oxide does notrepresent a diffusion barrier. Here we have made extensive use ofelectron microscopy to characterise the size, location and develop-ment of pores and cracks in the oxide. We have also developedtechniques based on the rate effect of capillary rise of an electrolytein these flaws on the impedence of the oxide; and a mercury porosi-meter, where the flow of mercury in the flaws was monitored byimpedance measurements, to give information o.. the size, depth andfrequency of these defects.

Studies of the stress generated in the oxide, and the recrys-tallization processes proceeding in the oxide have also beenimportant in reaching conclusions about the mode of oxide breakdown.

I have concluded that, because small pores are alwaysgenerated at the oxidation transition, whereas cracks in the oxide,although common, are not univerally observed, the development ofsmall pores in the oxide is the primary cause of the loss of theprotective nature of the pre-transition oxide. These pores arethought to develop at crystallites boundaries by virtue of therecrystallization processes occurring during oxide growth. Theprecise details of crystallite growth from specimen to specimenand alloy to alloy may be quite variable; however, the common endeffect is that pores develop which permit oxygen gas to flow throughmost of the oxide thickness. For Zircaloy-2 any remaining barrierat the oxide/metal interface is exceedingly thin, only a few nano-meters at most, whereas for some other alloys such as Zr-2.5% Nbthe pores appear only to penetrate through a much smaller fractionof the oxide thickness. Thus, unlike Zircaloy-2, where a pressurechange during oxidation results in a proportional change in oxida-tion rate, for Zr-2.5% Nb little effect of a pressure change isseen. By the combined application of the above techniques to studiesof the effects of variables such as heat treatment, alloying andirradiation it should be possible to predict the behaviour of alloysunder any given set of conditions. However, the effort involvedin such an approach would be massive, and it remains probable thatin most instances "ad hoc" solutions will remain the preferredapproach, unless such a solution is found to be too elusive.

In recent years my interests have expanded to cover theenvironmentally induced cracking of zirconium alloys, and their

9.

delayed hydride cracking. Whenever possible I have tried to adopta similar philosophy to that described above in tackling theseproblems. Broader responsibilities have also permitted me toapply this approach to other aspects of the physical metallurgyof zirconium. T^ particular, I have been able to encourage othermembers of the ivterials Science Branch at Chalk River to developnew techniques wi anever possible to aid in the understanding ofin-reactor creep, the defect structure of zirconium and orderedalloys. I expect to continue working in these areas in the future,and finish by thanking you for your attention, and the Kroll AwardCommittee for the honour bestowed on me.

A c h i:c(<:Cc dg CIT.V i> t$ :

It would take too long to list all the people who havecollaborated in, assisted with, or commented on this programmeover the years. The names of iay principal collaborators can beobtained from the co-authors listed in the bibliography. To themand the others acknowledged individually in individual papers Ioffer again my sincerest thanks for the help I have received.

Bj_b£ iognav'nij: [in •tei'&'VSc c hnonclcgij) ••

1. B. Cox, "Oxidation of Zirconium and its Alloys", Adv. inCorr. Sci. and Tech., Plenum, N.Y., Vol. V, p.173, 1976.

2. B. Cox, "Zirconium Alloys in High Temperature Water", Int.Conf. on High Temp., High Press Electrochem., Univ. of Surrey,Guildford, Jan. 1973, Pub. NACE 1976.

3. B. Cox and R.A. Ploc, Comments on the Origin of the CubicRate Law in Zirconium Alloy Oxidation, J.E.C.S., 1975, 122,1744.

4. B. Cox, "Environmentally Induced Cracking of Zirconium Alloys",in Revs, in Coatings and Corrosion, Vol. 1, No. 4, p.366,Freund, Tel Aviv 1975.

5. B. Cox, "Techniques for Studying the Rate Controlling Processesduring Metal Oxidation',' Proc. of 5th Int. Cong, on Met. Corr.Tokyo, 1972 (publ. NACE 1974) p.682.

6. B. Cox and J.C. Wood, "Iodine Induced Cracking of ZircaloyFuel Cladding - A Review", Proc. of Symp. on Corrosion Problemsin Energy Conversion and Generation, Electrochem. S o c , N.Y.,Oct. 1974, p.275.

7. B. Cox, "Stress Corrosion Cracking of Zircaloys in IodineContaining Environments" ASTM-STP-551 (1974) p.419.

10.

8. B. Cox, "A Correlation between Acoustic Emission during SCCand Fractography of Cracking of Zircaloys" Corrosion, 1974,30_, 191.

9. B. Cox and A. Donner, "The Morphology of Thick Oxide Films enZircaloy-2", J. Nucl. Mat., 1973, 47_', 1972.

10. B. Cox, "The Effect of Surface Films on the Initiation ofStress Corrosion Cracking of Zircaloy-2", Presented at the56th Annual C.I.C. meeting, June 1973 (AECL-4589).

11. B. Cox, "Stress Corrosion Cracking of Zircaloy-2 in NeutralAqueous Chloride Solutions at 25 C", Corrosion, 1973, 29̂ , 157.

12. B. CJX, "Accelerated Oxidation of Zircaloy-2 in SupercriticalSteam" Atomic Energy of Canada Limited, Report AECL-4448 (1073)

13. W. Iliibner and B. Cox, "Electrochemical Properties and Oxida-tion of some Zirconium Alloys in Molten Salt at 300'-500 C",Atomic Energy of Canada Limited, Report AECL-4431 (1973).

14. B. Cox, "Stress-Corrosion Cracking of Zirconium Alloys" inMaterials Research at AECL, Fall 1972, Atomic Energy of CanadaLimited, Report AECL-4 353.

15. B. Cox, "Environmentally Induced Cracking of Zirconium Alloys",Corrosion, 1972, 2_8_, 207.

16. B. Cox, "Comments on the Effect of an Applied Electric Fieldon the Oxidation of Aluminum in the Temperature Range 50-400°C"Oxid. of Met., 1971, 3^ 5 2 9-

17. B. Cox, "Catastrophic Oxidation of Zircaloys in Fused Saltsat 300°C", Oxid. of Met., 1971, 3_, 399.

18. B. Cox, "Comments on the Influence of Oxide Stress on theBreakaway Oxidation of Zircaloy-2", J. Nucl. Mat., 1971, 41,96.

19. B. Cox, "Environmentally Induced Cracking of Zirconium Alloys.Ill, Cracking in Hot and Fused Salts", Atomic Energy of CanadaLimited, Report AECL-3799 (1971) .

20. F.J. Shirvington and B. Cox, "A Study of Charge TransportProcesses during the Oxidation of Zirconium Alloys", J. Nucl.Mat., 1970, 35, 211.

21. B. Cox, "Factors Affecting the Growth of Porous Anodic OxideFilms on Zirconium", J.E.C.S., 1970, 117, 654.

22. B. Cox, "Environmentally Induced Cracking of Zirconium Alloys.II, Liquid Metal Embrittlement", Atomic Energy of CanadaLimited, Report AECL-3612 (1970).

11.

2J. B. C O X , "Scanning Electron Microscopy", in Materials Researcha; AECL, Sprinq 1970, Atomic Energy of Canada Limited, ReportA1XL-36 0 4 .

24. B. Cox, "Environmentally Induced Cracking of Zirconium All°vs.I, Topography of Stress Corrosion Cracking in Methanolic Solu-tions" Atomic Energy of Canada Limited, Report AECL-3551 (1970).

25. B. Cox, "Comments on the Use of the Nuclear Reactions 1EO(d,p)17Oto Study Oxygen Diffusion in Solids and Its Application toZirconium", J. Appl . Phys . , 1969, 4_0, 4669.

2f: . H. Cox, "Thermal Oxidation of Metals" in Materials Researchat Chalk River, Fall 1969, Atomic Energy of Canada Limited,Report AECL-3478.

27. B. Cox, "The Zirconium-Zirconia Interface", J. Aust. Inst.Met., 1969, 14, 123.

28. B. Cox, "Rate Controlling Processes during the Pre-TransitionOxidation of Zirconium Alloys", J. Nucl. Mat., 1969, 31, 48.

29. B. Cox, "Comments on Aqueous Corrosion of the Zircaloys atLow Temperatures", J. Nucl. Mat., 19 69, ^0, 3 51.

30. B. Cox, "Processes Occurring during the Breakdown of OxideFilms on Zirconium Alloys", J. Nucl. Mat., 1969, 329, 50.

31. B. Cox, "The Morphology of Zirconia Films and its Relationto the Oxidation Kinetics", Atomic Energy of Canada Limited.,Report AECL-3285 (1969) .

32. B. Cox, "Comments on the Influence of Thin Noble Metal Filmson Zirconium Oxidation", J.E.C.S., 1968, 115, 1259.

33. B. Cox, and A.R. Mclntosh, "The Oxide Topography on Crystal-barZirconium and Reactor-Grade Sponge Zirconium", Atomic Energyof Canada Limited, Report AECL-3223 (1968) .

34. B. Cox and J.P. Pemsler, "Diffusion of Oxygen in Growing Zirco-nia Films", J. Nucl. Mat., 1968, 2Q_, 73.

35. B. Cox, "Effects of Irradiation on the Oxidation of ZirconiumAlloys in High Temperature Aqueous Environments", J. Nucl.Mat., 1968, 2S_, 1.

36. B. Cox, "A Porosimeter for Determining the Sizes of Flaws inZirconia or Other Insulating Films in situ", J. Nucl. Mat.,1968, 2J7_, 1.

37. B. Cox, "Comments on the Dielectric Constant of Zirconia"Brit. J. Appl. Phys. (J. Phys. D.) 1968, Ser. 2, .L, 671.

38. J.S. Sheasby and B. Cox, "Oxygen Diffusion in Alpha-NiobiumPentoxide", J. Less Comm. Met., 1968, 15, 129.

12.

39. B. Cox, "Low Temperature (<300°C) Oxidation of Zircaloy-2in Water", J. Nucl. Mat., 1968, 2_5, 310.

40. B. Cox, "Rate Controlling Processes during the Oxidation ofZirconium Alloys", Atomic Energy of Canada Limited, ReportAECL-2777 (1967) .

41. B. Cox, "Oxidation cf Zirconium-Aluminum Alloys", Atomic Energyof Canada Limited, ReportAECL-2776 (1967).

42. B. Cox and D.L. Speirs, "Rectification by Oxide Films onZirconium Alloys", Atomic Energy of Canada Limited, ReportAECL-2690 (19G7).

43. B. Cox, "The Use of Electrical Methods for Investigating theGrowth and Breakdown of Oxide Films on Zirconium Alloys",Atomic Energy of Canada Limited, Report AECL-2668 (1967).

44. B. Cox, "Porosity in Oxide Films on Zirconium Alloys", Proceed-ings of 3rd Int. Cong, of Met. Corr., Moscow, 1966, Vol. IV,p. 341 (Izdat., Moscow, 1969).

45. B. Cox and C. Roy, "Transport of Oxygen in Oxide Films onZirconium Determined by the Nuclear Reactor O17(He3,a)O16",Electrochem. Tech. 1966, 4_, 122, and Atomic Energy of CanadaLimited, Report AECL-2350~(1965).

46. B. Cox and C. Roy, "The Use of Tritium as a Tracer in Studiesof Hydrogen Uptake by Zirconium Alloys", Atomic Energy ofCanada Limited, Report AECL-2519 (1965).

47. J.A.L. Robertson, and B. Cox, "Comparison of Water-CcoledFuels", Nuclear News, 1964, Oct., p. 34.

48. B. Cox and R.W. Ball, "A Study of Oxide Film Breakdown onZirconium Alloys by Capacitance Measurements", Atomic Energyof Canada Limited, Report AECL-2144 (1964) .

49. J.H. Chute (ed. B. Cox), "An Electron Microscope Study of theOxidation of Some Zirconium Alloys in Steam", Atomic Energyof Canada Limited, Report AECL-1999 (1964) .

50. J. Adam and B. Cox, "The Irradiation-Induced Phase Transfor-mation in Zirconia", Reactor Sci. & Tech. (J. Nucl. Eng.A/B), 1963, r7, 435.

51. B. Cox and Mrs. J.A. Read, "Oxidation of a Zr-2.5% Nb alloyin Steam and Air", UKAEA Report, AERE-R4459 (1963) .

52. B. Cox, "Some Effects of Pressure on the Oxidation of Zircaloy-2in Steam and Oxygen", J. Less Comm. Metals, 1963, 5_, 325; andProc. of Conf. on Zr Alloy Tech., Castlewood, Calif"., Nov. 1962,GEAP-4089.

13.

53. B. Cox and B.R. Harder, "The Effect of Dissolved Oxygen onthe Oxidation of Zircaloy-2 by Steam", J.E.C.S., 1963, 110, 1110.

54. B. Coy "The Effect of some Alloying Additions on the Oxidationof Zirconium in Steam", U.K.A.E.A. Report, AERE-R4458 (1963).

55. B. Cox, "Some Factors which Affect the Rate of Oxidation andHydrogen Absorption of Zircalcy-2 in Steam", U.K.A.E.A. ReportAERE-4348 (1963).

56. B. Cox and T. Johnston, "The Oxidation and Corrosion of Niobium(Columbium)", Trans. Met. Soc. AIME, 1963, 227, 36.

57. R.C. Asher and B. Cox, "The Effects of Irradiation on the Oxi-dation of Zirconium Alloys", Proceeding of Conf. on Corr.of Reactor Materials, Salzburg, 1962, p.209 (IAEA, Vienna).

58. B. Cox, P.G. Chadd and J.F. Short, "The Oxidation and Corrosionof Zirconium and its Alloys. XV, Further Studies of Zirconium-Niobium Alloys" U.K.A.E.A. Report, AERE-R4134 (1962).

59. B. Cox, "Hydrogen Absorption by Zircaloy-2 and Some OtherAlloys during Corrosion in Steam", J.E.C.S., 1962, 109, 6;and U.K.A.L'.A. Report, AERE-R3556 (1961).

60. B. Cox and T. Johnston, "The Oxidation and Corrosion of Zirco-nium and its Alloys. XIII, Some Observations of Hydride inZirconium and Zircaloy-2 and its Subsequent Effect on Corrosion"U.K.A.E.A. Report, AERE-R3881 (1962).

61. B. Cox, "Recent Developments in Zirconium Alloy Corrosion Tech-nology" in Progress in Nucl. Energy, Ser. IV, Vol. 4 PergamonLondon, 1962, p.166.

62. B. Cox, "Causes of a Second Transition Point Occurring duringOxidation of Zirconium Alloys", Corrosion, 1962, 18_, 336.

63. B. Cox and B.R. Harder, "An Attempt to Locate IntermetallicParticles in Zirconium Alloys using a Bitter Figure Technique",U.K.A.E.A. Report AERE-R3845 (1961).

64. R.C. Asher, B. Cox and J.K. Dawson, "Investigations Relatingto the Use of Zirconium Alloys in Steam-Cooled Reactors",Proc. of I.A.E.A. Symposium on Power Reactor Experiments,Vol. II, p.135, Vienna, 1961.

65. B. Cox, K. Alcock and F.W. Derrick, "The Oxidation and Corro-sion of Zirconium and its Alloys. VI, The Mechanism of theFission Fragment Induced Corrosion of Zircaloy-2", J.E.C.S.1961, 108, 129; and U.K.A.E.A. Report, AERE-R2932, (1959).

66. B. Cox, "The Oxidation and Corrosion of Zirconium and itsAlloys. V, Mechanism of Oxide Film Growth and Breakdown

14 .

on Zirconium and Zircaloy-2", J.E.C.S., 1961,108, 24? andU.K.A.E.A. Report AERE-R2931 (1959).

67. B. Cox, "Heavy Particle Irradiation Effects during th3 Reactionof Solids with Gases or Liquids", U.K.A.E.A. Report AERE-M742.

68. B. Cox, M.J. Davies and T. Johnston, "The Oxidation andCorrosion of Zirconium and its Alloys. XI, The OxidationKinetics of Zirconium-Niobium Binary Alloys in Steam at300-500°C", U.K.A.E.A. Report, AERE-R3257 (1968).

69. B. Cox, "The Effect of Fission Fragment and Neutron Irradiationon the Kinetics of Zirconium Oxidation", Proceedings of 4thInt. Symp. on the Reactivity of Solids, Amsterdam, 1960, p.425,eds. J.H. de Boer et al. (Elsevier).

70. B. Cox, "The Oxidation and Corrosion of Zirconium and itsAlloys. III. Oxide Film Breakdown in Arc-Melted SpongeZirconium", Corrosion, i960, 16_, 3806; and U.K.A.E.A. Report,AERE-R2874 (1959).

71. B. Cox, "The Oxidation and Corrosion of Zirconium and itsAlloys. II, The Effect of Carbide Inclusions on Oxide FilmFailure", Corrosion, 1960, 1_6_, 188t, and U.K.A.E.A. Report,AERE-R2873 (1969).

72. B. Cox, M.J. Davies and A.D. Dent, "The Oxidation and Corro-sion of Zirconium and its Alloys. X, Hydrogen AbsorptionDuring Oxidation in Steam and Aqueous Solutions", U.K.A.E.A.,AERE-M621 (1960).

73. B. Cox and T. Johnston, "The Oxidation and Corrosion of Zirco-nium and its Alloys. IX, Observation of a Second TransitionPoint during the Oxidation of Zirconium Alloys", U.K.A.E.A.Report, AERE-R3256 (1960).

74. K. Alcock and B. Cox, "The Oxidation and Corrosion of Zirconiumand its Alloys. VIII, The Effect of Neutron Irradiation onthe Dissolution Rate of ZrO2", U.K.A.E.A. Report, AERE-R3114(1959) .

75. B. Cox, "An Investigation of the Mechanism of Oxide FilmGrowth and Failure on Zirconium and Zircaloy-2", Proceedingsof AEC-EURATOM Conf. on Aqueous Corrosion of Reactor Materials,Brussels, Oct. 1969, U.S.A.E.C. Report TID-7587.

76. J. Adam and B. Cox, "Neutron and Fission Fragment Damage inZirconia',' Phys. Rev. Lett., 1959, 2_, 543.

77. J. Adam and B. Cox, "The Irradiation-Induced Phase Transfor-mation in Zirconium Solid Solutions", J. Nucl. En. A, ReactorScience, 1959, 11, 31; and U.K.A.E.A. Report, AERE-M415 (1959).

15.

78. K. Alcock and B. Cox, "The Oxidation and Corrosion of Zirco-nium and its Alloys. VII, Experience with High PressureEquipment for Use with Aqueous Solutions at High Temperatures"U.K.A.R.A. Report, AERE-R3029 (1959).

79. B. Cox, "The Oxidation and Corrosion of Zirconium and itsAlloys. IV, The Effect of Fission Fragment Irradiation on theSubsequent Corrosion of Two Zirconium Alloys", U.K.A.E.A.Report, AERE-R2875 (1959).

80. K. Alcock and B. Cox, "The Oxidation and Corrosion of Zirconiumand its Alloys. I, Destructive Internal Oxidation of Zircaloy-2in Steam Under Irradiation at 300°C",U.K.A.E.A. Report,AERE-C/R 2826 (1959).

81. B. Cox and K. Alcock, "The Effect of Surface Preparation andNeutron Irradiation on the Corrosion of Zircaloy-2 in HighTemperature Aqueous Corrosion", U.K.A.E.A. Report, AERE-C/M353 (1958).

P.-.. J.K. Dawson, B. Cox, R.Murdoch and R.G. Sowden, "SomeChemical Problems of Homogeneous Aqueous Reactors", Proc.of 2nd U.N. Conf. on the Peaceful Uses of Atomic Energy,Geneva, 1958, P.46.

16 .

TABLE I. Understanding the Thermal Oxidation of Zirconium Alloys

Is the oxidation controlled by diffusion through a protective oxide film?(deduce from kinetics and pressure dependence)

Study transport of ions and electrons through oxide film

Ions

1. Which is mobile ion?(metal, oxygen, both)

2. What is oxide defect structure?(p-type, n-type, junction)

3. What is macroscopic diffusioncoefficient?

4. What is microscopic diffusion process?

5. Identify diffusion route from studiesof oxide morphology.

Define ionic transport process

Electrons

If NO,

Study the migration of molecular species in the oxide filn

1. What is electron transport process?(diffusion, tunneling, emission)

2. What is mobile species?

3. What is microscopic transport routefor electron current?

4. What is contribution of surfaceconduction to overall electrontransport process?

Define electron transport process

Determine rate-controlling process

1. Study visual occurrence of cracks,pores, etc., in oxide.

2- Determine size, depth, and frequencyof these defects.

3. Study processing generating defects.a. Stress in oxideb. Recrystallization of oxide

Define oxide film breakdown process

Assess effect of variables (e.g. heat treatment, irradiation) on all processes"

Predict behaviour of Alloy under given Conditions

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