The differential aeration cell and the corrosion paradox

Abstract

Steel with a low volume of both anolyte and catholyte was used for monitoring of steel electrode corrosion rates under conditions in the environment of a differential aeration cell under the conditions of both stagnating and exchanging electrolyte. A 3% NaCl solution was used as the electrolyte. A low content of oxygen near the anode was ensured by purging the solution with nitrogen while a high content of oxygen near the cathode was obtained by saturating the solution with oxygen. All experiments were performed for 72 h. Corrosion rates of steel electrodes were identified by using the weight loss method in comparison with the resistometric method. The corrosion rate of an aerated electrode drops in stagnating electrolyte to a hundredth of millimetre per year (0.01 mm/a) as a result of the catholyte alkalization and subsequent surface passivation, and that it is lower than the corrosion rate of a non‐aerated electrode (0.12 mm/a). In a stagnating solution, the catholyte is alkalized (pH 11) whereas acidification of the anolyte is negligible. In the case of continuous exchange of the aerated catholyte and the oxygen‐depleted anolyte in the environment of a differential aeration cell, the cathode's corrosion rate is up to seven times higher (0.7 mm/a) than the corrosion rate of the anode (0.1 mm/a). The difference between the electrodes' corrosion rates is given by the IR drop in the electrolyte between the cathode and anode. It was experimentally proved that in the case of continuous electrolyte exchange in the differential aeration cell the cathode corrodes faster than the anode and that no corrosion paradox is observed.

Second‐harmonic generation technique for in situ study of passive film formation on carbon steel surfaces in aqueous solutions

“…This local increase in pH is hypothesized to shift the WE to the passive domain of the Pourbaix diagram, as shown schematically in Figure 9. [55][56][57][58][59][60][61][67][68][69][70][71][72][73][74][75] Thus, the application of a cathodic current is expected to promote the formation of a passive film, composed mainly of Fe 3 O 4 in the potential range of relevance here. While experimental evidence for the generation of F I G U R E 9 Schematic representation of the shift toward the passive region of the Pourbaix diagram of the WE (carbon steel) during cathodic protection in a non-buffered solution with initial pH 7.3, according to Leeds and Cottis.…”

“…[55] The pH of the WE during CP was estimated from corresponding data of pH at a current density of 0.5 A m −2 from the literature. [56][57][58][59][60][61][62][63][64][65][66] The black lines correspond to the equilibrium reactions of the iron species considering a soluble concentration of Fe 2+ in water of 10 −6 M. The dashed line corresponds to the reversible potential of the hydrogen evolution reaction (HER). [Color figure can be viewed at wileyonlinelibrary.com] alkaline conditions at the metal surface upon cathodic polarization has been widely provided in the literature, [55][56][57][58][59][61][62][63]67,68,71,72,75,76] limited experimental evidence is available that simultaneously and directly relates this raise in pH to passive film formation.…”

“…[79][80][81] Many researchers in the last 40 years showed that as a consequence of the application of the CP current alkalinization of the environment up to tens of centimeters from the steel occurred. [1,[56][57][58]60,62,63,67,[69][70][71][72]74,76,[82][83][84][85] Leeds, [55] Büchler et al, [86] Brenna et al, [87,88] and Glass et al [89] suggested that the application of a cathodic current, with the consequent increase in pH, promotes the formation of a protective film on the steel surface.…”

Corrosion Localization Analysis in T‐Shape Pipe Junction Based on Multielectrode Current Measurements

“…When flow passes through these elements, water velocity deviates from the mean value in different parts of the junction. Change in the flow rate leads to the different oxygen supply causing the formation of differential aeration galvanic couples between parts of the surface with dissimilar flow rate [6,7]. e operation of anodic region of galvanic couple accelerates metal dissolution and even leads to the penetration of the pipe wall within a short period.…”

Deeper insight into FAC corrosion mechanism

“…At present, there are few studies on this kind of problem (Fangteng, 1981; Yan et al , 2008; De Gruyter et al , 2001; Msallamová et al , 2015; Martins and Nunes, 2006), during which people can only use the method of hanging film in laboratory conditions to select two different oxygen concentrations for research, and are far from meeting the actual working condition of pipeline.…”