Mechanism of Action
Corrosion is a thermodynamically favorable, electrochemical process. Corrosion processes involve the oxidation of a metal and the corresponding reduction of another material (usually oxygen, under normal atmospheric conditions). Iron or steel in contact with aerated water, for example, undergoes the following overall chemical reaction:
This reaction is ultimately the result of two simultaneously
occurring reduction/oxidation reactions:
Electrons pass through the conductive metal substrate from anodic to cathodic corrosion sites (which typically shift in location with time), given that the suitable surface electrolyte is available to complete the electrochemical circuit or cell.
Formation of ferrous hydroxide, followed by oxidation and
dehydration, leads to the final reaction product, ferric oxide or
common `rust´:
Ferric oxide or rust delaminates easily from a steel surface to expose fresh metal, thus allowing for continued corrosion. Under normal atmospheric conditions (without inhibitors present), the ferric oxide does not form an adherent or so-called `passive´ layer on the metal surface. It is also well established that most organic coating films are not sufficiently impermeable to water and oxygen to prevent corrosion based on barrier properties alone. Effective corrosion inhibition by coatings therefore requires other protection mechanisms.
When a coating film containing MOLY-WHITE® pigments becomes exposed to moisture, molybdate ions will pass into solution and migrate to the metal surface due to the controlled solubility characteristics of these pigments. The ability of the molybdate ions to stabilize a thin, adherent and protective layer on iron and other metals is the key to the effectiveness of MOLY-WHITE® corrosion inhibitors. The oxide layer effectively insulates (i.e. passivates) the metal from attack and halts the corrosion process. The passivating properties of molybdates are well recognized and documented in the field of corrosion science.