Surfaces In Close Proximity Research Articles

A review of the theoretical modelling of crevice and pitting corrosion

A metal surface exposed to a corrosive environment may, under certain conditions experience attack at a number of isolated sites. If the total area of these sites is much smaller than the surface area then the metal is said to be experiencing localized corrosion. The rate of dissolution in this situation is often much greater than that associated with uniform corrosion and structural failure may occur after a very short period. Several different modes of localized corrosion may be identified. These are dependent on the type of metal undergoing corrosion and its environment at the time of attack. One of the most destructive forms is pitting corrosion which is characterized by the presence of a number of small pits on the exposed metal surface. The geometries of the pits depend on many factors such as the metal composition and the surface orientation. A similar mode of corrosion is crevice attack which occurs in situations where two or more surfaces in close proximity lead to the creation of a locally occluded region in which enhanced dissolution may occur. The rapidity with which localized corrosion can lead to the failure of a metal structure and the extreme unpredictability of the time and place of attack, has led to a great deal of study of this phenomenon. There are many difficulties associated with the experimental measurements necessary to understand the process and as a result a number of theoretical models of the various stages of localized corrosion have been developed. In this paper, a review of the literature on this modelling is presented with particular reference to crevice and pitting corrosion. The accuracy of the predictions of the models and the validity of the various approximations are discussed and both the achievements and the weak areas of the current state of corrosion modelling are demonstrated.

Calculation of the Aerodynamic Forces on Automotive Lifting Surfaces

A numerical technique was developed to investigate the performance of automotive lifting surfaces in close proximity to ground. The model is based on the Vortex Lattice Method and includes freely-deforming wake elements. The ground effect was simulated by reflection and both steady and unsteady pressures and loads on various wing planforms were considered. Calculated results are presented for wings having both positive and negative incidences, with and without ground effect. Also the transient lift of a wing in a plunging motion was analyzed in ground proximity and at a negative angle of attack. Finally the periodic lift fluctuations on the front winglet of a racing car, due to its suspension oscillations, were calculated and found to exceed approximately twice the steady-state value.

Neutralization and Focussing of Pulsed Beams with High Current Densities

The intense pulsed ion beams with energies about 70 keV produced by the multiplate chamber(MPC) under low gas pressure are extracted from the cathode hole. High power densities are evidently proved by the craters with diameters about 15 micrometers or less on the surface of metallic targets. The major processes of beam production and propagation could be explained by the factors of the field escalation effects in the MPC; fast charge neutralization by the simultaneous introduction of electrons on surfaces in close proximity to the beam; bunching of time of flight and focussing of the plasma lens. It is expected that the processes described above could be used experimentally for simulation of intense pulsed ion beams propagating in the low gas pressure.

Interaction between Surfaces in Close Proximity

ACCORDING to theories of the interaction of double layers1,2, two surfaces in close proximity are mutually repelled by a force which increases as the surfaces are brought together. Therefore, if an air bubble is pressed against a plane glass surface under water, the disjoining film of water should reach an equilibrium thickness at which the double layer repulsion is balanced by the excess pressure of the bubble.

Electroviscosity. I. The flow of liquids between surfaces in close proximity

The conditions governing the rate of flow of ionic and non-ionic liquids in narrow channels are examined, and it is demonstrated that in certain circumstances the electrical resistance to shear in ionic liquids, due to the effect of an electrical potential gradient on the ions of the surface layers, may assume considerable importance relative to the mechanical (viscous) resistance to shear. The results of this analysis are of important application in various fields, and offer an explanation of anomalous results obtained by numerous experimental workers.

Disk-Sphere Transformation in Mammalian Red Cells

ABSTRACT The purpose of this paper is to try to explain the phenomenon of the transformation of the mammalian erythrocytes from disks to spheres when a small drop of a saline suspension is placed between glass slide and cover-glass. Ponder (1934) several years ago reviewed the work which had been done in connexion with this phenomenon. As was pointed out by Ponder, who was the first to show clearly the necessity of having the glass surfaces in close proximity for the formation of perfect spheres (1928-9), the two explanations which have been put forth to explain the phenomenon are both inadequate. As will be seen later, that of Waller (1930) is somewhat closer to the true explanation than that of Brinkman & van Dam (1920).