Effect of phase distributions on the corrosion of quenched and self-tempered (QST) steel rebars

Author(s): Sooraj A.O. Nair, Radhakrishna G. Pillai
Paper category: Proceedings
Book title: Proceedings of the International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Electrochemistry in Civil Engineering
Editor(s): Lisbeth M. Ottosen
ISBN: 978-2-35158-176-6
e-ISBN: 978-2-35158-177-3
Publisher: RILEM Publications SARL
Publication year: 2016
Pages: 110-118
Total Pages: 9
Language : English

Abstract: Quenched and Self-Tempered (QST) steel reinforcing bar (rebar) has a composite phase microstructure with ductile core of ‘ferrite-pearlite (FP)’ and hard peripheral ring of ‘tempered martensite (TM)’. Typically, a good quality QST rebar will have a peripheral TM ring with uniform thickness and constituting about 25% of the cross-sectional area (CSA). This continuous TM microstructure at the periphery is expected to provide good corrosion resistance. However, a separate two-year laboratory study observed higher than expected rates of chloride-induced corrosion for QST rebars. This paper presents the research performed to find the reason behind this high corrosion rates of QST rebars. The variations in the distribution of TM and FP microstructure of rebars (8, 12, and 16 mm diameter) collected from India, Germany, Hungary, Italy and Russia were studied. Etching of rebar cross-sections using 5% Nital solution, revealed the discontinuous peripheral TM ring and the presence of exposed FP at the periphery of some rebars. Then, the possibility of corrosion-cell formations at these peripheral TM-FP interfaces was investigated. Five small coupon specimens each of FP and TM phases were extracted from the rebar and metallographic specimens were prepared. Then, a series of Open Circuit Potential (OCP) and Linear Polarization Resistance (LPR) tests were performed on these specimens immersed in simulated concrete pore solutions with various chloride concentrations. In general, the FP phase is found to be more vulnerable to chloride-induced corrosion than TM phase – indicating a high probability of localized corrosion in QST steel rebars with incomplete TM ring. Therefore, there is a need for better quality control in the manufacturing to ensure that complete TM ring is formed at the periphery of the QST rebars.

Online publication : 2016
Publication type : full_text
Public price (Euros) : 0.00