Towards a better understanding of the oxide film growth mechanism in E110 zirconium alloy under high-temperature oxidation in steam

2.1 Materials

In this study, samples of cladding tubes with a length of 30 mm, a diameter of 9.13 mm, and a wall thickness of 0.7 mm made of electrolytic-based E110 alloy (Zr-1Nb) were used. The cladding tubes were manufactured by a typical technology. The alloy microstructure represented an α-Zr recrystallized matrix containing β-Nb particles. The total content of major impurities in tubes (C, Si, Ni, P, Cl, N, F, Al) was about 140 ppm, which is approximately 2 times higher than in tubes manufactured on the basis of zirconium sponge.

2.2 High-temperature oxidation

High-temperature oxidation was performed in steam at the temperature of Т=1100°C in a special facility that consisted of a heating furnace and steaming unit (Figure 1). Heating was performed in a tube furnace. The sample was placed inside a tubular sealed channel located vertically and blown by a steam flow from bottom to top. An electric heater was located around the channel. The upper end of the channel was open and the steam released into the atmosphere. Pure steam was supplied from below. Only the pure steam was used in the experiment (demineralized water was used to produce steam); no carrier gas was used. The steam channel was hermetically sealed to not allow air inflow to the system, and hermeticity was regularly checked. Steam flow rate was 20 g/h and steam partial pressure was 1 atm.

Figure 1:

Scheme of the oxidation device.

The testing facility ensured a temperature field with a sample heating zone uniform in height and radius and controlled by a thermocouple. Thermocouple was placed along the tube channel wall at 5 mm above the upper edge of sample and 4 mm to the side from specimen. The accuracy of temperature measurement was 2.5°C at 1100°C, the same accuracy as for thermocouple calibration.

The sample heating rate was about 50°C/s (sample was pushed into the furnace already at a temperature). The samples were oxidized from both sides. The sample holding time at 1100°C was the time providing an ECR of 8%, 10%, 14%, and 18% (see Table 1). Five samples for each ECR were used for checking repeatability.

ECR was determined by the weight gain of the samples, which was continuously measured in the process of oxidation. The samples were cooled in the steam stream at a rate of about 40°C/s, gradually moving a sample from the heating zone to room temperature air.

The experimental facility allows achieving any ECR degree without limitations with exposure time from 20 s to 90,000 s and steaming flow rate of 10–20 g/h. More details of the high-temperature oxidation procedure and facility are given in Nikulin et al. (2011).

2.3 Analysis of oxide film morphology using scanning electron microscopy

Scanning electron microscopy (SEM) was used for analysis of the oxide film morphology on both of the sample surfaces and cross-sections, as well as from fractures obtained by compression of ring samples. For that, Jeol JSM-7800 F scanning electronic microscope was used. The surface of the E110 alloy samples oxidized to different ECRs has been studied in detail using SEM. Also, changes in the oxide film morphology in cross-sections of samples oxidized to different ECR degrees have been studied by SEM. All these qualitative observations have been used to identify specific details on the oxidation mechanism.

2.4 Analysis of oxide film microstructure using optical microscopy

Oxidized tube samples were cut to rings using an electrolytic erosion cutting machine and pressed in the mixture of epoxy resin in Simplimet 1000 mounting press at 150°C and 286 atm. After pressing, the samples were charged into the revolving chamber of a Buehler Vectоr Рhоeniх Betа grinding/polishing machine to make cross-sections by sequential polishing, starting with medium grinding paper and finishing with velvet having diamond suspension and an abrasive particle size of 0.5 micron. After polishing, the samples were washed with alcohol and water and dried in hot air. In order to eliminate the remaining scratches and identify microstructure, a “brightening” etching with H₂O+HNO₃+H₂SO₄+HF composition was applied.

The cross-sections were analyzed using Cаrl Zeiss “Aхiоvert 40 MAT” optical microscope at a magnification of ×50–1000. Pictures of cross-sections were taken by a digital camera connected to the microscope. The thickness of the oxide film and the depth of oxygen-stabilized α-layer in the photos (min. 100 measurements for each parameter) were measured. The scheme of the α-layer thickness (Hα) measurement is shown in Figure 2.

Figure 2:

Scheme of the α-layer (Hα) thickness measurements.

The length of individual α-layer platelets measured normal to the oxide film was considered as the α-layer thickness (mean value was calculated for 100 measurements with a standard error estimation). Moreover, the structure features (morphology) of the oxide film were studied using an optical microscope.

2.5 Analysis of oxide film microstructure using electron microscopy

Oxide film microstructure was studied both by SEM and transmission electron microscopy (TEM). The fracture surfaces of compressed ring samples have been used for the study of oxide films by SEM. The column grain sizes of the oxide film in fracture surfaces were measured using SEM. Around 200 column grains were measured in each sample using pictures obtained by SEM with a magnification of ×800 in Image Expert Pro software. The grain thickness was measured in two different places along the column height (at the surface of the oxide film and at oxide film/metal interface).

For the study of oxide film microstructure by TEM, special microsamples were prepared by focused ion beam (FIB) method. For that purpose, Strata FIB201 scanning ion microscope with gallium as an ion source was used. Microsamples of about 5×10 micron were cut using FIB through etching of two parallel “grooves” in the oxide film by an ion gun, thinning the remaining material (lamella) between the grooves, pulling out of lamella, and its transfer to a sample microholder for TEM. The microsamples were cut in the longitudinal direction of the cladding tube (in accordance with Figure 3) in two areas (near the oxide film surface – oxide film of white color, and at the oxide film/metal interface).

Figure 3:

Sketch of TEM sample cut up from the oxide film by the ion gun.

To cut samples at the interface, the film was prepolished to the required depth. To determine the grain size, the quantitative analysis of the oxide film structure was done by TEM pictures using Image Expert Pro software and chord method (100 measurements for each sample).

2.6 Measurement of oxide film microhardness

The microhardness of oxide films of the samples after oxidation was measured using Micrоmet 5100 microhardness tester having a Vickers-type tip. The measurement parameters were as follows: load, 100 gs; load application time, 10 s; magnification, ×500. Microhardness was measured in five different areas of the oxide film, depending on the degree of oxidation as follows: in the oxide film when it still has a single-layer structure (area 1), in the upper (area 2) and lower (area 3) parts of the nodule, in the upper (area 4), and the lower (area 5) layer of a two-layer oxide film. Five measurements were carried out in each area per specimen, followed by calculation of the mean value and standard error. The measuring areas are schematically presented in the section on the study results.

3.1 Oxidation kinetics

The typical oxidation kinetics of the E110 alloy for various ECR degrees are shown in Figure 4. All the curves presented are characterized by a parabolic law of oxidation without sharp deviations in the oxidation kinetics. The most representative curves for analysis are those for samples with a maximum oxidation degree of 14% and 18% ECR (Figure 4D). On these oxidation curves, the inflection points with ~1000 s of oxidation time have been observed (which corresponds to ECR between 10% and 14%). This inflection point is not strongly pronounced and difficult to see on the curve with the naked eye (this point was defined by a least square method in a sample oxidized to 18% ECR).

Figure 4:

Oxidation kinetics of E110 alloy samples during high-temperature oxidation at 1100°C to different ECR degrees: (A) 8% ECR, (B) 10% ECR, (C) 14% ECR, (D) 18% ECR.

Despite the fact that at this point there is no drastic change in the oxidation curve, some acceleration of corrosion takes place (after the inflection point, the oxidation curve has a linear character for some time), which indicates the sensitivity of the studied samples from E110 alloy to breakaway oxidation. The effect of “inflection” appears to happen in accordance with mechanisms similar to nodular corrosion (see section below).

3.2 Surface morphology of oxidized samples

The appearance of samples after high-temperature oxidation to different ECRs is shown in Figure 5. It can be seen that when the ECR is 8%, the sample surface is black, smooth, and shiny without any grossly visible white spots. When the ECR is 10%, single white spots can be seen near the sample edges. When the ECR is 14%, all the surface of the samples is covered with single widely spaced white spots (taking up about 20–30% of the surface). At last, when the ECR is 18%, most parts of the surface are covered with white spots merging into wide areas (taking up about 70–80% of the surface area). The inner surface of tube samples looked the same in general; approximately the same relation of white and black areas on the surface was observed.

Figure 5:

View of E110 alloy samples after high-temperature oxidation at 1100°C and different ECR degrees: (A) 8% ECR, (B) 10% ECR, (C) 14% ECR, (D) 18% ECR.

The ECR degree at which the first white spots can appear on the surface of the samples can depend on several factors, including the testing conditions and material characteristics. Thus, in the study (Yegorova et al., 2005)), the first white spots were already found under high-temperature oxidation of E110 alloy at 6.5% ECR.

If there is a white oxide film in the sample with a higher ECR (14–18%), it proves that under high-temperature oxidation, the corrosion rate is increased (“breakaway” phenomenon). The acceleration of high-temperature oxidation of E110 alloy in steam has been described in earlier studies (Yan et al., 2009; Nikulin et al., 2011) and associated with white spots, which would be nodular defects and combined with high absorption of hydrogen. In Grosse (2010) and Steinbrück et al. (2010), (2011), a “breakaway” phenomenon for E110 alloy cladding was observed in oxidation temperatures between 800 and 1040°C, including 900°C (a white spalling oxide was observed on the surface of the sample), while this effect was practically not observed at higher temperatures. Indeed, the temperature and boundary conditions of the tests are important factors on which the occurrence of the breakaway effect depends, but also the sensitivity of the cladding material itself, which may depend in particular on the content of impurities, which, in turn, is determined by the production technology of the claddings, is also an important factor.

The detailed analysis of the surface of a sample oxidized to 18% ECR (Figure 5D) demonstrates that the white spots do not build up above the formed black oxide film but are part of this black film (the black film having turned white in some areas). Figure 5D also shows that in the areas of white spots, the oxide film often peels off the surface and cracks. In these areas, a black oxide film can be sometimes seen under the white film. Thus, it can be suggested that in the areas of white spots, the oxide film is double.

The nodule morphology was studied in more detail using SEM. The typical view of nodules on the surface of oxidized samples is shown in Figure 6. It can be seen that the nodules on the surface look like round-shaped bulges (Figure 6A). The nodules can sometimes have microdefects on their surface (cracks, central pit, Figure 6B). These defects can be caused by mechanical damages resulting from the nodule growth.

Figure 6:

Appearance of nodules on the surface of oxidized samples (18% ECR): (A) group of several single nodules, (B) one nodule.

The formation of such microdefects in nodules can lead to the acceleration of corrosion rate as the defects are additional channels for diffusion of oxygen (and hydrogen) into metal. The effect of such kind was also noted in the study of Yan et al. (2009).

3.3 Microstructure of oxide films

3.3.1 Oxide film microstructure analyzed by optical microscopy

The analysis of metallographic pictures of cladding samples after high-temperature oxidation showed that the thickness and character of the oxide film vary in samples after applying different ECR degrees (Figure 7). The oxide film is quite uniform in the samples with a low ECR degree (8%), and on an average, its thickness does not exceed 30 microns. If the ECR degree rises to 10%, local thickened parts appear in the film, the oxide film/metal interface (boundary between oxide and α-layer; see Figure 2) becomes wavy, and the film thickness increases, on average, to ~40 microns. The samples with a medium ECR degree (14%) obtain single nodules that are likely to be formed from previous thickened areas of the oxide film. Such nodules form the second “inner” layer of the oxide film. In the areas of the nodules, the thickness of the oxide film reaches ~110 microns (the first and second oxide film layers in total). Mainly, the samples with a high ECR degree (18%) have a double oxide film with a total thickness of ~130 microns in the cross-sections. The oxide film/metal interface becomes smoother (no waviness), which was also noted in the study of Yan et al. (2009).

Figure 7:

View of oxide film on E110 alloy in the cross-section after application of different ECR degrees at Т=1100°C: (A) 8% ECR, (B) 10% ECR, (C) 14% ECR, (D) 18% ECR.

Figure 8 shows the relation of average thickness of E110 alloy oxide film and oxygen-enriched α-layer on the oxidation time. It can be seen that when the oxidation time increases, the average thickness of α-layer and oxide layer increases too. When the oxidation time reaches 1460 s (corresponds to 14% ECR), the thickness of the oxide film increases dramatically due to the formation of the second (“inner”) oxide layer. At the stage where the oxidation time is higher than 1460 s (over 14% ECR), the oxide film mainly grows due to an increase in thickness of the second (“inner”) oxide layer. At the same time, the growth rate of the oxygen-enriched α-layer increases, which indicates acceleration of oxygen diffusion (and probably hydrogen) into metal during the formation of nodules and their active growth. It should be noted that a sharp increase in the thickness of the oxide film after an oxidation time of 1460 s is apparently due to the fact that at this moment of time, the number and size of nodules significantly increased during oxidation. When metallographic measurements of the oxide film thickness have been performed on the circumferential surface of the samples oxidized for 1460 s and more, nodules were found more often, which contributed to an increase in the average oxide film thickness.

Figure 8:

Relation between oxidation time and oxide film/α-layer thickness for E110 alloy (dots correspond to different ECR levels).

It is well known that an increase in the oxidation time at Т=1100°C causes an increase in the thickness of oxide film and α-layer. In this case, the most noticeable increase in the thickness of the oxide film is observed starting from 1460 s of oxidation (corresponding to 14% ECR), when a sufficient quantity of nodules are formed on the surface of the samples forming the second “inner” layer of the oxide film.

3.3.2 Oxide film microstructure analyzed by electron microscopy

To analyze the structure of oxide films in fracture surfaces, oxidized samples were failed by compression. The analysis of the film fracture surfaces showed that after high-temperature oxidation, the structure of the oxide film was columnar throughout the whole thickness, which was typical for oxide films on zirconium alloys after oxidation under LOCA conditions. The view of oxide film in transverse fraction of a tubular sample is shown in Figure 9. In principle, the obtained pictures confirm the data obtained from metallography.

Figure 9:

Fracture of a sample in the area of oxide film and α-layer, SEM (18% ECR): (A) single oxide film, (B) double oxide film (nodule).

The picture also demonstrates that the thickness of oxide film columns in the surface layer is lower than at the oxide film/metal interface, which has also been noted earlier in a few studies. The results of quantitative measurements of grain sizes in the oxide film for samples with various ECRs are given in Table 2. It can be seen that for all ECRs, the thickness of a column grain is 3 or more times greater at the oxide film/metal interface than near the surface. The greater thickness of the oxide grain at the oxide film/metal interface seems to be caused by the fact that the surface layers of the oxide film have been formed during a shorter period of oxidation (at the initial stages of oxidation), and the layers near the oxide film/metal interface, during a longer period (with a totally longer period of oxidation). The data in Table 2 show that near the surface, the thickness of oxide grains does not significantly change when ECR increases and is about 0.4 micron, while near the oxide film/metal interface, the grain thickness increases a little (from ~1 to 1.8 microns while ECR increases from 8% to 18%).

Table 2:

Results of measuring column structure sizes as per SEM and TEM data.

Degree of oxidation (ECR%)	Thickness of columns in oxide film (micron)
	SEM		TEM
	Oxide film/metal interface	Surface	Oxide film/metal interface	Surface
8	1.03±0.1	0.39±0.1	0.5±0.02	0.6±0.02
10	1.51±0.1	0.43±0.1	0.8±0.04	0.6±0.02
14	1.83±0.1	0.46±0.1	1.2±0.05	0.5±0.01
18	1.81±0.1	0.44±0.1	2.5±0.05	0.6±0.02

Table 2 also shows data on measuring column structure sizes of oxide film as per data obtained from TEM pictures. Although in principle, the quantitative data vary a little from SEM data, the obtained results prove the common tendency discovered during measurements of oxide grain size using the SEM method; i.e. the thickness of grains is significantly lower in the surface layers of the film than near the oxide film/metal interface, while near the oxide film/metal interface, the grain thickness increases as ECR increases and does not vary significantly near the surface.

Generally, TEM pictures of the oxide film microstructure (Figures 10–12) show that the microstructure is characterized not only by main elongated (column) oxide grains but also by substructural elements inside those grains. As a rule, elongated elements (plates) are such substructural elements. Normally, their crosswise size is not greater than 100 nm. Often, these structural elements are oriented in different ways in adjacent oxide column grains; for example, in one grain they lay perpendicular to the surface, and in another grain they are at a certain angle to the surface. This fact is very evident when comparing pictures of one and the same area of the oxide film, taken in the light-field and dark-field mode (the dark and light areas in Figure 10 correspond to various oxide grains). However, sometimes, substructural elements of different orientations can be found within one column grain.

Figure 10:

Microstructure of oxide film on E110 alloy sample with 18% ECR near the surface, TEM: (A) light field, (B) dark field.

Figure 11:

Microstructure of oxide film on E110 alloy sample with 18% ECR near the surface, TEM (white oxide film): (A) and (B) different magnifications.

Figure 12:

Microstructure of oxide film on E110 alloy sample with 18% ECR near the oxide film/metal interface, TEM (black oxide film): (A) and (B) different magnifications.

Another typical element that is most frequently met mainly in the oxide film of surface layers (that is white) is transverse cracks (see Figure 11A). Besides, thin light strips can be often seen in all analyzed films along the main grains interface (see Figure 12A). They do not seem to be cracks but are paths of possible diffusion of oxygen and hydrogen into metal during oxidation.

In some cases, small black dots with a size of around 10–20 nm can be found inside the substructural elements of the oxide film. They are uniformly distributed over the whole surface. The view of those dots reminds of beta niobium particles in metal structures of Zr-Nb system alloys. However, the nature of such particles shall be additionally analyzed.

In general, the substructures of oxide film in areas near the surface (white film) and in areas near the oxide film/metal interface (black oxide film) do not differ significantly, except that cracks are more frequently found in a white oxide film.

Therefore, it is shown that when the degree of oxidation increases, the crosswise size of column grains in the oxide film near the oxide film/metal interface rises, while this size does not vary essentially near the surface itself. The microstructure of oxide films is presented by column grains inside of which there is a substructure of mainly elongated elements oriented differently in various grains. The substructure of oxide films in white and black films is not fundamentally different except for the fact that transverse cracks are most often found in white oxide film. An increase in the diffusion of oxygen and hydrogen with the appearance of white spots on the surface of samples, apparently, occurs not due to structural changes in the film but due to the formation of defects in this film.

3.3.3 Microhardness of oxide films

Microhardness was measured in five different areas of cross-sections of samples with various degrees of oxidation. The areas are shown in Figure 13. Area no. 1 (Figure 13A) is in the middle of the single oxide film thickness. The samples with the oxidation degree of 8–14% ECR were measured in this area, as when the oxidation degree was 18% ECR, a double oxide film already appeared on the samples surface (Figure 13B). Area no. 2 (Figure 13A) is the upper (outer) layer of the oxide film in a nodule, and area no. 3 (Figure 13A) is the lower (inner) layer of the oxide film in this nodule. Only the samples with the oxidation degree of 14% ECR were measured in these areas, when there were quite a great number of single nodules. Area no. 4 and no. 5 are the upper (outer) and lower (inner) layer of the oxide film, respectively, when it is nearly or completely formed as a double oxide film on the whole surface. Measurements in these areas were possible only for the sample with the oxidation degree of 14% ECR (when quite extended areas of the double oxide film could be observed) and 18% ECR (when the whole oxide film was completely double-layer).

Figure 13:

Microhardness measurement scheme in different areas of oxide film.

(A) Nodule area, (B) double oxide area.

The results of oxide film microhardness measurement in the specified points are given in Table 3. It can be seen that as the degree of oxidation increases from 8% to 14% ECR, the microhardness of single-layer oxide film decreases, while the most valuable decrease in microhardness could be observed as the degree of oxidation grew from 10% to 14% ECR. This seems to be due to the increase in oxide film defectiveness, which significantly rises at the oxidation degree of 14% ECR (Area 1), as the nodules appear on the surface more actively. The sample with the oxidation degree of 14% ECR showed all five possible variants of oxide film state: areas with just a single-layer oxide film (Area 1), areas where nodules had been formed (Areas 2, 3), and areas with an extended double film (Areas 4, 5), which represented several merged nodules.

Table 3:

Microhardness of oxide film in different areas of E110 alloy samples, HV, mean values, and standard errors.

State	Area 1	Area 2	Area 3	Area 4	Area 5
E110, 8% ECR	706±44	–	–	–	–
E110, 10% ECR	630±63	–	–	–	–
E110, 14% ECR	334±52	348±77	701±55	452±63	728±16
E110, 18% ECR	–	–	–	570±55	739±21

In samples with 14% ECR, the microhardness of the upper nodule part (Area 2) virtually did not differ from the microhardness of the oxide in the typical area, where the oxide film is single-layered (Area 1). At the same time, the microhardness of the lower nodule part is essentially greater, which shows that oxide film of a nodule part growing inside the metal (inner part, Area 3) is significantly denser than the upper part and contains fewer defects. The results of microhardness measurement in the areas of a sample where a double oxide film has been formed are similar – in the upper layer (Area 4), microhardness is less than in the lower layer (Area 5). A similar pattern is observed in the sample of alloy oxidized to 18% ECR.

Since the microstructure of the lower and upper parts of oxide does not differ much, the main factor that explains the difference in the microhardness is the presence of defects in the oxide that have been observed by TEM and SEM. However, the other possible factor could be a stoichiometric composition of the oxide or lattice differences, but it has not been studied in the current research and requires very local techniques to adequately measure it.

Even though the lower nodule part of oxide is relatively dense, it is still has some microdefects, including grain boundaries and vacancies for oxygen and hydrogen diffusion. There are no big defects like cracks found in the upper part of oxide, but there are still microdefects, which allows diffusion as well as it happens during the initial stages of oxidation (when the oxide layer is very thin and dense, it is still transparent for diffusion anyway).

Thus, the formation of nodules in the oxide film/metal interface is characterized by the fact that the nodule oxide has the highest microhardness and is denser (which, apparently, is associated with less defectiveness), while the upper part of the oxide above the nodule becomes less dense and looser. Defects in the upper part of the oxide (above the nodule) can cause acceleration of diffusion of oxygen (and hydrogen) in these areas and acceleration of oxidation in total.

4.1 General considerations on the oxide film growth

It is known that the “breakaway” phenomenon in the oxidation kinetics in conditions of high-temperature oxidation in steam can appear by implementation of two main oxidation mechanisms – uniform oxidation (model described by S. Leistikow and G. Schanz) and nodular oxidation (Yegorova et al., 2005)). From this point of view, the behavior of electrolytic-based E110 alloy produced by typical technology is not governed by the first model (uniform oxidation; Schanz & Leistikow, 1981) and follows the mechanism of nodular oxidation (Yan et al., 2009)).

As it was demonstrated by many studies and confirmed by this study, the external appearance of E110 alloy samples after high-temperature oxidation in steam resembles typical nodular corrosion, which is observed in zirconium alloys in BWR reactors and is also characterized by white oxide spots on the surface of a black oxide film (Allen et al., 2012). Zr-Nb alloys are also subjected to this type of corrosion, especially in boiling conditions and high oxygen content in the coolant (Shebaldov et al., 2000).

It should be mentioned that in LOCA studies, despite the fact the corrosion mechanism is usually mentioned as nodular, practically confirmed data on the fact that the mechanism was really the same as the mechanism of typical nodular corrosion were almost never proved.

However, apart from several features that indicate that this is the same mechanism (the view of white spots, cracks in the oxide spot, acceleration of hydrogen absorption), there are also features that distinguish both mechanisms from each other, e.g.

1. in typical nodular corrosion, a nodule is presented as a single integral lenticular defect (from cross-section view), while in LOCA conditions, there is a separate interface that separates the upper and lower oxide layers;

2. typical nodular corrosion occurs when thinner films are formed as compared with nodular corrosion in LOCA conditions;

3. in typical nodular corrosion, the thickness of oxide in the nodule area is usually tens of times greater than the thickness of oxide in a normal area, while the thickness of oxide in the nodule in LOCA conditions is only around 2 times greater than the thickness of black oxide;

4. in typical nodular corrosion, the nodule usually grows equally both directions (up and down), while in LOCA conditions, the nodule grows predominantly down (inside the metal), at least at the initial step;

5. in typical nodular corrosion, cracks can also be observed in a nodule, but they are commonly never associated with the growth of uniform oxide layer as this oxide is usually very thin. In LOCA conditions, the appearance of cracks in the nodule can be caused by mechanical stresses resulting from the growth of the uniform oxide film (which is thicker than in typical nodular corrosion) and the growth of nodule itself (Rudling & Wikmark, 1999). At the same time, for typical nodular corrosion, the mechanisms at the microlevel are not completely clear yet; various hypotheses are discussed (Rudling & Wikmark, 1999). In general, even though from the point of view of visual observations, both mechanisms are really similar, it must be stated that the degree of similarity of the two mechanisms should be determined through a more detailed separate study.

In general, the sequence of oxidation of zirconium alloys in LOCA conditions with the appearance of oxide nodules on the surface was shown in some studies (Chung, 2005; Yan et al., 2009). It was noted that in E110 alloy, nodules appear on its surface as separate spots of white oxide, which grow in all directions and merge with each other, which ultimately leads to delamination of the film on the surface of samples (Chung, 2005; Yan et al., 2009). However, these ideas were formulated on the basis of the observation of nodules from the surface of the samples than from the inside (from the cross section); in particular, it was not unambiguously clear exactly where in the oxide film and exactly how nodules begin to form. Only in some papers was this question discussed a little more in detail (Schanz & Leistikow, 1981). In addition, issues regarding the effect of impurities on the initiation of nodular oxidation and the aspects of possible formation of oxide having different crystal lattices (monoclinic and tetragonal) during oxidation have still not been studied in detail and in some cases remain controversial.

This work emphasizes that nodules form at the interface between the previously formed oxide film and the alpha-layer. It all starts with a sufficiently rapid growth of the oxide film inside the metal (into the alpha-layer) in some areas more than in other areas, where it continues growing at the same rate as before. Areas with thickened oxide film appear. It is the starting point of a nodule formation. These thickened areas of the oxide film facing the α-layer will then turn into nodules.

The most pressing question is why this happens in alloys based on electrolytic zirconium. Even though there is no final answer to this question yet, one of the most probable reasons is the high content of certain impurities, in particular, fluorine (Markelov et al., 2019), but at the microlevel, their mechanism of action is still not clear. The negative effect of fluorine on the corrosion of zirconium alloys was noted early in the 1960s in Berry (1963) and Lunde (1975), where it was shown that fluorine causes the appearance of white spalling oxide films. In Chung (2005), the negative effect of fluorine was described not only on the zirconium corrosion but also regarding some other materials, in particular steels, and some hypotheses were proposed on the mechanism of its influence. Regarding fluorine, which is initially present on the cladding surface (for example, due to application of fluorine-containing acid), it has been proposed that fluorine can quickly diffuse through the oxide film to the oxide film/metal interface.

Fluorine, which is initially inside the zirconium as a result of metallurgical production processes, also diffuses to the oxide film/metal interface and eventually transfers to the oxide. It is assumed that fluorine atoms in the oxide film occupy oxygen vacancies and promote the formation of a stoichiometric oxide and reduce the stability of the oxide tetragonal phase. These hypotheses, however, have not yet been experimentally confirmed on zirconium, although they have been partially confirmed on some other metals.

The possibility of experimental confirmation of such hypotheses appeared in recent years with the advent of new methods of sample preparation and research. Thus, in Elaish et al. (2017), using ultramicrotomy and analytical TEM energy-dispersive X-ray spectroscopy analysis, it was possible to observe the formation of a fluorine layer 5–10 nm thick at the oxide film/metal interface. The study was conducted on aluminum, which was subjected to anodizing in a solution containing sodium fluoride. The thickness of the formed oxide film was 110 nm.

Fluorine penetrated into the oxide film from the solution and diffused to the oxide film/metal interface. The diffusion rate of fluorine to the oxide film/metal interface was 2 times higher than that of oxygen. An increase in the fluorine content in the solution led to the appearance of pores in the oxide film. Earlier, a similar effect was also found on tantalum (Shimizu et al., 1997). Despite the fact that such data for zirconium are not yet available now (they have yet to be obtained), hypotheses related to the accumulation of fluorine at the oxide film/metal interface seem very likely.

But one way or another, in certain areas of the zirconium oxide film that has already been formed by the moment of nodule appearance, access of oxygen (and probably hydrogen) to the film/metal interface is enhanced (the possible reason is increased defectiveness of the oxide film). Previously, it has been observed that cracks in the preformed oxide (prior to high-temperature oxidation) can lead to the appearance of thickened areas in the oxide film/metal interface (Nagase et al., 2000).

At the initial stage, these thickened areas continue growing predominantly down (toward the alpha-layer) and to the sides, and only when they reach a certain size and it seems that the stress state of the oxide film in such areas change, the nodules starts growing up (toward the surface) and to “push out” the oxide film, which has been formed above it earlier. As a result, “bulges” appear on the surface. They are the white spots that are observed against the background of the black oxide film.

An important aspect is the formation of an interface in the nodule at a certain stage of its growth. In fact, this interface seems to be a crack and is the result of stresses in the growing oxide film. In the future, this crack will be the interface between layers of the double oxide film that is formed at higher degrees of oxidation. The appearance of such lamination in the oxide film can accelerate corrosion processes by enhancing the access of oxygen and hydrogen to the metal even more. Besides, when the nodule is formed in the oxide film/metal interface, it starts to “push out” the layer of the above oxide film upward. It is obvious that stresses appear in the upper oxide layer and additional cracks occur that enhance the access of oxygen and hydrogen to the metal. In this regard, it is important to note that the appearance of nodules in local points of the surface is worse than the even growth of the whole oxide film (according to the model described by Schanz & Leistikow, 1981), as the stresses and cracking in the local areas will be much greater than during the even growth of the oxide film. Thereinafter, the mechanism is briefly described as a sequence of steps from the mechanistic point of view.

4.2 Model of oxide film growth

The mechanism of oxide film growth can be described as a sequential process that includes several steps (Figure 14).

Figure 14:

Growth of oxide film under high-temperature oxidation in steam: (A–C) step 1, (D–F) step 2, (G–I) step 3, (J–L) step 4, (M–O) step 5, (P–R) step 6, (S–U) step 7.

Step 1 is defined by the appearance of a smooth dense oxide film, which is well adherent to the metal. At this step, the growth of the oxide film proceeds due to the diffusion of oxygen through the initial thin oxide film to the oxide film/metal interface and interaction with the metal to form a new part (thin sub-layer) of oxide. Thus, the film growth occurs due to continuous oxygen flow.

Step 2 is characterized by the appearance of small local thickened parts of the oxide film in the oxide film/metal interface. This effect is known in the literature as the “waviness” effect of oxide film.

In step 3, the thickened areas of the film turn into nodules, which are oxide lenticular formations. The appearance of nodules is characterized by the appearance of an interface (seems to be a crack), which divides nodules into the upper and lower parts and can be viewed using optical microscopy. Nodules appear under the oxide film, which has already been formed and grown to a certain thickness. The growth of nodules is accompanied with “bulging” of the upper layer of the oxide film in local areas (characteristic white “bulges” appear on the surface – white spots).

In step 4, the quantity of nodules increases, the existing nodules grow further, and the distance between them decreases. In fact, the third and fourth steps are one and the same step as they describe one and the same process. Just in the fourth step, this process flows more actively. The white spots on the surface of the preformed black oxide film become very clearly seen.

In step 5, the nodules located close to each other merge, which leads to the appearance of larger, expanded over the surface nodules, or rather areas that present a cluster of several nodules.

In step 6, the preformed merged nodules finally merge together, making an integral oxide layer around the whole annular surface of a tubular sample, which becomes the second inner “oxide” layer. In some areas, the outer white oxide film starts delaminating.

In step 7, the thickness of the double oxide layer grows a little and the whole surface of the sample is covered with a white oxide film; the process of delamination of the upper white oxide layer is promoted. For the process of high-temperature oxidation of E110 alloy at Т=1100°C, the first and second steps approximately correspond to 8–10% ECR; the third, fourth, and the fifth steps approximately correspond to 14% ECR; and the sixth and seventh steps approximately correspond to 18% ECR.

Therefore, the mechanism of high-temperature oxidation of electrolytic-based zirconium alloy E110 can be described as a sequential process from the mechanistic point of view. The process includes seven key steps and finally leads to the appearance of a white loose oxide film on the surface of samples.