Abstract
This paper presents a new approach to model the growth mechanism of the sweet corrosion product (i.e. iron carbonate scale) and to study the influence of the corrosion product morphology on the uniform corrosion rates. Corrosion rates were defined as the iron ion flux rate leaving the metal surface which is governed by the thickness and volumetric porosity of the corrosion product (scale). Nucleation and crystal growth were treated as the two committed steps in forming the scale. The population number and the critical size of the iron carbonate nuclei were determined using the classical nucleation theory. Supersaturation level was quantified by the effusion of iron ions from the metal surface and infusion of carbonate ions from the bulk solution. Crystal growth, followed by nucleation and termination of the supersaturation condition, was modeled using a moving boundary approach solving Fick’s diffusion equations in spherical coordinates. Under stagnant conditions, the formed scale layer thickness and the volumetric porosity were obtained based upon the crystal shape and size. Also, the time-dependent iron ion flux/corrosion rates were predicted. Further, the effects of operating parameters such as pressure and temperature on the competition between the nucleation rate and the growth rate of crystal nuclei were examined.
