Classic Evans’s Drop Corrosion Experiment Investigated in Terms of a Tertiary Current and Potential Distribution

Abstract

Background: Evans’s drop is a classic corrosion experiment that is nearly 100 years old, and it is analogous to other corrosion systems promoted by O2 gradients. The availability of more robust finite element software packages opens the possibility to reach a deeper understanding of these kind of corrosion systems. Methodology: In order to solve the problem, the model includes the governing mass transport diffusion and migration equation and the material balance in a nonsteady state by the finite element method. This is performed using COMSOL Multiphysics to predict the tertiary current and potential distribution considering the geometry, reaction kinetics, and mass transport for each ionic species. Significant Findings: A simulation of the tertiary current and potential distribution of the Evans’s drop corrosion experiment on an iron surface is presented. An oxygen concentration difference of 0.18 mol m−3 between the center and the drop periphery sets up a potential difference of 60 mV which acts as a corrosion driving force. Reaction kinetics are described by Tafel equations. Results include the evolution of concentration profiles for OH−, Fe2+, Fe3+, Fe(OH)2, and Fe(OH)3.

Experimental Investigation and Modeling: Considerations of Simultaneous Surface Steel Droplets’ Evaporation and Corrosion

“…Relation ( 11) is written to highlight the driving force and the mass transfer coefficient in the form (12). Since c, D v , δ and even y Bm in (10) depend on t g and t s, or more correctly on those differences, it was considered, for k g, as dependent on the heat transfer driving force t g − t s .…”

“…A balance for the duration from τ to τ + dτ, when denoting the interphase transfer surface (evaporation surface) S(τ), yields Equation (12). Here S(τ) depends on the instantaneous mass of the droplet (m p ) as follows from the coupling of Equations ( 6)-( 8), where we put V τ0y = m pτ /ρ a .…”

“…Now, the problem of atmospheric corrosion of steels is treated much more complexly, starting mainly from the fact that water reaches the steel structures through rain and through condensation in drops (dew), which determines two main cases of corrosion, namely film corrosion and corrosion drops. Both cases of atmospheric corrosion can be characterized by approaches which consider them to be cases of simultaneous transfer of momentum and mass [9,10], respectively, and of simultaneous transfer of heat and mass [11,12] which are associated with the electrochemical process that takes place at the corrosion surface. In the case of droplet corrosion, their appearance on the condensation surface is a relatively fast process, under 10-15 min [13], if the condensation conditions are met [14], as is shown by Equation (2), where p s is the water saturation pressure, t s and t g represent the surface and air temperature, respectively, U r gives the air relative humidity and t dew is the dew point temperature.…”

“…Considering these remarks, we focused in this study on the analysis of the kinetics within the corrosion process of the droplets, upon their evaporation. Many of the current approaches to modeling droplet corrosion refer to the Evans droplet model [12,15,16], which, in its current form [15], characterizes the onset of hemispherical droplet corrosion using unsteady-state species concentration field equations with reactions occurring on the surface at the cathodic and anodic (droplet-based) sites. For small hemispherical drops, the absence of movement in the droplet can be accepted [17], while for large drops this fact is no longer acceptable because here the evaporation of the droplet is accompanied by the Marangoni effect [18][19][20].…”

Corrosion-driven droplet wetting on iron nanolayers

“…Further reactions lead to the formation of iron oxides and oxy-hydroxides, green and brown rust. Both electrochemical reactions at the metal surface and further reactions inside the liquid droplet produce a dynamical system with spatial chemical gradients, potential gradients, diffusion and convection of species together with water movements 21 – 23 . However, only a little is known about the droplet spreading due to corrosion, e.g., lateral expansion of a NaCl-containing droplet concomitant with pit openings on a Cu surface 24 , formation of micro-droplets and lateral expansion of a thin water layer around the primary droplet in high-humidity conditions 21 , expansion of micro-droplets 25 , and influence of CO 2 level on the secondary spreading dynamics 26 .…”