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Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization. The dissolved organic and inorganic carbons in electrolyte solution were quantified by using a photometric method. The structures of the coatings were investigated before and after the electrochemical tests by microscopy and scanning electron microscope/EDX analysis. The results of the electrochemical impedance measurements show that the tested coatings all have a relatively low resistance, which is between 100 and 200 Ω. The binder and the surface porosity influence the degradation behavior of those coatings. Especially the organic binder reacts with the strong alkaline medium under dissolving of organic carbon.
The reduction of the chromate contents in cements affects the electrochemical reactions of galvanized reinforcing steels in fresh concrete. By means of electrochemical measurements and bar pull-out tests the effect of chromates on the formation of protective coatings at the zinc and the bond behaviour have been investigated using chromate-containing and chromate-reduced cements. The results have shown that the bond zone was affected as a result of chromate reduction for cement types CEM I and CEM II. The disturbance to the bond zone should be considered as long-term in relation to the pore structure which is formed. The effects on the bond are largely compensated for as the concrete ages. The results of the related bond strength with displacement paths of 0.1 and 0.5?mm, respectively, are still within the range of expected figures for non-galvanized reinforcing steel despite the structural disturbance. Under practical aspects it is not necessary that the existing structural disturbance with respect to bond strength is taken into consideration.
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.
Steel in concrete is protected by the alkaline pore water environment and the resulting formation of a protective passive layer against corrosion. Adverse environmental conditions due to carbonation or chloride ingress can destroy the passive layer on the steel surface. Corrosion processes starting in those areas lead to uniform corrosion or local corrosion like pitting corrosion. In comparison to uniform corrosion pitting corrosion is a form of increased local corrosion and thus leads to a progressive reduction in cross-section of the reinforcing steel. The corrosion products are first absorbed by the pores of the concrete matrix, without causing visible external changes at the concrete surface.
The corrosion resistance of aluminium surfaces is closely linked to the surfacecstate after a grinding process. For years, iron‐containing abrasive materials were suspected to lead to increased corrosion susceptibility after processing of aluminium surfaces. To prove a possible correlation between the iron content of an abrasive and the corrosion behaviour of aluminium components, scientific investigations and experimentally practical corrosion tests are necessary.
For the current investigation, specimens of a technical Al‐Si alloy from the same batch were used. The test specimens were mechanically ground with various resin‐bonded model abrasives containing different iron contents. The performed corrosion tests did not reveal a negative influence of the different iron‐containing abrasives on the corrosion behaviour of the Al–Si alloy.
However, the most sensitive measuring method (electrochemical noise)
showed differences in the surface activity depending on the type of abrasive.
The salinization and contamination of metal surfaces by chloride-containing aerosols is of great importance with regard to corrosion phenomena of damaged coated metal surfaces and stainless steels in the maritime sector and in offshore applications. Detailed questions have to be answered to clarify whether and to what extent salinization of the surface has an influence on the adhesion and durability of coatings in repairing applications and on pitting occurrence on stainless steels under atmospheric conditions. The questions arise which degree of salinization is reached after which period of time and how a defined and reproducible salinization can be simulated in the laboratory for further systematic investigations. The article is dedicated to these questions. Results of a field trial on the Island of Heligoland are presented and a simple procedure for a defined loading of metal surfaces with chlorides at a laboratory scale using a design of experiments (DoE) approach is introduced.
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.
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.