Studies on Pitting Corrosion of Al-Cu-Li Alloys Part III: Passivation Kinetics of AA2098-T851 Based on the Point Defect Model.

In this paper, the passivation kinetics of AA2098-T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098-T851 in NaHCO3 solution in a CO2 atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler-Volmer (B-V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface

Corrosion and Corrosion-Fatigue Behavior of 7075 Aluminum Alloys Studied By In Situ X-Ray Tomography

abstract: 7XXX Aluminum alloys have high strength to weight ratio and low cost. They are used in many critical structural applications including automotive and aerospace components. These applications frequently subject the alloys to static and cyclic loading in service. Additionally, the alloys are often subjected to aggressive corrosive environments such as saltwater spray. These chemical and mechanical exposures have been known to cause premature failure in critical applications. Hence, the microstructural behavior of the alloys under combined chemical attack and mechanical loading must be characterized further. Most studies to date have analyzed the microstructure of the 7XXX alloys using two dimensional (2D) techniques. While 2D studies yield valuable insights about the properties of the alloys, they do not provide sufficiently accurate results because the microstructure is three dimensional and hence its response to external stimuli is also three dimensional (3D). Relevant features of the alloys include the grains, subgrains, intermetallic inclusion particles, and intermetallic precipitate particles. The effects of microstructural features on corrosion pitting and corrosion fatigue of aluminum alloys has primarily been studied using 2D techniques such as scanning electron microscopy (SEM) surface analysis along with post-mortem SEM fracture surface analysis to estimate the corrosion pit size and fatigue crack initiation site. These studies often limited the corrosion-fatigue testing to samples in air or specialized solutions, because samples tested in NaCl solution typically have fracture surfaces covered in corrosion product. Recent technological advancements allow observation of the microstructure, corrosion and crack behavior of aluminum alloys in solution in three dimensions over time (4D). In situ synchrotron X-Ray microtomography was used to analyze the corrosion and cracking behavior of the alloy in four dimensions to elucidate crack initiation at corrosion pits for samples of multiple aging conditions and impurity concentrations. Additionally, chemical reactions between the 3.5 wt% NaCl solution and the crack surfaces were quantified by observing the evolution of hydrogen bubbles from the crack. The effects of the impurity particles and age-hardening particles on the corrosion and fatigue properties were examined in 4D.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Corrosion and Corrosion-Fatigue Behavior of 7075 Aluminum Alloys Studied By In Situ X-Ray Tomography

abstract: 7XXX Aluminum alloys have high strength to weight ratio and low cost. They are used in many critical structural applications including automotive and aerospace components. These applications frequently subject the alloys to static and cyclic loading in service. Additionally, the alloys are often subjected to aggressive corrosive environments such as saltwater spray. These chemical and mechanical exposures have been known to cause premature failure in critical applications. Hence, the microstructural behavior of the alloys under combined chemical attack and mechanical loading must be characterized further. Most studies to date have analyzed the microstructure of the 7XXX alloys using two dimensional (2D) techniques. While 2D studies yield valuable insights about the properties of the alloys, they do not provide sufficiently accurate results because the microstructure is three dimensional and hence its response to external stimuli is also three dimensional (3D). Relevant features of the alloys include the grains, subgrains, intermetallic inclusion particles, and intermetallic precipitate particles. The effects of microstructural features on corrosion pitting and corrosion fatigue of aluminum alloys has primarily been studied using 2D techniques such as scanning electron microscopy (SEM) surface analysis along with post-mortem SEM fracture surface analysis to estimate the corrosion pit size and fatigue crack initiation site. These studies often limited the corrosion-fatigue testing to samples in air or specialized solutions, because samples tested in NaCl solution typically have fracture surfaces covered in corrosion product. Recent technological advancements allow observation of the microstructure, corrosion and crack behavior of aluminum alloys in solution in three dimensions over time (4D). In situ synchrotron X-Ray microtomography was used to analyze the corrosion and cracking behavior of the alloy in four dimensions to elucidate crack initiation at corrosion pits for samples of multiple aging conditions and impurity concentrations. Additionally, chemical reactions between the 3.5 wt% NaCl solution and the crack surfaces were quantified by observing the evolution of hydrogen bubbles from the crack. The effects of the impurity particles and age-hardening particles on the corrosion and fatigue properties were examined in 4D.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Nonlinear ultrasonic evaluation of disorderedly clustered pitting damage using an in situ sensor network

Pervasive but insidious, pitting damage—from pitting corrosion in maritime structures through electrical pitting in bearings to debris cloud–induced pitting craters in spacecraft—is a typical modality of material degradation and lesion in engineering assets in harsh service environment. Pitting damage may feature hundreds of clustered, localized craters, cracks, and diverse microscopic defects (e.g. dislocation, micro-voids, and cracks) disorderedly scattered over a wide area. Targeting accurate, holistic evaluation of pitting damage (mainly the existence, location, and size of the pitted area), an insight into the generation of nonlinear features in guided ultrasonic waves (i.e. high-order harmonics) that are triggered by pitting damage, is achieved using a semi-analytical finite element approach, based on which a monotonic correlation between the nonlinear ultrasonic features and the holistic severity of pitting damage is established. With such correlation, a structural health monitoring framework is developed, in conjunction with the use of an in situ sensor network comprising miniaturized piezoelectric wafers, to characterize pitting damage accurately and monitor material deterioration progress continuously. The framework is experimentally validated, in which highly complex pitting damage in a space structure, engendered by a hypervelocity debris cloud, is evaluated precisely

Porous Topography Dependence of Mechanical properties and Biological Responses for 3D Printed Stainless Steel and Modified Bioimplant Devices

Materials used in biomedical engineering should contain certain properties in order to satisfy their roles, and orthopedic implant materials, commonly metals, required some specific properties, including sufficient mechanical strength, good durability, good biocompatibility and less cytotoxicity, to function in the animal or human body. Stainless steel was and continues to be one of the choices to be used as implant material for its relatively low cost, excellent strength, good corrosion resistance and relatively good biocompatibility. Additive layer manufacturing (ALM) allows the precise manufacture of implant in certain material, and porous structure, usually lattice, is found to be benefit to bone recovery. In this work, selective laser melting (SLM) is used to produce stainless steel lattices with different pore size in order to evaluate their capability to be used as orthopedic implant material. It was found that the surface of stainless steel lattices contains voids and partially melted stainless steel particles to affect their mechanical properties, but the strength and porosity of lattices are sufficient to be used to be implanted in human body. Study also found that the mechanical properties have a close relationship between pore size and unit cell size of lattices, which the lower the unit cell size, the higher the elastic modulus and ultimate tensile strength. A long-term submersion of lattices in stimulated body fluid is used to evaluate its durability in a stimulated body environment, and the results shows that there is no damage on sample surface and change in mechanical strength. Cytotoxicity tests and osteogenic characterizations show the stainless steel samples and their calcium sulphate modified samples have relatively good biocompatibility. At last, the lattice samples are implanted into rabbit distal femur, and a qualitative analysis on femur using Dual Energy Computed Tomography (DECT), Computed Tomography (CT), and Volume Rendering Technology (VRT) shows a relatively good bone growth after implantation of both lattice samples and modified samples. Tissues are also sliced and evaluated by pathology staining including HE, Masson and Von Kossa staining. Results suggest that the stainless steel lattice have sufficient mechanical strength, durability and biocompatibility, and have great potential to be used as orthopedic implants

Carbon Oxidation at the Atomic Level: A Computational Study on Oxidative Graphene Etching and Pitting of Graphitic Carbon Surfaces

In order to understand the oxidation of solid carbon materials by oxygen-containing gases, carbon oxidation has to be studied on the atomic level where the surface reactions occur. Graphene and graphite are etched by oxygen to form characteristic pits that are scattered across the material surface, and pitting in turn leads to microstructural changes that determine the macroscopic oxidation behavior. While this is a well-documented phenomenon, it is heretofore poorly understood due to the notorious difficulty of experiments and a lack of comprehensive computational studies. The main objective of the present work is the development of a computational framework from first principles to study carbon oxidation at the atomic level. First, the large body of literature on carbon oxidation is examined with regards to experimental observations of the pitting phenomenon as well as relevant theoretical studies on different aspects of the mechanistic details of carbon oxidation. Next, a comprehensive, atomic-scale kinetic mechanism for carbon oxidation is developed, which comprises only elementary surface reactions with reaction rates derived from first principles. The mechanism is then implemented using the Kinetic Monte Carlo (KMC) method. This framework for the first time allows the simulation of oxidative graphene etching at the atomic scale to relevant time- and lengthscales (up to seconds and hundreds of nanometers), and in a wide range of conditions (temperatures up to 2000 Kelvin, pressures ranging from vacuum to atmospheric pressure). The numerical results reveal information about the pitting process in heretofore unattained detail: Pit growth rates (and therefore intrinsic oxidation rates) are calculated and validated against a set of different experimental data at a wide range of conditions. Such information is crucial for modelling of material behavior on meso- and macroscales. The dependence of the pit geometry (hexagonal vs. circular) on temperature and gas pressure is assessed. This is important for utilizing oxidative etching as a manufacturing technique for graphene-based nanodevices. More subtle phenomena like pit inhibition at low pressures and temperatures are also discussed. Moreover, all these findings are examined with respect to the underlying reaction mechanism. This unveils the fundamental reasons for the observed reaction behavior, in particular different activation energies and reaction orders at low and high temperatures, as well as the transition of the pit geometry. The present work is a first step in an ongoing effort to develop predictive models for carbon oxidation in Thermal Protection Systems (TPS), with the ultimate goal of improved safety for hypersonic flight vehicles

Corrosion of bio implants

Chemical stability, mechanical behaviour and biocompatibility in body fluids and tissues are the basic requirements for successful application of implant materials in bone fractures and replacements. Corrosion is one of the major processes affecting the life and service of orthopaedic devices made of metals and alloys used as implants in the body. Among the metals and alloys known, stainless steels (SS), Co-Cr alloys and titanium and its alloys are the most widely used for the making of biodevices for extended life in human body. Incidences of failure of stainless steel implant devices reveal the occurrence of significant localised corroding viz., pitting and crevice corrosion. Titanium forms a stable TiO2 film which can release titanium particles under wear into the body environment. To reduce corrosion and achieve better biocompatibility, bulk alloying of stainless steels with titanium and nitrogen, surface alloying by ion implantation of stainless steels and titanium and its alloys, and surface modification of stainless steel with bioceramic coatings are considered potential methods for improving the performance of orthopaedic devices. This review discusses these issues in depth and examines emerging directions

Electrochemical behaviour of steel reinforced concrete during accelerated corrosion testing

Corrosion of reinforcing steel presents a major durability issue worldwide and is the focus of much research activity. The long time periods involved in replicating reinforcement corrosion within laboratories has resulted in a number of accelerated test methods being developed. The basis of this research presented in this paper was to examine the impressed current technique often used to induce reinforcement corrosion. The suitability of the technique to model chloride induced corrosion was investigated by examining the electrochemical nature of the test method. Corrosion was induced in prisms of differing characteristic strengths and cover thicknesses by applying a current for between 3 and 17 days. The gravimetrical and theoretical mass losses are compared and a modified expression based on Faraday’s law relating the electrical current to the mass loss is also proposed which accounts for the localised nature of chloride-induced corrosion. It was found that the technique is a suitable method to simulate reinforcement corrosion