On the Anodic reaction of the CO2 corrosion process
This figure shows a simulation of the experimental current transient (in red) obtained during CO2 corrosion of carbon steel electrode polished to 15 μm , three different nucleation events occurred at different times (cut lines). They are described by the equation in the figure. Their addition (green line) fit the experimental current. pH = 6.5.
A very fundamental concept that has evade the corrosion science wisdom, as a whole, is the nucleation and phase formation of the corrosion products (CP). In any type of corrosion, the solubility limit of the CP will be fulfilled at the interfase. This is because between the metal and the solution, the concentration of Fe+2 or Fe+3 is the highest. This condition will produce the CP molecule: FeCO3 (CO2 corrosion), FeS (H2S corrosion) or Fe2O3 (atmospheric corrosion). You name it. However, this is far from producing the CP crystals and the protection that could follow. These CP molecules needs to aggregate. The aggregates are called nucleuses. The nucleuses vary in number of molecules. Imaging a mist of CP molecules aggregates.....a colloid. Yes, you have a colloid at the interface in the first instances of any corrosion process. Once the number of CP molecules arrive to a certain size, called critical nucleus. The CP crystal precipitates and grow a layer. The formation of this critical nucleus will depend on the surface roughness, concentration of the colloid, temperature, flow, etc. These conditions are fundamental for any corrosion prediction tool. This is because the quality of the scale and its protectiveness will be determined by the nucleation process.
Imagine you discovered a mathematical equation that truly describes the corrosion rate with great precision!
This is the abstract on a paper about the nucleation of siderite on carbon steel.
Abstract
The anodic reaction of the carbonic acid corrosion process controls the outcome of the degradation of the material. This is because it might produce an FeCO3 scale. In turn, the quality of this crystal will influence the corrosion rate. At the interphase, a colloid of iron carbonate forms. This is fed by Fe+2 coming from the metal dissolution and reacting with the HCO3- coming from the solution. In the colloid, different aggregates of iron carbonate molecules occur. This is until a critical nucleus of the siderite phase occurs, thus forming a siderite crystal. All parameters that normally affect corrosion rate, will affect directly the nucleation, growth and quality of this crystal. Particularly, the metal surface texture and pH of the solution. The nucleation of siderite was studied with anodic chronoamperometry at different surface textures and pH values. It was found that the surface texture is determinant on the law governing siderite nucleation and therefore its scale. FeCO3 formation is required but not sufficient to produce a protective siderite scale. Even after producing such scale, it needs to grow to a certain thickness to offer any protection. Higher pH and rougher surfaces help to achieve good protection. Weight loss corrosion rates measured in autoclave experiments, is controlled by the siderite nucleation phenomena occurring at the interphase.
https://www.researchgate.net/publication/331454097_On_the_Anodic_reaction_of_the_CO2_corrosion_process