J. Fullea

Ab initio molecular dynamics simulation of hydrogen diffusion in alpha -iron

First-principles atomistic molecular-dynamics simulation in the microcanonical and canonical ensembles has been used to study the diffusion of interstitial hydrogen in α-iron. Hydrogen to iron ratios between θ=1/16 and 1/2 have been considered by locating interstitial hydrogen atoms at random positions in a 2×2×2 supercell. We find that the average optimum absorption site and the barrier for diffusion depend on the concentration of interstitials. Iron Debye temperature decreases monotonically for increasing concentration of interstitial hydrogen, proving that iron-iron interatomic potential is significantly weakened in the presence of a large number of diffusing hydrogen atoms

High Strength Steels Fracture Toughness Variation by the Media

The stress corrosion cracking process is at this moment an unknown mechanism of deterioration. It is a process that implies the joint action of the media, the presence of corrosion or a surface defect and of stress in the metal. Prestressing tendons can suffer SCC jointly with hydrogen embrittlement which dramatically changes not only the type of fracture (from ductile to brittle) but also the kinetics of the process leading to unexpected collapses. The metal should be resistant to this type of process which can be characterized by its toughness and therefore by its damage tolerance. This research shows that the Fracture Toughness change when the steel corrodes, questioning the idea that is an intrinsic characteristic of the material. The reduction in the fracture toughness of steel wires when they are in contact to aggressive media involve that the material fractures with a lower crack depth for the same stress level. That means that the material becomes less damage tolerant, which implies that it is necessary to detect defects of smaller size, as for example, small notch, pits or superficial cracks. In the paper some results of the percentage of decrease of the toughness of prestressing wires suffering corrosion are presented.

Hydrogen Embrittlement of High Strength Steels

Hydrogen embrittlement is believed to be one of the main reasons for cracking of structures under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). We compute the interaction of atomic hydrogen with iron using first principles. The interstitial hydrogen can be placed in two high symmetry positions: octahedral and tetrahedral sites. Our calculations provide diffusion barriers between these sites. These barriers have been analyzed to understand the propagation of hydrogen through the iron lattice. We analyze how these barriers can be modified by the hydrogen concentration. The results show the main site for high and low hydrogen density and they show the diffusion coefficient variation by the hydrogen density.

Hydrogen induced changes in structural properties of iron: Ab initio calculations

We have used Ab initio calculations to study the structural changes produced by the inclusion of light impurities into pure bcc iron. In order to obtain a clear picture of the mechanics of the phase changes Bain’s pathway was studied in detail for pure iron. The position that hydrogen atoms tend to occupy at high densities favours octahedral sites inside the bcc matrix, producing an internal stress field that suggests a deformation that matches the prediction of martensitic transformation predicted by Bain’s pathway. We have used Density Functional Theory in order to optimize the structures studied, obtaining the enthalpy of the configuration as a function of c/a, allowing a better understanding of the dynamics of the process of phase changes.

Steel Corrosion in a Chloride Contaminated Concrete Pore Solution with Low Oxygen Availability

It is commonly mentioned that in concrete chloride induced corrosion is controlled by the oxygen content in such a manner that in water saturated conditions no oxygen will be present and thus no corrosion can develop. In the present paper, experimentation has been made in low oxygen availability “pore” solutions with several amounts of chlorides. These situations may represent the case of a water saturated concrete. The results indicate that at very low oxygen contents, i.e. almost negligible because complete removal is very difficult, corrosion may develop in presence of chlorides. The presence or absence of corrosion is influenced by the amount of chloride, its corrosion potential and the steel surface condition.

Cold Drawn Steel Surface Analysis in Contact with Saline Solution: Analysis Using Electrochemical Atomic Force Microscopy

High strength steels in concrete structures used as reinforcement are usually passivated due to the physical barrier that concrete provides and by a protective passive film developed on the surface by the high alkaline media. Chloride ions can break the passive film. These ions can be found dissolved in the water or in structures in marine environment.

Observations on the Morphology of Oxide Formation due to Reinforcement Corrosion

The oxide formed during corrosion has an expansive character which induces the cracking of concrete cover. The properties of this oxide layer are focussing the interest due the need to introduce its mechanical characteristics in the models to calculate crack width. In a previous paper it was suggested that the oxide behave as “water” and in consequence it has to be modelled so, which fitted very well in the experiments and model associated an that time. The attribution to the water was based in the observation that the rust has not mechanical consistency and was like a suspension. However, much recent papers insist in attributing other much higher young modulus to the rust. Present paper tries to insist in the original concept by illustrating the formation of the rust in solution and in mortar. The oxides are of mixed colours evolving in function of the time to contact to open air and they have not mechanical consistency, being apparently hydrophilic by retaining large amount of water.