Crevice Mouth Research Articles

A physiological model of the release of gas bubbles from crevices under decompression

Moving bubbles have been observed in the blood during or after decompression using ultrasonic techniques. It has been proposed that these may grow from nuclei housed on the blood vessel wall. One candidate for bubble nucleation is hydrophobic crevices. This work explores the growth of gas pockets that might exist in conical crevices and the release of bubbles from these crevices under decompression. An existing dynamic mathematical model for the stability of gas pockets in crevices [Chappell, M.A., Payne, S.J., in press. A physiological model of gas pockets in crevices and their behavior under compression. Respir. Physiol. Neurobiol.] is extended to include the behavior as the gas pocket reaches the crevice mouth and bubbles seed into the bloodstream. The behavior of the crevice bubble is explored for a single inert gas, both alone and with metabolic gases included. It was found that the presence of metabolic gases has a significant effect on the behavior under decompression and that this appears to be due to the high diffusivity of these gases.

Hydrogen Mapping Across Crevices

Crevice corrosion was investigated in pure iron exposed to a strong acetate buffer solution. The specimens experienced the potential drop inside the crevice, which resulted in the formation of passive, active, and hydrogen evolution regions. The distribution of hydrogen evolution occurring inside the crevice was mapped using a silver decoration technique applied on the other side of the specimen. The results suggest that the entry of absorbed hydrogen show a variation in the intensity across the crevice. The crevice mouth and bottom portions showed the least hydrogen entry, whereas the intermediate portion yielded the significant amount which resulted in the accumulation of silver deposits. © 2005 The Electrochemical Society. DOI: 10.1149/1.1979367 All rights reserved.

THE IMPORTANCE OF A CREVICE ENVIRONMENT TO THE LIMPET HELCION PECTUNCULUS (PATELLIDAE)

Helcion pectunculus has an organized distribution within rock crevices with smaller, younger limpets towards the back of the crevice and larger, older limpets towards the crevice mouth. The crevice refuge provides H. pectunculus with a stable and buffered environment with higher relative humidities and lower rock surface temperatures than adjacent exposed rock surfaces. Limpet body temperatures were significantly lower in crevice refuges compared with limpets on adjacent exposed rock surfaces. Body temperatures never exceeded the rock surface temperatures. It is suggested that this is a result of morphological adaptations such as short spines on the shell and allometric growth (shell increasing in height faster than length). Compared with other South African patellids, less force is required to detach H. pectunculus from the substratum and individuals deprived of a crevice refuge experienced extremely high mortalities, 45% of limpets being lost during the first high tide period.

A study on the modeling of parameters affecting the Ir drop mechanism in the initiation stage of crevice corrosion

The sizable potential difference between the crevice mouth and the crevice tip is difficult to explain from theories mainly based on the chemical changes in crevice solution. In this study, the IR drop theory was adopted to explain the initiation of crevice corrosion in the framework of IR drop in the crevice electrolyte. Furthermore, the parameters linking the two components of potential drop — one due to accumulated charges from chemical changes of ionic species by mass transfer and the physical one caused by the current path through the crevice electrolyte — are examined. The mathematical model is constructed and the IR drop in the crevice is calculated using the FVM method. The chemical composition change in the crevice solution can affect the IR drop behavior through the intermediate parameter form of ion conductivity and viscosity, etc., together with the potential difference generated by the electric charge of locally charged ions. Viewed in this light, it can be concluded that the initiation mechanism only by IR drop is insufficient to explain the initiation of crevice corrosion in stainless steels that have very strong passivity.

Transport processes in steam generator crevices—I. General corrosion model

A new model for describing transport processes in Pressurized Water Reactor (PWR) Steam Generator (SG) tube/tube support plate crevices has been developed. The internal and external systems are coupled using the principle of charge conservation, recognizing that anodic and cathodic reactions may occur at any location. This model includes the influence of convective transport, diffusion, and migration of species in the crevice on the evolution of crevice properties. Calculations show that under normal operating conditions after an initial time period, the concentrations of all species and potential of the metal reach limiting values at the center of the crevice. After this period, the space in the cavity can be subdivided into two parts: (1) a region near the crevice mouth where concentration and potential gradients exist, and (2) a region near the crevice center where the concentrations of the various species and metal potential attain constant values. In the latter, the cathodic current density compensates for the corrosion current. The limiting values of the specie concentrations and the metal potential do not depend on the corrosion potential of the metal, but do depend on the available superheat and the bulk composition of the solution. For a sufficiently negative corrosion potential, the net current in the cavity can be negative, which must be balanced by a positive current on the external surface.

The effect of H 2S on the crevice corrosion of aisi 410 and ca6nm stainless steels in 3.5% nacl solutions

Crevice corrosion of two martensitic stainless steels of different composition were studied in sodium chloride solution with and without H 2S at pH 7.0 and 4.2. In the presence of H 2S, crevice corrosion of both studied steels has been found to occur in the active region of potentials. In the nitrogen de-aerated solutions the corrosive attack occurred inside the crevice while in H 2S-containing solutions the corrosion occurred outside crevice mouth.

Cooling Kinetics: Evidence for a Hydrogen Countercell in 304L Stainless Steel Crevices at High Temperatures

Cell current reversals deep within tubular 304L stainless steel crevices containing electrolyte at 290°C demonstrated reduction at previously anodic sites. Analysis of transient currents during cooling showed an increase in anodic activity adjacent to the crevice mouth as oxygen reduction at the mouth decreased, a direct indication that molecular hydrogen was diffusing outward and oxidizing in the active/passive transition region. Calculations confirmed that a similar hydrogen countercell was operating in crevices containing .

Oxidation of Mild Steel Crevices in high pressure carbon dioxide

AbstractThe oxidation of mild steel crevices in pressurized carbon dioxide containing small amounts of carbon monoxide, hydrogen and water is analysed mathematically. For bulk gas which is hydrogen‐rich with respect to the water‐gas shift reaction equilibrium, conversion of hydrogen to water on magnetite surfaces is rapid. This, and the differential diffusion of hydrogen and water, lead to crevice gas compositions very different from the bulk gas.In restricted crevices, protective oxidation causes enhanced carbon monoxide concentration. In less restricted crevices, enhanced water concentrations are possible. Both situations lead to shorter times to breakaway oxidation than in the bulk gas.Crevice breakaway oxidation rates may be enhanced by a factor of 1.7 for suitable crevice geometry, provided that the bulk gas is not equilibrated at the crevice mouth. In more restricted crevices the rate is depressed.These predictions of the analysis are substantiated by experimental oxidation of mild steel specimens within inert glass capsules having a leak path. Such capsules provide a convenient method of exposing material to a wide range of gas compositions within the same autoclave.