Modelling of Crevice Corrosion Kinetics in Stainless Steel

Abstract

Understanding corrosion kinetics is of high importance because it has a significant adverse effect on the economics and sustainability of various industries. My research work aimed to investigate the crevice corrosion kinetics in iron nuts and bolts. 2-Dimensional simulation models of iron nuts and bolts were simulated in Comsol Multiphysics software. This research provides insight into the crevice corrosion mechanism and factors that are affecting it. Corrosion kinetics for iron in 10 M and 0.01M NaCl solution, is different, especially inside the crevice. For instance, the peak pH value at the bottom of the crevice in 10M and 0.01M NaCl solution are 10.4 and 11.5, respectively. In addition to that, the lowest potential value at the bottom of the crevice in 10M and 0.01M NaCl solutions were found to be -365 mV and -790mV, respectively. In this work, the crevice corrosion mechanism is closely followed by the passive dissolution theory. One of the key findings in my research is that position in the geometry affects the oxygen concentration, and subsequently affects the pH and the corrosion thermodynamic characteristics. In 1M NaCl solution, in 20 mm crevice width, at the bottom of crevice, the oxygen concentration depleted after 8 min, where pH was estimated to be 8.5, and the metal entered the active region as shown in pourbaix diagram (fig. 4-9). Moreover, the relationship between crevice width-depth and oxygen concentration was investigated. The simulation revealed that the oxygen concentration inside the crevice depletes completely. On the contrary, the outer crevice did not deplete. The simulations showed that smaller crevice width and larger crevice depth have higher oxygen depletion rates. At the bottom of the crevice in geometry 1, oxygen depletes completely in 21 min for 50 mm and less than 2 minutes for 5 mm crevice width. Moreover, it took less than 0.5 min for a 5 mm crevice width to deplete its oxygen completely in the geometry that has twice the depth. Oxygen diffusion into the crevice was found to be limited, especially for the smaller and deeper crevice. The results presented in this thesis help the industrial partner Grundfos, to simulate the effect of salt solution and geometry in an occluded region.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Anthony Cook (Supervisor), Dirk Engelberg (Supervisor) & Nicholas Stevens (Supervisor)