Many authors state that, for corrosion process if energy of activation is above 20 kJ mol-1, whole process is surface controlled. Is it specific for only corrosion? How magnitude of Ea inflences surface control?
Pls. treat it as an academic discussion as reported in literature.Hopefully, it shall help you to make a headway
Definition - What does Activation Controlled Corrosion mean?
Activation controlled corrosion is a type of corrosion which arises from activation polarization. Activation is the changing of a passive surface of a metal to a chemically active state.
Activation polarization is usually a controlling factor during corrosion in media containing high concentrations of active species or strong acids. Mixed potential theory can be used to explain activation controlled corrosion.
Explaining Activation Controlled Corrosion
Corrosion is an activation-controlled chemical reaction, the rate of which is greatly affected by temperature. Typically, corrosion rate increases significantly as temperature increases. Activation polarization is usually the controlling factor during corrosion in strong acids.
The increased corrosion rate results from increased activation energy for chemical and electrochemical reactions, increased diffusion rates in the electrolyte, and increased transport through the electrolyte or environment and across films that may be formed on the metal surface. If a corrosion reaction is activation controlled, then stirring or increasing agitation has no effect on the corrosion rate.
Activation overpotential controls the electrode reaction at a low reaction rate. The cathodic reaction 2H+ + 2e- = H2 is, in the acid solution, one of the processes controlled by the activation overpotential. The activation overpotential varies with the kind of metal and the electrolytic condition. In most cases, metal dissolution and metal-ion deposition are controlled by the activation overpotential.
For purely activation controlled processes, each reaction can be described by a straight line on a potential, E versus log current, Log i plot, with positive Tafel slopes for anodic processes and negative Tafel slopes for cathodic processes. Mixed potential theory can be used to explain cases where corrosion processes are purely activation controlled or cases where concentration controls at least one of the corrosion processes.
To the best my knowledge and recently reported literatures, the boundary you stated in your question is widely applicable to the corrosion process under organic inhibition system since this system need enough spontaneous ability to replace the adsorb water and other species on the metal surface by the inhibitor molecule. Adsorption of organic compounds on metal surface as corrosion inhibitor at free energy of around -20 kJ/mol or lower indicates the spontaneous physical type of interaction between the metal surface and organic and can be defined as electrostatic interaction between the charged molecules and the charged metal surface (the negative sign indicate only for spontaneous direction) which is not enough to form stable layer blocking the surface against the corrosive species. Whereas, those around -40 kJ/ mol or higher indicate chemical adsorption type involve electrons transfer from the inhibitor molecules to the metal surface forming bonded layer able to making the required surface blocking and at between -30 and -40 kJ/ mol, the adsorption is interaction involving both physical and chemical types.
So, when the corrosion process involving organic inhibitor proceed with surface energy of activation above -20 kJ/ mol, the whole corrosion process become surface controlled by means of chemisorptions of inhibitor or mixed interaction of physical and chemical adsorption as rate determining step for corrosion progress. In this way the organic inhibitor is stable and effective to control the corrosion process by forming stable thin layer able to minimize the corrosive environment to reach the metal surface to some extent. These extents can defined as inhibition efficiency
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