How Simple is a Corrosion Process?

Figure 1 is an schematic of the diffusion occurring on a corroding metal. The x axis is the distance from the metal towards the solution. The y axis is the concentration of iron (+2). The thickness of the diffusion layer is around 0.1 nm. This means it is 100 water monolayers thick. The maximum concentration of Fe+2 occurs on the metal surface at zero monolayers of water. All these dissolved iron diffuse through the 100 water monolayers progressively diminishing its concentration in a process called diffusion. This occurs until the concentration gradient reaches the concentration of Fe+2 in the bulk solution. The slope of this concentration gradient is the corrosion rate (see Figure 1). When the flow of the solution increases, the diffusion layer gets closer to the metal surface. The slope for the concentration gradient gets steeper and thus corrosion rate increases.

However, Figure 2 presents that there are way more phenomena occurring in those 100 water monolayers at the same time. At the surface of the corroding metal, there is an excess of Fe+2 ions. This excess charge provokes the alignment of negative charges from the bulk solution. The water acts as an electrical insulator and you ends up having a pseudo capacitor.

Also, the corrosion products produce a scale that covers the metal protecting it from further corrosion. This produces another capacitor, where the corrosion product is the insulator. All these things are described by the infamous equivalent circuit.

Figure 3 is the equivalent circuit for corrosion where a film of corrosion products is produced. Rct is the resistance to the charge transfer (or resistance to polarization, Rp). This is not a resistor, OK? it is an element that describes the rate of the reaction, Fe ---> Fe+2 + 2 e-. If this resistance is low, the rate of this reaction is high. Cdl is the pseudocapacitor described in Figure 2. Rs is the resistance of the electrolyte, which is an ionic resistance! If you have a solution with higher concentration of NaCl, this resistance will be lower. Cc is the capacitance of the corrosion product film formed on the metal and Rpo is the porous resistance due to an ionic path occurring in the corrosion products scale.

Electrochemical impedance sense each element in the equivalent circuit and therefore, helps to better understand the corrosion process.