Corrosion characteristics of vanadium micro-alloyed steel reinforcement bars exposed in concrete environments and industrially polluted atmosphere

Abstract

Corrosion resistance performance of rebars rolled from vanadium micro-alloyed (VMA) steel was evaluated vis-à-vis rebars rolled from steel having similar chemistry to the (VMA) steel without vanadium and subjected to thermomechanical treatment after hot rolling (TMT). Three test environments namely simulated concrete pore solution, mortars, and industrially polluted atmosphere were used to assess the performance of these rebars. Mass loss, electrochemical impedance spectroscopy, DC polarization and potential-time studies were employed to acquire quantitative data and explain the observed results. Raman spectroscopy was used to identify the corrosion products formed on the surface of the exposed rebars in different environments. The corrosion resistance of VMA rebars was better than TMT especially with increased duration of exposure in chloride contaminated concrete environments. However, under the industrially polluted atmosphere, both rebars exhibited comparative corrosion rates. Lesser detrimental effect of chloride present in concrete environment on the VMA steel rebars is attributed to the higher grain boundary per unit area in these rebars than in the TMT rebars. It is suggested that owing to the stronger tendency of the higher-grain-boundary- material to interact with oxygen and moisture, a thicker film of Fe(OH)₃ was formed on its surface than on the surface of the TMT rebar, prior to their exposure to the test environments. This hydroxide film dissolved in alkaline environment of concrete, nucleated and grew as a protective FeOOH film on the surface of the rebars.

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.