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Corrosion
Published in J. T. McGuire, Pumps for Chemical Processing, 2020
The third basic galvanic cell, the concentration cell, develops a potential by a difference in electrolyte concentration. A particular version of this cell, the oxidation type, is the one that usually afflicts pumps. Either by design or as a product of debris accumulation, some regions become oxygen deficient, see Fig. 3.4. With materials dependent upon passivity for corrosion resistance, the oxygen-deficient regions become anodic and susceptible to rapid corrosion. The concentration cell is thought to be a factor in pitting corrosion, serving to accelerate the rate of corrosion as the pitting deepens, see Fig. 3.4.
Electrochemistry
Published in W. John Rankin, Chemical Thermodynamics, 2019
The Daniell cell is an example of a chemical galvanic cell in which the potential difference generated by the cell is due to a spontaneous chemical reaction. The other type of galvanic cell is the concentration cell in which the potential difference generated by the cell is due to a difference in concentration across a boundary. There are two types, electrolyte concentration cells and electrode concentration cells. An example of an electrolyte concentration cell isCu|CuSO4aq,lowcon||CuSO4aq,highcon|Cu
Corrosion of mild steel: a microbiological point of view
Published in Canadian Metallurgical Quarterly, 2022
Dissimilar metal couplings or the connection to a positive battery terminal can also remove electrons. When a deposit covers a metal surface, the metal outside the deposit is exposed to oxygen, while the metal beneath the deposit is protected from it. This promotes the formation of a corrosion cell, with metal ions developing in the anodic zone under the deposit, resulting in pitting. Outside the deposit, electrons will move to a metal surface to decrease oxygen (cathodic process), generating hydroxyl ions [42]. An oxygen concentration cell is a type of cell that is largely involved in microbial corrosion. Crevice corrosion is a kind of corrosion that is comparable to pitting corrosion. Pitting is caused by microbiological deposits on a smooth metal surface, whereas crevice corrosion occurs within fissures or other protected areas with the existence of a stagnant solution. Chemical concentration cells are another type of corrosion cell that could be caused by microbial activity in deposits [43].
The electric conductance of dilute sulfuric acid in water: a new theoretical interpretation
Published in Molecular Physics, 2021
In the stationary state of ionic solutions, under reversible movement of all ions, not only is the SiS a perfectly spherical, concentric shell, but the entire ion atmosphere is also spherical. At a characteristic radial distance 1/κ from the origin (i.e. from the centre of the chosen, β-ion), the ion atmosphere acquires its highest charge density, and the overall charge is distributed unevenly on both sides of the sphere of radius 1/κ, being smaller at a radial distance from the origin below 1/κ, and usually very large above 1/κ. At extremely high dilution, when 1/κ lies far away from the origin (being of the order of magnitude of hundreds of Å), the charge distribution about 1/κ becomes more even. This is the case under which activity coefficients are measured; for example, by potentiometric measurements of the voltage between two electrodes placed in solutions of a binary electrolyte at different concentration (the ‘concentration cell’), both solutions being ‘at rest.’
A critical overview of monitoring infrastructural health using corrosion probes
Published in Corrosion Engineering, Science and Technology, 2020
As shown in Figure 1(a), serious localised corrosion occurred on a relatively small civil structure, a galvanised steel lamp post, leading to a vital accident [32]. The lamp post was manually and visually inspected only three days before the accident, failing to identify serious localised corrosion on the lamp post, at a location approximately 3 cm below the damp soil surface. This accident raised major public concerns over the safety of lamp posts in Singapore since in its Tampines council alone, there were 4,500 such posts and more than 300 of them were corroded and replaced every year. In this structure, the ‘worst-case scenario’ corrosion occurred below the damp soil surface because this location had the most favourable corrosion conditions: differential aeration, high level of moisture and conductivity, and the existence of crevice between the lamp post and soil. Oxygen concentration cell effects and chemistry changes in the crevice area lead to significant electrochemical heterogeneity and rapid crevice corrosion at the location. This location was obviously the anodic area of a crevice corrosion cell where fast corrosion dissolution reactions occurred. The cathodic area would be at the interface of the air and the soil where high oxygen content and high level of moisture existed. There was a significant galvanic corrosion effect between anodic and cathodic areas, leading to localised corrosion at the location 3 cm below the damp soil surface.