Corrosion refers to the process in which a metal reacts with the environment thus leading to its destructiveness. The electrochemical and chemical reactions that occur because of the instabilities thermodynamics in the environment are what cause corrosion. (1) Typical corrosive environments include water (distilled, fresh, salt), steam, air and humidity, atmospheres (urban, industrial, natural, marine), and gases. Other materials like ceramic and plastics also undergo degradation just like metals. Nonetheless, it is only where metals are involved where the term corrosion is used. There are various technically significant attributes depicted by metals including temperature resistance, tensile strength, and thermal conductivity, among many others which make them distinct from other materials.
All the important components of cars, machines, and instruments are usually made of metal. As such, resistance to corrosion greatly affects their durability. The corrosion in storage (contamination, leaking), construction (bridges- steel reinforcements in concrete, buildings), industry (loss of product, efficiency, and money because of repairs), and transport experience a huge problem since corrosion is not usually visible up to the point when huge damage is already inflicted. (2) There is an extremely high cost associated with corrosion due to the extent of the damage. The average corrosion cost for most industrialized countries is between 3.5 and 4.5% GNP. (1) When adequate preventive measures are utilized, the damage and cost can be minimized. The rate of corrosion can be kept minimal sufficiently when materials and surface treatment are correctly chosen.
A lot of metals naturally exist in the formation of oxides and sulfides. In order to convert these into the pure metal form, a lot of energy will be needed. This implies that they are naturally thermodynamically unstable such that they can be spontaneously converted back to their original form. As such it is extremely difficult to avoid corrosion of metals. The corrosion process can be explained in the form of a galvanic cell.
A closed electric circle is represented by the cell and this includes an aqueous solution with two metal electrodes of cathode and anode. In the event that a conductor links the electrodes, the difference in potential-nobility will cause currents to flow from the cathode to anode. Electropotential series including all semimetals and metals are the ones that determine nobility and these are paced in the order of their potential’s value. (3) Metals that are resistant to oxidation are noble metals and these include Ag, Au, and Pt. Oxidation occurs on the anodes and this includes loss of material (1) and reduction occurs on the cathode (2)-(5). In the event that the metals are exposed to varying environments, then the reaction becomes different. The popularly known reactions include hydrogen formation (2), oxygen reduction in an acidic environment (3), oxygen reduction in an alkaline environment (4), and metal ion reduction (5). These reactions are written as follows:
Such a process will not always involve two varying metals. Similar metals can also be subjected to both oxidation and reduction. The surface of the metal is not completely homogenous most of the time. Various defects that can be experienced include kink sites, adsorption of ions from solution, edges, or a surface or steps contaminants such as an impurity metal’s presence. Redox reactions are caused to occur in a coupled manner because of potential differences in the event that the metal comes into contact with the electrolyte. Corrosion begins when micro galvanic cells are formed on the metal’s surface- the anodic and cathodic sites.
Thermodynamics of Corrosion Process
It is important to note that one cannot directly measure Icorr. Most of the time, this is estimated using the data of current versus voltage. The logarithmic current can be measured versus a potential curve over a range of like a half volt. The scan of the voltage can be centered on Eoc. The measured data is then fit on the corrosion process’s theoretical model. (4) The rates of both cathodic processes and the anodic processes which are controlled by the reaction at the surface of metal of kinetics are assumed by the corrosion process model that we utilize. The Tafel equation, Eq. 1 is obeyed by the electrochemical reaction.
In this equation:
I is the current resulting from the reaction
I0 is a reaction-dependent constant called the exchange current
E is the electrode potential
E0 is the equilibrium potential (constant for a given reaction)
β is the reaction’s Tafel constant (constant for a given reaction, with units of volts/decade.