1,721,014 research outputs found
Measuring corrosion rates: a novel AC method based on processing and analysing signals recorded in the time domain
No full textIn this work, a novel method for the determination of instantaneous corrosion rates icorr is proposed. It relies on recording the current response in the time domain upon imposing an AC voltage signal, and a computational procedure to remove non-faradaic currents and to analyse the data. The method was applied to steel in stirred, unstirred, and sand-filled 1 M KCl; the resulting icorr compared well with different conventional techniques. The proposed method is robust, non-destructive and fast. In addition to icorr, it provides also an estimate of the capacitance and Tafel slopes (thus, no assumption on constant B is required)
A critical analysis of proposed test methods for measuring the chloride threshold value in reinforced concrete
No full textThe chloride content at which reinforcement corrosion initiates in concrete is usually referred to as “chloride threshold value” or “critical chloride content” (Ccrit). As has been shown in a recent review [1], many aspects of chloride induced reinforcement corrosion in concrete are still incompletely understood and huge differences from virtually 0 to more than 3% chloride by cement weight are reported for Ccrit in the literature. There are various reasons for this, such as experimental inaccuracies (limited measurement precision, sampling methods, etc.) or localized corrosion initiation being a stochastic phenomenon. Nevertheless, also the numerous types of experimental procedures and setups being used by different laboratories are considered a major factor contributing to the scatter of literature values for Ccrit. This involves parameters such as specimen geometry, specimen preparation, concrete mix design, curing, methods to accelerate chloride ingress, electrochemical methods to detect corrosion initiation, sampling procedures, etc. This situation led in 2009 to the formation of a RILEM Technical Committee (TC 235-CTC) with the aim of proposing a realistic and feasible test procedure. Preliminary results of a round-robin test carried out with a test procedure selected on the basis of a careful state of the art report revealed that a few more challenges need to be addressed.
Thus an additional effort has to be made in developing a standardized test method for the determination of Ccrit. The outcome of the mentioned RILEM Technical Committee will provide a sound basis for this. It may be combined with new insights gained over the last years such as probabilistic aspects related to specimen size [2] or challenges with corrosion initiation detection with certain binders such as slag, e.g. Ref. [3]. Also the currently running research project funded by the Swiss Federal Roads Office that aims at determining Ccrit in samples taken from field structures (rather than laboratory made samples) will provide useful experience.
In this paper, advantages and disadvantages of the numerous test methods for measuring Ccrit proposed in the literature are discussed e.g. with respect to practice-relationship, experimental time / acceleration of real-world processes, applicability to different cement types, and relevance of the obtained results. The discussion also relates this to the different objectives that determining Ccrit may have, viz. if input parameters for service life modeling are the desired output or if the method is applied for comparative purposes only, e.g. for ranking materials (steel types, binders, etc.)
The size effect in corrosion greatly influences the predicted life span of concrete infrastructures
No full textChloride-induced corrosion of reinforcing steel in concrete is the main cause for premature degradation of concrete infrastructures worldwide. Since mid of the last century, the conceptual approach to tackle this challenge in science and engineering is based on a threshold chloride concentration (Ccrit) for corrosion initiation. Here, we present an experiment that shows that Ccrit depends strongly on the exposed steel surface area. The smaller the tested specimen, the higher and the more variable becomes Ccrit. This size effect can be explained by the local conditions at the steel-concrete interface, which exhibit pronounced spatial variability. The size effect has major implications for the future use of the concept of Ccrit. It questions the reproducibility of typically small-scale laboratory testing and the applicability of laboratory results to engineering structures. We show that the weakest link theory is suitable to transform Ccrit from small to large dimensions, which lays the basis for taking into account the size effect in science and engineering of corrosion of infrastructures
Potenzialmessung an Stahlbetonbauten
No full textDie Potenzialmessung geniesst als nahezu zerstörungsfreie Methode zur Erfassung des Korrosionszustands von Stahlbetonbauten in der Baupraxis einen hohen Stellenwert. Ein neues SIA-Merkblatt fasst nicht nur Messmethoden und -auswertung zusammen, sondern verpflichtet Auftraggeber, eine zertifizierte «Fachperson Potenzialmessung» beizuziehen
On the applicability of the Wenner method for resistivity measurements of concrete
No full textConcrete resistivity measurements are favored for durability assessments due to the relatively simple measurement procedure and the documented correlations between concrete resistivity and transport properties, as well as reinforcement steel corrosion rates. For the measurement, it is common to use a four-point probe with four equally spaced electrodes (spacing distance a) and convert the measured resistance R into apparent resistivity with the so-called Wenner expression, rho(app) = 2 pi aR. By means of experimental measurements and model simulations, the present study confirmed that the application of this formula is adequate, even if most of the assumptions underlying its derivation are violated. The reasons for this are systematically discussed. The experimental results also indicate that measurements with common four-point setups overestimate the resistivity with respect to bulk resistivity measurements. This is likely due to the presence of coarse aggregates that distort the current field at the sense electrodes
Forecasting chloride-induced reinforcement corrosion in concrete – effect of realistic reinforcement steel surface conditions
No full textThis paper presents experimental results on the effect of realistic steel surface conditions on the initiation of chloride-induced corrosion. Ten series of “as-received” reinforcement steel originating from different countries and having been subject to different exposure histories (storage, transport, handling, etc.) were studied. There were clear differences in visual appearance, which were reflected in electrochemical characterization (cyclic voltammetry). Metallographic analyses also indicated differences in terms of steel mi-crostructure. Neither the visual appearance nor the amount of initially present rust permitted any prognosis of resistance to chloride-induced corrosion. On the other hand, martensitic steels appeared to yield lower chlo-ride threshold values than ferritic-pearlitic microstructures.
The results illustrate the difficulties that realistic conditions cause for the concept of the critical chloride content. Given the low degree of confidence with which corrosion initiation can be predicted, the feasibility of the concept of the critical chloride content is discussed
Electrochemistry and capillary condensation theory reveal the mechanism of corrosion in dense porous media
Full text linkCorrosion in carbonated concrete is an example of corrosion in dense porous media of tremendous socio-economic and scientific relevance. The widespread research endeavors to develop novel, environmentally friendly cements raise questions regarding their ability to protect the embedded steel from corrosion. Here, we propose a fundamentally new approach to explain the scientific mechanism of corrosion kinetics in dense porous media. The main strength of our model lies in its simplicity and in combining the capillary condensation theory with electrochemistry. This reveals that capillary condensation in the pore structure defines the electrochemically active steel surface, whose variability upon changes in exposure relative humidity is accountable for the wide variability in measured corrosion rates. We performed experiments that quantify this effect and find good agreement with the theory. Our findings are essential to devise predictive models for the corrosion performance, needed to guarantee the safety and sustainability of traditional and future cements
Local electrochemistry of reinforcement steel - Distribution of open circuit and pitting potentials on steels with different surface condition
No full textLocal electrochemical measurements (open circuit potential (OCP), cathodic and anodic polarization curves, pitting potentials) were performed at random locations on 1 m long ribbed steel bars with different surface conditions. The results show that the local OCP values are statistically distributed. The standard deviation was found to be 45 mV for “as received” surface condition, decreasing to 10 mV only after prolonged immersion in sat. Ca(OH)2. A model based on local variations of the reduction current density could explain the variations in local OCP and allowed predicting the overall global OCP of the long bars when completely immersed in solution
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