Nodular Corrosion Research Articles

Determining the microstructure effects on the stress corrosion cracking initiation behavior of laser powder-bed-fusion printed 304L stainless steel in high-temperature hydrogenated water

This study investigated the microstructure effects on the stress corrosion cracking (SCC) initiation behavior of laser powder-bed-fusion (L-PBF) printed 304 L stainless steel in high-temperature hydrogenated water. The dislocation cells facilitate Cr transportation, thereby mitigating intergranular oxidation and SCC initiation. Compared to the warm-rolled dislocation cells, as-printed dislocation cells result in reduced strain localization, which is beneficial for suppressing SCC initiation. This is because the as-printed dislocation cells are stable and composed of abundant screw dislocations. Micro-inclusions can lead to nodular corrosion, thus increasing the depth of intergranular oxidation and promoting SCC initiation.

Corrosion behavior of the FeCrAl coating with different Cr and Al contents before and after Au-ions irradiation in stagnant LBE

The corrosion behavior of the FeCrAl coating with different Cr and Al contents before and after Au-ions irradiation was investigated in static LBE. The grain of the irradiated coating changed from nanocrystal to large columnar crystal. The irradiation accelerated the LBE corrosion rate, and the oxide scale changed from three layers to four layers. Nodular corrosion was found in the irradiated FeCrAl coating. The change of Cr and Al contents has a remarkable effect on the corrosion behavior of the irradiated coatings.

Susceptibility of Zr-2.5 (wt. %) Nb alloy to undergo nodular corrosion in water and steam environments: Effect of surface, cold work, temperature

Nodular corrosion can severely affect the service life of zirconium alloys in water cooled nuclear reactors. In this study, the susceptibility of pressure tube alloy to undergo nodular corrosion in water-steam with air surroundings at 250°C and 300°C is investigated. Accelerated corrosion has been studied in single phase steam at 500°C. The effect of specimen surface condition and cold work on nodular corrosion susceptibility has been studied in detail. The results indicate that surface condition of the specimen plays primary role in determining the propensity of the alloy to undergo nodular corrosion. Cold work has negligible effect on the initiation as well as growth of oxide nodules in Zr-2.5 (wt. %)Nb alloy. The effect of temperature is highly localized and is visible on the size of the nodules. Presence of non-uniform or discontinuous autoclave layer increases the susceptibility of pressure tube alloy to undergo nodular corrosion.

Uniform oxidation in steam and nodular corrosion in gas phase of Zr-2.5Nb pressure tube material - effect of nature of initial oxide

Oxide layers of similar starting thicknesses were grown on Zr-2.5Nb pressure tube material in air at 400 °C and in steam at 400 °C and 10 MPa. Both types of oxides were subjected to (a) further long term exposure up to 85 days in steam at 400 °C and 10 MPa and (b) nodular corrosion in a gas mixture of CO2 and O2 in presence of a contaminant, HCl at 350 °C. The air oxidized material gave an advantage of no hydrogen pick-up to start with as against steam oxidized material. Both types of oxides exhibited similar uniform oxidation and hydrogen pick up behaviour during long term exposure in steam at 400 °C and 10 MPa. On the other hand, the air-formed oxide was found to be more resistant to nodular corrosion in a gas mixture of CO2 and O2 at high temperature in presence of hydrochloric acid. The superior nodular corrosion resistance was attributed to a thicker impervious layer in the oxide rendering better protection from local oxide damage. This has been explained through the global mechanism for nodular corrosion in gas phase. The usefulness of air oxidation in the final steps of pressure tube fabrication has been discussed as well.

Microstructural understanding of general and localized corrosion of Fe13Cr4Al2Mo1.2Nb alloy in high temperature aerated water and superheated steam

The corrosion behaviors of a Fe13Cr4Al2Mo1.2Nb alloy in 360 °C aerated water and 400 °C superheated steam were investigated. A similar dual-layered oxide scale was formed under both conditions. However, the inner oxide layer was much thicker in 400 °C steam than in 360 °C water, indicating the occurrence of steam-accelerated corrosion. Nodular corrosion was observed following exposure to 360 °C water, while localized corrosion and Ni segregation along the grain boundary occurred after 120-d exposure to 400 °C steam. The Fe2Nb phase experienced delayed oxidation with respect to matrix, which involved transformation from amorphous Nb oxides to crystalline Nb2O5.

Nodular corrosion inside the crevice of Alloy 690 in deaerated high-temperature chloride solution

Crevice corrosion behavior of Alloy 690 in 290 °C deaerated chloride solution was investigated using a high-temperature crevice corrosion exposure device. The results show that the oxide film enriched in Cr formed inside the crevice is thinner than that enriched in Ni formed outside the crevice. A kind of nodular corrosion occurred inside the crevice. The autocatalytic effect resulting from the limitation of metallic ions diffusion caused by the crevice geometry promoted the development of nodular corrosion. The mechanism of oxide film and nodular corrosion formation during crevice corrosion are discussed.

Nodular corrosion of zirconium alloys in gaseous environment containing different contaminants

The nodular corrosion susceptibility of two zirconium alloys, namely Zr-2.5Nb and Zr-2.5Nb-0.5Cu, in CO2 + O2 containing different contaminants is established. The contaminants used were ethylene, chlorine, formic acid, nitric acid, sulphuric acid and moisture. Nodules formed on both alloys in presence of each contaminant with varied extent except for sulphuric acid. The presence of ethylene resulted in nodular corrosion only in presence of radiation. The occurrence and extent of nodular corrosion is discussed in light of the “global mechanism” for nodular corrosion of zirconium alloys in gaseous environments.

Oxidation behavior analysis of a ferritic ODS steel in supercritical Water

Abstract The corrosion behavior of 14Cr ODS steel exposed to the supercritical water (SCW) at 600 °C under 25 MPa with the dissolved oxygen content of 300 ppb was investigated. The weight gain measurement and corrosion layers analysis were used to determine the corrosion behavior and corrosion mechanism of the 14Cr ODS steel. The results indicate that the 14Cr ODS steel exposed in this work possesses better corrosion resistance than the other ODS steel with a similar composition. Both uniform corrosion regions and nodular corrosion regions can be seen on the exposed specimens, which is due to the bimodal microstructure of the ODS steel. The corrosion products have a multi-layer structure, including the outer oxide layer, the inner oxide layer, and the transition layer. The oxide in the inner layer is FeCr2O4. The oxides in the outer layer change from the initial Fe3O4 to Fe2O3 with the increase of exposure time. The possible corrosion mechanism is analyzed as well, combining with the microstructure characters of the 14Cr ODS steel.

Features of the Water-Chemical Mode of the I Circuit and Problems of Equipment Operation in the Reactor Units of Atomic Icebreakers

Analysis of the experience gained in operating atomic icebreakers, the specifics of operating equipment of the first circuit, and the organization of the water-chemical regime shows the relationship between the high content of gases in the coolant and the problems of ensuring reliable equipment operation. Spatial modeling of vapor-gas bubbles’ mass transfer in the SG-28 steam generator during damping confirmed the possibility of forming a gas cushion in the stagnant zone of the SG. The abrupt increase in the volumetric content of hydrogen during the transition from the liquid to the gas phase of the water coolant explains the anomalous hydrogenation in the area of weld no. 62 on the tubing of the AO SKBK design steam generators. The multiscale, multiphysical approach to conducting calculations was based on the adaptive use of different calculation codes in accordance with the required degree of analysis detail. Four computational models were developed—a common one based on the Relap5 code and three for the CFD code. An offline scheme of combining codes was used. Within its framework, a consistent decomposition of the general problem into computational domains was carried out; appropriate models for them were used, and organization of information exchange between models was based on the initial and boundary conditions. An analysis of the operating conditions and the nature of the mass transfer of vapor-gas bubbles, taking into account the patterns of their fragmentation, confirms the possibility of their entry into the active zone in transient conditions. The presence of oxygen in vapor-gas bubbles dramatically changes the corrosive environment and can cause the nodular corrosion of zirconium shells. To drastically reduce the content of noncondensable gases in the coolant of the first circuit, it was proposed to study the possibility of using a steam volume pressurizer instead of a gas pressurizer in promising projects of transport nuclear power plants (NPPs).

Microstructural investigation of the nodular corrosion of 304NG stainless steel in supercritical water

304NG stainless steel was exposed to deaerated 600 °C supercritical water for 300 h. The thin Cr-rich oxide film and corrosion nodules were characterized in detail by cutting edge techniques. The results show that the formation of MnCr2O4 is an important factor that deteriorates the protectiveness of the Cr-rich thin oxide film. Reaction zones ahead of the inner oxide are observed. The reaction zones are composed of oxide particles and remaining metal, indicating the occurrence of internal oxidation. The internal oxidation controls the further corrosion after the breakdown of Cr-rich thin oxide film, and subsequently the generation of corrosion nodules.

The influence of microtexture on the formation mechanism of nodules in Zircaloy-4 alloy tube

Corrosion resistance of Zircaloy-4 alloy tube in superheated steam at 673 K/10.3 MPa is anisotropic. A part of the surface undergoes uniform corrosion while the other suffers nodular corrosion. Narrow and wide nodules are observed after an exposure period of 3 and 30 days, respectively. A new matrix transformation method is established in order to study the formation mechanism of nodules in the cross-section (CS) of Zircaloy-4 alloy tube using the EBSD technique, while the CS perpendicular to axial direction (AD). The results reveal that the microtexture is a key factor behind the two types of corrosion. Furthermore, the oxide layers grow anisotropically over the corroded surface. A thick oxide layer forms over the nodular corrosion region on the grains with c-axis oriented in the range of 40° around tangential direction (TD), whereas a thin oxide layer over the uniform corrosion region is detected on the grains with c-axis oriented in the range of 68° around TD. In short, the anisotropic growth of oxide layer was caused by the change of microtexture of the Zr-4 alloy tube, and this anisotropic growth of oxide layer contributed to the nodules formation.

Features of Mass Transfer of Noncondensable Gases by Primary Coolant of Nuclear Icebreaker Reactors

To study the characteristics of the mass transfer of noncondensable gases in the primary loop of the nuclear icebreaker reactors, models of the core and the up-core space were developed using the Relap5 (USA) and Fluent (USA) codes and modeling of interphase mass transfer and transport of the gas phase by the water coolant was performed on these sections. Spatial non-uniformity of the content of undissolved gases, which arises as a result of vapor-gas bubbles concentrating in the vortex structures of the flow and resulting in a significant reduction of their dissolution rate, was revealed. This must be taken into account in the analysis of the operation of the primary loop equipment of nuclear icebreakers (hydrogen absorption in the titanium alloys of steam-generator piping, nodular corrosion of the zirconium cladding of the fuel rods).

Galvanic Corrosion Behaviour of Phosphate Nodular Cast Iron in Different Types of Residual Waters and Couplings

The aim of this paper is to analyze the corrosion process that occurs in galvanic couplings of different alloys. The study focuses on materials that can come into contact in submersible pumps used by water treatment plants. Because, the rotor, one of the pump main components must possess high chemical and mechanical properties, nodular cast iron is usually used. Therefore, this is exposed in the same environment with different types of materials, such as aluminum, copper, bronze, grey cast iron, low alloy steel or stainless steel from which other components are made. The tests have been performed in three types of residual waters with neutral, acidic and basic pH. According to this study, the nodular cast iron galvanic corrosion resistance is highly improved by the phosphate layer deposited on its surface. Keywords: galvanic corrosion, wastewater, nodular cast iron, phosphate layer, galvanic couple

Nodular Corrosion of Zr–0.85Sn–0.16Nb–0.37Fe–0.18Cr Alloy in 500 °C Steam Caused by High-temperature Processing

Zr–0.85Sn–0.16Nb–0.37Fe–0.18Cr alloy samples prepared by low-temperature process (LTP) and high-temperature process (HTP) show different corrosion behaviors in 500 °C steam. The former exhibits uniform corrosion, while the latter exhibits nodular corrosion casually. The occurrence of nodular corrosion on HTP samples is attributed to the formation of local alloying elements depletion region in the Zr matrix. During hot rolling at 800 °C, transformation of zirconium from α-phase to β-phase happens locally. Alloying elements migrate to β-phase from its neighboring α-phase. The local β-phase dissolves in the subsequent cold rolling and final annealing, leaving the microstructure with agglomeration of precipitate particles and neighboring alloying elements depletion regions in Zr matrix. And the undesirable microstructure becomes the causation of nodular corrosion.

Localized oxidation of Zr-2.5Nb alloy in high temperature carbon dioxide environment containing hydrochloric acid

Zr-2.5Nb pressure tube alloy with uniform oxide layer was shown to undergo extensive nodular corrosion when exposed to a gas mixture of CO2 and O2 at high temperature in presence of hydrochloric acid. Substantial hydrogen pick-up with radial hydrides at the nodule base was observed. A bare metal surface, however, did not undergo nodular corrosion. A thicker initial oxide was less prone to nodular corrosion. A global mechanism was proposed which postulates that a site with localized damage (chemical/mechanical) of the uniform oxide layer, yet surrounded by the remaining oxide, tends to oxidize at an enhanced rate resulting in nodules.

X-Ray Computer Tomography Study of Degradation of the Zircaloy-2 Tubes Oxidized at High Temperatures

ABSTRACT The investigations of high-temperature oxidation of zirconium alloys, applied for fuel pellets in nuclear power plants, are usually limited to oxidation kinetics, phase transformations and microstructural characterization. The purpose of this research was to characterize the degradation phenomena occurring within oxide layer and at the interface oxide/metal, on internal and external Zircaloy-2 tube surfaces, below and over crystalline transformation temperature of zirconium oxides. The commercial tubes were oxidized at 1273 K and 1373 K in calm air for 30 min and then examined with a technique novel for such purpose, namely a high-resolution X-ray computer tomography. The light microscopy was used to examine the cross-surfaces. The obtained results show that the form and intensity of oxide damage is significant and it is in a complicated way related to oxidation temperature and on whether external or internal tube surface is studied. The found oxide layer damage forms include surface cracks, the detachment of oxide layers, the appearance of voids, and nodular corrosion. The oxidation effects and damage appearance are discussed taking into account the processes such as formation of oxides, their phase transformation, stress-enhanced formation and propagation of cracks, diffusion of vacancies, formation of nitrides, diffusion of hydrogen into interface oxide-metal, incubation of cracks on second phase precipitates are taken into account to explain the observed phenomena.

Integrity control of an RBMK-1500 fuel rod locally oxidized under a bounding reactivity-initiated accident

In 2007, the license for the second reactor unit of the Ignalina nuclear power plant was renewed considering the safety-related modifications introduced in this reactor. The Safety Analysis Report for this reactor unit was prepared with more strict criteria. The bounding reactivity-initiated accident (RIA) performed by the Lithuanian Energy Institute could be mentioned as an example. The performed analysis demonstrated that even when the worst initial conditions and possible uncertainties are considered, the fuel cladding remains intact. However, the analysis was performed assuming a fresh fuel assembly. In this study, an analysis of the fuel rod cladding behavior in the RBMK-1500 reactor following a bounding RIA is performed using the computational codes FEMAXI-6 and RELAP5. The analysis is extended by modeling an oxide layer (nodular corrosion) on the external surface cladding. An uncertainty and sensitivity analysis was performed using a method developed by the Society for Plant and Reactor Safety, employing the Software for Uncertainty and Sensitivity Analyses, in order to evaluate the effect of the oxide layer on the inside and outside fuel rod temperatures. The results of the thermo-mechanical analysis (stress, strain, and enthalpy) for a local oxide layer with a thickness of 70 μm show that despite the exceeded limit of allowed linear power density, the fuel rod is under acceptable safety conditions.

Axial and Azimuthal Asymmetry of RBMK-1000 Fuel-Element Cladding Oxidation

The particularities of the axial and azimuthal asymmetry of nodular corrosion of RBMK-1000 fuel-element cladding operating in FA at power 1.7–2.2 MW are examined. The measured thickness of the oxides is compared with the concentration of the oxidative products of radiolysis in the coolant and the computed distribution of the thermophysical parameters in a fuel channel of the reactor. It is shown that the axial asymmetry of the oxidation of the fuel elements could be due to the temperature distribution in cladding and the non-uniform content of radiolytic oxygen and the radicals HO2 and $$ {\mathrm{O}}_2^{-} $$ over height in FA and the azimuthal distribution is due to the radial distribution of the fast and thermal neutron fluxes. The results of studies attesting the possibility of the appearance in the RBMK-1000 fuel channel of conditions where the corrosion rate of the cladding on the sections between the spacer grills is comparable to the oxidation rates of VVER-1000 fuel elements are presented.

Effect of molybdenum on properties of zirconium components of nuclear reactor core

The paper presents results of studies of the effect of Mo on properties of Zr and its alloys. In a structure of binary Zr-Mo alloys and Zr-Nb-Mo alloys a strong and corrosion-resistant phase ZrMo2 additionally appears. In Zr-Sn-Nb-Fe alloys Mo alloying leads to an increase in amount of second phase particles, reduces their dimensions and dissolves in Laves phase particles Zr(Nb, Fe)2. These structural changes led to increased strength of all studied Zr alloys. Alloying by Mo also improves corrosion properties of experimental Zr alloys containing Nb and Sn, the best being alloys with a content of 0.1–0.15% mass of Mo. Adding of 0.1% Mo in standard claddings from the E635 alloy (Zr-Sn-Nb-Fe) notably reduced their hydrogenation and propensity for nodular corrosion. The testing results and their comparison with a literature data made possible to determine the prospects and directions for further work on using of Mo for alloying Zr alloys.

High-temperature oxidation kinetics of sponge-based E110 cladding alloy

Two-sided oxidation experiments were recently conducted at 900°C–1200 °C in flowing steam with samples of sponge-based Zr-1Nb alloy E110. Although the old electrolytic E110 tubing exhibited a high degree of susceptibility to nodular corrosion and experienced breakaway oxidation rates in a relatively short time, the new sponge-based E110 demonstrated steam oxidation behavior comparable to Zircaloy-4. Sample weight gain and oxide layer thickness measurements were performed on oxidized E110 specimens and compared to oxygen pickup and oxide layer thickness calculations using the Cathcart-Pawel correlation. Our study shows that the sponge-based E110 follows the parabolic law at temperatures above 1015 °C. At or below 1015 °C, the oxidation rate was very low when compared to Zircaloy-4 and can be represented by a cubic expression. No breakaway oxidation was observed at 1000 °C for oxidation times up to 10,000 s. Arrhenius expressions are given to describe the parabolic rate constants at temperatures above 1015 °C and cubic rate constants are provided for temperatures below 1015 °C. The weight gains calculated by our equations are in excellent agreement with the measured sample weight gains at all test temperatures. In addition to the as-fabricated E110 cladding sample, prehydrided E110 cladding with hydrogen concentrations in the 100–150 wppm range was also investigated. The effect of hydrogen content on sponge-based E110 oxidation kinetics was minimal. No significant difference was found between as-fabricated and hydrided samples with regard to oxygen pickup and oxide layer thickness for hydrogen contents below 150 wppm.