Understanding Adsorption Theory in Metal Passivation

Adsorption theory provides insight into the passivation process of metals and their alloys, particularly those in the transition series, such as iron, chromium, nickel, molybdenum, and cobalt. This theory highlights that the unique electronic configurations of these metals—specifically the partly filled 3d or 4d shells—play a critical role in forming strong anodic passivation layers. As the theory suggests, these metals can interact with highly polarized water molecules and anions, facilitating electron sharing that helps create a protective passive state.

At the heart of adsorption theory is the concept of a dynamic steady state, wherein molecules continuously exchange between the adsorbed layer and the surrounding aqueous environment. Researchers like Uhlig have explored the composition required for optimal passivation in iron-chromium alloys, noting that a chromium content of around 13.6 atomic-% is necessary for effective passivation, closely aligning with empirical observations.

However, the adsorption theory must also address scenarios where metals fail to achieve passivity despite being in an active potential range. In these instances, the interaction dynamics change significantly. At lower potentials, metal atoms can lose electrons and form soluble cations, increasing their susceptibility to corrosion. Conversely, in the passive state, these interactions shift to facilitate the formation of stabilizing anions, which only occur above a specific passivating potential.

While the adsorption theory effectively explains instantaneous passivation, it has faced criticism for not fully accounting for thicker passive films often present on metals. This is where the film theory complements the adsorption perspective, emphasizing not only the nature of the protective film but also the underlying kinetics of dissolution and passivation processes. Together, they provide a more comprehensive understanding of how passivity operates in various metal systems.

The breakdown of passivity is critical to consider, as metals and alloys relying on this protective mechanism are vulnerable to corrosion when passivation fails. The nature of corrosion damage can reveal the underlying causes, whether due to widespread uniform dissolution or selective attack at isolated active sites. Understanding the interplay between active and passive states is essential, as localized corrosion can lead to severe material degradation, particularly in complex environments where mixed metal systems are involved.