Characterisation of the steel concrete interface submitted to chloride‐induced‐corrosion

This paper deals with the characterisation by means of electrochemical, gravimetric and analytical methods of chloride‐induced‐corrosion behaviour of steel coupons embedded in chloride‐containing‐cement pastes. Corrosion rates were estimated from electrochemical measurements as well as gravimetric ones. They vary from 2.6 to 5.7 µm/year for 5 and 10 g/L chloride‐containing‐cement pastes. Analytical characterisations (including optical and electronical microscopy and Raman micro‐spectroscopy) showed that corrosion patterns are not depending on the chloride content of the cement paste (5 and 10 g/L chloride in the interstitial solution). A localised corrosion pattern composed of pits growing inside the metallic substratum, a corrosion products layer (CPL) and a transformed medium (TM) was pointed out. CPL can be divided into two sub‐layers (CPL1 and CPL2), characterised by the presence or absence of calcium coming from the cement matrix.     

X‐ray micro‐computed tomography analysis of accumulated corrosion products in deep‐water shipwrecks

Accumulated corrosion products from two different shipwrecks which had lain on the seabed (2.5 km depth) for 73 years were systematically analysed by three‐dimensional imaging at high resolution using X‐ray micro‐computed tomography. Complementary surface and chemical characterization experiments were conducted to identify the morphological structure of the corrosion products. Goethite was observed as the main corrosion phase found in both the wreck's corrosion products. However, other corrosion products such as silica, lepidocrocite, maghemite, magnetite, benyacarite, jarosite and amorphous materials were noticed using Fourier transform infrared spectroscopy, Raman spectroscopy and X‐ray diffraction. In addition, mineralized microbial structures were also observed as significant constituents of the corrosion products. However, there were significant differences between samples from the two shipwrecks including porosity, distribution and volume percent of the corrosion products components. The mechanism of different corrosion products formation was proposed and discussed in detail.     

Oxidation of tungstate‐containing green rust (Cl–) in aqueous solution

The influence of tungstate on the oxidation of green rust [GR(Cl^–)], which contains both Fe(II) and Fe(III), was investigated by synthesizing suspensions of GR(Cl^–) containing tungstate and oxidizing them via injection of N₂ gas containing O₂. XRD and TEM analyses were used for characterizing the solid particles formed during synthesis and oxidation. The results showed that the formation of fine α‐FeOOH was enhanced by the addition of tungstate to the GR(Cl^–) suspensions, while GR(Cl^–) without tungstate was transformed primarily into γ‐FeOOH. The pH, oxidation‐reduction potential (ORP), and dissolved oxygen (DO) values of the aqueous solution were measured during oxidation of GR(Cl^–) with and without tungstate. The results showed that whereas the pH value of the solution was decreased and the ORP value was increased monotonically by oxidation of GR(Cl^–), the pH and ORP values during oxidation the GR(Cl^–) suspension containing tungstate revealed characteristic changes with time. XAS was also used for characterizing the chemical state and local structure of tungstate in the oxidized particles. The results indicated that the local structure of WO was essentially retained in the particles precipitated from GR(Cl^–) suspensions.     

Monitoring of corrosion processes in chloride contaminated mortar by electrochemical measurements and X‐ray tomography

Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. Electrochemical methods, e.g., potential mapping, provide information whether the steel reinforcement is still passive or depassivation has been initiated. By applying such techniques no information on the type of corrosion, its extent and distribution of corrosion products is available. Particular the corrosion progress is a significant problem. Especially in the case of macrocell corrosion in reinforced concrete structures, the development at the anode cannot be separated into corrosion damage resulting from macrocell corrosion or self‐corrosion. Until now also in laboratory tests it is impossible to collect such information without destroying specimens after electrochemical testing was performed. To overcome this problem it was tried to study the steel surface within the mortar specimens by X‐ray tomography (CT). Within the scope of these investigations it could be shown, that X‐ray tomography is suitable to make corrosion pits and their development visible which are embedded in a mortar with a cover thickness of about 35 mm. In this publication the time‐dependent corrosion damage of reinforced steel is documented by X‐ray tomography.     

Application of X‐ray tomography for the verification of corrosion processes in chloride contaminated mortar

Within the scope of examinations at steel specimens embedded in chloride contaminated mortar for the first time X‐ray tomography was used to analyse the areas, damaged by chloride induced corrosion. Damaged areas with dimensions of a few μm could be detected. The results from the X‐ray tomography were verified by inspection of surfaces of the bars after removing the cover mortar.     

Performance of galvanized and stainless steel rebars in concrete under macrocell corrosion conditions

In the recent past, the damage caused by rebar corrosion in concrete structures has been considered as one of the major durability problems affecting the service life of concrete structures. In order to prevent corrosion of steel reinforcement, various types of protective methods have been adopted. One of the protective measures is using galvanized reinforcement in concrete to enhance the service life. In the present study various types of galvanized rebars namely bare – Cold Twisted Deformed (CTD), bare – Thermo Mechanically Treated (TMT), galvanized – Cold Twisted Deformed, Galvanized – Thermo Mechanically Treated, galvanized and chromated – Cold Twisted Deformed, galvanized and chromated – Thermo Mechanically Treated and stainless steel rebars have been evaluated for their corrosion resistance in M30 grade concrete under macro cell corrosion condition over a period of one year. From the studies it has been observed that TMT bars performed better when compared to CTD bars. Among the rebars tested, stainless steel rebar has shown negligible corrosion in chloride contaminated concrete.     

Application of agent-based paradigm to model corrosion of steel in concrete environment

This paper explains developing an agent-based computer model to predict the corrosion of steel bars in the high alkaline environment in concrete. Modelling started with specific assumptions about ‘agents’, how they interact and how the population changes over time. A simulation approach, employing agent-based modelling, was formulated and corrosion of steel in the alkaline environment of concrete pore solution, with different chloride concentrations, was simulated. The electrochemical behaviour of steel in a chloride-contaminated concrete pore solution was also experimentally examined. Results of the simulations and the experiments were in good agreement.     

X‐Ray photoelectron spectroscopic investigations of powder coated steel

Polyester powder coatings are used for corrosion protection of different materials. XPS studies were performed in order to characterize such coatings on steel surfaces as a pre‐treatment for following applications like gluing. Different surface treatments by alkaline cleaner, plasma treatment (Plasma Treat® process), and the use of a silicoater (Pyrosil® process) were investigated. In dependence of such treatments, the resulting surfaces are found to be hydrophilic or hydrophobic. Although this behavior can also be investigated by contact‐angle measurements, the aim of our work is the element‐specific characterization of chemical bonding states (speciation).     

Experimental investigation of time dependent non-linear 3D relationship between critical carbonation depth and corrosion of steel in carbonated concrete

Abstract

This paper aims at the experimental investigation of time dependent non-linear relationship between critical carbonation depth and corrosion rate of steel in carbonated concrete by laboratory controlled experimentation under severe environmental condition. In this research, three-dimensional experimental observations are taken consecutively involving carbonation depth, half-cell potential and elapsed time as well as the gravimetric corrosion mass loss. The experimental observations revealed an interesting non-linear relationship between the above said measurements due to the varying resistivity of carbonated concrete. It is also found that the carbonation induced corrosion does not start until the carbonation depth reaches a certain critical level from the steel rebar and the half-cell potential values become constant after carbonation reaches the critical depth and then start rising again after carbonation reaches the rebar level.     

Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion

The durability of reinforced concrete structures becomes a matter of concern, due primarily to the increase of damage by the corrosion of steel reinforcements. This corrosion is not only related to the composition and to the procedure of concrete manufacturing (water/cement, sand/cement, etc.), but also to the aggressive agents as chlorides, carbon dioxide, etc. present in the surrounding medium (Cl⁻, CO₂, etc.). It is well known that the first kind of rebar corrosion (chloride) is more detrimental and that this process contains three basic components: chloride diffusion, electrochemical corrosion and concrete fracture. Therefore the early detection of possible degradation of structures by means of non-destructive testing is essential in order to ensure the functionality of these structures.

This paper presents the results of an experimental investigation on the use of acoustic emission during the corrosion of steel rebars embedded in mortar and immersed in sodium chloride solution. The process of corrosion is accelerated by various imposed potentials and is followed by acoustic emission coupled to electrochemical techniques. The experimental results show that electrochemical techniques can evaluate the corrosive character of the medium used. The acoustic emission showed an activity characteristic of the corrosion initiation phase and the corrosion propagation phase. Thus, it was significantly possible to highlight the acoustic signature of the concrete damage related to the porosity of the mortar and to chloride concentration. The results also show a perfect correlation between the evolution of the acoustic activity and the current of corrosion density.     

Characterisation and comparison of corrosion products originated in steel pipelines transporting sour gas and crude oil

In this work, X-ray diffraction and Mössbauer spectroscopy techniques were used to characterise and carry out a comparison of the corrosion products obtained during the pigging activities in two sour-hydrocarbon-transporting pipelines located in the Gulf of Mexico: the first one being a gas pipeline and the other one a crude oil pipeline. The results indicate that for these specific conditions, there are differences between the corrosion products formed in each pipeline. Iron sulphides and oxides were the main corrosion products and their presence is directly related to the operating conditions prevailing in the pipelines as well as to the hydrocarbon quality. As for the sour gas pipeline, higher concentrations of greigite and pyrite were observed, whereas more proportions of magnetite, mackinawite and marcasite were found in the sour crude oil pipeline. The most important parameters in the formation of the different types of corrosion products are the water content, hydrogen sulphide concentration and oxygen presence in the system.     

Quantification of Tafel coefficients according to passive/active state of steel carbonation‐induced corrosion in concrete

Statistical quantification of Tafel coefficients is investigated in this study for isolated steel rebar embedded in concrete. The survey is supported by a wide experimental campaign carried out earlier to characterize the passive and active states of carbonation‐induced corrosion of steel. Electrochemical measurements (polarization resistance, corrosion potential, Tafel coefficients) and gravimetric estimations of iron loss were regularly conducted over 417 days on 108 concrete specimens. The statistical analysis reveals that the mean value of Tafel coefficients, both cathodic and anodic, is higher under active corrosion, which seems to contradict the general tendency found for chloride‐induced corrosion, while their coefficient of variation is smaller. The statistical inference was based on the first step of distributions fitting the experimental data and then on the second step of goodness‐of‐fit tests. The most suitable of the distributions proposed were the Burr, Rayleigh, and Gamma distributions. A similar analysis was made for the corrosion potential and polarization resistance. The findings of the study will be valuable for probabilistic approaches to corrosion where probabilistic distributions are required.     

Influence of a biopolymer admixture on corrosion behaviour of steel rebars in concrete

Among the multitude of concrete structure pathologies, corrosion of rebars is one of the most important problems of concrete durability. In the context of sustainable development, it appears of primary importance to develop new means to protect the rebars against corrosion. This study aims to develop a new eco‐friendly and corrosion‐inhibiting admixture based on EPS 180 exopolysaccharides, biopolymers used in coatings already studied for the corrosion inhibition on steel in seawater. C15 rebars embedded in CEMI and CEMV cement paste containing EPS 180 were immersed in natural seawater and their electrochemical behaviour was studied using open circuit potential measurements and electrochemical impedance spectroscopy. These tests highlight the decrease of the cathodic reaction kinetics due to the EPS 180 action at the rebars surface, and the absence of effect on the passive layer. Capillary imbibition tests carried out on cement paste and mortars showed that although limiting the imbibition kinetics for cement pastes, the EPS 180 did not influence the water imbibition of mortars. Tests comparing capillary imbibition of soaked cement pastes and mortars with EPS 180 solution and the same samples containing the EPS 180 admixture highlight that the corrosion inhibition induced by EPS 180 admixture is more due to the modification of the cement – rebars interface than to the clogging of the cement porous network.