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Paper des Monats
- ja (1)
Five stainless steel alloys were tested and compared to find a test method for the qualification of stainless steel used in drinking water systems. Determination of the pitting/critical potentials from the conventional cyclic polarization results was shown to be difficult when stainless steels have high Mo and Cr content. The electrochemical impedance data indicated that results received with the conventional methods do not necessarily reflect the steady state. The low frequency time constant determined from the impedance measurements showed that the anodic polarization should be conducted with very slow scan rate to get information about the mechanism. The steady state investigations were compared with the conventional electrochemical results to propose an electrochemical-based route as a standard qualification method. The qualification method was tested to be valid by a European round-robin-test.
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
CO2-corrosion of injection pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers (CCS). The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, is demonstrated in laboratory experiments under an equivalent corrosive environment (T=60 °C,p=1-60 bar, aquifer water, CO2-flow rate of 3 l/h, 700 h8000 h heat treatment). Corrosion kinetics and microstructures were characterized using samples of the heat treated steel 1.7225 (AISI 4140, 42CrMo4) used for casing, and samples of the martensitic stainless injection-pipe steel 1.4034 (AISI 420, X46Cr13).
In engineering storage sites for Carbon Sequestration the CO2-induced corrosion of injection pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers. The reliability of the steels suitable for the geological onshore CCS-site (Carbon Capture and Storage) at Ketzin, Germany, is demonstrated in laboratory experiments in equivalent corrosive environment (T = 60 °C, p = 160 bar, aquifer water, CO2-flow rate of 3 L/h, 7008000 h exposure time). Corrosion kinetics and microstructures were characterized using specimens of the heat treated steel 42CrMo4 (1.7225) used for casing, and specimens of the martensitic stainless injection-pipe steel X46Cr13 (1.4034).
Although the complex changes at the steel-concrete interface due to cathodic polarisation are widely acknowledged to have a beneficial influence concerning the cathodic protection (CP) of steel in concrete, some questions concerning the repassivation of carbon steel in consequence of cathodic polarisation are still not satisfactorily clarified. In the recent literature, some indications are presented that repassivation occurs after a certain time of polarisation. Therefore, the investigations discussed in this paper aim to clarify, to what extent the re-passivation of carbon steel due to cathodic polarisation occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step, the corrosion state of five nominal equal test specimens was determined by electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarisation resistance, respectively, the specimens were polarised cathodically. Impedance data were recorded before, during and after polarisation. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and the impact of diffusion on the corrosion and polarisation behaviour. The results indicate that the reduction of oxides and oxygen diffusion during cathodic polarisation has strong impact on the systems behaviour and that repassivation effects occur after switching off the polarisation current and during depolarisation, respectively.
BETOSCAN - An instrumented mobile robot system for the diagnosis of reinforced concrete floors
(2009)
Glass bricks are important transparent building materials. They are produced by joining two halves of glass pressings at 600700 °C. During this production process alkali oxides evaporate and are redeposited at the cooler inner front surfaces of the bricks. This surface layer reacts with H2O and CO2 from the residual brick atmosphere, leading to the formation of an alkali-rich silicate-hydrate layer of ≥50 nm thickness, which could be evidenced leading to a reduced nano-hardness of similar thickness, and from which NaHCO3 crystals can finally grow. Climate chamber experiments (repeated cooling between at -8 and -14 °C and reheating to 0 to 15 °C) resulted in reversible NaHCO3 crystallization and redissolution, presumably influenced by water evaporation or condensation and driven by the NaHCO3 supersaturation of the silicate-hydrate layer. Depending on the timetemperature schedule, different crystal morphologies became visible in this closed system, e.g. isolated spherical crystals, crystals arranged in chains and in double-chains, respectively, which can limit already the transmittance of the glass bricks. When a crack occurs or the brick is opened, the hygroscopic NaHCO3 crystals take up more H2O from the ambient, react irreversibly with the glass surface, finally leading to a total loss of transmittance.
Electrochemical noise measurements on unalloyed steel in chloride-containing alkaline environment
(2007)
The investigations discussed in this paper aim to clarify whether electrochemical noise measurements (ENM) are capable for application in concrete and mortar, particularly with regard to pitting initiation in such environment. For these purposes, the results of several different test series are compared and discussed. Based on investigations on passivity and pitting corrosion in alkaline solutions with different chloride contents further results in cement paste, mortar and concrete are evaluated.
About the long-term protection behaviour of corrosion protection materials (filling materials) for prestressed systems under critical environmental conditions is only little known. The corrosion protection effect is usually based on theoretical considerations and is proven by short-term tests. The selection of the different products is mainly made according to economical or workability criteria. In a research project the barrier effect of different commercial corrosion protection materials (waxes as well as oil-based greases) against water, their tendency for undercutting as well as their additional corrosion protection effect were investigated. The exposure tests were carried out with non-stressed as well as stressed prestressing steel specimens which were subjected to critical conditions (condensed water, artificial soil solution, direct soil contact). Parallel to these long-term exposure tests the applicability of different electrochemical techniques and their significance with respect to testing the corrosion protection ability and water absorption was evaluated. Within the project a suitable method for simple testing the performance of corrosion protection materials under real conditions was developed. By means of a small compact cell submicroscopical reactions of the used sensors could be measured. The high sensitivity of this measuring technique enables the detection of degradation processes at thin protection layers.
Im Rahmen einer Bauwerksuntersuchung an einer in nachträglicher Verbundbauweise hergestellten Brücke wurden Anrisse an Spannstählen der Quervorspannung festgestellt, ohne daß Verpreßmängel oder korrosionsfördernde Substanzen vorlagen. Bei dem verwendeten Werkstoff handelt es sich um einen in der ehemaligen DDR hergestellten vergüteten Stahl der Festigkeitsklasse St 140/160. Ursache für die Anrisse ist die in Laborversuchen nachgewiesene Empfindlichkeit dieses Stahles gegenüber wasserstoffinduzierter Spannungsrißkorrosion, die unter ungünstigen Bedingungen dazu führen kann, daß Vorschädigungen vor dem Verpressen eintreten. Zusätzlich durchgeführte Magnetpulverprüfungen an ausgewählten Bereichen der Längsvorspannung ergaben keinerlei Hinweise auf Anrisse.
Translated Abstract
Stress corrosion cracking of prestressing steels
During the investigation of a post-tensioned bridge structure incipient cracks of the prestressing steels of the transverse prestressed members were observed. Defects related to non-injected ducts or the presence of corrosion inducing substances could not be detected. The prestressing steel used is a quenched and tempered steel, strength class St 140/160, which was produced in the former GDR. The cause for the cracks is the susceptibility of this type of steel to hydrogen-induced stress corrosion cracking as could be shown in laboratory tests. Under unfavourable conditions cracks can be initiated before grouting. Additional magnetic particle tests at selected areas of the longitudinal prestressed members did not indicate any signs for incipient cracks.
Investigations on hydrogen-induced embrittlement of quenched and tempered prestressing steels
(2000)
In Germany in recent years failures of more than 30-year-old prestressed concrete structures have been observed. In all these cases a quenched and tempered steel type, strength class St. 1420/1570 was used. In order to assess the corrosion risk, i.e. crack initiation and propagation, under depassivating conditions lifetime tests in environments relevant for building practice were carried out using different prestressing steels of this quenched and tempered type of material.
From the results it can be concluded that the specific susceptibility of certain prestressing steels is the determining factor with respect to the occurrence of cracks rather than environmental factors.
Translated Abstract
Untersuchungen zum wasserstoffinduzierten Sprödbruch vergüteter Spannstähle
In den letzten Jahren ist es verschiedentlich zu Schadensfällen von Spannbetonbauteilen gekommen, bei denen vergüteter Spannstahl der Festigkeitsklasse St 1420/1570 eingesetzt war. Durch systematische Untersuchungen an verschiedenen Typen dieser Stahlsorte sollte geklärt werden, ob unter depassivierenden Bedingungen Rißeinleitung und -ausbreitung unter praxisnahen Verhältnissen zu erwarten ist.
Die durchgeführten Untersuchungen lassen folgern, daß die spezifische Anfälligkeit des Spannstahls für das Auftreten von Rissen einen wesentlich höheren Einfluß besitzt als die elektrolytseitigen Parameter, da sich die verschiedenen Stähle auch bei vergleichbaren Korrosionsangriffen unterschiedlich verhalten.