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Relativenobility of precipitated phases in stainless steels: Evaluation with a combination of local probing techniques
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stainless steels often exhibit complex transformation and precipitation behaviour due to a high content of alloying elements. Secondary phases can be formed in the temperature range of 300-1000°C and are generally undesirable due to their detrimental effect on mechanical properties and corrosion resistance of stainless steels. Of all precipitate types, sigma phase is the major concern due to its effect on both toughness and resistance to corrosion. However, the effect of the phase itself cannot be separated from that of associated changes in the surrounding matrix if macroscopic techniques are used. The situation is similar for investigations of chromium nitrides, which are the precipitated phases frequently observed in high nitrogen stainless steels. High resolution techniques are required to characterize such small individual precipitates to be able to examine their actual effect on the corrosion resistance of stainless steels.

 

In this work, magnetic force microscopy (MFM) and scanning Kelvin probe force microscopy (SKPFM) were used to investigate the magnetic properties and the Volta potential difference of precipitated phases. The magnetic domain distribution was used to distinguish between ferrite (ferromagnetic), austenite (paramagnetic), and sigma phase (non-magnetic). The Volta potential differences reflect the relative nobility of the phases and thus their tendency to corrode. The MFM and SKPFM techniques are shown to achieve a high lateral resolution of at least 100 nm. This means that small particles or precipitated phases can be characterized separately from the surrounding matrix.

 

Two grades of duplex stainless steels, the standard 2205 and the super duplex 2507, and an electroslag strip weld with a multi-phase microstructure were investigated using a combination of local probing techniques. The duplex stainless steels underwent various heat treatments to precipitate a sufficient amount of secondary phases. Scanning electron microscopy in backscattered electron mode and energy dispersive X-ray spectroscopy yielded information on the phase distribution and chemical composition of individual phases. Detailed marking of the surfaces was used so that exactly the same areas could be investigated with the MFM and SKPFM techniques. Transmission electron microscopy (TEM) was also employed to characterize the chemical composition of depleted phase boundaries.

 

 

The general observation is that, austenite exhibited a higher Volta potential compared with ferrite, most likely due to a higher nickel content in the austenite phase. When sigma phase was present, it showed an intermediate Volta potential between the austenite and ferrite phases. This indicates that austenite is, in general, more noble than sigma phase, and that ferrite is the most active phase. However, austenite showed a lower Volta potential than sigma phase when a long-term isothermal heat treatment at 800°C was used. This is attributed to the depletion of alloying elements in the austenite phase as a result of the formation of chromium nitrides and sigma phase. Synergistic interactions between chromium, molybdenum, and nitrogen may contribute to the effect on the Volta potential, since such small changes of these elements result in reversed Volta potentials of the austenite and sigma phase.

 

Results from SKPFM and TEM analyses are in concordance and indicate local drops in Volta potential at the phase boundaries due to the depletion of alloying elements caused by sigma phase formation. Immersion tests in acidic mixtures also confirmed that these depleted regions are more susceptible to selective corrosion.

 

Precipitated chromium nitrides showed a higher Volta potential compared with the other phases. This indicates that any deterioration in the corrosion resistance is unlikely caused by the nitride particles themselves, but rather by the alloying element depleted regions surrounding the nitride particles. The size of nitride particles affected the measured Volta potential, and the measured Volta potentials of small particles are tended to be concealed by the surrounding matrix. When the size of nitride particles is below the resolution limit of the SKPFM technique, the Volta potential differences of these particles relative to the matrix could not be detected.

 

Volta potential measured in air with the SKPFM technique correlated better to the tendency to active dissolution than to pitting corrosion in acidic mixtures. The magnetic force showed a certain influence on the electrostatic force, thus Volta potential measurements are recommended to be performed with a non-magnetic tip. Although many factors may affect the measured Volta potential, the SKFPM technique combined with other local probing techniques is a promising approach to evaluate corrosion tendency of precipitated phases in multi-phase alloys. With optimal conditions, the detectable size was down to approximately 100 nm.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , ix, 67 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:15
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12829ISBN: 978-91-7415-634-8 (print)OAI: oai:DiVA.org:kth-12829DiVA: diva2:319081
Public defence
2010-05-28, F3, Lindstedtsvägen 26, KTH, Stockholm, 14:43 (English)
Opponent
Supervisors
Note
QC 20100518Available from: 2010-05-18 Created: 2010-05-12 Last updated: 2010-05-18Bibliographically approved
List of papers
1. Characterization of phases in duplex stainless steel by magnetic force microscopy/scanning kelvin probe force microscopy
Open this publication in new window or tab >>Characterization of phases in duplex stainless steel by magnetic force microscopy/scanning kelvin probe force microscopy
2008 (English)In: Electrochemical and solid-state letters, ISSN 1099-0062, E-ISSN 1944-8775, Vol. 11, no 7, C41-C45 p.Article in journal (Refereed) Published
Abstract [en]

A 2205 duplex stainless steel, which had undergone a slow cooling process in order to precipitate intermetallic phases, was characterized by means of magnetic force microscopy (MFM) and scanning Kelvin probe force microscopy (SKPFM), in addition to conventional scanning electron microscope and energy-dispersive spectroscopy analysis. MFM measurements yield information about the magnetic domain distribution, while SKPFM provides the variation in Volta potential between austenite (gamma), ferrite (alpha), and sigma phases (sigma). In general, paramagnetic austenite exhibits the highest Volta potential, followed by nonmagnetic sigma phase and ferromagnetic ferrite, respectively. Results show the applicability of MFM/SKPFM techniques for characterization of the individual phase properties of duplex stainless steel. Because a cross talk between magnetic and Volta potential signals has been observed, it is recommended to perform SKPFM measurements with nonmagnetic tips.

Keyword
THIN ELECTROLYTE LAYERS, ATMOSPHERIC CORROSION, IN-SITU, ALUMINUM-ALLOY, ATOMIC-FORCE, SIGMA-PHASE, PITTING CORROSION, METAL-SURFACES, DISSOLUTION, SKPFM
Identifiers
urn:nbn:se:kth:diva-12887 (URN)10.1149/1.2912601 (DOI)000255982800010 ()2-s2.0-44149086203 (Scopus ID)
Note
QC20100628Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2017-12-12Bibliographically approved
2. Depletion effects at phase boundaries in 2205 duplex stainless steel characterized with SKPFM and TEM/EDS
Open this publication in new window or tab >>Depletion effects at phase boundaries in 2205 duplex stainless steel characterized with SKPFM and TEM/EDS
2009 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 51, no 8, 1850-1860 p.Article in journal (Refereed) Published
Abstract [en]

2205 duplex stainless steel, which had undergone two different heat treatments, was characterized by means of SEM/EDS, TEM/EDS, magnetic force microscopy (MFM) and scanning Kelvin probe force microscopy (SKPFM). In general, paramagnetic austenite was found to exhibit the highest Volta potential, followed by non-magnetic sigma phase and ferromagnetic ferrite. The local Volta potential drop at the phase boundaries is clearly visible in the slowly cooled sample with sigma phase precipitate while it is not notable in the solution-annealed reference sample. Chemical composition line analyses with TEM/EDS showed the depletion of Cr and Mo at the phase boundaries.

Keyword
PROBE FORCE MICROSCOPY; SCANNING KELVIN PROBE; SIGMA-PHASE; CORROSION BEHAVIOR; PITTING CORROSION; IN-SITU; ATOMIC-FORCE; PASSIVE FILMS; COLD-WORKING; DISSOLUTION
Identifiers
urn:nbn:se:kth:diva-12895 (URN)10.1016/j.corsci.2009.05.012 (DOI)000268656300043 ()2-s2.0-67649831333 (Scopus ID)
Note
QC20100628Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2017-12-12Bibliographically approved
3. Study of nobility of chromium nitrides in isothermally aged duplex stainless steels by using SKPFM and SEM/EDS
Open this publication in new window or tab >>Study of nobility of chromium nitrides in isothermally aged duplex stainless steels by using SKPFM and SEM/EDS
2010 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 52, no 1, 179-186 p.Article in journal (Refereed) Published
Abstract [en]

The relative nobility of Cr2N in duplex stainless steels (DSSs) was investigated in isothermally aged 2205 and 2507 DSSs using scanning Kelvin probe force microscopy (SKPFM) and SEM/EDS. The specimens contained nitrides, austenite and sigma phase but no ferrite. In these materials, Cr2N exhibits a higher Volta potential than sigma phase and austenite, implying the highest practical nobility of Cr2N compare to the surrounding phases. The composition and alloying effect can explain the relative nobility of the phases. The apparent "size effect" of small Cr2N on the measured Volta potential difference is probably due to the influence of surrounding matrix.

Keyword
PROBE FORCE MICROSCOPY; HIGH-NITROGEN; PITTING CORROSION; PRECIPITATION; MOLYBDENUM; PASSIVATION; PASSIVITY; CR2N; RESISTANCE; RESOLUTION
Identifiers
urn:nbn:se:kth:diva-12894 (URN)10.1016/j.corsci.2009.08.057 (DOI)000272580000022 ()2-s2.0-74249094464 (Scopus ID)
Note
QC20100628Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2017-12-12Bibliographically approved
4. Evaluation of corrosion behaviour in a 317L stainless steel strip welding using scanning Kelvin probe force microscopy
Open this publication in new window or tab >>Evaluation of corrosion behaviour in a 317L stainless steel strip welding using scanning Kelvin probe force microscopy
2011 (English)In: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176, Vol. 62, no 12, 1092-1099 p.Article in journal (Refereed) Published
Abstract [en]

Scanning Kelvin probe force microscopy and magnetic force microscopy have been used in combination with SEM/EDS and immersion tests to study a 317L electroslag strip weld which contains austenite and interdendritic ferrite and sigma phase. The individual phases can easily be recognized from the compositional contrast, magnetic pattern and Volta potential variation. Austenite, which is paramagnetic, exhibits the highest Volta potential followed by non-magnetic sigma phase and ferromagnetic ferrite, respectively. Corrosion testing in acidic chloride solutions indicates that the Volta potential measured in air can be related to the tendency to uniform corrosion, while pitting corrosion shows different dependence. In both cases ferrite and sigma phase behaved in a similar manner, indicating that there was no specific detrimental effect of sigma phase on corrosion properties in this material.

Keyword
stainless steel, strip welding, scanning Kelvin probe force microscopy, Volta potential, uniform corrosion, pitting corrosion
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-12908 (URN)10.1002/maco.201005647 (DOI)000298064000001 ()2-s2.0-83655191721 (Scopus ID)
Note
QC 20120206Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2017-12-12Bibliographically approved
5. SKPFM Study of Chromium Nitrides in 2507 Super Duplex Stainless Steel: Implications and Limitations
Open this publication in new window or tab >>SKPFM Study of Chromium Nitrides in 2507 Super Duplex Stainless Steel: Implications and Limitations
Show others...
2011 (English)In: ELECTROCHIMICA ACTA, ISSN 0013-4686, Vol. 56, no 4, 1792-1798 p.Article in journal (Refereed) Published
Abstract [en]

The effect of chromium nitrides on localized corrosion resistance of 2507super duplex stainless steel was investigated in this study. The Voltapotential difference measured with scanning Kelvin probe force microscopy(SKPFM) indicates that chromium nitrides with the size range of 80‐230 nmprecipitated isothermally at the ferrite/austenite phase boundaries maydetrimentally affect the corrosion resistance due to the observed local Voltapotential drop at the phase boundaries. Small quenched‐in nitrides with thesize range of 50‐100 nm formed in the centre of the ferrite phase, on theother hand, may have small or no adverse effect on the corrosion resistancesince they showed no difference in Volta potential relative to the matrix.

Place, publisher, year, edition, pages
Oxford: PERGAMON-ELSEVIER SCIENCE LTD, 2011
Keyword
duplex stainless steels, chromium nitrides, scanning Kelvin probe force microscopy, critical pitting temperature
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12911 (URN)10.1016/j.electacta.2010.08.038 (DOI)000287951600014 ()2-s2.0-79151473371 (Scopus ID)
Note
QC 20100518 Uppdaterad från manuskript till artikel i tidskrift 20110407.Available from: 2010-05-18 Created: 2010-05-18 Last updated: 2011-04-07Bibliographically approved

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