| 1. |
- Fuertes, Nina, et al.
(författare)
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Use of SVET to evaluate corrosion resistance of heat tinted stainless steel welds and effect of post-weld cleaning
- 2017
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Ingår i: Materials and corrosion - Werkstoffe und Korrosion. - : Wiley. - 0947-5117 .- 1521-4176. ; 68:1, s. 7-19
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Tidskriftsartikel (refereegranskat)abstract
- 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. 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. It was seen that heat tints dissolve by electrochemical reactions that can be mapped with the SVET and correlated with the level of discolouration of the oxides, with the purple–brown oxide being the most active. 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.
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| 2. |
- Casals, Nuria Fuertes, et al.
(författare)
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Influence of annealing oxide structure and composition on pickling of the duplex stainless steel 1.4462
- 2011
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Ingår i: 7th European Stainless Steel Conference: Science and Market, Proceedings. - : Associazione Italiana di Metallurgia. - 9788885298842
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Konferensbidrag (refereegranskat)abstract
- In this work, correlation between the oxide formation and pickling behaviour of a conventional duplex material - 1.4462 is reported. The materials investigated are the duplex grade 1.4462 and two single phase alloys with compositions corresponding to the respective phases of the duplex material. Annealing was conducted at 1070 °C for 8 minutes. Topography and composition of the oxide layer were analysed with GDOES, FIB, XRD, SEM-EDS and CLSM. The ferritic steel formed a thick chromium-enriched oxide with a silica-rich sublayer. On the austenitic steel, ironenriched nodules were observed in the centre of the grains, together with oxide ridges on the top of the grain boundaries. The oxide on the duplex grade differed depending on the phase; nodule formation was observed on the austenitic grains while a more homogenous oxide formed on the ferritic grains. Pickling was performed in a mixed nitric/hydrofluoric acid bath. The fastest removal was observed for the austenitic sample, followed by the duplex and the ferritic samples. The oxide on the austenitic sample spalled from the surface while the oxide on the ferritic grade was removed by dissolution rather than by spallation. A mixed behaviour was observed on the duplex grade where some zones pickled faster than others.
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| 3. |
- Fuertes, Nuria, et al.
(författare)
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Effect of cold rolling on microstructure, corrosion and electrochemical response of the lean duplex stainless steel LDX 2101® by a correlative EBSD–SKPFM investigation
- 2020
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Ingår i: Materials and corrosion - Werkstoffe und Korrosion. - : Wiley. - 0947-5117 .- 1521-4176. ; 68:1, s. 1052-1065
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Tidskriftsartikel (refereegranskat)abstract
- This study elucidates the effect of cold rolling on the microstructure, corrosion and electrochemical response of the lean duplex stainless steel LDX 2101®. With thickness reductions of 0%, 20% and 40%, three different specimens are investigated in terms of microstructure (electron backscattered diffraction [EBSD] and energy-dispersive X-ray spectroscopy [EDS]), corrosion properties (ASTM G150, ASTM G61 and field testing), passive film properties (X-ray photoelectron spectroscopy) and electrochemical response (scanning Kelvin probe force microscopy [SKPFM]). It is shown that deformation of LDX 2101 leads to changes in the microstructure such as mechanical twinning and martensite formation. The combination of EBSD, EDS and SKPFM maps shows that the work function is clearly dependent on composition, deformation and local misorientation, but not on the crystallographic orientation of the grains in the microstructure. Zones with low work function are seen to have the highest pitting susceptibility, which includes deformed ferritic, martensitic phase and areas with a high concentration of dislocations such as grain boundaries and mechanical twins. The overall conclusion is that cold deformation up to a 40% thickness reduction has a significant influence on the microstructure, but a small impact on the corrosion resistance of LDX 2101.
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| 4. |
- Fuertes, Nuria, et al.
(författare)
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Influence of mechanical stress on the potential distribution on a 301 LN stainless steel surface
- 2015
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 162:9, s. C465-C472
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Tidskriftsartikel (refereegranskat)abstract
- The aim of the present work was to study the influence of the stress on the electrode potential of the austenitic stainless steel 301LN using Scanning Kelvin Probe (SKP). It was found that elastic deformation reversibly ennobles the potential whereas plastic deformation decreases the potential in both tensile and compressive deformation mode and this decrease is retained even 24 h after removal of the load. To interpret the stress effects, different surface preparations were used and the composition and thickness of the passive film were determined by GDOES. Slip steps formed due to plastic deformation were observed using AFM. The effect of plastic strain on the potential is explained by the formation of dislocations, which creates more a defective passive film.
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| 5. |
- Fuertes, Nuria
(författare)
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Microstructural and Corrosion studies of Super-duplex Stainless steels produced via Additive Manufacturing
- 2021
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Ingår i: Microstructural and Corrosion Studies of Super-Duplex Stainless Steels Produced via Additive Manufacturing. - : NACE International.
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Konferensbidrag (refereegranskat)abstract
- This work focuses on the influence of post-processing on microstructure and corrosion properties of additive manufactured (AM) super-duplex stainless steels (SDSS) 2507 consolidated via laser-powder bed fusion (L-PBF). Conventional heat treatment or hot isostatic press (HIP) with different cooling rates; 200 °C/min or 5 °C/min were carried out. The corrosion resistance was analyzed by critical pitting temperature (CPT) using ASTM G150 and immersion testing. Scanning Kelvin Probe Microscopy (SKPFM) was used to evaluate the electrochemical response.The results suggest that different thermal cycles led to variation in the microstructure and corrosion properties. HIP with a fast cooling rate is recommended to ensure good corrosion resistance of L-PBF 2507. Use of a slow cooling rate after HIP processing can lead to appreciably lower corrosion resistance compared to conventional manufactured 2507 due to the presence of detrimental phases. A conventional heat treatment of the AM components can also give acceptable corrosion resistance. The conclusion is that with the right post-processing, the L-PBF material had similar or even higher corrosion resistance than conventional material and thus a high potential to be used in off-shore applications or other corrosive environments.
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| 6. |
- Fuertes, Nuria, et al.
(författare)
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Review-passive film properties and electrochemical response of different phases in a Cu-alloyed stainless steel after long term heat treatment
- 2016
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Ingår i: Journal of the Electrochemical Society. - : Electrochemical Society Inc.. - 0013-4651 .- 1945-7111. ; 163:7, s. C377-C385
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Fuertes, Nuria
(författare)
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Use of local electrochemical techniques for corrosion studies of stainless steels
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Fuertes, Nuria
(författare)
-
Use of localised techniques to elucidate the influence of process variables on the corrosion of stainless steels
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Several factors during steel manufacture and fabrication can alter the passivity and corrosion behaviour of stainless steels. These include alloying, deformation, welding and heat treatments.In this work a combination of local techniques, such as Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM), and surface and microstructural analytical techniques, such as Electron Backscatter Diffraction (EBSD), Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) is used to elucidate the influence of process variables on the corrosion resistance of stainless steels. Different manufacturing processes, such as casting, rolling or additive manufacturing (AM) (Paper V), fabrication processes, such as welding (Paper III) and deformation (Paper I and IV), and post-processing, such as heat and high pressure treatments (Paper II and V) and post-weld cleaning (Paper II) are studied.The results showed that deformation can have a major impact on microstructure but a smaller impact on corrosion. Plastic deformation decreased irreversibly the Volta potential, whereas elastic deformation did not have any permanent effect. The potential was dependent on composition, passive film thickness and deformation but not on the crystallographic orientation. Thermal oxides formed after welding were detrimental to corrosion resistance. SVET showed anodic activity on the weld areas with oxides where pitting initiated, explained by the interplay between the composition and the thickness of the oxides. Brushing combined with pickling was recommended for restoring the corrosion resistance. Long-term ageing of a Cu-alloyed 2507 caused precipitation of Cu-rich ε particles which showed the lowest potential and were most severely pitted, plus nitrides which were the most noble. Intermetallic sigma phase observed in the AM 2507 after HIP heat treatment with slow cooling resulted in a decreased corrosion resistance. However, it was concluded that AM 2507 exhibits similar or even higher corrosion resistance than conventional 2507 if appropriate post-processing parameters are applied.This thesis provides scientific insights to facilitate the correct manufacturing, fabrication and use of stainless steels.
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| 9. |
- Fuertes, Nuria, et al.
(författare)
-
Use of SVET to evaluate corrosion resistance of heat tinted stainless steel welds and effect of post-weld cleaning
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 10. |
- Hosseini, Vahid, 1987-, et al.
(författare)
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Effect of sigma phase morphology on the degradation of properties in a super duplex stainless steel
- 2018
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Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 11:6
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Tidskriftsartikel (refereegranskat)abstract
- Sigma phase is commonly considered to be the most deleterious secondary phase precipitating in duplex stainless steels, as it results in an extreme reduction of corrosion resistance and toughness. Previous studies have mainly focused on the kinetics of sigma phase precipitation and influences on properties and only a few works have studied the morphology of sigma phase and its influences on material properties. Therefore, the influence of sigma phase morphology on the degradation of corrosion resistance and mechanical properties of 2507 super duplex stainless steel (SDSS) was studied after 10 h of arc heat treatment using optical and scanning electron microscopy, electron backscattered diffraction analysis, corrosion testing, and thermodynamic calculations. A stationary arc was applied on the 2507 SDSS disc mounted on a water-cooled chamber, producing a steady-state temperature gradient covering the entire temperature range from room temperature to the melting point. Sigma phase was the major intermetallic precipitating between 630 °C and 1010 °C and its morphology changed from blocky to fine coral-shaped with decreasing aging temperature. At the same time, the average thickness of the precipitates decreased from 2.9 μm to 0.5 μm. The chemical composition of sigma was similar to that predicted by thermodynamic calculations when formed at 800-900 °C, but deviated at higher and lower temperatures. The formation of blocky sigma phase introduced local strain in the bulk of the primary austenite grains. However, the local strain was most pronounced in the secondary austenite grains next to the coral-shaped sigma phase precipitating at lower temperatures. Microstructures with blocky and coral-shaped sigma phase particles were prone to develop microscale cracks and local corrosion, respectively. Local corrosion occurred primarily in ferrite and in secondary austenite, which was predicted by thermodynamic calculations to have a low pitting resistance equivalent. To conclude, the influence of sigma phase morphology on the degradation of properties was summarized in two diagrams as functions of the level of static load and the severity of the corrosive environment.
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