M.C. Alonso

Electrochemical response of natural and induced passivation of high strength duplex stainless steels in alkaline media

The passivation of two high strength duplex stainless steels (HSSS) was investigated in alkaline solutions simulating the pore solution of concrete by the growth of natural and induced passive films. Induced passive films were generated both by cyclic voltammetry and by chronoamperometry. Natural passive films were spontaneously grown by the immersion of the steel in the alkaline electrolyte. These passive layers were characterised by electrochemical impedance spectroscopy, corrosion current density (icorr) and corrosion potential (Ecorr) monitoring. The effect of significant parameters, such as the pH in the HSSS/alkaline solution interface, the composition of the duplex stainless steels and the ageing of the passive layer, on the electrochemical performance of both induced and spontaneously grown passive films has been analysed. The increase of alkalinity highly influences the electrochemical performance of the passive film by promoting the formation of a passive layer with a less resistant electrochemical response. The electrochemical behaviour of the passive layer is also affected by the alloying elements like Mo or Ni. Both natural and induced passive films show similar electrochemical trend with respect to significant parameters such as the pH and the composition of the steel. The ageing of the spontaneously grown passive layer promotes a higher resistive electrochemical response which might be related to the enrichment of the passive layer in non-conducting (or semi-conducting) oxides.

Study of the Microstructure Evolution of Low-pH Cements Based on Ordinary Portland Cement (OPC) by Mid- and Near-Infrared Spectroscopy, and Their Influence on Corrosion of Steel Reinforcement

Low-pH cements are designed to be used in underground repositories for high level waste. When they are based on Ordinary Portland Cements (OPC), high mineral admixture contents must be used which significantly modify their microstructure properties and performance. This paper evaluates the microstructure evolution of low-pH cement pastes based on OPC plus silica fume and/or fly ashes, using Mid-Infrared and Near-Infrared spectroscopy to detect cement pastes mainly composed of high polymerized C-A-S-H gels with low C/S ratios. In addition, the lower pore solution pH of these special cementitious materials have been monitored with embedded metallic sensors. Besides, as the use of reinforced concrete can be required in underground repositories, the influence of low-pH cementitious materials on steel reinforcement corrosion was analysed. Due to their lower pore solution pH and their different pore solution chemical composition a clear influence on steel reinforcement corrosion was detected.

Development and application of low-pH concretes for structural purposes in geological repository systems

Abstract: Several construction concepts of underground repositories for high-level waste (HLW) consider the use of low pH cementitious materials and different aspects related to their development are exposed in the present chapter. The general functional requirements and the special characteristics implied in the development and production of low-pH concretes are described, mainly focused on the design of a specific application: a low-pH concrete plug using the shotcrete technique. Considering the long life expected in HLW repositories, parameters related to the durability of the low-pH concretes are analysed, such as the interaction of low-pH cementitious materials with groundwaters and the risk of corrosion if reinforcements are needed.

Pitting corrosion and stress corrosion cracking study in high strength steels in alkaline media

Steel reinforcement in concrete is protected from corrosion by passivation due to the high alkalinity of the cement pore solution, but the presence of aggressive ions as Cl− could induce pitting corrosion processes thus decreasing the durability of the structure. This also occurs for prestressing concretes but the risk to suffer pitting corrosion in presence of aggressive ions, and hence stress corrosion cracking (SCC), increases due to the external mechanical load. The study of the corrosion processes by classical methods involves the determination of pitting potential (Epit) or the polarization resistance monitoring during long periods (icorr). However, the large number of parameters, which could potentially influence the determination of the Epit or the icorr values, result in a great variety of results in literature. The present study proposes a simple and reliable method to evaluate the susceptibility to suffer pitting corrosion process and stress corrosion cracking by the induction of pitting corrosion process by cyclic voltammetry. The pitting corrosion intensity is evaluated by means of the charge during the corrosion process, and related with the weight loss and the decrease of the mechanical properties of the steel under external load. The results show a linear and a volcano trend when pitting corrosion process and stress corrosion cracking are respectively evaluated.

Round Robin Test for Defining an Accurate Protocol to Measure the Pore Fluid pH of Low-pH Cementitious Materials

The present research belongs to an international project where several of the main nuclear waste management agencies have been involved. The main objective is the development of agreed procedures or protocols for measuring the pH value using low-pH cementitious products (LopHC). Pore Fluid Expression (PFE) identified as Reference method and Ex situ Leaching methods (ELS) with two variants (filtering and without filtering the suspension made) identified as Routine methods have been employed. Both methodologies are based on the extraction of the pore solution of the concrete before pH determination. The protocols employed were based on a broad literature review and in fitting the more critical parameters, such as the sample size, the carbonation affection, the leaching of cement hydrates during the measurement, etc. Moreover, the routine methods were validated with respect to the pore fluid expression results. According with the obtained results, the selected measuring methods were proposed. The proposed methodologies show very promising results having low deviations and high reproducibility when applied to LopHC materials that have given the possibility to develop agreed simple protocols for pH determination considering the chemical requirement in the construction of high nuclear waste repositories.

Degradation Reactions in Concretes Exposed to High Temperatures

The aim of this chapter is to present the relevant degradation processes occurring at the micro-level in concrete exposed to high temperatures, with special interest to the differences associated with High Performance Concrete (HPC) and Self Compacted Concretes (SCC) in respect to Ordinary Concrete. The microstructure of concrete can be identified using appropriate techniques sensitive to detect changes in concrete components that occur at micro level. The main items considered in present chapter are: 1. Chemical Degradation reactions 2. Physical degradation processes 3. Thermal changes.