Joseph R. McDermid

Physical Characterization of Cathodically-Activated Corrosion Filaments on Magnesium Alloy AZ31B

In this study, the filiform-like corrosion behavior of Mg alloy AZ31B was characterized using analytical microanalysis. Recent electrochemical measurements have shown that the corrosion filaments on AZ31B support enhanced cathodic reaction kinetics, but there has been little strong physical evidence published to explain this behavior. Hence, the specific aim of the investigation was to contribute to the understanding of the physical origin of the “cathodic activation” of the corrosion filaments. Highlights of this investigation include the presence of through-thickness cracks within the corrosion filaments, Al-Mn particles (identified as Al11Mn4 with energy-dispersive x-ray spectroscopy (EDS) quantification) in the preexisting films and filament corrosion products, and a significant enrichment of Zn at the corrosion filament/metal interface after aging of the filament. Diffraction patterns of the corrosion filaments indicated they were composed of nanocrystalline magnesium oxide (MgO), which was suggested...

Measuring the Mechanical Properties of Aluminum Sheets and Their Resistance Spot Welds at Large Strains Using Digital Image Correlation Coupled with a Modified Shear Test

Constitutive behavior of aluminum sheets and their resistance spot welds at large strains is critical for light weigh vehicle design analysis. However, data from uniaxial tensile tests are usually limited to small strains or material instability. A novel technique was developed using digital image correlation coupled with a modified shear test to directly measure stress – strain curves of aluminum sheets at large strains. The modified shear sample prevents end rotation of the shear zone compared to the ASTM standard B831 test. The results show that the effective stress and effective strain curves from shear tests match those obtained from uniaxial tension, but only by incorporating the material anisotropy using the Barlat–Lian yield function. The technique was then further applied to aluminum resistance spot welds to allow the first time to determine both the shear strength and stress-strain curves of spot welds at large strains. The results show that while there is slight change in yield strength and work hardening behavior for non heat treatable AA5754 spot welds, there is significant decrease in both yield strength and work hardening rate for heat treatable AA6022-T4 spot welds. Technique details and experimental validation will be discussed.

Effect of surface preparation on the corrosion resistance of friction stir linear lap welded AZ31B-H24

ABSTRACT The effect of surface preparations on the atmospheric (continuous NaCl salt fog) corrosion resistance of bare (uncoated) linear friction stir lap welded magnesium (Mg) alloy AZ31B-H24 joints was investigated. The surface preparations evaluated included as-received (as-welded for the weld zones), mechanically abraded and acid-cleaned. Electrochemical potentiodynamic polarisation, X-ray photoelectron spectroscopy and Auger electron spectroscopy measurements were made to elucidate the link between the surface preparation and the relative corrosion resistance observed across the microstructural zones of the joints. Heavy metal surface contamination of the as-received material, rather than the air-formed oxide or differences in alloy grain size resulting from variations in thermo-mechanical processing history, was found to be the controlling factor affecting the corrosion mode observed and, in turn, the relative corrosion resistance across the joint. Removal of the heavy metal contamination changed the corrosion mode from pit-like to filament-like, which in turn homogenised the corrosion resistance across the joint in the continuous salt fog exposure.

The fluctuating velocity field of a forced planar impinging gas jet

ABSTRACTThe fluctuating velocity field of a planar impinging gas jet is studied using particle image velocimetry, while anti-symmetrically forcing the jet column at the nozzle exit using planar synthetic jets. The jet exit velocity is set to  = 11 m/s providing a jet Reynolds number of , and the impingement distance is held constant at eight times the nozzle width W. Under forcing conditions near unstable frequencies, both the stream-wise and cross-stream fluctuating velocity fields of the jet have very different characteristics than that of the unforced jet. The fluctuating velocity fields reported here were also found to have qualitatively similar features to that of a high-speed self-excited impinging jet. For the highly periodic flow conditions tested, a triple decomposition, and a low order reconstruction of the fluctuating velocity field using proper orthogonal decomposition is employed. These methods allow for the extraction of the coherent velocity and turbulence fields from the forced planar impi...

Technical Note: Screening of Salt Fog Environments for Mg Alloy Stress Corrosion Cracking Susceptibility

U-bend samples of Mg alloy AZ31B were subjected to continuous near-neutral salt fog exposure to evaluate the effect of a variable salt content on stress corrosion cracking (SCC) susceptibility. Usi...

Effect of Surface Condition on the Localized Corrosion Behavior of Magnesium Alloy AZ31B

The localized corrosion behavior of AZ31B magnesium alloy sheet was studied in 0.01 M aqueous NaCl solution using the scanning vibrating electrode technique (SVET). Ongoing investigations by the authors have shown significant variation in corrosion mode and relative corrosion resistance depending on the alloy’s surface condition. Comparisons were made between the observed behavior of the as-received, acid-cleaned and mechanically-polished surfaces. The SVET work was conducted to elucidate variations in the local electrochemical behavior for each surface condition. SVET maps were coupled with images of the corroding surfaces in order to show the progression of corrosion as a function of time. Schematic corrosion process models are proposed in order to explain the observed differences between the three surface conditions.

Corrosion Behaviour of Friction Stir Welded AZ31 Joints for Automotive Applications

The corrosion behavior of friction stir welded (FSW) Mg alloy AZ31 used to fabricate an Mg alloy-intensive front end subassembly demo was investigated in this paper. The corrosion behavior of the uncoated FSW AZ31 joints was evaluated by ASTM B117 salt fog testing and electrochemical techniques including potentiodynamic polarization. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition of the surfaces on the stir zone and base material prior to exposure. Scanning electron microscope (SEM) was used to examine the mode and extent of the corrosion damage across the FSW joint. An improved corrosion resistance was observed in the stir zone compared to the base material in the as-received condition. However, the stir zone showed almost identical corrosion behavior as the base material after the as-received surface was cleaned. These results indicate the significant deteriorative effect of surface contamination on the corrosion resistance of AZ31, but the influence of grain size on the corrosion resistance was found to be insignificant.

Effect of Carbon Gradient on the Microstructure and Mechanical Properties of Fe-22Mn-C TWIP/TRIP Steels

High-manganese austenitic steels are promising emerging automotive steels demonstrating high strength and ductility. The main deformation products observed in these steels are mechanical twins and ε-martensite, where the dominant deformation products vary quite strongly with stacking fault energy (SFE), which in turn is a very strong function of the alloy carbon content. In this research, a Fe-22Mn-0.6C sheet steel was decarburized to achieve a variety of through-thickness C gradients, thereby varying the dominant deformation products through the sheet thickness, with the overall objective of producing unique microstructures and mechanical properties. Microstructural analyses after interrupted tensile testing indicated that the amount of both mechanical twins and ε-martensite increased with increasing true strain, where the deformation products changed from mechanical twins at the higher-C core to ε-martensite at the lower-C surface. The spring-back properties of the C graded steels were also compared with reference to the effect of differential carbon concentration gradient.

Effect of Corrosion on the Tensile Properties of Friction-Stir Welded AZ31B Sheet

To facilitate the use of magnesium and its alloys within automotive structures, it is necessary to characterize their possible mechanical property degradation in typical application environments. This work examines the effect of exposure to NaCl-based corrosive environments on the mechanical properties of friction stir welded (FSW) AZ31B magnesium alloy sheet. A complete microstructural, electrochemical, residual stress and mechanical property characterization of the as-received FSW panels was performed. Samples were subsequently exposed to 0.01 M and 0.1 M NaCl solutions at the corrosion potential for 24 h and any changes in mechanical properties, as a function of exposure, to the corrosive environment monitored. Friction stir welded panels exhibited large decreases in ductility compared to the base AZ31B material. Exposure to 0.1 M NaCl for 24 hours resulted in a degradation in mechanical properties for the base material, as well as the friction stir welded panel, whereas no significant changes were found for samples exposed to 0.01 M NaCl for 24 hours.

Effect of Some Microstructural Parameters on the Corrosion Resistance of Magnesium Alloys

The influence of the s-Mgi7Ali2 phase and, to a lesser degree, the solute content in the a-Mg matrix on the corrosion resistance of Mg alloys was investigated by cyclic potentiodynamic polarization and potentiostatic polarization tests using AZ31B, AM30 and AM60B in contact with a mildly aggressive near-neutral saline solution. Results showed that all three Mg alloys corrode in a partially protective state under open circuit conditions in the test solution. It was also determined that the surface film formed on each exhibits a similar apparent breakdown potential. This indicates that microstructure parameters such as the presence of the s-phase and the solute content of the a-Mg matrix do not strongly influence the factors controlling the breakdown of the surface films formed. It was further determined that the moderately improved protectiveness of the surface film, rather than the distribution of the s-MgnAli2 phase, is responsible for the improved (short-term) corrosion resistance exhibited by AM60B at potentials below the breakdown potential.