Chloride induced corrosion of steel in alkali activated slag concretes

Alkali activated slag (AAS) is an alternative cementitious material. Sodium silicate solution is usually used to activate ground granulated blast furnace slag to produce AAS. As a consequence, the pore solution chemistry of AAS differs from that of Portland cement (PC). Although AAS offers many advantages over PC, such as higher strength, superior resistance to acid and sulphate environments and lower embodied carbon due to 100% PC replacement, there is a need to assess its performance against chloride induced corrosion duo to its different pore solution chemistry. For PC systems, resistivity measurement, as a type of nondestructive test, is usually used to evaluate its chloride diffusivity and the corrosion rate of the embedded steel. However, due to the different pore solution chemistry present in the different AAS systems, the application of this test in AAS concretes would be questionable as the resistivity of concrete is highly dependent on its conductivity of the pore solution. Therefore, a study was carried out using twelve AAS concretes mixes, the results of which are reported in this paper. The AAS mixes were designed with alkali concentration of 4%, 6% and 8% (Na₂O% of the mass of slag) and modulus (Ms) of sodium silicate solution of 0.75, 1.00, 1.50 and 2.00. A PC concrete with the same binder content as the AAS concretes was also studied as a reference. The chloride diffusion coefficient was determined using a non-steady state chloride diffusion test (NT BUILD 443). The resistivity of the concretes before the diffusion test was also measured. Macrocell corrosion current (corrosion rate) for steel rods embedded in the concretes was measured whilst subjecting the concretes to a cyclic chloride ponding regime (1 day ponded with salt solution and 6 days drying). The results showed that the AAS concretes had lower chloride diffusivity with associated higher resistivity than the PC concrete. The measured corrosion rate was also lower for the AAS concretes. However, unlike the PC, in which a higher resistivity yields a lower diffusivity and corrosion rate, there was no relationship apparent between the resistivity and either the diffusivity or the corrosion rate of steel for the AAS concretes. This is assigned to the variation of the pore solution composition of the AAS concretes. This also means that resistivity measurements cannot be depended on for assessing the chloride induced corrosion resistance of AAS concretes.

Chloride transport and the resulting corrosion of steel bars in alkali activated slag concretes

As the relative performance of alkali activated slag (AAS) concretes in comparison to Portland cement (PC) counterparts for chloride transport and resulting corrosion of steel bars is not clear, an investigation was carried out and the results are reported in this paper. The effect of alkali concentration and modulus of sodium silicate solution used in AAS was studied. Chloride transport and corrosion properties were assessed with the help of electrical resistivity, non-steady state chloride diffusivity, onset of corrosion, rate of corrosion and pore solution chemistry. It was found that: (i) although chloride content at surface was higher for the AAS concretes, they had lower chloride diffusivity than PC concrete; (ii) pore structure, ionic exchange and interaction effect of hydrates strongly influenced the chloride transport in the AAS concretes; (iii) steel corrosion resistance of the AAS concretes was comparable to that of PC concrete under intermittent chloride ponding regime, with the exception of 6% Na2O and Ms of 1.5; (iv) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (v) the increase of alkali concentration of the activator generally increased the resistance of AAS concretes to chloride transport and reduced its resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the chloride transport and the corrosion resistance.

Monitoring of Corrosion in Structural Reinforcing Bars: performance comparison using in-situ fibre optic and electric wire strain gauge systems

Abstract This work addresses the problems of effective in situ measurement of the initiation or the rate of steel corrosion in reinforced concrete structures through the use of optical fiber sensor systems. By undertaking a series of tests over prolonged periods, coupled with acceleration of corrosion, the performance of fiber Bragg grating-based sensor systems attached to high-tensile steel reinforcement bars (ldquorebarsrdquo), and cast into concrete blocks was determined, and the results compared with those from conventional strain gauges where appropriate. The results show the benefits in the use of optical fiber networks under these circumstances and their ability to deliver data when conventional sensors failed.

KINETICS OF CORROSION OF RUTHENIUM DIOXIDE HYDRATE BY CEIV IONS

The kinetics of oxidative dissolution of RuO2 .xH2O to RuO4 by Ce(iv) ions are studied. Under conditions of a low [Ce(iv)] : [RuO2 .xH2O] ratio (e.g. 0.35 : 1) and a high background concentration of Ce(III) ions (which impede dissolution) the initial reduction of Ce(iv) ions is due to charging of the RuO2 .xH2O microelectrode particles. The initial rate of charging depends directly upon [RuO2 .xH2O] and has an activation energy of 25 +/- 5 kJ mol-1 Under conditions of a high [Ce(iv] : [RuO2 .xH2O] (e.g. 9 : 1) and a low background [Ce(III] the reduction of Ce(iv) ions is almost totally associated with the dissolution of RuO2 .xH2O to RuO4, i.e. not charging. The kinetics of dissolution obey an electrochemical model in which the reduction of Ce(iv) ions and the oxidation of RuO2 .xH2O to RuO4 are assumed to be highly reversible and irreversible processes, respectively, mediated by dissolving the microelectrode particles of RuO2 .xH2O. Assuming this electrochemical model, from an analysis of the kinetics of dissolution the activation energy for this process was estimated to be 39 +/- 5 kJ mol-1 and the Tafel slope for RuO2 .xH2O corrosion was calculated to be 15 mV per decade. The mechanistic implications of these results are discussed.

Susceptibility to stress corrosion cracking of NiTi laser weldment in Hanks’ solution

In this study, the susceptibility to stress corrosion cracking (SCC) of laser-welded NiTi wires in Hanks’ solution at 37.5 °C was studied by the slow strain-rate test (SSRT) at open-circuit potential and at different applied anodic potentials. The weldment shows high susceptibility to SCC when the applied potential is near to the pitting potential of the heat-affected zone (HAZ). The pits formed in the HAZ become sites of crack initiation when stress is applied, and cracks propagate in an intergranular mode under the combined effect of corrosion and stress. In contrast, the base-metal is immune to SCC under similar conditions. The increase in susceptibility to SCC in the weldment could be attributed to the poor corrosion resistance in the coarse-grained HAZ.

Fluorescence based fibre optic pH sensor for the pH 10–13 range suitable for corrosion monitoring in concrete structures

The design, development and evaluation of an optical fibre pH sensor for monitoring pH in the alkaline region are discussed in detail in this paper. The design of this specific pH sensor is based on the pH induced change in fluorescence intensity of a coumarin imidazole dye which is covalently attached to a polymer network and then fixed to the distal end of an optical fibre. The sensor provides a response over a pH range of 10.0–13.2 with an acceptable response rate of around 50 min, having shown a very good stability over a period of longer than 20 months thus far. The sensor has also demonstrated little cross-sensitivity to ionic strength (IS) and also excellent photostability through a series of laboratory tests. These features make this type of sensor potentially well suited for in situ long term monitoring of pH in concrete structures, to enhance structural monitoring in the civil engineering sector

Effect of Post-weld Heat-treatment on the Stress Corrosion Cracking Behaviour of Laser-welded Shape Memory NiTi Wires in Hanks’ Solution

In this study, the stress-corrosion cracking (SCC) behaviour of laser-welded NiTi wires before and after post-weld heat-treatment (PWHT) was investigated. The samples were subjected to slow strain rate testing (SSRT) under tensile loading in Hanks’ solution at 37.5 °C (or 310.5 K) at a constant anodic potential (200 mVSCE). The current density of the samples during the SSRT was captured by a potentiostat, and used as an indicator to determine the susceptibility to SCC. Fractography was analyzed using scanning-electron microscopy (SEM). The experimental results showed that the laser-welded sample after PWHT was immune to the SCC as evidenced by the stable current density throughout the SSRT. This is attributed to the precipitation of fine and coherent nano-sized Ni4Ti3 precipitates in the welded regions (weld zone, WZ and heat-affected zone, HAZ) after PWHT, resulting in (i) enrichment of TiO2 content in the passive film and (ii) higher resistance against the local plastic deformation in the welded regions.

Microabrasion-corrosion of cast CoCrMo alloy in simulated body fluids

Wear and corrosion of metal-on-metal hip replacements results in wear debris and metal-ion release in vivo, which may subsequently cause pain and hypersensitivity for patients. Retrieved metal-on-metal hip replacements have revealed that two-body sliding wear and three-body abrasive wear are the predominant wear mechanisms. However, there is a lack of understanding of the combined effects of wear/corrosion, especially the effect of abrasion-corrosion.

This study investigates the sliding-corrosion and abrasion-corrosion performance of a cast CoCrMo alloy in simulated hip joint environments using a microabrasion rig integrated with an electrochemical cell. Tests have been conducted in 0.9% NaCl, phosphate buffered saline solution, 25% and 50% bovine serum solutions with 0 or 1 g cm(-3) SiC at 37 degrees C. Experimental results reveal that under abrasion-corrosion test conditions, the presence of proteins increased the total specific wear rate. Conversely, electrochemical noise measurements indicated that the average anodic current levels were appreciably lower for the proteinaceous solutions when compared with the inorganic solutions. A severely deformed nanocrystalline layer was identified immediately below the worn surface for both proteinaceous and inorganic solutions. The layer is formed by a recrystallisation process and/or a strain-induced phase transformation that occurs during microabrasion-corrosion. (C) 2008 Elsevier Ltd. All rights reserved.

Influence of micro and macro cracks due to sustained loading on chloride-induced corrosion of reinforced concrete beams

Chloride-induced corrosion of steel is one of the most commonly found problems affecting the durability of reinforced concrete structures in both marine environment and where de-icing salt is used in winter. As the significance of micro-cracks on chloride induced corrosion is not well documented, 24 reinforced concrete beams (4 different mixes - one containing Portland cement and another containing 35% ground granulated blastfurnace slag at 0.45 and 0.65 water-binder ratios) were subjected to three levels of sustained lateral loading (0%, 50% and 100% of the load that can induce 0.1 mm wide cracks on the tension surface of beam - F_0.1) in this work. The beams were then subjected to weekly cycles of wetting with 10% NaCl solution for 1 day followed by 6 days of drying at 20 (±1) °C up to an exposure period of 60 weeks. The progress of corrosion of steel was monitored using half-cell potential apparatus and linear polarisation resistance (LPR) test. These results have shown that macro-cracks (at load F_0.1) and micro-cracks (at 50% of F_0.1) greatly accelerated both the initiation and propagation stages of the corrosion of steel in the concrete beams. Lager crack widths for the F_0.1 load cases caused higher corrosion rates initially, but after about 38 weeks of exposure, there was a decrease in the rate of corrosion. However, such trends could not be found in 50% F _0.1 group of beams. The extent of chloride ingress also was influenced by the load level. These findings suggest that the effect of micro-cracking at lower loads are very important for deciding the service life of reinforced concrete structures in chloride exposure environments. © 2014 4th International Conference on the Durability of Concrete Structures.

Chloride induced corrosion of steel bars in alkali activated slag concretes

The studies on chloride induced corrosion of steel bars in alkali activated slag (AAS) concretes are scarcely reported in the past. In order to make this issue clearer and compare the corrosion performance of AAS with Portland cement (PC) counterpart, an investigation was carried out and the results are reported in this paper. Corrosion properties were assessed with the help of rate of corrosion, electrical resistivity and pore solution chemistry. It was found that: (i) steel corrosion resistance of the AAS concretes was comparable or in some cases even worse than that of Portland cement (PC) concrete under intermittent chloride ponding regime; (ii) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (iii) the increase of alkali concentration of the activator generally reduced chloride resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the corrosion resistance.