Editors' Choice—Electrolyte Film Thickness Effects on the Cathodic Current Availability in a Galvanic Couple

Abstract

A combined modeling and experimental study was performed to understand the dependence of the cathodic current delivery capacity on the electrolyte film thickness and cathode size in a galvanic couple. The appropriate cathodic kinetics for the modeling were generated by use of a rotating disk electrode to simulate the electrolyte film thickness. These results provided boundary conditions for a finite element model which calculated the potential distribution along a metallic surface and the associated cathodic current supplied for electrolyte layers of varying thickness. The total cathodic current was then calculated through integration of the current density across the surface. Electrolyte layer domains were delineated by three limits which described, in order of decreasing film thickness, i) transition in exposure condition from full immersion to thick film, ii) the hydrodynamic boundary layer due to natural convection which defined the upper limit of the thin film regime, and iii) the relative dominance of ohmic resistance over mass transport in determining the total current output. This study also showed that for sufficiently thin films, this total current was independent of the size of the cathode and the nature of kinetics at the electrochemical interface, being solely driven by the ohmic resistance in solution. Dissimilar engineering alloys in close proximity and in electrical contact are frequently encountered in the architecture of high-value structural assets in the transportation, aerospace, and marine industries. In service, these structures are often exposed to atmospheric environments, such as sea spray, that result in the formation of a thin electrolyte layer on the surface sufficient to allow the dissimilar alloys to form galvanic couples. Under atmospheric conditions, the electrolyte can also form via deliquescence of salt as either a droplet or a thin film on the alloy surface, leading to the establishment of a corrosion cell. The extent of galvanic corrosion on the anode that results depends on a number of environmental, physicochemical, and geometric variables, which include relative humidity, temperature, electrolyte conductivity, electrolyte film thickness, in addition to the electrochemical kinetics on the alloy surface.1-7 Different exposure conditions can be modeled by varying the thickness of the electrolyte film (also termed the water layer, WL), which in turn affects solution resistance as well as cathodic kinetics, thus having a direct effect on the total cathodic current available to support corrosion of the galvanic system.The effects of the electrolyte film thickness on corrosion rate were recognized initially by Tomashov, 2 who qualitatively identified four regions of WL corresponding to different types of reaction control. As film thickness decreased, a transition from a plateau in corrosion rate when a constant diffusion layer was attained (conditions of full immersion) to cathodically controlled corrosion limited by diffusion of dissolved oxygen to the reaction surface (the corr...

A Review of the Application of Finite Element Method (FEM) to Localized Corrosion Modeling

“…These studies interrogated how each parameter explicitly influences the degree of localized corrosion in an effort to make FEM modeling more applicable and pertinent to a realistic corrosion environment. These parameters include: pit/ crevice geometry, [40][41][42][43] electrolyte layer thickness, 40,[44][45][46] corrosion inhibitors, [47][48] and transport of oxygen. [49][50] The advent of commercial off-the-shelf FEM software has greatly facilitated these computational efforts by easing the cost of entry as previously each investigator had to write their own code.…”

Application of Laplace Equation-Based Modeling into Corrosion Damage Prediction for Galvanic Coupling Between a Zinc Plate and Stainless Steel Rods Under a Thin Film Electrolyte

“…where conductivity (κ)=F 2 P j z j 2 u j c j . By enforcing the conservation of charge (Equation [4]),…”

“…1,25 The determination of this value has been discussed elsewhere. 4 The net result of the natural convection limit is that the dependence of the diffusion limited current density can be described by Fick's 1st Law as long as WL is smaller than 800 μm, whereas it is a SCIENCE SECTION constant regardless of WL thickness above that value. It should be also noted that only mass transfer limited cathodic kinetics of SS in terms of ORR were fitted and extrapolated as the activation region of ORR is not affected by the WL.…”

“…The solution to Nernst-Planck Equation results in a full transient expression of the distributions of concentration, potential, and current, but at the cost of very long processing time and increases computational complexity. Recent work [4][5][6][7][8][9] has demonstrated the utility of the use of Laplace's Equation in the modeling of electrochemical distributions by coupling it with experimentally derived electrochemical kinetics as boundary conditions along with estimations of the electrolyte conductivity. However, the accuracy of numerical solutions by the Laplace Equation needs to be assessed for predicting realistic experimental results.…”

“…Stainless steel (SS) is a typical cathode material involved in a number of galvanic coupling systems such as aluminum alloy (AA)/SS, [10][11][12][13] zinc (Zn)/SS, [14][15] and magnesium alloy/SS. [16][17][18] Especially for AA/SS or Zn/SS galvanic couples, the galvanic coupling potential is usually located at the cathodic masstransfer-limited region of SS based on mixed potential theory assuming a cathode-to-anode area ratio is 1∶1, 4,19 which implies cathodic diffusional kinetics of oxygen reduction reaction (ORR) on SS can further impact the degree of galvanic corrosion if corrosion exposure environment can affect the mass transfer of dissolved oxygen in the electrolyte. Zinc (Zn) is an electrochemically active metal material which generally experiences uniform corrosion when coupled with noble materials in saline solutions, leading to corrosion damage that is more straightforward to assess as compared to materials such as AAs which are prone to localized corrosion.…”