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- Stainless steel (5)
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- KorroPad (3)
- Korrosion (3)
- Pitting corrosion (3)
- Atmosphärische Korrosion (2)
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Organisationseinheit der BAM
Modelling of reinforcement corrosion - Influence of concrete technology on corrosion development
(2006)
Reinforcement corrosion is influenced by different parameters like resistivity of concrete, setting conditions and also by concrete technology. Moreover the presence of cathodic areas and the possibility of unhampered cathodic reaction influences the reinforcement corrosion. In this paper the development of corrosion without large cathodic areas, called self-corrosion, considering different concrete parameters, is studied.
In order to clarify under which practical conditions real risks for hydrogen-induced stress corrosion cracking of prestressing steels exist laboratory investigations in aqueous solutions and concrete as well as tests with stressed prestressing steels were carried out. While different distances between steel and zinc (cathode and anode) showed only a negligible influence on the polarisation of the steel a significant dependence on the area ratio steel/zinc was observed. A critical area ratio of 10:1 can be defined above which polarisation of the steel to hydrogen evolution potentials is unlikely. The results obtained in the laboratory concerning the critical area ratio were confirmed by on-site investigations in a precasting plant. Exposure tests with hydrogen loading did not lead to fractures during the testing time for approved prestressing steels. The characteristic mechanical values determined immediately after the hydrogen loading tests did not show significant changes. Thus, an indirect contact between prestressing steels and galvanised elements can be estimated as uncritical. But a direct contact has to be excluded by compliance with DIN 1045-1, i.e minimal distance of 2 cm between prestressing steel and galvanised elements.
Neben konstruktiven Anforderungen werden an nichtrostende Stähle im Bauwesen häufig hohe optische Ansprüche gestellt. Unerwartete Beeinträchtigungen des optischen Erscheinungsbildes nichtrostender Stähle unter wenig korrosiven Bedingungen sind in der Zwischenzeit ein weit verbreitetes Problemfeld und Gegenstand zahlreicher Streitfälle. Vielfach wird die Vermutung geäußert, dass eine veränderte Legierungszusammensetzung, veränderte Umweltbedingungen oder auch die unsachgemäße Verarbeitung die Ursache für diese Erscheinungen sind. Durch eine systematische Untersuchung zahlreicher Schadensfälle an Stabmaterial in den vergangenen zwei Jahren im Rahmen eines Forschungsvorhabens konnte gezeigt werden, dass die Ursachen in bekannten Werkstofffehlern zu finden sind. Hierzu gehören chemische Inhomogenitäten, wie zum Beispiel Ausscheidungen und örtliche Aufkohlung, sowie geometrische Fehler, wie zum Beispiel Hinterschnitte, Walzfehler und Schalenbildung. Die Fehler konnten damit eindeutig als Herstellungsfehler der jeweiligen Halbzeuge identifiziert werden.
Systematic laboratory and field exposure tests allow to compare nine different stainless steels (three ferritic, three austenitic, and three duplex grades) for civil engineering applications. The influence of surface finish was also taken into account by choosing five different industry-specific features. The three duplex stainless steels revealed excellent corrosion resistance under most of the test conditions. Especially for applications in civil engineering the lean duplex steels offer distinct advantages. The manganese-alloyed 1.4376 and the ferritic steel 1.4509 cannot be recommended as alternative materials as they did not perform satisfactorily.
Pitting corrosion is one of the most common mechanisms of local passivity breakdown on stainless steels. Electrochemical methods have preferentially been applied for the evaluation of pitting corrosion resistance of stainless steels in laboratory. Nevertheless, some of them are not reliable enough and in general the application of electrochemical methods in the field becomes difficult because of a required deep understanding of corrosion phenomena and measurement technology. A new approach for the evaluation of the pitting corrosion susceptibility of stainless Steel surfaces in the laboratory as well as in the field is frequently demanded by representatives of the metalworking industry. Therefore, the KorroPad-test was invented and patented as part of a research project at the BAM Federal Institute for Materials Research and Testing in Germany.
Nine different stainless steel alloys were exposed for 5 years under marine environment and their corrosion behaviour was compared and assessed. The investigation of four different surface finishes for all alloys tested further enabled to consider industry-specific features of the surface finish for the material comparison. The results of the exposure tests yield conclusions regarding the influence of alloy composition, surface finish and exposure duration under marine environment. The three duplex stainless steels revealed excellent corrosion resistance even in case of crevices during the 5 years of exposure under the given exposure conditions. Also the molybdenum-alloyed ferritic steel 1.4521 showed good corrosion resistance comparable to the classical austenitic materials 1.4301 and 1.4404.
Pitting corrosion is one of the most common mechanisms of surface damage on stainless steels. Electrochemical methods have been preferentially applied for the evaluation of the pitting corrosion resistance of stainless steels in the laboratory. Nevertheless, some of them are not reliable enough and in general the application of electrochemical methods in the field becomes difficult because of required deep understanding of corrosive phenomena and measurement technology. Therefore, new approaches for the evaluation of the pitting corrosion susceptibility of stainless steel surfaces in the laboratory as well as in the field are necessary. In the present paper two novel strategies including electrochemical noise measurements under anodic polarization for laboratory testing, and an indicator test to assess the susceptibility of stainless steel surfaces to pitting corrosion in the field are introduced. Experimental results concerning the influence of surface treatments on the pitting corrosion resistance on stainless steels have confirmed that final surface condition has a significant effect on their future pitting corrosion susceptibility. In addition, the pitting corrosion resistance of stainless steel surfaces was observed being specifically dependent on the achieved surface topography and in some cases independent on the roughness parameters of the surface.
Samples of the austenitic stainless steel grade X5CrNi18‐10 (1.4301, AISI 304) were ground industrially with various grinding parameters to study their influence on corrosion resistance. The ability of the mechanically ground surfaces to form a stable passive layer was evaluated by KorroPad test and a modified electrochemical potentiodynamic reactivation test based on a single loop (EPR‐SL). Furthermore, the surfaces were characterized by surface analytical methods. The main influence was determined regarding abrasive belt type. Surfaces mechanically ground with granulate abrasive belts constantly had a lower corrosion resistance than surfaces ground with single‐coated grain. The granulate abrasive belts generated more sensitized surface areas and left formations of welded sample material on the mechanically ground surfaces. A post‐treatment with a nonwoven abrasive proved to be an effective finishing process by which the surface defects and sensitized material got removed and the surfaces regained the expected corrosion resistance.
Investigation of chloride-induced pitting corrosion of steel in concrete with innovative methods
(2016)
The combination of electrochemical and 3D computed tomography (CT) investigations offers the possibility of verifying electrochemical measurements non-destructively. Determining the steel surfaces damaged by pitting corrosion allows developing specimens having damage pattern corresponding to practice. Corrosion phenomena like decoupled pitting corrosion cannot be verified by electrochemical measurements only, but with the combination of those two kinds of investigations it is possible. Another advantage is the minimization of the number of samples. The classical experimental procedure requires the destruction of samples after each damage step.This can be avoided by the use of 3D computed tomography. As long as the corrosion phenomena are completely within the examination zone shown by the 3D computed tomography, the electrochemical measurements can be calculated on an area basis to get the specific corrosion rate or polarization resistance.
The presented investigations have proved the principal suitability of the KorroPad method to assess the passive layer stability of stainless steels. The electrochemical mode of action could be described in detail and limitations of the applicability have been demonstrated. The influence of different surface finishes has been investigated and verified by known methods for describing corrosion resistances. As a result, the increased corrosion susceptibility of two ground surfaces has been detected, but also the corrosion resistance of further surface finishes could be confirmed.