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Use of local electrochemical techniques for corrosion studies of stainless steels
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. Swerea KIMAB AB.ORCID iD: 0000-0002-9045-9696
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The excellent corrosion resistance of stainless steels arises from the presence of a passive film on its surface. Above 10.5wt% Cr a chromium oxide of 1-3 nm is formed on the surface of the metal that in case of damage will reform and hinder further dissolution of the metal. However, the passivity of the stainless steel can be altered by material factors and external factors; such as the composition of the underlying phases, external loads or thermal treatments.

In this work the local electrochemical techniques Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM) and the local characterization techniques X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) have been used to investigate corrosion phenomena of stainless alloys based on measurements of corrosion current density, work function, thickness and composition of the oxide.

The effect on work function of the thickness of the passive film and composition of the underlying phases was investigated for 301LN austenitic stainless steel (Paper I) and a heat treated superduplex 25Cr7Ni type stainless steel (Paper II). It was shown that the work function can be an indicator of corrosion resistance of the phases in the microstructure, and that the composition of the underlying phases had a greater effect on the work function than the thickness of the passive film.

External factors such mechanical deformation (Paper I) and welding (Paper III) altered the passivity of the steel and work function. It was found that plastic deformation decreased irreversibly the work function, whereas elastic deformation did not have any permanent effect. Thermal oxides affected the passivity of stainless steels welded joints and were detrimental for its corrosion resistance. Anodic activity, observed with SVET, and pitting corrosion were detected at the heat tint and attributed to the interaction between the composition and the thickness of the oxide. Brushing combined with pickling was recommended for recovering the passivity of stainless steels.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 68 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:24
Keyword [en]
Stainless steel, passive film, thermal oxides, work function, pickling, SKPFM, SVET, XPS, AES
National Category
Corrosion Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-186206ISBN: 978-91-7729-002-5 (print)OAI: oai:DiVA.org:kth-186206DiVA: diva2:926129
Presentation
2016-06-09, sal K53, Teknikringen 28, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20160516

Available from: 2016-05-16 Created: 2016-05-04 Last updated: 2016-05-26Bibliographically approved
List of papers
1. Influence of Mechanical Stress on the Potential Distribution on a 301 LN Stainless Steel Surface
Open this publication in new window or tab >>Influence of Mechanical Stress on the Potential Distribution on a 301 LN Stainless Steel Surface
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2015 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, C465-C472 p.Article in journal (Refereed) Published
Abstract [en]

The aim of the present work was to study the influence of the stress on the electrode potential of the austenitic stainless steel301LN using Scanning Kelvin Probe (SKP). It was found that elastic deformation reversibly ennobles the potential whereas plasticdeformation decreases the potential in both tensile and compressive deformation mode and this decrease is retained even 24 h afterremoval of the load. To interpret the stress effects, different surface preparations were used and the composition and thickness ofthe passive film were determined by GDOES. Slip steps formed due to plastic deformation were observed using AFM. The effect ofplastic strain on the potential is explained by the formation of dislocations, which creates more a defective passive film.

Place, publisher, year, edition, pages
Electrochemical Society, 2015
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-186201 (URN)10.1149/2.0511509jes (DOI)
Note

QC 20160516

Available from: 2016-05-04 Created: 2016-05-04 Last updated: 2017-11-30Bibliographically approved
2. Review—Passive Film Properties and Electrochemical Response of Different Phases in a Cu-Alloyed Stainless Steel after Long Term Heat Treatment
Open this publication in new window or tab >>Review—Passive Film Properties and Electrochemical Response of Different Phases in a Cu-Alloyed Stainless Steel after Long Term Heat Treatment
2016 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 163, no 7, C377-C385 p.Article in journal (Refereed) Published
Abstract [en]

In this work the influence of copper (0–4 wt%) on the microstructure, passive film properties and local electrochemical response of 25Cr7Ni-type duplex stainless steel is investigated after long term heat-treatment at 800◦C for 6 months. This heat-treatment was done to promote the formation of different phases which could be studied in terms of passive film properties and electrochemical response. The unique microstructures of the alloys comprise austenite, sigma phase, Cr2N nitrides and, for the 2 wt% and 4 wt% Cu alloys, epsilon-Cu phase. The results show that alloying with Cu increases slightly the amount of isothermal Cr2N nitrides and epsilon-Cu phase, but decreases the sigma phase fraction. The location of pitting corrosion as well as the Electrochemical Potential (EP), or electron work function, measured with Scanning Kelvin Probe Force Microscopy (SKPFM) show that the epsilon-Cu phase has the lowest corrosion resistance. The EP appears to depend more on the composition of the underlying phase than on the thickness of the passive film. Cr-nitrides have the highest EP followed by sigma phase, austenite and epsilon-Cu phase. There is a clear decrease of EP of the austenitic phase when 2 wt% Cu is added in the alloy.

Place, publisher, year, edition, pages
Electrochemical Society, 2016
Keyword
Stainless steel, corrosion, passive film, SKPFM, volta potential
National Category
Corrosion Engineering
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-186200 (URN)10.1149/2.1101607jes (DOI)
Note

QC 20160513

Available from: 2016-05-04 Created: 2016-05-04 Last updated: 2017-11-30Bibliographically approved
3. Use of SVET to evaluate corrosion resistance of heat tinted stainless steel welds and effect of post-weld cleaning
Open this publication in new window or tab >>Use of SVET to evaluate corrosion resistance of heat tinted stainless steel welds and effect of post-weld cleaning
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect of heat tints on the corrosion resistance of a 2507 duplex stainless steel Tungsten Inert Gas (TIG) welded joint was assessed. The Scanning Vibrating Electrode Technique (SVET) was used to study oxide dissolution, initiation and propagation of corrosion on the weld at the open circuit potential (OCP) and at applied potentials. Small spot X-ray Photoelectron Spectroscopy (XPS) was used to investigate the composition and thickness of the heat tints. Both heat tinted and post-weld cleaned conditions were tested. Post-weld cleaning methods investigated were brushing, brushing plus polishing and brushing plus pickling paste. The results from the 2507 weld were also compared with results from a TIG 316L weld. SVET was shown to be an appropriate technique for characterising in-situ the activity of heat tints. It was seen that heat tints dissolve by electrochemical reactions that can be mapped with the SVET and correlated with the level of discoloration of the oxides, with the purple-brown oxide being the most active. The slow dissolution of the oxide gave a gradual decrease in current density over longer immersion times, leading to establishment of a passive state. The mechanical post-weld cleaning methods proved to be insufficient to remove the anodic activity in the heat tint. The most efficient process was brushing followed by pickling which resulted in a totally passive surface measured with SVET and a higher critical pitting temperature.

National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-186205 (URN)
Note

QS 2016

Available from: 2016-05-04 Created: 2016-05-04 Last updated: 2016-05-16Bibliographically approved

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