Effect of post-weld heat-treatment on the oxide film and corrosion behaviour of laser-welded shape memory NiTi wires

Post-weld heat-treatment (PWHT) was applied to NiTi weldments to improve the corrosion behaviour by modifying the microstructure and surface composition. The surface oxide film on the weldments is principally TiO2, together with some Ti, TiO, and Ti2O3. The surface Ti/Ni ratio of the weldments after PWHT is increased. The oxide film formed in Hanks’ solution is thicker on the weldments after PWHT. The pitting resistance of the weldments is increased by PWHT. The galvanic effect in the weldments is very small. The weldment with PWHT at 350 °C shows the best corrosion resistance among other heat-treated weldments in this study.

Electroless nickel composite (Ni-P/SiC) coating - A study on interface and corrosion characteristic

Surface behaviour is of paramount importance as failure and degradation tend to initiate from the surface. Electroless composite coating (NiP/SiC) was developed using SiC as reinforcing particles. As heat treatment plays a vital role in electroless nickel coating owing to the changes in microstructure, phase structure and mechanical properties, an insight at the interface changes in chemistry and micromechanical behaviour was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) and microindentation techniques. Corrosion performance was analysed using electrochemical impedance spectroscopy (EIS). Absence of zinc and migration of copper at the interface was detected. Brittleness and microcracks was seen long the interface when indenting at load of 500 gf (Vickers). Corrosion performance is weaker than particles free coating. However, a thin blanket of NiP could enhance the resistance to corrosive medium.

Enhancement of Wear and Corrosion Resistance of Beta Titanium Alloy by Laser Gas Alloying with Nitrogen

The relatively high elastic modulus coupled with the presence of toxic vanadium (V) in Ti6Al4 V alloy has long been a concern in orthopaedic applications. To solve the problem, a variety of non-toxic and low modulus beta-titanium (beta-Ti) alloys have been developed. Among the beta-Ti alloy family, the quaternary Ti-Nb-Zr-Ta (TNZT) alloys have received the highest attention as a promising replacement for Ti6Al4 V due to their lower elastic modulus and outstanding long term stability against corrosion in biological environments. However, the inferior wear resistance of TNZT is still a problem that must be resolved before commercialising in the orthopaedic market. In this work, a newly-developed laser surface treatment technique was employed to improve the surface properties of Ti-35.3Nb-7.3Zr-5.7Ta alloy. The surface structure and composition of the laser-treated TNZT surface were examined by grazing incidence x-ray diffraction (GI-XRD) and x-ray photoelectron spectroscopy (XPS). The wear and corrosion resistance were evaluated by pin-on-plate sliding test and anodic polarisation test in Hanks’ solution. The experimental results were compared with the untreated (or base) TNZT material. The research findings showed that the laser surface treatment technique reported in this work can effectively improve the wear and corrosion resistance of TNZT.

Electrical Based Sensors and Remote Monitoring System for Assessing the Corrosion Related Activity in Concrete at Hangzhou Bay Bridge

Concrete structures in marine environments are subjected to cyclic wetting and drying, corrosion of reinforcement due to chloride ingress and biological deterioration. In order to assess the quality of concrete and predict the corrosion activity of reinforcing steel in concrete in this environment, it is essential to monitor the concrete continuously right from the construction phase to the end of service life of the structure. In this paper a novel combination of sensor techniques which are integrated in a sensor probe is used to monitor the quality of cover concrete and corrosion of the reinforcement. The integrated sensor probe was embedded in different concrete samples exposed to an aggressive marine environment at the Hangzhou Bay Bridge in China. The sensor probes were connected to a monitoring station, which enabled the access and control of the data remotely from Belfast, UK. The initial data obtained from the monitoring station reflected the early age properties of the concretes and distinct variations in these properties were observed with different concrete types.

Chloride transport and the resulting corrosion of steel bars in alkali activated slag concretes

As the relative performance of alkali activated slag (AAS) concretes in comparison to Portland cement (PC) counterparts for chloride transport and resulting corrosion of steel bars is not clear, an investigation was carried out and the results are reported in this paper. The effect of alkali concentration and modulus of sodium silicate solution used in AAS was studied. Chloride transport and corrosion properties were assessed with the help of electrical resistivity, non-steady state chloride diffusivity, onset of corrosion, rate of corrosion and pore solution chemistry. It was found that: (i) although chloride content at surface was higher for the AAS concretes, they had lower chloride diffusivity than PC concrete; (ii) pore structure, ionic exchange and interaction effect of hydrates strongly influenced the chloride transport in the AAS concretes; (iii) steel corrosion resistance of the AAS concretes was comparable to that of PC concrete under intermittent chloride ponding regime, with the exception of 6% Na2O and Ms of 1.5; (iv) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (v) the increase of alkali concentration of the activator generally increased the resistance of AAS concretes to chloride transport and reduced its resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the chloride transport and the corrosion resistance.

Mineralogical characteristics and transformations during long-term operation of a zero-valent iron reactive barrier

Received for publication October 31, 2002. Design and operation of Fe0 permeable reactive barriers (PRBs) can be improved by understanding the long-term mineralogical transformations that occur within PRBs. Changes in mineral precipitates, cementation, and corrosion of Fe0 filings within an in situ pilot-scale PRB were examined after the first 30 months of operation and compared with results of a previous study of the PRB conducted 15 months earlier using X-ray diffraction and scanning electron microscopy employing energy dispersive X-ray and backscatter electron analyses. Iron (oxy)hydroxides, aragonite, and maghemite and/or magnetite occurred throughout the cores collected 30 mo after installation. Goethite, lepidocrocite, mackinawite, aragonite, calcite, and siderite were associated with oxidized and cemented areas, while green rusts were detected in more reduced zones. Basic differences from our last detailed investigation include (i) mackinawite crystallized from amorphous FeS, (ii) aragonite transformed into calcite, (iii) akaganeite transformed to goethite and lepidocrocite, (iv) iron (oxy)hydroxides and calcium and iron carbonate minerals increased, (v) cementation was greater in the more recent study, and (vi) oxidation, corrosion, and disintegration of Fe0 filings were greater, especially in cemented areas, in the more recent study. If the degree of corrosion and cementation that was observed from 15 to 30 mo after installation continues, certain portions of the PRB (i.e., up-gradient entrance of the ground water to the Fe0 section of the PRB) may last less than five more years, thus reducing the effectiveness of the PRB to mitigate contaminants. Abbreviations: EDX, energy dispersive X-ray • Fe0, zerovalent iron • PRB, permeable reactive barrier • SEM, scanning electron microscopy • XRD, X-ray diffraction

Microabrasion-corrosion of cast CoCrMo alloy in simulated body fluids

Wear and corrosion of metal-on-metal hip replacements results in wear debris and metal-ion release in vivo, which may subsequently cause pain and hypersensitivity for patients. Retrieved metal-on-metal hip replacements have revealed that two-body sliding wear and three-body abrasive wear are the predominant wear mechanisms. However, there is a lack of understanding of the combined effects of wear/corrosion, especially the effect of abrasion-corrosion.

This study investigates the sliding-corrosion and abrasion-corrosion performance of a cast CoCrMo alloy in simulated hip joint environments using a microabrasion rig integrated with an electrochemical cell. Tests have been conducted in 0.9% NaCl, phosphate buffered saline solution, 25% and 50% bovine serum solutions with 0 or 1 g cm(-3) SiC at 37 degrees C. Experimental results reveal that under abrasion-corrosion test conditions, the presence of proteins increased the total specific wear rate. Conversely, electrochemical noise measurements indicated that the average anodic current levels were appreciably lower for the proteinaceous solutions when compared with the inorganic solutions. A severely deformed nanocrystalline layer was identified immediately below the worn surface for both proteinaceous and inorganic solutions. The layer is formed by a recrystallisation process and/or a strain-induced phase transformation that occurs during microabrasion-corrosion. (C) 2008 Elsevier Ltd. All rights reserved.

Interpretation of electrochemical measurements made during micro-scale abrasion-corrosion

This paper brings together and analyzes recent work based on the interpretation of the electrochemical measurements made on a modified micro-abrasion-corrosion tester used in several research programmes. These programmes investigated the role of abradant size, test solution pH in abrasion-corrosion of biomaterials, the abrasion-corrosion performance of sintered and thermally sprayed tungsten carbide surfaces under downhole drilling environments and the abrasion-corrosion of UNS S32205 duplex stainless steel. Various abrasion tests were conducted under two-body grooving, three-body rolling and mixed grooving-rolling abrasion conditions, with and without abrasives, on cast F75 cobalt-chromium-molybdenum (CoCrMo) alloy in simulated body fluids, 2205 in chloride containing solutions as well as sprayed and sintered tungsten carbide surfaces in simulated downhole fluids. Pre- and post-test inspections based on optical and scanning electron microscopy analysis are used to help interpret the electrochemical response and current noise measurements made in situ during micro-abrasion-corrosion tests. The complex wear and corrosion mechanisms and their dependence on the microstructure and surface composition as a function of the pH, abrasive concentration, size and type are detailed and linked to the electrochemical signals. The electrochemical versus mechanical processes are plotted for different test parameters and this new approach is used to interpret tribo-corrosion test data to give greater insights into different tribo-corrosion systems. Thus new approaches to interpreting in-situ electrochemical responses to surfaces under different abrasive wear rates, different abrasives and liquid environments (pH and NaCl levels) are made. This representation is directly related to the mechano-electrochemical processes on the surface and avoids quantification of numerous synergistic, antagonistic and additive terms associated with repeat experiments. (C) 2010 Elsevier Ltd. All rights reserved.

Mineralogical Characteristics and Transformations during Long-Term Operation of a Zerovalent Iron Reactive Barrier

Design and operation of Fe⁰ permeable reactive barriers (PRBs) can be improved by understanding the long-term mineralogical transformations that occur within PRBs. Changes in mineral precipitates, cementation, and corrosion of Fe⁰ filings within an in situ pilot-scale PRB were examined after the first 30 months of operation and compared with results of a previous study of the PRB conducted 15 months earlier using X-ray diffraction and scanning electron microscopy employing energy dispersive X-ray and backscatter electron analyses. Iron (oxy)hydroxides, aragonite, and maghemite and/or magnetite occurred throughout the cores collected 30 mo after installation. Goethite, lepidocrocite, mackinawite, aragonite, calcite, and siderite were associated with oxidized and cemented areas, while green rusts were detected in more reduced zones. Basic differences from our last detailed investigation include (i) mackinawite crystallized from amorphous FeS, (ii) aragonite transformed into calcite, (iii) akaganeite transformed to goethite and lepidocrocite, (iv) iron (oxy)hydroxides and calcium and iron carbonate minerals increased, (v) cementation was greater in the more recent study, and (vi) oxidation, corrosion, and disintegration of Fe⁰ filings were greater, especially in cemented areas, in the more recent study. If the degree of corrosion and cementation that was observed from 15 to 30 mo after installation continues, certain portions of the PRB (i.e., up-gradient entrance of the ground water to the Fe⁰ section of the PRB) may last less than five more years, thus reducing the effectiveness of the PRB to mitigate contaminants.

Chloride induced corrosion of steel in alkali activated slag concretes

Alkali activated slag (AAS) is an alternative cementitious material. Sodium silicate solution is usually used to activate ground granulated blast furnace slag to produce AAS. As a consequence, the pore solution chemistry of AAS differs from that of Portland cement (PC). Although AAS offers many advantages over PC, such as higher strength, superior resistance to acid and sulphate environments and lower embodied carbon due to 100% PC replacement, there is a need to assess its performance against chloride induced corrosion duo to its different pore solution chemistry. For PC systems, resistivity measurement, as a type of nondestructive test, is usually used to evaluate its chloride diffusivity and the corrosion rate of the embedded steel. However, due to the different pore solution chemistry present in the different AAS systems, the application of this test in AAS concretes would be questionable as the resistivity of concrete is highly dependent on its conductivity of the pore solution. Therefore, a study was carried out using twelve AAS concretes mixes, the results of which are reported in this paper. The AAS mixes were designed with alkali concentration of 4%, 6% and 8% (Na₂O% of the mass of slag) and modulus (Ms) of sodium silicate solution of 0.75, 1.00, 1.50 and 2.00. A PC concrete with the same binder content as the AAS concretes was also studied as a reference. The chloride diffusion coefficient was determined using a non-steady state chloride diffusion test (NT BUILD 443). The resistivity of the concretes before the diffusion test was also measured. Macrocell corrosion current (corrosion rate) for steel rods embedded in the concretes was measured whilst subjecting the concretes to a cyclic chloride ponding regime (1 day ponded with salt solution and 6 days drying). The results showed that the AAS concretes had lower chloride diffusivity with associated higher resistivity than the PC concrete. The measured corrosion rate was also lower for the AAS concretes. However, unlike the PC, in which a higher resistivity yields a lower diffusivity and corrosion rate, there was no relationship apparent between the resistivity and either the diffusivity or the corrosion rate of steel for the AAS concretes. This is assigned to the variation of the pore solution composition of the AAS concretes. This also means that resistivity measurements cannot be depended on for assessing the chloride induced corrosion resistance of AAS concretes.

Corrosion resistance of self-growing TiC coating on SIMP steel in LBE at 600 °C

SIMP steel was newly developed as a candidate structural material for the accelerator driven subcritical system. The serious decarburization of SIMP steel because of the high Si content was used to successfully form a self-growing TiC coating on the surface, after the Ti deposition as a first step. This TiC layer can effectively protect the surface from the static liquid lead-bismuth eutectic (LBE) corrosion at 600 °C up to 2000 h in the low oxygen LBE. However, in the oxygen saturated LBE, the TiC coating is oxidized into porous TiO2 after only 500 h and fails to protect. Therefore, the self-growing TiC coating is desired only when the oxygen content of LBE is strictly controlled on a low level.

Performance evaluation of a zerovalent iron reactive barrier: Mineralogical characteristics

There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe° reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe° reactive barrier and (2) to examine the cementation and corrosion of Fe° filings in order to estimate the lifespan of this barrier. This field scale barrier (225' long x 2' wide x 31' deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe° portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe° section of the barrier. After 15 months in the barrier, most of the Fe° filings in the core samples were loose, and a little corrosion of Fe° filings was observed in most of the barrier. However, larger amounts of corrosion (~10-150 µm thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/ carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2-50 µm thick) were also found on the uncemented individual Fe° filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe° filings in the more corroded sections is 5 to 10 years, while the Fe° filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe° filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe° section of the barrier.

Integrating allowable design strains in composites with whole life value

Fibre-Reinforced Plastics (FRPs) have been used in civil aerospace vehicles for decades. The current state-of-the-art in airframe design and manufacture results in approximately half the airframe mass attributable to FRP materials. The continual increase in the use of FRP materials over metallic alloys is attributable to the material's superior specific strength and stiffness, fatigue performance and corrosion resistance. However, the full potential of these materials has yet to be exploited as analysis methods to predict physical failure with equal accuracy and robustness are not yet available. The result is a conservative approach to design, but one that can bring benefit via increased inspection intervals and reduced cost over the vehicle life. The challenge is that the methods used in practice are based on empirical tests and real relationships and drivers are difficult to see in this complex process and so the trade-off decision is challenging and uncertain. The aim of this feasibility study was to scope a viable process which could help develop some rules and relationships based on the fundamental mechanics of composite material and the economics of production and operation, which would enhance understanding of the role and impact of design allowables across the life of a composite structure.

Micro-abrasion mechanisms of cast CoCrMo in simulated body fluids

The abrasion seen on some of the retrieved CoCrMo hip joints has been reported to be caused by entrained hard particles in vivo. However, little work has been reported on the abrasion mechanisms of CoCrMo alloy in simulated body environments. Therefore. this study covers the mapping of micro-abrasion wear mechanisms of cast CoCrMo induced by third body hard particles under a wide range of abrasive test conditions. This study has a specific focus on covering the possible in vivo wear modes seen on metal-on-metal (MoM) surfaces. Nano-indentation and nano-scratch tests were also employed to further investigate the secondary wear mechanisms-nano-scale material deformation that involved in micro-abrasion processes. This work addresses the potential detrimental effects of third body hard particles in vivo such as increased wear rates (debris generation) and corrosion (metal-ion release). The abrasive wear mechanisms of cast CoCrMo have been investigated under various wear-corrosion conditions employing two abrasives, SiC (similar to 4 mu m) and Al(2)O(3) (similar to 1 mu m), in two test solutions, 0.9% NaCl and 25% bovine serum. The specific wear rates, wear mechanisms and transitions between mechanisms are discussed in terms of the abrasive size, volume fraction and the test solutions deployed. The work shows that at high abrasive volume fractions, the presence of protein enhanced the wear loss due to the enhanced particle entrainment, whereas at much lower abrasive volume fractions, protein reduced the wear loss by acting as a boundary lubricant or rolling elements which reduced the abrasivity (load per particle) of the abrasive particles. The abrasive wear rate and wear mechanisms of the CoCrMo are dependent on the nature of the third body abrasives, their entrainment into the contact and the presence of the proteins. (C) 2009 Elsevier B.V. All rights reserved.