Some thoughts on the mechanisms of in-reactor corrosion of zirconium alloys

In recent years sufficient new information has accumulated to change current views of which mechanisms of corrosion are operating in water-cooled reactors. The total number of publications is now so enormous that it is impossible for a short review to be completely comprehensive. This review concentrates on those studies that have resulted in changed views of the importance of various mechanisms. There seems to be insufficient evidence to support an hypothesis that increased corrosion rates in reactor result directly from displacement damage to the oxide by fast neutron bombardment. Replacing this hypothesis are the observations that redistribution of Fe from second phase particles (SPPs) into the Zr matrix by fast neutron recoil reduces the corrosion resistance of Zircaloy type alloys both in-reactor and in laboratory tests. There is little support for the idea that an irradiation induced phase change from monoclinic to tetragonal (or cubic) zirconia is important in-reactor, since such transformed zirconias are unstable in ∼300 °C water and revert to the monoclinic phase; as do chemically stabilised zirconias. New alloys with improved corrosion resistance in PWRs are generally low in Fe (and Sn), or have Fe in the form of more radiation resistant SPPs than those in the Zircaloys. Similarly, the hypothesis that nodular corrosion in BWRs was directly related to an effect of irradiation produced radical species in the water is unsupported. However, local dissolution of the oxide film by radiation produced species such as H2O2 may be occurring, and the close mechanistic relationship between nodular corrosion and ‘shadow corrosion’ is very evident. Thus, galvanic potentials between large SPPs (or clusters of SPPs) and the Zr matrix, aided by greatly increased electronic conduction of zirconia in irradiated systems appears to offer an hypothesis that provides a rationale for the observed effects of SPP sizes and numbers. Irradiation induced redistribution of Fe from the SPPs into the Zr matrix eliminates nodular corrosion susceptibility in Zircaloys.

[1]  H. Schaefer,et al.  OXYGEN DIFFUSION IN ULTRAFINE GRAINED MONOCLINIC ZRO2 , 1999 .

[2]  P. Shirvington.  ELECTRON CONDUCTION THROUGH OXIDE FILMS ON ZIRCALOY-2. , 1970 .

[3]  S. Sōmiya.  Hydrothermal Reactions for Materials Science and Engineering , 1990 .

[4]  J. Meunier,et al.  Corrosion du zircaloy dans divers milieux alcalins a haute temperature , 1962 .

[5]  M. Howlader,et al.  In situ measurement of electrical conductivity of Zircaloy oxides and their formation mechanism under electron irradiation , 1999 .

[6]  P. Lacombe,et al.  Etude par microscopie electronique en transmission de films minces d'oxyde obtenus par oxydation d'alliages de zirconium a teneur en element d'addition inferieure a 4% en poids , 1968 .

[7]  N. Ramasubramanian.  Localised electron transport in corroding zirconium alloys , 1975 .

[8]  B. Cox.  Is zirconium oxide morphology on fuel cladding largely determined by lithium hydroxide concentration effects , 1997 .

[9]  K. Pettersson,et al.  Phase transformation of stabilised zirconia in water and 1.0 M LiOH , 2001 .

[10]  Xin Guo.  Hydrothermal degradation mechanism of tetragonal Zirconia , 2001 .

[11]  B. Cox.  Rate controlling processes during the pre-transition oxidation of zirconium alloys , 1969 .

[12]  R. Ploc.  Electron diffraction analysis of ZrO2 on αZr(112̄0) , 1983 .

[13]  E. Ibe,et al.  Formation and dissolution of oxide film on zirconium alloys in 288°C pure water under γ-ray irradiation , 1997 .

[14]  S. Newcomb,et al.  Microscopy of oxidation 3 : proceedings of the Third International Conference on the Microscopy of Oxidation, held at Trinity Hall, the University of Cambridge, 16-18 September 1996 , 1997 .

[15]  P. Harrop,et al.  The effect of gamma dose on oxide films on zirconium and zircaloy-2 and its relevance to corrosion , 1965 .

[16]  V. Fidleris.  The Irradiation Creep and Growth Phenomena , 1988 .

[17]  B. Cox.  A porosimeter for determining the sizes of flaws in zirconia or other insulating films “In Situ” , 1968 .

[18]  T. Pajkossy,et al.  Oxide layers of Zr–1% Nb under PWR primary circuit conditions , 2001 .

[19]  A. Fiegna,et al.  Influence of Thin Noble Metal Films on Zirconium Oxidation , 1968 .

[20]  D. Gosset,et al.  Investigation on the zirconia phase transition under irradiation , 2000 .

[21]  X. Iltis,et al.  Microstructural study of oxide layers formed on Zircaloy-4 in autoclave and in reactor part 11: Impact of the chemical evolution of intermetallic precipitates on their zirconia environment , 1995 .

[22]  J. Whitton.  The Measurement of Ionic Mobilities in the Anodic Oxides of Tantalum and Zirconium by a Precision Sectioning Technique , 1968 .

[23]  R. Ploc.  Transmission electron microscopy of thin (<2000 å) thermally formed ZrO2 films , 1968 .

[24]  Xin Guo,et al.  Grain Boundary Blocking Effect in Zirconia: A Schottky Barrier Analysis , 2001 .

[25]  M. Griffiths,et al.  Phase instability, decomposition and redistribution of intermetallic precipitates in Zircaloy-2 and -4 during neutron irradiation , 1987 .

[26]  H. Sheikh,et al.  Redistribution of the alloying elements during Zircaloy-2 oxidation , 1997 .

[27]  R. Staehle,et al.  Advances in Corrosion Science and Technology , 1972 .

[28]  T. Matsui,et al.  Radiation damage in yttria-stabilized zirconia under Xe ion irradiation , 1998 .

[29]  R. Ploc.  A transmission electron diffraction study of ZrO2 on α-Zr (0001) , 1982 .

[30]  Steven J. Zinkle,et al.  Radiation effects in ceramics , 1994 .

[31]  Xin Guo,et al.  On the grain boundaries of ZrO2-based solid electrolyte , 1995 .

[32]  Liv Lunde,et al.  Special features of external corrosion of fuel cladding in boiling water reactors , 1975 .

[33]  C. Lemaignan.  Impact of β− radiolysis and transient products on irradiation-enhanced corrosion of zirconium alloys , 1992 .

[34]  B. Cox.  EFFECTS OF IRRADIATION ON THE OXIDATION OF ZIRCONIUM ALLOYS IN HIGH TEMPERATURE AQUEOUS ENVIRONMENTS. A REVIEW. , 1968 .

[35]  J. P. Pemsler,et al.  Diffusion of oxygen in growing zirconia films , 1968 .

[36]  B. Cox.  The Oxidation and Corrosion of Zirconium and its Alloys V . Mechanism of Oxide Film Growth and Breakdown on Zirconium and Zircaloy‐2 , 1961 .

[37]  A. L. Sutton,et al.  Electrochemical Measurements on Zirconium and Zircaloy‐2 at Elevated Temperatures II . 200°–300°C , 1965 .

[38]  B. Cox.  Some effects of pressure on the oxidation of zircaloy-2 in steam and oxygen , 1963 .

[39]  R. Ploc.  Electron diffraction from ZrO2 on αZr(101̄0) , 1983 .

[40]  B. Cox.  Comments on the paper ``the influence of oxide stress on the breakaway oxidation of Zircaloy-2'' by D. H. Bradhurst and P. M. Heuer , 1971 .

[41]  C. Britton,et al.  FURTHER STUDIES ON THE INHIBITION BY BORIC ACID OF THE OXIDATION OF ZIRCONIUM IN HIGH PRESSURE STEAM , 1965 .

[42]  R. Adamson,et al.  Precipitates in zircaloy: Identification and the effects of irradiation and thermal treatment , 1986 .

[43]  J. Bokros.  TRANSFORMATION KINETICS OF A ZIRCONIUM-URANIUM-HYDROGEN ALLOY , 1961 .

[44]  A. Motta,et al.  Zirconium Alloys in Nuclear Applications , 2006 .

[45]  R. Manzel,et al.  External corrosion of cladding in PWRs , 1975 .

[46]  M. Brown,et al.  Polarisation of zirconium and its alloys in high temperature water , 1977 .

[47]  K. Pettersson,et al.  Characterisation of pre-transition oxides on Zircaloys , 2001 .

[48]  G. David,et al.  Etude de la croissance de l'oxyde sur le zirconium et le zircaloy-2 , 1971 .

[49]  C. Britton,et al.  Inhibition by boric acid of the oxidation of zirconium in high pressure steam , 1962 .

[50]  D. Vermilyea,et al.  A Mechanism for the Effect of Heat‐Treatment on the Accelerated Corrosion of Zircaloy‐4 in High Temperature, High Pressure Steam , 1978 .

[51]  W. Ranken,et al.  Neutron‐Irradiation Damage in Stabilized ZrO2 , 1977 .

[52]  D. Franklin,et al.  Long-term corrosion of Zircaloy before and after irradiation , 2000 .

[53]  D. Silvester,et al.  Influence of Environment on the Corrosion of Zirconium and Its Alloys in High‐Temperature Steam , 1963 .

[54]  J. Draley,et al.  An Electrochemical Model for the Oxidation of Zirconium , 1965 .

[55]  D. Gosset,et al.  Analysis of the monoclinic–tetragonal phase transition of zirconia under irradiation , 2002 .

[56]  K. Pettersson,et al.  Oxidation of Zircaloy-2 and Zircaloy-4 in water and lithiated water at 360°C , 2001 .

[57]  X. Iltis,et al.  Microstructural study of oxide layers formed on Zircaloy-4 in autoclave and in reactor Part i: Impact of irradiation on the microstructure of the zirconia layer , 1995 .