Predicting Corrosion Potential of Alloy

Chemical process environment often contain corrosive components including acids, salts and their mixtures accompanied by various types of gases. These components potentially lead to significant corrosion issues, equipment failure, regular shutdowns and overhauls of the chemical plants.

Localized and general corrosion of alloys can be evaluated by analyzing electrochemical characteristics of the alloys, including the corrosion potential (CP), repassivation potential (RP), and depassivation pH (DP). In particular, CP and RP can be used together to predict the onset of localized corrosion and to estimate its propagation rate for different types of alloys. However, the electrochemical parameters such as CP or RP are usually obtained from laboratory tests in relatively simple corrosive environments and are not available at complex operational conditions. Therefore, establishing predictive models that could quantify the behavior of alloys in complex process environments is the great idea.

The Corrosion modelling / prediction can be summarized as below:

1. Computation of the thermodynamic properties of the aqueous environment in the bulk and at the surface constitutes the foundation of the model.

2. Alloy dissolution in the active state is modeled, which takes into account the adsorption of electrochemically active species and their surface coverage to calculate the metal dissolution current density.

3. The active-passive transition and consequently, the total anodic dissolution current (i.e., the active dissolution and passive dissolution) are expressed by solving an equation that considers the change of the passive layer coverage fraction with time in the steady-state limit.

4. The repassivation potential is calculated by solving the expression for the current density in an clogged localized corrosion environment. This expression takes into account the effects of competitive adsorption of active species (such as Cl? and H?S) on anodic dissolution. Adsorption of various aggressive species (e.g., Cl?) and inhibitive species (e.g., NO??or SO?2?) has an effect on alloy dissolution and the formation of metal oxide (or sulfide) layer, which are modeled to accompany the repassivation process.

By combining steps 1 through 3 as described above, the corrosion potential and the general corrosion rate are computed by calculating a synthesized polarization curve based on the mixed-potential theory. Taking into account that localized corrosion cannot occur at potentials below the repassivation potential, the calculated RP value (resulting from step IV) is then compared with the CP value to predict the occurrence of localized corrosion and to estimate the maximum propagation rate of localized corrosion as a function of environmental conditions.