42 VOLTAR AO INÍCIO ARTIgO Corros. Prot. Mater., Vol. 32, Nº 2 (2013) CARBONATED AND CHLORIDE CONTAMINATED CONCRETE STRUCTURE: THE ROLE OF MOLYBDENUM IN CORROSION OF STAINLESS STEEL REINFORCEMENT # Artigo submetido em Março de 2013 e aceite em Abril de 2013 A. Araujo (1)(*) , T. J. Mesquita (2) , E. Chauveau (2) , M. Mantel (2) , Z. Panossian (1) , C. A. Santos (1) and R. P. Nogueira (3) (1) CTMM Laboratory of Corrosion and Protection, Institute for Technological Research - IPT, Av. Prof. Almeida Prado 532, 05508-901, São Paulo, Brazil (2) Ugitech Research Center, Avenue Paul girod, 73403 Ugine Cedex France (3) LEPMI - INP grenoble, Rue de la Piscine, 38402 St. Martin d’Hères, France (*) Corresponding author, e-mail: aaraujo@ipt.br Abstract This paper is a contribution to the understanding of the role of molybdenum (Mo) in the pitting corrosion resistance of stainless steels in concrete contaminated with chloride ions after its carbonation. For the study, samples of ferritic, austenitic and duplex stainless steels were produced in laboratory with controlled levels of molybdenum (Mo). Samples of theses steels were immersed in simulated carbonated and noncarbonated concrete pore solutions, both with the addition of 3.5 % of NaCl. Other samples were embedded in concrete that was later carbonated and immersed in NaCl solution. The performance of the steels in concrete was verifed through corrosion potential monitoring (several months) followed by electrochemical experiments (anodic polarization). Finally, the optical and electronic microscopy techniques were used to analyze the corrosion attacked surfaces. Keywords: Corrosion, Concrete Structures, Stainless Steel, Molybdenum, Carbonation, Chloride Ions, Localized Corrosion ESTRUTURAS DE CONCRETO CARBONATADO E CONTAMINADO COM CLORETO: A INFLUÊNCIA DO MOLIBDÊNIO NA CORROSÃO DE ARMADURA DE AÇO INOXIDÁVEL Resumo Este trabalho é uma contribuição ao estudo da infuência do molibdênio (Mo) na resistência à corrosão por pite de aços inoxidáveis em concreto contaminado por íons cloreto, após a sua carbonatação. Para o estudo, amostras de aços inoxidáveis ferrítico, austenítico e dúplex foram preparadas em laboratório com níveis controlados de Mo. Amostras desses aços foram imersas em soluções alcalinas simulando água de poros de concreto carbonatado e não carbonatado, ambas com adição de 3,5 % de NaCl. Outras amostras foram embutidas em concreto, posteriormente carbonatado e imerso em solução de NaCl. O desempenho dos aços foi verifcado pela monitorização do potencial de corrosão (vários meses), seguido de ensaios eletroquímicos como de polarização anodica. Finalmente, os microscópios óptico e eletrônico foram usados para analisar as superfícies das armaduras atacadas pela corrosão. Palavras Chave: Corrosão, Estruturas de Concreto, Aço Inoxidável, Molibdenio, Carbonatação, Íons Cloreto, Corrosão Localizada 1. INTRODUCTION A large part of the world-wide transport and energy infrastructures uses reinforced concrete. Its durability is due to the excellent chemical stability of hydrated Portland cement and the passivity of carbon steel in the alkaline pore solution of concrete with a pH from 10 to 13.5. Corrosion of the reinforcement steels, induced by chloride ions penetrating into the concrete, is the main cause of early damage, loss of serviceability and safety of reinforced concrete structures [1, 2]. In order to prevent reinforcement steel corrosion in highly aggressive environments, the use of stainless steels (SS) reinforcing bars is becoming increasingly popular as they present much higher pitting corrosion resistance in concrete than carbon steel reinforcing bars. Furthermore, higher chloride content is required to initiate the localized corrosion and to depassivate the stainless steels surface [3]. The localized corrosion (pitting and crevice) and generalized attack of the steels are associated with chloride ions and pH lowering due to carbonation processes, respectively [4]. Thus, when the reinforced concrete structure is exposed to chloride contaminated environments and a fast carbonation process; the stainless steel reinforcing bars are probably a good choice to avoid several corrosion problems. In this context, the improvement of the SS corrosion resistance is the major priority, therefore the molybdenum (Mo) as an alloy element is a suitable metal to play this role. It has long been known that Mo additions in SS improve their corrosion resistance and also in favor of an easier repassivation [5-8]. Although there are several studies about electrochemical behavior of Mo and its role to increase the corrosion properties of the SS in acidic media, earlier studies showed an absence of the benefcial effect of Mo on pitting corrosion resistance of austenitic SS in alkaline solutions [1]. The studies of Ilevbare et al. [9] explain the action of Mo principally on nucleation and metastable pitting corrosion of austenitic SS. As these steps of pitting corrosion constitute the early stage of a pit growing, the probability associated with the successful generation of these events will affect the onset of stable pitting. According to Pardo et al. [6], the Mo addition enhances the pitting corrosion resistance of austenitic SS in 3.5 w % NaCl, because this element progressively reduces the corrosion rate, increasing the CPT (Critical Pitting Temperature) values and ennobling both the break potential (E pit ) and the corrosion potential (E corr ). Moreover, these authors believe that Mo promotes an important role on the repassivation process and, consequently, decreases the pitting propagation by formation of Mo oxides on the pit walls. These results are in agreement with the studies done by Lemaitre et al. [10] which describe the benefcial effect of the Mo addition on both in ferritic and austenitic SS regarding the resistance to local breakdown of the passive flm in neutral solutions. On the other hand, in alkaline medium, the Mo addition seems to have no positive effect on austenitic SS [1, 11, 12]. However, the electrochemical experiments done in solutions simulating the concrete environments are not really representative of the real conditions. Therefore, a method to study the corrosion properties of stainless steels in contaminated concrete environments is being developed by the European Community. Under this context, the aim of the present paper is to characterize the performance of the different stainless steel classes (austenitic, ferritic and mainly duplex) in a carbonated and chloride-contaminated concrete. Several-month surveys of corrosion potential followed by electrochemical experiments as well as microscopy analyses were done. Finally, the roles of the Mo addition on these different SS types were investigated in those aggressive corrosion conditions. 42-49 # Paper presented at the EUROCORR 2012.