Jernkontoret (Sweden)

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.

Two types of laboratory tests were used to evaluate the behaviour of a wide range of steels in chlorinating–sulphidising conditions: high temperature exposure after applying salt by dipping in an aqueous KCl/ZnCl2 solution and salt-bed test in a ZnCl2/NaCl/KCl/CaSO4 salt mixture. The exposures were performed at 500 °C in a gas comprising N2/HCl/SO2/O2/N2. For the alloy group with 20–30 wt-% chromium and 25–65 wt-% nickel, the extrapolated metal loss was below 0.2 mm/year in the salt dip, up to 6 mm/year in the salt-bed test with 10% Cl and up to 20 mm/year in the 20% Cl salt-bed test. The intermediate alloy group showed poorer performance in the salt-bed; Esshete1250 showed also large spallation in the salt dip test. Results were compared with plant-exposed samples. Further refinement of the salt dip test is suggested for material ranking in the superheater region of a waste or biomass-fired incinerator.

In this work, three different stainless steels (304L (CrNi‐18‐8), 253 MA®(CrNiSi‐21‐11), Kanthal® A‐1 (FeCrAl)) and a reference low‐alloyed ferritic steel (16Mo3 (Fe0.3Mo)) were exposed in a commercial biomass gasifier for three periods of 9 min, 580 and 1054 h in the temperature range 350–500 °C. Biomass is a fuel with generally higher amounts of chlorine and lower amounts of sulphur compared to coal and there is a current lack of data on materials performance in such environments. A high level of zinc sulphide was observed on the surfaces of all materials after exposure. It is argued that zinc plays a key role in capturing sulphur in this environment, thus preventing iron from sulphidation. Some incorporation of sulphur in the oxide scale was observed for Fe0.3Mo and CrNi‐18‐8. CrNiSi‐21‐11 showed some internal oxidation and pitting was observed for the FeCrAl material. All four materials showed acceptable performance with low total metal loss.

Stainless steels have been widely used as architectural and construction materials because of their high degree of corrosion resistance, unique aesthetic quality, and stability in an unpolluted atmosphere. Although stainless steel is highly corrosion resistant, localized corrosion can occur in certain environments, especially in marine atmospheric conditions if the appropriate grade is not used. Exposure of stainless steel to an environment more aggressive than the limiting conditions may be harmful to its aesthetic appearance and ultimately even to its load-bearing capacity. Selecting a suitable stainless steel grade requires knowledge of the actual location of the application and the atmospheric conditions. In terms of materials selection, the austenitic stainless steel grade 316/316L has proved a very popular choice for architectural applications in many locations, but it is not always suitable at demanding sites such as marine environments in the Middle-East. In such cases the use of a higher-performance grade, often in combination with a good surface finish and established cleaning routines, is required to maintain pristine surfaces. The main objective of this paper is to present information about the atmospheric corrosion resistance of a number of stainless steels in the Middle-East at a marine site. The results obtained are analysed and discussed in terms of factors affecting atmospheric corrosion of stainless steel such as the, alloying element level, surface roughness, surface treatment, and microclimate.

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.

Computational thermodynamics were used to predict the growth of intermetallic phases in superduplex stainless steels, assuming that it is the ferrite which primarily decomposes during heat treatments below 1000 °C. Sigma and chi phase were modeled to form at the edge of a 10 μm ferrite unit and the time from 0.1 s to the development of a 100 nm layer of the phase in question was evaluated. This approach encompasses thermodynamics and kinetics of growth of the intermetallic phases but omits nucleation. Nevertheless the calculations accurately predict the classic C-shaped TTT curves, in which “nose” the growth is most rapid with a critical combination of precipitation, diffusion, and driving force. Verification of the relevance of the model was done by comparison with a publication with experimental studies of molybdenum and tungsten alloyed superduplex stainless steel weld metals, showing that partial substitution of Mo of by W does cause a more rapid growth of intermetallic phases. In addition the effect of the partial substitution of nickel by copper was examined and showed that copper decreases the amount of sigma phase and also reduces its growth rate. The results also suggest that the time for nucleation is of less importance in this context than previously suggested and thermodynamics and kinetics are sufficient to give an adequate explanation to the experimental results.

steel research internationalVolume 83, Issue 1 p. 3-4 ContentsFree Access steel research int. 1/2012 First published: 03 January 2012 https://doi.org/10.1002/srin.201290001AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume83, Issue1January 2012Pages 3-4 RelatedInformation

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