Influence of tensile pre-strain and sensitization on passive films in AISI 304 austenitic stainless steel

doi:10.1016/j.matchemphys.2012.05.086

Materials Chemistry and Physics

Volume 135, Issues 2–3, 15 August 2012, Pages 973-978

https://doi.org/10.1016/j.matchemphys.2012.05.086 Get rights and content

Abstract

The degree of sensitization for tensile pre-strain (0%, 10%, 20%, 30%, 40%) of AISI 304 stainless steels and sensitization at 575 °C was investigated by the double loop electrochemical potentiokinetic reactivation technique and scanning electron microscopes (SEMs). The properties of passive films of all the sensitized specimens in borate buffer solution (pH = 9.2) with 5000 ppm Cl⁻ were investigated by the Mott–Schottky analysis. The degree of sensitization results showed that two reactivation current peak values were obtained for the sensitized specimens after 30% and 40% pre-strain. The degree of sensitization was decreased by pre-strain, except for specimen with 10% pre-strain. The results were further confirmed by the Mott–Schottky analysis.

Highlights

► Two reactivation current peaks occur for sensitized specimens with large strain. ► Change trend of acceptor concentration and degree of sensitization is similar. ► The Mott–Schottky analysis is effective in evaluating degree of sensitization.

Introduction

The austenitic stainless steels become more susceptible to sensitization in the temperature range from 773 to 1123 K. It is usually attributed to the precipitation of chromium carbides (M₂₃C₆) at the grain boundaries and the depletion of chromium and carbon around the precipitates [1]. Intergranular corrosion and intergranular stress corrosion cracking are caused by sensitization. In this process, if the local chromium content drops below 12 wt.%, then the chromium depleted zones become prone to local corrosion and intergranular stress corrosion cracking (IGSCC) [2].

Some studies have shown that the kinetics of sensitization can be influenced by prior deformation. The effects of tensile and cold rolling strain (up to 40%) over a range of grain sizes ranging from 300 μm to 10 μm on sensitization and desensitization were observed and compared for 304 and 316 stainless steel having a constant carbon content of 0.05% at 625 °C to 775 °C [1]. Sensitization was more rapid for cold rolling versus tensile straining in both stainless steels, and this was also true for both stainless steel which exhibited considerably less sensitization overall on the 10 μm grain size material. Mannan et al. [3] proposed that cold work 15% prior to sensitization of 316 austenitic stainless steel resulted in increasing rate of sensitization at 752 °C and remained constant up to 25%. Uniaxial deformation to 50% accelerated the sensitization/desensitization process in 316 stainless steel, especially at 670 °C and the deformed material showed more carbide precipitates (Cr₂₃C₆) at the grain boundaries and twin intersections than the nondeformed material [4]. Deformation prior to sensitization could greatly accelerate the kinetics of sensitization in 304 stainless steels [5]. Researches by Briant et al. [6] showed that deformation with martensite transformation caused rapid sensitization at temperature below 600 °C and produced rapid healing, while rapid healing was not observed without the presence of martensite. Besides, grain boundary was observed to have a dramatic effect on the sensitization. For example, the formation grain size about 10 nm with twin boundaries by surface mechanical attrition treatment (SMAT) suppressed sensitization duo to regular and coherent atomic structure and extreme low grain boundary energy of twin boundaries compared with those of other grain boundaries [7].

However, the majority results were the effects of cold rolling on sensitization and the effects of a few tensile strains and strain-induced martensite on degree of sensitization (DOS) were not consistent. Besides, the effects of sensitization on the donor or acceptor densities in stainless steels were very few. Therefore, an attempt has been made in the present paper to study the effect of tensile strain up to 40% on sensitization behaviour at 575 °C by XRD, the double loop electrochemical potentiokinetic reactivation technique (DLEPR) and the Mott–Schottky analysis.

Section snippets

Experimental details

The chemical composition of the investigated commercial grades AISI 304 austenitic stainless steel plate with the 2-mm thickness was shown in Table 1. For studying the effect of strain on sensitization, the as-received strips were uniaxially tensile strain to 10%, 20%, 30% and 40% engineering strain at strain rate of 1 × 10⁻⁴ s⁻¹ using a testing system (SANS ± 100 kN) in laboratory air. The as-received specimen was taken as a reference corresponding to a cold deformation state of 0%. The as-received

The XRD and DOS results

X-ray diffraction is used to determine the relative amounts of different phases formed in the austenitic stainless steel after tensile strain and sensitization. Fig. 1 compares the X-ray diffraction patterns of the strain specimens without and with sensitization. X-ray diffraction data at room temperature shows that the as-received stainless steel is single phase fcc (γ). The X-ray diffraction patterns indicate no existence of any other phase. With increasing the engineering strain, the

Conclusion

The present study shows that the tensile strain and sensitization has complex influence on characteristic of the DOS and passive film properties of AISI 304 stainless steel. The main conclusions are as follows.

(1)
Strain-induced martensite is not complete reversion when specimens (tensile engineering strain ≥30%) are heat-treated at 575 °C for 5 h.
(2)
Two reactivation current peak values are observed for the sensitized specimens with 30% and 40% pre-strains.
(3)
Two reactivation current peak values suggest

Acknowledgements

This work was financially supported by 973 Program.

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Influence of tensile pre-strain and sensitization on passive films in AISI 304 austenitic stainless steel

Abstract

Highlights

Introduction

Section snippets

Experimental details

The XRD and DOS results

Conclusion

Acknowledgements

J. Nucl. Mater.

Scr. Metall

Mater. Lett.

Corros. Sci.

J. Mater. Process. Technol.

Scr. Mater.

J. Nucl. Mater.

Mater. Charact.

Acta Metall. Mater.

Appl. Surf. Sci.

Electrochim. Acta

Electrochim. Acta