Introduction

Austenitic stainless steels of different grades are widely employed as structural materials in several industrial applications where high temperatures are encountered. The nitrogen substituted low carbon modification is receiving special attention on account of its resistance to pitting and intergranular corrosion coupled with improved creep strength. In some of the fast reactor applications such as sodium immersion heaters this material encounters very high temperatures (873-1073 K) so that oxidation becomes an important mode of material degradation and the consequent loss in mechanical strength. Such a situation of extremely high temperature may also arise in a reactor due to loss of coolant accident. Increasing loss in the cross section due to oxidation becomes a major concern under these conditions.

In order to protect stainless steel from oxidation loss, different methods of surface modifications are reported in the literature (Ref 1-5). In these cases only the modified surface interacts with the high temperature environment and the surface as such is capable of offering endurance against high rate of oxide growth and consequent spallation.

Among the oxide scales, those of aluminum, chromium and silicon are the ones that offer maximum protection against progress of oxidation. Therefore diffusion coatings are generally employed to increase the surface concentration of these elements. The deposit can be applied by either physical vapor deposition or chemical vapor deposition methods (Ref 6-9). Even though alumina scale has been reported to offer best protection, it suffers from poor ductility. In order to render the scale more ductile, modifying elements such as chromium, palladium and platinum are added. This addition is achieved by pre-treatment or co-deposition (Ref 10).

Ion-implantation is an effective method to introduce desired foreign elements into the metallic matrix without involving the rigors of alloy making (Ref 11). In this method, accelerated ions are allowed to impinge on the surface. The depth of penetration is in the range of 0.01-1.0 μm. The radiation damage introduced on the surface is rapidly annealed by heating to high temperature.

The application of laser to melt the surface of metal is an important method to bring about tailor made modification to the surface. Laser is capable of heating the surface without subjecting the matrix of the alloy to high temperature.

In the present investigation, the oxidation behavior of AISI 316 LN was studied at 1123 K for 3.6 Ms. The surface of the specimens was modified by three methods;

  1. (i)

    Aluminizing followed by high temperature pre-treatment.

  2. (ii)

    aluminizing followed by laser treatment

  3. (iii)

    ion-nitriding

Aluminizing was carried out by physical vapor deposition method. The paper discusses the effect of these surface treatments by comparing with the oxidation behavior of as-received material. Nitridation was also attempted as surface modification mainly with a view to impart better surface hardness. The effect of nitridation on the high temperature oxidation behavior has been analyzed.

Post-oxidation examinations were carried out using SEM and EDS to identify the nature of the phases formed and to examine the morphology of the surface.

Experimental

Preparation of the Specimen

The composition of the material selected for the investigation is given in Table 1.

Table 1 Composition of 316 LN SS
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The material was cut into sizes of 12 × 12 × 3 mm3. These were polished using successive grades of silicon carbide-coated paper and finally polished to 1 μm finish using diamond paste. The polished specimens were cleaned ultrasonically followed by the use of acetone.

Aluminizing

The aluminum coating was carried out using Resistance Heating Evaporation Technique (RHET) coupled with the application of vacuum of 10−5 Torr. Aluminum foils of purity 99.99% were used as the source. These foils were attached to a tungsten filament which promoted the evaporation of aluminum and its deposition on the specimen kept inside the vacuum chamber.

Pre-Treatment of Aluminized Specimen

High Temperature Diffusion-Annealing

High temperature pre-treatment was imparted to aluminized specimen to promote generation of adherent scale. Initially the specimens were heated at 873 K for 10.8 ks to promote the complete oxidation of the deposited aluminum. Subsequently, the specimens were heated at 1173 K for 90 ks. This treatment is to promote the complete conversion of γ-alumina generated initially to the θ-alumina. The θ-alumina undergoes ultimate conversion to α-alumina on prolonged exposure to high temperature. The α-alumina is a stable and protective scale.

Laser Annealing

The system employed was a multi-beam continuous wave carbon dioxide laser with a maximum power of 500 W. A power of 80 W and speed of 1500 μ/s were chosen for the modification of the alumina-coated surface. It was carried out by directing the laser beam on the specimen and simultaneously scanning the beam from one end to the other in the X-direction by moving the specimen. The irradiation was repeated by moving the specimen in the Y-direction also.

Ion-Nitriding

Ion-nitriding is a method of surface modification with a view to harden the surface. In the present experiment the nitrogen irradiation of the stainless steel surface was carried out by using an accelerator. The samples were mounted on a copper block kept inside the vacuum chamber of the accelerator. These samples were irradiated with 100 keV ions of N2 + at room temperature. The beam current was kept at 0.009-0.011 μA/mm2 to minimize heating effect. The fluence used was 5 × 1014 ions/mm2. The duration of ion-irradiation was 21.6 ks. A surface nitrogen concentration of 0.16% was achieved by the present nitriding process.

The Oxidation Experiment

The specimens were kept in a furnace at 1123 K. It was weighed periodically. The total duration of oxidation was 3.6 Ms. At the end, the specimens were examined by various post-oxidation examinations such as Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and x-ray Diffraction (XRD) in the glancing incidence mode.

Results and Discussion

Kinetics of Oxidation

The results of the mass gain experiment are shown in Fig. 1. The unmodified 316 LN SS exhibited the highest rate of oxidation. The initially formed scale was found to be not adherent and spalled to the extent so that the specimen even showed mass loss. The zigzag nature of the curve indicates that descaling has occurred after each time the scale has acquired a critical thickness. On the other hand, the alumina-modified specimen exhibited a uniform mass gain on oxidation commensurate with a parabolic rate behavior. Further more, the rate of oxidation was much lower than the bare specimen. Both the aluminized specimens (aluminized and diffusion-annealed and aluminized and laser treated) showed similar oxidation kinetics.

Fig. 1
figure 1

Mass gain data of specimens oxidized at 1123 K

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Nitridation did not result in the improvement in the oxidation behavior. However, nitridation enhanced the surface hardness from 250 VHN to 266 VHN without impairing the oxidation resistance.

The data on mass gain at specific intervals and the corresponding masses of the spalled oxide are listed in Table 2. The masses of the spalled oxide are maximum in the case of bare and nitrided specimens.

Table 2 Data of mass gain and corresponding masses of the spalled oxide
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The aluminized specimen showed a marginal deviation from parabolic oxidation kinetics after 2.88 Ms of oxidation. It is presumed that the scale has undergone cracking and a stage of enhanced oxidation has set in on account of exposure of bare surface to air. Investigation by EDS revealed the existence of granular structures, which are regions of enhanced rate of oxidation and spallation.

Analysis of Surface by SEM and EDS

The uncoated specimen showed the generation of heterogeneous oxide scale (Fig. 2). The alumina-coated specimen showed the generation of Al2O3 on the surface (Fig. 3a). Both laser treatment and diffusion annealing showed identical protective behavior. The microstructure of the laser-treated specimen is shown in Fig. 4a. Nodular structures were observed on the surface in both the cases(Fig. 3b, 4b, respectively) indicating the onset of rapid growth and spallation of the scale. The SEM structure of nitrided specimen is given in Fig. 5.

Fig. 2
figure 2

SEM micrograph of oxidized specimen not subjected to surface modification

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Fig. 3
figure 3

(a) SEM micrograph of aluminized sample after oxidation for 3.6 Ms. Pre-treatment was carried out by heating at 873 K for 10.8 ks followed by 1173 K C for 90 ks. 3 (b) SEM micrograph of specimen aluminized and oxidized (nodular region)

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Fig. 4
figure 4

(a) SEM micrograph of aluminized, laser annealed and oxidized specimen. (b) SEM microstructure of laser annealed and oxidized specimen showing nodular structure

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Fig. 5
figure 5

SEM microstructure of nitrided specimen after oxidation

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Analysis by EDS indicated that the uniform layer in the oxidized bare stainless steel is predominantly iron oxide with minor concentration of chromium on the surface scale. The surface compositions of different specimens are listed in Table 3.

Table 3 Surface composition of oxide scale
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The uncoated specimen showed the generation of oxides of chromium and iron. The aluminized specimen after oxidation formed predominantly the oxide of aluminum at the surface. However, the analysis of the nodular region revealed that a large part of iron ions have diffused through the pre-existing scales and appeared as iron oxide at the surface. The high diffusion rate of iron compared to chromium and nickel, through pre-existing scale has been reported as the reason for this behavior (Ref 12, 13). The laser-treated specimen also showed similar compositional variation of oxide scale. The nitrided specimen showed preferential generation of oxide of chromium. Nitridation probably caused the enrichment of chromium on the surface due to the formation of nitrides of chromium (both CrN and Cr2N are thermodynamically stable under present conditions). On subsequent oxidation these nitrides were converted to oxide. Iron does not form a stable nitride and hence has not segregated to the surface on nitridation. The presence of iron oxide at the surface is the result of high rate of diffusion of iron.

Characterization of Phases by Glancing Incidence X-ray Diffraction (GIXRD)

The oxides formed on the surface of the specimen were characterized by GIXRD using CuKα as the incident source. A glancing angle of 1° was employed. The oxide scales spallen from various specimens were also analyzed. Typical results of the analyses are given in Fig. 6a to c and the identified phases corresponding to all the specimens are summarized in Table 4.

Fig. 6
figure 6

(a) GIXRD spectra of thin film on the aluminized specimen after oxidation. (b) GIXRD spectra of thin film of laser-annealed specimen. (c) GIXRD spectra of thin film on the nitrided specimen

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Table 4 GIXRD of oxide scale on the surface of the specimen and that spalled during the experiment
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Mechanism of Nodular Growth and Scale Failure

When austenitic stainless steel is exposed to an oxygen-rich atmosphere, the protective scale expected to be developed is Cr2O3 as this is the most thermodynamically stable oxide among other constituents of stainless steel. For the alloy to sustain a continuous protective scale, the chromium content has to be in the range of 16-20% (Ref 14). If the chromium content is lower, the oxygen will encounter other elements present in the matrix such as iron, manganese and nickel. The transformation from a protective scale to a thicker scale has been reported in the literature. Wood et al. (Ref 15) proposed two probable mechanisms for the generation of iron-rich nodules on the surface. The first mechanism takes into account the entry of iron and nickel into the Cr2O3 film and the subsequent transformation into a spinel structure. Second theory is based on the stress associated with the growth of the scale which ultimately leads to the cracking. The crack exposes the underlying layer and leads to rapid growth of iron-rich oxide. The nodular growth is proposed to be aided by both these processes. In the aluminized specimen the nodular region was enriched in iron and the nodules also showed the generation of cracks. Thus the present investigation gives supportive evidence to Wood’s theory. The fast rate of diffusion of iron plays supportive role for the growth of nodular structure. Based on present results, the oxidation of stainless steel is proposed to proceed in three steps, viz;

  1. (i)

    Generation of protective chromia scale

  2. (ii)

    Penetration of the protective scale by fast diffusing ion such as that of iron. This results in the generation of nodules.

  3. (iii)

    Nodules breaks and the oxide spalls. Ultimately nodules grow longitudinally and interconnect resulting in the generation of a uniformly oxidized surface. In the present investigation, the aluminized specimen (both diffusion annealed and laser annealed) attained the second stage of generation of nodular structure. On the other hand, both the bare and nitrided specimen attained the third stage of rapid oxidation followed by intermittent spallation.

The mass gain data and SEM data provides supportive evidence of the above mechanism.

Conclusion

Specimens of stainless steel 316 LN were subjected to oxidation at 1123 K in the as-received condition and after subjecting to surface modifications by different routes. The mass changes were measured periodically followed by post-oxidative examination at the end. The following conclusions are drawn from this investigation.

  1. (i)

    Oxidation of stainless steel in the as-received condition resulted in rapid growth of scale followed by intermittent spallation.

  2. (ii)

    Surface modification by deposition of aluminum resulted in significant improvement in oxidation resistance due to the formation of adherent alumina scale. Special heat treatment was required to promote the generation of adherent α-Al2O3 scale. Pre-treatment by diffusion annealing and laser treatment provided almost similar protective behavior.

  3. (iii)

    For retarding the oxidation rate of iron-base alloys, it is necessary to device methods for reducing the diffusion rate of iron through the pre-existing scale.

  4. (iv)

    Nitridation is a good method to improve the surface hardness without adversely altering the oxidation properties.