FACTORS AFFECTING THE CORROSION RESISTANCE OF MATERIALSUNDER IRRADIATION IN THE EARLY STAGE OF GROWTH OF THEPASSIVE FILMS

D. GORSECECM-CNRS15 rue Georges Urbain94407, Vitry/Seine, France

ABSTRACT. The protective character ofthe passive films growing on eitherpure metals or alloysis largely modified either under or after irradiation, i.e. as observed during in-situ or ex-situelectrochemical tests respectively. In irradiation experiments, the projectile loses its energy in thetarget by nuclear collisions and various electronic excitation processes, depending on theprojectile - target couple, on the energy, flux and fluences considered. The electronic and/or ionictransport properties in the passive films can be affected by irradiation, depending on the testconditions. The changes in the electronic properties of the anodic films of titanium after ion­implantation are first briefly reviewed., i.e.under conditions of both nuclear and electronicstopping. When the target is a metal electrode whose back face is bombarded with alpha particlesand front face is exposed to the corrosive aqueous medium, the energy deposited by the alphaparticles in the target by electronic excitations can be a priori divided between the solid and theliquid phase, the passive film growing at the metal/solution interface being considered as part ofthe solid phase. The respective influence of radiations on the film and solution side of thepassivated materiallcorrosive aqueous medium interface is discussed for two different types ofmaterials, a 304 stainless steel and pure titanium passivated under alpha-irradiation. Only theearly stage ofgrowth of the passive film is considered. A reactivating or a passivating influenceof alpha-radiations is evidenced, depending on the energy range of the alpha-particles, underelectrochemical conditions "not corrosive enough" to produce this effect without irradiation(room T, dilute acidic aqueous solution).

1. Introduction

The factors affecting the growth and stability of anodic (or thermal) oxides grownin a radiative environment have been listed and discussed for over thirty years, onthe basis of a variety of experimental data [1]. They include the metallurgical andchemical factors, as well as environmental factors such as hydrodynamic conditionsand temperature, for example. The factors pertaining to the irradiation conditionsare well known. However, their respective influences on the corrosion resistance ofmaterials exposed to a corrosive environment are only established in a very fewcases, often due to a lack of well defined tests conditions. In other words, this state­of-the-art is due to the complexity of the passivity phenomenon, involving coupledmass and charge transport through three media (metal or metallic alloy, passive film,corrosive medium) and their respective interfaces.

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K. R. Trethewey and P. R. Roberge (eds.J, Modelling Aqueous Corrosion, 337-368.© 1994 Kluwer Academic Publishers.

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Whatever the projectile, the main processes of energy transfer to a target are, onthe one hand, nuclear collisions with the target screened nuclei (elastic contributionto the energy loss referred to as the nuclear stopping power for a given projectilein a material: So), and on the other hand electronic (possibly collective) excitationsand ionizations of the electronic shell of the target atoms (inelastic contribution tothe energy loss, referred to as the electronic stopping power: Se).

In this paper, by using light ions to irradiate the electrode during or afteranodization, we describe two different types of irradiation experiments showing theimportance of the different irradiation parameters, mainly the radiation flux and theenergy range of the projectile.

In ion-implantation experiments, both the nuclear and electronic contributions tothe stopping must be taken into account, the ratio SjSe depending on the energyand mass of the ion (keV/a.m.u.). The classical method used for simulating radiationdamage and doping effects in anodic films consists of forming the filmelectrochemically, irradiating the metal/film ensemble ex-situ with rare-gas ormetallic ions, and then studyingpost-morlem its electrochemical behaviour [2]. Thisprocedure requires the repassivation effects when the irradiated specimen isimmersed in the test solution after irradiation to be taken into account, since theelectronic properties of the unirradiated film could be, at least partiaUy, (and almostinstantaneously) restored in the uppermost layers of the passive film, even duringa short stay at open circuit before imposing an overvoltage - the influence ofrepassivation oon be compared to that of annealing.We first report on the irradiation effects in anodic films of titanium due to

implantation with "low" energy He+ ions (-few keV/a.m.u.) : a photoelectrochemicalstudy shows that these irradiation conditions leading to a non-negligible productionof point defects cause modifications of the electronic properties of the film, whichare strongly dependent on the stage of growth. Consequently, the electrochemicalbehaviour expected after irradiation could also depend on the stage of growth, andshould differ significantly from that without irradiation, more in the early than in theadvanced stage of growth.Then we concentrate on the irradiation effects occurring in the early stage of

growth of the anodic films which form under potential control. There are fewmethods which can be used in-situ to deal with this problem. One classic methoduses the gamma flux provided by a 6OCO source (for example) to irradiate theelectrochemical cell, and to test the "new" redox behaviour of the solution byelectrochemical measurements (Open Circuit Potential, Intensity-Potential plots,Electron Transfer Reactions) performed at more (Fe, Zr, Ti, some stainless steels... )or less (Pt) reactive metal electrodes. Some changes in the film have been oftenassumed under gamma irradiation, and this has proven to be the case experimentallywith different materials. One advantage is that the source can be introduced orwithdrawn easily, allowing to check the reversibility of the observed phenomena. Themain drawback is that this method does not allow proper discrimination betweendirect radiation effects in the film and indirect ones due to the solution radiolysis.

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This question of mixed direct and indirect radiation effects at the metal surfacecan be better dealt with by means of the following arrangement [3, 4]. By using asuitable radiation source, the backface of a thin metal foil electrode is bombardedwith alpha particles whose short range through matter (- few J.Lm) allows the control,at least partially. of the energy deposited at the front face, in the liquid phase closeto film/solution interface. Two different materials have been tested: pure titaniumand a 304 stainless steel. The change in the protective character of the film growingunder irradiation is followed as a function of the irradiation conditions. Therespective role played by ageing and f1uence in modifying the film stability isdiscussed. In parallel, the modifications of the electronic and optical properties ofthe film formed under alpha radiation are studied, by comparison with literaturedata. At last we attempt to relate the "new" electrochemical to the "new" electronicproperties induced by in-situ irradiation. We shall see that the results obtained withthe two different materials tested are consistent, in some aspects, with a one physicalmechanism involving both the film and solution side of the interface.

2. Irradiation effects in anodic films of titanium due to ion-implantation

A number of works have been devoted to the electrochemical behaviour of ion­implanted anodic oxide films. Investigations have focused on the influence ofradiation damage on the film stability as a function of the energy, dose and mass ofthe incident ions. However, until now, the role played by the irradiation flux for onegiven stage of growth, or reciprocally by the stage of growth for one given irradiationflux: is not established. It should be of importance for all systems giving rise toanodic films whose structure changes during growth [5].

2.1. GROwrn CONDITIONS

Prior to the implantation treatment, the chemically polished titanium electrodeswere anodized galvanostatically at high current density (I = 12 rnA cm'Z) in 1 MHzS04 up to two formation voltages, ymax = 20 Y and 60 Y SeE (followed by 10min ageing at ymax). Structural analysis by Transmission Electron Microscopy of thefilms separated from their substratum by dissolving Ti in a bromine-methanolsolution revealed that the anodic film grown at 20 Y is mainly amorphous, but thata significant density of crystallites is visible, spread at random in the amorphousmatrix, and that once reached the voltage of 60 y the film is well crystallized.Similar results were obtained in the literature: the crystallization degree of the filmattained for one defined (I, ymax) couple depends on the surface pretreatment, onthe electrolyte (pH, anionic species, temperature) as well as on the electrochemical(Y or I fixed) and ageing conditions imposed to the electrode.

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2.2. IMPLANTATION CONDITIONS

These films were implanted with 7 keY He+ ions for the films formed at 20 V, andwith 17 keY He+ ions for the film formed at 60 V, in order to keep the damageprofile in the inner part of the film. The radiation doses tested are ranging from 3x 1016 He+cm-2 up to 1.2 X 1017 He+cm-2, as indicated in Figure 1. In this range ofenergies, the He+ions lose their energy mainly via nuclear collisions with the targetions. The displacement rate was estimated to be of order 10-4 dpa S-I for the Ti ionsin the oxide. For He+ ions in Ti02, the sputtering yield is maximum at about 1 keYand decreases at higher energies.

2.3. PHOTOELECfROCHEMICAL STUDY

Ion-implantation produces structural damage in the film, which in turn will bereflected on its electronic structure: ion-implantation may lead to a nonzero DensityOf States (DOS) in the gap of the oxide. It may also modify the DOS in the gap ofthe anodic oxide films of titanium formed in various electrolytes depending on thenature of the incorporated anionic impurities, as already observed by differentauthors by using various in-situ or ex-situ spectrometries [7]. In principle, fortitanium, such a radiation-induced electronic disorder could be easily attained byspectro-photoelectrochemistry performed using illumination in the visible region,since the theoretical band gap of an ideal crystal of Ti02 in the rutile phase is about3 eV [8] (in principle "pure" Ti02 absorbs light only in the uv).The photocurrent spectra are recorded at 1 V SeE in 1 M H2S04 by the locking

technique, under chopped illumination at low frequency. The photocurrent isconverted to quantum e{{jciency by normalization to the photon flux. We focus onthe sub-band gap region. We compare in Figure 1 the Iph versus incident photonenergy spectra obtained after implantation, as a function of the stage ofadvancement of the anodization reaction.

It is a now classical result in semiconductor physics that ion bombardment ofcrystalline semiconductors gives rise to band tails of localized states in the bandstructure of the originally supposed defect free material, broadening the opticalabsorption edge, the so-called 'Urbach edge', when found exponential [9]. On theother hand, it is also a classical result that all amorphous semiconductors exhibit abroad absorption threshold. Such a behaviour is observed here, and particularly onthe "well" crystallized film at all radiation doses (see Figure 1b).More importantly, it is noteworthy that the influence of irradiation decreases with

increasing the formation voltage vmax, in other words the film thickness and (in manyinstances) its degree of crystallization. After implantation at the higher doses, thespectrum obtained on the 20 V film (Figure 1a) ressembles that obtained in theearlier stage of growth, namely on the amorphous film grown at vmax = 10 V undersimilar conditions, but not implanted. Further the onset of photoresponse is shiftedtoward longer wavelengths, as already observed for mixed Fe.Til_.Oy oxides which

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Figure 1 - Plot of the quantum yield versus photon ene'X)J under sub-band gap illumination, obtainedon anodic films of titanium after implantation with He' ions at various doses, at two different stages

ofgrowth: a) jI'''''' = 20 V SCE -df = 60 nm; b)V""" = 60 V SCE -dr = 125 nm [6J.

342

were tested in the eighties as possible photoanodes for electrochemical solar cells[10].For the less crystallized film grown at 20 V, a least square fit using gaussians has

given the peak positions at 1.9 eV, 2.1 eV and 2.3 eY. In contrast, for the filmgrown at 60 V, only one peak is found at 1.85 eV, whose position was unchangedafter irradiation. No extra peak is induced by irradiation, up to the higher dose(Figure 1b).Therefore, in the more advanced stage of growth, at vmax = 60 V, the radiation

effects have almost disappeared: increasing the dose up to 1.2 X 1017 He+cm-2 doesnot affect markedly the photocurrent spectrum. Comparable results were found fordose rates in the range f = 5.5 X 10-6 - 4 X 10-5 dpa S·I, when changing theanodization bath to ammonium tetraborate, once taken into account the fact thekinetics of crystallization are slower in this solution: the appearance of crystallineoxide is delayed to higher voltages. Further, it has been shown that when theradiation flux decreases below f = 10-6 dpa S-l, the irradiation effects becomenegligible for films grown in ammonium tetraborate: ageing in solution controls thefilm evolution as for unimplanted specimens [11].

It is suggested that if the irradiation flux used in that study could be sufficient toamorphize the crystallites present in the 20 V film, the conditions of amorphizationare not fulfilled for the larger crystallites obtained at higher voltages. In that case,rising the dose is inefficient.

2.4 ELECfRONIC AND OPTICAL PROPERTIES OFHE+-IMPLANTED ANODIC FILMS OFTITANIUM.

The question of the nature of the states in the gap, either intrinsic or extrinsic, isstill a matter of debate. This aspect has been studied in details for Ti02, even ifcertain ambiguities still exist concerning the evidence of "bulk" or "surface"phenomena. On n-Ti02 obtained by annealing in a reducing atmosphere, noPhotoluminescence band (PL) was detected in air between 1.65 eV (700 nm) and2.48 eV (500 nm), excluding a "bulk" effect, since PL probes bulk phenomena. Incontrast, by coupling PL and Electroluminescence (EL), it was inferred that aluminescence band detected by EL in the same range could result from "surface"states generated by specific adsorption of the SO/IOn at the electrode surface. Thisis known to interact strongly with Ti02. A peak at 1.45 eV (850 nm) was supposedto be due to radiative transitions from the conduction band to a mid-gap intrinsicstate, a result based on the coincidence of the EL and PL spectra) [12].Two different interpretations could be made regarding the hypothesis that the

sub-band gap absorption around 2 eV could be due to a bulk phenomenon,occurring within the film. The first is related to defect states in one given oxidestructure. For example, after Ar+ ion sputtering of the (110) surface of Ti02 singlecrystal, UPS and EELS studies revealed the presence of a surface state at 2.3 eV,0.7 eV below the conduction band edge (Ec,s), often associated with TP+ ions - V0

2­

complexes [13], better due to oxygen sub-surface vacancies in agreement with recent

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band structure calculations [14]. The second possible interpretation makes referenceto the series of existing titanium suboxides, Tin0 2n_1 with n ~ 36 [15]. A sub-bandgap photocurrent peak at - 2.2 eV detected on Ti02 ceramics heated in reducingatmosphere has been attributed to the formation of suboxides occurring duringreduction (16].Regarding the spectra presented in Figure 1, two arguments support the

hypothesis that such a structured sub-band gap PEC response could be probablyexplained by a "bulk" irradiation effect in the anodic oxide. The drastic change inshape with both the stage of growth and radiation dose, and also the fact that on notimplanted films grown and tested under same conditions, mean that the sub-bandgap response is a decreasing function of the stage of growth (up to 95 V [7]). Theadsorption of radicals is also possible, but their influence cannot be demonstratedin the present study. A more probable explanation of Figure1a at the higher dosesis that the irradiation causes both a change in the film composition (mixing effectat m/f) followed by a structural reorganization, leading to the formation ofsuboxides, and consequently to a completely different electronic transport processthrough the film (possibly via d-bands in the sub-oxides). In contrast, in Fig 1b, forthe well-crystallized film before irradiation, it may be assumed that these irradiationconditions, which do not affect the structure, only slightly enhance the peak of DOSlocated at 1.15 eV below Ec s'The irradiation conditions'described in this section are completely different from

those used in the following. Nevertheless, we shall see that the photoelectrochemicalbehaviour of films tested after He+ irradiation in the keV energy range resemblesclosely that observed on films grown under irradiation with alpha particles theenergy spectrum of which is extending up to the MeV range.

3. Alpha-irradiation effects on growth and stability of anodic films

When dealing with the problem of the influence of irradiation on growth andstability of the passive films in-situ, the first difficulty encountered concerns thechoice of the radiation source and of the associated experimental mounting, whichdetermines, in many aspects, the informations to be obtained. Until now,investigations of the passive state under irradiation using gamma sources (60CO, forexample) have mainly focused on the kinetics of redox reactions occurring at thefilm/solution interface, influenced by the solution radiolysis. In some cases, theproblem of growth and dissolution has been studied from a chemical point of view,by carrying out spectral analyses of the solution using such techniques as AtomicAbsorption Spectroscopy (AAS) or Inductively Coupled Plasma Atomic EmissionSpectroscopy (ICP/AES). Alternatively, measurements have been made of the filmremoved from the solution and tested (mainly by AES or XPS) at various stages ofthe passivation process.

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As already published, an alternative method, using an alpha source contacting ametal target electrode, appears to be the more suitable to study in situ theirradiation effects on film growth, since it minimizes the radiolysis effects in the"bulk" solution. This arrangement is better because the energy deposited in solutioncan be varied easily by changing the Ti thickness. However this method presentssome disadvantages. (i) The more important one is that, in principle, only theinfluence of electronic excitations can be tested (to our knowledge, irradiations withheavy ions which would yield nuclear damage have not been performed until now)(ii) The reversibility of the observed effects is at present unaccessible. (iii) Theirradiation effects in the keV range cannot be distinguished from those appearingat high energy, due to the uncertainties in both the target thickness (± 0.2""m) andthe calculated range of the alpha particles according to the available models; theenergy loss in the anodic film is unknown. For this reason, when the energy of thealpha particles is indicated in the following, this value will be assumed to be validat either mlf or fls. The energy loss in the film is considered negligible, at least solong as thin films (a few nm thick) are studied. This would not be the case in theadvanced stage of growth.

3.1. IRRADIATION CONDITIONS.

The working electrode consists of a thin metal foil « 15 ""m) whose back face isbrought into contact with an americium source e41Am) emitting alpha particles.Their initial energy, equal to 5.49 MeV, is degraded after passing throughsuccessively the protective coating on the source (to avoid contamination) and themetal target. The energy of the alpha reaching the metal/film interface (m/f) andthen penetrating the solution (if not stopped in the metal or the film) decreases withincreasing the metal thickness and density. Only the front face of the metal targetis exposed to the aqueous solution which is irradiated over a few ""m: less than 10""m, 20 ""m, and 34""m for alpha particles attaining mlf with 1.6 MeV, 3 MeV and4.8 MeV respectively.A plot of the electronic stopping power (Se) versus range of alpha particles in Fe

and Ti respectively, accounting for the energy distribution of the radiation source,is shown in Figure 2 [17]. The curve beginning at 8.3 ""m represents the assumedprofile of electronic energy deposit in water, on the solution side of the interface,for a 8.3 ""m thick Fe target [18]. Likewise for the titanium target, the samecomment is valuable for the curves whose starting points are at 9 ""m, 10 ""m and12.5 ""m.It is noteworthy that Se would be a maximum at fls for a mono-energetic flux of

1.28 MeV alpha when entering the solution. However, for alpha particles reachingmlf (or fls) with 1.6 MeV, or 3 MeV after passing through 12.5 ""m, or 9""m of Tirespectively, the maximum in electronic stopping is shifted deeply in the "bulk"solution, at a few ""m from fls, the peak of nuclear stopping being located in thesame range. It must be also pointed out that dE/dx Ie passes through a maximum at

345

Fe a70

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35

30N=,....

25~

....'8 20C.)

> 15Cl.l

~....... 10~

r:FJ.

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00 5 10 15

range I J..lrn20 25

Figure 2 - Electronic stopping power versus range for alpha particles: a) in iron and in water afterpassing through 8.3 JJJTI of iron; b) in titanium and in water after passing through 9 JJJTI, 10 JJJTI, 11JJJTI, 12.5 JJJTI, 13.5 JJJTI and 15 JJJTI of titanium. The vertical lines indicate the metallwater interface,

the energy loss in the growing film cannot be shown at this scale.

346

the Us interface for a titanium thickness of order 13.5 JLm, even if the amplitude ofvariation is only slight.

31. El.EcIROCHEMICAL STUDY OF A 304 SfAlNLESS SIEEL'i El.ECIRODE P~IVAlID UNDER AlJ'HA­IRRADIATION

The first study, using the experimental set-up previously described, used a 304stainless steel because it is one of the most commonly used alloys. However,difficulties arise from using this material related to the nature of the passive filmwhich forms on iron or iron base alloys. Indeed, in spite of the fact that thephenomenon of passivity has been the subject of an intense experimental andtheoretical activity [19, 20) for more than forty years, the relation between thechemical, structural, electronic properties and the stability of the passive film withrespect to the different degradation processes is still unknown. From a structuralpoint of view, it is well recognized that a certain degree of order can be achievedunder favorable electrochemical conditions, and that in most cases the electronicstructure resembles that of a highly disordered semi-conducting material. Withregard to the transport properties, a film growth mechanism due to anion or cationvacancy migration through the film, assisted by the strong electric field, supposedconstant or not, existing in very thin films, has been established long ago andapparently accounts for some experimental observations. Anyone of the abovementioned reasons renders a study of the passivation of a 304 stainless steel underalpha irradiation very complex. However we shall see that this study gives someresults which will be developed in the further study using a titanium target.

TABLE 1:304 stainless steel specimens used for the PEC study and ageing conditions

Symbol Thickness Surface area Ageing/JLm /cm-2

'" 1O.0±0.5 0.250 none

• 8.3±0.3 0.140 2 h at OCP

• 8.3±0.3 0.070 1 day in air

V 8.6±0.3 0.07 1 day in air+ 3 days at OCP

0 8.3±0.3 0.140 2 days in air

X 9.0±0.2 0.096 5 days in air

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Figure 3 . Photocurrent spectra recorded at 0.575 V SeE in borate buffer (pH 9.2) on stainless steelelectrodes oj various thicknesses under alpha·irradiation, the characteristics oj which are reported in

Table 1.

3.2.1 PhotoeLectrochemical study. The results of this preliminary study were consistentwith the existence of an irradiation effect on both the film and solution sides of theinterface. But the fact that these effects could be strongly coupled could not then beassessed. Moreover this study provides evidence of the role played by the air formedfilm.The thin steel foils were thinned electrochemically down to 7fJ-m, 8.3 fJ-m, 8.5 fJ-m

and 9 fJ-m respectively. The maximum energy of the alpha reaching the solutiondecreased from about 1.6 MeV for 7 fJ-m down to 1 MeV for 8.3 fJ-m. Then theywere cathodically reduced for 10 min at -1.5V SCE in the buffered borate solution(pH 9.2) before imposing a passivation voltage of 0.575 V SCE. This treatment didnot completely remove the native pre-existing oxide at the electrode surface.

348

In all cases, bombardment of the metal/film interface with 1 MeV alpha particlesincreases the above band gap photo-response while for both the irradiated 9,um and7 ,urn thin foils, no radiation effect was visible with respect to the not irradiatedelectrode. The fact that the irradiation modifies the native oxide is evidenced inFigure 3. This shows typical photocurrent spectra recorded at an electrode made ofa 8.3 ,urn thick 304 steel foil as a function of the illumination wavelength in thevisible region. The results show the effect of increasing the air ageing of theprepared electrode prior to the PEC test. With the native oxideexposed to alphairradiation, the photoresponse is markedly enhanced. In contrast, increasing theexposure time to the borate solution at open circuit under irradiation does not affectsignificantly Iph' It could be suggested that the amplitude of the photoresponse couldbe controlled by the irradiation flux, possibly "quenching" the ageing effect. Wewould argue that a positive space charge develops in the outer part of the film, asa result of a direct irradiation effect in the film: this hypothesis is consistent with thePEC behaviour reported above, and was corroborated by impedance measurementsperformed under similar electrochemical and irradiation conditions.

3.2.2. Influence of alpha-i"adiation on the rest potential. Electrodes prepared asdescribed above are exposed to the borate buffer (pH 9.2) for 360 h. The timeevolution of the open circuit potential (OCP) of three alpha-irradiated electrodesof 8.5 ,urn thickness is compared in Figure 4 with that of not irradiated electrodes.The bombardment of the metal/film interface with 1MeV alpha results in a steepand immediate increase in OCP, delayed by a few hours, and more gradual withoutirradiation. Moreover, the quasi-steady-state OCP attained after 360 h is more"noble" by approximately O.5V under irradiation.In order to establish the effects of alpha-irradiation on the redox behaviour of the

bulk solution, additional tests were performed with a platinum electrode whose frontface was irradiated with 4.8 MeV or IMeV alpha emitted by the Z41Am radiationsource located at 1 cm, 2 ,urn or 10 ,urn respectively from the platinum surface. Inthis case, only the radiolysis products of "sufficient" life time can attain the regionof the electrode surface whose OCP is only slightly and slowly modified (of 0.2 V)after 30 h of exposure.These results are in agreements with recent studies showing that introduction of

HzOz, Oz or Hz into the test solution leads to a limited open circuit potential changeat steady state. For example, an OCP rise < +0.3V was observed on a 304Lstainless steel exposed to a chloride solution in which HzOzwas added by successivedrops up to the level of 2.5 ppm [21]. Similar results were obtained in brine solutionat a Ti electrode whose OCP positive shift saturated after a few successive additionsof HzOz [221.However, the hypothesis that the instantaneous yield of oxidizing or reducing

species could affect or even control the OCP evolution of the electrode, as shownin Figure 4, cannot be refuted. A study of the chemistry of the solution under theinfluence of alpha radiation (primary species, recombination reactions...) should be

349

performed. Bombardment of the solution near the film/solution interface couldgenerate extrinsic surface states, induced by the adsorption of some radiolysisproducts, which, in turn, could favour either the electron or the hole transferprocesses at f/s, leading to abrupt OCP changes in either the cathodic or anodicdirection depending on the energy levels of these "new" radiation induced states.

a

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Figure 4 - Open circuit potential versus time plots in borate buffer (pH 9.2): a) for three alpha­irradiated ('-), and b) three un-irradiated (.I» electrodes made of 304 stainless steel foils of 8.5 ± 0.3

JJJn thickness (the standard deviation in the foil thickness is higher in this case than for Ti).

Moreover, corrosion reactions leading to either passive film formation ordissolution could also explain the results of Figure 4. In the literature, the corrosionrates of 304 stainless steels studied in an experimental loop were found to beenhanced by imposition of gamma-radiation at a dose rate of 500 Oy h,1 (due to a6OCo source, at 250°C with a 20 ppb content of O2 in the circulating water), fromthe "early" stage of exposure, This effect was consistent with the production ofoxidizing species under irradiation accelerating the oxidation reactions of ferrousions and thus leading to the formation of insoluble corrosion products [23].To conclude, a direct effect of irradiation in the film is assessed by PEC

measurements, while an indirect effect of irradiation on the film due to theadsorption of transient radiolysis products at Us is not inconsistent with OCP

350

measurements. These results motivated the choice of titanium, in place of an 304industrial alloy to go further, based on the fact that the electrochemistry of titaniumhas been accurately studied recently, in correlation with the structure andcomposition of the film, at the various stages of film growth [24].

3.3. ELECfROCHEMICAL STUDY OF TITANIUM PASSIVATED UNDER ALPHA-IRRADIATION

It is widely accepted in the literature that anodic films of titanium are highlyprotective against both localized and uniform corrosions, at least so long as theanodic layers are adherent in a "not too advanced stage of growth", allowing the useof this material in many applications. As reported above, ion-implantation causeschanges in electronic properties of the anodic layers, but leave not significantlychanged the stability of the film after implantation: only limited repassivation effectsafter implantation were shown, which may be largely induced by a surfaceroughening due to the bombardment of the surface with heavy ions of low energy(of order - few keV/amu.).We show in the two following sections that irradiation with alpha particles may

either apparently reinforce the passivation process or enhance drastically both theelectrochemical and chemical corrosion of the electrode, depending on the energyspectrum of the alpha particles [25, 26].

3.3.1. Intensity-potential plots. The measurements were made in 1 M H2S04 (pH 1)at 1 mV S-1 scan rate from -0.7 V up to 4 V after 3 min of potential stabilization atopen circuit. The solution (200 cm3) was degassed by N2 bubbling for 2 h prior tointroduction of the electrode in the main compartment of the electrolytic cell. Thetests were repeated with and without circulation of the solution through a pump ata rate of 3.5 I min- t , without changing the results. All potentials are referred to thesaturated calomel electrode.Figure 5 shows the influence of the polishing treatment on the electrochemical

behaviour. Figure 5a shows an intense anodic dissolution peak for bulk Ti polishedto mirror finish, followed by apparently uniform thickening of the anodic film on thefirst cycle in the region of constant current density, and an hysteresis effectappearing on the second cycle. (I passes through zero at a more anodic voltage andremains one order of magnitude lower until the range of oxygen evolution). For atitanium electrode covered with its native oxide (Figure 5b) the plateau of currentdensity attributed to film growth is conserved on the 1'1 cycle, but the anodicdissolution peak disappeared on the 1sl cycle while the anodic current densityremained higher on the 2nd cycle than for a mirror-polished surface. This is evidencethat the passivating influence of the remaining native oxide is not as great as the"true" anodic film grown under polarization to hinder anodic dissolution during the2nd scan.By comparison, Figure 6 shows six successive current-voltage (W) cycles recorded

at a Ti electrode (still covered with its native oxide)and irradiated with alpha

351

(OJ

I -2e~Acm

NI

E 0L> ( b)<{::L--..>-.......U1C(l)

V

4-'C(l)1-1-::J 0U

I11

-1 012Potential JVvs

3SeE

4

Figure 5 - Current-potential plots recorded at 1 mV S·l scan rate for titanium electrodes in 1M H2S04

at room temperature without irradiation: a) bulk freshly mechanically polished Ti electrode; b) gentlymechanically polished titanium foil, 15 Jl.In thick, covered with its native oxide.

352

o 1 234Potential / Vvs SeE

I II8~A.cm·2 I I f: : ',_, ,/1' x5

" I~, //

/'1'\ I I, I "-_//,I x.:: ..

/1"1\'\ '. I~ \ \ I I4-

1L' I 1

I I I n I:! /1I I I I,/! / V1I I I ,1 !x 5 / /1/f:fj\ I I /': /1/,1\ ~ // '1Ij'I:/I~~ ~4rl~/ V':;

t'I \:~:7'--- /~.,1----- 4/~--+-. 1 ..--==--=-~-..----__j;'//.-- I -

f· I I I _(1)

. I I ------- (2)I --- (3)I I I __ (4)I - ---- (S)I I I I --.--(6)

I I I

......CQ)L­L-::JU

NI

Eu<{:::i.

--->.......--UlCQ)"'0

Figure 6 - Current density-potential plots recorded in 1M H2S04 at a 12.5 JU1I thick titanium electrodeirradiated with alpha particles of energy <1.6 MeV, in the presence of the native oxide: the three firstcycles are recorded successively, starting with a ''fresh'' electrode; 15 h ageing at OCP precedes the two

following ones; and the sixth cycle is recorded after 15 h of additional ageing at OCP.

353

(c)

I8~A.cm-2

0( b)

NI

Eu<{::L...........>-

+J

U'lC a(lJ

"U (a)

+J

c(lJLL::JU

-1 a 1 2 3Potential I V vs SeE

4

Figure 7 - Same as Figure 6 for three alpha-irradiated Ti target electrodes of different thicknesses (only

the 1" and 2"d cycles are shown): a) dr, = 11 IJlTI so that E./ph4 < 2 MeV at ml!; b) dTi = 10 IJlTI so

that E./ph4 < 24 MeV at ml!; c) dr, = 9 IJlTI so that E./ph4 < 3 MeV at mit

354

particles of energy ranging from zero to about 1.6 MeV. It appears immediately thatthese irradiation conditions produce an instability of the electrode, since waves ofcurrent density apparently superimposed to a gradual anodic current rise startingonce passed through zero, are observed instead of the current plateau observedwithout irradiation. These peaks of current density are approximately located at VI= 0.8 V , V2 = 1.24 V on 1'1 cycle. VI disappears in the 2nd cycle while two newpeaks appear at Vo = -0.16 V and VI = 0.55 V while V2 shifts slightly towards higherpotentials (V2 = 1.35 V/SCE).Moreover, since the shape of the IIV plot remains unchanged after a long (- 55

h) exposure to the corrosive medium under irradiation, the successive cycles beingqualitatively reproducible even after a stay of more than 10 h at OCP between twoof them, it is temptatively assumed that these results reflect a radiation flux effect,destabilizing the passive layer and "quenching" the ageing effect of the electrode. Werecall that this reasoning has already been applied in the case of the 304 steelelectrode passivated under similar irradiation conditions. In addition, the hypothesisthat irradiation in this range of energies (E < 1.6 MeV) leads to the formation of aporous film can be advanced, concerning the results shown in Figure 6.It could be suggested tentatively that these findings involve a "surface

phenomenon" induced by the irradiation of the solution near f!s, related to theadsorption of radiolysis products, altering the classical film growth process. However,with increasing the energy spectrum of the alpha up to -3 MeV, by using Ti foils ofdecreasing thickness, the irradiation tends progressively to passivate the electrode,making it more blocking with respect to the ionic transport than without irradiationwhen the energy attains 3 MeV at m/f. This passivating influence of irradiationpersists on the second cycle and following (see Figure 7 a,b,c).

3.3.2. Criteria of stability of the Ti electrodes under alpha irradiation . Threeparameters are used to characterize the stabilizing/destabilizing influence of alphairradiation on the metal electrode, as a function of the thickness of the titaniumelectrode target: (i) the anodic charge passed Qan (in mC cm·2) below 2 V SCEwhere gas evolution becomes visible (as observed in Figure 5), (ii) the potentialdelay for growth defined as the potential difference between the voltage of zerocurrent density, V (1=0), noted during the first forward scan, and that of beginninggrowth, Vgrowth' taken at the intersect of the tangent to the IIV plot with the potentialaxis, (iii) the titanium concentration detected in solution by Inductively CoupledPlasma Atomic Emission Spectroscopy converted to the equivalent charge density,assuming that Ti dissolves as Ti4+, according to QICP/AES = Cri (z e VN 1M S) withz = 4, M = 47.9, V and S referring respectively to the volume of solution andirradiated surface area.A plot of Qan vs. dTi represented in Figure 8 evidences the destabilizing influence

of radiation on the passive film increasing with the projectile energy provided thatE is less than about 1.6 MeV (referred to as regime I), followed by a stabilizinginfluence of radiation whose efficiency enhances markedly when E attains about 3

355

MeV for dTi = 9 JA-m (referred to as regime II). The anodic charge (12.2 mC cm·2)is lower than found without irradiation on a Ti surface covered with its native oxide(14.7 mC cm·2) while the corresponding IN curve is similar in shape.

2220181210o4-.~..._T_T"""I'"'.,....,._I__r..........,....,................................................,....,..................l8

30.......----........------------.

5

~e 20("I

~ 15r-~v_/Tt---.:~;;;;;;;;;;:;;:;:::::==::;t-1"-

CI 10

25

Figure 8 - Plot of the anodic charge (me cm·2) passed at the Ti electrode under alpha-irradiation,

before the beginning of gas evolution. versus titanium thickness. calculated for the 1st cycle (opencircle) and 2"d cycle (black circle) respectively (--: eyes guide). The energy of the alpha-particle

reaching mlf increases from right (jar dTi = 15 JJftl. E.lpha < O.4MeV) to left (jar dn = 9 JJftl, E.lpha <3 MeV). Same quantity calculated without irradiation either on a freshly mechanically polished surface

(upper continuous line), or in the presence of the native oxide film (lower continuous line).

It is noticeable that bombardment with alpha particles of energy < 1.6 MeV leadsto the reactivation of the electrode surface still covered with the native oxide (at thebeginning of the test), since the anodic charge measured (26.4 mC cm·2) iscomparable to that obtained without irradiation at a titanium surface polished tomirror. However, it is also visible that the sequence of electrochemical reactionsdiffer under irradiation from the case without irradiation.Recalling that increasing the maximum energy of the projectile from 1.6 MeV to

3 MeV should not modify markedly the energy deposited in the thin layer of chargedsolution near fls (including not only the Helmholtz double layer but also the socalled Gouy layer), these results were unexpected. In order to interpret them, we

356

must assume that Figures 6, 7 and 8 reflect a mixed irradiation effect on both thefilm and solution side of the interface. This hypothesis is not inconsistent with theresults of Figure 9 showing the delay for growth plotted against target thickness from15 ,urn (stopping almost all alpha in metal) to 9 J.l.m (less than the thicknesscorresponding to the maximum of stopping power in the metal). Such a potentialdelay, function of the sweep rate, is found in the literature, as a result of ageing atopen circuit in not too corrosive media [27]. Consequently, it appears thatirradiation in regime I retards the ageing process, while irradiation in regime IIaccelerates the ageing process.The results of Figure 7 suggest that a threshold electric field must be passed to

observe the beginning of film growth. Figure 9 shows that this threshold value forthe electric field increases with increasing the energy of the projectile in regime II.These findings tend to prove that the degree of order in the film changes under thedirect influence of alpha-irradiation in regime II.

3 MeV 2.4 MeV 1. MeV 4 keVl-r---..L.---l--,r-----J..-;,.-....--,-----:.~::..:...;,

~u 0.8c:J:l

.-=II 0.6...>'-"

:: 0.4~e..llIl

> 0.2

16151411 12 13dTi I /lID

109o+r"T""T"T""I'"""I""T'",.........r-r-j-,-,,.......,~""T""T""I""T"T""T"T""T""'I'""T'""I""T'"~8

Figure 9 - Plot of the potential delay for film growth, .dV = Vgmwlh • V(I=O), versus thickness of the Titarget. Is also shown (upper scale) the higher estimated energy of the alpha particles reaching mlf after

passing through the corresponding Ti thicknesses. In inset: same quantity versus time spent at OCPbefore beginning the ?,d cycle for not irradiated specimens. (---: eyes guide).

357

ANot irradiatedbulk Ti

--I-tr- Q

8DOdic- .. - ~CPlAES

~

./

~... -Ii.~.

I.SIV I SCE

f90~:ln

60 3,---------------- ;._! 300l:~:::7t-:;;-;;E~~~::lo 0.5

E£ure IDc

4OT----------m-a-x-~ISO

Ea -2MeV

12032

ISO

120

30

o2I.S0.5

V I SCE

o-I -0.5Eme lOa

40

32

;6

OT------------"'TlSO

O.j..,.............-<~:....,...,. .........,...,..T"""'"........,...................+Oo 0.5 1 I.S 2.5E!!IIre IOd V I SCE

120

30

_ - - - 90.0...:l

60~:l ;.

maxEa -1.6MeV

-8

120 32

./

1.60.8 1.2V I SCE

:t:-~...~j~:,:w~..~~.. ~~=::=t30o 0.4E£ure lOb

40,.--------....,N~O~T::-:irr-a....,di....,ated~1SO

d,;= 15 J.lffi32

~E24'"

Figure 10 - Left scale: IntelJsity-potential plot recorded in the anodic region below oxygen evolution(open circle) and right scale: anodic charge passing through the interface versus potential (open

triangle), charge deduced from ICPIAES measurements (black triangle), making the hypothesis thattitanium dissolves as Ti4 + (Q/cPIAEs):without irradiation for a) bulk titanium polished to mirror, b) thin

titanium foil (15 J.IPl) covered with its native oxide; for a-irradiated titanium electrodes of differentthicknesses: c) dn = 11 J.IPl so that E.lpluo < 2 MeV at mlf; d) dn = 12.5 J.IPl so that Ed/pluo < 1.6

MeV at m/f

We have already seen that, for alpha particles, the main process of energytransfer to matter involve electronic excitations and ionizations may be caused by thealpha particles passing through the film. These cause the ejection of electrons to theconduction band of the anodic oxide (by hypothesis instantaneously exhausted fromthe film due to the electric field, in the absence of a potential barrier at theinterface with the metal) and leave holes in the valence band possibly accumulatingnear the film/solution interface. Thus a positive space charge will develop at fls andthe field strength will increase near the film/solution interface, which in turn couldaffect the ionic transport process in the film. However the accumulation of holes

358

near fls is made possible if the rate of hole capture by reduced species in solutionis sufficiently slow. And this capture process will be strongly affected by the yield inradiolysis products generated by the alpha particles interacting with the liquid phaseclose to the interface.Thus the respective irradiation effects on the film (via the electric field), and on

the solution (via the adsorption of radiolysis products) can be determined. We shallsee in the following section, how, by photoelectrochemistry, the interaction of alphaparticles with the film may occur with sufficient efficiency to alter the electronicstructure of the film the band structure of the film, when the irradiation isperformed in "a certain energy range".The third parameter allowing the determination of the stability of the titanium

electrode as a function of the energy spectrum of the incident alpha particles is theconcentration of titanium detected in solution below 2V SCE by means of ICP/AES,converted to the charge QiCP/AES' Only Ti dissolution is taken into account in thecalculation of the charge. In fact, the chemical and electrochemical dissolutions ofthe native oxide also contribute to Cri' The detected concentrations are reported inTable 2.The current density, Q an and Q'CP/AES are plotted as a function of the applied

voltage in Figure 10 without irradiation (Figure lOa and b) and under irradiation(Figure 10 c and d) respectively. By comparison with Figure lOc, an intense chemicaldissolution is observed under irradiation in regime I (energy of the projectile ~ 1.6MeV), followed by a reprecipitation process at the electrode surface (see FigurelOd).In contrast, the titanium concentration detected during irradiation in regime II

is significantly reduced (compare Figure lOb and Figure IOd), and tends to the leveldetected without irradiation for the surface initially covered with the native oxide.Any attempt to establish an electrochemical scenario accounting for these results

would be illusory (for example involving oxidation of Ti to Tj2+, then of Tj2+ toTi02+, competing with HTi03', both being in turn oxidized to TiOr or partiallyconverted to Ti02 precipitating at the electrode surface) due to the diversity ofprimary radiolysis products which may form complexes of long life time either withTi cations or sulfate anions whose influence could be visible at the time scale of thetest (for which a scan rate of 1 mV sec-I has been chosen).

3.4. PHOTOELECfROCHEMlCAL STUDY OF TI ELECfRODES PASSIVATED UNDERALPHA-IRRADIAnON

We have measured the photocurrent generated by a front monochromaticillumination in the visible and near ultraviolet region at a titanium electrode,irradiated at the same time with alpha particles at the back face. In this case, theanodic film is grown potentio-statically by imposing an anodic potential to theelectrode for one hour before beginning the photocurrent measurements. Here againthe native oxide film is still present at the electrode surface at the time of immersion

359

in 1 M H2S04 when the formation potential is fixed. The photocurrent (Iph) isdetected by using the lock-in technique and chopping the incident light at lowfrequency (30 Hz): only the non-steady photoresponse is studied.

TABLE 2:Titanium concentration in 1M H 2S04 electrolyte (el) in mg I-t measured byICP/AES for different formation potentials during anodisation of titanium at1mY s-t scan rate, at room temperature. Materials: unirradiated bulk Ti andthin Ti foil; several a-irradiated titanium thicknesses (the concentrations are

corrected for the quantities removed by sampling).

Volts Ti(el) (mgl") Ti(el) (mg I") Ti(eJ) (mg I") Ti(el) (mg 1"') Ti(eI) (mg 1.1)SCE Ti bulk dT;=15 ~m Ti bulk Ti bulk Ti bulk

un-irradiated un-irradiated E.=2 MeV E.=1.6 MeV E.=1.3 MeVS=1.2 em2 S=1.56 em2 irradiated irradiated irradiatedV=21O em' V=21O em' S=1.56 em2 S=1.95 em2 S=1.43 em2

V=21O em' V=170 em' V=170 em'

-0.62 0.00hO.002

-0.55 0.0086±0.0016

-0.46 0.D11±0.002

-0.2 0.013±0.003

0.0 0.0071 ±0.0005 0.006±0.002

0.3 0.01O±0.0005 0.013±0.002

0.4 0.021 ±0.001 0.015±0.003

0.46 0.0094±0.0017

0.6 0.0177±0.OO08 O.126±0.002

0.8 0.020±0.002 0.0132±0.0005 0.175±0.OOO3

0.9 0.016±0.003

1.0 0.027±0.001 0.015±0.003 0.064±0.002

1.2 0.D187±0.0006 0.121 ±0.004 0.069±0.OO2

1.6 0.031±0.001 O.125±0.0004

1.7 0.0215±0.0008 0.034±0.003 0.075±0.002

1.8 0.037±0.001 0.145±0.004

2.0 0.042±0.003 0.023±0.002 0.0333±0.0015 0.081±0.002

2.5 0.178±0.002

3.0 O.105±0.003

360

3.4.1. Optical band gap of passive films grown on titanium under alpha irradiation. Ithas been shown recently that the optical band gap of anodic films varies noticeably(by more than 0.2 eV) with the anodization conditions. Such a result could beexpected for a system whose structure and composition are dependent on theanodization conditions. Without irradiation, a significant decrease of E\, by steps,from about 4 eV on thin amorphous films to less than 3.6 eV for thick crystallizedfilms has been obtained by fitting an exponential law to the data (Figure 11).Assuming that the absorption edge obeys a power law, ahv oc [hv • Egopt]n, based

on the approximation of parabolic bands in the region of the band edges (with n=2,1/2 for indirect or direct transitions respectively), comparable results were obtainedby Leitner under similar electrochemical conditions [28].

4

3.9

3.8Q,Q

Of)

~3.7

3.6

3.50

NOT

20 40 60V I SeE

80 100

Figure 11 - Plot of the optical band gap as a function of the formation voltage reached at a Tielectrode anodized in 1M H~04 under I = 12 rnA cm·2• Eg~1 is deduced from a fit of the optical

absorption edge using an exponential law.

The localization of the electronic states in the region of the band edges leads tothe appearance of a mobility gap, larger than the band gap of the defect free oxide:it may explain the apparent increase of the gap obtained for the thinner films, inwhich a long range order exist no more. But it is not the only reason. A change ofthe gap can also be caused by either chemical or structural factors, not involvingnecessarily a disordering effect. For example, the introduction of oxygen vacancies,in the form of an ordered array of vacancies, may cause a band narrowing and shiftthe band mean energy which may also increase the band gap, without making

361

necessarily the hypothesis of the introduction of localized states. In contrast, whenchemical changes produce the appearance of new bands, a decrease of the gap canbe expected. These processes may explain the gap decrease shown in Figure It.This tendency is more pronounced under irradiation in regime II: alpha­

irradiation causes a steep decrease of the optical gap for increasing formationvoltages in a range in which the film was found amorphous without irradiation (blacksquares in Fig 12). By comparison with the results of Figure 11, these results couldbe consistent either with chemical changes caused by irradiation as suggested above,or with an accelerated (radiation-induced) crystallization process, once passed athreshold electric field.In regime I where the curve representing the optical gap against ymax crosses that

obtained without irradiation, it should be necessary to pursue the study at highervoltage to conclude.

4.1

~ 1.6 MeV4

;>~

...... 3.9Cl.0

01)

~ 3.8

3.7 NOTE ~ 2.4 MeV

20168 12V / SeE

43•6+-,......,---r......,....--.--~...--.,1"""""'T.....,..--r-.,.........--,......,---,--.---.----r-Io

Figure 12 - Same as Figure 11 for a Ti electrode anodized potentiostatically under alpha-in-adiation inregime I (.) and 11 (.) respectively.

3.4.2 Optical thickness. !Ph is converted into quantum yield (h) by normalization tothe photon flux. In the thin film limit, provided that the absorption spectrum isknown, the measurement of the quantum yield as a function of the illuminationwavelength (A.) gives the film thickness according to,

dr = -{In (l-h(A))}/2aA

362

We have used the absorption coefficients deduced for CVD Ti02 layers [29].Therefore the results reported in Figure 13 must be considered cautiously, as onlyindicative of a tendency: under irradiation in regime I, the anodic film would be verythin (with a threshold of film growth around 4 V), whereas under irradiation inregime II, the film thickness would apparently approach that obtained withoutirradiation according to literature data [30).

65

air.10 Jlrn Ti

•

,'X

J/ U Ir., 12.5 Jlrn Ti

2 3 4V vs. SeE I V

25

=20c"-~

~15c~C.I....c~10

C;C.I...~

Q.50

00 1

Figure 13 - Optical thickness versus electrode potential, estimated from photocurrent measurements onfilms grown potentiostatica/ly on a 12.5 !JlTl alpha-irradiated titanium foil (dotted-dashed fitting line),

on a 10!JlTl alpha-irradiated titanium foil (black symbols); comparison with an un-irradiatedelectrode (open symbols), and with data from [30J (solid line).

3.4.3. Sub-band gap photoresponse. Under the same electrochemical and opticalconditions, a completely different type of optical absorption edge is observed at thepassivated Ti electrode, by only varying the energy range of the alpha particlesbombarding Us. The quantum yield is plotted as a function of incident photon energyin Figure 14 and 15 for Ti electrodes respectively irradiated in the previously definedregime I (EalpllaS 1.6 MeV) and II (EalpllaS 2.4 MeV).A featureless photocurrent spectrum is obtained for all anodic films formed under

irradiation in regime I: Iph rises gradually near the energy gap, as observed in Figure14 for all applied voltages, from 1 V to 4 V/SCE. There is no difference betweenthese spectra and that which would be recorded on the passive films formed on Feor Fe-Cr alloys for example. Furthermore, this result corroborates the previousfindings concerning the film thickness. If we make the hypothesis that under these

363

./

.=~

·4 33 10 ,----------------,---,6 10·l

I

I

II . •

4 VI';> i ...-/l$.5y': ,,/ -210. 3

3 V. "/ " " 2 V'

/' , / ... "<;t- 1 V ~:::,-< '

o10° +--:-;;::.~,.;;:...=..~.. :;:...;:..~...~...:;..~...:;;:..:.. ~~~~.....-.......--~ 0 10°2 2.5 3 3.5

hv / eV

Figure 14 - Plots of the sub-band gap quantum yield versus incident photon energy, measured at atitanium electrode irradiated in regime II (dTi = 10 J.l.m, so that EaJpha < 2 MeV at mlf) passivated

in 1M H2SO.at different voltages: IV, 2V, 3V and 4V seE.

.=

310· 4"T"'""-----:::-_-,-------"""':"'""--,--r61O·3

\4V\ --l>.

~

......410. 3

IV.... 210. 3

/I <l--

o10°2 2.5 3 3.5

hv/ eV

o10° +-=r-.,.~-r-.-.--r-r-"T"'""'1;-r--.-~ ......--.--r-r-r-+-1.5

Figure 15 - Same as Figure 14 for a titanium electrode irradiated in regime I (dTi = 12.5 J.l.m, sothat EaJpha < 1.6 MeV at mlf).

364

particular irradiation conditions, the film forms according to a dissolution!reprecipitation process (completely different from that established for Ti withoutirradiation), then the observed photoelectrochemical behaviour is consistent with theexistence of a photoactive region of the film significantly reduced with respect tothat of unirradiated films.In contrast, even at the lower potential of 1 V SCE, a well structured sub-band

gap photoresponse is visible in Figure 15 for Ti electrodes irradiated in regime II.The quantum yield increases and shifts slightly towards lower photon energies withincreasing the formation potential, while the absorption edge steepens. It is worthnoting that such a photocurrent spectrum, exhibiting a broad subband gap peak(centered at 2.25 eV) has not been still reported in the literature at 1 V SCE, nearthe potential threshold above which the titanium electrode becomes photo-active.In the range 1.5 eV - 3 eV, a least squares fit with gaussians has been carried out,

revealing the presence of two photocurrent peaks respectively located at E1 = 2.3± 0.05 eV and E2 = 2.8 ± 0.05 eV for IV SCE formation potential and of oneadditional peak at lower energy Eo = 2.05 ± 0.05 eV for the higher formationpotential of 4V SCE. The sub-gap peaks at Eo and E1 were already found at 5V SCEwithout irradiation. Therefore, apparently, once reached the formation voltage of 4V, the film formed at the titanium electrode irradiated in regime II exhibits aphotocurrent response whose spectral dependence is typical of that found on a notirradiated passive Ti electrode under comparable electrochemical and illuminationconditions.These PEC results are consistent with the previous electrochemical study and

analysis of the solution, from which it could be suggested that under irradiation inregime II, the growth mechanism and hence the stability of the film grown withoutirradiation are recovered.This change in spectral response with the energy range of the bombarding alpha

particles could be considered puzzling, but in fact is consistent with all previouslyobtained results, including those gained by PEC with the 304 stainless steelelectrode.The question of the origin of the subband gap peaks has already been discussed

in 2.2.2. Apart from the peak at 2.8 eV, a similar reasoning leads to the conclusionthat intrinsic surface states participate to the photoprocess, as for the He+-implantedanodic oxides whose PEC behaviour is described in section 2.However it is not still proved that extrinsic surface states do not play a role.

Without irradiation, this point may be checked by changing the electrolyte, the pHand the hydrodynamics conditions. In the present case, a chemical study of theirradiated solution is missing, for example performed in parallel with a study of thephotocurrents transients to check the adsorption effects at f/s.More generally, one may wonder what could be the role attributed to the solution

side of the interface in the electrochemical and photoelectrochemical processesoccurring under alpha-irradiation in the two regimes noted. Even if they are notstudied here and if they are limited to the surface region, the radiolysis effects in

365

solution exist in all cases and may affect the potential drop in the Helmholtz layer.In the liUerature, it is observed that at sufficiently low (or likewise high) pH, the pHcan be considered as "pinned" [31]. Apparently it would not be the case under"certain" irradiation conditions. We suggest that the radiolysis species produced nearUs with energetic alpha particles can produce in some cases local pH changes (andpossibly flat band potential shifts).However, it may also happen that the species which could be responsible for a pH

change, might be produced and consumed simultaneously, hindering any change inpH. In this case, only the direct irradiation effect in the film will be detected (for the304 stainless steel or pure titanium as well): increase in band bending in the filmnear fls leading to an increase in field strength and to an enhanced dissolution rate.This mechanism was suggested by Vermilyea to explain the growth of anodic filmsof tantalum under uv illumination [32] (in this example, the uv illumination plays thepart taken here by irradiation). Further, the film formed by adissolution/reprecipitation mechanism will be photoactive only over a thin layer (lessthan the film thickness), and the results obtained when irradiating in regime I wouldthus be explained. The results of the study on the 304 stainless steel are consistentwith that obtained on titanium.Now, if we pursue this reasoning, when the energy spectrum of the alpha is

extended up to 3 MeV, there is no argument for assuming that the irradiation effectin the film would vary. Thus, the hypothesis may be advanced that the irradiationof the solution will change the near surface pH: this effect may counterbalance thedirect irradiation effect in the film, and the band bending without irradiation wouldthen be recovered. For example, an increased potential drop in the Helmholtz layerwill result of a surface acidification. This process could explain the striking similaritybetween the results detected under irradiation in regime II and without irradiation.

4. Conclusions

This paper deals with the question of ex-situ and in-situ irradiation effects on thefilms which form anodically on two different types of material, a 304 industrial alloyand titanium, in aqueous solution at room temperature.In a first part, we concentrated on the influence of radiation damage, produced

by means of a front irradiation of titanium anodic films, ex-situ, with light ions in thekeV range.A photoelectrochemical study, performed after irradiation, has shown that the

modifications of the electronic and optical properties of the film caused byirradiation become less visible with advancing the stage of growth and disappear inan advanced stage, for a flux of 10-4 dpa S-I. This study enlightens the role of theradiation flux in modifying the properties of anodic oxides whose crystallizationdegree is varying with the experimental conditions. Increasing the energy of theprojectile in the MeV range, the production of point defects in the film becomes

366

negligible.In the second part, we have studied in-situ the irradiation effects on the passive

films, by means of a back irradiation of a thin metal foil with alpha particles ofenergy spectrum extending up to the MeV range. In this case, the stopping ispredominantly electronic, in the solid and liquid phases (except at the end of thetrajectory, far from the electrode surface). The experimental arrangement usedallows to vary the energy of the alpha particles reaching the metal/film interface andhence deposited in solution after passing through the metal target and the (native+) growing film, by simply changing the metal thickness.This study highlights the role of the irradiation on the growth and stability of the

film which forms either on titanium or on a 304 industrial alloy in the early stage ofgrowth.As for titanium, two growth regimes are apparent under alpha-irradiation.

i) A dissolution/reprecipitation process for Ti, bombarded with alpha particlesreaching mlf with an energy < 1.6 MeV. In this case, the electrochemical,photoelectrochimical and ICP/AES results are consistent with the formation of athin, porous, non-protective film.

ii) In contrast, extending the energy spectrum progressively up to 3 MeV, all theresults obtained without irradiation are progressively recovered. They are consistentwith the formation of a pore free film, more protective than without irradiation oncereached 3 MeV.

This study suggests that the energy transferred by electronic excitations to the nativefilm in air and during the time spent at OCP before imposing I or V modifiesstrongly the films properties. Moreover, the results of the study of titanium underalpha-irradiation are consistent with an interplay between direct "bulk" irradiationeffects in the film and indirect "surface" effects due to the solution radiolysis.The influence of alpha-irradiation in the advanced stage of growth of anodic films

on titanium and the reversibility of the observed phenomena are currently underinvestigation.

References

1. Cox B., Journal of Nuclear Materials, Vol. 28, 1 (1968)

2. Elfenthal L., Schultze l.W. and Meyer 0., Corrosion Sci., Vol. 29, 343 (1989)

3. Gorse D. Rondot B. and Da Cunha Bela M., Corrosion Sci., Vol. 30, 23 (1990)

4. GorseD. and Sakout T., Solid State Phenomena, Vol. 30-31,451 (1993)

367

5. Leach J.S.L. and Pearson BR, Co"osion Sci., Vol. 28, 43 (1988)

6. Serruys Y. Sakout T. and Gorse D., Surface Sci., Vol. 282, 279 (1993)

7. Gorse D., Serruys Y. and Sakout T., in preparation

8. Munnix S. and Schmeits M., Phys. Rev B, Vol. 30,2202 (1984)

9. Mott N.F. and Davis E.A., in Electronic Processes in Non-Crystalline Materials,Clarendon Press, Oxford (1979)

10. Danzfuss B. and Stimming U., J. Electroanal. Chern., Vol. 164,89 (1984)

11. Gorse D., Serruys Y. and Rondot B., in proceedings of the International Symposiumon Radiation Materials, Alushta, USSR (1990)

12. Smandek B. and Gerischer H., Electrochirnica Acta, Vol. 34, 1411 (1989)

13. Henrich V.E., Dresselhaus G. and Zeiger H.1., Vol. Phys. Rev. L., 36, 1335 (1976)

14. Munnix S. and Schmeits M., Phys. Rev B, Vol. 31,3369 (1985)

15. Goodenough J.B., Les Oxydes des Metaux de Transition, Gauthier-Villars editeur,Paris (1973)

16. Houlihan J.F., Bonaquist RF., Dirstine RT. and Madacsi D.P., Mat. Res. Bull., Vol.16,659 (1981)

17. Ziegler J.F., The Stopping and Ranges of Ions in Matter, Vol 4, Pergamon Press,Oxford (1977)

18. Northcliffe L.c. and Schilling RF., Nuclear Data Tables, Vol. A7, 233 (1970)

19. Vetter KJ. Electrochimica Acta, Vol. 16, 1923 (1971)

20. Cabrera N. and Mott N.F., Rep. Progr. Phys., Vol. 12, 163 (1949)

21. Marsh G.P., Taylor KJ., Bryan G. and Worthington S.E., Co"osion Sci., Vol. 26,971 (1986)

22. Kim Y.1. and Oriani RA., Co"osion, Vol. 43,92 (1987)

23. Ishigure K, Ikuse H., Oshima K, Fujita N. and Ono S., Radiat. Phys. Chern., Vol.21, 281 (1983)

368

24. Delplancke J.-L., Winand R., Eleclrochimica Acta, Vol. 33, 1539 and Vol. 33, 1551(1988)

25. Gorse D., Sakout T. and Serruys Y., in proceedings of the International Symposium onModifications of Passive Films, Paris, 15-17 February (1993)

26. Rouchaud J.c., Fedoroff M., Sakout T. and Gorse D., in Symposium on AnalyticalSciences - SAS 93, Deauville, 4-6 Mai (1993), to be published in Analusis

27. Blackwood D.I., Peter L.M. and Williams D.E., Electrochimica Acta, Vol. 33,1143 (1988)

28. Leitner K., Schultze I.W. and Stimming D., 1. Electrochem. Soc., Vol. 133, 1561 (1986)

29. Moller F., Tolle H.I. and Memming R., J. Electrochem. Soc., Vol. 121, 1160 (1974)

30. McAleer I.F. and Peter L.M., Faraday Disc. Royal Soc. Chem., Vol. 70, 67 (1980)

31. Salvador P., Electrochimica Acta, Vol. 37,957 (1992)

32. Vermilyea D.A~ J. Electrochem. Soc., Vol. 104,212 (1957)