Search

A calcium phosphate coating was directly synthesized on AZ91D magnesium (Mg) alloy. Resistance of this coating to corrosion in a modified-simulated body fluid (m-SBF) was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Mechanical properties of the bare and coated alloy were investigated using slow strain rate tensile (SSRT) and fatigue testing in air and m-SBF. Very little is reported in the literature on human-body-fluid-assisted cracking of Mg alloys, viz., resistance to corrosion fatigue (CF) and stress corrosion cracking (SCC). This study has a particular emphasis on the effect of bio-compatible coatings on mechanical and electrochemical degradations of Mg alloys for their applications as implants. The results suggest the coating to improve the general as well as pitting corrosion resistance of the alloy. The coating also provides visible improvement in resistance to SCC, but little improvement in CF resistance. This is explained on the basis of pitting behaviour in the presence and absence of the coating. [ABSTRACT FROM AUTHOR]

Magnesium (Mg) alloys have recently attracted great attention for potential biodegradable implant applications. Cracking/fracture of metallic implants under the simultaneous action of corrosion and mechanical stresses, viz., corrosion fatigue (CF) or/and stress corrosion cracking (SCC) is an obviously critical criterion before any new material could be deployed as implants. This article presents a review of the available literature on CF of Mg alloys in corrosive environments including simulated-body-fluid (SBF) and the associated fracture mechanism, and identifies the knowledge gap. A brief overview of the mitigation strategies to combat the possible CF failures of Mg alloys is also presented. [ABSTRACT FROM AUTHOR]

Abstract: Stress corrosion cracking of the magnesium alloy Elektron 21 (ASTM–EV31A) and AZ91E was studied using constant load test in 0.1M NaCl solution (saturated with Mg(OH)2), and slow strain rate test using glycerol, distilled water and Mg(OH)2 saturated, 0.01M and 0.1M NaCl solutions. Slow strain rate test indicated that EV31A was less susceptible to stress corrosion cracking than AZ91E. Under less intense loading of constant load, EV31A was found to be resistant to stress corrosion cracking. Fractography of EV31A specimens showed little evidence of hydrogen embrittlement. The superior resistance of EV31A is attributed to a more robust oxide/hydroxide layer. [Copyright &y& Elsevier]

Abstract: When used as implants, magnesium alloys will simultaneously encounter mechanical loading and corrosive physiological environment, which can cause a premature failure due to stress corrosion cracking (SCC). Therefore, it is essential to characterize the SCC behavior of magnesium alloys, before they can be actually used as implants. In the present study, the SCC behavior of a common magnesium alloy, AZ91D, and a biocompatible alloy, Mg–3wt.% Zn–1wt.% Ca, was evaluated in the physiological environment using slow strain rate tensile (SSRT) testing. The susceptibility of the alloys to SCC was confirmed by analyzing the fracture surfaces using scanning electron microscope. The slow strain rate tensile testing results, together with fractography, confirmed that both the magnesium alloys were susceptible to SCC in the physiological environment. [Copyright &y& Elsevier]

Abstract: This paper discusses a new approach to determination of threshold stress intensity factor for stress corrosion cracking (K ISCC) of stainless steel in 42% MgCl2 environment at 154°C. K ISCC of solution-annealed and sensitized AISI 304 stainless steel in chloride environment has been determined using circumferential notch tensile (CNT) technique. K ISCC data generated using CNT technique have been compared with those generated using traditional techniques such as compact tension and double cantilever beam (reported in the literature). The results presented here validate the ability of CNT technique for determination of K ISCC of sensitized as well as solution-annealed austenitic stainless steel. This paper also discusses the mechanistic aspects of the difference in fractographic features of the sensitized and solution-annealed stainless steel. [Copyright &y& Elsevier]

Abstract: Threshold stress intensity factor for stress corrosion cracking of heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. HAZ microstructure produced during manual metal arc welding of grade 250 steel was simulated over a length of 35mm of CNT specimens, using a thermo-mechanical simulator. Inter-granular stress corrosion cracking has been confirmed with scanning electron microscope. The results presented here validate the ability of CNT technique for determination of KISCC of HAZ and base metal. [Copyright &y& Elsevier]

Abstract: The role of microbiological environments containing pure and mixed cultures of sulfate-reducing bacteria in stress corrosion cracking of carbon steel in biotic and abiotic environments was investigated. Carbon steel specimens were subjected to slow strain rate testing in a synthetic seawater environment (3.5% sodium chloride solution), with and without the microbiological cultures. Specimens tested in biotic (microbiological) conditions showed a considerable loss of ductility, as compared to those tested in abiotic conditions. Fractography of the specimens tested in abiotic solutions suggested features of only ductile failure (dimples), whereas those tested in biotic conditions had additional features of brittle cracking. Results also suggested the role of sulfate-reducing bacteria in promoting hydrogen-assisted cracking. [Copyright &y& Elsevier]

The paper presents results of the experimental investigation of the role of strain rate and temperature on stress corrosion cracking (SCC) of mild steel at elevated temperatures. For investigation of SCC-susceptibility, slow strain rate tests were carried out at various strain rates (in the range of 0.12×10-6–0.55×10-6 s-1) in an industrial caustic solution and an inert environment (liquid paraffin) at 90 and 150 °C. The mode of the SCC crack propagation and the fractographic details of the specimens tested in the two environments have been compared. The investigation suggests susceptibility of the mild steel to SCC in the given caustic solution at 150 °C only when the tests were carried out at a strain rate <0.25×10-6s-1, and no susceptibility at 90 °C. [Copyright &y& Elsevier]

Abstract: The paper presents a new strategy based on combined analytical and finite element (FE) solution to hydrogen assisted stress corrosion crack growth. The diffusion process is solved analytically through both one-and two-dimensional modelling. These solutions are adopted with two-dimensional FE based cohesive zone model of crack extension study. The results fit well with published experimental data and show improvement over the predictions by full FE approach. The new solution approach helps to reduce time required for simulation/computation. The study has produced a relationship between concentration dependent reduction in cohesive strength and plastic strain rate. [Copyright &y& Elsevier]

Abstract: The threshold stress intensity factor for stress corrosion cracking (KIscc ) of the simulated heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. The HAZ microstructure produced upon manual metal arc welding of grade 250steel was simulated over a length of 35mm of CNT specimens, using a thermo-mechanical simulator. Inter- and trans-granular stress corrosion cracking has been confirmed using a scanning electron microscope. The results presented here validate the ability of CNT technique for the determination of KIscc of HAZ and Base metal. Crack growth rates have also been determined using CNT technique. Further, the effect of microstructures on KIscc and crack growth rate is discussed in the present study. The determined KIscc of Base metal and simulated heat affected zone in 30% caustic solution is 24 and 45MPam1/2, respectively. [Copyright &y& Elsevier]

It is essential for any temporary implant to possess adequate strength to maintain their mechanical integrity under the synergistic effects of mechanical loading characteristics of human body and the corrosive physiological environment. Such synergistic effects can cause stress corrosion cracking (SCC). The aim of the present study is to investigate the effect of the addition of bovine serum albumin (BSA) to Hanks' solution in corrosion and SCC susceptibility of AZ91D magnesium alloy. The electrochemical impedance spectroscopy (EIS) results indicated that the addition of BSA increased corrosion resistance of the alloy during the first 48 h of immersion and then decreased it rapidly. The energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses indicated adsorption of BSA on the alloy surface during initial hours of immersion. However, with the increasing immersion time, BSA chelated with the corrosion products causing disruption of the protective film; thus, it accelerated the corrosion of the alloy. Both the mechanical data and fractographic evidence have confirmed susceptibility of the alloy to SCC. However, in the presence of BSA, the alloy suffered greater SCC which was attributed to its increased susceptibility towards localized corrosion. [ABSTRACT FROM AUTHOR]