Identification of Defects of Stainless Steel Sold with Brand Names, by Studying their Surface Thin Films with Scanning Electron Microscope (SEM)

Abstract:

In the present paper, we have studied the thin films formed on the commercial stainless steel sold under different brand names in Pakistan with the help of SEM, EDX and XRD and have further investigated that how such stainless steels lose their brightness and become dull. A sample of recycled stainless steel melted in induction furnace was taken for study. The analysis of the layer formed is as follows: wt. %, C = 6.55, O = 6.20, Na = 0.47, Mg = 0.31, Al = 2.28, Si = 1.23, Cr = 15.42, Mn = 1.06, Fe = 59.58, Ni = 6.91 Total = 100, Atomic %, C = 21.02 O = 14.93, Na = 0.79, Mg = 0.49, Al = 3.26, Si = 1.68, Cr = 11.43, Mn = 0.74, Fe = 41.12, Ni = 4.53. Stainless steel has austenite structure, is non magnetic and forms fine passive film of Cr (III) oxide, (Cr₂O₃) which prevents surface corrosion and avoids further corrosion from spreading in metal’s internal structure. Mn and Ni stabilize austenite structure. XRD spectra show presence of αFe₂O₃ and Cr₂O₃ and spinal oxides in the film formed on the sample taken for study. SEM images show how this thin film tears. In a sound stainless steel, the film formed is too thin to be visible and metal remains lustrous. The film quickly reforms when surface is scratched (passivation). Mostly stainless steel is recycled and average stainless steel objects manufactured and marketed are composed of above 60% recycled material. Oxide spinals of Mn, Ni, Cr with iron oxide are not very resistive to the corrosion. Recently Duplex stainless steel has been manufactured which contains 50:50 austenite and ferrite and have twice the strength compared to austenite stainless steel. Keywords Recycle stainless steel, composition of thin film with EDX, SEM images, investigation on αFe₂O₃ and Cr₂O₃ spinals. In the present paper, we have studied the thin films formed on the commercial stainless steel sold under different brand names in Pakistan* with the help of SEM, EDX and XRD and have further investigated that how such stainless steels lose their brightness and become dull. The sample is austenitic stainless steel.

• Periodical: Key Engineering Materials (Volumes 622-623)
• Pages: 53-63
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.622-623.53
• Online since: September 2014
• Authors: Pervaiz Habibullah*
• Keywords: Composition of Thin Film with EDX, Investigation on Cr₂O₃ Spinals, Investigation on αFe₂O₃ Spinals, Recycle Stainless Steel, Sem Images

References

[3] 92.

[4] 02.

[4] 25.

[4] 54.

[5] 01 It can be observed that the ratio of Fe/Cr becomes larger with the increase of degree of sensitization. This means that chromium depletes more greater than iron. The difference can be explained by carbide (Cr23C6) precipitation during sensitization process which leads to chromium depletion [5] Passive films electrochemically grown on chromium metal have a duplex structure with an upper more hydrated layer and an underlying less hydrated layer Shlepa Kov and Su Khotin (1985).

[3] Experimental.

[3] 1 Scanning Electron Microscope (SEM)* The surface and cross section of the sample of austenitic stainless steel sold under different brand names was observed at Scanning Electron Microscope (SEM).

DOI: 10.4028/www.scientific.net/kem.622-623.53

[3] 2 EDX – Energy Dispersion X-ray Spectroscopy* The analysis of the surface layer, where oxides particles (spinals) are found were analyzed by EDX method.

[4] Results.

[4] 1 Scanning Electron Microscopy (SEM).

[4] 1. 1 Flat surface observation.

[1] SEM image of the surface of the sample at 200x. Fig. 2 (a).

[2] SEM image of the surface of the sample at 500x. Fig. 2 (c).

[3] Surface of the sample as seen at 1500x the spinal of oxides are visible. Fig. 2 (f).

[4] The particle of the spinal in the pit formed on the surface, at 2000x. Fig. 2 (g).

[5] Layers of oxides are visible at 2000x. Fig. 2 (h) *By courtesy of PITMAEM Pakistan Council of Scientific and Industrial Res. Lahore Pakistan4. 1. 2 Cross section of the sample.

[1] Cross section of the sample at 200x. Thin oxide film on the cross section of the sample. Fig. 2 (b).

[2] Cross section of the sample at 500x Thin oxide film in fig. 2(d) is more visible at 500 mag. Fig. 2 (d).

[3] Cross section of the sample at 1000x thin film is visible as penetrated in the sample. Fig. 2 (e) (a) SEM image of the surface of the sample at 200x (b) Cross section of the sample at 200x. Thin oxide film on the cross section of the sample (c) SEM image of the surface of the sample at 500x (d) Cross section of the sample at 500x Thin oxide film in fig. 2(d) is more visible at 500 mag. (e) Cross section of the sample at 1000x thin film is visible as penetrated in the sample (f) Surface of the sample as seen at 1500x the spinal of oxides are visible (g) The particle of the spinal in the pit formed on the surface, at 2000x (h) Layers of oxides are visible at 2000x Fig. 2 SEM images of thin film formed on commercial stainless steel.

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[4] 2 EDX – Energy Dispersion X-ray Spectroscopy The analysis of the surface layer, where oxides particles (spinals) are found, were analyzed by Energy Dispersion X-ray Spectroscopy (Fig. 3) Results are as follows: Table 2 Analysis EDX Fig. 3 Fig. 3.

[5] Conclusions.

[1] In a sound stainless steel, the film formed is too thin to be visible and metal remains lustrous. The film quickly reforms when surface is scratched (passivation). Mostly stainless steels sold under brand names are recycled and average stainless steel objects manufactured and marketed are composed of above 60% recycled material.

[2] Stainless steel having austenite structure, is non magnetic and forms fine passive film of Cr (III) oxide, (Cr2O3) which prevents surface corrosion and avoids further corrosion from spreading in metal's internal structure. Mn and Ni stabilize austenite structure.

[3] XRD spectra show presence of aFe2O3 and Cr2O3 and spinal oxides in the film formed on the sample taken for study. SEM images show how this thin film tears (see text). Oxide spinals of Mn, Ni, Cr with iron oxide are not very resistive to the corrosion. The presence of oxide spinals play an important role in making the stainless steel dull and impairing its brightness by affecting the process of passivation.

[4] The analysis of the layer formed is as follows: wt. %, C = 6. 55, O = 6. 20, Na = 0. 47, Mg = 0. 31, Al = 2. 28, Si = 1. 23, Cr = 15. 42, Mn = 1. 06, Fe = 59. 58, Ni = 6. 91 Total = 100, Atomic %, C = 21. 02 O = 14. 93, Na = 0. 79, Mg = 0. 49, Al = 3. 26, Si = 1. 68, Cr = 11. 43, Mn = 0. 74, Fe = 41. 12, Ni = 4. 53.

DOI: 10.11606/d.44.2008.tde-02092008-163850

[5] The chemical comp. of film formed on duplex stainless steel has been studied by A. Kochijan [3] and likewise, has shown the presence of two main oxides i. e. Fe and Cr. The oxides of the alloying elements Ni and Mo were negligible compared to the bulk. A slight decrease of Cr content close to the oxide / chloride soln. interface at higher anodic potential was also observed.

[6] Further research Further study on the morphology and properties of oxide spinals formed in the thin films of stainless steel may be conducted and their composition may be analyzed as the function of the depth of the layer.

[7] Acknowledgement Author is grateful to the scientists of SEM section of PITMAEM, Pakistan Council of Scientific and Industrial Res. Lahore – Pakistan for scanning electron microscopy of stainless steel samples.

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