James C. Earthman

Corrosion inhibition by aerobic biofilms on SAE 1018 steel

Abstract Carbon steel (SAE 1018) samples were exposed to complex liquid media containing either the aerobic bacterium Pseudomonas fragi or the facultative anaerobe Escherichia coli DH5α. Compared to sterile controls, mass loss was consistently 2- to 10-fold lower in the presence of these bacteria which produce a protective biofilm. Increasing the temperature from 23 °C to 30 °C resulted in a 2- to 5-fold decrease in corrosion inhibition with P. fragi whereas the same shift in temperature resulted in a 2-fold increase in corrosion inhibition with E. coli DH5α. Corrosion observed with non-biofilm-forming Streptomyces lividans TK24 was similar to that observed in sterile media. A dead biofilm, generated in situ by adding kanamycin to an established biofilm, did not protect the metal (corrosion rates were comparable to those in the sterile control), and mass loss in cell-free, spent Luria-Bertani (LB) medium was similar to that in sterile medium. Confocal laser scanning microscopy analysis confirmed the presence of a biofilm consisting of live and dead cells embedded in a sparse glycocalyx matrix. Mass-loss measurements were consistent with microscopic observations of the metal surface after 2 weeks of exposure, indicating that uniform corrosion occurred. The biofilm was also able to withstand mild agitation (60 rpm), provided that sufficient time was given for its development.

On the mechanism of grain formation during spray atomization and deposition

Abstract The mechanisms governing the morphological changes in the microstructure of spray atomized and deposited Ni3Al were studied, with particular emphasis on the formation of a spheroidal grain morphology. Accordingly, the various microstructural features present in the spray deposited material were rationalized on the basis of thermal energy considerations. The formation of spheroidal grains was proposed to evolve from: (a) the homogenization of dendrites that did not deform extensively during deposition; and (b) the growth and coalescence of deformed or fractured dendrite fragments. Support for this suggestion was provided by experimental results and numerical analyses which show that the microstructure of Ni3Al is exposed to a high temperature anneal during deposition. Moreover, the results show that during high temperature annealing, the deformed or fractured dendrite fragments that were initially present in the spray deposited materials grew and coalesced leading to the development of a spheroidal grain morphology. On the basis of a coarsening mechanism, the relative annealing time under a particular cooling rate may be calculated and converted into a spheroidal grain size, dsph, from the following equation, dsph = 15.2· Ṫ−0.35. The experimental results were observed to concur with this relationship.

The influence of bacteria on the passive film stability of 304 stainless steel

The stability of stainless steel 304 in the presence of aerobic and anaerobic bacteria was investigated in a continuous flow system using electrochemical impedance spectroscopy and scanning electron microscopy (SEM) examinations. The results show that the open circuit potential (OCP) of stainless steel was ennobled (shifted in the electropositive direction) by about 150 mV in the presence of the aerobic bacteria. The observed change of OCP in the electropositive direction can be explained by an increase in the rate of the cathodic reaction. Also the presence of an aerobic biofilm led to a decrease in the polarization resistance of stainless steel is not only due to the growth of micropits, as shown from SEM micrographs, but also due to thinning of the passive film. In the presence of only Pseudomonas fragi, the electrochemical impedance response showed a capacitive behavior with RP on the order of 500 kV. The addition of anaerobic sulfate reducing bacteria (SRB) to the test medium decreased RP to 12 kV due to an increase in localized corrosion, as indicated by SEM examination.

CAVITATION AND CAVITY-INDUCED FRACTURE DURING SUPERPLASTIC DEFORMATION

The characteristics of fracture by cavitation in superplastic materials are reviewed. Particular attention is paid to the theoretical developmental aspects of cavity nucleation, cavity growth and cavity interlinkage. Various factors, including grain boundary sliding, impurity atoms or particles, phase proportion, deformation temperature, strain rate, strain and grain size, are discussed. Finally, methods for controlling cavitation during superplastic deformation are summarized, and problems which require further work are also presented.

Axenic aerobic biofilms inhibit corrosion of SAE 1018 steel through oxygen depletion

Abstract Corrosion inhibition of SAE 1018 steel by pure-culture biofilms of Pseudomonas fragi and Escheri-chia coli DH5α has been evaluated in complex Luria-Bertani medium, seawater-mimicking medium, and modified Baars medium at 30 °C. In batch cultures, both bacteria inhibited corrosion three to six fold compared to sterile controls, and the corrosion was comparable to that observed in anaerobic sterile media. To corroborate this result, a continuous reactor and electrochemical impedance spectroscopy were used to show that both P. fragi K and E. coli DH5α decreased the corrosion rate by 4- to 40-fold as compared to sterile controls; this matched the decrease in corrosion found with sterile medium in the absence of oxygen and with E. coli DH5α grown anaerobically. In addition, the requirement for live respiring cells was demonstrated by the increase in the corrosion rate that was observed upon killing the P. fragi K biofilm in continuous cultures, and it was shown that fermentation products do not cause an increase in corrosion. Hence, pure-culture biofilms inhibit corrosion of SAE 1018 steel by depleting oxygen at the metal surface.

Effect of Cd on superplastic flow in the Pb-62 wt% Sn eutectic

Abstract A detailed investigation of the superplastic deformation of two grades of the Pb-62 Sn eutectic (where the compositions here and subsequently are in weight per cent) was conducted under identical experimental conditions of grain size (about 8 μm), temperature (390–420 K) and stress (0·1–10 MPa). The first grade (grade 1) is Pb-62 Sn containing 890 wt p.p.m. Cd while the second grade (grade 2) is very-high-purity Pb-62 Sn. The experimental results on grade 1 (Pb-62Sn doped with Cd) show the presence of a sigmoidal relationship between strain rate and stress. This sigmoidal behaviour exhibits firstly an intermediate-stress region (region II) which is characterized by a stress exponent n of about 1·7 and an activation energy Q similar to the activation energy Q gb anticipated for grain-boundary diffusion and secondly a low-stress region (region I), where n > 2·6 and Q > Q gb. By contrast, the experimental results on grade 2 (high-purity Pb-62 Sn) show that the characteristics of superplastic flow at...

Natural tooth intrusion and reversal in implant-assisted prosthesis: Evidence of and a hypothesis for the occurrence

Based on clinical observation, a hypothesis of the mechanism of intrusion of natural teeth in an implant-assisted prosthesis is suggested. Engineering principles are presented that establish an energy absorption model as it relates to the implant-assisted prosthesis. In addition, in the course of patient treatment it has been discovered that the intrusion of natural teeth can be reversed. Patient histories that demonstrate intrusion reversal are reviewed. The possible mechanisms for the intrusion/reversal phenomenon are presented and preventative recommendations are given.

Importance of biofilm formation for corrosion inhibition of SAE 1018 steel by axenic aerobic biofilms

To investigate if corrosion inhibition by aerobic biofilms is a general phenomenon, carbon steel (SAE 1018) coupons were exposed to a complex liquid medium (Luria–Bertani) and seawater-mimicking medium (VNSS) containing fifteen different pure-culture bacterial suspensions representing seven genera. Compared to sterile controls, the mass loss in the presence of these bacteria (which are capable of developing a biofilm to various degrees) decreased by 2- to 15-fold. The extent of corrosion inhibition in LB medium depended on the nature of the biofilm: an increased proportion of live cells, observed with confocal scanning laser microscopy (CSLM) and image analysis, decreased corrosion. Corrosion inhibition in LB medium was greatest with Pseudomonas putida (good biofilm formation), while metal coupons exposed to Streptomyces lividans in LB medium (poor biofilm formation) corroded in a manner similar to the sterile controls. Pseudomonas mendocina KR1 reduced corrosion the most in VNSS. It appears that only a small layer of active, respiring cells is required to inhibit corrosion, and the corrosion inhibition observed is due to the attached biofilm.

Static stretch affects neural stem cell differentiation in an extracellular matrix-dependent manner.

Neural stem and progenitor cell (NSPC) fate is strongly influenced by mechanotransduction as modulation of substrate stiffness affects lineage choice. Other types of mechanical stimuli, such as stretch (tensile strain), occur during CNS development and trauma, but their consequences for NSPC differentiation have not been reported. We delivered a 10% static equibiaxial stretch to NSPCs and examined effects on differentiation. We found static stretch specifically impacts NSPC differentiation into oligodendrocytes, but not neurons or astrocytes, and this effect is dependent on particular extracellular matrix (ECM)-integrin linkages. Generation of oligodendrocytes from NSPCs was reduced on laminin, an outcome likely mediated by the α6 laminin-binding integrin, whereas similar effects were not observed for NSPCs on fibronectin. Our data demonstrate a direct role for tensile strain in dictating the lineage choice of NSPCs and indicate the dependence of this phenomenon on specific substrate materials, which should be taken into account for the design of biomaterials for NSPC transplantation.

Two dimensional modeling of momentum and thermal behavior during spray atomization of γ-TiAl

Abstract A two dimensional model was formulated for the study of the momentum and thermal behavior of atomized droplets of γ-TiAl. The velocity, temperature, flight time, cooling rate and solidification behavior of droplets were numerically investigated as a function of the axial distances from the atomization zone and the radial distances from the central line of the spray cone. The velocity, temperature, cooling rate, flight time and solidification behavior of a droplet strongly depend on the initial position ( r 0 ) and the diameter of the droplet ( D ). The two dimensional droplet size distribution in the spray cone changes from being heterogeneous to being almost homogeneous as axial flight distance increases. The two dimensional distribution of the fraction solidified in the spray cone is heterogeneous. The fraction solidified in the spray at any given axial distance increases with increasing radial distance from the spray axis.