Corrosion characteristics of vanadium micro-alloyed steel reinforcement bars exposed in concrete environments and industrially polluted atmosphere
Introduction
Use of steel reinforcement bars (rebars) in construction of concrete infrastructures has revolutionized the construction sector technically as well as commercially. Since the accidental discovery by Joseph Monier [1] regarding a considerable improvement in the strength of his iron-nail-embedded Portland cement cast flowerpots, phenomenal improvements had taken place on the shape, size, strength, ductility, and chemistry of steel rebars. Imparting tensile strength and ductility to brittle concrete had been the main goal of embedding rebars in concrete. Iron metal without alloying elements, especially carbon, is extremely weak. Initially, to meet the demand of high strength and toughness for the construction of bridges and high-rise concrete buildings, carbon was added in concentrations higher than 0.35% to steels used to roll rebars. However, this high carbon concentration posed a major challenge to achieving high ductility and meeting the requirement of good weldability. To overcome these problems, thermo-mechanically treated (TMT) rebars having a soft-core pearlite-ferrite microstructure and a tough outer rim of tempered martensite were introduced in the 1970s. To this date, these rebar are widely used in the construction sector. This technology, which is also known as quenched and tempered technique of achieving improved strength and ductility, enabled reduction of the carbon content in steels to the level of 0.15%–0.25%. A new approach to achieve strength and toughness of rebars based on alloying with small amount of elements such as vanadium, was reported in 1970s. Addition of very low concentrations (about 0.1%) of such elements to carbon-manganese steels was found to have considerable positive effects on the strength, ductility, and toughness of the resultant steels. This improvement is attributed to the precipitation hardening and grain reinforcement [[2], [3], [4]]. An exhaustive literature search reveals that many researchers have compared the fatigue properties, atmospheric corrosion, oil-field corrosion, and saline environment degradation of such grain refined steels vis-à-vis normal carbon-manganese steel [[5], [6], [7], [8], [9], [10], [11]]. However, very limited information is available on the corrosion behavior of rebars rolled from such steels exposed to concrete environments. Since these types of rebars are increasingly being used in the construction sector, it is important to understand their corrosion characteristics as well as their performance vis-à-vis TMT rebars, which are currently used in construction on a mass scale. This communication presents the results of systematic long term exposure test results of vanadium micro alloyed steel rebars by exposing them in chloride contaminated concrete pore solution, mortars and industrially polluted environments.
Section snippets
Test materials
Hot rolled VMA and TMT steel rebars 12 mm in diameter, manufactured in an integrated steel plant, were used in the present investigation. The chemical compositions of the steel rebars determined by an inductively coupled plasma optical emission spectrometer are presented in Table 1. The microstructure of the cross section of rebars polished and NITAL etched surface and examined by optical microscope are given in Fig. 1.
Test environments
The simulated concrete pore solution (SPS) representing concrete with high
Microstructure
Optical microphotographs of the cross sections of the VMA steel rebar and the outer tempered-martensite rim of the TMT rebar are shown in Fig. 1. The microphotograph of the VMA rebar reveals uniformly distributed ferrite and pearlite in its microstructure. The white structures in the microphotograph of the VMA rebar are ferrite, whereas the dark portion is pearlite. Image analysis revealed a ferrite content of 39.8% in the microstructure of this rebar. This value is closer to previously
Discussion
The comparative studies of the corrosion resistance performances of the rebars produced by two approaches (thermomechanical treatment and addition of small content of vanadium in steel) suggest that the rebars produced by the VMA approach have an edge over those produced by thermomechanical treatment. The corrosion resistance of metals is controlled by the protective passive film formed on their surfaces in a particular environment. The nature of the film is controlled by the composition of the
Conclusions
The VMA micro-alloyed steel rebars exhibit better corrosion resistance than the TMT rebars in all the tested environments, i.e. simulated concrete pore solution, mortars, and an industrial atmosphere. All of the obtained evidences indicate that this enhanced performance of the VMA rebars is a result of the availability of higher concentrations of iron cations during the initial nucleation and growth of the passive film on the rebar surface. Raman spectroscopy of the corrosion products formed on
Declaration of competing interest
There is no conflict of Interest.
Acknowledgement
This Project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (15-BUI4983-02).
Raja Rizwan Hussain: Associate Professor, He received his Ph.D. and M.Sc. in Civil Engineering from the University of Tokyo, Japan in 2008 & 2006, for which he was ranked as outstanding and was awarded the best research thesis prize and medal from the University of Tokyo. He also completed his Ph.D. in a record short period of just 2 years from a university ranked among top 20 in the world. He has authored more than 225 publications with reputed publishers in the past decade of his post Ph.D.
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Raja Rizwan Hussain: Associate Professor, He received his Ph.D. and M.Sc. in Civil Engineering from the University of Tokyo, Japan in 2008 & 2006, for which he was ranked as outstanding and was awarded the best research thesis prize and medal from the University of Tokyo. He also completed his Ph.D. in a record short period of just 2 years from a university ranked among top 20 in the world. He has authored more than 225 publications with reputed publishers in the past decade of his post Ph.D. tenure and has received several awards, prizes and distinctions throughout his research and academic career. His research interest is in the corrosion of steel reinforced concrete.
Abdulaziz Al-Negheimish: Professor, He received his BS and MS from the University of Michigan, Ann Arbor, MI, and his PhD from the University of Texas at Austin, Austin, TX, in 1988. His research interests include corrosion of steel reinforced concrete, quality scheme for ready mixed concrete and hot-weather concreting.
Abdulrahman Alhozaimy: Professor, He received his PhD from Michigan State University, East Lansing, MI, in concrete materials and technology in 1993. His research interests include the durability and sustainability of concrete materials, especially corrosion.
D.D.N.Singh: Ex-Chief Scientist and Professor, Dr. Singh obtained his PhD from Banaras Hindu University (BHU), India, in 1980. His research interests lie in concrete reinforcement corrosion and control, coatings, inhibitors etc.