Dr Bobby Mathan

Magnesium, a light-weight engineering metal, is a potential biomaterial for orthopaedic biodegradable mini-implant applications due to its compatible mechanical properties, biodegradability and biocompatibility. However, magnesium-based implants will become a reality only if the degradation rate of magnesium is controlled. Alloying has shown to reduce the degradation rate of magnesium to some degree, but magnesium alloys are highly susceptible to localised degradation, which can affect the mechanical integrity during service. In orthopaedic applications, mechanical integrity of the implant is a key factor to be considered, especially during the initial service period. Hence, surface modification techniques to delay the general and localised degradation behaviour of magnesium-based materials have gained increased interest in recent years. This chapter reviews the use of biodegradable polymers as coating materials on magnesium-based materials for enhancing the general and localised degradation resistance.

The plasma electrolytic oxidation (PEO) process, which converts the surface of magnesium alloys into a hard, ceramic-like top coat, has been found to be capable of providing enhanced wear and corrosion resistance. By changing the process parameters such as electrolyte composition, current density and duration, a wide variety of coatings can be produced and adapted to applications ranging from automotive to medical. However, the latter applications are still limited. This chapter provides an overview of state-of-the-art PEO processing for magnesium biomedical applications, including processing parameters, coating compositions and properties.

There is enormous interest in nanostructured biomaterials as they can stimulate tissue-biomaterial interaction more effectively. Research is focused on developing advanced nanostructured biomaterials. The major strategies include fabrication of nanocomposites, surface nanostructuring and nanostructured coatings. A potential issue of concern in terms of biocompatibility is the corrosion resistance of the modified biomaterials. This chapter presents the current trends in this area with emphasis on load-bearing orthopaedic and dental implants. A detailed account on the general and the localized corrosion of conventional metallic implants is provided. Novel fabrication strategies of nanostructured hydroxyapatite-based coatings and their roles as barrier coatings are presented. Impacts of nanoscale surface modifications on the corrosion resistance of permanent implants and novel bioresorbable implants based on magnesium alloys are highlighted. © 2012 Woodhead Publishing Limited. All rights reserved.

Stress corrosion cracking (SCC) of Mg alloys is intergranular (IGSCC) or transgranular (TGSCC). A continuous or nearly continuous second phase, typically along grain boundaries, causes IGSCC by micro-galvanic corrosion of the adjacent Mg matrix. IGSCC is expected in all such alloys, typical of most creep resistant alloys, because each known second phase has a more positive corrosion potential than the matrix a-Mg; the degree of severity depends on the electrochemical properties of the second phase; these electrochemical properties need to be studied. Nearly continuous second phases can be avoided by Mg alloy design. TGSCC is most likely caused by an interaction of hydrogen (H) with the microstructure. A study of H-trap interactions is needed to understand this damage mechanism, and to design alloys resistant to TGSCC. Understanding is urgently needed if wrought alloys are to be used safely in service, because prior research indicates that many Mg alloys have a threshold stress for SCC of about half the yield stress in common environments including high-purity water. © 2011 The Commonwealth of Australia. All rights reserved.

The stress corrosion cracking (SCC) behaviour of aluminium alloys has been studied for the past five decades and is still a research area of high interest due to the demand for higher strength aluminium alloys for fuel saving. This chapter brings out the general understanding of the SCC mechanism(s) and the critical metallurgical issues affecting the SCC behaviour of aluminium alloys. The developments made so far with regard to alloying and heat treatment of aluminium alloys for high SCC resistance are discussed. An overview of the available literature on the SCC of aluminium alloy weldments and aluminium alloy metal matrix composites is also presented. © 2011 The Commonwealth of Australia. All rights reserved.

This chapter summarizes stress corrosion cracking of copper and copper-based alloys in different chemical, thermal, and electrochemical environments. The chapter begins with description of different copper-based alloys and their common application. A description of different operating SCC mechanisms for the copper-based alloys is provided. Information regarding the chemical, thermal and electrochemical conditions that could induce SCC of the copper and copper-based alloys and additional details on the SCC mechanism of particular copper-based alloys in specific chemical conditions are also detailed. The role of secondary phase particles on SCC of copper-based alloys is discussed. SCC mitigation strategies for the copper- based alloys are discussed. A summary of the chapter is provided in the end. The aim of this chapter is to develop an understanding of SCC susceptibility of different copper-based alloys in different chemical environments. © 2011 The Commonwealth of Australia. All rights reserved.

Smart micro-arc oxidation (MAO)/epoxy resin (EP) composite coatings were formed on AZ31 magnesium (Mg) alloy. Mesoporous silica nanocontainers (MSN) encapsulated with sodium benzoate (SB) corrosion inhibitors were strategically incorporated in the MAO micropores and in the top EP layer. The influence of the strategic positioning of the nanocontainers on the corrosion protective performance of coating was investigated. The experimental results and analysis indicated that the superior corrosion resistance of the hybrid coating is ascribed to the protection mechanisms of the nanocontainers. This involves two phenomena: (1) the presence of the nanocontainers in the MAO micropores decreased the distance between MSN@SB and the substrate, demonstrating a low admittance value (~ 5.18 × 10-8 O-1), and thus exhibiting significant corrosion inhibition and self-healing function; and (2) the addition of nanocontainers in the top EP layer densified the coating via sealing of the inherent defects, and hence the coating maintained higher resistance even after 90 days of immersion (1.13 × 1010 O cm2). However, the possibility of corrosion inhibitors located away from the substrate transport to the substrate is reduced, reducing its effective utilization rate. This work demonstrates the importance of the positioning of nanocontainers in the coating for enhanced corrosion resistance, and thereby providing a novel perspective for the design of smart protective coatings through regulating the distribution of nanocontainers in the coatings.

In this study, the corrosion inhibition efficiency of thioridazine hydrochloride (TH), an antipsychotic drug, on mild steel (commonly used pipeline material in the oil and gas industry) in 1 M hydrochloric acid (HCl) was evaluated using electrochemical techniques and weight loss method. Electrochemical impedance spectroscopy (EIS) results suggest that TH significantly enhances the polarization resistance (Rp) of mild steel. Similarly, potentiodynamic polarization results showed that the corrosion current density (icorr) of mild steel decreased significantly with addition of TH. To understand the long-term effect of TH, mild steel was tested for 7 days in 100 ppm TH containing electrolyte. EIS results showed that the Rp did not change significantly after 24 h exposure as compared to 2 h exposure; whereas the Rp increased by 28% after 7-day exposure. Weight loss measurements revealed that the inhibition efficiency of TH is remarkably high (98.8%) after 7-day exposure. The adsorption free energy calculation suggests that at the initial stage (1-day) of mild steel exposure, TH was physically adsorbed onto the surface. However, at a later stage (7- day) the binding of TH was chemical, and hence the corrosion protection increased with increase in the exposure period. As compared to the wide range of corrosion inhibitors reported in the literature, TH has shown to be highly effective for mild steel. Thus, it can be suggested that TH drug waste is a potential corrosion inhibitor for mild steel pipelines in the oil and gas industry.

Molecular recognition was utilized to fabricate bioinspired calcium phosphate (Ca-P) coating on bioabsorbable magnesium alloys through small biomolecules such as Vitamin C (VC). Ca-P and VC hybrid coating (Ca-PVC) was successfully fabricated on AZ31 Mg alloy. The surface morphology and chemical composition of the coatings were investigated using SEM, XRD, and FTIR together with XPS. The results showed that the Ca-PVC coating was composed of bamboo leaf-like Ca-P particles with a thickness of about three times that of the Ca-P coating. The surface roughness of the Ca-PVC coating (1.12 ± 0.12 µm) was lower than that (3.14 ± 1.93 µm) of Ca-P coating, suggesting the formation of refined Ca-P particles resulting from the VC addition. The corrosion resistance of the coated samples was characterized via electrochemical polarization, impedance spectroscopy, and immersion hydrogen evolution tests. The cell toxicity of the coated samples was evaluated utilizing mouse MC3T3-E1 pre-osteoblasts. The charge transfer resistance (Rct) of the Ca-PVC coated alloy increased as compared to the bare and Ca-P coated alloy samples. The Ca-PVC coated alloy exhibited minimal corrosion current density (1.36 × 10-6 A cm-2), which is one order of magnitude lower in comparison to that of the Ca-P coated alloy. These results confirm that VC addition greatly enhanced the coating resistance on AZ31 Mg alloy. It was also noticed that the Ca-PVC coated samples rapidly induced the formation of apatite after immersion in Hank's solution. VC was mainly transformed to L-Threonic acid, which facilitated the nucleation process of the Ca-PVC coating and significantly increased the thickness, density, and bonding strength of the coating. With enhanced corrosion resistance property and excellent biocompatibility, Ca-PVC coating has great potential for application in biodegradable Mg-based alloys.

Magnesium is a candidate metal for biodegradable implant applications for its biodegradation tendency and excellent biocompatibility. Unfortunately, the high degradation rate of magnesium and also its localized degradation in physiological conditions are the main issues for its successful implant applications. The degradation rate of magnesium has been reduced to some degree via alloying, but the localized degradation susceptibility is a great concern. For many years, hydroxyapatite (HAp), a biocompatible ceramic material, has been extensively used for bio-implant applications. Recently, a substantial amount of research has been carried out on coating HAp on magnesium-based materials for improved degradation resistance in particular and also to enhance the biocompatibility. This review article focuses on the different methods of HAp coating on magnesium-based materials and also the recent cutting-edge advancements made in the coating process for improved degradation resistance and biocompatibility. The mechanical stability of the HAp coated magnesium-based materials is also discussed.

Mg alloys possess biodegradability, suitable mechanical properties, and biocompatibility, which make them possible to be used as biodegradable implants. However, the uncontrollable degradation of Mg alloys limits their general applications. In addition to the factors from the metallic materials themselves, like alloy compositions, heat treatment process and microstructure, some external factors, relating to the test/service environment, also affect the degradation rate of Mg alloys, such as inorganic salts, bioorganic small molecules, bioorganic macromolecules. The influence of bioorganic molecules on Mg corrosion and its protection has attracted more and more attentions. In this work, the cutting-edge advances in the influence of bioorganic molecules (i.e., protein, glucose, amino acids, vitamins and polypeptide) and their coupling effect on Mg degradation and the formation of protection coatings were reviewed. The research orientations of biomedical Mg alloys in exploring degradation mechanisms in vitro were proposed, and the impact of bioorganic molecules on the protective approaches were also explored.

The influence of intermetallic Al-Mn particles on the corrosion behavior of in-situ formed Mg-Al layered double hydroxide (Mg-Al-CO32--LDH) steam coating on AZ31 Mg alloy was investigated. The alloy was pretreated with H3PO4, HCl, HNO3 or citric acid (CA), followed by hydrothermal treatment, for the fabrication of Mg-Al-LDH coating. The microstructure, composition and corrosion resistance of the coated samples were investigated. The results showed that the surface area fraction of Al-Mn phase exposed on the surface of the alloy was significantly increased after CA pretreatment, which promotes the growth of the Mg-Al-LDH steam coating. Further, the LDH-coated alloy pretreated with CA possessed the most compact surface and the maximum coating thickness among all the coatings. The corrosion current density of the coated alloy was decreased by three orders of magnitude as compared to that of the bare alloy.

Protein exerts a critical influence on the degradation behavior of absorbable magnesium (Mg)-based implants. However, the interaction mechanism between protein and a micro-arc oxidation (MAO) coating on Mg alloys remains unclear. Hereby, a MAO coating was fabricated on AZ31 Mg alloy. And its degradation behavior in phosphate buffer saline (PBS) containing bovine serum albumin (BSA) was investigated and compared with that of the uncoated alloy. Surface morphologies and chemical compositions were studied using Field-emission scanning electron microscope (FE-SEM), Fourier transform infrared spectrophotometer (FT-IR) and X-ray diffraction (XRD). The degradation behavior of the bare Mg alloy and its MAO coating was studied through electrochemical and hydrogen evolution tests. Cytotoxicity assay was applied to evaluate the biocompatibility of Mg alloy substrate and MAO coating. Results indicated that the presence of BSA decreased the degradation rate of Mg alloy substrate because BSA (RCH(NH2)COO¿) molecules combined with Mg2+ ions to form (RCH(NH2)COO)2Mg and thus inhibited the dissolution of Mg(OH)2 by impeding the attack of Cl¿ ions. In the case of MAO coated Mg alloy, the adsorption of BSA on MAO coating and the formation of (RCH(NH2)COO)2Mg exhibited a synergistic effect and enhanced the corrosion resistance of the coated alloy significantly. Furthermore, cell bioactive assay suggested that the MAO coating had good viability for MG63 cells due to its high surface area.

Magnesium (Mg) and its alloys have become a research frontier in biodegradable materials owing to their superior biocompatibility and excellent biomechanical compatibility. However, their high degradation rate in the physiological environment should be well tackled prior to clinical applications. This review summarizes the latest progress in the development of polymeric coatings on biodegradable Mg alloys over the last decade, regarding preparation strategies for polylactic acid (PLA), poly (latic-co-glycolic) acid (PLGA), polycaprolactone (PCL), polydopamine (PDA), chitosan (CS), collagen (Col) and their composite, and their performance in terms of corrosion resistance and biocompatibility. Feasible perspectives and developing directions of next generation of polymeric coatings with respect to biomedical Mg alloys are briefly discussed. Statement of Significance: Magnesium (Mg) and its alloys have become a research frontier in biodegradable materials owing to their superior biocompatibility and suitable biomechanical compatibility. However, the principal drawback of Mg-based implants is their poor corrosion resistance in physiological environments. Hence, it is vital to mitigate the degradation/corrosion behavior of Mg alloys for safe biomedical deployments. This review summarizes the latest progress in development of polymeric coatings on biomedical Mg alloys regarding preparation strategy, corrosion resistance and biocompatibility, including polylactic acid (PLA), poly (latic-co-glycolic) acid (PLGA), polycaprolactone (PCL), chitosan (CS), polydopamine (PDA), collagen (Col) and their composite. In addition, functionalized polymer coatings with Mg alloys exhibits a promising prospect owing to their ability of degradation along with biocompatibility, self-healing, drug-delivery and osteoinduction.

Magnesium and its alloys have been widely studied in recent years for load-bearing biodegradable implant applications due to their biodegradability and biocompatibility. Unfortunately, there are two major concerns with these materials, i.e., high degradation rate and localized degradation susceptibility which can affect the in-service mechanical integrity. This review paper focuses on the potential use of calcium phosphates as biocompatible coatings on magnesium-based materials to control their degradation rates and also delay their localized degradation tendency.

In this study, the effects of various process parameters on the adhesive strength of copper electrodeposits on stainless steel cathodes produced in a laboratory scale electrowinning cell were investigated. The experimental results showed that the chloride ion concentration and the current density had a significant effect on the adhesive strength. Electrolyte temperature and cupric ion concentration also influenced the adhesive strength to some extent, whereas cobaltous ion concentration and Guar dosage showed no noticeable effect. Microstructure analysis of the copper electrodeposits indicated that the process parameters which affected the grain size of the deposit at the substrate/deposit interface influenced the adhesive strength, i.e., fine grains exhibited higher adhesive strength than coarse grains.

Hydrogen permeation of nanostructured bainitic steel, produced at two different transformation temperatures, i.e., 473.15 K (200 °C) BS-200 and 623.15 K (350 °C) BS-350, was determined using Devanathan¿Stachurski hydrogen permeation cell and compared with that of mild steel. Nanostructured bainitic steel showed lower effective diffusivity of hydrogen as compared to the mild steel. The BS-200 steel, which exhibited higher volume fraction of bainitic ferrite phase, showed lower effective diffusivity than BS-350 steel. The finer microstructural constituents (bainitic ferrite laths and retained austenite films) and higher dislocation density in the bainitic ferrite phase of BS-200 steel can be attributed to its lower effective diffusivity as compared to BS-350 steel and mild steel.

Calcium phosphate (CaP) was electrochemically coated on a magnesium-calcium (Mg-Ca) alloy using an unconventional electrolyte and a pulse-potential method. The CaP particles of the coating were relatively large, flat, and irregularly oriented; however, they covered the entire alloy surface with a coating thickness of 5 ?m. Cytocompatibility tests using L929 cells inoculated in Eagle minimum essential medium supplemented with 10% (v/v) fetal bovine serum (E-MEM+FBS) revealed that CaP coating improved the cytocompatibility of the alloy. It also showed effective suppression of Mg2+ ion release from the substrate of the coated alloy and consequently reduced the pH increase of the medium. In vitro degradation experiments using electrochemical techniques in simulated body fluid (SBF) also suggested significant enhancement of the alloy degradation resistance by CaP coating. Potentiodynamic polarization results showed that the corrosion current density of the coated alloy was ?95% lower than that of the bare metal. Electrochemical impedance spectroscopy results revealed that the polarization resistance (RP) of the coated alloy was more than an order of magnitude higher than that of the bare metal after 2 h of immersion in SBF. Interestingly, after 72 h of immersion, the measured RP had decreased by ?82%, and the coating appeared cracked and damaged. The results suggest that SBF is more aggressive than E-MEM+FBS cell culture medium.

A thin film of hydroxyapatite (HA) was coated on a magnesium-calcium (Mg-Ca) alloy using radio frequency (RF) magnetron sputtering. The thickness of the HA coating was in the range of 550-750 nm. In vitro degradation behaviour of the HA coated alloy was evaluated in simulated body fluid (SBF) using electrochemical methods. The ultrathin film coating has significantly improved the degradation resistant of the alloy. The polarisation resistance (R<inf>p</inf>) of the coated alloy was more than two-order of magnitude higher and the corrosion current density (i<inf>corr</inf>) reduced by ~98% as compared to the base alloy.

Magnesium-zinc alloy is a potential base material for biodegradable implant applications. In this study, the influence of zinc content (0.5-3 wt.%) in as-cast magnesium-zinc binary alloys towards the microstructure, mechanical properties and in vitro corrosion behaviour was studied. Increase in zinc content reduced the grain size of magnesium-zinc alloy. Mechanical properties such as yield strength, tensile strength and hardness improved with increase in zinc content. Potentiodynamic polarization results suggest that increase in zinc content enhanced the in vitro corrosion resistance of the alloy, which could be attributed to the combined effect of grain size refinement and even distribution of zinc on the alloy surface resulting in better passive film formation.

In this study, the influence of living cells (L929) on the in vitro degradation behaviour of a magnesium-calcium alloy was investigated using an electrochemical technique in the Eagle's minimum essential medium (EMEM) with 10% foetal bovine serum (FBS) under 5% CO2 atmosphere. The degradation of the alloy increased significantly in the medium containing the cells as compared to that without cells. Post-degradation analysis revealed localized degradation in the vicinity of the cells. It is suggested that the cell metabolic activity has induced local pH drop and as a result increased the alloy degradation.

In this study, the in vitro degradation behaviour of titanium-tantalum (Ti-Ta) alloys (10-30 wt.% Ta) was investigated and compared with conventional implant materials, i.e., commercially pure titanium (Cp-Ti) and titanium-aluminium-vanadium (Ti6Al4V) alloy. Among the three Ti-Ta alloys studied, the Ti20Ta (6.3 × 10- 4 mm/y) exhibited the lowest degradation rate, followed by Ti30Ta (1.2 × 10- 3 mm/y) and Ti10Ta (1.4 × 10- 3 mm/y). All the Ti-Ta alloys exhibited lower degradation rate than that of Cp-Ti (1.8 × 10- 3 mm/y), which suggests that Ta addition to Ti is beneficial. As compared to Ti6Al4V alloy (8.1 × 10- 4 mm/y), the degradation rate of Ti20Ta alloy was lower by ~ 22%. However, the Ti30Ta alloy, which has closer elastic modulus to that of natural bone, showed ~ 48% higher degradation rate than that of Ti6Al4V alloy. Hence, to improve the degradation performance of Ti30Ta alloy, an intermediate thin porous layer was formed electrochemically on the alloy followed by calcium phosphate (CaP) electrodeposition. The coated Ti30Ta alloy (3.8 × 10- 3 mm/y) showed ~ 53% lower degradation rate than that of Ti6Al4V alloy. Thus, the study suggests that CaP coated Ti30Ta alloy can be a viable material for load-bearing permanent implants.

The aim of this work was to understand the effect of microgalvanic degradation on secondary phase particles in magnesium alloys under in vitro condition. Pure magnesium and Mg17Al12 (ß-phase) were galvanically coupled in simulated body fluid and the degradation behavior was studied using electrochemical impedance spectroscopy. The galvanic coupling produced a phosphate/carbonate layer on the ß-phase, which initially increased the degradation resistance. However, the deposited phosphate/carbonate layer rapidly degraded once the galvanic coupling was removed, and ß-phase exhibited similar degradation resistance to that of pure magnesium. A phenom-enological model has been presented, demonstrating the galvanic coupling effect.

An attempt was made to develop a self-dissolution assisted coating on a pure magnesium metal for potential bone fixation implants. Magnesium phosphate cement (MPC) was coated successfully on the magnesium metal in ammonium dihydrogen phosphate solution. The in vitro degradation behaviour of the MPC coated metal was evaluated using electrochemical techniques. The MPC coating increased the polarisation resistance (RP) of the metal by ~150% after 2 h immersion in simulated body fluid (SBF) and reduced the corrosion current density (icorr) by ~80%. The RP of the MPC coated metal remained relatively high even after 8 h immersion period. However, post-degradation analysis of the MPC coated metal revealed localized attack. Hence, the study suggests that MPC coating alone may not be beneficial, but this novel coating could provide additional protection if used as a precursor for other potential coatings such as biodegradable polymers or calcium phosphates.

In this study, polyaniline was coated on a nanostructured bainitic steel to improve the corrosion resistance of the steel. A galvanostatic method with current density varied from 1-20 mA cm-2 was used for synthesising and coating polyaniline on the nanostructured bainitic steel. The electrochemical experiments showed that the polyaniline coating formed using 5 mA cm-2 exhibited the highest protection as compared to that formed at other current densities, which can be attributed to the low porosity in 5 mA cm-2 coating.

In this study, a dual layer inorganic coating was formed on a pure magnesium metal using electrochemical methods to delay the biodegradation of the metal for potential bone fixation implant applications. Firstly, a layer of silicate-based coating was formed on the base metal using the plasma electrolytic oxidation (PEO) method. A second layer of calcium phosphate (CaP) was formed on the PEO coating using the electrodeposition method. in vitro electrochemical degradation testings showed that the double layer coating (PEO-CaP) has significantly improved the initial degradation resistance of the metal. Localized degradation was not evident on the PEO-CaP coated metal even after 72 h exposure to simulated body fluid (SBF). © 2014 Elsevier B.V.

The aim of this study was to investigate whether the cathodic activity of magnesium alloys plays a significant role on the electrochemical deposition of calcium phosphate (CaP). CaP was deposited electrochemically on two magnesium alloys, i.e., magnesium-calcium (Mg-Ca) and magnesium-aluminium-zinc (AZ91), with different electrochemical degradation behaviour. The in vitro degradation behaviour of the CaP coated samples was evaluated using electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF). The polarisation resistance (RP) of the CaP coated Mg-Ca alloy was ~85% lower than that of the CaP coated AZ91 alloy. Fourier transform infrared (FTIR) analysis showed no difference in the chemical nature of the coatings. However, scanning electron microscopy (SEM) analysis revealed that the coating particles on Mg-Ca alloy were less densely packed than those on the AZ91 alloy. This can be attributed to the higher dissolution rate of Mg-Ca alloy as compared to AZ91 alloy. As a result, the former exhibited higher cathodic charge density which produced higher hydrogen evolution, thereby affecting the coating process. © 2014 Elsevier B.V.

In this study, the corrosion performance of nanostructured bainitic steel was compared with martensitic steel in chloride-containing solution using electrochemical techniques. Electrochemical impedance spectroscopy (EIS) results showed that the polarization resistance (Rp) for nanostructured bainitic steel (3400Ocm2) was higher than that of martensitic steel (2000Ocm2). Potentiodynamic polarization results showed an 85% lower corrosion current density for nanostructured bainitic steel compared to martensitic steel. Galvanostatically polarized martensitic steel revealed high localized corrosion i.e., intergranular corrosion (IGC) as well as localized attack in the grains. However, the nanostructured bainitic steel when polarized galvanostatically exhibited only a marginal selective dissolution of retained austenite. Thus, it can be suggested that nanostructured bainitic steel will perform better than martensitic steel in chloride-containing environments. © 2013 Elsevier Ltd.

Nanostructured bainitic steels, containing bainitic ferrite laths and retained austenite films, formed at two different isothermal temperatures were compared for corrosion behavior in chloride-containing solution using electrochemical techniques. The potentiodynamic polarization results suggest that nanostructured bainite formed at 200°C exhibits marginally higher corrosion resistance compared with that at 350°C. Post-corrosion analysis of the galvanostatically polarized samples revealed localized corrosion for both the steels, but the degree of attack was higher in the 350°C steel than in the 200°C steel. The localized corrosion attack was due to selective dissolution of the retained austenite phase. The higher volume fraction and larger size of retained austenite in the 350°C steel as compared to that of the 200°C steel contributed to the pronounced corrosion attack in the 350°C steel. Nanostructured bainitic steels, containing bainitic ferrite laths and retained austenite films, formed at two different isothermal temperatures (i.e. 200 and 350°C) are compared for the corrosion behavior in chloride-containing solution. Selective dissolution of the retained austenite phase is observed for both the steels, but the corrosion attack is higher in the 350°C steel due to the difference in the volume fraction of retained austenite. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In vitro degradation behaviour of a pulsed constant current silicate-based PEO coating on pure magnesium was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation techniques. The PEO coating increased the polarisation resistance (Rp) of magnesium by an order of magnitude and reduced the corrosion current (icorr) by 65%. EIS modelling indicated that the inner compact layer resistance of the PEO coating was critical for the overall degradation resistance of the material. However, the breakdown potential from potentiodynamic polarisation curves and the post-degradation analysis suggested that the porous outer layer of the PEO coating played an important role in the stability of the inner compact layer. © 2013 Elsevier B.V.

In this study, the in vitro degradation behaviour of AZ91 magnesium alloy with two different surface finishes was investigated using electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF). The polarisation resistance (R p ) of the rough surface alloy immersed in SBF for 3 h was ~30% lower as compared to that of the smooth surface alloy. After 12 h immersion in SBF, the R p values for both the surface finishes decreased and were also similar. However, localised degradation occurred sooner, and to a noticeably higher severity in the rough surface alloy as compared to the smooth surface alloy. © 2013 Published by Elsevier B.V.

The effect of a hybrid coating, calcium phosphate (CaP) + polylactic acid (PLA), on a magnesium alloy on its in vitro degradation (general and localized) behaviour was studied for potential load-bearing biodegradable mini-implant applications. CaP was coated on a magnesium alloy, AZ91, using an electrochemical deposition method. A spin coating method was used to coat PLA on the CaP coated alloy. In vitro degradation performance of the alloy with hybrid coating was evaluated using electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF). The EIS results showed that the hybrid coating enhanced the degradation resistance of the alloy by more than two-order of magnitude as compared to the bare alloy and one-order of magnitude higher than that of the CaP coated alloy, after 1 h exposure in simulated body fluid (SBF). Long-term (48 h) EIS results also confirmed that the hybrid coating performed better than the bare alloy and the CaP coated alloy. Importantly, the hybrid coating improved the localized degradation resistance of the alloy significantly, which is critical for better in service mechanical integrity. © 2013 Elsevier B.V. All rights reserved.

In this study, the role of recystrallized grains on the environment-assisted cracking (EAC) susceptibility of a high strength aluminium alloy (Al-Zn-Mg-Cu) was examined using slow strain rate testing (SSRT) and U-bend test methods in chloride-containing solution. Experimental results suggest that the recrystallized grains in the peak-aged alloy are more prone to EAC. However, by altering the morphology and chemistry of the grain boundary precipitates of the recrystallized grains by overaging heat treatment, the alloy susceptibility to EAC reduced significantly. © (2013) Trans Tech Publications, Switzerland.

The electrochemical corrosion behaviour of WE54 magnesium alloy in 0.5 wt.% NaCl solution was studied using electrochemical techniques. Polarization results suggested that the rareearths in WE54 alloy enhanced the passivation tendency of the alloy and decreased the corrosion current by ~30% compared to pure magnesium. Pitting corrosion resistance was also higher in WE54 alloy than that in pure magnesium. Long-term electrochemical impedance results showed that the polarization resistance of WE54 alloy was more than two times higher than that of pure magnesium even after initial passivity breakdown. © (2013) Trans Tech Publications, Switzerland.

In this study, polyaniline (PANI) was coated on a steel using galvanostatic polymerisation technique. The coating was performed under two different electrolyte bath conditions, i.e. unstirred and stirred. The unstirred-condition coating (UC) showed large defects, whereas the stirred-condition coating (SC) exhibited only a few fine defects. The performance of the coatings was tested using electrochemical techniques in chloride-containing solution. Electrochemical impedance spectroscopy (EIS) results showed that the SC sample exhibited almost two orders of magnitude higher polarisation resistance (R p) than that of the UC sample. Long-term EIS results revealed that the Rp for UC sample dropped gradually with exposure time (R p = 1200 O cm2, after 78 h exposure), while the SC sample showed an initial increase in Rp value (Rp = 4300 kO cm2, after 78 h exposure) and then gradually decreased to Rp = 700 kO cm2, after 168 h exposure. However, it should be noted that the Rp value of the SC sample even after 168 h exposure was close to three orders of magnitude higher than the UC sample after 78 h exposure. Potentiodynamic polarisation results confirmed the higher corrosion resistance of the SC sample as compared to the UC sample. The study clearly suggests that stirring the electrolyte bath during polymerisation of aniline on steel produces high performance coating. © 2013 Elsevier B.V. All rights reserved.

In this study, a magnesium alloy (AZ91) was coated with calcium phosphate using potentiostatic pulse-potential and constant-potential methods and the in vitro corrosion behaviour of the coated samples was compared with the bare metal. In vitro corrosion studies were carried out using electrochemical impedance spectroscopy and potentiodynamic polarization in simulated body fluid (SBF) at 37 C. Calcium phosphate coatings enhanced the corrosion resistance of the alloy, however, the pulse-potential coating performed better than the constant-potential coating. The pulse-potential coating exhibited ~ 3 times higher polarization resistance than that of the constant-potential coating. The corrosion current density obtained from the potentiodynamic polarization curves was significantly less (~ 60%) for the pulse-deposition coating as compared to the constant-potential coating. Post-corrosion analysis revealed only slight corrosion on the pulse-potential coating, whereas the constant-potential coating exhibited a large number of corrosion particles attached to the coating. The better in vitro corrosion performance of the pulse-potential coating can be attributed to the closely packed calcium phosphate particles. © 2012 Elsevier B.V.

In this study, the corrosion behavior of twinning-induced plasticity (TWIP) steel i.e. Fe-Mn-Al-Si steel subjected to cold-working (0, 20, and 35%), was examined in acidic (0.1 M H2SO4), alkaline (0.1 M NaOH) and chloride-containing (3.5% NaCl) environments, using potentiodynamic polarization experiments. Interestingly, cold-working did not show any significant change in the corrosion susceptibility of TWIP steel in all the three environments. However, TWIP steel showed the highest corrosion susceptibility in acidic environment and the lowest in alkaline environment. Scanning electron microscope analysis of the corroded TWIP steel samples revealed high-localized attack in acidic environment and some pitting corrosion in chloride-containing solution. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this study, an attempt was made to improve the packing density of calcium phosphate (CaP) coating on a magnesium alloy by tailoring the coating solution for enhanced degradation resistance of the alloy for implant applications. An organic solvent, ethanol, was added to the coating solution to decrease the conductivity of the coating solution so that hydrogen bubble formation/bursting reduces during the CaP coating process. Experimental results confirmed that ethanol addition to the coating solution reduces the conductivity of the solution and also decreases the hydrogen evolution/bubble bursting. In vitro electrochemical experiments, that is, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization showed that CaP coating produced in 30% (v/v) ethanol containing coating solution (3E) exhibits significantly higher degradation resistance (i.e., ~50% higher polarization resistance and ~60% lower corrosion current) than the aqueous solution coating. Scanning electron microscope (SEM) analysis of the coatings revealed that the packing of 3E coating was denser than that of aqueous coating, which can be attributed to the lower hydrogen evolution in the former than in the latter. Further increase in the ethanol content in the coating solution was not beneficial; in fact, the coating produced in 70% (v/v) ethanol containing solution (7E) showed degradation resistance much inferior to that of the aqueous coating, which is due to low thickness of 7E coating. © 2012 Wiley Periodicals, Inc.

Polylactic acid (PLA) was coated on a biodegradable magnesium alloy, AZ91, using spin coating technique for temporary implant applications. The degradation behaviour of the coated alloy samples was evaluated using electrochemical impedance spectroscopy (EIS) method in simulated body fluid (SBF). EIS results suggested that the PLA coating enhanced the degradation resistance of the alloy significantly. Increase in the PLA coating thickness was found to increase the degradation resistance, but resulted in poor adhesion. Long-term EIS experiments of the PLA coated samples suggested that their degradation resistance gradually decreased with increase in SBF exposure time. However, the degradation resistance of the PLA coated samples was significantly higher than that of the bare metal even after a 48 h exposure to SBF. © 2012 Elsevier B.V.

A magnesium alloy, AZ91, was coated with calcium phosphate using pulse-potential method in ethanol containing coating solution for improved degradation resistance of the alloy in body fluid. Electrochemical impedance spectroscopy and potentiodynamic polarization experiments in simulated body fluid suggested that the degradation resistance of the pulse-potential coated alloy was higher than that of the constant-potential coated alloy. Scanning electron microscopy analysis revealed that the morphology of the pulse-potential coating played a critical role in enhancing the coating performance in simulated body fluid. © 2012 Elsevier B.V. All rights reserved.

In this study, the pitting corrosion susceptibility and its role on the hydrogen embrittlement behaviour of AZ80 magnesium alloy were studied using slow strain rate testing (SSRT), electrochemical technique and immersion test method. The electrochemical and immersion tests in chloride-containing solution revealed severe pitting corrosion in the alloy. The SSRT results of the alloy under continuously-exposed conditions in chloride-containing solution and in distilled water showed that the mechanical properties of the alloy deteriorated considerably in both the solutions. Pre-exposure of the alloy in distilled water did not show any considerable change in the mechanical properties of the alloy, however in chloride-containing solution a significant loss in the mechanical properties was noticed. Cleavage facets were observed in the vicinity of the localized attacked region of the alloy pre-exposed in chloride-containing solution. Interestingly, desiccating the pre-exposed (in chloride-containing solution) samples reduced the loss in the mechanical properties, which could be attributed to reversible hydrogen. Thus, the study suggests that pitting corrosion facilitates hydrogen entry into the alloy and causes hydrogen embrittlement. © 2012 Elsevier Ltd.

Rare earths containing magnesium alloy, WE54, exhibited a marginally higher in-vitro degradation resistance than that of pure magnesium. Heat-treatment procedure had an influence on the degradation behaviour. However, comparing with AZ91 magnesium alloy the in-vitro degradation resistance of WE54 magnesium alloy was significantly lower, which suggests that the passivating capacity of rare earths is inferior to that of aluminium under in-vitro condition. © 2010 Elsevier B.V.

The study suggests that the rare-earths containing magnesium alloys ZE41 and QE22 exhibit a poorer corrosion resistance than the AZ80 magnesium alloy. Electrochemical experiments showed that the two rare-earths containing alloys are highly susceptible to localized corrosion. Post corrosion analysis revealed intergranular and pitting corrosion in ZE41, whereas QE22 alloy underwent only pitting corrosion. © (2011) Trans Tech Publications.

In this study, the influence of surface roughness on the passivation and pitting corrosion behaviour of AZ91 magnesium alloy in chloride-containing environment was examined using electrochemical techniques. Potentiodynamic polarisation and electrochemical impedance spectroscopy tests suggested that the passivation behaviour of the alloy was affected by increasing the surface roughness. Consequently, the corrosion current and the pitting tendency of the alloy also increased with increase in the surface roughness. Scanning electron micrographs of 24. h immersion test samples clearly revealed pitting corrosion in the highest surface roughness (Sa 430) alloy, whereas in the lowest surface roughness (Sa 80) alloy no evidence of pitting corrosion was observed. Interestingly, when the passivity of the alloy was disturbed by galvanostatically holding the sample at anodic current for 1. h, the alloy underwent high pitting corrosion irrespective of their surface roughness. Thus the study suggests that the surface roughness plays a critical role in the passivation behaviour of the alloy and hence the pitting tendency. © 2010 Elsevier Ltd.

In this study, the in vitro degradation behaviour of a friction stir processed AZ31 magnesium alloy was investigated. Electrochemical experiments in simulated body fluid suggest that friction stir processing marginally enhances the degradation resistance of the alloy, which could be attributed to the dissolution of secondary phase particles. Homogenisation of the microstructure reduces galvanic corrosion. It is envisaged that the beneficial effect would be more pronounced for magnesium alloys which contain high volume fraction of galvanic corrosion inducing secondary phase particles. © 2011 Springer Science+Business Media, LLC.

Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the mechanical integrity of an AZ91 magnesium alloy was studied using a constant extension rate tensile (CERT) method. The samples in two different geometries that is, circumferentially notched (CN), and circumferentially notched and fatigue cracked (CNFC), were tested in air and in simulated body fluid (SBF). The test results show that the mechanical integrity of the AZ91 magnesium alloy decreased substantially (~50%) in both the CN and CNFC samples exposed to SBF. Fracture surface analysis revealed secondary cracks suggesting stress corrosion cracking susceptibility of the alloy in SBF. © 2010 Wiley Periodicals, Inc.

The mechanical integrity of resorbable implants during service, especially in load bearing orthopaedic applications, is critical. The high degradation rate of resorbable magnesium and magnesium-based implants in body fluid may potentially cause premature in-service failure. In this study, a magnesium alloy (AZ91) was potentiostatically coated with hydroxyapatite at different cathodic voltages in an attempt to enhance the mechanical integrity. The mechanical integrity of the uncoated and hydroxyapatite coated alloys was evaluated after in vitro testing of the coated samples in simulated body fluid (SBF). The uncoated alloy showed 40% loss in the mechanical strength after five days exposure to SBF. However, the hydroxyapatite coated alloy exposed to SBF showed 20% improvement in the mechanical strength as compared to that of the uncoated alloy. The alloy coated potentiostatically at -2 V performed better than the -3 V coated alloy. The cross-sectional analysis of the coatings revealed relatively uniform coating thickness for the -2 V coated alloy, whereas the -3 V coated alloy exhibited areas of uneven coating. This can be attributed to the increase in hydrogen evolution on the alloy during -3 V coating as compared to -2 V coating. The scanning electron micrographs of the in vitro tested alloy revealed that hydroxyapatite coating significantly reduced the localized corrosion of the alloy, which is critical for better in-service mechanical integrity. Thus, the study suggests that the in vitro mechanical integrity of resorbable magnesium-based alloy can be improved by potentiostatic hydroxyapatite coating. © 2011 IOP Publishing Ltd.

The aim of the work was to inhibit recrystallization in Al-Zn-Mg-Cu-Zr alloy and to evaluate the stress corrosion cracking (SCC) behavior of the alloy in the peak aged condition. For this purpose, scandium addition was made to an Al-Zn-Mg-Cu-Zr alloy as the former inhibits recrystallization. The scandium-containing alloy was heat-treated to peak aged condition and compared with the base peak aged alloy which contained recrystallized grains. The SCC susceptibilities of the alloys were evaluated using slow strain rate testing (SSRT) and U-bend techniques. While, the base alloy having recrystallized grains showed drastic loss in ductility in the corrosive environment (3.5 wt.% NaCl solution), the scandium-containing alloy having un-recrystallized and fine grains showed no significant loss in ductility in the similar environment. The fracture surface analysis revealed typical intergranular cracking of recrystallized grains in the base alloy, whereas in the scandium-containing alloy predominant ductile failure was observed. Thus, the study clearly indicated that inhibiting recrystallization in Al-Zn-Mg-Cu-Zr alloy through scandium addition, the SCC resistance of the alloy can be substantially improved even in the peak aged condition. © 2009 Elsevier Ltd. All rights reserved.

The study shows that the microstructural difference between the fine-grained die-cast and coarse-grained sand-cast magnesium-based alloys has no significant effect on the in-vitro degradation behaviour. However, the post-degradation analysis of the alloys suggest that the high volume fraction of secondary phase particles in the die-cast alloy may not be suitable for biodegradable implant applications, primarily due to the high stability of the secondary phase particles in physiological conditions. © 2010 Elsevier B.V. All rights reserved.

Successful application of magnesium-based alloys as biodegradable biomaterials is critically dependent on controlling the degradation rate of the alloy. The present study suggests that electrochemical deposition of calcium phosphate on magnesium alloy at an optimal voltage enhances the degradation resistance of the alloy significantly. © (2010) Trans Tech Publications.

To understand the in vitro degradation mechanism of magnesium alloy, electrochemical experiments viz., electrochemical impedance spectroscopy and potentiodynamic polarization, were carried out on AZ91 magnesium alloy under different experimental conditions. The study suggests: (i) the body temperature decreases significantly the corrosion resistance of the alloy, (ii) alkalitreatment of the alloy enhances the corrosion resistance, and (iii) although chloride in simulated body fluid minimizes the corrosion resistance, the presence of other constituents viz., phosphate, calcium, and carbonate, enhances the film forming tendency and hence increases the corrosion resistance of the alloy. © 2009 Wiley Periodicals, Inc.

In this study, an attempt was made to enhance the degradation resistance of magnesium alloys for potential biodegradable implant applications through surface treatment. AZ91 magnesium alloy was taken as the test sample and was alkali-treated for two different periods of time and then the in vitro degradation behaviour of the alloy was studied using electrochemical impedance spectroscopy and polarization techniques in simulated body fluid. The study suggests that alkali-treatment reduces the degradation rate in AZ91 magnesium alloy. © (2009) Trans Tech Publications, Switzerland.

This paper reviews the environmentally-assisted cracking studies of some of the key engineering materials i.e. austenitic stainless steels, aluminium alloys and magnesium alloys. Some of the salient work carried out by the research laboratories and institutions in India and other countries, in this critical research area, are brought out in this paper. © 2009, by Walter de Gruyter GmbH & Co. All rights reserved.

Recent developments in a group of super ductile Fe-Mn-Al-Si steels with high-manganese content demands for more research in the corrosion behavior of such steels. The corrosion properties of the Fe-Mn-Al-Si steel was studied in acidic (0.1 M H2SO4), alkaline (0.1 M NaOH) and chloride-containing (3.5% NaCl) environments, using immersion and polarization experiments and compared with that of interstitial-free (IF) steel. In acidic solution, the Fe-Mn-Al-Si steel exhibited significantly lower corrosion resistance than that of IF steel. Though the Fe-Mn-Al-Si steel showed lower corrosion resistance as compared to IF steel in chloride solution, the difference was not as substantial as observed in acidic medium. However, in alkaline solution, the Fe-Mn-Al-Si steel showed no significant difference in the corrosion resistance in comparison with that of IF steel, and moreover exhibited substantially high corrosion resistance than in acidic and chloride solution. The post-corrosion characterization studies showed higher corrosion attack of the Fe-Mn-Al-Si steel exposed to acidic solution as compared to that in alkaline and chloride solutions, which is consistent with the corrosion measurement data. © 2008 Elsevier Ltd. All rights reserved.

The successful applications of magnesium-based alloys as degradable orthopaedic implants are mainly inhibited due to their high degradation rates in physiological environment and consequent loss in the mechanical integrity. This study examines the degradation behaviour and the mechanical integrity of calcium-containing magnesium alloys using electrochemical techniques and slow strain rate test (SSRT) method, respectively, in modified-simulated body fluid (m-SBF). Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) results showed that calcium addition enhances the general and pitting corrosion resistances of magnesium alloys significantly. The corrosion current was significantly lower in AZ91Ca alloy than that in AZ91 alloy. Furthermore, AZ91Ca alloy exhibited a five-fold increase in the surface film resistance than AZ91 alloy. The SSRT results showed that the ultimate tensile strength and elongation to fracture of AZ91Ca alloy in m-SBF decreased only marginally (~15% and 20%, respectively) in comparison with these properties in air. The fracture morphologies of the failed samples are discussed in the paper. The in vitro study suggests that calcium-containing magnesium alloys to be a promising candidate for their applications in degradable orthopaedic implants, and it is worthwhile to further investigate the in vivo corrosion behaviour of these alloys. © 2008 Elsevier Ltd. All rights reserved.

In this study, polyoxadiazole-based coatings were molecularly designed by attaching two different functional groups, i.e., diphenyl-ether and diphenyl-hexafluoropropane, in the main polymer chain for the purpose of low water permeability and eventually for high corrosion protection of AM50 magnesium alloy. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the coating performance of the two polymers. Electrochemical experiments showed that POD-6FP (poly(4,4'-diphenyl-hexafluoropropane-1,3,4-oxadiazole)) coated alloy exhibited 3-4 orders of magnitude higher corrosion resistance as compared to the POD-DPE (poly (4,4'-diphenyl-ether-1,3,4-oxadiazole)) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain. © 2008 Elsevier B.V. All rights reserved.

Stress corrosion cracking (SCC) of the high-performance rare-earth containing magnesium alloys ZE41, QE22 and Elektron 21 (EV31A) was studied using slow strain rate test (SSRT) method in air, distilled water and 0.5 wt.% NaCl solution. For comparison, the well-known AZ80 alloy was also studied. All alloys were susceptible to SCC in 0.5 wt.% NaCl solution and distilled water to some extent. AZ80 had similar SCC susceptibility in distilled water and 0.5 wt.% NaCl solution. ZE41, QE22 and EV31A had higher susceptibility to SCC in 0.5 wt.% NaCl solution than in distilled water. EV31A had the highest resistance to SCC compared to AZ80, ZE41 and QE22 in both distilled water and 0.5 wt.% NaCl solution. The fractography was consistent with (i) largely transgranular SCC (TGSCC) in distilled water for AZ80, ZE41 and QE22 and also for AZ80 in 0.5 wt.% NaCl solution, and (ii) a significant component of intergranular SCC (IGSCC) in 0.5 wt.% NaCl solution for QE22, ZE41 and EV31A. The TGSCC fracture path in AZ80, ZE41 and QE22 is consistent with a mechanism involving hydrogen. In each case, the IGSCC appeared to be associated with the second-phase particles along grain boundaries. For IGSCC of EV31A and QE22, the fractography was consistent with micro-galvanic acceleration of the corrosion of a-magnesium by the second-phase particles, whereas it appeared that the second-phase particles had corroded itself in the case of ZE41 in 0.5 wt.% NaCl solution. The study suggests that rare-earth elements in magnesium alloys can improve SCC resistance significantly as observed in the case of EV31A. However, the SCC resistance also depends on the other critical alloying elements such as zinc (in ZE41) and silver (in QE22) and the microstructure. © 2007.

Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the stress corrosion cracking susceptibility of sand-cast Mg-Al-Zn alloy in modified-simulated body fluid was evaluated using the slow strain rate test method. The study suggests that the stress corrosion cracking susceptibility of the sand-cast magnesium alloy is not substantial and this aspect should not be a concern for its implant applications. © 2008 Acta Materialia Inc.

Coarse intermetallic particles (larger than 1 µm in size) in Al-Zn-Mg-Cu-Zr (7010) alloy were found to significantly influence the crack initiation of the over aged alloy while not affecting the more susceptible peak aged alloy, when subjected to slow strain rate testing (SSRT) in 3.5% NaCl solution. A detailed study was undertaken to examine the causes of such an observation. The study shows that the galvanic action and/or dealloying of the coarse intermetallic particles are responsible for the crack initiation in the over aged alloy. However, this phenomenon is not seen in the peak aged alloy due to its inherent environmentally assisted cracking (EAC) susceptibility and the consequent failure in shorter duration, before the coarse particles can exert an influence. © 2007 Springer Science+Business Media, LLC.

In this study, the stress corrosion cracking (SCC) behavior of samples fabricated from AZ31 Mg alloy by laser butt welding was investigated in slow strain rate tensile (SSRT) and constant load tests. The results show that the laser butt welded AZ31 Mg samples are highly susceptible to SCC. The failure of the samples occurred in the fusion boundary when tested in corrosion environment in contrast to failures in the fusion zone (FZ)/base region observed for tests in air. It is suggested that weakening of the fusion boundary is due to the localized corrosion attack along the region. © 2006 Elsevier B.V. All rights reserved.

In this study, the stress corrosion cracking (SCC) behaviour of a friction stir welded (FSW) AZ31 Mg alloy was examined using the slow strain rate tensile (SSRT) test method. Electrochemical impedance spectroscopy (EIS) and salt spray tests were conducted to understand the general and localized corrosion behaviour of the base material and the FSW joint. It was found that the FSW AZ31 Mg samples exhibited higher SCC susceptibility than the base material under SSRT testing in the corrosive environment. The failure of these FSW AZ31 Mg samples occurred in the stir zone (SZ), whereas failure occurred in the thermo mechanically affected zone (TMAZ) for samples tested in air. The EIS and salt spray test results demonstrate that the SZ exhibits higher uniform and pitting corrosion resistance than the base material. The present study suggests that the hydrogen assisted cracking mechanism predominates in the failure of FSW AZ31 Mg samples during SSRT test in the corrosive environment. © 2007 Elsevier B.V. All rights reserved.

The aim of this paper is to show that laser treated magnesium-based alloys can be a potential candidate for biodegradable implants. Magnesium alloys are biocompatible, lightweight and have good mechanical properties. The poor corrosion resistance of magnesium based alloys can be advantageously used as biodegradable implants. However, the key issue is that, magnesium corrosion is too high in the physiological system which deteriorates the mechanical integrity of the alloy very rapidly. This study explores the possibility of enhancing the corrosion resistance of magnesium alloy (AZ31) through laser treatment, which has an advantage of unfaltering the bulk chemical composition and thus the mechanical property is not compromised. Electrochemical polarisation experiments, showed increment in general corrosion resistance for laser treated alloy. High pitting corrosion resistance of laser treated alloy was observed in the salt spray tests. The stress corrosion cracking resistance was also significantly higher in the laser treated alloy as compared to the untreated alloy. The higher corrosion resistance in the laser treated alloy is attributed to the fine grain size, uniform distribution of the secondary phase particles along the grain boundary and aluminium enrichment on the surface. The study shows that suitable biodegradable magnesium-based orthopaedic implants can be achieved through laser treatment. © 2007, Taylor & Francis Group, LLC.

In this study, the electrochemical polarization behaviors of Al-Zn-Mg-Cu-Zr (7010) alloy in three different heat treatments, namely, underaged, peak-aged, and overaged, were examined in 3.5 wt pct NaCl solution. Experimental results show that the cathodic current increases marginally in the order of underaged < peak-aged < overaged alloys, which has been attributed to an increase in copper content of the precipitates in general and the grain boundary precipitates (GBPs) in particular. The change in the precipitate chemical composition has been found to affect the anodic polarization behavior even in a more significant way. Thus, both the anodic polarization curves of underaged and peak-aged alloys exhibit two distinct breakdown potentials and current reversal immediately below the second breakdown potential, whereas such a phenomenon is found to be absent in the overaged alloy. The overaged alloy exhibits only one breakdown potential without any current reversal. Detailed study of the polarization data and corroded surfaces of the alloy shows that the anodic current reversal is due to H2 evolution on the alloy surface just after the occurrence of passive film breakdown along the grain boundary. Notably, it is only those heat treatments that are prone to intergranular corrosion (IGC) seems to exhibit the tendency to reduce H+ ions, when they are anodically polarized. The chemical composition of the precipitates that can be altered by heat treatments is responsible for this behavior. The addition of 0.25 wt pct scandium to type 7010 Al alloy did not show any improvement in the corrosion resistance of the alloy. The Ecorr of scandium containing alloy shifted toward the active direction as compared to the base alloy. Noticeably, the peak-aged scandium containing alloy also exhibited two distinct breakdown potentials in the anodic polarization curve similar to the peak-aged base alloy, thus revealing its susceptibility to IGC and pitting corrosion. © The Minerals, Metals & Materials Society and ASM International 2007.

The hydrogen-induced-cracking (HIC) behaviour of the magnesium alloy AZ80 was evaluated using the slow strain rate test method. The study shows no evidence of HIC in AZ80 alloy after pre-exposure or pre-charging in distilled water. However, under continuous charging in distilled water during straining, the AZ80 alloy shows evidence of HIC. The contradiction reported in the literature is addressed. © 2007 Acta Materialia Inc.

Slow strain rate testing (SSRT) was carried out on over aged 7010 Al-alloy in laboratory air, glycerin and 3.5 wt.% NaCl solution with and without cathodic charging to study the hydrogen embrittlement susceptibility of the alloy in over aged condition. It was found that the over aged alloy exhibited high resistance to stress corrosion cracking (SCC) than hydrogen embrittlement (HE). The high SCC resistance is due to the modification in the grain boundary precipitate morphology and chemistry due to over aging, however it is suggested that the dislocations in the alloy are not completed annealed during over aging to arrest HE.

The purpose of this work is to show that during slow strain rate testing (SSRT), the laboratory air might not be a non-corrosive atmosphere for evaluating the Iscc (stress corrosion cracking susceptibility index) of high strength Al alloys. Instead, it is proposed that glycerin could be effectively used as the non-corrosive atmosphere to find out the correct Iscc. This is demonstrated while examining 7010 Al alloy in the under aged condition. © 2004 Acta Materialia Inc. Published by Elsevier Ltd. All right reserved.

The role of multistep aging on the stress corrosion cracking (SCC) behavior of aluminum alloy 7010 has been investigated. The material in the form of 5-mm-thick sheets was heat-treated to two different tempers. The heat treatments are peak-aged, obtained by a two-step aging (i.e., solution treated at 465°C, water quenched at room temperature, and aged at 100°C/8 h and 120°C/8 h) and over-aged, obtained by a three-step aging (i.e., solution treated at 465°C, water quenched at room temperature, and aged at 100°C/8 h followed by 120°C/8 h and 170°C/8 h). SCC behavior was investigated using slow strain rate testing (SSRT) in air and 3.5% sodium chloride (NaCl) solution (environment). U-bend testing was carried out as a qualitative test. SSRT results show a remarkable improvement in the SCC resistance in three-step aged samples. For example, the percent reduction in area dropped from 28.1% (in air) to just 24.4% (in environment), and there was no drop in the percent elongation (~ 10%) when tested at a strain rate of 10-6/s. Two-step aged samples were found to be inferior compared with the three-step aged sample-the percent reduction in area of two-step aged samples dropped from 9.9% (in air) to 3.3% (in environment) and percent of elongation dropped from 10% (in air) to 3% (in environment). U-bend test results are also in conformity with the SSRT results, showing high resistance to SCC in the three-step aged alloy, though no direct correlation can be made. Anodic dissolution along the grain boundaries leading to intergranular cracking was evident in the SSRT and U-bend fractured specimens of the two-step aged alloy, whereas, such an attack was less prominent in the three-step aged alloy leading to a relatively more ductile fracture. The breaking of grain boundary precipitate network in the three-step aged condition could be a possible reason for better SCC resistance of the three-step aged alloy.

Magnesium alloys are attractive for use as biodegradable materials for temporary implant applications. However, the high localized degradation of magnesium alloys in physiological conditions is a major concern, which can affect the mechanical integrity of the implant during service. Calcium phosphate (CaP) coating is a suitable method to delay the initiation of localized attack in magnesium alloys. This paper will discuss the challenges and opportunities in electrochemically coating CaP on magnesium and its magnesium alloys for biodegradable implant applications. © (2014) Trans Tech Publications, Switzerland.

Magnesium alloys are attractive for use as biodegradable materials for temporary implant applications. However, the high localized degradation of magnesium alloys in physiological conditions is a major concern, which can affect the mechanical integrity of the implant during service. Calcium phosphate (CaP) coating is a suitable method to delay the initiation of localized attack in magnesium alloys. This paper will discuss the challenges and opportunities in electrochemically coating CaP on magnesium and its magnesium alloys for biodegradable implant applications. © (2014) Trans Tech Publications, Switzerland.

An attempt was made to seal the porous silicate-based plasma electrolytic oxidation (PEO) layer on pure magnesium (Mg) with a biodegradable polymer, poly(l-lactide) (PLLA), to delay the localized degradation of magnesium-based implants in body fluid for better in-service mechanical integrity. Firstly, a silicate-based PEO coating on pure magnesium was performed using a pulsed constant current method. In order to seal the pores in the PEO layer, PLLA was coated using a two-step spin coating method. The performance of the PEO-PLLA Mg was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The EIS results showed that the polarization resistance (R p ) of the PEO-PLLA Mg was close to two orders of magnitude higher than that of the PEO Mg. While the corrosion current density (i corr ) of the pure Mg was reduced by 65% with the PEO coating, the PEO-PLLA coating reduced the i corr by almost 100%. As expected, the R p of the PEO-PLLA Mg decreased with increase in exposure time. However, it was noted that the R p of the PEO-PLLA Mg even after 100 h was six times higher than that of the PEO Mg after 48 h exposure, and did not show any visible localized attack. © 2014 Elsevier B.V. All rights reserved.

In this study, polyaniline (PANI) was coated on a steel using galvanostatic technique. The corrosion performance of the coating was studied using electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation in 3.5 wt. % NaCl solution. The electrochemical experiments showed that PANI coating produced using a current density of 5 mA cm-2 enhanced the corrosion resistance of the steel significantly, the corrosion current density (icorr) decreased by ~ 88 % and the polarisation resistance (RP) increased by ~ 70%. Higher current density coatings produced large pores and have deteriorated the coating performance.

This paper brings out the developments on heat-treatment and alloying to improve the stress corrosion cracking (SCC) behavior of 7010 Al-alloy. The role of microstructures including the grain boundary precipitates and recystallized grains and the relation of intergranular corrosion (IGC) on the SCC behavior of 7010 Al-alloy have been discussed. © (2010) Trans Tech Publications.

In this work, the effect of recrystallization inhibition in Al-Zn-Mg-Cu (7010) alloy towards the stress corrosion cracking (SCC) behavior was studied. For this purpose, Sc addition (0.25 wt.%) was made to Al-Zn-Mg-Cu alloy and the SCC behavior of the base and Sc-bearing alloys in peak-aged condition was examined using slow strain rate testing (SSRT). The base 7010 Al alloy showed 10% elongation, 9.9% reduction in area and 561 MPa ultimate tensile strength (UTS), when tested in air. The ductility of the base alloy dropped to 3% and 3.3% in terms of elongation and reduction in area, respectively, when tested in 3.5% NaCl solution, showing its high susceptibility to SCC. On the other hand, the 0.25 wt.% Sc containing alloy showed a significant improvement in ductility not only in air but also in 3.5% NaCl solution, without any loss in the UTS. Thus, the 0.25 wt.% Sc containing alloy exhibited 13.4% elongation, 15.8% reduction in area and 560 MPa UTS in air and 12.5% elongation, 16.4% reduction in area and 560 MPa UTS in 3.5% NaCl solution. The study shows that inhibiting recrystallization in 7010 alloy through Sc addition, improves the SCC resistance substantially even in the peak aged condition.

In this study, the stress corrosion cracking susceptibility of Mg-Al-Zn alloy in simulated body fluid (SBF) was evaluated using slow strain rate test (SSRT) method. The SSRT results showed that the mechanical properties such as ultimate tensile strength and elongation-to-fracture decreased marginally (17% and 21%, respectively) in comparison with these properties in air, suggesting that stress corrosion cracking is not substantial for Mg-Al-Zn alloy in simulated physiological condition.

In this work, the electrochemical polarisation behaviour of twinning induced plasticity (TWIP) steel (Fe-Mn-Al-Si) has been studied in three different environments viz., acidic, basic and chloride containing solutions, and compared with interstitial-free (IF) steel. The electrochemical polarisation results showed that TWIP steel exhibits lower corrosion resistance as compared to IF steel in all the three environments. However, the corrosion resistance of the TWIP steel in alkaline and chloride solutions were not as substantially higher as observed in acidic medium. The results were consistent with the post-corrosion SEM analysis. The plausible mechanisms are discussed in this paper.

Laser assisted surface modification has been used to produce variety of surface/sub-surface microstructures in AZ91 alloy. Refined microstructures thus produced have been characterized by metallography. Electrochemical corrosion behaviour of these variations is studied using polarization carves and compared to that of sand-cast alloy. Scanning electron microscope is used to determine post corrosion morphology. The corrosion behaviour is correlated to refined microstructures developed upon laser treatment.

Magnesium is the fourth major cation of the human body, is biocompatible as opposed to other implant materials that have toxic effect on long time exposure, low density, acquires high strength through suitable alloying and has favourable electrochemical properties, which makes magnesium attractive for degradable orthopaedic implant applications. Unfortunately, magnesium corrosion is too high in the physiological system which deteriorates the mechanical integrity of the alloy very rapidly. This study explores the possibility of enhancing the corrosion resistance of magnesium alloys through calcium addition. In-vitro corrosion studies were carried out to evaluate the corrosion/degradation behaviour of calcium containing magnesium alloys in simulated body fluid. Potentiodynamic polarisation, electrochemical impedance spectroscopy and immersion test results have shown that, calcium addition to magnesium alloy improves the general and localized corrosion.

Stress corrosion cracking (SCC) susceptibility of Al-Zn-Mg-Cu-Zr base 7010 alloy of two different tempers viz. T6 (peak aged - two step aging) and T73 (over aged - three step aging) have been investigated in air and an aqueous environment (non-deaerated 3.5 wt.% NaCl solution) by slow strain rate testing (SSRT) as well as U-bend testing. The tests have been carried out at 10-5/s and 10-6/s strain rates. In over aged temper (T73), the alloy exhibits substantial improvement in SCC resistance with minimal reduction in the ultimate tensile strength as compared to the peak aged temper (T6). For example, percent reduction in area of peak aged samples dropped from 9.9 % (in air) to 3.3 % (in environment), whereas percent reduction in area of over aged samples dropped from 28.1 % (in air) to just 24.4 % (in environment) when tested at a strain rate of 10-6/s. U-bend test results are also in conformity with the SSRT results. Fractographs of samples tested in air and the environment revealed mixed mode of failure for both the treatments. However, the over aged samples as compared to the peak aged samples showed greater ductile mode of fracture. This appears to be a clear benefit of the over aging process.